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Electrical stimulation of the orbicularis oculi to restore eye blink

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Electrical stimulation of the orbicularis oculi to restore eye blink

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ELECTRICAL STIMULATION OF THE ORBICULARIS

OCULI TO RESTORE EYE BLINK

by

Nicholas Alexander Sachs

A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(BIOMEDICAL ENGINEERING)

August 2007

Copyright 2007 Nicholas Alexander Sachs

ii
Dedication
I would like to dedicate this to my grandfathers, whose strength of character and love
of learning have been a constant inspiration to me, and to my family, whose love and
support have guided me through these seemingly endless years.

iii
Acknowledgements
First and foremost I would like to thank my advisor, Jim Weiland, for giving me the
opportunity to pursue this research, for pushing me when I needed to be pushed, and
for being understanding through the entire process. I always felt like a colleague in
his presence and the confidence, experience, and vision that I have gained from
working with him are assets I will carry with me throughout my career.
I would like to thank Eli Chang for making those long dark days in the vivarium
bearable. His dedication to both professional excellence and the enjoyment of life are
an example of the balance that I strive for in my own life.
I would like to thank Jerry Loeb for his uncompromising commitment to excellence.
Over the years he has taught me to raise my expectations of myself and has given me
the opportunity to develop myself as a teacher, for which I am truly grateful.
I would also like to acknowledge all of the other professors, mentors, and advisors
that have influenced me throughout my graduate career, especially Kathy Allen,
David D’Argenio, Mark Humayun, Michael Khoo, Florian Mansfeld, Danielle
Mihram, Michael Quick, Frances Richmond, Armand Tanguay, and Chad Topaz.
Each has been an inspiration to me in his or her own way and allowed me to develop
different aspects of myself that I will carry into my professional career.

iv
I would like to acknowledge all of the fellow students and staff that I have shared a
lab or a classroom with over the years. In particular I would like to thank Alice Cho,
Raj Khalsa, Jason Lee, Tim Odell, Ray Peck, Vivek Pradeep, Claudia Rodriguez,
Brandon Sorenson, Neha Vyas, and Jeff Young, who each put in countless hours to
help design instrumentation or analyze data that contributed directly to this thesis. I
would also like to mention Aditi, Adrian, Alan, Ashish, Carla, Brooke, Christian,
Devyani, Jack, Leanne, Matthew, Michelle, Mohammed, Neha, and Roberto who
have made my days at Doheny so enjoyable.
I would like to thank my close friends, both old and new, who have supported me
over the years and waded through the insanity of the last few months with me. I
hesitate to mention names because I know that I will inevitably forget someone
important, but hopefully those who have touched my life realize the impact that they
have had and will forgive the oversight. I want to especially thank Phil, who has
been there for me since I first set foot on campus and whose unwavering support has
helped me through not only this but all of my endeavors. I also want to thank Dave,
Jessica, and Hilton. We started this journey together, and though our paths may have
veered slightly along the way, they have provided support and insight that have
helped to keep me on track, and for that I am extremely grateful. I would also like to
give special thanks to Adam, Amanda, Daisy, Daniel, Daren, Devyani, DJ, Jacques,
Jen, Jeni, Jennifer, Jeremy, Joe, Mira, Molly, Peter, Rachel, Rich, Ronalee, Ryan,
Sisi, Stan, and Tim for encouraging me and allowing me to be a part of their lives.

v
I cannot thank my family enough for the love and support that they have shown me
over the years. I know it has taken a lot of patience and sacrifice on their part for me
to get this far, but hopefully they feel that paying off now the same way that I do. I
would like to thank my Mom and Dad for always being there for me in whatever
capacity I needed and for making the trek out to Los Angeles so many times to visit
over the years. I would like to thank Scott and Andy for their brotherly support and
encouragement. I would like to especially thank my Grandma Shirley for taking such
an interest in my education and celebrating that with me on so many occasions. The
rest of my family is simply too large to name, but they have all played a role in my
growth over the years and have my heartfelt thanks for that. I am looking forward to
celebrating this accomplishment with them.
I would like to thank the Doheny Eye Institute and the National Institutes of Health
for providing the resources and funding that made this work possible.
And finally, the University of Southern California has been my home for the past 10
years. I am truly thankful for the friendships I have made and the opportunities that
have been provided to me during my tenure here. It is a bittersweet feeling to bid my
final farewell to the cardinal and gold, but I do so with excitement for what lies
ahead and the knowledge that whichever way my path leads from here I will always
be proud to call myself a member of the Trojan family.

vi
Table of Contents
Dedication ii

Acknowledgements iii

List of Tables x

List of Figures xi

Abstract xv

1. Introduction 1
1.1. Relevant Anatomy and Physiology 1
1.1.1. The facial nerve 2
1.1.2. The eyelids 9
1.1.2.1. The orbicularis oculi 11
1.1.2.2. The levator palpebrae superioris 18
1.2. Facial Nerve Palsy 22
1.2.1. Incidence and etiologies 22
1.2.2. Complications 27
1.2.2.1. Psychological distress 28
1.2.2.2. Functional deficits 30
1.2.3. Current treatments 30
1.2.3.1. Medication (corticosteroids and acyclovir) 34
1.2.3.2. Nerve decompression 35
1.2.3.3. Nerve repair and substitution 35
1.2.3.4. Muscle transfer 38
1.2.3.5. Surgical suspension and lower lid procedures 39
1.2.3.6. Physical therapy 41
1.2.3.7. Artificial tears and ointments 42
1.2.3.8. Lid loading with gold weights 43
1.2.3.9. Implantation of mechanical springs 45
1.2.3.10. Tarsorraphy 50
1.3. Neuromuscular Stimulation & Neural Prostheses 50
1.3.1. Biophysics of electrically excitable tissue 52
1.3.1.1. Anatomy of nerve and muscle cells 52
1.3.1.2. Cell membranes and ion channels 56
1.3.1.3. Passive electrical properties of nerve and muscle cells 61

vii
1.3.1.4. Action potential formation and propagation 66
1.3.1.5. Electrical elicitation of action potentials 70
1.3.1.6. Difficulties of stimulating denervated muscle 78
1.3.2. History of neuromuscular stimulation 81
1.3.2.1. Upper extremities 83
1.3.2.2. Lower extremities 84
1.3.2.3. Bowel and bladder 86
1.3.2.4. Respiratory function 87
1.3.2.5. Future directions 88
1.3.3. Previous studies of orbicularis oculi stimulation 90
1.3.3.1. Tobey & Sutton 91
1.3.3.2. Rothstein & Berlinger 92
1.3.3.3. Otto (et al.) 93
1.3.3.4. Salerno et al. 96
1.3.3.5. Somia et al. 98
1.3.3.6. Gittins et al. 101
1.4. Goals of this Research 102

2. Denervation and Reinnervation in the Rabbit Orbicularis Oculi 103
2.1. Neurophysiology of Denervation and Reinnervation 103
2.1.1. Facial nerve microanatomy 104
2.1.2. Mechanisms of nerve degeneration and regeneration 106
2.1.2.1. Cell body 106
2.1.2.2. Axon 107
2.1.2.3. Muscle 108
2.1.3. Classification of nerve injury and expected recovery 109
2.1.3.1. First degree (neurapraxia) 109
2.1.3.2. Second degree (axonotmesis) 110
2.1.3.3. Third degree (neurotmesis) 110
2.1.3.4. Fourth degree (neurotmesis) 110
2.1.3.5. Fifth degree (neurotmesis) 111
2.1.4. Denervation and reinnervation following facial paralysis 111
2.2. Electrophysiological Investigation of Orbicularis Reinnervation 112
2.2.1. Background 112
2.2.2. Materials and Methods 114
2.2.2.1. Facial nerve dissection 114
2.2.2.2. Strength-duration curves 116
2.2.2.3. Electrophysiological nerve tracing 116
2.2.3. Results 118
2.2.3.1. Facial nerve dissection 118
2.2.3.2. Strength-duration curves 118
2.2.3.3. Electrophysiological nerve tracing 120
2.2.4. Conclusions and Discussion 122

viii
3. Acute Electrical Stimulation of the Orbicularis Oculi in Rabbit 130
3.1. Background 130
3.2. Materials and Methods 132
3.2.1. Dissection of the facial nerve 132
3.2.2. Verification of paralysis 134
3.2.3. Electrode placement 135
3.2.4. Electrical stimulation protocol 136
3.2.5. Blink recording and data analysis 138
3.2.6. Histological analysis 138
3.2.7. Statistical analysis 140
3.3. Results 140
3.3.1. Surgical procedures 141
3.3.2. Strength-duration curves for lid twitch 141
3.3.3. Eyelid closure with single stimulation pulses 149
3.3.4. Eyelid closure with stimulating pulse trains 155
3.3.5. Histological analysis of muscle atrophy 159
3.4. Conclusions and Discussion 159

4. Chronic Electrical Stimulation of the Orbicularis Oculi in Rabbit 165
4.1. Background 165
4.2. Materials and Methods 167
4.2.1. Development of chronic stimulating electrodes 168
4.2.1.1. Design 168
4.2.1.2. Fabrication 168
4.2.1.3. Implantation 171
4.2.2. Electrical stimulation protocol 173
4.2.2.1. Chronic stimulation 176
4.2.2.2. Stimulation for data recording 178
4.2.3. Data collection and analysis 180
4.3. Results 183
4.3.1. Chronic stimulating electrodes 183
4.3.2. Tolerance and effect of chronic stimulation 184
4.3.3. Strength-duration curves for lid twitch 185
4.3.4. Quantitative analysis of lid closure 188
4.4. Conclusions and Discussion 195

5. Restoration of Symmetric Blink Function via Electrical Stimulation 197
5.1. Kinematics of Electrically Elicited Eyelid Closure 197
5.1.1. Background 198
5.1.2. Materials and Methods 199
5.1.2.1. Dissection of the seventh cranial nerve 199
5.1.2.2. Verification of paralysis 200
5.1.2.3. Electrode placement 201

ix
5.1.2.4. Electrical stimulation protocol 203
5.1.2.5. Blink recording and analysis 203
5.1.3. Results 204
5.1.4. Discussion and Conclusions 209
5.2. Contralaterally Triggered Orbicularis Oculi Stimulation 209
5.2.1. Background 211
5.2.2. Materials and Methods 212
5.2.2.1. Dissection of the seventh cranial nerve 212
5.2.2.2. Chronic electrode implantation 213
5.2.2.3. Verification of paralysis 214
5.2.2.4. EMG triggering instrumentation 216
5.2.2.5. EMG recording and triggering protocol 219
5.2.2.6. Blink recording and data analysis 221
5.2.3. Results 224
5.3. Discussion and Conclusions 229

6. General Conclusions 231
6.1. Key Findings 231
6.2. Future Directions 232
6.2.1. Comments on the rabbit model 233
6.2.2. Tolerability of stimulation in humans 235
6.2.3. Functional benefits of contralateral triggering 235
6.2.3.1. Bilateral LPS relaxation 236
6.2.3.2. Bell’s phenomenon 237
6.2.4. Determination of aesthetic requirements 237
6.2.5. Control methods for eliciting a proportional response 238
6.2.6. Ipsilateral triggering methods for bilateral palsy 239
6.2.7. Electrode design and stability 240
6.2.8. Potential use of reinnervation 240
6.3. Final Words 242

References 243

x
List of Tables
Table 1.1 The Cranial Nerves 3
Table 1.2 The Innervation and Action of the Facial Muscles 8
Table 1.3 Etiology and Incidence of Facial Palsy in the US 23
Table 1.4 Resting Ion Distribution in the Nerve Cell 58
Table 1.5 Charge Density Limits for Electrode Materials 73
Table 2.1 Threshold Values for Electrophysiology Twitch Stimulation 119
Table 3.1 Chronaxie and Rheobase Values for Acute Stimulation 143
Table 3.2 Maximum Closure Values for Acute Stimulation 152
Table 3.3 Histological Comparison of Muscle Fiber Atrophy 162
Table 4.1 Chronaxie and Rheobase Values for Chronic Stimulation 187

xi
List of Figures
Figure 1.1 Diagram of Facial Nerve Anatomy 4
Figure 1.2 Primary Branches of the Peripheral Facial Nerve 6
Figure 1.3 Anatomy of the Peripheral Facial Nerve 7
Figure 1.4 Regions of the Eyelid Surface 10
Figure 1.5 Anatomy of the Eyelids 12
Figure 1.6 Anatomy of the Orbicularis Oculi Muscle 13
Figure 1.7 Neuromuscular Junction Distribution in the Orbicularis Oculi 15
Figure 1.8 Muscle Fiber Distribution in the Orbicularis Oculi 16
Figure 1.9 The Lacrimal Pumping System 17
Figure 1.10 LPS and OO Activity During Gaze and Blinking 20
Figure 1.11 Percent Incidence for Major Etiologies of Facial Palsy 24
Figure 1.12 Patient with Unilateral Facial Paralysis 29
Figure 1.13 Lagophthalmos on Attempted Lid Closure with Facial Palsy 31
Figure 1.14 Paralytic Ectropion of the Lower Lid Due to Facial Palsy 32
Figure 1.15 Corneal Damage as a Result of Deficits in Eyelid Function 33
Figure 1.16 Wedge Resection to Treat Paralytic Ectropion 40
Figure 1.17 Gold Weight Implantation Procedure 44
Figure 1.18 Facial Palsy Patient with Gold Weight Implant 46
Figure 1.19 Implantation of Open and Closed Palpebral Springs 48

xii
Figure 1.20 Facial Palsy Patient with Palpebral Spring Implant 49
Figure 1.21 Facial Palsy Patient with Medial and Lateral Tarsorrhaphy 51
Figure 1.22 Anatomy of a Neuron 53
Figure 1.23 Anatomy of the Neuromuscular Junction 55
Figure 1.24 Passive Electrical Properties of a Neuron 62
Figure 1.25 Timecourse of an Action Potential 69
Figure 1.26 Current Flow in a Fiber Induced by a Monopolar Electrode 74
Figure 1.27 Chronaxie Curve 76
Figure 1.28 Electrode Configurations Investigated by Somia et al. 100
Figure 2.1 Classification and Expected Recovery of Neural Injury 105
Figure 2.2 Strength Duration Curves for Electrophysiological Testing 121
Figure 2.3 Electrophysiological Nerve Tracing for Rabbit BL48 (8 weeks) 123
Figure 2.4 Electrophysiological Nerve Tracing for Rabbit BL49 (8 weeks) 124
Figure 2.5 Electrophysiological Nerve Tracing for Rabbit BL51 (16 weeks) 125
Figure 2.6 Electrophysiological Nerve Tracing for Rabbit BL52 (16 weeks) 126
Figure 2.7 Electrophysiological Nerve Tracing for Rabbit BL53 (16 weeks) 127
Figure 2.8 Electrophysiological Nerve Tracing for Rabbit BL54 (16 weeks) 128
Figure 3.1 Acute Electrode Placement 133
Figure 3.2 Data Analysis Procedure 139
Figure 3.3 Strength-Duration Curves for Twitch Threshold 142
Figure 3.4 Strength Duration Curve for Reinnerverted Twitch Threshold 147
Figure 3.5 Stimulus-Response Curves for Single Pulses 150

xiii
Figure 3.6 Stimulus-Response Curves for Single Pulses at 1 Week 151
Figure 3.7 Stimulus-Response Curves for Pulse Trains 156
Figure 3.8 Stimulus-Response Curves for Pulse Trains at 1 Week 157
Figure 3.9 Sample Histology Image from a Rabbit Eyelid 160
Figure 3.10 Histological Comparison of Muscle Atrophy 161
Figure 4.1 Typical Design of Skull Mounted Pedestal 169
Figure 4.2 Photograph of Chronic Electrode 172
Figure 4.3 Chronic Electrode Placement 174
Figure 4.4 Photograph of Implanted Pedestal 175
Figure 4.5 Example of Chronic Stimulation 177
Figure 4.6 Rabbit in Restraint Box 179
Figure 4.7 Mirror System for Bilateral Blink Recording 181
Figure 4.8 Data Analysis Procedure 182
Figure 4.9 Strength-Duration Curves for Twitch Threshold 186
Figure 4.10 Bilateral Recording of OO Stimulation 190
Figure 4.11 Stimulus Response Curves for BL58 (Unstimulated Control) 191
Figure 4.12 Stimulus Response Curves for BL62 (Chronic Stim) 192
Figure 4.13 Stimulus Response Curves for BL71 (Chronic Stim) 193
Figure 5.1 Acute Electrode Placement 202
Figure 5.2 Image Processing for Kinematic Analysis 205
Figure 5.3 Eyelid Kinematics for Single Pulses 206
Figure 5.4 Eyelid Kinematics for Pulse Trains 208

xiv
Figure 5.5 Kinematics of Normal Rabbit Eye Blink 210
Figure 5.6 Chronic Electrode Placement 215
Figure 5.7 Instrumentation for Blink Triggering 217
Figure 5.8 EMG Triggering Circuit 218
Figure 5.9 Rabbit in Restraint Box 220
Figure 5.10 Mirror Setup for Bilateral Blink Recording 222
Figure 5.11 Image Analysis for Blink Symmetry 223
Figure 5.12 EMG Recording with Chronic Electrode 225
Figure 5.13 EMG Threshold Signal Generation 226
Figure 5.14 Contralateral EMG Triggered Blink Response 227
Figure 5.15 Bilateral Kinematics for EMG Triggered Stimulation 228

xv
Abstract
Brought about by dysfunction of the seventh cranial nerve, facial paralysis results in
the inability to contract the facial muscles and leads to host of functional and
psychological deficits. The most significant clinical outcome of facial nerve palsy is
the loss of the ability to blink the eye, which results in corneal exposure and can lead
to such complications as corneal ulceration, permanent vision loss, and even
potential loss of the eye. Current methods for restoring eye closure include the
implantation of gold weights or mechanical springs into the eyelid, the use of
artificial tears, and tarshorrhaphy. These passive measures are largely ineffective and
both surgically and cosmetically unappealing. The use of electrical stimulation to
reactivate the paralyzed orbicularis oculi muscle, which generates eye blink in
healthy individuals, has the potential to provide a much more elegant and effective
means of eliciting eye closure.
This thesis investigated the use of electrical stimulation of the orbicularis oculi
muscle as a means of restoring blink function in a surgically induced animal model
of orbicularis oculi paralysis. Both acute and chronic stimulation protocols were
carried out, as well as a study that used electromyographic activity from a healthy
eyelid to trigger the delivery of stimulation pulses to its paralyzed counterpart. This
research represents the first quantitative assessment of eyelid closure generated by

xvi
electrical stimulation of the paralyzed orbicularis oculi. It also represents the only
known study to examine the effects of stimulation at multiple time-points following
paralysis, both with and without chronic stimulation, and to note the effects of
orbicularis oculi reinnervation on the ability to generate electrically stimulated eyelid
closure. Finally, it provides the first quantitative evidence of the ability to restore
synchronized blink function in a unilateral model of facial paralysis.
The ultimate goal of this work is to advance the understanding of orbicularis oculi
stimulation and enable the transition from animal models to human studies. This
research will hopefully lay the groundwork for the successful development of a
clinical neuroprosthesis for the restoration of eye blink function.

1
Chapter 1 : Introduction
The work presented in this thesis involves a neural engineering approach to replacing
specific function that is lost due to damage of the facial nerve. This chapter provides
necessary background for understanding the complexity of the problem as well as the
reasoning behind the approach chosen. The following sections will focus on the
anatomy and physiology of the facial nerve and muscles of the eyelid, the causes and
complications of facial palsy along with a review of current treatments, the history of
neuromuscular stimulation, and the goals of the work presented.
1.1 Relevant Anatomy
This section provides basic anatomy for the structures involved in normal blink
function that are affected by facial paralysis. Much attention is paid to the seventh
cranial (facial) nerve, which is responsible for innervating the facial musculature,
including the muscles that close the eye. The general structure of the eyelids is also
detailed, with particular focus on the muscles that control eye blink: the orbicularis
oculi (which closes the eye) and the levator palpebrae superioris (which opens it).

2
1.1.1 The facial nerve
The facial nerve is listed 7
th
out of the 12 pairs of cranial nerves (see Table 1.1). As
with the other cranial nerves, it is differentiated from the peripheral nervous system
by its origin within the cranium, rather than at the spinal cord. Anatomically, its cell
bodies are located in the pontine-facial nucleus, where they synapse with axonal
projections originating from various supranuclear structures, including the voluntary
motor cortex, internal capsule, extrapyramidal system, upper midbrain, and lower
brainstem. From the pontine nucleus, the facial nerve emerges from the brainstem to
make its way through a series of narrow canals (internal auditory and fallopian)
within the temporal bone before exiting at the stylomastoid foramen. Figure 1.1
shows a diagram of the main anatomical features of the facial nerve.
After exiting the cranium the external facial nerve has been described to “branch like
a tree”. It begins this process approximately 2 cm after exiting the stylomastoid
foramen by bifurcating into upper and lower divisions that run through the parotid
gland and continue to divide in a pseudorandom fashion that is characterized by five
main branches (listed superior to inferior: temporal, zygomatic, buccal, mandibular,
and cervical) (R. A. Davis, Anson, Budinger, & Kurth, 1956). The main trunk of the
facial nerve is approximately 3 mm in diameter and becomes progressively smaller
as the approximately 10,000 axons (each with an individual diameter of 3-20 µm)
disperse, often crossing paths and intermingling with other branches before re-

3

TABLE 1.1
The Cranial Nerves
Number Common Name
Functional
Classification
Specific Function
I Olfactory Sensory (special) Smell
II Optic Sensory (special) Vision
III Oculomotor Motor (somatic)
Levator palpebrae superioris muscle;
Inferior, medial, and superior rectus muscles
IV Trochlear Motor (somatic) Superior oblique muscle
Motor (bronchial) Muscles of mastication
V Trigeminal
Sensory (general)
Sensation for the head, neck, sinuses, meninges,
and external surface of tympanic membrane
VI Abducens Motor (somatic) Lateral rectus muscle
Motor (bronchial) Muscles of facial expression and stapedius
Motor (visceral)
Lacrimal and salivary glands
(parasympathetic, excluding parotid)
VII Facial
Sensory (special) Taste (anterior two-thirds of tongue)
VIII Vestibulocochlear Sensory (special) Hearing and balance
Motor (bronchial) Stylopharyngeus muscle
Motor (visceral) Parotid gland
IX Glossopharyngeal
Sensory (special) Taste (posterior one-third of tongue)
Motor (bronchial) Muscles of the pharynx and larynx
X Vagus
Motor (visceral)
Muscles of the neck, thorax, & abdomen
(parasympathetic)
XI Spinal Accessory Motor (bronchial) Trapezius and sternocleidomastoid muscles
XII Hypoglossal Motor (somatic) Muscles of the tongue (excluding palatoglossal)
List of the names and functions of the 12 pairs of cranial nerves.

4

Figure 1.1 Diagram of Facial Nerve Anatomy. The facial nerve travels alongside other nerves
and branches at specific locations as it makes its way from the pontine facial nucleus, where it
originates, to the stylomastoid foramen, where it exits the cranium. The right column represents
diagnostic tests that can be used to determine lesions at different locations along its path. Adapted
from (M. May, Fria, Blumenthal, & Curtin, 1981).

5
dividing and sorting themselves out en route to their final destinations (see Figure
1.2 and Figure 1.3) (M. May, 2000a).
The primary physiological functions of the facial nerve include the following:
1. Motor innveration of the facial muscles for both voluntary and spontaneous
emotional expression as well as protective function.
2. Motor innervation of the stapedius muscle, which acts in reflex to dampen
loud noises in the lower frequency regions.
3. Motor innervation of the lacrimal and salivary glands (with exception of the
parotid).
4. Sensory innervation of the anterior two-thirds of the tongue to provide taste
function.
Of the 10,000 axons in the facial nerve, approximately 7000 are myelinated motor
axons destined to innervate the facial muscles (Van Buskirk, 1945). Each axon
innervates a single motor unit of a specific facial muscle, which is comprised of
approximately 25 muscle fibers (M. May, 2000b). The individual muscles, along
with their innervating branch(es) of the facial nerve and primary action are listed in
Table 1.2.

6

Figure 1.2 Primary Branches of the Peripheral Facial Nerve. Note the amount of variety in
facial nerve branching and the degree of normal branch intermingling (particularly in the superior
branches). Adapted from (R. A. Davis et al., 1956).

7

Figure 1.3 Anatomy of the Peripheral Facial Nerve. Note the substantial degree of branching as
well as intermingling of branches prior to final termination in the facial muscles. Adapted from (M.
May, 2000a).

8

TABLE 1.2
The Innervation and Action of the Facial Muscles
Muscle Facial Nerve Branch Primary Action
Posterior auricular Pulls ear backward
Occipital belly of epicranius
(occipitofrontalis)
Posterior auricular
Moves scalp backward
Anterior auricular Pulls ear forward
Superior auricular Raises ear
Frontal belly of epicranius
(occipitofrontalis)
Moves scalp forward
Corrugator supercilli
Pulls eyebrow medially and downward
(vertical wrinkles)
Procerus
Temporal
Pulls medial eyebrow downward
Orbicularis oculi Temporal and zygomatic Closes eyelids and contracts around eye
Zygomaticus major Zygomatic and buccal Elevates corners of mouth
Zygomaticus minor Elevates upper lip
Levator labii superioris
Elevates upper lip and midportion
nasolabial fold
Levator labii superioris
alaeque nasi
Elevates medial nasolabial fold and nasal
ala
Risorius Aids smile with lateral pull
Buccinator
Pulls corner of mouth backward and
compresses cheek
Lvator anguli oris
Pulls angles of mouth upward and toward
midline
Orbicularis oris Closes and compresses lips
Nasalis (Dilator naris) Flares nostrils
Nasalis (Compressor naris)
Buccal
Compresses nostrils
Depressor anguli oris Buccal and mandibular Pulls corners of mouth downward
Depressor labii inferioris Pulls down lower lip
Mentalis
Mandibular
Pulls skin of chin upward
Platysma Cervical Pulls down corners of mouth
Stapes stylohyoid and digastric muscles not included. Adapted from (Schaitkin & Eisenman, 2000).

9
1.1.2 The eyelids
The eyelids reside adjacent to the anterior surface of the eye and play an important
role in maintaining the health of the eye and of the corneal surface in particular.
They accomplish this primarily through the following mechanisms:
1. Lacrimal secretions by glands contained in the eyelids lubricate the corneal
surface with a protective tear film of high optical quality.
2. Voluntary, spontaneous, and reflex blinks spread lacrimal secretions across
the corneal surface to maintain the integrity of the tear film and serve to
protect the corneal surface from external insult and airborne debris.
3. Sensory function of the eyelashes (cilia) triggers reflex blinking in response
to potential external insult.
In addition to their protective role, the mechanical properties of the eyelids help to
maintain the physical position of the eye within the orbit (the bony cavity in which
the eye resides, also known as the eye socket) and regulate the amount of light that is
allowed to enter the eye (Hart Jr., 1992).
The external surface and boundaries of the eyelids are divided into specific regions
that are useful anatomical descriptors (see Figure 1.4). The exposed portion of the

10

Figure 1.4 Regions of the Eyelid Surface. The palpebral fissure is bounded vertically by the
upper and lower lid margins and horizontally by the medial and lateral canthi. Adapted from (Hart Jr.,
1992).

11
eye surface is known as the palpebral fissure. It is bounded vertically by the upper
and lower lid margins and horizontally by the medial and lateral canthi. The
palpebral fissure measures approximately 8-10 mm in vertical width and 27-30 mm
in horizontal length with the eyelids in their open resting position for normal adults
(Hart Jr., 1992).
Anatomically, the eyelids are composed of five principal layers of tissue, each with
discrete functions. From anterior to posterior these include the skin, the orbicularis
oculi muscle, the levator palpebrae superioris aponeurosis, the tarsal plate, and the
conjunctiva (see Figure 1.5). The skin, tarsal plate, and conjunctiva all have
protective and structural functions while the orbicularis oculi and levator palpebrae
superioris are responsible for eyelid movement. These two muscles will be described
in more detail in the following subsections.
1.1.2.1 The orbicularis oculi
The orbicularis oculi (OO) is the sphincter-like muscle that is located about the orbit
and is responsible for eye closure. It is innervated by the temporal and zygomatic
branches of the facial nerve, and is divided into two main anatomical regions, and
one smaller region in between (see Figure 1.6). The palpebral (pretarsal) portion of
the OO is located within the eyelids. This region is responsible for normal blinking
function and is under both voluntary and reflex control. The orbital portion, which
surrounds the eyelids, on the other hand, is responsible for forced eye closure and is

12

Figure 1.5 Anatomy of the Eyelids. Note the five principal layers that make up the eyelid: the
skin, orbicularis oculi muscle, levator palpebrae superioris aponeurosis, tarsal plate, and conjunctiva.
Adapted from (Hart Jr., 1992).

13

Figure 1.6 Anatomy of the Orbicularis Oculi Muscle. The muscle is divided into the (A)
orbital, (B) preseptal, and (C) palpebral (or pretarsal) regions. The palpebral region is responsible for
blinking while the orbital region is responsible for forced closure. Adapted from (Hart Jr., 1992).
A
B
C

14
only under voluntary control. The preseptal region lies between the orbital and
palpebral regions.
Orbicularis oculi muscle fibers average approximately 23.13 µm in diameter
(Freilinger, Happak, Burggasser, & Gruber, 1990). The OO is classified as a phasic
muscle and is comprised of over 80% type II fibers. Each motor unit consists of
approximately 25 muscle fibers. Neuromuscular junction distribution is region
dependant, appearing in three distinct bands within the orbital/preseptal portion and
demonstrating a more uniform distribution within the palpebral part (see Figure 1.7)
(Lander, Wirtschafter, & McLoon, 1996). Individual muscle fibers originate at the
nasal portion of the frontal bone, the fontal process of the maxilla, and the medial
palpebral ligament (Schaitkin & Eisenman, 2000) and span only a portion of the
length of the eyelid, connecting in series to additional OO fibers that, when
combined, span the length of the eyelid (Figure 1.8). Due to the elliptical shape of
the palpebral fissure, the fibers of the palpebral OO tend to be oriented roughly
horizontally, while the orbital fibers are more circular in their orientation.
Full contraction of the OO results in a substantial downward excursion by the upper
eyelid, accompanied by a smaller upward movement of the lower eyelid. There is
also a notable medial sliding of the skin, toward the bony attachments of the OO
muscle (Doane, 1980). This lateral movement is believed to be important in
sweeping debris to the medial canthus for expulsion as well as encouraging lacrimal
flow (see Figure 1.9). This combination of eyelid closure and tearing function

15

Figure 1.7 Neuromuscular Junction Distribution in the Orbicularis Oculi. Neuromuscular
junctions in the preseptal region form three distinct bands, indicating the presence of in-series muscle
fibers. Adapted from (Lander et al., 1996).

16

Figure 1.8 Muscle Fiber Distribution in the Orbicularis Oculi. Note that OO muscle fibers
traverse only a portion of the eyelid before connecting in series to additional OO fibers. Adapted from
(Lander et al., 1996).

17

Figure 1.9 The Lacrimal Pumping System. (A) Tears that are secreted by the lacrimal glands
near the lateral part of the upper eyelid make their way medially to the lacrimal puncta via capillary
action and eyelid closure. (B) When the orbicularis oculi contracts, it pulls the lacrimal sac open,
causing it to fill with tears. (C) When the orbicularis oculi relaxes, the lacrimal sac collapses and tears
drain through a valve system into the inferior meatus of the nose. Adapted from (M. May, Levine,
Patel, & Anderson, 2000).
A B C

18
maintains the tear film layer that is necessary for corneal protection. This will be
expanded upon in Section 1.2.2.
1.1.2.2 The levator palpebrae superioris
The levator palpebrae superioris muscle (LPS) is the primary antagonist of the OO
and is responsible for raising the upper eyelid. During waking hours, the LPS
contracts tonically to maintain the open position of the eyelids and relaxes during
eyelid closure. Unlike the OO and other facial muscles, the LPS is innervated by the
oculomotor nerve (cranial nerve III) and not the facial nerve, a fact that plays a key
role in both normal and impaired eyelid function.
LPS muscle fibers travel anteriorly from their origin behind the eye, wrap across the
superior portion of the globe, and terminate at a broad aponeurosis that crosses
through the OO and inserts vertically downward into the tarsal plate and skin of the
upper eyelid (see Figure 1.5). Thus, the action of LPS contraction is to pull the upper
eyelid upward and back across the eye, widening the palpebral fissure and exposing
the corneal surface. Incidentally, the insertion of the LPS tendon into the anterior
surface of the skin is what causes the fold (known as a superior palpebral furrow)
that is generally present across the upper lid, and it is a lack of such insertion
(leaving only insertion into the tarsal plate) that results in its absence in Asian races
(Hart Jr., 1992; Sayoc, 1956).

19
The LPS is not under strict voluntary control, but rather responds to the state of other
muscles. Specifically it plays a primary role in both vertical lid saccades and eye
blinks (Hart Jr., 1992; P. J. May, Baker, & Chen, 2002). Figure 1.10 shows the
activity of both the LPS and OO during blinking and during changes in vertical gaze.
The action of the LPS during gaze is closely tied with that of the superior rectus
muscle, such that the LPS increases contraction (raising the position of the upper lid)
when looking up and decreases contraction (lowering the position of the upper lid)
when looking down (see Figure 1.10). This serves to maintain the ideal position of
the upper lid, placing it out of the path of the pupil but in close proximity should a
sudden blink be necessary to protect the cornea from potential danger. As a note, the
OO does not modulate its activity to affect lid position during gaze, but only
responds to generate lid closure. Neither does gravity play a role in adjusting lid
position, as vertical lid movements accompanying saccades will even occur with the
body in an inverted position. Thus it is LPS activity in combination with the passive
mechanical properties of the upper eyelid that dictate its changing position during
gaze (Hart Jr., 1992).
With the exception of its role in lid modulation accompanying gaze, the LPS has a
reciprocal relationship with the OO. The normal waking condition, with the eyelids
open, is accomplished by tonic contraction of the LPS and a lack of activity within
the OO. During blinking these patterns of activity reverse, such that the LPS relaxes
just prior to the initiation of OO contraction at the onset of a blink and resumes

20

Figure 1.10 LPS and OO Activity During Gaze and Blinking. (A) Reciprocal EMG activity of
the LPS (upper trace) and OO (lower trace) during blinking. (B-F) Decreasing degrees of LPS activity
as upper lid descends during slow lowering of vertical gaze. Each trace between B and F represents a
drop of 10 degrees in vertical gaze. Note that OO activity does not increase and therefore does not
contribute to gaze related lowering of the upper lid. Adapted from (Hart Jr., 1992).

21
contraction immediately following OO relaxation at the completion of a blink (see
Figure 1.10). This role reversal also occurs during sleep, with tonic OO activity and
LPS relaxation serving to maintain prolonged lid closure (Hart Jr., 1992).
The left and right LPS muscles are innervated as yoke muscles, meaning that they act
as a team, demonstrating bilateral symmetry in their activity and innervation (Hart
Jr., 1992). This bilateral innervation follows Hering’s law and remains present in
unilateral deficits, such that modification of unconscious activity to accommodate on
the involved side will affect the uninvolved side as well, and symmetric LPS activity
will remain intact despite changes in actual lid movement caused by differences in
the physical condition of the eyelid or other muscles (Gay, Salmon, & Windsor,
1967).
The action of the LPS is supplemented by the smooth muscle of Müller, which helps
to regulate the resting position of the upper eyelid when the eyes are open (Hart Jr.,
1992). Müller’s muscle arises from the undersurface of the LPS and its fibers insert
into the border of the upper tarsus. This function is also supplied in the lower eyelid
by a similar smooth muscle known as the inferior tarsal muscle. These muscles have
negligible effect during blinking, however, and will therefore be largely ignored
and/or lumped with the LPS for purposes of this thesis.

22
1.2 Facial Nerve Palsy
Dysfunction of the facial nerve brought about by trauma or disease results in
paralysis of the facial muscles. The nature of facial palsy is expansive, encompassing
a variety of etiologies and resulting in a wide range of impairments, complications,
and durations of effect. This section reviews the causes and effects of facial palsy, as
well as current methods for managing the associated ophthalmic complications.
1.2.1 Incidence and Etiologies
A survey of the etiologies and incidence of facial palsy based on existing literature
was compiled by Bleicher et al (1996). The authors categorized etiologies of similar
origin and presented their results based on annual incidence in the US. Their findings
are summarized in Table 1.3 and Figure 1.11. This is the most complete single report
of facial palsy incidence available. Most other surveys focus solely on Bell’s palsy,
which is reported to account for approximately half of all facial palsy cases (Bleicher
et al., 1996; Schaitkin, May, & Klein, 2000a). The estimated numbers reported for
incidence of Bell’s palsy vary depending on region and method of collection, falling
between 13 and 40 cases per 100,000 based on medical evaluation and increasing
based on door-to-door surveys (Bleicher et al., 1996; Boerner & Seiff, 1994;
Brandenburg & Annegers, 1993; Katusic, Beard, Wiederholt, Bergstralh, & Kurland,
1986; Nicoletti et al., 2002; Ohye & Altenberger, 1989; Savettieri et al., 1996; Tovi,
Hadar, Sidi, Sarov, & Sarov, 1986). Projecting an average of 26.5 cases per 100,000

23

TABLE 1.3
Etiology and Incidence of Facial Palsy in the US
Etiological
Category
Included Etiologies
Annual
Incidence
(per 100,000)
Annual
Incidence
(Total)
Percentage
Idiopathic Idiopathic (Bell’s palsy) 25.0 75,396 49.6
Ramsay Hunt syndrome
(Herpes zoster infection)
Lyme disease
Infectious
Otitis media
7.7 23,222 15.3
Acoustic neuroma
Parotid tumor
Primary cholesteatoma
Neoplastic
Glomus jugular tumor
6.8 20,508 13.5
Cerebrovascular accidents
Guilain-Barre syndrome
Neurologic
Melkerson-Rosenthal
Syndrome
6.8 20,508 13.5
Birth-related trauma
Traumatic
Temporal bone fracture
4.1 12,365 8.1
Total 50.4 151,999 100.0
Based on results reported by Bleicher et al (1996) and adjusted for current US Census population
estimate of approximately 301,584,000 (January 1, 2007).

24

Figure 1.11 Percent Incidence for Major Etiologies of Facial Palsy. Adapted from (Bleicher et
al., 1996).

25
into the estimated US population of 301,584,000 for January 1, 2007, gives a total
annual incidence for Bell’s palsy of 79,920 and an annual incidence of facial palsy
based on all etiologies of approximately 159,840. These numbers are slightly higher
than those reported by Bleicher et al. The vast majority of facial palsy cases are
unilateral, and prognosis depends on the particular etiology and extent of seventh
nerve involvement. Unfortunately, reliable statistics for long-term outcomes of most
etiologies are not available (Lohne, Bjornsborg, Westerby, & Heiberg, 1986).
The most common etiology of facial paralysis is idiopathic, also known as Bell’s
palsy. It is named after Sir Charles Bell, who is credited with first describing the
seventh cranial nerve and its role in innervating the muscles of facial expression
(Schaitkin, May, Podvinec et al., 2000). Bell’s palsy accounts for approximately half
of reported cases of facial palsy (Bleicher et al., 1996; Schaitkin, May et al., 2000a).
It demonstrates no predilection based on age or race, however, does have increased
rates of incidence in pregnant women (Adour, Byl, Hilsinger, Kahn, & Sheldon,
1978; Peitersen, 1982; Yanagihara, Mori, Kozawa, Nakamura, & Kita, 1984). Of
those affected by Bell’s palsy, 71% experience complete recovery, 13% retain a
slight degree of residual palsy, and the remaining 16% are left with significant
impairment (Bleicher et al., 1996; Peitersen, 1992). Recovery typically begins either
within the first few weeks or after several months. Those who begin to recover
earliest have the best prognosis, while those whose recovery begins after three
months invariably develop sequelae. Bell’s palsy is typically unilateral, and can recur

26
in 4 to 13% of patients (Schaitkin, May, Podvinec et al., 2000; Yanagihara et al.,
1984). The exact cause of Bell’s palsy is unknown, however the leading theories
involve a connection to herpes simplex virus and seventh nerve damage due to
pressure caused by inflammation of the seventh or adjacent nerves within the narrow
bounds of the canals in the temporal bone.
Infection is the most prominent known cause of facial palsy, accounting for
approximately 15.3% of diagnosed cases (Bleicher et al., 1996). The primary
infectious causes of facial paralysis include herpes zoster cephalicus (also known as
Ramsay Hunt syndrome), lyme disease, and otitis media, although a host of other
infectious processes have been indicated in medical literature (Schaitkin, May et al.,
2000a). Lyme disease is a common cause of bilateral facial palsy.
Neoplastic (tumor-related) etiologies are estimated to account for approximately
13.5% of facial palsy cases (Bleicher et al., 1996). These primarily include tumors of
the parotid gland and acoustic neuromas, but are by no means limited to these areas
(Schaitkin, May et al., 2000a). Damage to the facial nerve can result from growth of
the tumor itself or from its surgical removal.
Neurologic etiologies also form the basis for approximately 13.5% of facial paralysis
cases (Bleicher et al., 1996). These can include such causes as cerebrovascular
accidents, Guillain-Barre syndrome, and Melkerson-Rosenthal syndrome among
others (Schaitkin, May et al., 2000a).

27
The last main category of facial palsy etiologies is traumatic, which is estimated to
account for 8.2% of cases (Bleicher et al., 1996). Injury of the seventh cranial nerve
during delivery is the most prevalent single cause for traumatic facial palsy (Falco &
Eriksson, 1990; J. D. Smith, Crumley, & Harker, 1981). It occurs in approximately
1.8 out of every 1000 live births with a recovery rate of 91%. Other traumatic causes
include temporal bone fractures resulting from such things as vehicle accidents and
falls, as well as a myriad of other less common injuries (Schaitkin, May et al.,
2000a).
1.2.2 Complications
Facial paralysis is debilitating from both a physical and a psychological perspective.
The exact manifestation depends on the level of injury and can sometimes be used as
a diagnostic tool (Schaitkin, May, & Klein, 2000b). Unilateral paralysis of all facial
muscles indicates a lesion of the ipsilateral facial nerve or facial nucleus, while
involvement of only the lower half of the face is indicative of a contralateral
supranuclear intracranial lesion (M. May, 2000a; Vlastou, 2006). This is due to the
fact that the OO and forehead muscles are controlled by fibers projecting from both
cerebral cortices, while the lower facial muscle are only controlled by contralateral
projections. Supranuclear lesions can also result in a loss of voluntary function with
preserved emotional response and deficits of other motor functions such as in the
tongue and hand. The primary indicator of a supranuclear lesion, however, is the
presence of muscle tone, reflex activity, and evoked electromyographic (EEMG)

28
activity due to the fact that supranuclear lesions do not destroy the peripheral facial
nerve axons and consequently do not result in denervation of the facial muscles (M.
May, 2000a).
The most immediate consequence of a lower facial nerve injury is loss of tone and
movement on the involved side caused by denervation of the affected facial muscles.
In unilateral facial palsy this results in marked asymmetry that is exacerbated by the
lack of voluntary, reflex, and emotional response of the paralyzed muscles (see
Figure 1.12). Again, depending on the level of injury, this paralysis can be
accompanied by other deficits that can be used diagnostically. For example, lesions
at the level of the cerebellopontine angle or internal auditory canal are often
accompanied by damage to the nervus intermedius or vestibuloacoustic nerves,
which can affect tearing, taste, submandibular salivary flow, hearing, and balance
(see Figure 1.1) (M. May, 2000a)
1.2.2.1 Psychological distress
Patients suffer a great deal of psychological distress as a result of facial paralysis
(Lee, Currie, & Collin, 2004; Lohne et al., 1986; Schaitkin & May, 2000a; Vlastou,
2006). Initially this can be due to the misunderstanding of the cause or severity of
their condition, often fearing they have contracted a life-threatening disorder.
Following reassurance that this is not the case, many patients experience anxiety and
depression caused by the physical deformity resulting from facial palsy. This is due

29

Figure 1.12 Patient with Unilateral Facial Paralysis. Facial palsy patient attempting to contract
the facial muscles bilaterally. Note the flaccid appearance and lack of movement on the involved
(patient’s right) side.

30
to both patients’ consciousness of their distorted appearance, and the lack of ability
to display emotion that facial expression generally conveys.
1.2.2.2 Functional deficits
Functionally, the most significant effect of facial nerve palsy is paralysis of the OO
muscle, which results in an inability to close the eye during spontaneous, reflex, and
forced closure (M. May & Hughes, 1987). This residual opening of the palpebral
fissure during attempted eye closure is known as lagophthalmos (see Figure 1.13)
and can lead to corneal exposure. The lack of muscle contraction disrupts the normal
lacrimal pumping function of the eyelids (see Figure 1.9) and can lead to impaired
drainage and excessive watering of the eye known as epiphora. The loss of tone due
to atrophy of the OO muscle can cause the lower lid to hang away from the eye, a
condition known as paralytic ectropion (see Figure 1.14), as well as resulting in
abnormal lid retraction and widening of the palpebral fissure. Left untreated, such
functional deficits can lead to corneal damage (see Figure 1.15), infection,
perforation, blindness, and loss of the eye (C. Conley & May, 2000; Lee et al., 2004;
M. May et al., 2000; Rahman & Sadiq, 2007; Vlastou, 2006).
1.2.3 Current treatments
While there is no cure for facial nerve palsy, there are a variety of treatments in use
that are directed toward restoration of movement and/or function to either all or part
of the face. General treatments include the use of corticosteroids and acyclovir, nerve

31

A
B
Figure 1.13 Lagophthalmos on Attempted Lid Closure with Facial Palsy. Facial palsy patient
(A) with eyes open and (B) on attempted closure. Note the severe lagophthalmos on the involved
(patients right) side. The cornea is still reasonably protected due to substantial Bell’s phenomenon.
Adapted from (M. May et al., 2000)

32

A
B
Figure 1.14 Paralytic Ectropion of the Lower Lid Due to Facial Palsy. Facial palsy patient with
severe ectropion of the lower lid on the involved (patient’s right) side (A) before and (B) after
corrective surgery. Adapted from (M. May et al., 2000).

33

Figure 1.15 Corneal Damage as a Result of Deficits in Eyelid Function. Corneal perforation
resulting from severe neurotrophic and exposure keratitis secondary to seventh nerve impairment.
Adapted from (M. May et al., 2000).

34
decompression, nerve repair and substitution, muscle transfer, surgical suspension,
and physical therapy (Boerner & Seiff, 1994; C. Conley & May, 2000; Henkelmann
& May, 2000; M. May, 2000c, 2000d, 2000e; Schaitkin & May, 2000a; Swartz,
2000; Vlastou, 2006). In addition, several methods for dealing specifically with
ophthalmic complications have been developed. These include the use of artificial
tears and ointments, lid loading with gold weights, the implantation of mechanical
springs, and tarsorraphy (Abell, Baker, Cowen, & Porter, 1998; Arion, 1972;
Kinney, Seeley, Seeley, & Foster, 2000; Lee et al., 2004; M. May et al., 2000;
Rahman & Sadiq, 2007). While all of these methods can helpful in preserving the
health of the eye, none of them, even used in combination, are fully effective.
Additionally, these techniques are often inconvenient, cosmetically unacceptable,
and subject the patient to multiple surgical procedures. This section will briefly
describe each of the current methods for general and ophthalmic management of
facial palsy, including the advantages and disadvantages of each.
1.2.3.1 Medication (corticosteroids and acyclovir)
Historically, prednisone and acyclovir have both been prescribed as a general
treatment for patients with Bell’s palsy as well as facial palsies of known viral origin,
based on the implication that Bell’s palsy is brought on by virally induced
inflammation or edema of the facial nerve (Schaitkin, May, Podvinec et al., 2000).
The results of controlled studies have been largely contradictory, however, and
evidence from the Cochrane database shows no benefit to the use of either

35
corticosteroids or acyclovir in the treatment of Bell’s palsy (Lee et al., 2004; M.
May, Klein, & Taylor, 1985; Prescott, 1988; Rahman & Sadiq, 2007; Salinas,
Alvarez, Alvarez, & Ferreira, 2002; Sipe & Dunn, 2001).
1.2.3.2 Nerve decompression
Another measure targeted at general recovery from idiopathic and viral palsies
specifically is surgical decompression of the facial nerve (Fisch, 1981; Fisch &
Esslen, 1972; Rahman & Sadiq, 2007; Schaitkin, May, Podvinec et al., 2000). This
treatment method is also based on the implication that Bell’s palsy is caused by an
inflammatory response within the confined space of the canals of the temporal bone
resulting in excessive damaging pressure on the facial nerve axons. Decompression
of the nerve by removal of some of the surrounding bone and opening of the sheath
formed by the epineurium and perineurium is designed to relieve that pressure.
Decompression can lead to unwanted postoperative complications such as
conduction loss and partial hearing loss, however, and has not demonstrated
conclusive benefit over the natural progression of the disease (M. May & Klein,
1983; M. May et al., 1985; Schaitkin, May, Podvinec et al., 2000).
1.2.3.3 Nerve repair and substitution
In cases of peripheral facial nerve trauma, one of the most successful treatment
methods is nerve repair (Boerner & Seiff, 1994; C. Conley & May, 2000; Lee et al.,
2004; M. May, 2000d). When possible, this is performed by direct suturing of the
divided ends of the facial nerve. Otherwise it is performed by attaching a donor

36
nerve graft, commonly harvested from the great auricular or sural nerve, between the
proximal nerve stump and distal segment of the facial nerve. The graft acts as a
conduit for regenerating nerve fibers within the proximal stump to reach the distal
segment, which then directs them to the previously denervated muscle (Nath &
Mackinnon, 2000). This method is highly dependant upon the timing of the
procedure (preferably within 30 days post-injury) as well as the location and
condition of a viable proximal stump and distal segment. When successful it can
restore both voluntary movement and spontaneous mimetic facial expression,
however, even with careful alignment and graft selection, nerve repair typically
results in partial recovery and at least some degree of synkinesis.
In the absence of a viable facial nerve segment, nerve substitution can provide an
alternative neural-based reanimation strategy (Boerner & Seiff, 1994; C. Conley &
May, 2000; Lee et al., 2004; M. May, 2000e; Vlastou, 2006). This includes both
cross-face nerve grafts as well as direct substitution from other cranial nerves. Cross-
face nerve grafts attempt to reinnervate muscles in a way that ties their function to
the contralateral partner. Thus, voluntary or spontaneous contraction of the muscle
on the healthy side of the face will also cause its partner on the involved side to
contract, restoring some sense of symmetry. The primary complicating factor in this
procedure is the large distance that regenerating fibers must cross to reach the target
muscle, and results from surgical procedures have had limited success (M. May,
2000e).

37
Cranial nerve XII (hypoglossal nerve), is the most common candidate for direct
nerve substitution (Boerner & Seiff, 1994; C. Conley & May, 2000; Lee et al., 2004;
M. May, 2000e; Vlastou, 2006). The hypoglossal nerve supplies the motor fibers to
the tongue, so in sectioning it to anastomose to the distal segment of the facial nerve,
function on the ipsilateral side of the tongue is sacrificed. Following the procedure,
patients regain tone on the affected side of the face within 4-6 months, and develop
some voluntary facial movement over the next 18 months (M. May, 2000e). With
dedicated training and re-education, patients can adjust to using their new motor
system, however there is limited potential for emotional expression or selective
muscle contraction, which tends to be overshadowed by mass movement.
Additionally, tone and movement tend to be concentrated within the lower face
(Boerner & Seiff, 1994; Lee et al., 2004; M. May, 2000e). Roughly half of patients
experience good or excellent results from hypoglossal-facial nerve anastomosis, with
best results following early substitution (J. Conley & Baker, 1979; Gavron & Clemis,
1984; Luxford & Brackmann, 1985; M. May, 2000e; Pensak, Jackson, Glasscock, &
Gulya, 1986).
Both nerve repair and substitution are dependant upon the regenerative capabilities
of the nerve. Thus, they are both limited by the growth rate of regenerating fibers,
which is approximately 1 mm/day. The time required for function to manifest itself
is defined by the distance regenerating fibers must travel from the proximal stump to
the denervated muscle, and is typically on the order of several months. Concurrently,

38
the distal segments of the nerve are undergoing Wallerian degeneration while the
denervated muscle is experiencing atrophy, both of which can inhibit reinnervation
over time. For these reasons, timing becomes a critical issue in the success of any
neural reanimation technique (Nath & Mackinnon, 2000).
1.2.3.4 Muscle transfer
When the original facial muscles are not viable for reinnervation, particularly in
cases of long-standing facial paralysis, the transfer of other somatic muscles can be
used in combination with nerve grafting or substitution to provide some function
(Boerner & Seiff, 1994; C. Conley & May, 2000; Lee et al., 2004; M. May, 2000c;
Swartz, 2000; Vlastou, 2006). Techniques include the transposition of regional
muscles such as the temporalis, masseter, and digastric as well as free flaps from a
variety of donor muscles such as the gracilis, latissmus dorsi, inferior rectus
abdominus, and pectoralis minor. These can be constructed to restore smiling by
raising or lowering the corner of the mouth, or eyelid closure by forming an
encircling sling.
Transferred muscles are typically supplied by the ipsilateral or contralateral facial
nerve via nerve graft but can also be supplied directly by the hypoglossal nerve or
the cranial nerve V, which innervates the masseter muscle. For free flaps, this is
generally a two-stage procedure in which a graft is placed first, allowing donor nerve
fibers time to traverse its length, followed by the transfer of the muscle and
neurovascular pedicle, which must be anastomosed to the nerve graft and local

39
vessels. Single-stage free flap procedures involving donor muscles with motor
neurons long enough to reach the supplying nerve are also advocated, but require
extra care in finding proper muscle placement that allows for both satisfactory
function and nerve location (Lee et al., 2004; M. May, 2000c; Swartz, 2000; Vlastou,
2006).
The use of muscle transfers tends to have good results in raising or extending the
corner of the mouth to recreate a somewhat mimetic smile, however, reanimation of
the lower lip and eyelids are more difficult and controversial (Boerner & Seiff, 1994;
Vlastou, 2006).
1.2.3.5 Surgical suspension and lower lid procedures
Surgical suspensions are generally used as complimentary procedures to raise the lip
and face or support the lower eyelid (Boerner & Seiff, 1994; Lee et al., 2004; M.
May et al., 2000; Rahman & Sadiq, 2007). These are generally static, but can be
dynamic in nature when passed around the masseter or temporalis muscles. Dynamic
suspensions create mass movement effects similar to synkinesis, which can be
troubling. Additionally, they do not address the problems of lower lid ectropion or
closing of the eye at night. Lid tightening and shortening procedures such as wedge
resection, on the other hand, can be useful for dealing with paralytic ectropion (see
Figure 1.16) (M. May et al., 2000).

40

Figure 1.16 Wedge Resection to Treat Paralytic Ectropion. (A) A small wedge-shaped region of
the lower lid is removed, (B) it is sutured closed in layers, (C) and the lower lid is tightened. Adapted
from (M. May et al., 2000).

41
1.2.3.6 Physical therapy
Physical therapy is used to manage the state of facial palsy patients who experience
partial but incomplete recovery or have undergone nerve substitution or muscle
transfer techniques (Henkelmann & May, 2000). It should be noted that this does not
facilitate additional recovery, but rather trains patients to use their new neuromotor
system in the most effective way. The goal of such therapy is to teach patients how
to actively isolate specific muscles, which may involve new neural pathways, and
control the amplitude of movement while avoiding synkinesis (Henkelmann & May,
2000; Lee et al., 2004; Rahman & Sadiq, 2007).
Once a stable degree of recovery has been achieved (usually several months after
onset or treatment to allow for the possibility of spontaneous recovery and an
established state of reinnervation), patients undergo a battery of tests to determine
their baseline functional levels. Specific dysfunctions that are assessed include
muscle weakness, loss of individual muscle control, excessive muscle tension and
hypertonicity, and synkinesis. This is followed by educational sessions for the patient
on muscle physiology and explanations of individual muscle actions and interactions
to perform synergistic movements. Patients are then prescribed one or a combination
of therapies, which may include muscle stimulation and relaxation techniques or
exercises incorporating the use of mirrors or surface EMG biofeedback (Henkelmann
& May, 2000; Lee et al., 2004; Rahman & Sadiq, 2007). Ideal outcomes include the

42
restoration of subconsciously activated symmetric and near normal looking
movements.
1.2.3.7 Artificial tears and ointments
The most basic treatment for lubricating the eye following facial paralysis is the use
of eye drops and ointments (Lee et al., 2004; M. May et al., 2000; Rahman & Sadiq,
2007). There are a wide variety of products available, with a range of viscosities.
One complicating factor is the frequency with which drops must be applied. Those
with higher viscosity tend to have a longer lasting effect, however, they also have a
higher degree of opacity and tend to blur vision. Highly viscous ointments are very
good at providing lasting lubrication, but severely blur vision. For this reason, such
ointments tend to be used at night, often in combination with taping of the eyelids
closed or application of a moisture chamber, which both prevent the ointment from
being wiped away and protect the cornea from physical irritation due to accidental
scratching. Additional drawbacks of ointments include the fact that they do not
address the loss of blink symmetry brought on by facial paralysis, and they do not
protect the cornea from physical insult. For these reasons, they are generally not the
ideal treatment for chronic palsy. In Bell’s palsy, however, this type of treatment is
universally prescribed at least until a determination regarding the possibility of
spontaneous recovery can be made (Tucker & Santos, 1999).

43
1.2.3.8 Lid loading with gold weights
The most common surgical ophthalmic procedure for chronic facial nerve palsy is
the use of lid loading (Jobe, 1993; Lee et al., 2004; M. May et al., 2000; Rahman &
Sadiq, 2007; Tucker & Santos, 1999). The implantation of gold weights in the upper
lid has been practiced as a surgical treatment for lagophthalmos for over 40 years
(Smellie, 1966). During implantation an incision is made across the upper eyelid
along the superior palpebral furrow and a thin gold weight weighing between 0.6 and
1.2g is inserted into a pocket made just anterior to the tarsal plate and sutured in
place (see Figure 1.17).
It is actually the action of the LPS that makes upper lid loading effective. The weight
of the implant is carefully calibrated (in a fitting procedure prior to implantation)
such that after it is inserted the tonic activity of the LPS will maintain the eyelid in
its open position despite the gravitational force provided by the presence of the gold
weight. When the LPS relaxes during a normal blink, the gravitational force
provided by the gold weight pulls the upper lid down over the eye resulting in a
substantial, but typically incomplete, degree of closure (Abell et al., 1998). This
partial closure is exacerbated by the fact that the short duration of spontaneous and
reflex blinks does not allow enough time for the weight of the implant to overcome
the inertia of the eyelid and therefore these movements cannot be truly restored (M.
May et al., 2000). Increasing the frequency of voluntary blinking, which involves
slower kinematics and can therefore be restored, enhances the effect of the gold

44

Figure 1.17 Gold Weight Implantation Procedure. (A) An incision is made along the superior
palpebral furrow, (B) the weight is placed in a pocket adjacent to the tarsus and sutured in place, and
(C) the wound is closed. Adapted from (M. May et al., 2000).
A B C

45
weight. Taking this into account, in most cases, a weight can be selected such that
this is sufficient to provide corneal wetting.
One of the advantages of gold weight implantation is that it is relatively inexpensive
and the weight is relatively easy to remove, allowing it to be performed in the early
stages of facial palsy and reversed in the event that reinnervation occurs (Snyder,
Johnson, Moore, & Ogren, 2001). While it can restore a substantial degree of lid
closure and corneal wetting, however, its passive nature precludes it from preventing
lower lid ectropion or restoring lacrimal pump function. Additionally, because it
relies on gravity to lower the upper lid, it is ineffective in the prone position and is
completely ineffective during sleep. As a result of these factors, the use of lid
loading typically requires additional complimentary measures to effectively maintain
eye health. Complications can include infection, implant migration, skin erosion,
poor cosmetic appearance, and extrusion (Jobe, 1993; Pickford, Scamp, & Harrison,
1992; Rahman & Sadiq, 2007; Smellie, 1966; Tucker & Santos, 1999). Figure 1.18
shows an example of a facial palsy patient with a gold weight implant.
1.2.3.9 Implantation of mechanical springs
The implantation of mechanical springs is an alternative to upper lid loading (Lee et
al., 2004; Levine & Shapiro, 2000; M. May et al., 2000; Rahman & Sadiq, 2007).
The technique of palpebral springs was also pioneered over four decades ago (Morel-
Fatio & Lalardrie, 1964). Such springs have a single small coil that rests hear the
lateral canthus, and two long wire ends. Two types of springs have been used to elicit

46

A
B
Figure 1.18 Facial Palsy Patient with Gold Weight Implant. (A) Eyes open and (B) attempted
closure. This particular patient has a poor cosmetic result due to bulging of the implant and some
degree of residual lagophthalmos on the involved (patient’s left) side.. Adapted from (M. May et al.,
2000).

47
eyelid closure, one in which the tension of the spring pushes the wire ends apart, and
one in which the tension of the spring pulls the wire ends together (see Figure 1.19).
The former spring has one wire end that is embedded near the superolateral edge of
the orbit while the other is routed across the upper eyelid. Thus, the tendency of the
spring to want to open puts pressure on the upper lid, forcing it down. The latter
spring is inserted such that one wire end is routed across the upper eyelid while the
other is routed across the lower eyelid. The tendency of the spring to want to close
puts pressure on the lids, forcing them together.
The effect of the palpebral spring is based on the same principles as lid loading, such
that the tension of the spring is insufficient to counteract the tonic activity of the
LPS, but sufficient to cause the lids to close when the LPS relaxes. The primary
benefits of the palpebral spring over lid loading are the fact that springs can provide
more rapid closure than gold weights and that they do not rely on gravitational forces
and will therefore work independent of patient position (M. May et al., 2000).
Therefore they can not only provide a more complete spontaneous or reflex blink,
but can be used in the prone position and should be effective during sleep. The
procedure, however, is much more complex than gold weight insertion, and
increased rates of complications such as bulging through the skin, infection,
extrusion, and revision secondary to these effects tend to make the use of palpebral
springs a lesser alternative (M. May, 1987; Rahman & Sadiq, 2007). Figure 1.20
shows an example of a facial palsy patient with a palpebral spring implant.

48

A
B
Figure 1.19 Implantation of Open and Closed Palpebral Springs. (A) Open spring has one end
fixed to the superolateral region of the orbit, while the other end pushes the upper lid down. (B)
Closed spring has one end in the upper eyelid and the other end in the lower eyelid, pulling the two
eyelids together. Adapted from (M. May et al., 2000).

49

A B
C D
Figure 1.20 Facial Palsy Patient with Palpebral Spring Implant. (A) Eyes open and (B)
attempted closure prior to implant. (C) Eyes open and (D) attempted closure immediately following
implantation. Involved side is on patient’s right. Adapted from (M. May et al., 2000).

50
1.2.3.10 Tarsorraphy
Tarsorraphy is the practice of suturing the eyelids together. It has long been used as
both a temporary and a permanent technique for protecting the cornea by decreasing
the exposed area of the palpebral fissure and increasing passive lid tension to
facilitate closing (Lee et al., 2004; M. May et al., 2000; Rahman & Sadiq, 2007).
Recent techniques have employed the use of cyanoacrylate glue and removable
sutures to gain temporary effect (Donnenfeld, Perry, & Nelson, 1991; Gossman,
Bowe, & Tanenbaum, 1991). Today, tarsorraphy is becoming less common with
increased employment of other surgical measures, however, it is still frequently used
as a complementary technique to enhance the effect of such procedures (Rahman &
Sadiq, 2007; Tucker & Santos, 1999). Figure 1.21 shows an example of a patient
with a medial and lateral tarsorraphy.
1.3 Neuromuscular Stimulation / Neural Prostheses
Neural prostheses are devices that interface with the nervous system or other
electrically excitable tissues in order to restore function. Examples of such systems
include pacemakers to restore cardiac rhythm, cochlear implants to restore hearing,
retinal implants to restore vision, and neuromuscular implants to restore movement.
Electrical stimulation of the orbicularis oculi muscle may have potential for restoring
eyelid movement. This section provides a brief review of the biophysical principles

51

Figure 1.21 Facial Palsy Patient with Medial and Lateral Tarsorrhaphy. Adapted from (M. May
et al., 2000).

52
on which the concept of neuromuscular stimulation is based, as well as a brief
history that includes previous studies of its application to the OO.
1.3.1 Biophysics of electrically excitable tissue
The concept of neuromuscular stimulation is made possible by the fact that the
membranes of nerve and muscle cells are electrically excitable. This section gives a
brief overview of the underlying biophysical principles on which this electrical
excitability is based. This material is adapted largely from (Kandel, Schwartz, &
Jessell, 2000; Malmivuo & Plonsey, 1995).
1.3.1.1 Anatomy of nerve and muscle cells
Most nerve cells have three main components: the cell body (soma), the dendrites,
and the axon (see Figure 1.22). The soma is similar to the body of other cells and
contains all of the major components of a typical cell, including the nucleus,
mitochondria, and other organelles. Thus it forms the life center for the neuron. From
a signaling point of view, it also forms the decision center of the cell, where all
inputs are summed and outputs are generated. The numerous dendrites radiate from
the cell body and form synaptic connections with incoming axonal projections from
other neurons. Input signals in the form of post-synaptic potentials travel down the
dendrites to the cell body. The single axon forms a long fiber that extends outward
from the soma. It acts as the sole output projection for the neuron and carries signals
to other cells. Axons usually vary in diameter from 1 to 20 µm and can extend to

53

Figure 1.22 Anatomy of a Neuron. Incoming signals (post-synaptic potentials) at the dendrites
and cell body are summed in order to determine if an output signal (action potential) is sent along the
single axon, which communicates with other cells at synapses. Adapted from (Kandel et al., 2000).

54
greater than 1m in length. They come in two main varieties: myelinated and
unmyelinated. Myelin is a periodic sheath formed by Schwann cells that wrap around
the axon and has implications in the speed of nerve transmission. Its significance will
be elaborated upon below.
Synapses form connections between neurons and allow them to communicate (see
Figure 1.22). When axons terminate they typically synapse with the dendrites of
other neurons. An electrical output signal (known as an action potential) that is
transmitted down an axon causes neurotransmitter to be released from the
presynaptic terminal into the synaptic cleft. The neurotransmitter chemically
stimulates the receiving cell when it binds at the postsynaptic terminal, causing either
an excitatory or inhibitory electrical signal (postsynaptic potential) to propagate
down the dendrite to the cell body.
Motor nerves are a particular class of neurons with myelinated axons that innervate
muscles. They synapse with groups of muscle fibers at neuromuscular junctions (see
Figure 1.23). Each motor axon with all of the fibers it innervates is known as a motor
unit. (Each motor axon innervates many muscle fibers, however each muscle fiber is
generally innervated by only one motor nerve.) When an action potential reaches the
presynaptic terminal of a neuromuscular junction it causes release of the
neurotransmitter acetylcholine, which causes an action potential to be generated
within the innervated muscle fiber and leads to muscle contraction.

55

Figure 1.23 Anatomy of the Neuromuscular Junction. Incoming action potentials from the motor
neuron cause acetylcholine release at the presynaptic terminal. Nicotinic acetylcholine receptors in the
muscle fiber open ion channels that lead to local membrane depolarization and muscle action potential
generation. Adapted from (Kandel et al., 2000).

56
Muscle cells come in three varieties: smooth, striated, and cardiac. Smooth muscles
are innervated by the autonomic nervous system and are responsible for involuntary
movements. Striated muscles, which get their name from the alternating light and
dark bands that form their substructure, are responsible for voluntary movements.
Striated muscles are also known as skeletal muscles because of their anatomical
attachment and functional relationship to the skeleton. Cardiac muscle composes the
heart. Structurally, it is similar to skeletal muscle, but has important physiological
differences. Particular emphasis will be placed on striated muscle, as it is the variety
responsible for eyelid movement.
Each muscle fiber is composed of an individual cell. Skeletal muscle fibers range in
diameter from 0.01 to 0.1 mm and in length from 1 to 40 mm. From a general
perspective, striated muscle fibers are similar to large unmyelinated nerve fibers, but
are distinguished by the presence of a transverse (T) tubule system that aids in signal
propagation as well as actin and myocin filaments that interact to cause
morphological changes in the length of the fiber. Action potentials initiate a series of
events that culminate in these myofilaments sliding past each other, causing the
structural units of the muscle fiber, which are known as sarcomeres, to shorten and
the muscle fiber to contract.
1.3.1.2 Cell membranes and ion channels
The membranes of neurons and muscle fibers, like those of all cells, are composed of
an essentially impermeable bilayer phospholipid backbone. Embedded within this

57
backbone are channels through which specific ions flow. These channels and the
flow of ions that they facilitate form the basis of bioelectric phenomena.
In the resting condition, there is a substantial difference between intracellular and
extracellular ion concentrations (see Table 1.4). Because each of these ions carries a
charge, this results in an unequal distribution of positive and negative charges across
the cell membrane. This charge separation gives rise to a difference in electrical
potential within the cytoplasm relative to the extracellular fluid that is referred to as
the membrane potential (V
m
). At rest there is a surplus of negatively charged ions
within the nerve cell, creating a resting membrane potential that is generally in the
range -60 to -70 mV.
These intracellular and extracellular ions are subject to forces imposed on them by
two different factors: the chemical driving force resulting from their concentration
gradients, and the electrical driving force resulting from the potential difference
across the cell membrane. For example, there is a higher concentration of potassium
(K
+
) ions within the cytoplasm, resulting in a chemical driving force that attempts to
push K
+
ions out of the cell. At the same time K
+
ions are positively charged,
resulting in an attraction to the negatively charged inner surface of the cell
membrane and an electrical force that attempts to pull K
+
ions into the cell. The
membrane potential at which the electrical force would balance the chemical force
supplied by the concentration gradient for a specific ion (X) is called its equilibrium
potential and can be calculated by the Nernst equation:

58

TABLE 1.4
Resting Ion Distribution in the Nerve Cell
Species of Ion
Intracellular Ion
Concentration (mM)
Extracellular Ion
Concentration (mM)
Equilibrium Potential
(mV)
Na
+
15 150 +61
K
+
150 5.5 -88
Cl
-
9 125 -70
Measured in the cat motoneuron. Adapted from (Kandel et al., 2000).

59

!
E
x
=
RT
zF
ln
[X]
o
[X]
i

(1.1)
where R is the gas constant, T is the temperature (in Kelvin), z is the valence of the
ion, F is the Faraday constant, [X]
o
is the ion concentration outside of the cell and
[X]
i
is the ion concentration inside of the cell. If the cell were selectively permeable
to only one type of ion (such as a glial cell, which is only permeable to K
+
), ions
would pass through its membrane until the membrane potential was equal to the
equilibrium potential for that ion, at which point the ionic flux caused by the
electrical force would equal that caused by the chemical force resulting in zero net
ionic flux. (Note that this assumes the ionic redistribution necessary to achieve
equilibrium would not significantly alter the intracellular or extracellular ion
concentrations.) As a result, the resting membrane potential would equal the
equilibrium potential for that particular ion. The equilibrium potentials for the ions
involved in nerve conduction are listed in Table 1.4.
The permeability of a cell’s membrane to specific ions is determined by the
distribution of channels that are selective to each particular ion. As mentioned
previously, glial cells have ion channels that are selective to K
+
only, resulting in a
resting membrane potential that is equal to the equilibrium potential for K
+
. Nerve
cells, on the other hand, have separate ion channels that are selective for K
+
, sodium
(Na
+
), and chloride (Cl
-
) ions. Because each ion species has a different equilibrium
potential, there is no resting membrane potential at which a condition of zero ion flux

60
can be reached. Instead, the resting membrane potential must create a balance
between the inflow and outflow of ionic charge, which will necessarily result in a
membrane potential that is somewhere between the highest and lowest equilibrium
potentials of the ions involved. The exact value for the resting membrane potential
depends on its relative permeabilities to these ions and can be calculated by the
Goldman equation:
!
V
m
=
RT
F
ln
P
K
[K
+
]
o
+P
Na
[Na
+
]
o
+P
Cl
[Cl
-
]
i
P
K
[K
+
]
i
+P
Na
[Na
+
]
i
+P
Cl
[Cl
-
]
o
(1.2)
where P
X
is the permeability of the membrane to X. In a nerve cell there are many
more resting K
+
channels than there are Na
+
channels, and the permeability ratios of
K
+
, Na
+
, and Cl
-
have been measured at
!
P
K
:P
Na
:P
Cl
=1.0:0.04:0.45
(1.3)
Thus, the resting membrane potential V
m
approaches E
K
and can be calculated as
approximately -70 mV. Note that in this calculation, V
m
is essentially equal to E
Cl

resulting in zero net flux of Cl
-
ions. Because V
m
is less than E
Na
and greater than E
K
,
however, it will result in a net influx of Na
+
ions and a net outflow of K
+
ions. If this
were allowed to continue, the passive flow of ions through the resting membrane
channels would eventually cause a change in the ionic concentration gradients,
resulting in a depletion of the chemical driving force and change in the effective
equilibrium potentials for both ions. In order to maintain the appropriate ionic

61
concentrations, the cell employs what is know as a sodium-potassium (Na
+
-K
+
)
pump, which uses energy harnessed from ATP to actively bring K
+
into the cell and
export Na
+
out of the cell. This balances the passive flow of ions through the ion
channels and maintains a zero net flux of ions in the resting state. The Na
+
-K
+
pump
also plays an important role in resetting the ionic gradients following the active
signaling process of action potential generation, a phenomenon that will be described
below.
1.3.1.3 Passive electrical properties of nerve and muscle cells
The major conducting structure of the nerve cell is the axon. The passive properties
of the axon compare to those of a leaky cable (see Figure 1.24). The impermeable
portion of its membrane is essentially a capacitor that becomes charged to the
membrane potential when positive and negative ions line up along its surface due to
the differences in internal and external ionic concentrations described above; the ion
channels act as conductors (or resistors) by allowing charge to flow through the
membrane; and the cytoplasm acts as a conductor for ionic charge within the cell.
Outside of the cell, the extracellular fluid acts as a relatively low resistance
conductor and can be thought of as an electrical short.
The values of the electrical components of the axon depend on the physical
properties of the cell, which can be assumed as a cylinder. Capacitance (C) is a
function of the area of the charging plates (A) and the thickness of the dielectric (d):

62

Figure 1.24 Passive Electrical Properties of a Neuron. The axon can be modeled as a leaky
cable, with an axial resistance (r
a
) determined by the cytoplasm, membrane resistacne (r
m
) determined
by the membrane ion channels, and membrane capacitance (c
m
) determined by the lipid bilayer and
myelin sheath. Adapted from (Kandel et al., 2000).

63

!
C =k
A
d
(1.4)
The membrane capacitance (C
m
) therefore depends on the surface area and the
thickness of the membrane. The surface area of a cylinder is defined as
!
A
s
=(2"a)l
(1.5)
where a is the radius and l is the length. Therefore the capacitance per unit length
(c
m
) is directly proportional to the radius (or diameter) of the axon. The membrane
thickness is defined by the lipid bilayer (in an unmyelinated axon), which has a
uniform and consistent thickness of approximately 4 nm for all biological cells.
The membrane resistance (R
m
) depends on the density of ion channels, and the
surface area over which those channels are distributed. A greater number of ion
channels, as determined by a greater density or greater surface area, results in a
greater ionic conductance and lower resistance. As such, the membrane resistance
times unit length (r
m
) for cells with similar intrinsic membrane properties (channel
densities) is inversely proportional to the axon diameter.
The axial resistance (R
a
) depends on the conductive properties of the conducting
medium (cytoplasm) as well as the cross-sectional area through which charges are
allowed to pass and the distance they must travel. An increased area allows more
charged particles to pass unimpeded (decreased resistance), while an increased

64
distance requires more electric force to drive through (increased resistance). The
cross-sectional area of a cylinder is defined as:
!
A
c
="a
2
(1.6)
Because the axial resistance per unit length (r
a
) is inversely proportional to the cross-
sectional area, this means that it is inversely proportional to the square of the
diameter.
A brief summary of the geometric dependence of the electrical properties of the axon
gives the following proportionalities:
!
c
m
"d; r
m
"
1
d
; r
a
"
1
d
2
(1.7)
These properties interact to determine the passive response of the axon to local
electrical changes. If a DC current were injected into an axon with a perfectly
insulated membrane, the current would flow through the axon resulting in linear
potential gradients defined by r
a
. Because of the permeability of the membrane,
however, some of this current will flow through the membrane ion channels at a rate
defined by r
m
, altering these potential gradients. More current will flow through the
membrane channels closest to the site of current injection (initial voltage deflection)
because of the greater electrical driving force defined by the greater local membrane

65
potential. The resulting decay in potential with distance (x) will be exponential, as
expressed by
!
"V(x) ="V
0
e
#x/$
(1.8)
where !V
0
is the potential change at the site of injection and " is the membrane
length constant. The length constant is the point at which the initial membrane
potential change has decayed by 1/e, and is calculated by
!
" = r
m
/r
a
( )
(1.9)
Substituting the geometric proportionalities for r
m
and r
a
into this equation reveals
the fact that the length constant is proportional to #d. Thus potential changes will
passively propagate further in axons of larger diameter, making them more efficient
conductors.
The axon’s membrane has both a resistive and capacitive component, resulting in a
membrane time constant as defined by
!
" =r
m
c
m

(1.10)
Membrane current can either flow through ion channels or to the capacitive surface
of the membrane to charge it. Thus, the effect of the capacitive component of the
membrane surface is to slow membrane potential changes resulting from membrane

66
current. Similarly, a propagating current within the cytoplasm must charge the walls
of the axon as it passes. Increased capacitance requires more charge to be deposited,
effectively slowing conduction velocity. From this analysis it becomes apparent that
decreasing the membrane capacitance will speed membrane charging and increase
conduction velocity.
The cellular solution to this is myelin. Myelin is a sheath formed by Schwann cells
that surrounds an axon (see Figure 1.22). This has the dual effect of substantially
decreasing the membrane capacitance by increasing the dielectric thickness and
increasing the membrane resistance by preventing the proliferation of ion channels
across its surface. As a result, myelinated axons have a much greater conduction
velocity than unmyelinated axons. Electrically, muscle cells behave essentially like
large diameter unmyelinated axons.
1.3.1.4 Action potential formation and propagation
From the analysis of the passive properties of nerve and muscle cells, it is apparent
from the spatial decay equation that passively propagated electrical signals would not
be very effective at traveling the length of axons. Rather, they would die out long
before reaching the presynaptic terminal. The solution to this dilemma is the
presence of special voltage gated ion channels that allow large amplitude signals to
be triggered and regenerated as they travel along the length of the axon. The signals
that they generate are known as action potentials (AP’s).

67
The typical resting membrane potential for a nerve cell is approximately -70 mV.
This means that the inner surface of the membrane is negatively polarized with
respect to the extracellular environment. Changes in the local environment can cause
membrane currents to flow, changing the local membrane potential. If the membrane
potential becomes increasingly negative it is said to be hyperpolarized, while if it
approaches zero it is said to be depolarized.
There are specific categories of ion channels that are voltage gated, such that they are
closed when the membrane is at its typical resting potential and open when the
membrane potential crosses a specific threshold value. These channels tend to have a
much greater ionic permeability than the resting ion channels. Of particular note are
voltage gated Na
+
channels that substantially increase the permeability of the axon
membrane to Na
+
when the membrane is depolarized above a threshold value of
approximately -50 mV. The effect of the increase in Na
+
permeability is a rapid
influx of Na
+
ions, resulting in a rapid depolarization of the membrane toward the
equilibrium potential for Na
+
(+61 mV). In practice, the membrane potential never
reaches E
Na
. Instead, slower acting voltage gated K
+
channels are also activated
while inactivation gates cause the Na
+
channels to deactivate. The resulting outflow
of K
+
ions and halt of Na
+
influx tends to drive V
m
back toward E
K
(-88 mV). As the
membrane repolarizes, the voltage gated K
+
channels close and V
m
eventually settles
back to its resting potential (-70 mV). Following the action potential, the local
membrane enters a brief refractory period and the Na
+
-K
+
pump restores the original

68
ion concentrations. The timecourse of local potential changes involved in generating
a single action potential is approximately 1 ms (see Figure 1.25).
As the local membrane depolarizes due to the opening of voltage gated Na
+
channels,
the passive electrical properties of the axon cause the incoming ionic current to flow
down the interior of the axon, which causes adjacent regions of the membrane to
become depolarized above threshold as well. Thus, once an action potential is
generated, it tends to propagate along the entire length of the axon.
Because threshold must be reached passively in order for voltage gated ion channels
to activate, the same properties that affect the speed of passive conduction within an
axon affect the speed of action potential propagation. Thus, larger axons tend to have
greater action potential conduction velocities than smaller axons. In addition, action
potentials tend to propagate differently in myelinated axons than in unmyelinated
axons. The entire length of unmyelinated axons is populated with both resting and
voltage gated ion channels. Thus, action potentials are continuously regenerated
along the length of the axon. Myelinated axons, on the other hand, have small gaps
in the myelin sheath known as nodes of Ranvier, at which the membrane is densely
populated with ion channels. Thus action potentials are rapidly but passively
propagated from node to node, with little loss in amplitude due to the insulating
properties of the myelin between nodes. When the depolarization reaches the
adjacent node, propagation slows temporarily as the action potential is renewed to its

69

Figure 1.25 Timecourse of an Action Potential. Note how the interaction of Na
+
and K
+

conductance lead to changes in membrane potential. The entire action potential takes approximately 1
ms. Adapted from (Kandel et al., 2000).

70
original amplitude. Thus, the action potential tends to “jump” from node to node.
The resulting propagation is much faster than in unmyelinated axons.
1.3.1.5 Electrical elicitation of action potentials
Action potentials typically originate at the axon hillock, a specialized area of the
neuron at the junction between the cell body and axon. This area has a high density
of voltage gated ion channels. Incoming excitatory and inhibitory postsynaptic
potentials that reach the cell body tend to sum at this region. When threshold is
reached, an action potential is generated and propagates unidirectionally down the
length of the axon. This action potential generation is an all-or-nothing response to
the aggregate incoming input activity for the neuron.
In neural prosthesis applications, action potentials are generated by currents injected
into the extracellular space. These currents produce voltage gradients in the vicinity
of the axon that induce capacitive membrane currents and transmembrane potential
changes. When the membrane potential change reaches threshold, an action potential
is generated that is identical to a naturally elicited action potential, except for the fact
that it travels in both directions away from the source of activation. The proximally
directed action potential will be annihilated when it reaches the cell body, however
the distally propagating action potential will cause neurotransmitter release at the
axon’s presynaptic terminal generating the same effect as a naturally evoked action
potential. Because of this fact, the ideal neural prosthesis would simply recreate the
firing pattern of the functional axon. In practice, however, this is complicated by

71
many factors, primary among which is the fact that electrical stimulation pulses do
not target individual axons, but rather groups of axons within the generated electric
field.
The injection of current requires the use of two electrodes, an active electrode and a
return electrode. These can be placed in a monopolar configuration where the return
electrode is far away and the current radiates out from the active electrode, or in a
bipolar configuration where the return electrode is relatively close to the active
electrode and current is steered toward the return electrode. Because currents that run
parallel to the axon are most effective at stimulating neurons, the use of properly
oriented bipolar stimulation can result in more selective activation.
Electrodes pass current into the extracellular fluid by charging and discharging the
capacitance between the metal surface and body fluid. This converts the electronic
current in the electrode to ionic current in the surrounding fluid. Different tissues
tend to contain different concentrations of ions and have different physical structures
that result in tissue-specific conductance. In general, however, it can be assumed that
the body acts as a large volume conductor. The capacitive flow of current between
the electrode surface and surrounding fluid is a fully reversible process and can
continue as long as the electrode surface potential remains below the value at which
electrolysis can occur (typically ±0.8 VDC). Once this potential has been exceeded,
irreversible reactions will take place that will actually strip molecules from the
electrode surface and cause gas formation that results in pH changes. The amount of

72
charge that can be injected without exceeding this voltage depends on the electrode
material and surface area. Safe charge density limits for commonly used electrode
materials are listed in Table 1.5. In order to prevent such reactions from taking place,
biphasic stimulation pulses can be used. Thus, the first phase of the pulse charges the
electrode surface and the opposite phase discharges it back to its resting state. This is
particularly important for implanted neural prosthetic systems.
In order for an action potential to be generated, the local cell membrane must be
depolarized above threshold, which requires the use of cathodic (negative) current. A
cathodic extracellular current will cause an outward (depolarizing) current in the cell
membrane. The outward current at the point of depolarization, however, must be
balanced by inward current at other locations. In a monopolar configuration, this
results in what are known as “virtual anodes” surrounding the depolarized region. If
the stimulus intensity is great enough, these virtual anodes (which tend to cause local
hyperpolarization, but to a lesser degree) can actually become strong enough to block
the propagating action potential in what is called “anodal block”. Much the same
way, a monopolar anode will result in weaker “virtual cathodes” in the surrounding
vicinity, which can be used to cause stimulation if the current is high enough. See
Figure 1.26 for more detail. Because most neural prosthetic applications try to limit
the amount of current necessary for stimulation, cathodic currents are more
commonly used.

73

TABLE 1.5
Charge Density Limits for Electrode Materials
Material Charge Density Limit (mC/cm
2
)
Au 0.490
TiN 0.687
Pt 4.134
Ir 17.078
IrO
x
28.450
IrO
x
(after activation) 95.100
Adapted from (Slavcheva, Ewe, Schnakenberg, & Mokwa, 2004).

74

Figure 1.26 Current Flow in a Fiber Induced by a Monopolar Electrode. Induced membrane
voltages along the length of a nerve fiber resulting from (A) anodic and (B) cathodic stimulation. Note
the smaller amplitude virtual cathodes in (A) and virtual anodes in (B). Cathodic stimulation causes
the greatest degree of local depolarization. Adapted from (Ranck, 1975).

75
From the cellular point of view, the membrane properties are very important in
determining a neuron’s reactivity, particularly the diameter and degree of
myelination. The ability for a cell to depolarize depends on the ability to discharge
its capacitive membrane. Thus, neurons with shorter membrane time constants can
be activated with shorter pulses. Additionally, larger myelinated axons have greater
spacing between their nodes of Ranvier, resulting in greater transmembrane voltages
due to extracellular currents. As a general rule the largest, most myelinated structures
tend to be activated first.
Electrical stimulation pulses tend to be rectangular and have three basic parameters:
pulse width, pulse amplitude, and pulse frequency. The combination of pulse width
and pulse amplitude determine the charge injected, while the passive electrical
properties of the surrounding tissue determine its distribution and resulting potential
gradients. The current density will be greatest in the vicinity of the electrode and
decrease with distance as it spreads through the volume conductor comprised of the
body fluid, exposing neurons closer to the electrode to larger voltage gradients. The
minimum stimulus intensity necessary to activate a neuron depends on the duration
of the stimulation pulse, and is defined by LaPicque’s Equation:
!
I =b(1+c/d) (1.11)
where I is the stimulus intensity, b is the rheobase value, c is the chronaxie, and d is
the pulse duration (see Figure 1.27). The rheobase value is the minimum stimulus

76

Figure 1.27 Chronaxie Curve. Normalized strength-duration curves for a nerve cell plotted on
(A) linear and (B) log scales. Note the pulse duration with the lowest energy requirement to achieve
stimulation is equal to the chronaxie. Adapted from (Geddes, 2004).

77
intensity required to elicit a response with an infinitely long pulse and is defined by
the membrane properties of the neuron and the coupling between the neuron and the
stimulating electrode. Thus, increasing the distance from the stimulating electrode
will increase the rheobase value and therefore increase the necessary stimulus
intensity for eliciting a reaction at all pulse durations proportionately. The chronaxie
value is the pulse duration at which the stimulus intensity necessary to elicit a
reaction is twice the rheobase value. The chronaxie is directly related to the time
constant of the neuron (!) by the equation:
!
c ="[ln(2)] (1.12)
and is therefore purely a property of the cell itself. Solving the above equation for the
electrical energy required to stimulate a neuron, it can be shown that the most
electrically efficient stimulation pulse is one with a duration equal to the chronaxie
of the cell being activated, even though it requires less charge to activate the same
cell with shorter pulses.
Stimulation pulses can be either voltage controlled or current controlled. The local
potential gradient, which determines whether a neuron is depolarized, is a direct
result of the current injected. The problem with voltage controlled pulses is that the
current delivered to the tissue depends in part on the impedance of the electrode
itself, which can fluctuate over time. Thus, injected currents can vary within the
same stimulation system. Regulating the current, however, ensures that the voltage

78
gradients produced within the tissue will be more consistent, and gives a direct
measure of the charge density delivered across the electrode-electrolyte interface,
helping to prevent unwanted electrolysis. For these reasons current controlled pulses
are more commonly used for neural prosthesis applications.
1.3.1.6 Difficulties of stimulating denervated muscle
During normal physiological muscle activation, action potentials in a motor axon
cause acetylcholine release at the neuromuscular junctions of all muscle fibers in the
motor unit that it innervates. On each muscle fiber, nicotinic acetylcholine receptors
at the motor endplate open Na
+
/K
+
channels that cause local depolarization and open
adjacent voltage gated ion channels, resulting in the generation of an action potential
that propagates in both directions down the length of the fiber. As these action
potentials propagate, they cause Ca
++
influx from the T tubule system, bringing about
the conformational changes in actin and myosin binding that result in contraction.
In principle, muscle fibers have similar electrical properties to large, unmyelinated
nerve axons. Therefore, the voltage gated ion channels in their membranes can be
activated by externally generated electrical fields to produce action potentials in
much the same way as nerves are electrically activated. The primary differences are
that the chronaxie values for muscle fibers tend to be very long compared to nerves,
and the end result is not transmitter release, as it is in axonal activation, but rather
contraction of the individual muscle fiber.

79
Most neuromuscular prosthesis applications rely on stimulation of the motor axons
that innervate the target muscles, rather than direct activation of the muscle fibers
(Peckham & Knutson, 2005; Ranck, 1975). There are two primary reasons why
stimulation of motor axons is more efficient than stimulation of denervated muscle
fibers: the chronaxie values of myelinated motor axons are orders of magnitude
shorter than those of bare muscle fibers (Geddes, 1999), and activation of a single
motor axon will cause activation of all the muscle fibers within the motor unit it
innervates while denervated muscle fibers must be activated individually.
The difference in chronaxie values between motor axons and muscle fibers is due to
the differences in their anatomical structures and can be explained by the underlying
biophysics. As noted in equation 1.12, the chronaxie of a cell is proportional to its
membrane time constant. Motor axons are smaller than muscle fibers and are
myelinated. This may give motor axons a greater membrane resistance, however, it
also gives them in a substantially lower membrane capacitance. This difference in
membrane capacitance translates to motor axons having a much shorter membrane
time constant, and therefore chronaxie value, than muscle fibers.
As a result of their longer membrane time constants, activation of denervated muscle
fibers requires the use of relatively long duration pulses, which have a greater
potential to simultaneously activate unmyelinated pain fibers in the vicinity. This
also results in a high amount of charge injection with each stimulation pulse, placing
greater constraints on power supply and electrode design. Additionally each

80
individual denervated muscle fiber must be activated directly, meaning that sufficient
charge to induce activation must reach every fiber that is to be recruited (as opposed
to indirect activation through motor axons, which only requires activating currents to
reach the motor nerve in order to recruit all innervated muscle fibers). Given the
rapid fall off of current density with distance from the electrode, very high amplitude
stimulation pulses are generally required to activate large regions of denervated
muscle. These factors combine to make it very electrically expensive to activate
denervated muscle and increase the probability of discomfort associated with the
stimulation.
Unfortunately, many debilitating conditions involve lower motor neuron damage and
do not afford the luxury of intact motor axons. Therefore, despite these drawbacks, a
considerable amount of effort has gone into research directed at the electrical
activation of denervated muscle (Eichhorn, Schubert, & David, 1984; Kern et al.,
1999; Kern, Salmons, Mayr, Rossini, & Carraro, 2005; Modlin et al., 2005).
Previous work has been primarily focused on the lower extremities, which are
comprised of large bulky muscles that must support large amounts of weight in order
to be functional. The OO muscle, on the other hand is a small, non-weight-bearing
muscle. Additionally, it need only be activated for short periods of time with a low
duty cycle, limiting the potential for fatigue that can become problematic in
chronically denervated muscles that tend to experience extensive atrophy (Salmons
et al., 2005). For these reasons, the OO muscle seems to be a much better candidate

81
than previously studied muscles for replacement of function with electrical
stimulation despite the complications caused by denervation.
1.3.2 History of neuromuscular stimulation
The previous section described how artificially generated electrical impulses can be
used to activate paralyzed muscles through the process of neuromuscular stimulation.
Applications involving neuromuscular stimulation can be divided into three separate
categories: therapeutic electrical stimulation (TES), neuromodulatory stimulation
(NMS), and functional electrical stimulation (FES) (G. E. Loeb & Lan, 2001). TES
is essentially electrically induced exercise, in which the desired effect is achieved not
by the movement itself but rather by the trophic changes it induces in the stimulated
muscle. TES applications include stimulation of the paralyzed shoulder muscles in
stroke patients to prevent or reverse shoulder subluxation caused by disuse atrophy
(Faghri et al., 1994) and electrical stimulation to reduce spasticity following spinal
cord injury (Stefanovska, Vodovnik, Gros, Rebersek, & Acimovic-Janezic, 1989).
NMS involves the use of preprogrammed stimulation patterns to pace or modulate
some neural function without the use of any control signals. Examples of NMS
applications include pacing of the phrenic nerves to achieve respiration in patients
with central hypoventilation (Glenn & Phelps, 1985) and sacral nerve stimulation for
bladder voiding (Brindley & Rushton, 1990). FES involves the use of neuromuscular
stimulation in combination with command signals and control feedback to reanimate
muscles for functional tasks. Examples of FES applications include gait-triggered

82
peroneal nerve stimulation to correct foot-drop in stroke patients (Wieler et al., 1999)
and electrical stimulation of the finger muscles based on the residual activity in the
proximal or contralateral limb to provide assisted grasp function in quadriplegics
(Prochazka, Gauthier, Wieler, & Kenwell, 1997).
The origins of neuromuscular prostheses are derived from the early cardiac
pacemakers of the 1950s, which used single channel stimulation to modulate cardiac
function (G. E. Loeb, 2001). This technology was originally extended to
neuromuscular applications in both the upper and lower extremity in the 1960s and
has led to additional developments in the areas of bowel and bladder function and
respiration (Peckham & Knutson, 2005). Despite its early promise and the decades of
research that have followed, neuromuscular stimulation has historically enjoyed
modest clinical and limited commercial success. One reason for this was the choice
of early applications. The field began rather ambitiously, and subsequently much
effort has been directed at the goal of making paraplegics walk, a high-risk activity
that requires very sophisticated control of the muscles involved (G. E. Loeb & Lan,
2001). It might be said that researchers originally bit off more than they could chew,
and the difficulties of dealing with such complicated FES systems slowed progress in
the field. Since then, a refocus on more attainable milestones and lower-risk
applications (such as those embodied by TES, NMS, and FES in the upper
extremities) have led to steady progress that is beginning to manifest itself in the
clinic. The following review describes current clinically available systems for FES

83
(and some NMS) applications based on functional region, as well as some of the
directions being pursued by current research efforts (Peckham & Knutson, 2005).
1.3.2.1 Upper extremities
Upper extremity FES applications are directed at restoring the capability of
performing activities of daily living in paralyzed subjects (generally those with either
C5 or C6 level spinal cord injury), and thus have tended to focus on hand function.
Four systems have reached commercial status, three of which are still on the market.
Handmaster (NESS Ltd., Ra’anana, Israel; BioNESS Inc., Valencia, CA) combines
an adjustable wrist-hand orthosis with push-button operated surface stimulation to
activate the thumb and finger muscles (Snoek, MJ, in 't Groen, Stoffers, & Zilvold,
2000). Bionic Glove, which was developed at the University of Alberta, consisted of
a fingerless glove and sleeve that covered self-adhesive surface electrodes that used
residual wrist movement to trigger electrically stimulated grasp and release
(Prochazka et al., 1997). It is no longer being marketed. FESMate (NEC Medical
Systems, Tokyo, Japan) uses percutaneous stimulation of large numbers of muscles
based on numerous command signals to restore hand, forearm, elbow, and shoulder
movement (Handa & Hoshimiya, 1987). Freehand is an implanted system that was
developed at Case Western Reserve University and the Cleveland VA Medical
Center to restore lateral and palmar grasp (Keith et al., 1989; B. Smith, Peckham,
Keith, & Roscoe, 1987). Studies evaluating the use of these systems have indicated
that patients generally do achieve improved function (Alon & McBride, 2003;

84
Peckham et al., 2001; Popovic et al., 1999). Complications with the wearable
systems have included donning requirements and selectivity of stimulation (Popovic
et al., 1999), while the implanted system has experienced minimal but existent lead
failure and electrode infection (Kilgore et al., 2003).
1.3.2.2 Lower extremities
FES in the lower extremities has tended to focus on three specific applications:
prevention of footdrop, restoration of standing and transfer, and restoration of
walking.
Footdrop is a hemiplegic or hemiperetic condition secondary to many neurological
disorders in which the muscles involved in lifting the foot during gait fail to function
properly. As a result, the toe tends to drag, requiring patients to severely alter their
gait pattern. Footdrop prevention via triggered stimulation of the peroneal nerve was
one of the first researched uses for FES (Liberson, Holmquest, Scot, & Dow, 1961).
Since then many subsequent research efforts have been undertaken and currently
there are several FES system to combat footdrop that are either commercially
available or undergoing clinical investigation. Surface stimulation systems include
the Footlifter (Elmetec A/S, Arhus, Denmark) and Odstock Footdrop Stimulator
(Burridge, Taylor, Hagan, Wood, & Swain, 1997), both of which use a footswitch as
a trigger, as well as Walkaide (Innovative Neurotronics, Inc., Bethesda, MD), which
uses a built-in tilt sensor to detect step intention (Wieler et al., 1999). Implanted
systems include ActiGait (Neurodan A/S, Aalborg, Denmark), which is implanted in

85
the thigh and uses a four-channel cuff electrode in combination with a wireless
external footswitch (Haugland et al., 2004) and the Finetech Dropped Foot System
(Finetech Medical Ltd., UK), which has a dual-channel stimulator implanted below
the knee to stimulate two branches of the common peorneal nerve based on an
external footswitch signal (van der Aa et al., 2002). Trials evaluating FES systems to
prevent footdrop have indicated benefits of increased walking speed and decreased
walking effort (Burridge et al., 1997; Haugland et al., 2004).
A second application of FES in the lower extremities involves assisting paraplegics
to stand from a seated position and transfer between surfaces. This is much more
demanding than the correction of footdrop, but requires less sophistication than the
restoration of walking. This can be achieved with the use of a balance aid by
stimulating the quadriceps to extend the knee and glutei muscles to extend the hip
(Kuzelicki, Kamnik, Bajd, Obreza, & Benko, 2002). There are no commercially
available systems to restore standing, however, there is one implantable system in
clinical trials (J. A. Davis, Jr. et al., 2001). Reports of subject perception indicate that
they are satisfied with the performance of the device (Agarwal et al., 2003).
The holy grail of lower extremity neuroprostheses has been restoration of the ability
to walk. There actually does exist one FDA-approved FES system to restore
ambulation. Parastep
TM
(Sigmedics, Inc., Fairborn, OH) uses four to six channels of
bilateral surface stimulation to activate the quadriceps muscle for knee extension, the
peroneal nerve to elicit a knee withdrawal reflex that substitutes for the gait swing

86
phase, and the glutei muscles (if necessary) for hip extension. This has been used to
enable patients with paraplegia caused by level T4 to T12 spinal cord injury to walk
with the use of a walker over limited distances (Bajd, Kralj, Turk, Benko, & Sega,
1983; Graupe & Kohn, 1998; Kralj, Bajd, Turk, Krajnik, & Benko, 1983). Other
percutaneous and implantable systems have also been developed (R. Davis, Patrick,
& Barriskill, 2001; Kobetic, Triolo, & Marsolais, 1997; Kobetic et al., 1999;
Marsolais & Kobetic, 1987; Rushton et al., 1997). The number of subjects, however,
has been limited and none has reached commercial deployment. All FES systems for
restoration of ambulation require the use of some external support or assistance as
well as lengthy periods of muscle conditioning and the expenditure of large amounts
of energy. The combination of these factors discourages the potential use of FES-
assisted walking as a primary means of mobility for paraplegics.
1.3.2.3 Bowel and bladder
Loss of bowel and bladder function due to spinal cord injury above the level of the
sacral nerves or other neurological disorders can result in a host of complications
such as urinary tract infections, renal deterioration, bladder or kidney stones, and
autonomic dysreflexia. Electrical stimulation can be used to improve micturition and
continence, as well as bowel function. Following bilateral posterior rhizotomies of
the S2 to S4 spinal nerves in order to prevent involuntary detrusor contraction,
micturition is accomplished by stimulation of the sacral nerves, which results in co-
contraction of the detrusor and urethral sphincter. The slower relaxation time of the

87
smooth detrusor muscle compared to the striated urethral sphincter muscle results in
a brief period of increased bladder pressure with urethral sphincter relaxation
following stimulation that allows for a brief period of micturition. Using intermittent
stimulation allows the bladder to be emptied in spurts (Brindley, 1993). The
Finetech-Brindley Bladder System (Finetech Medical, Ltd., UK) has been implanted
in more than 2500 patients to restore bladder function with high levels of satisfaction
and hardware robustness (Brindley, 1995; Creasey et al., 2001; Rijkhoff, 2004). This
system can also be used to aid in bowl evacuation (Binnie, Smith, Creasey, &
Edmond, 1991; MacDonagh, Sun, Smallwood, Forster, & Read, 1990) and penile
erection (Brindley, 1994; Egon et al., 1998; van der Aa, Alleman, Nene, & Snoek,
1999). Further developments are targeted at achieving the same effect without need
of the posterior rhizotomy (Grill & Mortimer, 1995; Kilgore & Bhadra, 2004).
1.3.2.4 Respiratory function
Individuals with high cervical spinal cord injury or central alveolar hypoventilation
cannot control their diaphragm and therefore lose the ability to breathe. Electrical
pacing of the phrenic nerves to induce diaphragm contraction is an alternative to the
use of mechanical respirators. Originally developed in the 1960s (Glenn et al., 1964),
this technique has utilized in more than 1200 patients and has become a clinically
accepted intervention (DiMarco, 1999; Glenn et al., 1964). There are two
commercially available systems for diaphragm pacing. The Avery Mark IV (Avery
Laboratories, Commack, NY), which uses monopolar or bipolar nerve cuff

88
electrodes (Glenn & Phelps, 1985), and the Atrostim system (Atrotech Ltd, Tampere,
Finland), which uses quadripolar nerve cuff electrodes (Baer et al., 1990). The four
contacts in the Atrostim system are cycled with the intention of reducing muscle
fatigue by activating each motor unit only one fourth of the time compared to
monopolar stimulation. After a period of diaphragm conditioning, electrical pacing
can restore normal tidal volumes and be used full-time. Reported success and usage
rates are good, with the vast majority of patients achieving complication-fee pacing
and a split between patients who use stimulation during waking hours, during sleep,
and during both. Additionally, the rates of hardware failure, infection, and nerve
trauma were low (Elefteriades et al., 2002; Weese-Mayer et al., 1996). Current
research is directed at stimulation of the intercostals muscles for patients who are not
eligible for phrenic nerve pacing because of issues such as denervation of the
diaphragm (DiMarco, 2001; DiMarco, Supinski, Petro, & Takaoka, 1994), and the
use of intramuscular diaphragm electrodes rather than phrenic nerve cuffs for more
minimally invasive surgical implantation (Onders, Dimarco, Ignagni, Aiyar, &
Mortimer, 2004).
1.3.2.5 Future directions
Past FES systems have generally used open loop configurations developed for
specific applications. Additionally, they have favored either surface stimulation,
which has issues with high power dissipation, poor muscle selectivity, and system
donning requirements, or leaded implantable systems, which have extensive surgical

89
requirements and potential for lead breakage. A great deal of current FES research is
being directed at the development of platform technologies suitable for multiple
applications, minimally invasive implantable systems, and the acquisition and
incorporation of command and feedback signals for closed loop control.
BIONs
TM
are a class of injectable wireless microstimulators that are being developed
by the A. E. Mann Foundation and the A. E. Mann Institute at the University of
Southern California (G. E. Loeb, Peck, Moore, & Hood, 2001). Individual BIONs,
which measure 2 mm x 16 mm, can be inserted through a 12-gauge needle into
paralyzed muscles in a minimally invasive procedure. Once implanted, each BION
forms a single stimulation channel that can selectively activate nerves in its vicinity.
BIONs receive power and command signals via inductive coupling from an external
transmitting coil. Because each transmitting coil can power and control up to 256
implants, multiple BIONs can be activated by a single controller, allowing for
incremental buildup of multichannel FES systems. First generation BIONs are
suitable for a wide range of open loop applications and have been investigated for
treatment and/or prevention of shoulder subluxation, knee osteoarthritis, urinary urge
incontinence, and footdrop (Dupont et al., 2004; Weber et al., 2004). More advanced
BIONs are currently being developed that will incorporate such features as built-in
sensing of command and feedback signals that can be telemetered out to an external
controller and used for closed-loop control of more sophisticated FES applications
(G. Loeb et al., 2004; G. E. Loeb & Davoodi, 2005; Sachs & Loeb, 2007).

90
By FES standards, neural prosthetic reanimation of the eyelids for blink restoration is
a relatively simple, low-risk application. There are technical challenges (many of
which will be dealt with in this thesis), however the level of control needed to restore
useful function is low and the consequences of system failure are tolerable. Blink
restoration essentially involves a single muscle (OO) acting in a binary fashion
(relaxing when the eye is open or contracting to close it), that can either be paced or
triggered by a readily accessible and robust command signal (the closing of the
contralateral eye in patients with unilateral facial paralysis). If eye blink is not fully
restored by the stimulation, the patient will not suffer any immediate physical harm
(such as would be result from a fall due to inadequate coordination in FES walking)
and can simply be reverted to one of the pre-existing ophthalmic treatment methods
for facial palsy. This makes the pursuit of FES-enabled eyelid reanimation a natural
step in the current progression of clinical neuromuscular stimulation.
1.3.3 Previous studies of orbicularis oculi stimulation
Several authors have previously investigated electrical stimulation of the orbicularis
oculi muscle in animal models (Otto, 1997; Otto, Gaughan, Templer, & Davis, 1986;
Rothstein & Berlinger, 1986; Salerno, Bleicher, & McBride, 1991; Salerno, Bleicher,
& Stromberg, 1990; Salerno, Bleicher, Stromberg, & Cheng, 1990; Salerno,
McClellan, Bleicher, Stromberg, & Cheng, 1991; Somia et al., 2001; Tobey &
Sutton, 1978), and at least one known study has performed limited investigation in

91
humans (Gittins, Martin, Sheldrick, Reddy, & Thean, 1999). This section will briefly
describe the methods and outcomes of these studies.
1.3.3.1 Tobey & Sutton
In the first known publication of electrical stimulation for facial reanimation, Tobey
and Sutton investigated the effects of contralaterally triggered stimulation of the OO
in four rabbits (1978). After unilaterally sectioning and ligating the facial nerve
trunk, they bilaterally inserted multistranded wire into an unspecified region of the
OO and routed it through an acrylic block on the skull. The wire on the normal side
was used to record EMG and the wire on the paralyzed side was used to deliver
voltage controlled stimulation pulses. Stimulation was monopolar and varied
between 3 and 5V at 200 ms. EMG activity was used as a trigger for eliciting
electrical stimulation in one of three modes: single stimulus of fixed intensity, single
stimulus of variable intensity, and trains of stimuli with variable duration and
intensity. They reported the ability to generate blink and to produce a reasonably
symmetric response to contraction on the normal side. Limiting factors were reported
as controlling the location of response and synkinesis due to activation of adjacent
muscles, however duration and intensity matching were good. Rabbits were
implanted for up to five months, however the stimulation protocol within that time
frame was not specified. The authors reported collecting video recordings of
experiments, however none were presented. Although this was an early publication,
it has been the most ambitious in its scope. Unfortunately, no follow up was reported

92
despite a list of future directions including the use of bipolar electrodes to limit
current spread, exploration of continuous operation for the stimulator, and
development of a fully implanted system.
1.3.3.2 Rothstein & Berlinger
Eight years after Tobey and Sutton’s study, Rothstein and Berlinger investigated the
electrical reanimation of facial paralysis in 12 rabbits (1986). They collected
strength-duration data for innervated muscles (OO and zygomaticus major) and
denervated muscles two weeks following facial nerve section, however it was not
specified that these were for twitch movement. They used voltage controlled
stimulation pulses delivered at 2 and 33 pulses/second with pulse widths that varied
from 0.1 to 2.0 ms. Stimulation was bipolar with stainless steel electrodes that were
“implanted percutaneously at opposite ends of each of the denervated muscle
groups”. Additionally, EMG activity was recorded from healthy OO muscles at
different degrees of closure. The results section stated that “in most cases there was a
significant increase in minimum stimulation threshold for the respective muscles
after denervation”. There seemed to be very little difference, however, in the
rheobase and chronaxie values for innervated versus denervated muscles listed in
their tables, and these rheobase values seem to be substantially greater than the
minimum voltages in the actual strength-duration curves, so it is unclear how these
values were reached. Mean OO EMG activity was reported, however it was not
specified if this was rectified mean (or root-mean-square) which would be the only

93
possible way of reporting a variable mean value from the largely AC EMG signal.
Mean EMG values ranged from 114 µV at rest to 1100 µV when shut tightly.
Curiously, a mean OO EMG value of 103 µV was reported for blink, which is less
than the reported resting value. The authors stated that stimulation of denervated
muscles “yielded mimetic function that was modifiable by varying the voltage output
and the rate of pulse generation” and that eyelid closure “could be achieved without
spread of the electrical stimulus to the adjacent muscles”. No details for this part of
the experiment were reported, however, as the methods and data only focused on
strength-duration curves and EMG. Discussion alluded to development of a circuit
for delivering stimulus pulses determined by contralaterally generated voltages and
included demonstration of a trigger generating comparator circuit, however no
follow up was reported.
1.3.3.3 Otto (et al.)
Otto, Gaughan, Templer, and Davis reported on the electrical restoration of the blink
reflex in a dog model of experimentally induced facial paralysis (1986). The
palpebral branch of the auriculopalpebral nerve was unilaterally sectioned and
ligated in five dogs. Three to four weeks later bipolar cardiac pacing electrodes, with
spacing of approximately 1 cm, were implanted in the superior portions of both the
normal and paralyzed OO muscles, near the medial canthus. Approximately three
months after the onset of paralysis, EMG activity from reflex induced blinks in the
normal side was recorded under mild sedation and used to trigger stimulation pulses

94
in the paralyzed OO using a single square-wave pulse. The current amplitude was
varied to determine a value “sufficient to elicit the blink reflex, but at a level below
the apparent pain threshold” for each dog. The values for stimulus pulse width and
pulse amplitude were not reported. The authors reported the restoration of
“functional symmetry” achieved by stimulating a blink that “appeared normal” and
very good tolerance of the system. They stated that video of dog facial responses was
recorded, but none were presented. In the discussion they note additional research
questions and indicate that their current treatment method “mandates investigation
into possible alternative solutions”.
Eleven years later, Otto published a second manuscript on electrical stimulation to
restore function in the paralyzed OO (1997). This study was performed in rabbits. It
included an acute pilot phase to determine anatomy for facial nerve dissection and
stimulus parameters capable of achieving complete eyelid closure, a chronic pilot
phase in which one test animal was stimulated, and an investigational phase in which
six rabbits were chronically stimulated for a duration of 30 days. During the
investigational phase, the facial nerves were bilaterally sectioned and ligated at the
pes anserinus and rabbits were implanted with exposed stainless steel wire electrodes
that spanned the upper and lower eyelids. Nerve interruption was confirmed
intraoperatively using electrical stimulation of the nerve trunk and postoperatively by
observing fibrillation potentials and a lack of activity in the OO during attempted
EMG recording. Beginning on the 11
th
day following nerve dissection, a battery

95
powered backpack stimulator was used to deliver single biphasic voltage controlled
stimulation pulses between the electrode in the upper lid and the electrode in the
lower lid on one side, while the other side was used as a control. Stimulation pulses
were delivered every 10 seconds for 24 hours each day. Stimulus amplitude was
generally fixed around 4.8V and pulse duration generally ranged from approximately
20 to 90 ms with a mean of 37.18 ms but was reported to require little adjustment.
Photographic evidence of electrically stimulated closure equivalent to recorded
normal closure was provided. The rabbits were reported to “flinch with each pacing
signal during the first few hours of pacing”, but acclimated over time. A reduction of
weight was reported that the author believed was due to stress possibly caused by
interruption of the normal sleeping patterns of the rabbits. Histological evaluation
reported no difference between experimentally paced and control eyelid tissue. This
study represents one of the most complete analyses of OO stimulation reported, and
the author concludes with the statement that the reported research demonstrates that
“functional restoration of the rabbit orbicularis oculi muscle can be accomplished
with direct electrical stimulation of peripherally denervated muscle for a short period
of time without evidence of injury to muscle and without obvious undue discomfort”.
He also lists a host of future investigations intended to answer remaining questions
regarding OO stimulation, however no further publications were produced and no
clinical system was developed.

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1.3.3.4 Salerno et al.
Salerno, Bleicher, Stromberg, and Cheng studied the electrophysiological properties
of the denervated OO in dog (1990). They unilaterally ligated and transected the
facial nerve in 10 dogs and performed acute percutaneous stimulation with EMG
needle electrodes under general anesthesia an average of 54 days later. The authors
stimulated the normal OO and the palpebral nerve on the intact side and the
paralyzed OO directly on the operated side. For OO stimulation, two electrodes were
inserted into the upper lid area, however the exact locations were not reported. For
palpebral nerve stimulation, two monopolar electrodes were inserted into the
perineurium of branches of the facial nerve innervating the upper and lower lids,
respectively, with a ground electrode placed in the ear. Single square wave current
controlled galvanic stimulation pulses were used to elicit first a muscle twitch and
then a blink for each configuration over a range of pulse widths and the minimum
threshold values were recorded for each. Strength-duration data were reported,
including rheobase and chronaxie values, for both twitch and blink for palpebral
nerve stimulation, normal OO stimulation, and denervated OO stimulation. The most
noteworthy findings were that the denervated OO had significantly lower twitch
rheobase values and higher twitch and blink chronaxie values than either the normal
OO or palpebral nerve. Mean denervated OO chronaxie values ranged from 30 to 48
ms, while normal OO ranged from 0.08 to 0.14 ms and palpebral nerve was
approximately 0.06 ms. There were no significant differences between twitch or
blink parameters for normal OO and palpebral nerve. The majority of twitch and

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blink threshold data were reported for pulse widths ranging from 0.03 to 2.0 ms.
Amplitudes for producing a twitch in denervated OO within this range were greater
than those required to produce a full blink in normal OO and palpebral nerve. A
separate denervated twitch curve was presented for pulse widths ranging from 5 to
400 ms, however the graphical and tabular data for this curve do not correspond and
there is a distinct discontinuity between the denervated twitch curves for 0.03 to 2.0
ms and 5 to 400 ms. Denervated blinks were only reported for pulse widths ranging
from 5 to 400 ms, and the graphical and tabular data for this curve do not correspond
either.
The authors did produce two follow up publications, each focusing on therapeutic
rather than functional benefits of chronic electrical stimulation of the OO following
nerve transection. The stimulation protocol in both of these studies involved daily
anesthetization and insertion of a monopolar EMG electrode into both the upper and
lower eyelid five days/week beginning on postoperative day three and lasting until
postoperative day 75. Monophasic current pulses with a pulse width of 5 ms were
delivered at 0.5 Hz for 10s, followed by 10s without stimulation. This pattern was
repeated for two sessions of 20 minutes each, with a 20 minute break period in
between. The stimulus amplitude was set to a level that achieved complete eyelid
closure (10-13 mA). The first study investigated the effects of chronic electrical
exercise on the OO following facial nerve transection and immediate neurotmesis in
three dogs compared to three nonstimulated controls (Salerno, Bleicher, &

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Stromberg, 1990). It demonstrated that electrical stimulation may have a therapeutic
effect in speeding recovery of the blink reflex resulting from reinnervation after
seventh nerve damage. The second study investigated the effects of electrical
exercise on the electrophysiological properties of denervated OO in four dogs
compared to four nonstimulated controls (Salerno, McClellan et al., 1991). It
reported a possible temporary decrease in the electrical threshold to achieve
complete blink that reversed over time, as well as probable muscle fiber type
conversion leading to greater percentages of type II fibers in denrvated OO muscles
that had undergone chronic stimulation. No clinical investigations were initiated as a
result of this work.
1.3.3.5 Somia et al.
Somia et al. investigated the use of multichannel stimulation for restoring paralyzed
OO function (2001). Bilateral seventh nerve resection was performed in eight dogs
and a segment of the zygomatic branch was removed on each side. Fourteen days
after surgery, the dogs were lightly sedated and percutaneous wire electrodes were
inserted into the OO muscles on both sides through a 25-gauge hypodermic needle.
One side received single channel bipolar stimulation between contacts in the upper
lid superior to the medial and lateral canthi. The other side received multi-channel
stimulation consisting of two bipolar stimulation channels in the upper lid and two in
the lower lid. See Figure 1.28 for reported electrode placements. The stimulation
protocol involved the use of trains of pulses lasting a total of 300 ms and delivered at

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60 Hz. The pulse widths investigated ranged from 0.3 to 1.0 ms (note that these are
labeled incorrectly in the published manuscript and were corrected via personal
communication) and the amplitudes ranged from 0.1 to 4.9 mA. It was not explicitly
stated whether monophasic or biphasic pulses were used, however the figures
presented seem to indicate monophasic pulses. Strength-duration curves for eliciting
a muscle twitch, as well as complete closure were reported. Multi-channel
stimulation produced a twitch with significantly lower amplitude than single channel
stimulation at all pulse widths, and only multi-channel stimulation was able to
produce full closure over the range of pulse widths tested. Interestingly, the range of
pulse widths seems rather short for a study focused on the stimulation of denervated
muscle, which tends to have reported chronaxie values greater than 10 ms. The
authors discussed the ability of multiple channels to contain stimulation currents and
focus them in the pretarsal region of the OO, where they reported achieving the most
effect in producing electrically elicited eyelid closure. They also discussed the
possibility of tailoring stimulation such that different regions of the OO
receivedifferent stimulus amplitudes to compensate for changes in required current
intensity for produce an acceptable blink resulting from gradual OO reinnervation.
They do not give any physiological basis, however, for how these various muscle
zones may be organized. They conclude by acknowledging that additional
experiments would be required to “take this technology to the clinical setting”,
including long-term stimulation and the incorporation of contralateral feedback to
produce functional synchrony, however no follow up has been reported.

100

A
B
Figure 1.28 Electrode Configurations Investigated by Somia et al. Electrode placements for (A)
single channel and (B) multichannel stimulation. Adapted from (Somia et al., 2001).

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1.3.3.6 Gittins et al.
Gittins, et al. produced the only known publication involving electrical stimulation
of the paralyzed OO in humans (1999). Their study investigated therapeutic, rather
than functional effects. Transcutaneous electrical stimulation was performed in 10
individuals with chronic facial nerve palsy. Electrodes were placed near the medial
and lateral canthi and monophasic voltage pulses were delivered with a nerve
stimulator that allowed for frequencies between 2 and 200 Hz and pulse widths
between 50 and 200 µs. The parameters used for the study included 200 µs pulses
delivered at 10 Hz. The patients set the voltage to a comfortable level (reported at
approximately 25V) and initiated the stimulation themselves for approximately one
hour/day with a duty cycle of 2s on and 1s off. The treatment period lasted for three
months and assessments were made every four weeks. The authors reported that the
use of similar stimulation in normal individuals could cause eyelid movement and
even closure by increasing pulse frequency to 40 Hz, however, no movement was
observed in any of the facial palsy patients. Nonetheless, they did report an increase
in voluntary eye movement following the therapy that they believed to be a result of
“reduction in stiffness of the eyelid apparatus”. The lack of ability to produce
electrically induced OO contraction is not surprising, given the fact that stimulus
pulse widths used in the study were in the range appropriate for nerve activation but
not for activation of denervated muscle. At such short pulse widths, extremely high
amplitudes would be required to activate the denervated OO. The authors concluded

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with a statement that further study of alternative stimulation protocols was
underway, however, no additional studies have been published.
1.4 Goals of this Research
The previous studies of OO stimulation described above have generally reported
promising results, however, they have thus far failed to lead to the development of a
clinical system. This is most likely due to gaps in the research that this thesis will
attempt to address. The research presented here includes the first quantitative
assessment of eyelid closure generated by electrical stimulation of the paralyzed OO.
It also represents the only known study to examine the effects of stimulation at
multiple time-points beyond paralysis, both with and without chronic stimulation.
Finally, it provides quantitative evidence of the ability to restore synchronization of
eyelid closure by triggering electrical stimulation based on contralateral eye closure
in unilateral facial paralysis.
The ultimate goal of this work is to advance the understanding of OO stimulation
and enable the transition from animal models to human studies. It is my hope that
this research will lay the groundwork for the successful development of a clinical
neuroprosthesis for the restoration of eye blink function.

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Chapter 2 : Denervation and Reinnervation in the Rabbit
Orbicularis Oculi
As indicated in Chapter 1, one of the primary complicating factors for both facial
paralysis and functional electrical stimulation (FES) research is the concept of
denervation. Facial paralysis patients undergo differing degrees of both temporary
and chronic denervation that ultimately determine the prognosis of natural recovery,
while most FES applications steer clear of chronic denervation because of the
associated complications with the electrical activation of denervated muscle. Thus,
an understanding of denervation and reinnervation is important for the development
of a neural prosthetic system to restore function following facial paralysis.
Additionally, experiments in our animal model of surgically induced seventh nerve
palsy consistently revealed reinnervation of the orbicularis oculi (OO) that may or
may not correlate with clinical reinnervation patterns in facial paralysis patients. This
chapter provides detailed background on the neurophysiology of denervation and
reinnervation, and describes an experiment that was performed to help characterize
the reinnervation that was observed in our rabbit model of facial paralysis.
2.1 Neurophysiology of Denervation and Reinnervation
Within hours of a neural injury, the damaged structures begin a series of changes
that, under ideal conditions, will lead to regeneration of damaged fibers and

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reinnervation of muscles that were denervated by the original insult. Injured axons
undergo different degrees of damage depending on the type of insult, which in turn
has implications on the level of natural spontaneous recovery that might occur (see
Figure 2.1). This section describes the natural processes that take place when
damaged nerves degenerate and regenerate, as well as the corresponding changes
that take place in the muscles they innervate. This is intended to provide a greater
understanding of the role of denervation and reinnervation in facial paralysis.
2.1.1 Facial nerve microanatomy
The facial nerve consists of approximately 10,000 axons, including 7,000 motor
axons destined to innervate the facial muscles. Each of these motor axons is
myelinated, with diameter between 3 and 20 µm and spacing between the nodes of
Ranvier of 0.1 to 1.8 mm, providing a normal conduction velocity of approximately
70 to 110 m/s (M. May, 2000b). Following injury, faulty remyelination can result in
markedly slower conduction velocity and depolarization threshold increase. When
considering facial nerve injury, it is important to remember that each individual axon
is an independent structure that can be damaged or spared, resulting in variable
degrees of functional loss depending on the number of axons affected. Additionally,
each damaged axon must undergo independent regeneration, resulting in variable
degrees of functional recovery.

105

Figure 2.1 Classification and Expected Recovery of Neural Injury. First degree: compression
without structural disruption. Second degree: compression with interruption of the myelin and
axoplasm. Third degree: compression with disruption of the endoneurium. Fourth degree: disruption
of the endoeurium and perineurium. Fifth degree: complete nerve transection with interruption of the
endoneurium, perineurium, and epineurium. Adapted from (Schaitkin & May, 2000b).

106
The nerve is composed of three primary membranes: the epineurium, perineurium,
and endoneurium (see Figure 2.1). The epineurium surrounds the perineural sheath
and contains both blood and lymphatic vessels. The perineurium forms a
metabolically active diffusion barrier that protects the nerve and maintains its
internal pressure. The endoneurium is the connective tissue within the funiculus that
encircles each individual nerve fiber and plays an integral role in axonal
regeneration. Which membranes are involved in nerve damage has implications on
prognosis for recovery.
2.1.2 Mechanisms of nerve degeneration and regeneration
When a nerve fiber is injured, it triggers specific and unique reactive responses in
different regions of the cell. These begin within hours of injury and, under ideal
conditions, will lead to full recovery of any lost function. If necessary conditions are
not met, the window for spontaneous natural recovery can close, and over time,
continued degeneration can potentially lead to the loss of any opportunity for
reinnervation.
2.1.2.1 Cell body
Within seven hours of injury the cell body begins to undergo a process referred to as
chromatolysis. The volume of the cell body increases due to an increase in metabolic
processes. This persists for 10 to 21 days, however the regenerative capacity of the
cell body itself persists indefinitely, provided the cell body survives the initial injury.

107
There is evidence to indicate that the probability of cell body survival in the facial
nerve is dependent upon the location of injury, with a larger percentage of cell body
deaths associated with proximal transection of the nerve than distal transection (Dai,
Kanoh, Li, & Wang, 2000).
2.1.2.2 Axon
The proximal and distal segments of injured axons undergo very different processes
that together promote the replacement of damaged fibers with new healthy ones.
Within the first five hours of axonal transection, the process known as Wallerian
degeneration begins within the distal segment. Signs of degeneration become evident
within 12 hours and the process is well advanced by 36 to 48 hours after injury.
During Wallerian degeneration, Schwann cells proliferate and become macrophages,
engulfing the myelin and other debris from degeneration, which is mostly removed
by 12 to 14 days following the injury. These Schwann cells form endoneural tubes,
known as Bünger bands, which strongly support axonal regeneration.
Within three days of injury, the proximal stump begins to enlarge and form axonal
sprouts known as growth cones. These sprouts grow at a rate of 1 mm/day,
approximately the same rate as the peripherally moving cellular transport system.
Therefore, the anticipated time to recovery under ideal conditions can be determined
by measuring the distance from the site of injury to the site of original innervation.
Following the initial stump enlargement, axons decrease in diameter, allowing for

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natural spontaneous decompression and the ability of regenerating axons to grow
around obstacles.
Neurotrophic factors encourage the growth of axonal sprouts into the endoneural
tubes of the distal segment. These tubes direct the regenerating axons to the location
of original innervation, where they form new neuromuscular junctions. While there
is a natural preference for muscle fibers to reform neuromuscular junctions with the
appropriate cell body, the lack of topographic organization in the facial nerve leads
to faulty reinnervation. When there is division of the endoneurium, regenerating
axons are free to grow into any endoneural tube, leading to the easy misdirection of
fibers and resulting in high probability of synkinesis. Over time, the number of
regeneration encouraging Schwann cells decreases and within three to four months
the Bünger bands degenerate and are replaced by connective tissue, eliminating the
possibility of reinnervation along the original nerve tract.
2.1.2.3 Muscle
Following denervation, muscle fibers demonstrate an increased sensitivity to
acetylcholine along the length of the fiber, encouraging the development of new
neuromuscular junctions at an increased number of locations. The source of this
reinnervation can be the regenerating axons from the originally injured nerve, or
aberrant sprouting from other nerves in the vicinity. If reinnervation does not take
place, the fiber will begin to atrophy over a period of weeks to months, and
eventually will be replaced by fibrous tissue. The general rule of thumb is that

109
muscle fibers must be reinnervated within a year of denervation, however, the
presence of atrophic muscle fibers has been noted in the facial muscles as late as 36
years after initial denervation (Schwarting, Schroder, Stennert, & Goebel, 1984).
Reinnervated muscles will demonstrate a reversal of atrophy correspondent to a
return of function.
2.1.3 Classification of nerve injury and expected recovery
There are five main classes of nerve injury depending upon the type of insult and
extent of structures damaged (see Figure 2.1) (Sunderland, 1978). Each level of
injury carries with it a different prognosis for recovery. The first three categories of
injury are consistent with nerve compression, while the last two are consistent with
nerve disruption. Another common method of classification refers to the two lowest
degrees of nerve injury as neurapraxia and axonotmesis, while grouping the three
most severe under the name neurotmesis (Seddon, 1943). Different axons within a
single nerve can undergo different levels of injury from the same insult.
2.1.3.1 First degree (neurapraxia)
In a first degree injury, increased neural pressure causes a physiological block of
nerve conduction. If the pressure is released, return of normal function can be
expected immediately or within three weeks.

110
2.1.3.2 Second degree (axonotmesis)
In a second degree injury, unrelieved neural pressure causes obstruction of venous
drainage leading to local swelling and damming up of axoplasm. This causes
compression of the arterioles and interruption of nutrient flow. As a result, axons are
lost, but the immediate connection to endoneural tubes is maintained. Once pressure
is released, newly generating axons are free to grow down the endoneural tubes to
their original site of inneration. Because it involves the regeneration of axons, the
time to recovery will depend on the duration of injury and the distance from the site
of injury to the site of innervation, generally taking three weeks to two months in the
facial nerve. Because there is no loss of connection with the neural tubes, however,
recovery should be essentially complete.
2.1.3.3 Third degree (neurotmesis)
In a third degree injury, intraneural pressure continues until the point where there is
endoneural tube loss. Regenerating axons will travel down any remaining endoneural
tube they can find, resulting in variable degrees of recovery and synkinesis. Because
of the extended duration of injury, spontaneous recovery will not be noted for two to
four months.
2.1.3.4 Fourth degree (neurotmesis)
Fourth degree injury results from partial transection of the nerve, involving
disruption of the perineurium. Because this also includes substantial disruption of the
endoneural tubes, little or no spontaneous recovery should be expected and surgical

111
repair is generally indicated. Note that there is no way to ensure proper matching of
endoneural tubes with regenerating axons during surgical repair, and consequently
functional return is highly variable and synkinesis can be rather severe.
2.1.3.5 Fifth degree (neurotmesis)
Fifth degree injury includes complete transection of the nerve, including both the
perineurium and epineurium. Similar to fourth degree injury, recovery should not be
expected without surgical repair.
2.1.4 Denervation and reinnervation following facial paralysis
Facial paralysis can result from either compressive or disruptive etiologies and cases
therefore span the spectrum of neural injury and recovery profiles. Compressive
injury typically results from such causes as viral inflammation as seen in herpes
zoster cephalicus and as possibly indicated in Bell’s palsy. This explains the high
rates of recovery in Bell’s palsy, which generally fit with the descriptions for first
through third degree injury. It should also be noted that delayed recovery in Bell’s
palsy is generally associated with more variable degrees of recovery and greater
degrees of synkinesis, which are typical of third degree injuries. Disruptive injuries
are common in traumatic cases and those resulting from surgical transection during
tumor removal. These are generally referred for surgical repair in an attempt to
encourage reinnervation. Additionally, evidence of aberrant regeneration along the
routes of the fifth nerve, ninth nerve, tenth nerve, autonomic pathways, and cervical

112
plexus, as well as axonal sprouting from adjacent muscles, particularly along the
midline in unilateral facial palsy, also exists.
2.2 Electrophysiological Investigation of Orbicularis Reinnervation
This section describes experiments that were performed to help characterize the
somewhat unexpected reinnervation that was observed in our rabbit model of
surgically induced facial nerve paralysis. Electrophysiological tracing of the nerve
branches reinnervating the orbicularis oculi demonstrated variability in the source of
reinnervation.
2.2.1 Background
The research presented in this thesis investigated electrical stimulation of the OO in a
rabbit model of surgically induced facial nerve paralysis. The original goal was to
create a chronic model of OO denervation by surgically disrupting the seventh nerve
that would allow study of electrical stimulation beginning after various durations of
initial denervation progressing out to several months. To accomplish this, we
completely transected and removed a portion of the branch(es) of the facial nerve
innervating the OO. We then cauterized the area around the proximal and distal
segments. Despite these efforts, however, reinnervation of the OO was observed to
consistently occur beginning sometime between four and eight weeks following

113
nerve disruption. This reinnervation progressed through the 16
th
week after surgery,
at which point a substantial degree of reinnervation was consistently observed.
These observations provided a shift in focus, allowing investigation of the effects of
reinnervation on outcomes of electrical stimulation for blink restoration. This has
relevance in the treatment of facial nerve palsy because at least some degree of
reinnervation of the facial muscles following the onset of facial paralysis is common.
The fact that reinnervation had occurred in our animal model at all, let alone so
consistently, however, was puzzling given the extent of nerve injury induced.
Additionally, the rabbits seemed to possibly have some level of control over the
reinnervated OO. Did this mean that the original facial nerve axons had regenerated
and crossed the gap between the proximal and distal segments despite our efforts to
prevent it, or that axonal sprouting from adjacent nerves we had left intact had
innervated the OO and the rabbits were possibly learning to adapt to a new motor
system? The time at which reinnervation began to manifest itself seemed consistent
with the time necessary for regenerating nerve fibers to traverse the distance from the
proximal stump to the eyelid, however, the degree of damage caused by the nerve
transection procedure seemed severe enough to prevent successful regeneration. In
order to gain a better understanding of this observed reinnervation, an
electrophysiological experiment was designed to trace the reinnervating fibers back
to their origin and determine the source of reinnervation.

114
2.2.2 Materials and Methods
A total of six rabbits were used for this experiment. The zygomatic branch of the
facial nerve, which innervates the OO muscle, was located visually and confirmed
with intraoperative electrical stimulation. The nerve was transected, a portion was
removed, and the surrounding area was cauterized. Total nerve disruption was
confirmed via electrical stimulation and the absence of a corneal blink reflex. Two
rabbits were tested at 8 weeks and four rabbits were tested at 16 weeks following
nerve transection. Strength-duration curves for OO twitch threshold were generated
to confirm the presence of reinnervation and a stimulating probe was used to trace all
innervating branches on both the operated side and the intact contralateral side to
their origin (or as close as was surgically possible). The paths of the facial nerve
trunk and main branches were also investigated.
2.2.2.1 Facial nerve dissection
The seventh cranial nerve was identified and divided on the right (OD) side of all six
rabbits, resulting in paralysis of the orbicularis oculi muscle. The left (OS) side was
unaltered and used as a control. Prior to surgery, rabbits were anesthetized using an
intramuscular injection of ketamine (50-80 mg/kg) and xylazine (5-10 mg/kg). After
the skin of the cheek was shaved, a 1 cm vertical incision was made through the skin,
1 cm inferior to the center of a line drawn from the lateral canthus to the external
auditory meatus, and just anterior to the mandibular ramus. A combination of sharp

115
and blunt dissection was used to divide the subcutaneous tissue and the parotid
gland.
The facial nerve trunk and its three large branches were identified on the surface of
the masseter muscle. Electrical stimulation of the nerve with a 0.5 ms, 1 mA biphasic
current pulse near the point of branching produced simultaneous eye closure and ear
movement. A 7 mm section of the zygomatic branch, which supplies the orbicularis
oculi muscle, was removed and both ends were cauterized. Complete interruption
was confirmed by stimulation of the proximal stump and observation of absent blink
but maintained ear movement. Stimulation of the distal nerve resulted in only eye
blink. Additionally, mechanical stimulation of the cornea, which had produced a
corneal blink reflex prior to the procedure failed to produce one after it. Wounds
were closed using 4-0 Vicryl interrupted sutures. Rabbits were administered
Buprenex (0.03mg/kg) and Vetropolycin ointment every 12 hours, as well as Baytril
(2.5mg/kg) every 24 hours, for the first 48 hours following surgery.
After the rabbits were allowed to recover, paralysis was verified at regular intervals
by lightly touching the cornea with the tip of a cotton swab and gauging the animal’s
response. A healthy eyelid demonstrated smooth and complete closure of the
palpebral fissure with little or no noticeable eye retraction. A paralyzed eyelid
demonstrated substantial eye retraction accompanied by narrowing of the palpebral
fissure and lateral sliding of the nictitating membrane. The movement had a
somewhat wavy profile. This is consistent with expected reaction after facial nerve

116
section when the retractor bulbi muscle is left intact (Leal-Campanario, Barradas-
Bribiescas, Delgado-Garcia, & Gruart, 2004). The health of the eye was maintained
during paralysis by the action of the retractor bulbi, which acted in combination with
the nictitating membrane to provide adequate protection and lubrication.
2.2.2.2 Strength-duration curves
After the specified duration between the onset of paralysis and electrophysiological
investigation, each rabbit was again anesthetized using an intramuscular injection of
ketamine (50-80 mg/kg) and xylazine (5-10 mg/kg). A pair of 25-gauge hypodermic
needles was inserted percutaneously into the upper eyelid, such that one was near the
medial canthus and the other was near the lateral canthus. These were used as
electrodes to deliver stimulation pulses for measuring twitch thresholds used to
generate strength-duration curves. Thresholds for eliciting muscle twitch were found
by increasing the pulse amplitude from zero until the first sign of eyelid movement
coincident with electrical stimulation was visible. Threshold amplitudes for eliciting
muscle twitch were recorded for biphasic pulses with pulse widths ranging from 0.1
to 100 ms per phase. This was performed first on the operated side (OD) and then on
the intact side (OS).
2.2.2.3 Electrophysiological nerve tracing
Following the collection of threshold data, the procedure described in Section 2.2.2.2
was used to expose the facial nerve. After the skin of the cheek was shaved, a 1 cm
vertical incision was made through the skin, 1 cm inferior to the center of a line

117
drawn from the lateral canthus to the external auditory meatus, and just anterior to
the mandibular ramus. A combination of sharp and blunt dissection was used to
divide the subcutaneous tissue and the parotid gland, exposing the facial nerve trunk
and branches.
A 25-gauge hypodermic needle was inserted percutaneously into the back of each
rabbit to act as a ground electrode and another 25-gauge needle was used to deliver
monophasic cathodic stimulation pulses with 0.5 ms duration and 0.5 mA (or lower)
amplitude. These pulse parameters were found to be capable of stimulating the facial
nerve, but only within a distance of approximately 1 mm, allowing for selective
stimulation of individual nerve branches.
The area surrounding the eye on the OD side was explored with the stimulation
probe until a notable twitch of the OO muscle was observed, indicating the location
of the innervating branch(es). The branch was then traced back to its origin, or as far
as was surgically possible, by probing the area around it and tracking the locations
where stimulation continued to elicit a response. The same procedure was performed
for any other branches of the facial nerve that could be located in the vicinity,
particularly those innervating the ear and cheek.
Following thorough investigation of the operated OD side, the same procedures were
performed to trace the healthy intact facial nerve on the OS side.

118
2.2.3 Results
Six rabbits were investigated in this experiment. Their ID numbers were BL48,
BL49, BL51, BL52, BL53, and BL54. Of these rabbits BL48 and BL49 were
examined 8 weeks following nerve transection, while BL51, BL52, BL53, and BL54
were studied 16 weeks after the initial surgery.
2.2.3.1 Facial nerve dissection
Facial nerve transection produced complete paralysis of the OO muscle in all six
rabbits investigated. Disruption of the seventh nerve was confirmed intraoperatively
by the difference in response to electrical stimulation of the proximal and distal
segments as well as the absence of a corneal blink reflex. Paralysis was verified to
persist for several weeks by abnormal response to corneal stimulation.
2.2.3.2 Strength-duration curves
Threshold data for invoking a muscle twitch on both sides for all rabbits are included
in Table 2.1. Of these, the OS sides of all rabbits demonstrated threshold values
consistent with typical motor neuron stimulation, indicating that the intact facial
nerve was being activated to cause the OO to twitch. The OD sides of all rabbits
except for BL54 demonstrated threshold values inconsistent with a typical strength-
duration curve for neuromuscular stimulation. These threshold values, on the other
hand, resembled what might be expected if there were multiple strength-duration
curves for structures with very different chronaxies superimposed on top of each

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TABLE 2.1
Threshold Values for Electrophysiology Twitch Stimulation
Pulse Width (ms)
Rabbit
ID
Side
0.1 0.5 1 2 5 10 25 50 100
OD 0.70 0.80 1.10 1.20 1.50 2.10 2.50 3.40 6.60
BL48
OS 0.20 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.60
OD 0.05 0.05 0.06 0.11 0.20 0.40 0.70 0.80 1.90
BL49
OS 0.50 0.50 0.50 0.50 0.50 0.60 0.60 0.60 1.00
OD 0.09 0.09 0.13 0.31 0.49 0.63 0.89 1.60 2.90
BL51
OS 0.44 0.44 0.44 0.45 0.46 0.50 0.58 0.69 1.30
OD 0.05 0.06 0.15 0.22 0.23 0.32 0.35 0.36 0.78
BL52
OS 0.19 0.20 0.20 0.20 0.21 0.21 0.22 0.22 0.39
OD 0.09 0.09 0.10 0.17 0.24 0.40 0.64 0.73 1.80
BL53
OS 0.32 0.33 0.34 0.35 0.36 0.40 0.43 0.50 1.10
OD 0.12 0.13 0.13 0.14 0.15 0.17 0.20 0.25 0.66
BL54
OS 0.19 0.19 0.19 0.20 0.23 0.26 0.31 0.34 0.62
All stimulus amplitudes given in mA.

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other (see Figure 2.2). In particular, the regions of their strength-duration curves for
longer pulse widths were consistent with what could be expected from stimulation of
denervated muscle, while the regions of their chronaxie values for shorter pulse
widths were consistent with what could be expected from stimulation of
motorneurons. This seems to indicate the presence of partial reinnervation within the
OO of these rabbits, particularly near the electrode contacts. The threshold values for
the OD side of BL54 were consistent with motor nerve stimulation for all pulse
widths tested, indicating more complete reinnervation in the OO of that particular
rabbit.
2.2.3.3 Electrophysiological nerve tracing
Stimulation on the intact (OS) side revealed that the main trunk of the facial nerve
originated deep and traveled superficially approximately midway along a line drawn
from the lateral canthus to the external auditory meatus before branching. Two or
three main branches could be found running superficially along the surface of the
massater muscle. One of these traveled superiorly and posteriorly to innervate the
ear, while another traveled anteriorly to innervate the eyelids. In most cases a third
branch could be found traveling inferiorly and anteriorly to innervate the cheek.
Sometimes nose twitch could be elicited along one of the branches. Branches that
run deep may not have been located during the experiment. Attempts to trace the
facial nerve trunk to its origin at the styolmastoid foramen consistently resulted in
damage to the close-traveling carotid artery. The purpose of these tracings was not to

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Figure 2.2 Strength Duration Curves for Electrophysiological Testing. ‘OD’ represents the
mean threshold values recorded from the OD sides of BL48, BL49, BL51, BL52, and BL53, while
‘OS’ represents the mean threshold values recorded from the OS sides of all rabbits. (BL54 OD was
excluded because it demonstrated a more substantial degree of reinnervation than the other rabbits.)
Error bars represent standard deviation. Plots are staggered slightly for differentiation of error bars.
‘Nerve 1’ represents a theoretical strength-duration curve for a motor nerve with chronaxie = 0.17 ms
and rheobase = 0.32 mA. ‘Nerve 2’ represents a theoretical strength-duration curve for a motor nerve
with chronaxie = 0.17 ms and rheobase = 1.0 mA. ‘Muscle represents a theoretical strength-duration
curve for a denervated muscle fiber with chronaxie = 10 ms and rheobase = 0.18 mA. Note the
similarity between ‘OS’ and ‘Nerve 1’ for all pulses, between ‘OD’ and ‘Nerve 2’ for short duration
pulses, and between ‘OD’ and ‘Muscle’ for long duration pulses. The strength-duration curve for
‘OD’ seems to indicate the probable crossing of strength-duration curves for denervated muscle and
motor nerve, indicating the presence of partial reinnervation.

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completely characterize the course of the external facial nerve, but to give context to
compare the reinnervation patterns on the contralateral side. Results of
electrophysiological nerve tracings on individual rabbits can be found in Figures 2.3
through 2.8.
Nerve tracing using electrical stimulation revealed that all six rabbits had undergone
at least some level of reinnervation of the OO on the previously paralyzed side. This
reinnervation seemed to come from different sources in different rabbits, sometimes
originating from the facial nerve, and sometimes not. Approximately half of the
rabbits investigated demonstrated reinnervation from a location near where the
original nerve appeared to have branched, while half demonstrated reinnervation
from some other location. Additionally, at least one rabbit (BL51) demonstrated
evidence of reinnervation from multiple sources. Results of electrophysiological
nerve tracings on individual rabbits can be found in Figures 2.3 through 2.8.
2.2.4 Conclusions and Discussion
All six rabbits investigated demonstrated reinnervation of the OO muscle within 8 or
16 weeks following complete transection of the facial nerve, partial removal of the
branch(es) innervating the eyelids, and cauterization of the surrounding area. The
source of reinnervation, however, was found to vary among rabbits, originating from
the facial nerve near the area of injury approximately half of the time and originating
from other sources the other half. Additionally, evidence of reinnervation from

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Figure 2.3 Electrophysiological Nerve Tracing for Rabbit BL48 (8 weeks)
Left: Stimulation on the previously paralyzed (OD) side at site (A) resulted in strong isolated
contraction of the OO muscle. Movement of the stimulation probe along the blue line continued to
cause strong OO contraction. This was determined to be the reinnervating branch. Stimulation at site
(B) caused strong isolated ear movement. The red line was determined to be the innervating branch
for the ear. Stimulation at site (C) caused weak contraction of both the eyelid and ear. It was
determined that the reinnervating branch to the OO had regenerated from the facial nerve, but deep to
its original location.
Right: Stimulation on the intact (OS) side at site (A) resulted in strong eyelid contraction with weak
cheek contraction. This persisted along the blue line, which was determined to be the primary branch
innervating the OO. Stimulation at site (C) resulted in strong cheek contraction with very weak eyelid
movement. This persisted along the green line, which was determined to be the primary innervating
branch of the cheek. Stimulation at site (B) caused strong isolated ear movement. This persisted along
the red line, which was determined to be the innervating branch of the ear. Stimulation at site (D)
caused simultaneous ear, eyelid, and cheek movement and was determined to be the location of the
facial nerve trunk. The nerve trunk originated deep and traveled to the surface before branching
superficially.

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Figure 2.4 Electrophysiological Nerve Tracing for Rabbit BL49 (8 weeks)
Left: Stimulation on the previously paralyzed (OD) side at site (A) resulted in strong contraction of
the OO muscle, with possible eye retraction. Movement of the stimulation probe along the blue line
continued to cause strong OO contraction. This was determined to be the reinnervating branch. At the
point where the line becomes dotted, the nerve branch began to dive deep. In attempting to locate its
origin, which appeared to be somewhere near the orbit, the nerve was accidentally transected by the
stimulating probe. Stimulation at site (B) caused strong isolated ear movement. This persisted along
the red line, which was determined to be the innervating branch for the ear. Stimulation at site (C)
caused strong isolated cheek movement. This persisted along the green line, which was determined to
be the innervating branch for the cheek. Stimulation at site (D) resulted in strong contraction of both
the ear and cheek. This was determined to be the branching point of the intact facial nerve trunk. A
branch from the facial nerve to the eyelid was noticeably absent, and it was determined that the
reinnervating branch to the OO had not regenerated from the facial nerve.
Right: Stimulation on the intact (OS) side at site (A) resulted in strong eyelid contraction with weak
cheek contraction. This persisted along the blue line, which was determined to be the primary branch
innervating the OO. Stimulation at site (C) resulted in strong cheek contraction with weak eyelid
movement. This persisted along the green line, which was determined to be the primary innervating
branch of the cheek. Stimulation at site (B) caused strong isolated ear movement. This persisted along
the red line, which was determined to be the innervating branch of the ear. Stimulation at site (D)
caused simultaneous ear, eyelid, and cheek movement and was determined to be the location of the
facial nerve trunk. The nerve trunk originated deep and traveled to the surface before branching
superficially.

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Figure 2.5 Electrophysiological Nerve Tracing for Rabbit BL51 (16 weeks)
Left: Stimulation on the previously paralyzed (OD) side at site (A) resulted in strong contraction of
both the cheek and the OO muscle in the regions marked (2). Movement of the stimulation probe
along the blue line continued to cause contraction of both the cheek and eyelid in these regions.
Stimulation at site (C) and along the green line caused isolated contraction of the cheek without eyelid
movement. It was determined that fibers innervating the cheek had sprouted to innervate part of the
OO. Deep stimulation at the site marked with a star (*) caused isolated contraction of the eyelid in
region (1). This source of reinnervation could not be traced to its origin, but appeared to be coming
from inside the orbit. Stimulation at site (B) caused strong isolated ear movement. The red line was
determined to be the innervating branch for the ear, however the facial nerve was determined to
branch deep, as superficial stimulation could not cause simultaneous contraction of the ear with other
structures. It was determined that the OO had been reinnervated from multiple sources, including
sprouting from the branch of the facial nerve previously dedicated to the cheek and an unknown
source near the orbit. These sources had reinnervated distinct and separate regions of the eyelids.
Right: Stimulation on the intact (OS) side at site (A) resulted in strong eyelid contraction
accompanied by nose twitch. This persisted along the blue line, which was determined to be the
primary branch innervating the OO. Stimulation at site (B) caused strong isolated ear movement. This
persisted along the red line, which was determined to be the innervating branch of the ear. Stimulation
at site (C) caused simultaneous ear, eyelid, cheek, and nose movement and was determined to be the
location of the facial nerve trunk. The individual branch traveling to the cheek was not located.

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Figure 2.6 Electrophysiological Nerve Tracing for Rabbit BL52 (16 weeks)
Left: Stimulation on the previously paralyzed (OD) side at site (A) resulted in strong contraction of
the upper eyelid along with mass forehead movement. Lower lid contraction was minimal. This
persisted along the blue line, which was determined to be the reinnervating branch for the OO.
Stimulation at site (B) caused strong isolated ear movement. The red line was determined to be the
innervating branch for the ear. Simultaneous contraction of the eyelid could not be elicited with any
structure other than the forehead. Additionally, eyelid movement could not be isolated from forehead
movement. There was no apparent retraction with stimulation. No source of strong lower lid
contraction could be found. It was determined that axonal sprouting from the forehead had resulted in
reinnervation of the superior portion of the OO.
Right: Stimulation on the intact (OS) side at site (A) resulted in strong isolated eyelid contraction.
This persisted along the blue line, which was determined to be the primary branch innervating the
OO. Stimulation at site (C) resulted in movement of the nose and forehead, which persisted along the
green line. Stimulation at site (B) caused strong isolated ear movement. This persisted along the red
line, which was determined to be the innervating branch of the ear. Stimulation at site (D) caused
simultaneous movement of all aforementioned structures and was determined to be the location of the
facial nerve trunk. The nerve trunk originated deep and traveled to the surface before branching
superficially. This branching pattern seemed to be atypical.

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Figure 2.7 Electrophysiological Nerve Tracing for Rabbit BL53 (16 weeks)
Left: Stimulation on the previously paralyzed (OD) side at site (A) resulted in strong isolated
contraction of the OO muscle. Movement of the stimulation probe along the blue line continued to
cause strong OO contraction. This was determined to be the reinnervating branch. Stimulation at site
(B) caused strong isolated ear movement. The red line was determined to be the innervating branch
for the ear. Stimulation at site (C) caused strong contraction of both the eyelid and ear. It was
determined that the reinnervating branch to the OO had regenerated from the facial nerve near its
original location.
Right: Stimulation on the intact (OS) side at site (A) resulted in strong eyelid contraction
accompanied by nose twitch. This persisted along the blue line, which was determined to be the
primary branch innervating the OO. Stimulation at site (B) caused strong isolated ear movement. This
persisted along the red line, which was determined to be the innervating branch of the ear. Stimulation
at site (C) caused simultaneous ear, eyelid, and nose movement and was determined to be the location
of the facial nerve trunk. The nerve trunk originated deep and traveled to the surface before branching
superficially. The branch innervating the cheek was not located.

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Figure 2.8 Electrophysiological Nerve Tracing for Rabbit BL54 (16 weeks)
Left: Stimulation on the previously paralyzed (OD) side at site (A) resulted in strong isolated
contraction of the OO muscle. Movement of the stimulation probe superficially along the blue line
continued to cause strong OO contraction. This was determined to be the reinnervating branch.
Stimulation at site (D) resulted in contraction of both the nose and eyelid. Stimulation deep along the
dotted green line to site (C) caused nose twitch. Stimulation at site (B) caused strong isolated ear
movement. The red line was determined to be the innervating branch for the ear. This did not seem to
connect to the branches for the nose and eyelid, however, and may have joined the facial nerve trunk
through a circuitous route. It was determined that the reinnervating branch to the OO had regenerated
from the facial nerve, probably near its original location.
Right: Stimulation on the intact (OS) side at site (A) resulted in strong eyelid contraction
accompanied by nose twitch and persisted along the blue line. Stimulation at site (C) resulted in
contraction of both the cheek and eyelid, which persisted along the green line. It was determined that
the OO was innervated by multiple branches. Stimulation at site (B) caused strong isolated ear
movement. This persisted along the red line, which was determined to be the innervating branch of
the ear. Stimulation at site (D) caused simultaneous movement of all aforementioned structures and
was determined to be the location of the facial nerve trunk. The nerve trunk originated deep and
traveled to the surface before branching superficially.

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multiple sources was demonstrated to have occurred in at least one rabbit. This
indicates that chronic denervation cannot be expected following this type of lesion in
a rabbit model. The variety of reinnervation sources, while not controlled, may
provide a somewhat relevant model for reinnervation following facial paralysis in
humans, however, which has been demonstrated to occur through both normal and
aberrant routes (J. J. Conley, 1964; M. May, 2000a). If a model of chronic
denervation is required, it has been suggested that the use of neurotoxins may have
some effect in preventing unwanted reinnervation (Paydarfar & Paniello, 2001; Yian,
Paniello, & Gershon Spector, 2001).

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Chapter 3 : Acute Electrical Stimulation of the Orbicularis
Oculi in Rabbit
This study investigated the quantitative effects of acute electrical stimulation of the
orbicularis oculi (OO) muscle in a rabbit model of facial nerve paralysis. The goal
was to demonstrate that electrical stimulation of the denervated OO can elicit a
satisfactory degree of eyelid closure and to determine optimal stimulation parameters
(including pulse width, pulse amplitude, pulse frequency, number of pulses
delivered, number of stimulation channels, and stimulating contact placement). A
modified version of this chapter was published as (Sachs, Chang, Vyas, Sorensen, &
Weiland, 2007).
3.1 Background
(For additional background information see Chapter 1.)
Dysfunction of the seventh cranial (facial) nerve brought about by trauma or disease
leads to paralysis of the facial musculature. The incidence of facial paralysis in the
United States resulting from various etiologies is approximately 50 cases per
100,000. Of those affected, more than 40% demonstrate satisfactory to complete
recovery, while the remaining are left with some level of chronic impairment
(Bleicher et al., 1996). Clinically, the most significant effect of seventh nerve lesion

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is paralysis of the OO muscle, which results in inability to close the eyelids during
both voluntary and reflex blinking (M. May & Hughes, 1987). Because eyelid
closure is the primary protective mechanism of the eye, functional deficits in the
ability to close the eyelids can lead to corneal damage, infection, perforation, and
loss of the eye. Current methods for ensuring eye closure following facial paralysis
include the implantation of gold weights in the eyelid, the implantation of
mechanical springs, the use of artificial tears, nerve and muscle transfer, and
tarsorrhaphy (Abell et al., 1998; Arion, 1972; Boerner & Seiff, 1994; Kinney et al.,
2000; Lee et al., 2004). All of these are helpful in preserving the eye, however none
of these techniques, even used in combination, are fully effective. Additionally, these
techniques are often inconvenient, cosmetically unacceptable, and subject the patient
to multiple surgical procedures. Electrical stimulation of the OO muscle has the
potential to provide a more elegant and effective method for eliciting eyelid closure.
Previous studies have investigated electrical stimulation of the OO muscle in dog
and in rabbit (Otto, 1997; Otto et al., 1986; Rothstein & Berlinger, 1986; Salerno,
Bleicher, Stromberg et al., 1990; Somia et al., 2001; Tobey & Sutton, 1978).
Reported effects have all been qualitative, and stimulation strategies and parameters
have varied widely among experiments. To our knowledge, our work is the first
quantitative study that measures degree of lid closure as a function of stimulus pulse
parameters. The range of pulse width and amplitude assessed was wider than most
previous studies. Also, a direct comparison between single pulses and pulse trains

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was made. Finally, this is the first study to assess stimulus efficacy at multiple times
after the onset of paralysis. These results will aid in the specification of requirements
for chronically implanted devices for restoration of eyelid function.
3.2 Materials and Methods
The OO muscle was paralyzed in 20 rabbits by sectioning the seventh cranial nerve.
These were divided into the following groups based on the duration of paralysis prior
to stimulation: 1-week (n = 5), 4-week (n = 5), 8-week (n = 5) and 16-week (n = 5).
In addition, a group of normal rabbits (n = 5), with facial nerve intact, was used for
comparison. After the specified period, each rabbit was anesthetized and an electrode
was acutely inserted into the subcutaneous plane near the margin of the upper eyelid,
such that platinum metal contacts rested in the subcutaneous space of the upper
eyelid in the vicinity of the medial and lateral canthi (see Figure 3.1). These contacts
were used to deliver biphasic, current controlled stimulation pulses. A high-speed
digital video camera was used to record the response of the eyelid to stimulation and
image processing software was used to quantify lid closure.
3.2.1 Dissection of the facial nerve
The seventh cranial nerve was identified and divided in rabbit resulting in paralysis
of the OO muscle. Prior to surgery, rabbits were anesthetized using an intramuscular
injection of ketamine (50-80 mg/kg) and xylazine (5-10 mg/kg). After the skin of the

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Figure 3.1 Acute Electrode Placement. Diagram of electrode inserted subcutaneously into the
upper eyelid of a rabbit and secured with a suture external to the insertion site. The first and fifth
contacts (indicated by arrows) were used to deliver stimulation pulses.

134
cheek was shaved, a 1 cm vertical incision was made through the skin, 1 cm inferior
to the center of a line drawn from the lateral canthus to the external auditory meatus
and just anterior to the mandibular ramus. A combination of sharp and blunt
dissection was used to divide the subcutaneous tissue and the parotid gland. The
facial nerve trunk and its three large branches were identified on the surface of the
masseter muscle. Stimulation of the nerve with a 0.5 ms, 1 mA biphasic current pulse
produced simultaneous eye closure and ear movement. A 7 mm section of the
zygomatic branch, which supplies the OO muscle, was removed and both ends were
cauterized to help prevent reinnervation. Complete interruption was confirmed by
stimulation of the proximal stump and observation of absent blink but maintained ear
movement. Stimulation of the distal nerve resulted in only eye blink. Wounds were
closed using 4-0 Vicryl interrupted sutures. Rabbits were administered Buprenex
(0.03 mg/kg) and Vetropolycin ointment every 12 hours, as well as Baytril (2.5
mg/kg) every 24 hours, for the first 48 hours following surgery.
3.2.2 Verification of paralysis
Persistence of paralysis was verified at regular intervals and immediately prior to
electrode insertion by lightly touching the cornea with the tip of a cotton swab and
gauging the animal’s response. A healthy eyelid demonstrated smooth and complete
closure of the palpebral fissure with little or no noticeable eye retraction. A
paralyzed eyelid demonstrated substantial eye retraction accompanied by narrowing
of the palpebral fissure and lateral sliding of the nictitating membrane. The

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movement had a somewhat wavy profile. This is consistent with expected reaction
after facial nerve section when the retractor bulbi muscle is left intact (Leal-
Campanario et al., 2004). The health of the eye was maintained during paralysis by
the action of the retractor bulbi, which acted in combination with the nictitating
membrane to provide adequate protection and lubrication.
3.2.3 Electrode placement
A stimulating electrode was surgically inserted into the upper eyelid after the
specified duration of paralysis for each rabbit, and immediately prior to initiating the
electrical stimulation protocol. Prior to surgery, rabbits were anesthetized using an
intramuscular injection of ketamine (50-80 mg/kg) and xylazine (5-10 mg/kg). After
the area surrounding the eyelids was shaved, a small cutaneous stab incision was
made using a #11 blade, approximately 5 mm lateral to the lateral border of the
upper eyelid. A 14-gauge angiocatheter was inserted through the stab incision and
into the subcutaneous plane across the length of the eyelid, 2 mm superior to the
lower border of the eyelid. The stylet of the angiocatheter was removed, and a depth
electrode (Spencer Probe, Ad-Tech Medical Instrument Corporation, Racine, WI)
was threaded into the subcutaneous space, through the lumen of the angiocatheter.
The angiocatheter was withdrawn leaving the electrode in the subcutaneous space of
the eyelid. A 4-0 silk anchoring suture was used to secure the electrode to the skin of
the rabbit, 2 cm lateral to the entry site. See Figure 3.1 for a diagram of electrode
placement.

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The probe was 1 mm in diameter and included six cylindrical platinum contacts,
each 2.3 mm long and spaced 5 mm apart. A minimum of five contacts fit in the
subcutaneous space of the upper eyelid. The first and fifth contacts were used for
electrical stimulation, giving a dipole spacing of approximately 2 cm.
In addition to the 25 rabbits that were acutely stimulated, 2 normal rabbits received
chronically implanted sham electrodes in order to evaluate the degree to which the
presence of an implanted electrode would affect normal lid closure. Sham implants
were inserted using the same procedure mentioned previously, however, the sham
implants were cut at the entry site and the external portion of the probe was removed.
The entry site was sutured closed, leaving the remaining portion of the probe
implanted subcutaneously. These rabbits were allowed to heal and later observed to
qualitatively gauge the degree to which the presence of the implant affected normal
lid closure during spontaneous and reflex blinks.
3.2.4 Electrical stimulation protocol
Following electrode placement, rabbits were maintained under anesthesia using
periodic injections of ketamine and xylazine throughout the stimulation portion of
the experiment. Biphasic square wave current pulses were delivered using a
multifunction DAQ (PCI-6025E, National Instruments Corporation, Austin, TX) and
analog stimulus isolator (Model 2200, A-M Systems, Inc., Carlsborg, WA). The
stimulation protocol was divided into three stages. During the first stage, thresholds

137
for eliciting muscle twitch were found by increasing the pulse amplitude from zero
until the first sign of eyelid movement coincident with electrical stimulation was
visible. Threshold amplitudes for eliciting muscle twitch were recorded for biphasic
pulses with pulse widths ranging from 0.1 to 100 ms per phase. During the second
stage, single biphasic stimulation pulses with amplitudes ranging from 1 to 7 mA
were delivered over the range of pulse widths from 0.5 to 100 ms per phase and the
response to stimulation was measured in terms of the overall lid closure. During the
third stage, pulse trains consisting of 5 and 10 pulses with pulse widths ranging from
0.5 to 10 ms per phase were delivered at a rate of 50 Hz for pulse amplitudes ranging
from 1 to 7 mA and the response to stimulation was again measured in terms of
overall lid closure. Each variation of pulse parameters was delivered twice, with an
inter-stimulus spacing of 1s. Trials for individual parameter settings were separated
by 20s. The electrode voltage was monitored to ensure that the compliance voltage
of the stimulator did not limit output. During the course of the experiments, it was
determined that the maximum current that could be consistently delivered was
slightly above 7 mA.
Following the stimulation protocol, rabbits were euthanized with an intracardiac
injection of Pentobarbitol (120 mg/kg) and tissue samples were taken from the upper
and lower eyelids on both sides for histological analysis.

138
3.2.5 Blink recording and data analysis
A high-speed video camera (1M75-SA, Dalsa Corporation, Ontario, Canada) was
used to record the response of the eyelid to stimulation. Video was captured and
recorded at a rate of 192 frames/second with a resolution of 0.083 mm. An interface
was created using LabVIEW (National Instruments Corporation, Austin, TX) to
coordinate the recording of video and delivery of stimulation pulses. Eyelid
separation was measured with National Instruments, Vision Assistant software by
tracing the outline of the palpebral fissure created by the margin of both eyelids and
calculating the enclosed area. The minimum exposed area during stimulation for
each recording was divided by the area prior to stimulation in order to determine
peak percent closure (see Figure 3.2).
3.2.6 Histological analysis
Tissue samples were fixed in 10% formalin and embedded in paraffin. Serial sections
with 5 !m thickness were taken transverse to the longitudinal direction of the muscle
fibers near the midpoint along the eyelid. Sections were stained with Masson’s
trichrome and observed under a microscope at magnifications up to 40x. Digital
images were taken at 40x from two rabbits within each paralyzed group one normal
control and the cross-sectional areas of a portion of muscle fibers were measured
using ImageJ software (National Institutes of Health, USA) by tracing the outline of
each individual fiber. These were recorded from both the operated (OD) and intact

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Figure 3.2 Data Analysis Procedure. The outline of the palpebral fissure was traced in video
frames captured prior to stimulation (A) and at the peak of lid closure (B). Percent closure was
calculated as 1 – area of (B) / area of (A).

140
(OS) sides. Areas were normalized according to the mean OS muscle fiber area and
compared between the OS and OD for each animal in order to assess muscle atrophy.
3.2.7 Statistical analysis
Five rabbits were electrical stimulated within each group. Pair-wise statistical
comparison of the chronaxie and rheobase values among groups was performed
using a Student’s t test compensated for multiple comparisons using the Bonferroni
method. Similarly, statistical comparison of percent closure values among groups
was also performed using a Student’s t test with compensation for multiple
comparisons. An initial P value of less than 0.05 (prior to compensation) was
indicative of significant difference between data.
Samples from two rabbits within each paralyzed group and one normal control were
histologically analyzed to assess muscle atrophy. Statistical comparison of muscle
fiber cross-sectional areas between the OD and OS for each animal was performed
using a Student’s t-test. A P value of less than 0.05 was indicative of significant
difference between samples.
3.3 Results
Results are presented for each phase of the experiment, along with some brief
discussion of key findings.

141
3.3.1 Surgical procedures
Surgical procedures for nerve division and electrode insertion were simple, effective,
and repeatable. All rabbits exhibited loss of blink function following seventh nerve
division. This was verified by response to both electrical stimulation of the divided
nerve stump and mechanical stimulation of the cornea. Narrowing of the palpebral
fissure in response to mechanical stimulation of the cornea with a cotton swab
increased to nearly complete closure at 8 and 16 weeks, however this closure was
still accompanied by movement of the nictitating membrane. Whether this is due to
recovered control of active lid movement corresponding with reinnervation of the
OO muscle or simply the result of strengthening and increased control of the
retractor bulbi muscle due to increased use is unknown. The electrode could be
consistently placed but did cause some deformation of the upper lid. Normal rabbits
that received sham implants were capable of achieving 100% lid closure during both
spontaneous and reflex blinking.
3.3.2 Strength-duration curves for lid twitch
Strength-duration curves plotting lid twitch threshold are shown in Figure 3.3 and the
associated chronaxie and rheobase values are compared in Table 3.1. Rheobase
values were measured using 100 ms pulse width and chronaxie values were
calculated using a least squares fit, according to the Lapicque law for stimulation
(Geddes & Bourland, 1985; Lapicque, 1909, 1926), which is given by

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Figure 3.3 Strength-Duration Curves for Twitch Threshold. Plots are threshold for eliciting lid
twitch v. pulse width. Twitch was defined as minimum visible movement of any part of the lid
coincident with stimulation. Mean threshold values are plotted for each group with error bars
representing standard deviation. All data were collected with pulse widths of 0.1, 0.5, 1, 2, 5, 10, 25,
50, or 100ms. Plots are staggered slightly to allow differentiation of error bars.

143

TABLE 3.1
Chronaxie and Rheobase Values for Acute Stimulation
Rheobase (mA) Chronaxie (ms)
Normal: 0.480 ± 0.259 0.367 ± 0.111
1-week: 0.044 ± 0.011 50.97 ± 10.85
4-week: 0.054 ± 0.042 47.34 ± 21.35
8-week: 0.034 ± 0.015 48.96 ± 23.49
16-week: 0.570 ± 0.342 0.518 ± 0.549
Rheobase values were measured at 100 ms pulse width and chronaxie values were calculated using a
least squares fit according to equation (3.1). All values are listed as Mean ± SD.

144

!
I =b(1+c/d) (3.1)
where I is the threshold current, b is the rheobase, c is the chronaxie, and d is the
pulse duration.
The chronaxie values for normal and 16-week rabbits were each significantly lower
than those for 1-week rabbits. Distinct differences were also noted for normal and
16-week rabbits when compared with 4-week and 8-week rabbits, however these
were not found to be statistically significant. Similarly, the rheobase values for
normal and 16-week rabbits were each higher than those for 1-week, 4-week and 8-
week rabbits although these differences were not found to be statistically significant.
As described in Section 1.3.1, differences in membrane biophysics result in much
longer chronaxie values for denervated muscle than for muscle that is innervated by
motor axons. Chronaxie values for lid twitch in 1-week, 4-week and 8-week rabbits
were comparable to those reported by Salerno et al. for denervated OO muscle
twitch in dog (1990) as well as those reported for denervated human skeletal muscle
(Geddes, 1999). Chronaxie values for normal and 16-week rabbits were much closer
to those those reported by Salerno et al. for normal OO muscle and palpebral nerve
twitch than for denervated muscle (1990). These chronaxie values were also
comparable to those reported for motor nerve in both human and cat (Geddes, 1999).
Salerno et al. reported that the rheobase values for stimulation of normal OO muscle
in dog were significantly higher than those for stimulation of denervated OO muscle

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(1990). Rheobase values for muscle twitch in normal and 16-week rabbits were
higher than those for 1-week, 4-week and 8-week rabbits in this study (although
differences were not statistically significant). This is most likely the result of an
increased sensitivity to electrical stimulation in local denervated muscle fibers
combined with weaker coupling between the stimulating electrode and the nearest
innervating motor axons, which would tend to be separated by a greater distance.
The fact that strength-duration data for threshold response in the 16-week rabbits
closely resembled that of normal innervated rabbits as well as reported data for
normal muscle and nerve stimulation, but did not resemble that of other paralyzed
rabbits or reported data for denervated muscle stimulation, seems to indicate that the
16-week rabbits experienced some degree of reinnervation.
Qualitative assessment of the twitch movement elicited by threshold level
stimulation suggests that the contraction was generally limited to a small area
focused immediately adjacent to one of the electrode contacts. For most rabbits, this
movement was consistent in both magnitude and location for all pulse widths. One 4-
week rabbit and four 8-week rabbits, however, exhibited a singular change in the
location of lid movement as pulse width was decreased. The transition generally
occurred at or near 1 ms pulse width. For longer pulse widths, the twitch associated
with stimulation was localized in the area near a single electrode, while for shorter
pulse widths, movement associated with twitch stimulation occurred over a larger
area and was centered at a different location that varied among rabbits and was

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generally not isolated to the vicinity of either electrode. The current necessary to
elicit twitch movement with short duration pulses was also slightly less in these
rabbits than what was observed in the majority of paralyzed rabbits. An example of
threshold data from a single 8-week rabbit demonstrating such characteristics is
shown in Figure 3.4.This difference in twitch response for long versus short pulse
widths may indicate partial reinnervation of the OO. The fact that short pulse widths
generated movement in a different location of the eyelid than long pulse widths
seems to indicate that different structures were being activated over different regions
of the strength-duration curve. Additionally, the threshold values depicted in the
strength-duration curves for these rabbits demonstrated characteristics of denervated
muscle for long pulse widths and characteristics of innervated muscle for short pulse
widths (as demonstrated by the representative 8-week rabbit data depicted in Figure
3.4). If a portion of the OO muscle in the vicinity of one of the electrode contacts
remained denervated, it would have a low activation current for long pulse widths
and would therefore define the threshold in that region of the strength-duration
curve. If other portions of the OO had been reinnervated, however, they would have
a lower activation current than the denervated portion of the muscle for short pulse
widths and would therefore define the threshold in that region of the strength-
duration curve. This is precisely the behavior that was observed in these rabbits.
Because the presence of such partial reinnervation only affected thresholds for short
pulse widths (generally less than 1 ms), the measured rheobase values were defined
by activation of the denervated portion of the muscle and the calculated chronaxie

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Figure 3.4 Strength Duration Curve for Reinnerverted Twitch Threshold. Plots are threshold
for eliciting lid twitch v. pulse width for a single 8-week rabbit compared to the groups of innervated
(normal and 16-week) and denervated (1-week and 4-week) rabbit. This particular 8-week rabbit
demonstrated characteristics of innervated OO at pulse widths shorter than 2ms and characteristics of
denervated OO at pulse widths longer than 2ms, most likely indicating the presence of partial
reinnervation. Mean threshold values are plotted for each group with error bars representing standard
deviation. All data were collected with pulse widths of 0.1, 0.5, 1, 2, 5, 10, 25, 50, or 100ms. Plots are
staggered slightly to allow differentiation of error bars.

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values resembled those of denervated muscle. Thus, evidence of such partial
reinnervation was not reflected in the chronaxie and rheobase values.
Based on these observations from the strength-duration data for lid twitch thresholds,
it is inferred that the OO muscle became denervated following the nerve sectioning
procedure described above, remained denervated through a period of at least 4
weeks, then began to demonstrate progressive reinnervation that was evident but
partial at 8 weeks and more thorough, if not complete, at 16 weeks. This pattern
seemed to be fairly consistent, with only two rabbits falling outside these windows
(one 4-week rabbit demonstrated evidence of partial reinnervation and one 8-week
rabbit did not).
The anatomical source of this reinnervation was not investigated but could be due to
either regeneration of the transected branch of the seventh nerve or in-growth of
motor axons from the surrounding musculature, which was supplied by branches of
the seventh nerve that were left intact. The time at which reinnervation manifested is
consistent with the distance from the proximal stump to the OO muscle given the
normal regeneration rate of approximately 1 mm/day, however the extent of injury
induced would seem to preclude regeneration (see Chapter 2.) Patients suffering
from facial palsy can experience neuronal in-growth or cross-innervation following
seventh nerve lesion (Lee et al., 2004). While this may not allow for volitional
control, the results of this study indicate that it may be useful for improving
electrically elicited lid closure, and is an interesting opportunity for future study.

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3.3.3 Eyelid closure with single stimulation pulses
Single biphasic pulses were delivered over a range of amplitudes from 1 to 7 mA and
a range of pulse widths from 0.5 to 100 ms per phase. A stimulus-response curve
plotting percent closure as a function of pulse width for all groups (with pulse
amplitude fixed at 5 mA) is shown in Figure 3.5. A similar stimulus-response curve
plotting percent closure as a function of pulse width for a single representative group
of rabbits (1-week) at 3, 5, and 7 mA is shown in Figure 3.6. The maximum degree
of closure achievable with a single biphasic pulse over the range of parameters tested
is listed for all groups in Table 3.2. Maximum closure was generally achieved with
the greatest stimulation current (7 mA). For single pulses with 5 mA amplitude, there
was a significant difference between normal rabbits and both 1-week and 4-week
rabbits for pulse widths less than or equal to 10 ms. No other statistically significant
differences were found for single biphasic pulses.
Lid response was strongest in normal rabbits. It decreased in 1-week rabbits as a
result of dennervation of the OO, and continued to degrade in 4-week rabbits,
presumably as a result of increased atrophy of the muscle fibers due to persistent
denervation. Lid response improved substantially in 8-week and 16-week rabbits as a
result of reinnervation of the OO, although it did not return to the same level as that
of normal rabbits. Thus, there was a decrease in response of the OO to electrical
stimulation following denervation that was partially recovered following
reinnervation. In contrast to previous studies, complete lid closure could not be

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Figure 3.5 Stimulus-Response Curves for Single Pulses. Plots are percent closure v. pulse
width for single biphasic pulses with pulse amplitude fixed at 5mA. Mean closure values are plotted
for each group with error bars representing standard deviation. All data were collected with pulse
widths of 0.5, 1, 2, 5, 10, 25, 50, or 100ms. Plots are staggered slightly to allow differentiation of
error bars. Statistically significant differences were noted between normal rabbits and both 1-week
and 4-week rabbits for pulse widths less than or equal to 10ms per phase.

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Figure 3.6 Stimulus-Response Curves for Single Pulses at 1 Week. Plots are percent closure v.
pulse width for single biphasic pulses with varied pulse amplitude. All data are from a single group
(1-week). Similar trends were visible for all other groups. Mean closure values are plotted for each
pulse amplitude with error bars representing standard deviation. All data were collected with pulse
widths of 0.5, 1, 2, 5, 10, 25, 50, or 100ms. Plots are staggered slightly to allow differentiation of
error bars.

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TABLE 3.2
Maximum Closure Values for Acute Stimulation

Single Pulses
(% Closure)

Pulse Trains
(% Closure)
Normal: 91.0 ± 8.7 98.2 ± 3.8
1-week: 74.2 ± 15.2 81.3 ± 12.8
4-week: 70.5 ± 13.4 77.4 ± 12.4
8-week: 83.9 ± 14.1 92.5 ± 10.2
16-week: 81.5 ± 18.0 93.1 ± 7.8
Maximum percent closure values measured for single biphasic pulses and pulse trains over the full
range of parameters tested. All values are listed as Mean ± SD.

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consistently achieved with a single pulse (Otto et al., 1986; Salerno, Bleicher,
Stromberg et al., 1990; Tobey & Sutton, 1978). Factors that possibly may have
affected this include limitations on the current source output and added stiffness in
the upper lid due to the presence of the implant.
Qualitative assessment of the movement elicited by single biphasic pulses suggests
that movement of both the lower and upper lid contributed to overall eye closure. It
was also evident that both phases of the stimulation pulse elicited independent
contractions. This was particularly noticeable with pulse widths greater than 25 ms.
Eyelid movement was more rapid during both closure and opening in normal rabbits
than in paralyzed rabbits.
The stimulus-response curves for single biphasic pulses generally demonstrated an
increase in closure with longer pulse widths as well as with increased pulse
amplitudes, however increases in pulse width beyond 10 ms per phase actually
resulted in a decrease in the amount of closure achieved in normal rabbits (see Figure
3.5). This is because biphasic pulses caused independent activation of the OO with
each phase. For longer pulse widths the overall response began to resolve as two
separate twitches, occurring at the onset of each phase and centered over whichever
electrodes was cathodic during that phase. Because the eyelids opened slightly
during the interval between twitches, the maximum closure that was achieved
declined, an effect that became more pronounced as the pulse width, and interval
between twitch onsets, increased. For shorter pulse widths, the onset of the two

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stimulus phases was closer together, causing the twitches to fuse. This created a
synergistic effect, generating greater maximal closure. The separation of twitches at
long pulse widths was not evident in denervated rabbits, which demonstrated slower
kinematics in response to electrical stimulation (Sachs, Chang, & Weiland, 2006).
The independent activation of portions of the OO with each phase of stimulation can
be explained by the fact that individual OO muscle fibers span only a portion of the
eyelid (Lander et al., 1996). During each phase of a biphasic stimulation pulse, a
different contact on the bipolar electrode would be cathodic and cause stimulation of
neuromuscular tissue in the surrounding areas. The effects of such localized
activation would be most pronounced in denervated muscle, where individual muscle
fibers must be stimulated directly, and therefore only fibers closest to the cathode
would be activated. This could explain why Somia, et al. reported lower thresholds
for achieving lid closure with multichannel stimulation than with single channel
stimulation, since multichannel stimulation would result in multiple sources of
activation, each interacting with a different local region of independent muscle fibers
(2001). While we did not include monopolar or monophasic stimulation in this study,
our observations seem to suggest that the use of bipolar electrodes and symmetric
biphasic pulses may have an advantage over other strategies, provided the pulse
width and contact spacing are optimized to take advantage of the potential synergy
resulting from fusion of twitches caused by such independent local activation.

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3.3.4 Eyelid closure with stimulating pulse trains
Trains of biphasic pulses were delivered over a range of amplitudes from 1 to 7 mA
and a range of pulse widths from 0.5 to 10 ms per phase. These trains consisted of
either 5 or 10 pulses delivered at a frequency of 50 Hz. A stimulus-response curve
plotting percent closure as a function of pulse amplitude for all groups (with a train
of 10 pulses at 10 ms per phase) is shown in Figure 3.7. A similar stimulus-response
curve plotting percent closure as a function of pulse amplitude for a single
representative group of rabbits (1-week) with trains of 1, 5 and 10 pulses at 10 ms
per phase is shown in Figure 3.8. The maximum degree of closure achievable with a
train of 10 pulses over the range of parameters tested is listed for all groups in Table
3.2. Maximum closure was generally achieved with the greatest stimulation current
(7 mA). No statistically significant differences were found for pulse trains consisting
of 10 pulses at 10 ms per phase.
Stimulation with pulse trains achieved greater lid closure than single pulses. The
stimulus-response curves for pulse trains generally demonstrated an increase in
closure for increased amplitude as well as increased number of pulses. The slope of
the stimulus-response curve for pulse trains was greatest between 1 and 2 mA and
began to plateau above 4 mA. This shows that the majority of muscle fibers
contributing to lid closure with a train of 10 ms per phase pulses could be recruited
with a pulse of 3 to 4 mA for the electrode configuration used. The effect of
increasing the number of pulses used seemed to plateau as well, peaking at about 10

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Figure 3.7 Stimulus-Response Curves for Pulse Trains. Plots are percent closure v. pulse
amplitude for trains of 10 biphasic pulses delivered at 50Hz with pulse width fixed at 10ms per phase.
Mean closure values are plotted for each group with error bars representing standard deviation. All
data were collected with pulse amplitudes of 1, 2, 3, 4, 5, or 7mA. Plots are staggered slightly to allow
differentiation of error bars. No statistically significant differences between groups were noted.

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Figure 3.8 Stimulus-Response Curves for Pulse Trains at 1 Week. Plots are percent closure v.
pulse amplitude for single pulses and trains 5 and 10 biphasic pulses. All pulse widths were fixed at
10ms per phase and pulse trains were delivered at 50Hz. All data are from a single group (1-week).
Similar trends were visible for all other groups. Mean closure values are plotted for each pulse
waveform with error bars representing standard deviation. All data were collected with pulse
amplitudes of 1, 2, 3, 4, 5, or 7mA. Plots are staggered slightly to allow differentiation of error bars.

158
pulses. The relative response among groups for pulse train stimulation was similar to
that for single pulses. Lid response was strongest in normal rabbits. This decreased in
1-week rabbits, and again in 4-week rabbits as a result of the effects of denervation.
This was followed by a substantial improvement in lid response in 8-week and 16-
week rabbits resulting from reinnervation, however response did not return to the
same level as that achieved in normal rabbits.
Qualitative assessment of the response to pulse trains suggested that trains of pulses
with amplitude less than 4 mA caused isolated contraction of the OO, while trains of
pulses greater than 4 mA tended to activate portions of the surrounding musculature
(particularly the cheek and forehead). This was also evident with single pulses but
more pronounced with pulse trains. This could indicate that the current required for
complete activation of the denervated OO muscle with the electrode configuration
used may be high enough to cause unwanted stimulation of adjacent nerves. In our
anesthetized animals we could only observe response to motor activation, however,
this does not rule out the potential stimulation of cutaneous or other sensory fibers in
the vicinity of the electrode. In chronic stimulation of the OO in awake rabbit, Otto
reported a reactive movement by his animals in response to initial stimulation that
habituated over time (1997). These findings could be related.

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3.3.5 Histological analysis of muscle atrophy
A sample histology image taken at 4x is shown in Figure 3.9 and sample muscle
fiber comparisons at 40x are shown in Figure 3.10. Values for normalized cross-
sectional muscle fiber areas for each rabbit analyzed are reported in Table 3.3. The
OD muscle fiber areas were significantly smaller than those of the OS in both 1-
week rabbits, one 4-week rabbit, and one 8-week rabbit. No difference was noted in
the second 4-week rabbit, the second 8-week rabbit, or either of the 16-week rabbits.
This supports the hypothesis that muscle atrophy is present within the first few
weeks of denervation and may therefore contribute to decreases in closure observed
at these times. This atrophy seems to be reversed over time, presumably by OO
reinnervation.
3.4 Conclusions and Discussion
This study quantitatively measured the effects of different electrical stimulation
parameters for restoring function to the paralyzed OO in rabbit. This was
investigated at different points in time following the onset of paralysis. It was
demonstrated that denervation caused a decrease in the degree of closure that could
be achieved by electrical stimulation and that reinnervation resulted in improvement.
The use of 10 ms biphasic pulse trains delivered at 50 Hz was demonstrated to be an
effective means of eliciting lid closure, achieving greater than 70% closure in

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Figure 3.9 Sample Histology Image from a Rabbit Eyelid. Image of the lower margin of the
OD upper eyelid of BL01 (paralyzed 4 weeks). (A) Tarsus. (B) Region of muscle fibers from which
sample was taken for analysis. (C) Hole where stimulating probe had been inserted. Image has been
rotated 180° and recorded at 4x magnification.
A
B
C

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A
B
Figure 3.10 Histological Comparison of Muscle Atrophy. (A) OO muscle fibers from the OD
and OS in BL01 (paralyzed 4 weeks). The OD muscle fibers were significantly smaller than those of
the OS, indicating muscle atrophy. (B) OO muscle fibers from the OD and OS in BL07 (normal
control). There was no statistical difference between the cross-sectional areas of the OD and OS
muscle fibers. Images were recorded at 40x magnification.
OD OS
OD OS

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TABLE 3.3
Histological Comparison of Muscle Fiber Atrophy
OS OD

Fibers
Counted
Cross-Sectional Area
(Normalized)

Fibers
Counted
Cross-Sectional Area
(Normalized)
BL07 (Normal): 59 1.000 ± 0.599 62 0.821 ± 0.424
BL04 (1-week): * 69 1.000 ± 0.546 139 0.351 ± 0.156
BL05 (1-week): * 70 1.000 ± 0.524 125 0.700 ± 0.328
BL01 (4-week): * 76 1.000 ± 0.488 108 0.374 ± 0.205
BL03 (4-week): 59 1.000 ± 0.650 54 0.817 ± 0.380
BL09 (8-week): * 47 1.000 ± 0.658 82 0.628 ± 0.267
BL13 (8-week): 57 1.000 ± 0.492 59 0.867 ± 0.463
BL06 (16-week): 74 1.000 ± 0.527 55 1.262 ± 0.771
BL08 (16-week): 75 1.000 ± 0.616 50 1.171 ± 0.580
Cross-sectional areas are all normalized to the mean OS value. All values are listed as Mean ± SD.
* Indicates a statistically significant difference between the OS and OD areas for that particular rabbit.

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denervated rabbits and greater than 90% closure in normal and reinnervated rabbits
with stimulus currents of 5 mA or less.
These maximum degrees of lid closure were generally substantial but incomplete.
While at least one study has shown that the majority of spontaneous blinks in
humans are incomplete (Doane, 1980), it is unclear what degree of closure would
need to be consistently achieved to maintain the health of the eye. Anecdotally,
oculoplastic surgeons have told of patients with no blink ability and perfectly healthy
corneas, so the required degree of closure will vary from person to person.
Maximizing the degree of closure achieved by electrical stimulation would
undoubtedly improve its clinical applicability. Increases in pulse width, pulse
amplitude, and number of pulses have been shown to improve effective closure,
however each of these provides diminishing returns as it increases above a certain
point. Additionally there is a trade-off between the use of long duration pulses,
which are necessary to activate denervated muscle, and the synergy of muscle
contraction resulting from independent activation with the opposite phases of
biphasic pulses trains, which has been shown to improve closure. There is also a
limit on pulse amplitude above which current spread may cause activation of
adjacent neural structures. The use of long duration pulses with relatively high
stimulus currents would place constraints on the size and material of electrodes used
for chronic stimulation in order to maintain safe current density levels and avoid
electrode deterioration and tissue damage (McCreery, Agnew, Yuen, & Bullara,

164
1990; Rose & Robblee, 1990; Weiland, Anderson, & Humayun, 2002). Whether
these factors will be a barrier to clinical use of electrical stimulation has yet to be
determined, but these results indicate that investigation of strategies for increasing
effective closure with lower stimulation currents are warranted.
The only known clinical study of OO stimulation, which was performed by Gittins et
al., investigated the effect of transcutaneous electrical stimulation as a therapeutic
option for improving eyelid function in facial paralysis patients (1999). The authors
reported some improvement in voluntary blink but no significant improvement in
spontaneous blink following the therapy. Their stimulus regimen utilized pulses with
a maximum duration of 200 !s. While this could elicit blink response in normal
subjects, it was reported that it could not cause noticeable lid closure in facial
paralysis patients. This finding is consistent with the results of this study, in that
effective stimulation of denervated OO muscle requires a much longer pulse width
than normal OO. In order to achieve a response, a stimulus regimen similar to that
reported here (10 ms pulse train at 50 Hz) would be more effective. Therefore, the
parameters suggested by this study may have clinical significance in progressing the
field and creating a system to restore eyelid function using electrical stimulation.

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Chapter 4 : Chronic Electrical Stimulation of the
Orbicularis Oculi in Rabbit
This study investigated the effects of chronic electrical stimulation of the orbicularis
oculi (OO) muscle in a rabbit model of facial nerve paralysis. The goals were to
demonstrate that electrical stimulation of the denervated OO can consistently elicit a
satisfactory degree of eyelid closure in an awake animal using tolerable stimulation
levels and to quantify the response to electrical stimulation at different time points
following paralysis in animals that had received chronic stimulation compared to a
non-stimulated control.
4.1 Background
Our study of acute electrical stimulation of the orbicularis oculi demonstrated that it
can elicit a substantial degree of eyelid closure in an animal model of surgically
induced facial paralysis (Sachs et al., 2007). Whether or not this will translate into
the ability to restore functional blinking, however, depends on how well the
stimulation is tolerated on a chronic basis when the subject is awake. The fact that
stimulus amplitudes necessary to achieve maximum eyelid closure frequently caused
movement of adjacent muscles in anesthetized animals seemed to suggest that such
stimulation pulses may also activate nearby sensory nerves that could cause an
unwanted reaction when used in awake animals. If large amplitude stimulus currents

166
are required to elicit eyelid closure, this could potentially cause painful sensation and
therefore negate the possibility of using electrical stimulation for a clinical prosthetic
system to restore eye blink. This study was initiated to test whether satisfactory
eyelid closure could be elicited in awake rabbits using tolerable stimulation levels
and to track the response of the OO to electrical stimulation on a chronic basis.
One published study has previously investigated the use of OO stimulation to restore
blink function on a chronic basis (Otto, 1997) and one has investigated the effects of
daily electrical exercise on the electrophysiology and morphology of the paralyzed
OO (Salerno, McClellan et al., 1991). Otto reported the ability to consistently
achieve lid closure in awake rabbits, but also noted that his animals demonstrated a
reactive movement to initial stimulation pulses that habituated over time. Salerno et
al reported a temporary decrease in the minimum stimulus intensity for evoking a
complete blink in animals whose paralyzed OO had been electrically exercised that
lasted from 10 to 30 days after treatment was initiated. This temporary decrease then
reverted to its original value, which matched that of OO muscles that had not been
exercised. Additionally, Salerno et al reported no difference in muscle fiber
diameters between denervated OO muscles that had and had not been electrically
exercised.

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4.2 Materials and Methods
A total of 16 rabbits had their facial nerve surgically transected and a chronic
stimulating electrode implanted across their upper eyelid. Of these, eight electrodes
stopped functioning due to wire breakage soon after surgery. (This was primarily due
to a design flaw that was corrected in the description below.) Additionally, four
rabbits caused extrusion of the electrode lead shortly after the implantation surgery
by scratching open their sutures and one rabbit was euthanized due to an unrelated
injury. Of the remaining three rabbits, one was used as a control, one was stimulated
chronically for a total of 8 weeks and one was stimulated chronically for a total of 16
weeks. At periodic intervals, a high-speed digital video camera was used to record
the response of the eyelid to stimulation and image processing software was used to
quantify lid closure.
Adaptation of stimulation and data acquisition protocols for chronic use in awake
animals posed a number of interesting challenges. First, a robust electrode and
connector interface was developed to enable the delivery of electrical stimulation
pulses to the paralyzed OO. Second, a protocol for chronic stimulation with periodic
quantitative evaluation of response to electrical stimulation was devised. And finally,
instrumentation and methods for the recording and analysis of lid response in awake
animals were developed.

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4.2.1 Development of chronic stimulating electrodes
A robust interface was of critical importance for enabling chronic stimulation. This
section outlines the design considerations, fabrication, and implantation procedure of
a custom built chronic stimulating electrode for eyelid stimulation.
4.2.1.1 Design
A skull-mounted pedestal design (see Figure 4.1) was utilized because it represented
a well-established robust method of attachment and straightforward interfacing for
external electronics (G. E. Loeb & Gans, 1986). The location of the connector was
easily accessible to investigators, but not to the rabbit by either by chewing or
scratching with the hind limbs. Additionally, the location was close to the eyelids,
minimizing the distance leads must be routed from the connector.
Rigid and machinable titanium (Ti) was chosen for the pedestal material and flexible
silicone was chosen for the leads. Sturdy but flexible Teflon coated platinum-iridium
(Pt-Ir) was selected for lead wires and iridium (Ir) foil, which provides a reasonably
high charge density limit (Mokwa, 2004), was chosen for the contacts. All of these
materials are both biocompatible and compatible with steam sterilization.
4.2.1.2 Fabrication
A 14 mm long flanged cylindrical pedestal with 9/32” ID was machined from 3/8”
diameter rod stock titanium. Two 1 mm slots were machined into the base of the
pedestal with angular separation of 180°.

169

Figure 4.1 Typical Design of Skull Mounted Pedestal. Adapted from (G. E. Loeb & Gans,
1986).

170
Silicone tubing (0.020” ID x 0.037” OD) was cut into two 8” lengths. One 4” and
one 5” length of Teflon coated (0.002” bare diameter) Pt-Ir (90/10 wt%) wire was
threaded into each length of tubing. The lengths of wire were brought through the
wall of the tubing at distances of 3” and 4” from the proximal end by inserting a 25G
needle through the wall of the tubing into the lumen, threading the wire into the
lumen of the needle, and removing the needle. The Teflon coating was burned from
the distal end of the wire via a small flame.
Iridium foil (0.003” thick) was cut into four 3 mm x 4 mm rectangles. Each rectangle
was pressed into a mold and wrapped around a 1 mm diameter metal rod to form a 3
mm long cylinder. The cylinders were then slightly expanded, placed around the
silicone tubing just distal to the locations where the Pt-Ir wire exited, and crimped
tightly around the tubing. A resistance welder (Miyachi Unitek) with custom weld
tips curved to fit the Ir cylinders was used to seal the seam of the Ir cylinders and
bond the distal end of each Pt-Ir wire to the outside of the nearest Ir cylinder.
Silicone elastomer (MDX4-4210) was mixed, loaded into a syringe, and centrifuged
to remove air bubbles. A straight 20G dispensing tip was inserted into the proximal
end of the silicone tubing and used to backfill the tubing with elastomer to a distance
approximately 1 cm beyond the distal Ir contact. The portion of the wire exiting
through the wall of the tubing, the weld between the Pt-Ir wire and the Ir contact, and
the edges of the contact were all also coated with a thin layer of elastomer. The
elastomer was allowed 24 hours to cure.

171
The proximal ends of the Pt-Ir wire were clipped to a length of approximately 5/16”
and the tips exposed using a small flame. The wires of the two leads were soldered to
a 12-pin circular connector (Omnetics, Inc.). A 1/2" length of Teflon coated stainless
steel wire (0.005” bare diameter) was also soldered to one of the connector pins to
act as a ground wire. The connector was glued to the inside of the Ti pedestal with
cyanoacrylate such that the connector end was flush with the top of the pedestal and
the silicone leads fit into the notches at the base of the pedestal. The base of the
pedestal was then potted with silicone elastomer such that the leads and ground wire
ran out through the slots and the elastomer was allowed 24 hours to cure. The
electrode was then tested for continuity and sterilized in an autoclave. A photograph
of the final electrode design is shown in Figure 4.2.
4.2.1.3 Implantation
A 3 cm incision was made on the scalp along the centerline between the eye sockets.
Dissection was carried down to the bone of the skull. The periosteum was dissected
away from the bone exposing a region of bare skull approximately 15 mm in
diameter. Three pilot holes were drilled, 2 mm in depth. Flat-ended screws were
placed within the pilot holes and screwed tightly into the skull, leaving
approximately 0.5 mm between the skull and screw heads. The pedestal was placed
between the screws and held against the skull. Methylmethacrylate was mixed and
immediately injected through a syringe with a 20G dispensing tip coating the area
between the skull and pedestal as well as the screw heads and flanged pedestal base

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Figure 4.2 Photograph of Chronic Electrode.

173
with a single glob of acrylic. The acrylic was allowed to cure for 5-10 minutes,
anchoring the pedestal to the skull and screw heads.
Once the acrylic had set, a 14G angiocatheter was inserted from a small incision
(approximately 1 cm lateral to the lateral canthus) to the scalp incision. The trochar
was removed, the lead was threaded through the lumen of the angiocatheter, and the
angiocatheter was removed leaving the lead in place. An angiocatheter was inserted
approximately 5 mm medial to the medial canthus across the length of the upper
eyelid, 2 mm superior to the lower border, exiting through the previously made
incision near the lateral canthus. A 1 cm loop was made in the lead, and it was
threaded through the angiocatheter after the trochar had been removed. The
angiocatheter was withdrawn, leaving the electrode in the subcutaneous space of the
eyelid. The excess portion of the lead near the medial canthus was cut and the loop
near the lateral canthus was tucked into a subcutaneous pocket made near the initial
small incision. Leads were placed bilaterally and wounds were closed using 4-0 silk
sutures. Figure 4.3 shows a diagram of final electrode placement and Figure 4.4
shows a photograph of an implanted pedestal with attached connector.
4.2.2 Electrical stimulation protocol
One week following the surgical transection of the facial nerve and implantation of
the chronic electrode, electrical stimulation of the paralyzed OO was begun. Chronic
stimulation consisting of a train of stimulation pulses delivered every 10s was

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Figure 4.3 Chronic Electrode Placement. A connector was housed in a Ti pedestal that was
mounted on the skull with methylmethacrylate. A silicone lead containing Pt-Ir wires connected to Ir
contacts was routed subcutaneously across the upper eyelid.

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Figure 4.4 Photograph of Implanted Pedestal. The location on the skull was easily accessible to
researchers but minimized damage due to scratching or chewing by the rabbit.

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performed 5 days/week for a total of 4 hours/day. Quantitative data regarding lid
closure were collected 1-2, 4, 8, and 16 weeks following surgery. Stimulation
focused on the use of 10 ms per phase biphasic current pulses delivered in trains at
50 Hz (essentially a 50 Hz square wave), which were found to the most effective
stimulus parameters for activating denervated OO in previous acute experiments
(Sachs et al., 2007).
4.2.2.1 Chronic stimulation
While conscious, each rabbit was placed in a plastic container that allowed it to
move freely within a limited space (see Figure 4.5). The impedance between the
electrode contacts in the paralyzed eyelid was measured prior to each stimulation
session. The electrode was connected to the output of an isolated pulse stimulator
(Model 2100, A-M Systems, Inc., Carlsborg, WA). The stimulator was set to deliver
a train of ten 10 ms biphasic current pulses at 50 Hz every second. The pulse
amplitude was increased from zero until the rabbit began to show signs of discomfort
(flinching coincident with each stimulation pulse). The amplitude was then decreased
to slightly below the threshold for visible discomfort and the degree of eyelid closure
elicited was qualitatively estimated and recorded. Once the stimulus amplitude was
set, the delivery of pulse trains was changed from every second to every 10s.
Stimulus pulses were delivered every 10s for a period of 4 hours/day, 5 days/week.
One rabbit underwent chronic stimulation for a period of 7 weeks and another for a
period of 15 weeks, each beginning 1 week after implantation surgery.

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A
B
Figure 4.5 Example of Chronic Stimulation. (A) Eye open prior to stimulation. (B) Eye closed
during delivery of electrical stimulation (as indicated by green LED on stim box). Note that the rabbit
does not react adversely.

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4.2.2.2 Stimulation for data recording
While conscious, each rabbit was placed in a plastic restraint box (see Figure 4.6).
The impedance between the electrode contacts in the paralyzed eyelid was measured
to verify that all connections were intact. Biphasic square wave current pulses were
delivered using a multifunction DAQ (PCI-6025E, National Instruments
Corporation, Austin, TX) and analog stimulus isolator (Model 2200, A-M Systems,
Inc., Carlsborg, WA).
The stimulation protocol was divided into two stages. During the first stage,
thresholds for eliciting muscle twitch were found by increasing the pulse amplitude
from zero until the first sign of eyelid movement coincident with electrical
stimulation was visible. Threshold amplitudes for eliciting muscle twitch were
recorded for biphasic pulses with pulse widths ranging from 0.1 to 100 ms per phase.
During the second stage, 10 ms per phase biphasic stimulation pulses with
amplitudes ranging from 0.1 to 3.0 mA were delivered as single pulses and in trains
of 5 and 10 pulses at 50 Hz. If the rabbit demonstrated obvious discomfort at any
point during stimulation, the amplitude was not increased beyond that point for that
set of pulse parameters. Lid response was recorded and measured in terms of overall
lid closure. Each variation of pulse parameters was delivered twice, with an inter-
stimulus spacing of 1s. Only the response to the first stimulus was analyzed to avoid
complications from the possibility of conditioning effects that might result if the
animal had conscious sensation of pulse delivery (Leal-Campanario et al., 2004).

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Figure 4.6 Rabbit in Restraint Box. The rabbit was restrained such that only its head was free
and the pedestal was accessible for connection to instrumentation for electrical stimulation. Restraint
was necessary for stable image recording.

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Trials for individual parameter settings were separated by 20s. One of the chronically
stimulated rabbits (BL71) had quantitative data recorded 1, 4, and 8 weeks following
surgery while the other chronically stimulated rabbit (BL62) had quantitative data
recorded 1, 4, 8, and 16 weeks following surgery. The unstimulated control (BL58)
had quantitative data recorded 2, 4, 8, and 16 weeks following surgery.
4.2.3 Data collection and analysis
A high-speed video camera (1M75-SA, Dalsa Corporation, Ontario, Canada) was
used to record the response of the eyelids to stimulation. Video was captured and
recorded at a rate of 199 frames/second. In order to simultaneously record movement
in both eyes, a mirror setup was designed that reflected the images of both eyes by
270° and combined them into a single plane that could be recorded by the camera
(see Figure 4.7). An interface was created using LabVIEW (National Instruments
Corporation, Austin, TX) to control the video recording.
Individual frames from each video recording were exported in high quality jpg file
format and eyelid separation was measured with ImageJ software (National Institutes
of Health, USA) by tracing the outline of the palpebral fissure created by the margin
of both eyelids and calculating the enclosed area. The minimum exposed area during
stimulation for each recording was divided by the area prior to stimulation in order to
determine peak percent closure (see Figure 4.8).

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Figure 4.7 Mirror System for Bilateral Blink Recording. A set of mirrors was devised to reflect
both eyes through 270° and project them both into the camera lens simultaneously while maintaining
their normal orientation. This was necessary because rabbits are lateral-eyed creatures and have eye
separation of ~180°.

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Figure 4.8 Data Analysis Procedure. The outline of the palpebral fissure was traced in video
frames captured prior to stimulation (A) and at the peak of lid closure (B). Percent closure was
calculated as 1 – area of (B) / area of (A).
A
B

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4.3 Results
Results are presented for each aspect of the experiment, along with some brief
discussion of key findings.
4.3.1 Chronic stimulating electrodes
Initial design for chronic electrodes incorporated the welding of the Pt-Ir wire to the
inside of the Ir foil prior to crimping the Ir foil around silicone tubing. This design
led to frequent breakage of the wire near the edge of the Ir contact due to pinching of
the wire at the edge of the Ir foil during bending. The revised design (as outlined in
Section 4.2.1.2) successfully addressed this issue. Further revision incorporated the
coiling of the Pt-Ir wire around a 0.008” mandrel prior to insertion in the silicone
tube. This provided flexibility that further improved the robustness of the electrode
design, however, this design change was not incorporated in the electrodes used for
this study. Average inter-contact impedance following implantation was
approximately 7-9 k" at 1 kHz and stable over time.
Additionally it should be noted that wounds must be closed in layers following
implantation in order to prevent re-opening of the wound by the rabbit and extrusion
of the implant. While the cyanoacrylate held the connector within the pedestal and
was resistant to forces exerted when inserting and removing the connector in a linear
fashion, it did not hold as well in the presence of torque. Rabbits tended to push their

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heads down into the corners of their containers on occasion, which made the use of
cables that came straight up from the head difficult, as they would be constantly
bumped against the sides of the container. A cable that was pre-bent toward the rear
of the rabbit helped prevent stress from bumping. Unfortunately, the rabbits had a
tendency to turn around in circles and over the length of a stimulation period this
caused the cables to become twisted and exert a torque on the connector that
weakened the cyanoacrylate bond. The addition of a set screw to hold the connector
in place could help prevent the connector from twisting within the pedestal and
breaking this bond. Cables had to be suspended in such a way that they were kept out
of reach of the rabbits in order to prevent them from chewing through the cables.
4.3.2 Tolerance and effect of chronic stimulation
Chronic electrical stimulation of the paralyzed OO was well tolerated in conscious
animals at levels that consistently produced nearly complete to complete eyelid
closure. An example of the response to such a stimulation pulse is shown in Figure
4.5. The threshold for eliciting an unwanted reaction (flinch) varied between rabbits,
but remained consistent over time. In BL62 the amplitude for eliciting a flinching
reaction was slightly greater than 1.0 mA and in BL71 it was slightly greater than 1.4
mA. It was also noted that the stimulus amplitude necessary to elicit a flinching
reaction at the beginning of the stimulation session was generally lower than the
threshold at the end of the stimulation session by approximately 0.2-0.3 mA. This is
similar in nature to the observation made by Otto, in which his initial stimulus pulses

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caused a flinching reaction that habituated over time (1997). The degree of eyelid
closure achieved with the same stimulus amplitude was generally somewhat lower at
the end of the stimulation session than at the beginning. This could be due to either
muscle fatigue or the rabbit actively preventing closure by contracting the
antagonistic LPS muscle. Distracting the rabbit near the end of the stimulation
session seemed to result in an increase in lid closure, but not a return to the original
degree of closure from the beginning of the session. It is likely that this increase was
due to a lack of LPS contraction, which would require some degree of concentration
from the rabbit. The residual difference would likely be the result of muscle fatigue.
It should be noted that the rate of one blink every 10s is significantly more frequent
than the normal blink rate in rabbits, which blink spontaneously less frequently than
one blink every minute and can go for as long as 20-30 minutes between blinks (Lo
& Zhu, 1997; Maurice, 1995).
4.3.3 Strength-duration curves for lid twitch
Strength-duration curves plotting lid twitch threshold are shown in Figure 4.9 and the
associated chronaxie and rheobase values are compared in Table 4.1. Rheobase
values were measured using 100 ms pulse width and chronaxie values were
calculated using a least squares fit, according to the Lapicque law for stimulation
(Geddes & Bourland, 1985; Lapicque, 1909, 1926), which is given by

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Figure 4.9 Strength-Duration Curves for Twitch Threshold. (A) BL58: unstimulated control,
paralyzed for a total of 16 weeks. (B) BL62: paralyzed and chronically stimulated between 1 and 16
weeks. (C) BL71: paralyzed and chronically stimulated between 1 and 8 weeks. Plots are threshold for
eliciting lid twitch v. pulse width. Twitch was defined as minimum visible movement of any part of
the lid coincident with stimulation.

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TABLE 4.1
Chronaxie and Rheobase Values for Chronic Stimulation
Rheobase (mA) Chronaxie (ms)
2 week: 0.03 8.39
4 week: 0.08 2.46
8 week: 0.22 0.26
BL58
16 week: 0.27 0.31
1 week: 0.03 12.47
4 week: 0.03 9.11
8 week: 0.21 0.85
BL62
16 week: 0.23 0.24
1 week: 0.02 12.35
4 week: 0.03 4.26
BL71
8 week: 0.08 1.33
Rheobase values were measured at 100 ms pulse width and chronaxie values were calculated using a
least squares fit according to equation (4.1).

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!
I =b(1+c/d) (4.1)
where I is the threshold current, b is the rheobase, c is the chronaxie, and d is the
pulse duration.
Chronaxie and rheobase values were compared with those found in anesthetized
rabbits during acute stimulation (Sachs et al., 2007). The calculated chronaxie values
for denervated OO in this study were up to an order of magnitude lower than those
calculated for acute rabbits, however, these were still well above reported values for
motor nerve (Geddes, 1999). There was a change in the strength-duration curve
(increased rheobase and decreased chronaxie) that was noticeable at approximately
four to eight weeks that seems to indicate the presence of reinnervation. This
confirms similar observations made in acute rabbits. This change was more gradual
than expected, however, and the exact progress of reinnervation is therefore difficult
to discern.
4.3.4 Quantitative analysis of lid closure
Biphasic current pulses at 10 ms per phase were delivered individually and in trains
of 5 and 10 pulses at 50 Hz. The pulse amplitude was varied from 0.1 to 3.0 mA, or
until the rabbit displayed obvious discomfort coincident with stimulation. Data were
recorded at 1-2 week(s), 4 weeks, 8 weeks, and 16 weeks following facial nerve
transection and electrode implantation. An example of the recorded response to

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electrical stimulation is shown in Figure 4.10. Stimulus-response curves plotting
percent closure as a function of pulse amplitude are displayed for BL58
(unstimulated control) in Figure 4.11, BL62 (chronic stimulation up to 16 weeks) in
Figure 4.12, and BL71 (chronic stimulation up to 8 weeks) in Figure 4.13.
There was little apparent difference among rabbits with regards to stimulus effect for
single pulses or pulse trains. There was a difference between the stimulus-response
curves for stimulation at one and two weeks with pulse trains when compared to
stimulation at other time points for each individual rabbit. Subtle differences in this
comparison of stimulation at different time points within individual rabbits,
particularly when combined with the results of the strength-duration data, seem to
implicate different causes for these seemingly similar changes in stimulus response
over time. Further analysis will focus on response to pulse trains (particularly trains
of five pulses), as these demonstrated the most notable differences.
In BL58 (unstimulated control, see Figure 4.11b), stimulation at 2 weeks with trains
of five pulses demonstrated a gradual increase in response with increase in
amplitude, whereas stimulation at 4, 8, and 16 weeks resulted in a rapid increase in
effect with low amplitudes that quickly reached a plateau. The strength-duration
curves for BL58 (see Figure 4.9a) indicated likelihood of reinnervation at eight
weeks. It is also possible that reinnervation could have been present at four weeks,
even though evidence from the strength-duration data for twitch thresholds did not
necessarily confirm this, because the threshold levels recorded at 10 ms pulse width

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A
B
Figure 4.10 Bilateral Recording of OO Stimulation. Frames captured (A) prior to stimulation
and (B) at peak closure during stimulation. Stimulation was unilateral after 4 weeks of paralysis and
consisted of a train of five 10 ms biphasic pulses delivered at 50 Hz with 1.5 mA amplitude.

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Figure 4.11 Stimulus Response Curves for BL58 (Unstimulated Control). All pulses were
biphasic with a pulse width of 10 ms per phase. Pulse trains were delivered at 50 Hz. There is an
apparent difference between the response to pulse trains measured at 2 weeks when compared to
stimulation at all other time points. This may be the result of reinnervation occurring after 2 weeks.

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Figure 4.12 Stimulus Response Curves for BL62 (Chronic Stim). All pulses were biphasic with
a pulse width of 10 ms per phase. Pulse trains were delivered at 50 Hz. There is an apparent difference
between the response to pulse trains measured at 1 week when compared to stimulation at later time
points. This seems to be the result of conditioning of the muscle by electrical stimulation after 1 week
and/or reinnervation occurring after either 1 or 4 weeks.

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Figure 4.13 Stimulus Response Curves for BL71 (Chronic Stim). All pulses were biphasic with
a pulse width of 10 ms per phase. Pulse trains were delivered at 50 Hz. There is an apparent difference
between the response to pulse trains measured at 1 week when compared to stimulation at later time
points. This seems to be the result of conditioning of the muscle by electrical stimulation.

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were substantially lower than the amplitudes used during stimulus response
recording. If there were partial reinnervation with a motor point further from the
electrode contact it would have a higher threshold than the local denervated portion
of the muscle and not be activated during twitch threshold recording, however, it
would be activated at higher amplitude pulses and therefore its effect would be seen
in the stimulus-response curves. This appears to be the case with BL58. These results
seem to point to the fact that reinnervation played a key role in determining lid
response.
In BL62 (chronic stim, see Figure 4.12b), stimulation at 1 week with trains of five
pulses demonstrated a gradual increase in response with increase in amplitude,
whereas stimulation at 4, 8, and 16 weeks resulted in a rapid increase in effect with
low amplitudes that quickly reached a plateau. The slope of the curve for 4 weeks,
however, was more gradual than those for 8 and 16 weeks. The strength-duration
curves for BL62 indicated likelihood of reinnervation at 8 weeks, along with
evidence of at least partial denervation at 4 weeks. The response to stimulation at 4
weeks was much greater than that at 1 week and seems to indicate at least the
possibility of increased effect due to muscle conditioning by the chronic stimulation.
Whether this was augmented by reinnervation is unclear. These results seem to point
to the fact that both muscle conditioning and reinnervation most likely played a role
in determining lid response.

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In BL71 (chronic stim, see Figure 4.13b), stimulation at 1 week with trains of five
pulses demonstrated a gradual increase in response with increase in amplitude,
whereas stimulation at four and eight weeks resulted in a similar response but with a
steeper slope and plateau region. The strength-duration curves for BL71 indicated
possible but not conclusive evidence of reinnervation at eight weeks. Reinnervated
muscles seemed to demonstrate more of an abrupt change in the stimulus-response
curve than was evident in any of the weeks for BL71. The lack of abrupt response
change seems to indicate that substantial reinnervation was most likely not present.
This therefore seems to indicate that the increased effect seen at four and eight weeks
was primarily due to muscle conditioning by the chronic stimulation. It is possible
this may have been augmented by reinnervation, however, the shape of the stimulus-
response curves indicates that this did not play a significant role at lower amplitudes.
These results seem to point to the fact that muscle conditioning played the primary
role in determining lid response.
4.4 Conclusions and Discussion
This study investigated the effects of chronic electrical stimulation of the paralyzed
OO in rabbit. Strength-duration curves for twitch thresholds demonstrated evidence
of reinnervation approximately four to eight weeks following facial nerve
transection, confirming similar observations from rabbits in a previous acute
stimulation study. Chronic stimulation was well tolerated at a level that consistently

196
produced nearly complete to complete eyelid closure in awake rabbits. The stimulus
amplitude for generating complete closure was generally very similar to the
minimum amplitude that caused an unwanted (flinch) reaction. This is similar to
results seen in acute stimulation of anesthetized animals where stimulation
amplitudes necessary to achieve maximum eyelid closure were generally at a level
where other nearby muscles started to be recruited. The stimulus levels for achieving
eyelid closure in awake rabbits with chronic electrodes, however, were lower than
those for anesthetized rabbits with acutely implanted electrodes (see Chapter 3).
Chronic electrode impedance and threshold amplitude for eliciting a flinching
reaction were stable over time. Both muscle conditioning and reinnervation caused
substantial increases in response to electrical stimulation with 10 ms pulses,
particularly in response to pulse trains. A more thorough study of the effects of
muscle conditioning on the response of the OO to electrical stimulation would
require the development of a better model of chronic facial nerve paralysis that does
not demonstrate evidence of reinnervation.

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Chapter 5 : Restoration of Symmetric Blink Function via
Electrical Stimulation
The vast majority of facial palsy cases are unilateral (Bleicher et al., 1996; Schaitkin,
May et al., 2000a). The lack of symmetry that results from this unilateral deficit is a
major cause of psychological distress (Lee et al., 2004; Lohne et al., 1986; Schaitkin
& May, 2000a; Vlastou, 2006). Normal blinking function in humans exhibits marked
bilateral symmetry (Stava, Huffman, Baker, Epstein, & Porter, 1994). The onset of
eyelid movement in patients recovering from facial palsy retains this conjugacy
(Coulson, O'Dwyer, Adams, & Croxson, 2006). If electrical stimulation can restore
the normal dynamics of blink function, then timing the onset of stimulation to the
action of the healthy contralateral eyelid may have the potential to restore this blink
conjugacy and a semblance of lost symmetry resulting in cosmetic, psychological,
and possibly even functional benefit.
5.1 Kinematics of Electrically Elicited Eyelid Closure
This study investigated the kinematics of electrically elicited eyelid closure in
rabbits. The goal was to determine the effect of paralysis state, stimulus pulse
duration, and stimulus burst length on eyelid kinematics during electrical stimulation
of the orbicularis oculi (OO) muscle. By comparing these results with the kinematics
of normal eyelid closure, an optimal set of stimulus parameters for generating a

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natural looking blink was determined. A modified version of these results was
presented as (Sachs et al., 2006).
5.1.1 Background
The ability to elicit eye blink using electrical stimulation of the orbicularis oculi
muscle has been demonstrated in both dog and rabbit (Otto, 1997; Otto et al., 1986;
Rothstein & Berlinger, 1986; Sachs et al., 2007; Salerno, Bleicher, Stromberg et al.,
1990; Somia et al., 2001; Tobey & Sutton, 1978). This has been performed in both
normal animals and animals with surgically induced orbicularis paralysis. Until
recently, quantitative analysis of lid closure due to electrical stimulation had not been
reported (Sachs et al., 2007). The methods used to quantify lid closure are adaptable
to studies of lid movement over time.
Normal eyelid movements have been studied extensively and their kinematics have
been widely reported (Bour, Aramideh, & de Visser, 2000; Doane, 1980; Evinger,
Manning, & Sibony, 1991; Evinger, Shaw, Peck, Manning, & Baker, 1984; Gruart,
Blazquez, & Delgado-Garcia, 1995; Gruart, Schreurs, del Toro, & Delgado-Garcia,
2000; Huffman et al., 1996; Hung, Hsu, & Stark, 1977; Leal-Campanario et al.,
2004; Sibony, Evinger, & Manning, 1991; VanderWerf, Brassinga, Reits, Aramideh,
& Ongerboer de Visser, 2003). Studies have included a variety of species (human,
cat, rabbit, and guinea pig) and types of response (voluntary, spontaneous, reflex,
and conditioned), and have even included patients with varying degrees of facial

199
nerve palsy. To our knowledge, however, this is the first study to investigate the
kinematics of eyelid closure associated with electrical stimulation of the OO muscle.
These results are relevant to the overall understanding of the nature of lid movement
caused by electrical stimulation and its potential for restoring an effective and
natural-looking blink following facial paralysis.
5.1.2 Materials and Methods
The OO muscle was paralyzed in 12 rabbits by sectioning the seventh cranial nerve.
Three rabbits were paralyzed for each of the following durations: 1 week, 4 weeks, 8
weeks, and 16 weeks. In addition, three normal, non-paralyzed rabbits were used for
comparison. At the end of the specified period, each rabbit was anesthetized and an
electrode was inserted into the upper eyelid such that platinum metal contacts were
positioned in the subcutaneous space near both the medial and lateral canthus.
Biphasic, current controlled stimulation pulses were delivered through the electrode
and a high-speed digital video camera was used to record the response of the eyelid
to stimulation. Image-processing software was used to quantify lid movement over
time.
5.1.2.1 Dissection of the seventh cranial nerve
The seventh cranial nerve was identified and divided to create paralysis of the OO
muscle. With the rabbit under anesthesia, a 1 cm vertical incision was made through
the skin, 1 cm inferior to the center of a line drawn from the lateral canthus to the

200
external auditory meatus and just anterior to the mandibular ramus. A combination of
sharp and blunt dissection was used to divide the subcutaneous tissue and the parotid
gland, revealing the facial nerve trunk and its three large branches adjacent to the
surface of the masseter muscle. Stimulation of the intact nerve with a 0.5 ms, 1 mA
biphasic current pulse produced simultaneous eye closure and ear movement. A 7
mm section of the nerve and its branches was removed and both ends were
cauterized. Complete interruption was confirmed by observation of only ear
movement with stimulation of the proximal stump and only eye blink with
stimulation of the distal nerve.
5.1.2.2 Verification of paralysis
Persistence of paralysis was verified at regular intervals and immediately prior to
electrode insertion by observing the rabbit’s response to a light touch of the cornea
with the tip of a cotton swab. Normal eyelids demonstrated smooth and complete
closure of the palpebral fissure with little or no eye retraction, while paralyzed
eyelids demonstrated substantial eye retraction accompanied by narrowing of the
palpebral fissure and lateral sliding of the nictitating membrane. This movement is
consistent with the expected reaction after facial nerve section when the retractor
bulbi muscle is left intact (Leal-Campanario et al., 2004). Eye health was maintained
during paralysis by the retractor bulbi and nictitating membrane, which acted in
combination to provide adequate protection and lubrication.

201
5.1.2.3 Electrode placement
With the rabbit under anesthesia, a cutaneous stab incision was made, approximately
5 mm lateral to the lateral border of the upper eyelid. A 14-gauge angiocatheter was
inserted through the stab incision and subcutaneously advanced across the length of
the upper eyelid, 2 mm superior to the lid margin. The stylet of the angiocatheter was
removed, and a stimulating electrode (Spencer Probe, Ad-Tech Medical Instrument
Corporation, Racine, WI) was threaded through the lumen of the angiocatheter. The
angiocatheter was withdrawn leaving the electrode in the subcutaneous space of the
eyelid. An anchoring suture was used to secure the electrode to the rabbit’s skin 2 cm
lateral to the entry site. See Figure 5.1 for a diagram of electrode placement.
The stimulating probe was 1 mm in diameter and included six 2.3 mm long
cylindrical platinum contacts spaced at 5 mm intervals. The first and fifth contacts
were used to deliver stimulation pulses, giving a dipole spacing of approximately 2
cm.
In addition to the stimulated rabbits, two normal rabbits received sham implants in
order to evaluate whether the presence of an implanted electrode would substantially
affect eyelid closure. Following implantation, these rabbits were allowed to heal and
later observed to qualitatively gauge the degree to which the sham implant affected
normal lid closure during spontaneous and air-puff induced reflex blinks.

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Figure 5.1 Acute Electrode Placement. Diagram of electrode inserted subcutaneously into the
upper eyelid of a rabbit and secured with a suture external to the insertion site. The first and fifth
contacts (indicated by arrows) were used to deliver stimulation pulses.

203
5.1.2.4 Electrical stimulation protocol
With the rabbit under anesthesia, biphasic square wave current pulses were delivered
through the implanted electrode using a multifunction DAQ (PCI-6025E, National
Instruments Corporation, Austin, TX) and analog stimulus isolator (Model 2200, A-
M Systems, Inc., Carlsborg, WA).
The stimulation protocol was divided into three phases: (1) Thresholds for generating
lid twitch were measured over a range of pulse widths from 0.1 to 100 ms per phase.
(2) Single biphasic pulses with amplitudes ranging from 1 to 7 mA were delivered
with pulse widths ranging from 0.5 and 100 ms per phase. (3) Trains consisting of 5
and 10 pulses with amplitudes ranging from 1 to 7 mA and pulse widths ranging
from 0.5 to 10 ms per phase were delivered at a rate of 50 Hz.
Trials for individual parameter settings were separated by 20s. The electrode current
was monitored to ensure that the stimulator compliance voltage did not limit output.
During the experiments, it was determined that the maximum current that could be
consistently delivered was slightly above 7 mA.
5.1.2.5 Blink recording and data analysis
A high-speed video camera (1M75-SA, Dalsa Corporation, Ontario, Canada) was
used to record the response of the eyelid to stimulation. Video was captured with a
resolution of 0.083 mm and recorded at a rate of 192 frames/second. An interface
was created using LabVIEW (National Instruments Corporation, Austin, TX) to

204
coordinate the recording of video and delivery of stimulation pulses. Measurement of
eyelid separation was automated using National Instruments, Vision Assistant
software (see Figure 5.2). A grayscale threshold value was set that allowed the
exposed area of the pupil and iris to be automatically isolated from the rest of the
image by the nature of their color contrast. The height of the exposed area was
measured along a vertical axis crossing through the center of the pupil, giving a
value for lid separation in terms of pixels. This was repeated for each frame and
normalized with respect to lid separation prior to stimulation.
5.1.3 Results
Strength-Duration curves for twitch thresholds indicated that rabbits demonstrated
persistent denervation at 1 and 4 weeks, but demonstrated evidence of at least partial
reinnervation at 8 and 16 weeks (see Chapter 3 for more detail). For comparison
purposes, rabbits were therefore grouped into the following categories: normal,
denervated (1-week and 4-week rabbits), and reinnervated (8-week and 16-week
rabbits). Rabbits within each of these groups demonstrated similar movement
characteristics as a result of stimulation (i.e. 1-week rabbits demonstrated a similar
response to 4-week rabbits, and 8-week rabbits demonstrated a similar response to
16-week rabbits).
Eyelid movement in response to single biphasic pulses is shown in Figure 5.3 as a
function of time. Pulse amplitude was fixed at 5 mA for pulse widths ranging from

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Figure 5.2 Image Processing for Kinematic Analysis. The central region of the palpebral
fissure was isolated from (A) the raw image using (B) a contrast threshold. (C) The resulting binary
image underwent a series of processing steps including (D) smoothing, (E) hole filling, and (F) edge
detection. A caliber function was then used to measure the vertical dimension of the binary shape,
which corresponds to the vertical width of the palpebral fissure. The procedure was performed for
each frame of a recorded video clip and all distances were normalized to the maximum distance
measured during the clip.
A B
C D
E F

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Figure 5.3 Eyelid Kinematics for Single Pulses. Plots are of eyelid response to single
symmetric biphasic current pulses delivered with amplitude 5 mA for (A) normal rabbits, (B) rabbits
demonstrating evidence of reinnervation, and (C) rabbits demonstrating evidence of persistent
denervation. Pulses were initiated at Time = 0.5s. Pulses are listed according to duration per phase.
Legend and axis labels apply to all plots. Normalized value of 1 is equivalent to maximum eyelid
separation prior to stimulation and 0 is equivalent to complete closure.

207
0.5 to 100 ms per phase. Data presented are averages within each group for a single
biphasic stimulus.
Eyelid movement in response to trains of biphasic pulses is shown in Figure 5.4 as a
function of time. Pulse amplitude was fixed at 5 mA and pulse width was fixed at 10
ms per phase for single pulses and trains of 5 and 10 pulses. Data presented are
averages within each group for a single biphasic stimulus or stimulus train.
Normal rabbits and rabbits with evidence of reinnervation demonstrated similar
kinematics in response to electrical stimulation of the OO, with reinnervated rabbits
exhibiting a slight decrease in both closing and opening velocity (as demonstrated by
the decreased slope of the response curves). Denervated rabbits demonstrated a much
slower closing and opening velocity than the other two groups. This is most evident
in the comparison of response to single biphasic pulses with pulse width of 100 ms
per phase. Independent activation occurred with each phase for all groups, however,
the slower kinematics of the denervated rabbits resulted in a synergistic effect while
the faster kinematics of the normal and reinnervated rabbits resulted in two nearly
independent contractions.
Increases in pulse width and number of pulses did not have a significant effect on
closing velocity, as all response curves within each group tended to following the
same slope during the closing phase. These did affect the duration of lid movement,
however, and therefore the overall amount of closure, with increases in pulse width

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Figure 5.4 Eyelid Kinematics for Pulse Trains. Plots are of response to trains of symmetric
biphasic current pulses delivered with amplitude 5 mA at a rate of 50 Hz for (A) normal rabbits, (B)
rabbits demonstrating evidence of reinnervation, and (C) rabbits demonstrating evidence of persistent
denervation. Pulses were initiated at Time = 0.5s. Pulses are listed according to duration per phase x
number of pulses. Legend and axis labels apply to all plots. Normalized value of 1 is equivalent to
maximum eyelid separation prior to stimulation and 0 is equivalent to complete closure.

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and number of pulses generally leading to decreases in the amount of eyelid
separation at the peak of closure.
Qualitative assessment of spontaneous and reflex blinks with sham implants did not
indicate a noticeable effect on normal lid closure due to the presence of the implant.
Quantitative analysis with sham implants was not performed.
5.1.4 Discussion and Conclusions
Reported values for natural blinks in humans range from approximately 80 to 100 ms
for the duration of the closing phase and from approximately 150 to 250 ms for the
duration of the opening phase (VanderWerf et al., 2003). The blink dynamics of
rabbits are similar to those of humans (see Figure 5.5) (Evinger et al., 1984; Gruart et
al., 2000). These values fall within the range of durations reported here for eyelid
movement in response to electrical stimulation of the OO in rabbit. Specifically,
trains of five 10 ms pulses had a similar profile to those reported for normal blinking
and therefore make the optimal candidate for restoration of symmetric blink
function.
5.2 Contralaterally Triggered Orbicularis Oculi Stimulation
This study investigated the use of electromyography (EMG) from a normal OO
muscle as a signal for triggering an electrically elicited blink in the contralateral

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Figure 5.5 Kinematics of Normal Rabbit Eye Blink. Response of rabbit upper eyelid to a 50
ms, 3 k/cm
2
air puff presented to the ipsilateral cornea. Adapted from (Gruart et al., 2000).

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paralyzed eyelid. The goal was to demonstrate that such triggering can restore a
symmetric blink in a unilateral model of OO paralysis. Stimulus parameters
determined from the study reported in Section 5.1 were used and quantitative
analysis of bilateral blink kinematics was performed.
5.2.1 Background
The use of contralateral control signals for the electrical stimulation of unilaterally
paralyzed axial muscles was originally proposed by Zealear and Dedo (1977). Since
then, other researchers have investigated the use of facial muscle contraction to
trigger stimulated contraction of the contralateral pair (Broniatowski et al., 1991;
Broniatowski, Ilyes, Jacobs, Nose, & Tucker, 1989; Broniatowski et al., 1987; Otto
et al., 1986; Tobey & Sutton, 1978). These have reported the ability to restore
bilateral symmetry, but have provided no quantitative evidence.
EMG activity has been investigated as a control signal for many neural prosthetic
applications (Peckham & Knutson, 2005; Saxena, Nikolic, & Popovic, 1995;
Sennels, Biering-Sorensen, Andersen, & Hansen, 1997). EMG activity represents a
broadband signal with a bandwidth of approximately 0.1-3 kHz and amplitude that is
generally in the range around or below 1 mV when recorded intramuscularly.
Acquisition of the EMG signal can be accomplished using well-established
techniques (G. E. Loeb & Gans, 1986). The amplitude of the EMG envelope (usually
extracted by amplifying, rectifying, and lowpass filtering the raw EMG signal) tends

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to be largely proportional to the force generated by a contracting muscle. This study
investigated the use of EMG activity recorded from a healthy OO muscle as a signal
for triggering an electrically elicited contraction in the paralyzed contralateral OO.
5.2.2 Materials and Methods
A single rabbit underwent surgery to induce unilateral OO paralysis and implantation
of a chronic electrode with bilateral leads that were routed across the eyelids. After
four weeks of paralysis, the lead on the healthy side was used to record the OO EMG
response during reflex eyelid closure induced by a puff of air directed at the cornea.
The lead on the paralyzed side was used to deliver a train of stimulation pulses to the
paralyzed OO. The pulses were triggered when the envelope of the healthy OO EMG
crossed a preset threshold. The movement of both eyelids was recorded
simultaneously using a high-speed digital video camera with an integrated mirror
setup and image processing software was used to quantify eyelid closure.
5.2.2.1 Dissection of the seventh cranial nerve
The seventh cranial nerve was identified and divided to create unilateral paralysis of
the OO muscle on the right (OD) side of a single rabbit. With the rabbit under
anesthesia, a 1 cm vertical incision was made through the skin, 1 cm inferior to the
center of a line drawn from the lateral canthus to the external auditory meatus and
just anterior to the mandibular ramus. A combination of sharp and blunt dissection
was used to divide the subcutaneous tissue and the parotid gland, revealing the facial

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nerve trunk and its three large branches adjacent to the surface of the masseter
muscle. Stimulation of the intact nerve with a 0.5 ms, 1 mA biphasic current pulse
produced simultaneous eye closure and ear movement. A 7 mm section of the nerve
and its branches was removed and both ends were cauterized. Complete interruption
was confirmed by observation of only ear movement with stimulation of the
proximal stump and only eye blink with stimulation of the distal nerve, as well as a
lack of blink response to mechanical stimulation of the cornea.
5.2.2.2 Chronic electrode implantation
A 3 cm incision was made on the scalp along the centerline between the eye sockets.
Dissection was carried down to the bone of the skull. The periosteum was dissected
away from the bone exposing a region of bare skull approximately 15 mm in
diameter. Three pilot holes were drilled, 2 mm in depth. Flat-ended screws were
placed within the pilot holes and screwed tightly into the skull, leaving
approximately 0.5 mm between the skull and screw heads. The pedestal of the
chronic electrode was placed between the screws and held against the skull.
Methylmethacrylate was mixed and immediately injected through a syringe with a
20G dispensing tip coating the area between the skull and pedestal as well as the
screw heads and flanged pedestal base with a single glob of acrylic. The acrylic was
allowed to cure for 5-10 minutes, anchoring the pedestal to the skull and screw
heads.

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Once the acrylic had set, a 14G angiocatheter was inserted from a small incision
(approximately 1 cm lateral to the lateral canthus) to the scalp incision. The trochar
was removed, the lead was threaded through the lumen of the angiocatheter, and the
angiocatheter was removed leaving the lead in place. An angiocatheter was inserted
approximately 5 mm medial to the medial canthus across the length of the upper
eyelid, 2 mm superior to the lower border, exiting through the previously made
incision near the lateral canthus. A 1 cm loop was made in the lead, and it was
threaded through the angiocatheter after the trochar had been removed. The
angiocatheter was withdrawn, leaving the electrode in the subcutaneous space of the
eyelid. The excess portion of the lead near the medial canthus was cut and the loop
near the lateral canthus was tucked into a subcutaneous pocket made near the initial
small incision. Leads were placed bilaterally and wounds were closed using 4-0 silk
sutures. A single ground wire was also attached to the connector and placed in the
subcutaneous space. Figure 5.6 shows a diagram of final electrode placement.
5.2.2.3 Verification of paralysis
Persistence of paralysis was verified at regular intervals and immediately prior to
electrical stimulation by observing the rabbit’s response to a light touch of the cornea
with the tip of a cotton swab. The normal eyelid demonstrated smooth and complete
closure of the palpebral fissure with little or no eye retraction, while the paralyzed
eyelid demonstrated substantial eye retraction accompanied by narrowing of the
palpebral fissure and lateral sliding of the nictitating membrane. This movement is

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Figure 5.6 Chronic Electrode Placement. Leads were placed bilaterally. The lead in the healthy
eyelid was used to record EMG signals while the lead in the paralyzed eyelid was used to deliver
stimulation pulses. A ground wire was also placed subcutaneously.

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consistent with the expected reaction after facial nerve section when the retractor
bulbi muscle is left intact (Leal-Campanario et al., 2004). Eye health was maintained
during paralysis by the retractor bulbi and nictitating membrane, which acted in
combination to provide adequate protection and lubrication.
5.2.2.4 EMG triggering instrumentation
A system was designed in which the EMG output from the healthy OO muscle was
used to trigger delivery of electrical stimulation pulses to the paralyzed OO, while
the movement of both eyelids was simultaneously recorded (see Figure 5.7). The
lead contacts from the healthy eyelid and the ground wire were connected to the
input of a pre-amplifier (Model P55, Grass, West Warwick, RI), such that the
voltages recorded from the lead contacts were differentially amplified and bandpass
filtered. The gain of the pre-amplifier was set to 10,000 and the filter bandwidth was
set to 30-3000 Hz. The output of the pre-amplifier was monitored on an oscilloscope
(TDS2024, Tektronix, Richardson, TX) and connected to the input of a triggering
circuit (see Figure 5.8).
The triggering circuit produced an envelope of the amplified EMG signal by full-
wave rectifying the input and then lowpass filtering it with a cutoff frequency of 14.5
Hz. The amplified EMG envelope was compared to a threshold value, producing a
binary output. The comparator output was used to activate a 555 timer, generating a
trigger signal lasting 2.2s. The predefined duration for the trigger signal prevented
inadvertent triggering based on stimulus artifact. The trigger signal was output to an

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Figure 5.7 Instrumentation for Blink Triggering. An air puff was used to stimulate a reflex
blink in the healthy eyelid. EMG activity from the healthy OO was amplified and passed through an
envelope detector circuit that produced a trigger signal when a set threshold value was crossed. The
trigger signal was sent to an isolated pulse stimulator that delivered a train of current pulses to the
paralyzed OO. The blink response was recorded bilaterally using a high speed digital camera and
mirror setup to divide the recorded image such that both the right and left eyes were recorded
simultaneously.

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Figure 5.8 EMG Triggering Circuit. The pre-amplified EMG signal is input at (A) and (B). The
signal is then full-wave rectified and lowpass filtered to generate the signal envelope. The envelope is
input to a comparator and compared to a threshold value that is defined by a potentiostat (C). The
output of the comparator is used to activate a 555 timer and generate a trigger signal (D), the duration
of which provides adequate delay to prevent reactivation by the stimulus artifact from the triggered
stimulation pulse.
A
B
C
D

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isolated pulse stimulator (Model 2200, A-M Systems, Inc., Carlsborg, WA), which
delivered electrical stimulation pulses to the paralyzed eyelid.
5.2.2.5 EMG recording and triggered stimulation protocol
Prior to EMG recording and OO stimulation, the rabbit was placed in a plastic
restraint box (see Figure 5.9). While the rabbit was restrained, the lead from the
healthy side of the implanted electrode was connected to the EMG amplifier and
triggering circuit. In order to elicit an eye blink, a puff of air was blown through a
straw at the cornea on the healthy side. A substantial but incomplete reflex blink was
observed and recorded using the system described in Section 5.2.2.6 and the EMG
envelope and output trigger signal were monitored on an oscilloscope. The trigger
threshold was adjusted such that it was consistently activated by the EMG signal
generated by the reflex blink, but was not activated inadvertently by the background
noise.
Following verification that a satisfactory blink could be elicited in the healthy eye,
the implanted electrode was disconnected from the EMG amplifier and triggering
circuit. It was then connected to the isolated pulse stimulator and trains of five 10 ms
current controlled biphasic stimulation pulses were delivered to the lead in the
paralyzed eyelid. (This pulse train was selected as the closest anticipated match for
duration of eyelid closure based on the results of the kinematic analysis presented in
Section 5.1) The amplitude of the stimulus pulses was adjusted until the amount of

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Figure 5.9 Rabbit in Restraint Box. The rabbit was restrained such that only its head was free
and the pedestal was accessible for connection to instrumentation for EMG recording and electrical
stimulation. Restraint was necessary for stable image recording.

221
eyelid closure elicited approximately matched that observed in the reflex blink of the
healthy eyelid, and response to untriggered electrical stimulation was recorded.
Once the amplitude of the stimulus pulse had been appropriately tuned, the input of
the EMG triggering circuit was reconnected to lead on the healthy side and its output
was connected to the isolated pulse stimulator. A puff of air was blown through a
straw at the cornea on the healthy side to elicit a reflex blink, and the recorded EMG
was used to trigger the delivery of electrical stimulation pulses to the paralyzed
eyelid. The response of both eyelids was recorded.
5.2.2.6 Blink recording and data analysis
A high-speed video camera (1M75-SA, Dalsa Corporation, Ontario, Canada) was
used to record the response of the eyelids to stimulation. Video was captured and
recorded at a rate of 192 frames/second with a resolution of approximately 0.083
mm. In order to simultaneously record movement in both eyes, a mirror setup was
designed that reflected the images of both eyes by 270° and combined them into a
single plane that could be recorded by the camera (see Figure 5.10). An interface was
created using LabVIEW (National Instruments Corporation, Austin, TX) to control
the video recording.
A program was written in MATLAB (The Mathworks, Inc.) that used contrast to
isolate the iris and pupil, which comprised the majority of the exposed area of the
palpebral fissure (see Figure 5.11). A grayscale threshold value was set that allowed

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Figure 5.10 Mirror Setup for Bilateral Blink Recording. A set of mirrors was devised to reflect
both eyes through 270° and project them both into the camera lens simultaneously while maintaining
their normal orientation. This was necessary because rabbits are lateral-eyed creatures and have eye
separation of ~180°.

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Figure 5.11 Image Analysis for Blink Symmetry. Grayscale contrast was used to isolate the
region encompassed by the pupil and iris, which comprised the majority of the exposed area of the
palpebral fissure. The area of the exposed region was measured in terms of pixels for each frame of a
recorded video clip and all values were normalized to the maximum area measured during the clip.
This was performed for each eyelid to provide comparative analysis of eyelid kinematics and blink
symmetry.

224
the exposed area of the pupil and iris to be automatically isolated from the rest of the
image by the nature of their color contrast. The area of the exposed region was
measured in terms of pixels. This was repeated for each frame and normalized with
respect to lid separation prior to stimulation.
5.2.3 Results
EMG signals were robust with high signal to noise ratio. A sample of the amplified
EMG signal recorded from a rabbit during a spontaneous blink is shown in Figure
5.12. The triggering circuit worked reliably and the delay between the onset of EMG
activity and the activation of the trigger signal was minimal (< 5 ms). Activation of
the EMG triggering circuit is demonstrated in Figure 5.13.
Stimulation of the paralyzed OO in response to EMG activity recorded from the
healthy contralateral OO resulted in a symmetric looking blink. Sample frames from
a recording of triggered eyelid stimulation are shown in Figure 5.14. From high
speed video there is evidence of slight lag in the closing of the paralyzed eyelid, as
well as slight mismatch in the residual opening of the palpebral fissure at peak
closure, however these differences were not noticeable in real time. Plots of the
kinematics for both eyelids during reflex blinking with no trigger, during untriggered
electrical stimulation, and during triggered stimulation based on a reflex blink are
shown in Figure 5.15. There are differences in the overall amplitude of closure as
well as the closing velocity between the two sides during triggered stimulation, with

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Figure 5.12 EMG Recording with Chronic Electrode. Sample EMG of a spontaneous blink
recorded from an implanted chronic electrode. The signal was bandpass filtered between 30-3000 Hz
and amplified by 1000.

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Figure 5.13 EMG Threshold Signal Generation. (CH3) EMG of a reflex blink bandpass filtered
between 30-3000Hz and amplified by 10,000. (CH1) Envelope of EMG signal from CH3 produced by
full wave rectification and lowpass filtering at 14.5 Hz. (CH2) Threshold level for trigger signal.
(CH4) Trigger signal.

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A
B
C
D
E
Figure 5.14 Contralateral EMG Triggered Blink Response. Images of eyelids recorded (A) prior
to air puff and EMG triggered stimulation, (B) during eyelid closing, (C) at the peak of closure, (D)
during eyelid opening, and (E) immediately after opening. The eye on the right side of the images
(rabbit’s left eye) was healthy and stimulated with an air puff to create a reflex blink, while the eye on
the left side of the images (rabbit’s right eye) was electrically stimulated based on contralateral EMG.

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Figure 5.15 Bilateral Kinematics for EMG Triggered Stimulation. (A) Reflex blink initiated by
an air puff in OS with no electrical stimulation of OD. (B) Electrical stimulation of OD with no reflex
initiation in OS. (C) Electrical stimulation of OD triggered by EMG recorded from OS during an air
puff initiated reflex blink. Recordings of both eyes were made synchronously and plotted with the
initiation of movement at Time = 0.0s. Each plot is based on the average of five trials.

229
the electrically stimulated side demonstrating slightly less overall closure and a
slightly slower closing velocity. Despite these differences, however, the blink
profiles were quite similar and qualitatively the blinks appeared conjugate.
5.2.4 Discussion and Conclusions
These results demonstrate that it is possible to generate a symmetric blink response
by triggering electrical stimulation of a unilaterally paralyzed OO based on EMG
activity recorded from its healthy contralateral pair. In a clinical system, the stimulus
amplitude will most likely be set to ensure eyelid closure sufficient for maintaining
the cornea or to produce complete eyelid closure, assuming this is well tolerated. As
an alternative, it may also be possible to adjust the stimulus amplitude based on the
level of EMG activity in the contralateral OO to create a proportionate, rather than
binary response in the paralyzed eyelid. The EMG activity of the OO muscle has
demonstrated a direct relationship to the degree of static closure (Rothstein &
Berlinger, 1986) and changes in the parameters of stimulus pulses delivered to the
paralyzed OO have been demonstrated to achieve varying degrees of closure (Sachs
et al., 2007). This approach would require a method for eliminating the stimulus
artifact from the recorded EMG signal, as well as quantifying EMG activity more
specifically. This has been performed in FES (Sennels et al., 1997), however the
method of turning off the recording amplifier during stimulation pulses that is used
with neural stimulation is not compatible with the long duration pulses necessary for
stimulation of denervated muscle. Whether this is achievable or has practical

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application in the OO is an interesting research question, but is beyond the scope of
this study.
Rabbits have a unilateral blink reflex, which is in contrast to the bilateral blink
response exhibited in humans (Stava et al., 1994). During blinking the levator
palpebrae superioris (LPS) muscle in humans relaxes bilaterally, which may provide
functional benefits to triggering stimulus during a normal blink, since the tonic
activity of the LPS would not have to be overcome in order to generate eyelid
closure. Thus, triggered closure would most likely be achieved with lower stimulus
amplitude. Unfortunately, this effect cannot be investigated in the rabbit model.

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Chapter 6 : General Conclusions
The work presented in this thesis investigated various aspects of electrical
stimulation of the orbicularis oculi (OO) muscle for eyelid reanimation in a rabbit
model of facial nerve paralysis. This chapter briefly summarizes the key findings of
this research and the recommendations for future research directions that will bridge
the remaining gap and lead to the development of a clinical system for eye blink
restoration based on functional electrical stimulation (FES).
6.1 Key Findings
Key findings are summarized by the following points:
1) Transection of the branches of the facial nerve that innervate the OO muscle in
rabbit near their origin at the nerve trunk resulted in paralysis of the OO muscle
followed by progressive reinnervation. This reinnervation consistently became
evident after approximately eight weeks, however the source of reinnervation
varied among rabbits.
2) Electrical stimulation of the denervated OO muscle could produce substantial
eyelid closure. OO muscles that demonstrated evidence of reinnervation were
more sensitive to electrical stimulation.

232
3) Trains of 10 ms biphasic current pulses delivered at 50 Hz produced the greatest
degree of eyelid closure out of parameters tested. Trains of five such pulses
produced similar eyelid kinematics as have been reported for normal blinking.
4) Chronic electrical stimulation of the OO consistently produced nearly complete
to complete eyelid closure for periods up to 16 weeks following facial nerve
transection. At stimulus levels that produced significant closure, there was no
evidence of discomfort in the rabbits, which were awake during this procedure.
Electrode impedance and thresholds for eliciting adverse reaction (flinching)
remained consistent over time.
5) Triggering electrical stimulation of the denervated OO muscle based on EMG
activity recorded from the contralateral healthy eyelid produced a symmetric
blink in a unilateral model of facial paralysis.
6.2 Future Directions
This work has contributed to the development of a clinical system for restoring eye
blink function in humans suffering from facial paralysis. The use of electrical
stimulation to accomplish this goal could have significant advantages over currently
available methods for restoring eyelid closure. The most significant contribution
from this work is the determination of optimal stimulation parameters for restoring
blink function. These parameters have been demonstrated to be effective in

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activating the denervated OO muscle in rabbits and are different from what has been
previously used in attempts to activate the denervated OO in humans (Gittins et al.,
1999; Sachs et al., 2007). Progress has also been made toward demonstrating that
stimulation is tolerable on a chronic basis and that symmetric blink can be restored
by triggering stimulation based on contralateral EMG recording. This work opens up
additional areas of research that must be addressed in order to continue moving
toward clinical development, as well as areas that would not be required but may
have potential application.
6.2.1 Comments on the rabbit model
While there are a host questions that remain to be answered before a final clinical
system can be designed, many of these may not be addressable in a rabbit model.
Some limitations of the rabbit model that we have discovered through our research
include the following:
1) The consistent evidence of reinnervation in our model of facial paralysis prevents
the study of long-term denervation. It may be possible to inhibit reinnervation
with the use of neurotoxins (Paydarfar & Paniello, 2001; Yian et al., 2001),
however, this would require further investigation and the effects on the electrical
excitability of the muscle following such treatment would need to be determined.

234
2) The tarsal plate of the rabbit is much more loosely organized and softer than that
of the human. This has implications in surgical implantation techniques as well
as possibly in the current distribution that will result from electrodes implanted in
the eyelid.
3) Rabbits have an accessory muscle known as a retractor bulbi that contributes to
eyelid closure (Leal-Campanario et al., 2004). The retractor bulbi pulls the globe
back into the eye socket and the passive tension of the rabbit’s nictitating
membrane and eyelids cause them to close over the eye. This system remains
intact following facial nerve paralysis limiting the possibility of using corneal
health as an outcome measure in eyelid reanimation studies.
4) Rabbits blink very infrequently compared to humans. Spontaneous blinks in
freely moving rabbits have been reported to occur less than once a minute (Lo &
Zhu, 1997), and interblink intervals greater than 20 minutes have been observed
in rabbits under stress (Maurice, 1995). This limits the ability to study
spontaneous blinking in rabbits, particularly with methods that require the
restriction of animal movement.
5) Rabbits have a unilateral blink reflex, which is contrary to what is observed in
humans (Stava et al., 1994). Thus the bilateral relaxation of the LPS muscle that
occurs during a normal human blink will not be present in the rabbit reflex blink.
An OO muscle that is stimulated during a contralateral reflex blink would

235
therefore have to overcome the antagonistic LPS in order to generate closure in
the rabbit, whereas in the human it would not.
6.2.2 Tolerability of stimulation in humans
The most immediate question that must be addressed is whether the delivery of
electrical stimulation pulses in the human eyelid can cause adequate contraction of
the denervated OO muscle to elicit eyelid closure at tolerable stimulation levels. This
work has come a long way in defining a new set of stimulation parameters that can
provide nearly complete to complete eyelid closure at tolerable stimulation levels in
rabbits, but if this does not translate to humans then further research to refine
stimulation techniques is essentially meaningless. This question must be answered in
a clinical setting.
6.2.3 Functional benefits of contralateral triggering
The work presented in this thesis demonstrated that a reasonable degree of symmetry
can be restored by using contralateral EMG as a trigger signal for delivering
stimulation to the denervated OO in unilateral facial palsy. Besides the obvious
cosmetic benefits of restoring blink symmetry, there are potential functional benefits
as well. These benefits are derived from the bilateral nature of blinking in humans.
Due to their infrequency of spontaneous blinking and the unilateral nature of the

236
blink reflex exhibited in the lateral eyed animals, however, the rabbit makes a poor
model for studying these effects.
6.2.3.1 Bilateral LPS relaxation
During normal blinking, the OO and levator palpebrae superioris (LPS) muscles
demonstrate reciprocal activity (Hart Jr., 1992). Additionally, the LPS muscles,
which are innervated by the third cranial nerve and therefore not affected by facial
nerve palsy, act in concert in humans and other frontal eyed animals such that they
relax bilaterally during a normal blink regardless of the state of the OO muscle. If the
paralyzed OO muscle were stimulated during a normal blink, the antagonistic LPS
muscle would coincidentally be relaxed. Thus the OO would not have to overcome
the action of the LPS muscle in order to cause eyelid closure and it is likely that the
stimulus amplitude required to generate a blink would therefore decrease. This has
implications in both power consumption of an implanted device as well as the spread
of current to adjacent nerves and muscles. If humans are similar to rabbits, in that the
threshold for generating complete lid closure without consideration of LPS activity is
comparable to the threshold for eliciting sensation, this strategy could potentially
lower the threshold for eyelid closure below the range for sensation. This could play
a significant role in making a system for restoration of blink function with electrical
stimulation practical.

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6.2.3.2 Bell’s phenomenon
During forced eyelid closure the eyes rotate upward in the majority of people
(Takagi, Abe, Hasegawa, & Usui, 1992). This movement, which is known as Bell’s
phenomenon, causes the cornea to roll up and back in the eye socket. If the goal of
electrical stimulation of the OO muscle were simply to facilitate corneal wetting, a
strong Bell’s phenomenon would mean that the upper eyelid would not need to
traverse as far in order to produce adequate coverage. Thus, triggering stimulation to
occur during a period of forced eye closure would most likely decrease the stimulus
amplitude necessary to preserve eye health by decreasing the amount of eyelid
movement that must be generated.
6.2.4 Determination of aesthetic requirements
Normal eyelid movements in humans are highly conjugate (Stava et al., 1994). We
have demonstrated that the use of contralateral electromyography (EMG) to trigger
electrical stimulation of the unilaterally paralyzed OO muscle can restore a large
degree of symmetry in eyelid movement, however these movements are not perfectly
matched. Normal blinks have some degree of variation in their profiles, which the
natural motor system that controls bilateral blinking compensates for to maintain
conjugacy. Using a simple stimulation profile to generate binary blinking function
will not adjust for these differences and will inherently lead to some degree of
mismatch in movement between the lids of the two eyes. This could take on the form
of differences in the timing for movement initiation, the velocity of lid closing or

238
opening, or the overall duration of movement. The degree of difference that can be
tolerated before eyelid movements appear abnormal is unknown and an interesting
question with regards to the potential ability for restoring symmetry. This could have
implications on system design and patient acceptability.
6.2.5 Control methods for eliciting a proportional response
Strategies discussed thus far are strictly for binary restoration of eyelid function. In
normal subjects, however, there is fine control of lid position that is exhibited during
such actions as squinting. This work has demonstrated that adjustment of various
electrical stimulation parameters can provide a wide range of degrees of lid closure.
Development of a method for tracking healthy contralateral eyelid closure for use as
a control signal may allow for the generation of a proportional response via electrical
stimulation of the unilaterally paralyzed eyelid. It may be possible to do this using
contralateral OO EMG, which has been demonstrated to vary as a function of lid
position (Rothstein & Berlinger, 1986). This would require the development of novel
techniques for stimulus artifact cancellation, however, given the duration of pulses
necessary to activate denervated OO muscle. Other methods of tracking eyelid
movement, such as strain gauges, may also be employed (Broniatowski et al., 1991;
Broniatowski, Ilyes et al., 1989; Broniatowski et al., 1987). Such strategies could
also be developed to match the duration of stimulation with the duration of closure in
the healthy contralateral lid.

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6.2.6 Ipsilateral triggering methods for bilateral palsy
The use of contralateral lid movement as a trigger signal is only valid in patients with
unilateral facial paralysis. Bilateral pacing in patients with bilateral facial palsy may
be the simplest way to restore symmetry, however this would not provide any of the
functional benefits of triggering during a normal blink that were mentioned above.
Additionally, there is suppression of visual activity during normal blinking that helps
to preserve the continuity of the visual scene despite frequent eye closure (Manning,
Riggs, & Komenda, 1983; Volkmann, 1986; Volkmann, Riggs, Ellicott, & Moore,
1982; Volkmann, Riggs, & Moore, 1980). It is possible that pacing regardless of
blink intention would cause disruption of this continuity that may lead to adverse
effects. There may also be psychological benefits to patients being able to trigger
their own eyelid closure rather than having it paced. And finally, some patients with
unilateral facial paralysis may be averse to having an electrode lead or other sensor
implanted in their only remaining healthy eyelid, so an ipsilateral triggering
alternative may be advantageous for them. These reasons indicate that exploration of
ipsilateral methods of triggering stimulation may be worthwhile.
Recording of EMG from the LPS is a potential ipsilateral trigger, as the LPS muscles
relax during normal blinking, resulting in elimination of EMG activity. In addition to
relaxation during blinks, however, the LPS relaxes during changes in vertical gaze
that are not accompanied by changes in OO activity (Evinger et al., 1984; Hart Jr.,

240
1992). Therefore a method of distinguishing between the two types of relaxation
would be necessary. Other methods of ipsilateral triggering may also be available.
6.2.7 Electrode design and stability
The electrode design employed in this research involved a lead that was routed
subcutaneously across the eyelid to position metal contacts near the medial and
lateral canthi. Surgically, this type of implantation is fairly straightforward, however
the constant movement of the lead wire may cause fatigue and breakage during the
more than two million blinks that occur each year (based on one blink every 6s for
16 hours/day). Before clinical implantation, final electrode designs would need to
undergo substantial fatigue testing. Alternatively, other implantation strategies that
do not route leads across the eyelid and therefore minimize wire movement could be
investigated, as the optimal sites for stimulation were found to be near the canthi,
rather than near the center of the lid.
6.2.8 Potential use of reinnervation
These experiments demonstrated that reinnervation of the OO muscle results in a
substantial increase in the effect of OO stimulation. Stimulating innervating motor
axons, rather than bare muscle fibers can be accomplished with much shorter pulse
widths and much greater electrical efficiency (Peckham & Knutson, 2005; Salmons
et al., 2005). At least some degree of spontaneous OO reinnervation is common in

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many etiologies of facial paralysis, particularly Bell’s palsy (Schaitkin & May,
2000b). Depending on the severity of the injury and the time-course of recovery,
however, this is often associated with some degree of synkinesis resulting from the
misdirection of regenerating nerve fibers.
With regards to electrical stimulation, the effect of reinnervation is independent of
the source and therefore the benefits can be derived with reinnervation from non-
native nerve fibers and in patients where such reinnervation does not translate into
volitional control. Thus reinnervation, even that associated with high degrees of
synkinesis, could greatly facilitate electrical stimulation of the OO. It may also be
possible to use electrical stimulation in concert with other techniques for
encouraging reinnervation, such as nerve repair, or the implantation of nerve muscle
pedicles (Broniatowski et al., 1991; Broniatowski, Grundfest-Broniatowski et al.,
1989; Broniatowski, Ilyes et al., 1989; Broniatowski et al., 1987).
Some research has linked electrical stimulation with the potential for reinnervation,
particularly from native nerve fibers (Byers, Clark, & Thompson, 1998; Zealear et
al., 2002). It has also been reported that electrical stimulation of the OO muscle
following facial nerve transection and anastomosis by epineural repair results in
faster recovery of the blink reflex (Salerno, Bleicher, & Stromberg, 1990). While the
role of electrical stimulation in reinnervation and natural recovery remains
controversial, such studies may indicate that the use of electrical stimulation of the

242
OO can facilitate recovery in facial paralysis patients. Further investigation of the
relationship between electrical stimulation and OO reinnervation is warranted.
6.3 Final Words
Facial paralysis is a debilitating problem that causes a great deal of suffering for
those who are affected by it. Current treatment measures are simply inadequate and
medical technology has advanced too far to allow this to continue. Research in the
area of electrical stimulation for eyeblink reanimation has demonstrated promising
results in multiple animal studies that span as far back as 30 years. It is time to
declare that this is no longer simply an interesting exercise in electrophysiological
research and recognize it as grounds for development of an actual clinical system
that can provide tangible benefit for these patients. This thesis has provided a
thorough description of stimulation parameters and strategies that can be imported
directly into studies of electrical stimulation of the OO in humans. It is imperative
that such clinical investigation be performed and published so that it can finally be
demonstrated whether this strategy of eye blink reanimation is feasible as a medical
option for facial paralysis patients and development of a commercial medical device
can begin.

243
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Abstract (if available)

Abstract Brought about by dysfunction of the seventh cranial nerve, facial paralysis results in the inability to contract the facial muscles and leads to host of functional and psychological deficits. The most significant clinical outcome of facial nerve palsy is the loss of the ability to blink the eye, which results in corneal exposure and can lead to such complications as corneal ulceration, permanent vision loss, and even potential loss of the eye. Current methods for restoring eye closure include the implantation of gold weights or mechanical springs into the eyelid, the use of artificial tears, and tarshorrhaphy. These passive measures are largely ineffective and both surgically and cosmetically unappealing. The use of electrical stimulation to reactivate the paralyzed orbicularis oculi muscle, which generates eye blink in healthy individuals, has the potential to provide a much more elegant and effective means of eliciting eye closure.

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Asset Metadata

Creator Sachs, Nicholas Alexander (author)

Core Title Electrical stimulation of the orbicularis oculi to restore eye blink

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Biomedical Engineering

Publication Date 07/09/2007

Defense Date 06/22/2007

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag electrical stimulation,eye blink,facial paralysis,OAI-PMH Harvest,orbicularis oculi

Language English

Advisor Weiland, James D. (committee chair), Chang, Eli (committee member), Humayun, Mark S. (committee member), Loeb, Gerald E. (committee member), Mansfeld, Florian B. (committee member)

Creator Email nsachs@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-m593

Unique identifier UC1192928

Identifier etd-Sachs-20070709 (filename),usctheses-m40 (legacy collection record id),usctheses-c127-530678 (legacy record id),usctheses-m593 (legacy record id)

Legacy Identifier etd-Sachs-20070709.pdf

Dmrecord 530678

Document Type Dissertation

Rights Sachs, Nicholas Alexander

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Repository Name Libraries, University of Southern California

Repository Location Los Angeles, California

Repository Email cisadmin@lib.usc.edu