Thesis Formatted

Home , Chihuahua (dog), Hypoesthesia

Identifying and Treating Neuropathic Pain in Dogs with Syringomyelia

Thesis

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in

the Graduate School of The Ohio State University

Ashley C. Hechler, DVM

Graduate Program in Comparative Veterinary Medicine

The Ohio State University

2019

Thesis Committee

Sarah A. Moore, DVM, MS, DACVIM, Advisor

Lynette K. Cole, DVM, DACVD

Laurie B. Cook, DVM, DACVIM

Eric T. Hostnik, DVM, MS, DACVR

Copyrighted by

Ashley C. Hechler, DVM

2019

Abstract

Syringomyelia (SM) is a debilitating condition in the cavalier King Charles spaniel

(CKCS) that results in neuropathic pain and diminished quality of life. Von Frey aesthesiometry (VFA) is a method of mechanical quantitative sensory testing that provides an objective sensory threshold (ST) value and can be used to quantify neuropathic pain and monitor response to therapy. The utility of VFA has been previously established in client-owned dogs with acute spinal cord injury and osteoarthritis but the technique has not been evaluated in dogs with SM. The goal of this study was to evaluate ST, as determined by VFA, in dogs with and without SM, to assess the utility of VFA in quantifying NP in SM-affected dogs. We hypothesized the SM- affected CKCS would have lower ST values consistent with hyperesthesia, when compared to control CKCS. Additionally, we hypothesized that ST values in SM-affected dogs would be inversely correlated with syrinx size on MRI and with owner-derived clinical sign scores.

ST values for the thoracic and pelvic limbs differed significantly between SM-affected and control CKCS (p=0.027; p=0.0396 respectively). Median ST value (range) for the thoracic limbs was 184.1 grams (120.9-552) for control dogs, and 139.9 grams (52.6- ii

250.9) for SM-affected dogs. The median ST value (range) for the pelvic limbs was

164.9 grams (100.8-260.3) in control dogs and 129.8 grams (57.95-168.4) in SM-affected dogs. The ST values in SM-affected dogs did not correlate with syrinx height on MRI

(r=0.314; p=0.137). Owner-reported clinical sign scores showed an inverse correlation with pelvic limb ST values, where dogs with lower ST values (hyperesthesia) were reported by their owners to display more frequent and severe clinical signs (r=-0.657; p=0.022).

ST values were lower in SM-affected CKCS compared to control dogs, suggesting the presence of neuropathic pain. Dogs with lower ST pelvic limb values were perceived by their owners to have more severe clinical signs classically associated with SM. Our results suggest that VFA may offer an objective assessment of neuropathic pain in SM- affected dogs and could be useful for monitoring response to therapy in future clinical studies.

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Dedication

In dedication to Maverick, my childhood cavalier King Charles Spaniel, whose lifelong battle with Chiari-like malformation and syringomyelia fueled my desire to help others like him by becoming a veterinary neurologist. To my fiancé Stephen Jones, our CKCS

Patrick, and golden retriever Finnegan, who have been a constant source of love and encouragement throughout my life, especially while in pursuit of my graduate degree. I also dedicate this thesis to my family for their unrelenting support over the years.

Acknowledgments

I wish to thank my advisor and mentor, Dr. Sarah Moore for her guidance and support throughout this entire process. I would also like to thank Dr. Cole, Dr. Cook and Dr.

Hostnik for their expertise and assistance as members of my masters committee. I gratefully acknowledge Ms. Amanda Disher, Ms. Heather Anderson, Ms. Lane

Bookenberger, and Ms. Josey Sobolewski for their assistance with patient care and data collection. Finally, I would like to thank all the clients and cavalier King Charles spaniels that offered their time and support to the project.

Vita

May 2006….……………………………….. Father Lopez Catholic High School

May 2010…………………………………... B.S. Animal Science, Clemson University

May 2015…………………………..………. D.V.M., University of Florida

2015 to 2016…………………...…………... Intern, Metropolitan Veterinary Hospital

2016 to present……………………………... Residency in Neurology and Neurosurgery,

Veterinary Medical Center, The Ohio State

University

Publications

1. Black LJ, Hechler AC, Duffy ME, and Beatty SSK. "Presumed lupus erythematosus

cells identified in bronchoalveolar lavage fluid from a Mexican Hairless dog." Veterinary

Clinical Pathology. 2017; 46 (2): 354-359

2. Hechler AC, Moore SA. “Understanding and treating Chiari-like malformation and

syringomyelia in dogs.” Topics in Companion Animal Medicine. 2018; 33(1):1-11

Fields of Study

Major Field: Comparative Veterinary Medicine

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Table of Contents

Abstract ...... ii

Dedication ...... iv

Acknowledgments ...... v

Vita ...... vi

Publications ...... vi

Fields of Study ...... vii

List of Tables ...... xi

List of Figures ...... xiii

List of Abbreviations...... xvi

Chapter 1. Introduction ...... 1

Chapter 2. Literature Review ...... 3

2.1. Craniocervical Junction ...... 3

2.1.1. Craniocervical Junction Anatomy ...... 3

2.1.2. Craniocervical Junction Anomalies ...... 5 viii

2.2 Syringomyelia ...... 16

2.2.1. Classification of Syringomyelia ...... 16

2.2.1 Cerebrospinal Fluid Flow ...... 17

2.2.2 Etiology and Pathogenesis ...... 18

2.3. Clinical Presentation ...... 21

2.3.1. Signalment ...... 21

2.3.2. Clinical Signs ...... 22

2.3.3. Diagnostics ...... 27

2.3. Pain ...... 32

2.3.1. Classification of Pain ...... 34

2.3.2 Substance P ...... 36

2.4. Quantitative Sensory Testing ...... 38

2.4.1. Von Frey Anesthesiometry...... 40

2.4.2. Alternative Quantitative Sensory Testing in Veterinary Medicine ...... 42

2.5. Treatment ...... 43

2.5.1. Medical Management...... 43

2.5.2. Surgical Management ...... 53

2.5.3. Alternative Therapies ...... 54

2.6. Prognosis ...... 56

Chapter 3. Mechanical quantitative sensory testing in Cavalier King Charles Spaniels with and without syringomyelia ...... 59

3.1. Abstract ...... 59

3.2. Introduction ...... 61

3.3. Results...... 63

3.4. Discussion ...... 70

3.5. Conclusion ...... 74

3.6. Materials and Methods ...... 74

3.7. Ethics approval and consent to participate ...... 80

3.8. Funding ...... 80

3.9. Acknowledgements ...... 80

Chapter 4. Conclusions and Future Directions ...... 81

References ...... 84

List of Tables

Table 1. - Results of published morphometric studies and their implications with respect to the etiology of Syringomyelia. (Reprinted from Hechler, 201834) ...... 9

Table 2. Summary of terms and definitions relevant to neuropathic pain. (Reproduced with permission128)...... 36

Table 3 - Common medications used in the clinical management of Chiari-like

Malformation and Syringomyelia in dogs. (Reprinted from Hechler, 2018 34) ...... 44

Table 4. Demographic factors (age, gender) for 29 CKCS with and without syringomyelia...... 64

Table 5. MRI findings for 10 control and 19 syringomyelia (SM)-affected CKCS...... 65

Table 6. Owner-derived clinical sign scores for 10 syringomyelia-affected CKCS enrolled on or after January 1, 2017. Patient number corresponds to the number listed in tables 1 and 2...... 67

Table 7. Chiari malformation (CM) and syringomyelia (SM) severity scoring strategy, as proposed by the British Veterinary Association (BVA) for use in stratifying dogs with

CM/SM114...... 76

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List of Figures

Figure 1. The ligamentous (A) and bony (B) anatomy of the craniocervical junction and the single joint space (shaded area). (Reprinted with permission. 24) ...... 4

Figure 2. T2 weighted midsagittal MRI of the brain and cranial cervical spinal cord in a

CKCS with CM/SM. Skeletal changes labeled in yellow, neural parenchymal changes in red, and secondary SM in blue. (Reprinted from Knowler, 2018. 31) ...... 8

Figure 3. Transverse section through the spinal cord. Dorsal lamina layers are designated by roman numerals. Ascending spinothalamic and spinocervicothalamic tracts labeled in appropriate anatomic regions. (Reprinted from Nalborczyk et al., 2017.99) ...... 25

(D). Note the malformation of the caudal aspect of the occiput (C and D; arrow head) and caudal displacement of the cerebellum into the foramen magnum (C and D; arrow)

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typical of CM. T2 hyperintensity consistent with fluid accumulation within the spinal cord parenchyma is typical of SM (D; asterisk). (Reprinted from Hechler, 2018 34) ...... 29

Figure 5. Grading scheme for Chiari-like malformation (CM; A) and syringomyelia (SM;

B) recommended by the British Veterinary Association for use in breeding protocols.

(Reprinted from Hechler, 201834.) ...... 31

Figure 6. Pathway of pain transmission. (Reproduced with permission.117)...... 33

Figure 7. The electronic von Frey anesthesiometer. The device consists of a load cell

(A), handle (B), recording device (C) and tip (D). (Reprinted with permission.15).……..63

Figure 8. Comparison of median and range sensory threshold (ST) values for thoracic (a) and pelvic (b) limbs in control and SM-affected CKCS. ST values are significantly lower in the thoracic and pelvic limbs of SM-affected dogs compared to controls...... 66

Figure 9. Comparison of median and range sensory threshold (ST) values for thoracic (a) and pelvic (b) limbs in SM1 and SM2-affected CKCS. There is no significant difference in thoracic or pelvic limb ST between SM1 and SM2-affected dogs (P> 0.05 in all cases).

...... 68

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Figure 10. Relationship between sensory threshold (ST) values for pelvic limbs and owner-derived clinical sign scores for SM-affected dogs (n=10). There is a significant inverse correlation between pelvic limb ST values and clinical signs, suggesting that animals with more pronounced hyperesthesia showed more clinical signs of SM at home

(r=-0.657; p=0.022)...... 69

List of Abbreviations

AOO: Atlanto-occipital overlap

BVA: British Veterinary Association

C1: Atlas

C2: Axis

CCF: Caudal cranial fossa

CCJ: Craniocervical junction

CJA: Craniocervical junction abnormality

CKCS: Cavalier King Charles spaniel

CM: Chiari-like malformation

CMI: Chiari malformation type 1

CMII: Chiari malformation type 2

COMS: Caudal occipital malformation syndrome

CPG: Central pattern generators

CT: Computed tomography

ET: Eustachian tube

FM: Foramen magnum

MRI: Magnetic resonance imaging xvi

NP: Neuropathic pain

NMDA: N-methyl-D-aspartate

OM: Otitis media

PSOM: Primary secretory otitis media

QST: Quantitative sensory testing

SM: Syringomyelia

SM-NP: Syringomyelia-associated neuropathic pain

ST: Sensory threshold

TCA: Tricyclic antidepressant

VFA: von Frey aesthesiometry

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Chapter 1. Introduction

Chiari-like malformation (CM) and syringomyelia (SM) are two debilitating conditions affecting the craniocervical junction and spinal cord of small breed dogs, such as the

Cavalier King Charles Spaniel, that results in neuropathic pain and negatively affects the quality of life 1-5. CM is typified by a complexity of malformations of the calvarium and craniocervical junction which result in crowding of the caudal cranial fossa (CCF) and displacement of the cerebellum and occasionally brainstem, into or through the foramen magnum6. CM has recently been recognized as a breed-specific characteristic with an incidence of 100% in the CKCS and Chihuahua1,5. Approximately 50-70% of dogs with

CM eventually develop SM, although not all develop clinical signs7,8.

Syringomyelia is currently defined within the veterinary literature as the development of fluid-containing cavities within the parenchyma of the spinal cord secondary to abnormal

CSF dynamics 6. While some dogs with SM remain asymptomatic, many dogs demonstrate behaviors interpreted as a manifestation of neuropathic pain7,9-11.

Neuropathic pain has been shown to negatively affect quality of life and overall well- being in both people and dogs if it is not treated appropriately 12-14. Unfortunately, in veterinary medicine we currently must rely on owner-reported symptoms and clinical 1

observations to determine the presence of CM/SM related neuropathic pain behaviors.

Our dependence on recognizing and interpreting these signs can delay diagnosis, possibly leaving patients suffering from chronic pain with suboptimal or no treatment.

In people, neuropathic pain can be objectively diagnosed with quantitative sensory testing

(QST) which provides a numerical value for the patient’s sensory threshold (ST). The ST indicates the lowest mechanical or thermal stimulus required to induce a painful response. In a patient with hypoesthesia (decreased sensitivity to stimuli) the value is increased above normal, while in a patient with hyperesthesia or allodynia (increased sensitivity) the value is lower than normal. The ST can then be used to guide initiation of treatment and monitor therapeutic success. Recently, QST has gained attention in the veterinary clinical realm as a reliable means of evaluating sensation in dogs with SM, spinal cord injury, and osteoarthritis 15-19.

Von Frey anesthesiometry (VFA) is a type of QST that utilizes a mechanical stimulus applied to skin. Thus far, no reports have evaluated the efficacy of VFA as a measure of

QST in CKCS with and without SM. Given the prevalence of both asymptomatic CKCS and those with suffering from neuropathic pain in the CM/SM affected population, an objective method to evaluate ST is needed. Not only does QST have the potential to serve as a non-invasive diagnostic test for CKCS with CM/SM, it could ultimately guide treatment to better manage neuropathic pain by monitoring the response to therapy.

Chapter 2. Literature Review

2.1. Craniocervical Junction

2.1.1. Craniocervical Junction Anatomy

The occipital bone, the atlas (C1), the axis (C2) and associated ligaments comprise the craniocervical junction (CCJ) 20,21. The occipital bone is made up of four distinct pieces that fuse together to form the boundaries of the foramen magnum (FM). The largest piece, and most dorsal, is the supraoccipital bone. The lateral aspects of the foramen magnum are made up of the two exoccipital bones which house the occipital condyles.

The most ventral aspect of the foramen magnum is bordered by the basioccipital bone 22.

The foramen magnum is the opening in the occipital bone that allows the spinal cord to join the medulla oblongata within the cranium.

The first cervical vertebra (C1), also known as the atlas, articulates with the exoccipital condyles to form the atlanto-occipital joint 22. Multiple membranes and ligaments stabilize this joint, including the dorsal atlanto-occipital membrane and the ventral atlanto-occipital membrane which make up the joint capsule. The second cervical vertebra (C2), the axis, makes up the caudal portion of the atlantoaxial joint. This joint is a complex articulation to permit rotational motion of the head (Figure 1). The cranioventral aspect of C2 is the odontoid process, also known as the dens. The dens is 3

positioned in the ventral aspect of the spinal canal of C1 and secured in place by the transverse ligament. Stabilization is provided by the joint capsule as well as a series of ligaments. Arising from the most cranial aspect of the dens is the apical ligament that attaches to the ventral aspect of the foramen magnum and the two alar ligaments that attach to the occipital bone 23.

Figure 1. The ligamentous (A) and bony (B) anatomy of the craniocervical junction and the single joint space (shaded area). (Reprinted with permission. 24)

2.1.2. Craniocervical Junction Anomalies

The Chiari malformations fall under a broad category of developmental defects called craniocervical junction anomalies (CJA). Chiari-like malformation (CM) is the most commonly diagnosed malformation affecting the CCJ in the canine patient25. Other reported CJA include occipitoatlantoaxial malformations (OAAMs), atlanto-occipital overlap (AOO), atlanto-axial instability, dorsal constriction at the C1-C2 vertebral junction, and dorsal angulation of the dens. Each anomaly has been individually reported in the literature to be causative of syringomyelia (SM) 25. Despite each malformation being an individual anatomic entity, they result in similar clinical signs, making them difficult to distinguish on presentation. To complicate matters further, multiple CJA may exist concurrently, and some dogs with CJA are asymptomatic.

Chiari Malformations

The Chiari malformations are a group of congenital anomalies of the human caudal fossa that are graded based on severity. Much debate has surrounded the initial classification of these malformations, as such, they have been referred to by multiple names. The first description of hindbrain herniation and associated syringomyelia was documented by

Theodore Langhans in 1881; however, Hans Chiari was the first to describe and name the four subtypes in 189126,27. CM is analogous to the human Chiari type I malformation

(CMI). Hans Chiari described this form after performing an autopsy on a 17-year-old

woman with hydrocephalus. The malformation was initially defined as displacement of the cerebellar tonsils and medial divisions of the inferior lobes of the cerebellum into the cranial cervical spinal canal28. Chiari type II (CMII) malformation is the displacement of the vermis, pons, medulla, and fourth ventricle into the spinal canal29. However, the nomenclature surrounding this subtype remains controversial. In 1907 CMII was renamed Arnold-Chiari malformation after Julius Arnold, who described a similar case of cerebellar and fourth ventricle herniation in 1894, prompting the name change 27. Chiari malformation type III (CMIII), the most severe, is also the most uncommon. CMIII is a type of cervical spina bifida where the cerebellum and brainstem herniate through a defect in the tentorium cerebelli 27. Chiari malformation type IV, cerebellar hypoplasia without herniation, is the final original subtype 29. At the time, Hans Chiari attributed all hindbrain herniation subtypes he documented to an increase in intracranial pressure resulting from hydrocephalus 27. Over time, as more information was gathered about each subtype, more accurate definitions were assigned to the existing subtypes to more appropriately portray the anatomic changes observed.

Recently, two additional subtypes have been added in the human literature. Chiari 0 malformation does not have herniation as a component; there is just a decreased amount of space in the posterior fossa that results in syringomyelia. Chiari 1.5 malformation falls somewhere between CMI and CMII, as these patients predominantly have herniation of the tonsils but also have some brainstem involvement 30.

Chiari-like Malformation

Chiari-like malformation is defined as a decrease in the volume of the canine caudal cranial fossa (CCF) that results in caudal descent of the cerebellum, and occasionally brainstem, into or through the foramen magnum6. However, recent work has challenged this definition by identifying morphometric changes affecting the entire brain and craniocervical junction 31. CM is referred to by numerous names in the literature including brachycephalic obstructive CSF channel syndrome (BOCCS), caudal occipital malformation syndrome (COMS), occipital hypoplasia, Chiari malformation, and hindbrain herniation. However, the Chiari-like Malformation and Syringomyelia

Working Group round table designated by consensus, that the condition should be referred to as Chiari-like malformation (abbreviated CM) in the context of the canine condition 6. CM is now recognized as a breed-specific characteristic, with an incidence of 100% in the CKCS 1,19,32. Approximately 50-70% of dogs with CM go on to develop syringomyelia (SM), though not all show clinical signs associated with the condition7,8.

CM/SM is typified by a complexity of malformations of the calvarium and occipital bone, which result in crowding and in some cases caudal displacement of the cerebellum

(CM) as well as fluid accumulation within the spinal cord parenchyma, termed syringomyelia (SM) (Figure 2).

Figure 2. T2 weighted midsagittal MRI of the brain and cranial cervical spinal cord in a CKCS with CM/SM. Skeletal changes labeled in yellow, neural parenchymal changes in red, and secondary SM in blue. (Reprinted from Knowler, 2018. 31)

The cavalier King Charles spaniel lines originate back to the Toy Spaniel in the 17th century. Since then, the phenotype of the skull has undergone a number of transformations, ultimately resulting in the brachycephalic conformation we see today 33.

Brachycephalicism has been thoroughly investigated as the underlying factor leading to the prevalence of CM in the toy and small breed population. However, the full complexities of the morphologic as well as pathologic presence of CM in the brachycephalic population are not understood. A summary of morphometric studies, their results, and implications regarding the development of SM are summarized in Table 1.

Table 1. - Results of published morphometric studies and their implications with respect to the etiology of Syringomyelia. (Reprinted from Hechler, 2018. 34)

Underlying Trait Mechanism Supportive Findings Refs 39, 3, Brachycephalicism results Early closure results in • Decreased skull width:length ratio is from early closure of decreased skull length and protective against SM 35 spheno-occipital compensatory lengthening • A less domed skull with more caudally synchondrosis and of other calvarial bones distributed cranium is protective predisposes to CM/SM • CM dogs have a smaller FM to pons length • CM CKCS have a smaller spheno-occipital to atlas length and spheno-occipital angle

39, 3 Overcrowding of the • Cerebral length:cranial length is smaller in whole brain, causes caudal dogs with CM than control brachycephalics displacement of the • Smaller cerebral:cranial length correlates to cerebellum and brainstem. degree of cerebellar herniation • CM dogs have a smaller FM to pons length • Rostral forebrain flattening and a combination of short basicranium but tall cranial height is seen in CM affected CKCS

52, Overcrowding is due to a • CKCS with SM have smaller CCF volume small caudal cranial fossa than mesaticephalic dogs 219 • Reduced caudal CCF volume in CM/SM CKCS compared to CM alone 50, 58, Brain parenchyma size Overcrowding is • CKCS have a larger caudal fossa results in overcrowding secondary to large parenchyma than other small breed dogs 218, cerebellum with similar CCF sizes 221 • CKCS with SM have a larger CCF parenchyma size but similar CCF size than those without • The cerebellum is the portion of CCF parenchyma that is larger 40, 41, Craniocervical junction Concomitant CJA • CKCS have higher incidence of AOO than anomalies contribute to contribute to clinical signs other toy/small breed dogs 42, 44, clinical signs associated and development of SM • Atlantoaxial bands are associated with more 48, 49, with CM/SM severe SM and clinical signs • 66-68% of CKCS with CM have dorsal angulation of the dens

Table continued on next page

Underlying trait Mechanism Supportive findings Refs 49, 56, CM results in secondary CM results cerebellar and • Higher medullary kinking index was effects that have brainstem herniation associated with presence and severity of SM 60, 107 questionable influence on • Presence nor severity of cerebellar SM development herniation correlate to development to SM • CKCS with CM/SM have greater cerebellar pulsation during systole than cm alone which may result in further disruption of CSF flow

49, 62 Occipital hypoplasia • The size of cerebellar herniation is develops overtime associated with increased FM size over time • The height of the FM and length of cerebellar herniation increase over time

58 Concomitant • CM/SM have larger ventricular volumes ventriculomegaly is seen than CM alone in CKCS with CM

50, 220 A shortened skull base can • CKCS with CM/SM have a smaller JF decrease size of jugular volume than CM alone. Secondary venous foramen and increase ICP congestion may influence CSF pulse pressures and result in SM. AOO: Atlanto-occipital overlap, CCF: Caudal cranial fossa, CJA: Craniocervical junction anomalies, CKCS: Cavalier King Charles spaniel, CM: Chiari-like malformation, CSF: Cerebrospinal fluid, FM: foramen magnum, ICP: Intracranial pressure, SM: Syringomyelia

Recently, more attention has been directed at investigation of whole-skull morphometry, suggesting CM is not merely a problem of the caudal cranial fossa. Multiple morphometric changes that result in the brachycephalic skull appearance contribute to entire brain crowding, leading to the most significant disruption in the volume/size ratio in the CCF. This has led to the recent proposition to alter the definition of CM to a skull and craniocervical junction malformation that causes disruption in the CSF flow and/or pain secondary to brain parenchyma compromise 31.

The cephalic index, modified from the human classification scheme by Evans, is used to classify dogs as brachycephalic, dolichocephalic, or mesaticephalic 22. This is where the width of the cranium is divided by the length, and a higher value is positively associated with the development of SM35. The spheno-occipital synchondrosis (growth plate) is responsible for longitudinal growth of the skull 36. Brachycephalic breeds have an early spheno-occipital synchondrosis closure compared to mesaticephalic breeds, with CKCS having an even earlier closure 37. An early spheno-occipital synchondrosis closure is the putative cause of the short but wide skull shape of CKCS 37,38.

To date, a single morphometric characteristic has not been implicated as the primary factor in the development of SM 3,39,40. Instead, multiple bony and soft tissue changes affecting the skull and craniocervical junction resulting from selection for a brachycephalic skull shape, concurrently play a role in the development of pain and syringomyelia.

Atlanto-occipital Overlap

With atlanto-occipital overlap (AOO) the atlas is displaced rostrally, causing compression of the cerebellum and resulting in loss of the cerebellomedullary cistern. In certain cases C1 is so severely shifted, the rostral most aspect is located within the caudal cranial fossa24,41,42. AOO can occur traumatically, however, more often it is a developmental condition seen in toy and small breed dogs. Minimal research has

investigated the co-existence of AOO with CM and the impact it has on the disease progression, but there are two available studies that report the prevalence to range from

20-86% in the CKCS41,42. In one paper Gonzalez et al found AOO correlated with the presence of SM, and furthermore, the severity of AOO directly related to the severity of the syringes42. The postulated reasoning is based on the belief that syringomyelia results due to obstruction to the flow of CSF or blood flow through the basilar artery in the region of the CCJ 41-44. The malpositioning seen with AOO may result in obstruction of either CSF or blood flow, or possibly both.

AOO resembles a condition in human medicine called basilar invagination (BI), which is often incorrectly termed basilar impression. Basilar invagination is a developmental condition in which the odontoid process migrates through the foramen magnum, while basilar impression is acquired, secondary to softening of the bones at the skull base45.

Approximately 25-35% of patients with basilar invagination have concurrent Chiari malformation, syringomyelia, and/or hydrocephalus45. Basilar invagination results in symptoms of neuropathic pain and neurologic dysfunction that correlate with the severity of the displacement46. Although, neuropathic pain and neurologic deficits are reported with AOO in dogs, the relationship between clinical signs, morphologic severity, and the pathogenesis of disease is not completely understood.

Dorsal Atlantoaxial Bands

Dorsal constriction (DC) at the level of C1-C2 results in various degrees of focal spinal cord compression by abnormal dura or ligamentum flavum. The tissue dorsal to the atlanto-axial joint is thickened, fibrosed, and can be ossified, likely secondary to excessive movement 44,47-49. Histopathologic features of the dural bands indicate a chronic process that causes the calcification and ossification47. This dorsal constriction is referred to by numerous terms in human and veterinary medicine: dural band, fibrous band, atlanto-axial band, dorsal compressive band, and dural/fibrous band. Here we will refer to these all under the term “dorsal constriction” (DC).

A definitive relationship between DC, CM, and SM has not been defined in the veterinary patient, although, chronic obstruction of the subarachnoid space is suggested to cause alterations to normal blood and CSF flow, resulting in syringomyelia 24,41,47,49. Dorsal compressive bands have been identified in 38% of toy and small breed dogs with CM and

88% of CKCS with CM41,48. Comparatively, all cases of Chiari-type 1 malformation

(CMI) in people have thickening of the dura at the craniocervical junction47. A 2015 paper by Cerda-Gonzalez et al. found the presence and severity of DC were significantly associated with the presence and severity of SM48. When present together the syrinx is typically located just caudal to the compression49. This is in direct contrast to a more recent study by the same authors that found although there was association between DC

and the presence of SM, the severity of the compression did not influence the size of the syrinx42.

Congenital Dorsal Angulation of the Dens

The odontoid process, also known as the dens, is subject to many developmental anomalies that fall under the general term dens abnormality (DA). The most relevant DA in veterinary medicine is congenital dorsal angulation. With this condition, upward angulation of the dens, documented in 77% of CKCS, results in compression of the ventral aspect of the spinal cord 49,51,52. Recent studies evaluating cranial morphology identified dorsal angulation in 66-68% of all dogs with CM 41,49. The human literature documents an 81-84% prevalence of odontoid angulation, also termed retroverted dens, in patients with CMI, with more severe angulations correlating with the presence of syringomyelia 53,54. DA is often associated with increased tonsillar descent in CMI patients 55. While of recent interest, the clinical implications of this malformation are poorly understood in veterinary medicine.

Concurrent Anomalies

Medullary kinking (MK) is a structural anomaly often identified in dogs with CM/SM and is defined as elevation of the medulla from the ventral surface of the calvarium at the cervicomedullary junction, independent of external boney compression 25,41,49,56. This term was adopted from the human literature, and is seen in 70% of people with CM 57.

MK has a reported prevalence of 66-100% in CKCS with CM 10,49,52 and a higher degree has been associated with increased likelihood of clinical signs associated with CM/SM 56.

CKCS with CM/SM often have concurrent ventriculomegaly that is more severe than that typically present in dogs with CM alone. This finding may be a reflection of more significant CSF flow disturbances at the craniocervical junction 58. However, obstruction to CSF drainage through the cribriform plate, secondary to rotation of olfactory bulb associated with brachycephaly, has recently been proposed as the mechanism for ventriculomegaly 31. In people with CM/SM hydrocephalus was only identified in approximately 8% of children who had a syrinx present at initial diagnosis 55.

Cerebellar herniation occurs with a varying degree of severity in patients with CM. The presence nor degree of herniation have correlated with the development of SM, implying other factors contribute to the disruption in CSF flow 49,59. Investigations into abnormal pulsation of the canine cerebellum, also seen in human patients with CMI, have found more pronounced caudal movement of the cerebellum following systole in CKCS with

CM/SM 60. This pulsation is a normal finding in people and historically was referred to as the piston movement 61. The size of cerebellar herniation is associated with an increased size of the foramen magnum, indicating there is some degree of supraoccipital bone resorption likely due to cerebellar pressure 49. This was previously referred to as occipital hypoplasia/dysplasia, but recently was suggested to be acquired pressure

necrosis 31,49. The change in morphology of the occipital bone was also demonstrated in a longitudinal study by Driver et al. that found the height of the foramen magnum and length of cerebellar herniation increased over time in CKCS62.

2.2 Syringomyelia

Syringomyelia is directly translated from Latin as “cavity within the spinal cord”. The term was introduced by Ollivier d’Anders in 1827 to describe what ultimately turned out to be the normal central canal 63,64. SM is currently defined within the veterinary literature as the development of fluid-containing cavities within the parenchyma of the spinal cord resulting from abnormal CSF movement6. This single term is now used to encompass the previously distinct entities of syringomyelia and hydromyelia. A singular accumulation of fluid is termed a “syrinx” and the plural of syrinx is “syringes”.

2.2.1. Classification of Syringomyelia

Historically syringomyelia was further classified as either “communicating” with the fourth ventricle, or “non-communicating”. This nomenclature lost its utilization when

Mihorat found in the majority of people, the central canal does not communicate with the fourth ventricle65.

Four different classifications of syringomyelia are recognized in human medicine. Type

1 develops in association with CMI or other foramen magnum (FM) obstructive lesions and will be the focus of this paper. Type II is idiopathic, meaning no underlying cause can be identified. Type III forms in conjunction with spinal cord tumors, inflammatory disease, and trauma. Type IV is the dilation of the central canal that usually coincides with hydrocephalus 66. Early in the course of the development of syringomyelia, edema forms within the spinal cord that has not yet coalesced into a distinct cavity and is termed a “pre-syrinx”.

2.2.1 Cerebrospinal Fluid Flow

Cerebrospinal Fluid (CSF) makes up 10% of the intracranial fluid volume67, 68 and is produced by the transport of water, potassium, chloride, and bicarbonate from the choroid plexuses into the ventricles 68. The process of CSF production is facilitated by carbonic anhydrase C, sodium and potassium ATPases, and aquaporins in the epithelial lining of the choroid plexus 68. Aquaporin-4, the most abundant aquaporin found in the brain and spinal cord, is responsible for water transport within the central nervous system.

Alterations in the function of this protein have major implications on CNS homeostasis69.

The pathway of CSF flow starts in the lateral ventricles, courses through the interventricular foramen, and into the third ventricle. From there it passes through the

mesencephalic aqueduct into the fourth ventricle. The majority of CSF passes into the subarachnoid space through the lateral apertures adjacent to the foramen magnum, with only a small portion passing through the central canal 70. CSF flow rate, pressure, and drainage are influenced by body position, physical activities, gravity, blood pressure, and accelerating forces 70,71,72. Alterations in CSF flow have been implicated as a major contributing factor to the development of SM and there are several theories as to how this may occur.

2.2.2 Etiology and Pathogenesis

The “water hammer theory” was first described in 1950 by Drs. James Gardner and

Robert Goodall, two physicians who reasoned that cerebellar herniation obstructed the normal caudal movement of CSF through the foramen magnum during systole and forced ventricular CSF into the central canal instead 73. This theory was later rejected as a plausible cause of SM in people due to the infrequency of ventricle to central canal connections in people. However, patency between these structures has been observed in canines with an unknown significance in the pathogenesis of syringomyelia 74,75.

In 1976 Williams proposed the “suck theory”, which suggested that a pressure differential between the head and spinal column forced fluid into the cisterna magna during periods of increased intra-abdominal or thoracic pressure. The cerebellar tonsils

then obstruct CSF efflux through the subarachnoid space causing fluid to be shunted into the central canal and leading to dilation 76. The same year, after observing dilated perivascular channels in people with SM, Melvyn Ball proposed the theory that obstruction of CSF return allowed tracking of fluid into the spinal cord parenchyma by way of the Virchow-Robin space 77. Documentation of higher pressure gradient within a syrinx when compared to the subarachnoid space argues against both of these theories 43.

In 1994, Oldfield et al. proposed a new mechanism for the development of syringomyelia and countered previous theories by documenting that all human syringes were

“noncommunicating” with the fourth ventricle. His “piston theory” proposed that caudal displacement of the cerebellum during systole lead to an increased pulse pressure causing fluid to move through the perivascular or interstitial spaces 74. These pulse waves were ultrasonically observed during surgery and subsequently resolved following decompression 74. A limitation of this theory is the fact that an increased pressure within the subarachnoid space would theoretically collapse a syrinx, not expand it 78.

Several authors have proposed syringomyelia could be the result of failure of outflow, leading to fluid accumulation. Suggested mechanisms of altered outflow range from foramen magnum overcrowding leading to obstruction of cranial cerebrospinal fluid drainage 79, impairment of extracellular fluid absorption by increased venous pressure 80, and perivascular space obstruction 81.

Most recently, the “intramedullary pulse pressure theory” suggested that SM is secondary to mechanical distension of the spinal cord and the fluid within the syringes is extracellular fluid not CSF 7. With CM, or any other condition causing obstruction of the subarachnoid space, there is a decrease in pressure just caudal to the lesion but an increase in pressure in the spinal cord at the site of obstruction. This results in distension of the spinal cord in the region of decreased subarachnoid pressure, extracellular fluid accumulation, and dilation of the central canal 82,83. The extracellular fluid then coalesces into distinct cavities. The partial obstruction seen with CM can be explained by

Bernoulli’s principle, which states the mechanical energy of flowing fluid remains constant. Therefore, with increased velocity in the narrowed subarachnoid space there is an equivalent decrease in the subarachnoid hydrostatic pressure resulting in distention of the spinal cord 82. The formation of a syrinx results in progressive loss of subarachnoid space and ultimately extension of syringes distal to this region. A related theory, the

“vascular theory”, proposes that increased CSF pressure causes cranial collapse and caudal dilation of blood vessels. These vascular changes disrupt the blood-spinal cord barrier permitting leakage of a low protein fluid into the spinal cord parenchyma 43.

The composition and origin of the fluid within a syrinx is not well defined; however, studies support the notion that it may not be of CSF in origin but rather composed of extracellular fluid. Several studies have compared the protein of CSF to that of syrinx

fluid with mixed results 84-88. One study identified a higher protein concentration in the post-traumatic syrinx, 0.35-3.9g/L, when compared to CSF, 0.1-0.44g/L, while others found indistinguishable protein concentrations 84,85.

Despite substantial research focusing on the mechanism by which syringomyelia develops, its genesis remains unclear. As theories have evolved, recent studies have moved away from the belief that the fluid is CSF in origin and instead suggested an extracellular origin of the fluid. Still, the interaction between subarachnoid and spinal cord pressures, systolic pulses, and fluid outflow remains unsettled.

2.3. Clinical Presentation

2.3.1. Signalment

The cavalier King Charles spaniel is the most widely recognized breed affected by

Chiari-like malformation and secondary syringomyelia1,48,52,89. CM occurs exclusively in small breed dogs of varied skull morphologies such as the Yorkshire terrier, Pug,

Maltese, Boston terrier, Shih tzu, Bichon frise, Affenpinscher, Miniature Dachshund,

Pomeranian, Miniature Pinscher, West Highland White Terrier, Tibetan spaniel,

Pekingese, and French bulldog4,41,90. The Brussels Griffon and the Chihuahua are two breeds also gaining attention for the malformation. Approximately 60-80% of Brussels

Griffons and 100% of Chihuahua are affected by CM with 66% of the latter going on to develop SM 5,90. Recently, the dysmorphic skull shape of the CKCS, as determined by eye examination alone, was associated with an increased risk of developing SM91.

In the CKCS, clinical signs of CM/SM may develop anywhere between 0.3-4.5 years of age, with a mean of 2.2 years 9. Anecdotally, dogs that present with clinical signs at less than 2 years of age often suffer from a more severe clinical course 25.

2.3.2. Clinical Signs

Clinical signs observed in dogs with CM/SM may be reflective of neuropathic pain, cervical myelopathy, brainstem, cerebellar and/or vestibular dysfunction. The most consistent and debilitating symptom reported by people with CM/SM is pain, which can manifest in a variety of ways 92,93 including headache or radiating pain affecting the extremities or intrascapular area. Paresthesias (abnormal spontaneous sensations) such as pin prick, burning, or stretching sensations are also commonly reported 92. People with headaches resulting from CM typically describe them as suboccipital headaches that worsen with exertion 94,95. Dermatomal hypersensitivity and trigeminal pain are also reported in association with SM in people 92. Conversely, another common presentation in people is a “cape-like distribution” of sensory loss over the back and shoulders when the spinothalamic tracts are involved 92.

For obvious reasons, dogs with CM/SM-associated pain cannot self-report; however, several studies have documented clinical exam findings and owner-reported behaviors suggestive of neuropathic pain in affected dogs 7,9-11. Cervical hyperesthesia may be observed on spinal palpation during examination; although, pain can also be nonspecific, intermittent, and spontaneous (e.g. not caused by an obvious stimulus). CKCS with SM- associated neuropathic pain may cry out spontaneously or if lifted from under the front limbs 32. Owners may report that their dog sleeps with its head in an elevated position to relieve discomfort, as flexion has been reported to increase the length of cerebellar herniation 96.

Phantom scratching is a common but unique clinical sign of SM32. Clinicians must be careful to differentiate true pruritus from phantom scratching due to the high prevalence of primary secretory otitis media (PSOM) and other dermatologic conditions in breeds where SM is also prevalent. A defining feature of phantom scratching, as compared to true pruritus, is that typically no contact is made with the skin and phantom scratching is often oriented toward only one side of the body, generally correlating with lateralization of a syrinx on advanced imaging 10,32.

Phantom scratching has been suggested to represent a behavioral expression of paresthesia or allodynia, two manifestations of neuropathic pain that are reported by people with CM/SM 97. Alternatively, phantom scratching may be a manifestation of

fictive scratch, a neurologic phenomenon originally described by Sherrington in 1906.

Fictive scratch was initially observed following experimental spinal cord transection, where a cutaneous stimulus caudal to the lesion would induce a scratching movement that did not make contact with the skin98,99. Fictive scratch is thought to result from hypersensitization of central pattern generators (CPG) in the lumbosacral plexus.

Various rhythmic motor patterns, including scratching, breathing, and even walking, can be initiated by CPGs, which are local neuronal circuits throughout the spinal cord 100.

Nalborczyk et al. suggested that SM-associated damage to the cervical spinal cord dorsal horn (lamina I) (Figure 3) might influence descending projections to the lumbosacral

CPGs and resulting in disinhibition of scratching neural circuits 99. The similarities between fictive scratch and phantom scratch are intriguing and emphasize the importance of further investigation into the influences of the superficial dorsal lamina on the regulation of lumbosacral CPG’s.

Beyond phantom scratching, the remainder of the neurologic examination in dogs with

CM/SM may be normal or may reveal proprioceptive deficits, lower motor neuron signs to the thoracic limbs, ataxia, and/or paresis. Cranial nerve deficits have been identified in some dogs with CM/SM, possibly caused by medullary compression or traction on the nerves 102. Other neurologic abnormalities reported in dogs and people with CM/SM include sleep apnea, dysphonia, dysphagia, exotropia, unilateral tongue atrophy, and absent gag reflex93,95,102,103. Young animals, particularly those with large dorso-lateral syringes, may develop torticollis and scoliosis due to loss of unilateral general proprioceptive afferent input important in regulating posture 10,104,105. A common mistake in SM-affected dogs is misinterpreting mild torticollis as a head tilt, leading the clinician

to generate an inappropriate list of differential diagnoses for the patient 106. Clinical signs of CM/SM are often exacerbated during periods of excitement, stress, or contact by a neck collar or other tactile stimulus. Owners may also report that affected dogs are more symptomatic during high temperatures, higher humidity, or changes in barometric pressure.

Although well documented in people, the importance of CM in the absence of SM as a cause of clinical signs in dogs is unclear. Some dogs display behaviors suggestive of neuropathic pain when CM is present as a sole abnormality 32,40; however, CM is essentially universal in the CKCS breed, with many dogs apparently asymptomatic for the condition 1. Previous reports suggested ventriculomegaly secondary to CM may result in seizures7. This theory was later refuted when no correlation was found between the presence of seizures and ventriculomegaly in CM/SM affected patients107. Instead, idiopathic epilepsy should be the top differential in an otherwise healthy, young CKCS with exclusively CM/SM and ventriculomegaly on MRI, with a normal CSF analysis.

While SM is more classically associated with clinical signs, 70% of asymptomatic CKCS have CM/SM diagnosed on advanced imaging8, 108. This suggests that the development of clinical signs with both CM and SM may be multifactorial or that certain disease- modifying elements may play a role 12,13,40,109. Given the apparent complexities in the relationship between CM, SM, and the development of clinical signs, a wealth of recent

studies have focused on evaluating the genetic, structural, and biomechanical contributors to this syndrome. Of primary focus related to the pathogenesis of CM/SM have been skull morphology, presence of concurrent CCJ anomalies, and alterations in CSF flow dynamics. At this time, no single parameter has been identified to predict which CKCS will have symptoms or go on to develop them later in life.

2.3.3. Diagnostics

High field magnetic resonance imaging (MRI) is the gold-standard for diagnosing

CM/SM, although computed tomography (CT) and low-field MRI are comparable at making a diagnosis of SM110. CT can be useful to identify concurrent CJAs which may be difficult to appreciate on MRI due to poor bone detail, but may influence surgical approach and possibly long-term outcome 25, 41, 44.

An MRI study should at minimum, extend from the intrathalamic adhesion to C5 and consist of T1 weighted (T1W) and T2 weighted (T2W) sagittal and transverse images6;however, because syringes often extend in to the thoracic spinal cord, it is the standard of care at the authors’ practice to obtain images at least to the level of T2

(Figure 4). Multiplanar imaging allows adequate assessment of cerebellar vermis herniation, syrinx dimensions and locations, and the presence of secondary ventricular dilation.

Although the abnormalities typical of both CM and SM can be readily observed with

MRI, there is no specific imaging parameter that can universally predict severity of clinical signs or progression of disease. Due to the poor correlation between morphologic characteristics and the severity of clinical signs the utility of Cine MRI, to quantify CSF flow velocity and turbulence, has been proposed in both human and veterinary medicine111,112. Cine MRI provides a movie-like image of CSF flow. It can identify obstructed CSF flow at the foramen magnum in CKCS, with turbulent flow correlating to the presence and severity of SM113. Cardiac-gated cine MRI provides additional information on how CSF flow is affected during systole and diastole and detects increased cerebellar pulsation during systole60.

Figure 4. T2-weighted sagittal images of brain and cervical spinal cord from a normal mesaticephalic dog (A and B), a Cavalier King Charles Spaniel (CKCS) with Chiari-like malformation (CM) but no evidence of syringomyelia (SM; C) and a CKCS with CM/SM (D). Note the malformation of the caudal aspect of the occiput (C and D; arrowhead) and caudal displacement of the cerebellum into the foramen magnum (C and D; arrow) typical of CM. T2 hyperintensity consistent with fluid accumulation within the spinal cord parenchyma is typical of SM (D; asterisk). (Reprinted from Hechler, 2018 34)

The British Veterinary Association (BVA) has published CM/SM grading guidelines for use in classifying structural severity of disease114 (Figure 5). A diagnosis of presence or absence of CM should be made on the mid-sagittal T2W image. The severity is then graded based on the appearance of the cerebellum and subarachnoid space, with less emphasis placed on the morphology of the supraoccipital bone. In order to be classified as Grade 0 CM the cerebellum must be rounded with CSF present between the vermis and the FM. Grade 1 CM includes dogs with cerebellar indentation by the supraoccipital bone, but CSF is still visible. When the cerebellum becomes impacted into or through the FM dogs are classified as Grade 2 CM. Positioning during imaging acquisition is an important consideration as severity of cerebellar herniation increases significantly with a flexed head position96.

Syringes are identified using standard T1W and T2W imaging sequences. Since syrinx fluid has similar composition to CSF, it is generally hypointense (dark) on T1W and hyperintense (bright) on T2W images. Syringes are most commonly located within the cervical and high thoracic spinal cord; however, they may extend caudally to the lumbar spinal cord segments or cranially into the brainstem (syringobulbia) 7, 115, 116. The severity of SM is also evaluated using the BVA grading scheme, where syrinx diameter is measured in the transverse plane at the level of the subjectively largest diameter. Dogs are classified as normal, Grade 0, if there is no syrinx or central canal dilation. They are classified as Grade 1 if a dilated central canal with an internal diameter of <2mm is

present. If the central canal is >2mm, a separate syrinx or presyrinx is present with or without central canal dilation, then they are a Grade 2. The grade is further classified by the addition of a letter that represents the patient’s age group. Patients greater than 5 years of age fall into the “a” age group, those between 3 and 5 years are subtype “b”, and those less than 3 are catalogued in “c”. A presyrinx appears as an intraparenchymal T2W hyperintensity with T1W iso- to hypointensity.

Figure 5. Grading scheme for Chiari-like malformation (CM; A) and syringomyelia (SM; B) recommended by the British Veterinary Association for use in breeding protocols. (Reprinted from Hechler, 201834.)

Sampling and analysis of CSF should be performed in all patients undergoing a workup for suspected CM/SM in order to rule out concurrent inflammatory neurologic diseases and those that might mimic clinical aspects or the MRI appearance of SM. Despite the high prevalence of caudal displacement of the cerebellum in dogs with CM/SM, in the authors’ experience, cisternal puncture for CSF collection does not carry increased risk beyond the average patient.

2.3. Pain

Pain is a term that encompasses any unpleasant sensation incurred from nociceptive transmission, following damage to tissue. The perception of pain first requires a noxious stimulus to be transduced, transmitted, and modulated before it can be projected to the thalamus and recognized by the cerebral cortex as unpleasant (Figure 6). The entire pathway must be intact for normal pain perception. First, a stimulus is detected by receptors on the primary afferent neurons and results in depolarization, if the mechanical, chemical, or thermal insult reaches threshold. Ad and C fibers are the neurons primarily responsible for the transmission of noxious stimuli, while Aß fibers transmit non-noxious stimuli. Transmission is the propagation of the action potential from the afferent neuron to the dorsal root ganglion and ultimately the dorsal horn of the spinal cord.

Figure 6. Pathway of pain transmission. (Reproduced with permission.117)

The dorsal horn is divided into laminae numerically named I through VI, with I being the most superficial (Figure 3). Each lamina contains the projections of specific primary afferent fiber types. Ad fibers terminate in superficial lamina I while C fibers terminate in lamina II where they both synapse on interneurons 118. The synapses with various interneurons allow modulation that is dependent on the type of neurotransmitters present and the effect of excitatory and inhibitory influences. The final output of the dorsal horn is an amalgamation of all neurochemical events through the spinothalamic and spinocervicothalamic pathway11,118,119. The pathways ascend to the thalamus for the final

step, perception of pain. Thalamic nuclei project third order neurons to different regions of the cerebral cortex for discrimination and emotional reaction to pain. Pain serves the normal function of protecting the body from persistent and repeated damage, however, pain can become a disease itself when persistent beyond resolution of the initial injury or inappropriate activation. Any disruption in one or more components of the pain pathway can result in the development of neuropathic pain.

2.3.1. Classification of Pain

In order to better characterize and ultimately treat pain, understanding the etiology is necessary; therefore, pain is further divided into two categories; neuropathic and nociceptive. Nociceptive pain is the transmission of a noxious stimuli through a normal functioning somatosensory system, while neuropathic pain is caused by damage to the central or peripheral nervous system120. Neuropathic pain can further be subdivided, in a classification scheme designed by the International Association for the Study of Pain

(IASP), into “above-level”, “at-level”, or “below-level” 121. Each of these terms describes the distribution of dermatomes affected when compared to the level of injury.

At-level pain occurs in the dermatomes of the spinal cord segments affected by the injury

122 and is often the immediate result of an injury affecting the nerve roots or spinal cord and may resolve over time. In people, at-level neuropathic pain is often described as a burning or shooting sensation with concurrent allodynia and hyperalgesia. If the

symptoms arise greater than three dermatomes distal (caudal) to the site of injury, this is below-level pain 123 which often develops later in the course of the injury, due to central sensitization, and persists longer 124. Below-level pain has been associated with higher pain scores than at-level neuropathic pain in people 125 and is often more difficult to treat

126. A classic feature of below-level pain is increased sensitivity to temperature suspected to be caused by dysfunction of the spinothalamic tract 126.

The manifestation of neuropathic pain is not determined by the location or etiology of lesion; instead, much overlap in symptomology exists. In an attempt to classify individual symptoms, numerous terms have been used to describe what the patient is experiencing (Table 2). The sensation of pain can be evoked from a typically non-painful stimuli, referred to as allodynia, or a patient may just be more sensitive to normally noxious stimuli, termed hyperalgesia. In the veterinary patients, it is impossible to differentiate between the two manifestations of neuropathic pain because the patients cannot report which sensations normally elicit pain. Hyperesthesia is the increased sensitivity to any stimulus, encompassing both hyperalgesia and allodynia. Dysesthesia refers to any abnormal and painful sensation such as burning, while paresthesia is abnormal, but not an unpleasant sensation 127. Furthermore, these sensations can be either spontaneous or evoked by a stimulus.

Table 2. Summary of terms and definitions relevant to neuropathic pain. (Reproduced with permission. 128)

Term Definition Interoceptive stimulus A sensory stimulus that originates from within the body Exteroceptive stimulus A sensory stimulus that originates from outside the body Pain An unpleasant sensory and emotional experience provoked by a damaging or potentially damaging stimulus Nociceptive pain Pain caused by a noxious stimulus that is processed by a normally functioning somatosensory system Neuropathic pain Pain caused by a disease or lesion causing dysfunction of the somatosensory system Mixed pain Condition of coexisting nociceptive and neuropathic pain Allodynia Pain provoked by a stimulus that does not normally cause pain Hyperesthesia Increased sensitivity to stimulation

Paresthesia An abnormal sensation (burning, tingling, skin crawling) that can be either spontaneous or evoked Hyperpathia An abnormally painful reaction to a stimulus Hyperalgesia Increased pain in response to a stimulus that would normally be painful Hypoalgesia Diminished pain in response to a stimulus that would normally be painful Analgesia Absence of pain in response to a stimulus that would normally be painful Central sensitization Response of nociceptive neurons within the central nervous system to normally non-painful or sub-threshold sensory stimulus

2.3.2 Substance P

Although numerous neurotransmitters are involved in the sensation of pain, a few have received attention for their potential role in the development of neuropathic pain.

Substance P is a tachykinin neurotransmitter that acts on neurokinin receptors (NKR) 1,

2, and 3 92 to play a role in the transmission of nociception and signal transmission for 36

chronic itch and alloknesis 129. Substance P has been implicated in the pathology of neuropathic pain due to the high concentration and location within the central nervous system. Immunohistochemical analysis of the human spinal cord identified substance P in the dorsal horn, lateral lamina V, central gray, intermediate gray and tract of Lissauer; with the highest concentration in the superficial lamina I130. In people with SM, experimental studies have identified higher quantities of substance P in the dorsal horn laminae below the level of the syrinx while there was a significant reduction in quantity at and above the level of the syrinx 92,131. This finding is consistent with the reported hypoesthesia people experience at the level of the syrinx. The decreased amount of substance P in the dorsal horn is postulated to result from either interference of the syrinx with axonal transport of SP or destruction of the intrinsic supply 131. Further evidence supporting the role in generation of neuropathic pain is seen following experimental application of substance P to the spinal cord, resulting in enhanced nociceptive pain responses as well as excitation of neurons that normally respond to painful stimuli 92.

Part of the increased excitation is postulated to be secondary to up regulation of the neuropeptide’s NK1 receptors in lamina I of the dorsal horn 92. This laboratory data clinically translates to CKCS, with CM/SM related neuropathic pain, that have higher concentrations of substance P in cerebrospinal fluid (CSF) when compared to those

CKCS without neuropathic pain 132. The utility of CSF substance P levels in the detection of neuropathic pain in CKCS with CMSM is unknown.

Neuropathic pain is a complex, multimodal state of chronic discomfort resulting from an abnormal functioning somatosensory system. It likely remains an under-diagnosed condition in the veterinary patient population due to poor understanding, difficulty in identifying clinical signs, and lack of a reliable, objective testing method. The known detrimental effects of chronic neuropathic pain on quality of life 12,101 emphasize the need for the development of objective methods to allow an accurate diagnosis and to monitor response to therapy. There is a growing trend for the use of quantitative sensory testing

(QST) to objectively evaluate neuropathic pain in human and veterinary medicine 15-

17,19,122,133-140. QST could allow early diagnosis of neuropathic pain and accurate monitoring of the response to treatment.

2.4. Quantitative Sensory Testing

Quantitative sensory testing is a group of objective methods used to evaluate the somatosensory system through stimulation of various receptors. Afferent nerve receptors are tested through the use of different stimuli, most commonly pin-prick, pressure, thermal, and vibrations, to provide a single sensory threshold (ST) value 17,19,140-142. The

ST is the lowest numerical representation of the stimulus applied that elicits pain. When discussing QST in the context of veterinary patients, the ST is a behavioral response that is interpreted by the operator as a conscious response to pain15,133.

The necessity for a multimodal diagnostic approach stems from the complex presentation of patients with neuropathic pain. This is especially true for people diagnosed with SM.

Pain can manifest as spontaneous or provoked, continuous or paroxysmal, or a combination of these in one or multiple areas of the body 142. Although hyperesthesia is most frequently reported in a dermatomal pattern, a subset of people describe an atypical distribution 92,142. For this reason, people use sensory body maps to focus QST on the body area experiencing the most pain or sensory loss, as described by the patient.

Unfortunately, in veterinary medicine we instead must select the testing region based on behavioral indications of pain and dermatomal patterns associated with common sites for

SM135. To complicate matters further, SM-associated neuropathic pain is described in people as a mix of hypoesthesia and hyperesthesia, with both occurring in the same region 135,142. This symptomology makes it very challenging to select a region and time to perform QST in a patient that cannot verbalize the sensations they are experiencing.

Due to the variability in neuropathic pain, rigorous training is required to become proficient in a testing method and to ensure accuracy. Each sensory test evaluates one or more specific afferent nerve fiber by way of a validated protocol140. Both cold and hot thermal testing primarily evaluate the C fibers responsible for thermoception 139.

Mechanical allodynia and hyperalgesia can be evoked in three different manners. Light contact results in dynamic hyperalgesia, pin-prick results in punctate, and pressure causes a static hyperalgesia. Punctate and dynamic hyperalgesia are the most commonly

recognized mechanically evoked sensations reported by patients, and therefore the most commonly identified on testing 139. Dynamic hyperesthesia, mediated by Ab fibers, results in primary hyperalgesia at the site of the injury with further extension past the site and is best evaluated with brush-stroke testing139. Punctate hyperesthesia most often follows a dermatomal pattern of distribution. The Aδ-fibers are predominantly responsible for this type of pain, with minor influence from C fibers, and they are evaluated with von Frey monofilaments or pins139. Static hyperesthesia is the least commonly recognized form and is typically short-lasting and restricted to the site of injury. Application of superficial and deep pressure is used to test for the presence of this type of hyperesthesia 139. The ability to identify hyperesthesia and allodynia relies heavily on the appropriate selection of mechanical sensory test and body region evaluated. This becomes challenging in veterinary patients that cannot describe the type of neuropathic pain they are experiencing.

2.4.1. Von Frey Anesthesiometry

Electronic von Frey anesthesiometry (VFA) is a method of quantitative mechanical sensory testing used to detect the presence of punctate hyperalgesia by measuring the sensory threshold. Von Frey filaments were initially designed by Maximilian von Frey in the year 1896. Although the original method has undergone modification since this time, the underlying concept has remained constant. The filaments were used to provide a 40

constant punctate force to the skin and identify the tested area as hyper or hyposensitive based on the patients’ reactions. Today, convenience has led to the development of an electronic version of these filaments. The handheld device measures the amount of pressure required to elicit a response in the patient by stimulating A-δ and C fibers 143.

Hypoalgesic patients (those with decreased sensation) will exhibit higher ST values, while those with hyperesthesia have lower values. The ability to detect ST change in the non-verbal patient has great potential to guide diagnoses and treatment regimens.

The functionality of von Frey relies heavily on patient behavior, the experience of the operator, and sensory threshold. The operator is trained to watch for behavioral responses to an unpleasant stimulus; however, the quality of this response may vary between patients and can make interpretation particularly difficult in stoic animals.

Despite appropriate execution of the test, external environmental factors, patient anxiety, and temperament can also affect behavioral responses. A recent study reported moderate inter-observer agreement but excellent intra-observer agreement when testing ST using von Frey anesthesiometry in normal dogs 144. The modified dorsal technique used in this study was previously shown to be a reliable method demonstrating a difference in ST in dogs with and without spinal cord injury secondary to intervertebral disc herniations 15,16.

Further promising results have identified VFA’s usefulness in evaluating the efficacy of anti-nociceptive medications 134,145,146 and sensory variations in orthopedic disease147,148.

Overall, these results do not yet support comparing values between operators and

institutions but are encouraging to evaluate an individual patient and their response to treatment.

2.4.2. Alternative Quantitative Sensory Testing in Veterinary Medicine

Von Frey Anesthesiometry is the most common form of QST in the veterinary literature, although sporadic publications have investigated alternative methods. A recent study utilized hemostatic forceps to evaluate static hyperesthesia in CKCS with and without

CMSM, and found no significant difference in the ST values of the two groups19. As static hyperesthesia is the least recognized form of neuropathic pain in people, it may have contributed to the lack of difference observed in this paper. The same group also performed both heat and cold thermal testing on the same population of dogs, again with no significant difference between CKCS with and without CM/SM 19. Further use of thermal QST has been attempted in the veterinary literature with mixed results. On study concluded that cold thermal testing did not consistently yield a response in a population of normal dogs, while VFA was more reliable to produce a reaction in the same population 17. On the contrary, a study utilizing heat testing found consistency in the repeated measurements of a patient’s sessions, proving to be a reliable measurement of

ST values in dogs with and without osteoarthritis 137. With increased familiarity in veterinary medicine, QST could serve as a non-invasive, diagnostic tool in patients with

neuropathic pain, and ultimately allow better therapeutic outcomes by monitoring response to treatment.

2.5. Treatment

2.5.1. Medical Management

The goals of medical management for CM/SM target the treatment of neuropathic pain and phantom scratching. Treatment consists of pain medications and drugs that aim to decrease CSF production. An assortment of pain management strategies have been reported for use in the treatment of CM/SM and response to treatment is usually gauged by the clinician and owner by taking into consideration owner- reported observations related to frequency and severity of pain behaviors 12,149. Table 3 provides a summary of medications frequently used in the management of CM/SM along with their recommended dosing ranges.

Table 3 - Common medications used in the clinical management of Chiari-like Malformation and Syringomyelia in dogs. (Reprinted from Hechler, 2018. 34)

Class Drug MOA Dose Comments Anti-epileptics Gabapentin Binds voltage-gated 10-30mg/kg Sedation/ataxia Pregabalin Ca channels q8h Human liquid preparations Suppress itch 4mg/kg q12h contain 300mg/ml xylitol proprioceptive transmission Topiramate Modulation of 5-10mg/kg q8h Inappetence voltage-gated Na and Irritability Ca channels, potentiation of GABA Tricyclic Amitriptyline Antagonize voltage- 3-4mg/kg q12h Contraindicated with Antidepressants gated Na channels, MAOIs glutamate receptors Caution with KCS, and NMDA receptors glaucoma, arrhythmias, diabetes, adrenal tumors and hepatic disorders NMDA Receptor Amantadine NMDA receptor 3-5mg/kg q12- Agitation Antagonists antagonist 24h Diarrhea Enhances effects of Caution with kidney and NSAIDs, gabapentin, liver disease, glaucoma, and opioids and congestive heart failure Opioids Tramadol Mu-receptor agonist 4-5mg/kg q8h Contraindicated with MAOIs SSRIs and TCAs can inhibit metabolism NSAIDs Carprofen Inhibition of COX, 2.2mg/kg q12h Vomiting/Diarrhea Meloxicam PLA2, and 0.1mg/kg q24h Inappetence prostaglandin Hepatocellular damage synthesis. GI ulceration Renal toxicity Corticosteroids Prednisone Inhibit induction of 0.5mg/kg q12h Iatrogenic COX-2 and PLA2 hyperadrenocorticism Dexamethasone Decrease substance P 0.07mg/kg HPA axis suppression Decrease CSF q12h PU/PD/PP production GI ulceration Muscle wasting PPIs Omeprazole Reduce Na+-K+- 0.5-1mg/kg Decreases bioavailability of ATPase activity in q24h gabapentin if administered the choroid plexus within 2 h of each other

Possible GI bacterial colonization with long term use >3mos Ca: calcium, CSF: cerebrospinal fluid, COX: cyclooxygenase, GI: gastrointestinal, HPA: hypothalamic- pituitary-adrenal, K: potassium, KCS: keratoconjunctivitis sicca, MAOI: monoamine oxidase inhibitor, MOA: mechanism of action, NA: sodium, NMDA: N-methyl D-aspartic acid, PD: polydipsia, PLA2: phospholipase A2PP: polyphagia, PPI: proton pump inhibitors, PU: polyuria, SSRI: selective serotonin reuptake inhibitor, TCA: tricyclic antidepressant

Anti-Epileptics

Antiepileptic medications such as gabapentin, pregabalin, and topiramate are commonly prescribed medications for neuropathic pain. Gabapentin and Pregabalin are believed to exert analgesic properties by binding the α-2 δ-1 and α-2 δ-2 subunits respectively, of voltage-gated calcium channels which are upregulated in the superficial laminae of the spinal cord dorsal horn during a neuropathic pain state150. Both also appear to suppress phantom scratching in CM/SM-affected dogs, possibly by exerting effects on GABAergic inhibitory dorsal horn interneurons that are responsible for control of itch transmission through the proprioceptive circuits151-153. Several veterinary studies have evaluated the efficacy of gabapentin for pain management and have produced mixed results; however, the dosing frequency used in these studies was 12 hours, as opposed to the recommended

8 hour interval154-156. Additionally, they all focused on acute, post-operative nociceptive pain, not chronic neuropathic pain where the strongest rationale for the use of gabapentin exists. Pregabalin has a similar mechanism of action to gabapentin but a longer half-life and higher oral bioavailability and can therefore be administered every 12 hours157,158.

Anecdotally, pregabalin is effective in some dogs with CM/SM that are refractory to high doses of gabapentin and should be considered in this subset of patients. The current cost of pregabalin may limit its current utility as a first line treatment for management of neuropathic pain.

Topiramate’s mechanism of action for management of neuropathic pain is thought to be via modulation of voltage-gated sodium and calcium ion channels, potentiation of

GABA-activated chloride channels, and blockade of excitatory glutamate transmission.

The drug also has a minor role in inhibiting carbonic anhydrase 159,149. A recent prospective, randomized, cross-over study evaluating topiramate and gabapentin as adjunctive treatments to carprofen in CKCS with CM/SM did not find a difference between the two medications using a visual analogue scale; however, gabapentin did improve quality of life when compared to carprofen alone 149.

Tricyclic Antidepressants

Although infrequently used for this purpose in veterinary medicine, tricyclic antidepressants (TCAs) are considered a first-line therapy for treating neuropathic pain in people 160,161. The precise mechanism for treating neuropathic pain is unclear, but TCAs are proposed to work primarily by inhibiting pre-synaptic reuptake of noradrenaline and serotonin, two neurotransmitters involved in nociception transmission. The descending inhibitory pain pathway, originating in the brainstem and coursing caudally in the dorsal horn, is composed of noradrenergic and serotonergic projections. The importance of noradrenaline specifically has gained attention as a primary mechanism because of the efficacy of TCAs and serotonin and norepinephrine reuptake inhibitors (SNRIs) in treating neuropathic pain, coupled with the limited efficacy of selective serotonin reuptake inhibitors (SSRIs) 162. TCAs also antagonize voltage-gated sodium channels and

N-methyl-D-aspartate (NMDA) receptors160. Presently, no clinical trials have investigated the use of TCAs for management of neuropathic pain in veterinary patients, so their use is based on extrapolation from the human literature. A recent case series describes variable results using amitriptyline to treat neuropathic pain in 3 dogs and highlights the need for controlled studies evaluating the utility of the drug for dogs with

CM/SM 163.

NMDA Receptor Antagonists

Amantadine, an N-methyl-D-aspartate (NMDA) receptor antagonist, was first developed as an antiviral medication in 1964 but is now more commonly used to treat neuropathic and osteoarthritic pain in people. NMDA receptors are believed to play a role in allodynia, the sensation of pain from a non-noxious stimulus. In chronic pain states, normally inactive NMDA receptors become activated to produce a state commonly known as “wind up”. Amantadine is used to treat wind-up associated with this chronic pain state via inhibition of NMDA responses during prolonged depolarization, accomplished by maintaining the NMDA receptors in a closed position164. Amantadine has many proposed benefits including the potential to decrease central sensitization, decrease opioid tolerance, and enhance the effects of other pain medications such as

NSAIDs, gabapentin, and opioids. Limited evidence is available evaluating response rate for neuropathic pain in veterinary patients; however, a randomized, placebo-controlled trial found clinical improvement in dogs with osteoarthritis pain when amantadine was

used in combination with meloxicam165. While evidence is currently low for the use of amantadine as a sole agent, the use should be considered in combination with other medications for cases of CM/SM that are refractory to initial treatments.

Opioids

Opioids have been shown to decrease the intensity of neuropathic pain in people.

However, their association with significant side effects and lack of long-term proven benefit make their use for the management of neuropathic pain controversial 126,166,167.

Opioid use in people has been associated with acute opioid tolerance and opioid-induced hyperalgesia. Acute opioid tolerance refers to an increased dose requirement to control pain and is caused by upregulation of opioid receptors following repeated administration of high doses. Opioid-induced hyperalgesia refers to a condition of actual decrease in pain threshold that occurs after chronic administration of opioids, the mechanism for which is poorly understood167. Tramadol, a weak mu-receptor agonist and serotonin and norepinephrine uptake inhibitor168, is the fourth most commonly prescribed treatment for dogs with CM/SM, despite limited clinical evidence for its use in the context of chronic neuropathic pain169. Current veterinary literature is mixed with respect to the utility of tramadol for managing pain in dogs, and most studies focus on its use for acute nociceptive pain145,158,170-174. This, coupled with the high side-effect profile and potential for abuse suggests that tramadol may not be indicated in the management of most patients with CM/SM.

NSAIDs

The veterinary evidence for use of cyclooxygenase (COX) inhibitors in the treatment of chronic neuropathic pain are limited and pathophysiologic reasoning behind prescription of COX inhibitors is in dogs with CM/SM- associated pain are controversial; however, anecdotal reports suggest some clinicians find them useful7,11. COX-1 and COX-2 synthesis are upregulated following spinal cord and peripheral nerve injury, leading in turn to the synthesis of prostaglandins that may contribute to the eventual development of neuropathic pain. Studies evaluating the role of COX upregulation in the development of neuropathic pain using experimental rat peripheral nerve injury models have produced mixed results175,176 when objective quantitative sensory threshold testing was compared.

Clinical veterinary studies are limited, but so far COX inhibitors have not been shown to improve the quality of life in affected CKCS149 despite being the second most commonly prescribed treatment169. The role of COX inhibitors for preventing neuropathic pain in cases of CM/SM seems limited due to the chronicity of the condition in most dogs at the time of therapy initiation.

Corticosteroids

Corticosteroids may have multidimensional benefits in treating patients with CM/SM.

Corticosteroids inhibit induction of COX-2 and phospholipase A2 which prevent the release of a host of proinflammatory mediators177 as well as decrease substance P 11,178. A potential secondary benefit provided by glucocorticoids is a decrease in CSF

production179. Because of their unfavorable side-effect profile, corticosteroids may be most rationale for pulse dosing during flare-ups of CM/SM-related pain, or as a second or third-line treatment for patients that are refractory to other drug combinations. In the authors’ experience, some dogs with CM/SM may require chronic corticosteroid administration to manage clinical signs, in which case starting with an anti-inflammatory dose for several weeks followed by tapering to the lowest effective dose, preferably toward an every-other-day dosing scheme, is recommended.

Proton Pump Inhibitors

Omeprazole, a proton pump inhibitor, has been suggested to decrease CSF production via reduction in Na+-K+-ATPase activity in the choroid plexus180. A decrease in CSF production might result in a decrease in CSF pressure and syrinx progression. The case for the use of omeprazole in this way is based on an observed 26-50% decrease in CSF production immediately following intravenous or intraventricular administration of the drug in an experimental setting 180,181. Whether a similar decrease in CSF production occurs with chronic oral administration is questionable. Recently, Girod et. al did not find a reduction in CSF production following 14 days of oral omeprazole administration in healthy beagle dogs, calling in to question the sustained utility of oral omeprazole for treatment of CM/SM182. Additionally, veterinary clinicians should be aware that the over- the-counter formulation of omeprazole contains magnesium, which can decrease oral

bioavailability of gabapentin by 32-39% if the drugs are administered within 2 hours of each other183.

H2 Receptor Antagonists

Cimetidine and ranitidine, two Histamine-2 (H2) receptor antagonists, have also been suggested as treatment options to decrease the production of cerebrospinal fluid. The presence of H2 receptors in the choroid plexus allow the drug to block these receptors and reduce secretion. Cimetidine and ranitidine have been shown to significantly decrease the production of CSF following intravenous administration for approximately 6

½ hours, when the dose is greater than 10mg/kg 184. H2 receptor antagonists have been associated with delirium in people, that improves following transition to PPIs 185.

Palmitoylethanolamide

Palmitoylethanolamide (PEA) is an anti-inflammatory, endogenous fatty acid that also has neuroprotective effects186,187. PEA works by down-regulating the degranulation of mast cells and activation of microglia and indirectly activating cannabinoid receptors 1 and 2 187. In clinical trials and laboratory studies, PEA has been shown to alleviate neuropathic pain or work synergistically with other analgesic medications 186,187. The owners of CKCS enrolled in a clinical trial of PEA reported an improvement in clinical signs within one week of medication administration. The dose reported in this study was

30mg/kg twice daily. The only side effects reported were increased panting and restlessness at high doses 188.

Itch Reflex Inhibitors

Maropitant citrate is a neurokinin 1 (NK-1) receptor antagonist used to treat chronic itch.

NK-1 receptor expressing neurons are found in the dorsomedial superficial dorsal horn of the spinal cord and are responsible for the sensation of alloknesis, a condition where a normally non-pruritic stimulus is perceived as an itch189,190. The use of NK-1R antagonists have been proposed to treat phantom scratching, under the assumption that this is a manifestation of neuropathic itch 99. The idea of using this novel class of drugs for treatment of phantom scratching in dogs with CM/SM may not be supported by recent neuroanatomic studies evaluating the genesis of scratching in these dogs99;however, further studies are needed to assess the utility of itch reflex inhibitors in the treatment of

CM/SM.

Oclacitinib (ApoquelÒ) is an immune modulator that blocks Januskinase, an enzyme that plays an important role in itch and inflammation. The off-label use of this medication, at 0.4-0.6mg/kg twice daily for 14 days, has been reported in two CKCS.

The pilot results demonstrated resolution in scratching symptoms191.

2.5.2. Surgical Management

Surgical intervention should be considered in dogs that are refractory to medical management, those that experience adverse drug effects, or those with continued progression of neurologic signs despite medical treatment. The goal of surgical decompression is to prevent progressive syrinx enlargement and to improve frequency and severity of pain-related behaviors. The surgical procedures performed to address

CM/SM are referred to by many names including cranial cervical decompression, suboccipital decompression, foramen magnum decompression, and foramen magnum decompression with cranioplasty, depending on the procedure performed. Universally, decompression of the foramen magnum is achieved by removing a portion of the occiput and the dorsal aspect of the atlas. A portion of the dura may be removed or marsupialized, depending on surgeon preference and the presence or absence of dorsal dural constrictive bands. Previous reports describe application of gel foam in the surgical site, however, this is suspected to contribute to later scar formation9,25,109,192. Some surgeons also perform a cranioplasty in conjunction with the decompressive procedure and may use multiple titanium screws anchored into the occipital bone to attach a titanium mesh and PMMA plate or other apparatus to reconstruct the caudal aspect of the occiput 192. The necessity of including a cranioplasty with decompression is controversial although some authors suggest that this may decrease recurrence of scar tissue at the surgical site and may improve long-term outcome 25,109,192.

Surgical outcomes for CM/SM have been evaluated on a short and long-term basis. The short-term success rate is approximately 80-94%, with a better improvement correlating with shorter duration of clinical signs9,109,192,193. In the long term, FMD alone has been associated with a 25-47% relapse rate, due to scar formation at the surgical site9,192.

Addition of a cranioplasty to the procedure is suggested by some authors to decrease likelihood of re-operation due to scar tissue formation 25,109,192; however, no long-term studies have compared outcome or complications between the two variations in procedure. Syrinx resolution does not typically occur after surgery, despite short-term improvement of clinical signs in most dogs 9,194. This is in contrast to people with

CM/SM, for which syrinx resolution after surgery is common195,196. Lack of syrinx resolution after surgical decompression of the foramen magnum (both with and without cranioplasty) in CKCS calls into question whether current surgical procedures fully and adequately address the canine condition.

2.5.3. Alternative Therapies

Acupuncture

In human medicine, treatment of chronic pain is the most prevalent clinical target for acupuncture treatments197. A meta-analysis of human clinical trials, found that patients with chronic neck and back pain, arthritis, and headaches experience less pain following

acupuncture than those with no acupuncture or sham treatment198. Only one case report exists in the veterinary literature that evaluates dry needling as a treatment option for

CKCS with CM/SM associated neuropathic pain. In this case report the patient responded to treatment based on owner-assessment and subjective clinical assessment199.

Despite the lack of current clinical evidence for use in the treatment of CM/SM, acupuncture can be considered as an option for dogs refractory to medical management, those with significant medication side-effects, or for those whose owners would like to pursue non-pharmaceutical approaches to pain management.

Pulsed Electromagnetic Field Therapy

Pulsed electromagnetic field (PEMF) therapy is a non-invasive treatment option to promote healing and decrease pain. The proposed mechanisms include increased production of nitric oxide which decreases inflammation and enhances vasodilation, promotion of bone repair, increased expression of heat shock proteins, and reduction in prostaglandins and inflammatory cytokines200. In veterinary medicine, PEMF therapy has been found to speed recovery in dogs with tibial osteotomies201, improve pain in stifle osteoarthritis202, and improved incision pain and enhanced proprioceptive function in post-operative hemilaminectomies203,204. Prospective clinical trials are needed to assess efficacy of PEMF in treating neuropathic pain secondary to CM/SM.

2.6. Prognosis

People living with neuropathic pain experience a decrease in their overall quality of life and an increase in neurobehavioral disorders (anxiety, depression, fear, sleep disturbances, restricted activities) 14. Approximately 87% of people living with CM report moderate to high anxiety and 25% experience substantial depression101.

Understandably, neurocognitive and neurobehavioral consequences of chronic neuropathic pain are more challenging to document in dogs because veterinarians must rely on owner-reported behavioral manifestations that may suggest a problem exists.

Only 9% of owners of CM/SM-affected dogs report their pets to have a poor quality of life; however, the presence of neuropathic pain behaviors in dogs has been associated with stranger-directed fear, non-social fear, separation-related fear, attachment behavior, excitability and fear of pain-associated activities such as grooming and nail trims12.

Phantom scratching is the most common owner-reported behavior in CM/SM-affected dogs, and progression of scratching behavior is often associated with owner-perceived decline in the dog’s quality of life. Persistent scratching motions, often exacerbated by activity, can preclude dogs from going on leash walks and often causes the owner to avoid activities causing excitement. Decreased exercise often leads to weight gain which is also correlated with owner-reported worsening of quality of life12. A recent study investigated owner-reported clinical signs with the presence and severity of SM. No

correlation was identified between owner-reported neuropathic pain scores, phantom scratching, and syrinx size32. The lack of correlation between owner-reported pain behaviors and structural markers of neuropathic pain may support the notion that scratching behavior is not a surrogate marker for neuropathic pain and highlights need for more objective tests. Quantitative sensory testing could facilitate a more objective assessment of response to both medical and surgical approaches to the treatment of

CM/SM.

The progression of CM/SM-related symptoms over time is one of the most common concerns expressed by owners. In a prospective, cohort study, Plessas et al. followed 48

CM affected CKCS with or without SM diagnosed on MRI to document the long-term progression and outcome of medically managed dogs. Approximately 75% of SM- affected dogs displayed progression of clinical signs by the end of the study period (39 months +/-14.3). Despite this progression, three-quarters of these were still alive with an acceptable quality of life as reported by their owners. Fifteen percent of this population was euthanized during the study period for clinical signs related to CM/SM13. Another recent study compared BVA CM and SM grades between younger and older affected dogs and found no difference in CM grade based on age; however, SM grade was higher in older dogs 1. The authors of this study suggest these data support the idea that SM progresses with time. Cerda-Gonzalez et al. also followed a cohort of both symptomatic and asymptomatic SM-affected CKCS and found that 32% of asymptomatic dogs were

symptomatic at re-evaluation with a mean follow-up of 71 months. Of initially symptomatic dogs, 56% had progression of clinical signs despite medical management, while only 13% improved40. Despite worsening of clinical signs, overall owner-reported severity was mild to moderate. Recent studies support that clinical signs associated with

CM/SM are progressive in the majority of CKCS. They also highlight the fact that while certainly there is a subset of dogs with CM/SM that progress to debilitating pain and euthanasia12, a larger subset may be adequately managed via medical approaches with good owner-reported quality of life. There are no published studies comparing progression of clinical signs and outcome between medical and surgical management in

CM/SM-affected dogs.

Chapter 3. Mechanical quantitative sensory testing in Cavalier King Charles

Spaniels with and without syringomyelia

3.1. Abstract

Syringomyelia (SM) is a debilitating condition in the cavalier King Charles spaniel

ST, as determined by VFA, in dogs with and without SM, to assess the utility of VFA in quantifying NP in SM-affected dogs. We hypothesized the SM-affected CKCS would have lower ST values, consistent with hyperesthesia, when compared to control CKCS.

Additionally, we hypothesized that ST values in SM-affected dogs would be inversely correlated with syrinx size on MRI and with owner-derived clinical sign scores.

ST values for the thoracic and pelvic limbs differed significantly between the SM- affected and control CKCS (p=0.027; p=0.0396 respectively). Median ST value (range) for the thoracic limbs was 184.1 grams (120.9-552) for control dogs, and 139.9 grams

(52.6-250.9) for SM-affected dogs. The median ST value (range) for the pelvic limbs was 164.9 grams (100.8-260.3) in control dogs and 129.8 grams (57.95-168.4) in SM- affected dogs. The ST values in SM-affected dogs did not correlate with syrinx height on

MRI (r=0.314; p=0.137). Owner-reported clinical sign scores showed an inverse correlation with pelvic limb ST values, where dogs with lower ST values (hyperesthesia) were reported by their owners to display more frequent and severe clinical signs (r=-

0.657; p=0.022).

3.2. Introduction

Syringomyelia (SM) is an acquired neurologic disorder that results in debilitating pain in approximately 50% of the cavalier King Charles spaniel (CKCS) breed. SM is the development of fluid-filled cavitations in the cervical, thoracic, and occasionally lumbar spinal cord 10,115, most often affecting the dorsal horn, resulting in presumptive abnormalities of sensory processing due to disruption of the normal pathways. In CKCS,

SM results secondary to a malformation of the skull termed Chiari-like malformation, both of which are diagnosed using magnetic resonance imaging (MRI). Clinical signs of

SM include a vast array of behaviors such as compulsive scratching of the neck and flank, unprovoked vocalizations, and apparent sensitivity to light touch 10-13,32,105,205.

While these clinical signs have long been suspected to represent behavioral manifestations of neuropathic pain, studies using objective measures of sensation to document a neuropathic pain state in SM-affected dogs are limited 19.

In addition to being a disease of substantial importance within the CKCS breed, SM- affected dogs may represent an important clinical animal model through which to study new interventions aimed at treating neuropathic pain (NP). A canine clinical model of neuropathic pain would complement existing experimental laboratory animal models by offering the ability to study a naturally occurring disease, in a clinical setting, with canine patients who are diagnosed and managed in a similar fashion to people with the same

disease. Of equal importance, developing objective measures of NP in dogs with SM might allow more targeted studies of medical and surgical interventions aimed at improving outcome in affected canine patients and their human counterparts. Von Frey aesthesiometry (VFA) is a non-invasive, mechanical quantitative sensory test that delivers a known amount of punctate pressure to a body region until a behavioral manifestation of pain is elicited (Figure 7). The value obtained is termed the sensory threshold and alterations in this number can detect the presence of hyperesthesia and hypoesthesia.

The goal of the present study was to conduct a pilot investigation of mechanical quantitative sensory testing (QST), an objective technique for measuring neuropathic pain, using VFA in CKCS with and without SM. The primary aim was to identify the presence of hyperesthesia in SM-affected dogs manifested by lower sensory threshold

(ST) values compared to control dogs. A secondary aim was to evaluate the association between VFA-acquired ST values and severity of SM as documented on magnetic resonance imaging (MRI), and via a previously validated owner-derived score of clinical signs using a questionnaire specifically designed for CKCS with SM 12. We hypothesized that dogs with SM would have significantly lower ST values than dogs without SM, consistent with hyperesthesia. We also hypothesized that ST values in SM- affected dogs would be inversely correlated with MRI severity of SM and with severity of owner-reported clinical signs of SM.

Figure 7. The electronic von Frey anesthesiometer. The device consists of a load cell (A), handle (B), recording device (C) and tip (D). (Reprinted with permission.15)

3.3. Results

Case Selection

Nineteen SM-affected and 10 control CKCS were prospectively enrolled from a population of 46. Demographic factors (age, gender) for all dogs included in the present study are summarized in Table 4. Control dogs ranged from 1 to 2 years of age (median

1.5 years) and SM-affected dogs ranged from 1 to 8 years of age (median 4 years). SM- affected dogs were significantly older than control dogs (p = 0.037), but the gender did not differ significantly between groups.

Table 4. Demographic factors (age, gender) for 29 CKCS with and without syringomyelia Patient Age (years) Sex CM Grade SM Grade Control 1 1.5 MN 1 0 2 1 FI 2 0 3 1.5 MN 1 0 4 1.5 FS 1 0 5 1.5 MI 1 0 6 1 MI 2 0 7 1.5 MN 2 0 8 1 FI 1 0 9 2 MN 1 0 10 1 MN 1 0 SM-Affected 1 5 MN 2 1 2 8 FS 2 2 3 7 MN 2 2 4 5 FS 2 2 5 8 FS 2 2 6 6 FS 2 2 7 6 MN 1 2 8 6 MN 1 2 9 4 MN 2 2 10 3 MN 1 1 11 1.5 FI 1 1 12 1.5 FI 1 1 13 1.5 MI 2 2 14 1.5 FI 1 2 15 1 FS 2 2 16 5 FS 2 2 17 3 FS 2 2 18 1 FS 2 1 19 1 MI 2 2 CM: Chiari-like malformation; SM: syringomyelia MI: intact male; MN: neutered male; FI: intact female; FS: spayed female

Magnetic resonance imaging findings

Individual MRI findings for control and SM-affected dogs are summarized in Table 5.

Ten dogs were free of evidence of SM and served as control dogs, while 19 dogs had evidence of SM on MRI and were termed “SM-affected”. Using the BVA SM grading scale 114 to further delineate the severity of syringes on MRI, a diagnosis of grade 1 SM 64

(SM1) was present in 5 dogs and grade 2 SM (SM2) in 14 dogs. The syrinx height ranged from 0.87 to 1.7mm (median 1.2mm) in SM1 dogs, and 2.2 to 9.26 mm (median

3.6mm) in SM2 dogs.

Table 5. MRI findings for 10 control and 19 syringomyelia-affected CKCS.

Patient PSOM Syrinx Height (mm) Control 1 N -- 2 N -- 3 N -- 4 L -- 5 B -- 6 N -- 7 N -- 8 N -- 9 L -- 10 N -- SM-Affected 1 N 1.119 2 N 6.028 3 L 4.585 4 R 4.94 5 R 9.26 6 N 3.537 7 N 3.059 8 N 2.2125 9 N 2.438 10 R 0.875 11 N 0.933 12 B 1.422 13 N 2.755 14 R 2.689 15 N 3.748 16 B 3.2 17 N 4.212 18 R 1.726 19 R 4.668 B: bilateral; L: left; N: normal; PSOM: primary secretory otitis media; R: right SM: syringomyelia

Mechanical Quantitative Sensory Testing

The median sensory threshold (ST) value for the thoracic limbs was 184.1 grams (range

120.9-552) in control dogs, and 139.9 grams (range 52.6-250.9) in SM-affected dogs.

The median ST value (range) for the pelvic limbs was 164.9 grams (100.8-260.3) in control dogs and 129.8 grams (57.95-168.4) in SM-affected dogs. ST values were significantly lower in the thoracic limbs (p =0.027) and pelvic limbs (p = 0.039) of SM- affected dogs when compared to controls (Figure 8a and b).

Owner-derived clinical signs scores

Using a previously validated questionnaire to assess owner perception of SM-associated clinical signs 12, SM-affected dogs were assigned a composite score for the severity of clinical signs obtained by averaging the frequency of an assortment of behaviors across several categories. Results for individual animals are summarized in Table 6. This information was available only for SM-affected dogs enrolled in the study on or after

January 1, 2017 (n=10). The median owner-derived composite clinical signs score for

SM-affected dogs was 1.3 (range 0.5-1.8).

Table 6. Owner-derived clinical sign scores for 10 syringomyelia-affected CKCS enrolled on or after January 1, 2017. Patient number corresponds to number listed in tables 1 and 2.

Patient 10 11 12 13 14 15 16 17 18 19 Compulsive scratching 2 4 4 3 3 3 3 2 3 2 Facial rubbing 1 1 3 3 3 3 4 1 2 2 Hypersensitivity to light touch 2 0 4 3 3 2 1 3 2 0 Unexplained yelping 3 0 2 0 2 1 4 0 2 1 Not willing to lift head 2 0 1 1 1 0 1 0 2 1 Not willing to bend neck to eat 2 0 1 0 0 0 1 0 2 0 Weakness or ataxia 2 0 0 0 0 0 0 1 2 0 Strange behaviors 1 0 0 0 0 0 0 0 0 0 Reluctance to defecate 0 0 0 0 0 0 0 0 0 0 Sleep with head elevated 1 0 0 2 2 0 0 0 3 2 Composite Score 1.6 0.5 1.5 1.2 1.4 0.9 1.4 0.7 1.8 0.8 0: Never; 1: Seldom, 2: Sometimes; 3: Usually; 4: Always

Sensory threshold values and syrinx size

When SM-affected dogs were grouped by BVA grade, the median ST value for the thoracic limbs was 104.6 grams (range 93-250.9) for the SM1 dogs and 142.8 grams

(range 49.9-178.9) for the SM2 dogs. In the pelvic limbs, the median ST value was 121.1 grams (46-148.3) for SM1 dogs and 114.4 grams (58-171.2) for SM2 dogs. There was no significant difference in thoracic or pelvic limb ST values between SM1 and SM2 dogs

(p=0.75 and p=0.89, respectively; Figure 9). The association between ST value and syrinx height, in millimeters, was evaluated for dogs with SM2. There was not a significant correlation between syrinx height and thoracic (p=0.137) or pelvic limb

(p=0.324) ST values.

ST values and owner-derived SM clinical sign scores

The relationship between owner-derived clinical sign scores and ST values was evaluated for all dogs where this information was available (n=10; Figure 10). A significant inverse correlation was observed between ST values and owner-reported clinical signs for the pelvic limbs (r=-0.657; p=0.022), but not the thoracic limbs (r= -0.347; p=0.16).

3.4. Discussion

Results of the present study suggest that thoracic and pelvic limb ST values, as measured by VFA, differ between control and SM-affected dogs. SM-affected dogs display lower

ST values consistent with a neuropathic pain phenotype. This is not an unexpected finding, as hyperesthesia secondary to central sensitization has long been implicated as the cause of clinical signs in SM-affected dogs; however, this has not previously been objectively quantified. In people, mechanical QST techniques such as VFA are classically used to detect the presence of both hyperalgesia and allodynia. While, in veterinary patients, it is impossible to differentiate between the two manifestations of neuropathic pain because animals cannot report which sensations normally elicit pain.

Both hyperalgesia and allodynia can be further classified as dynamic, punctate, or static depending on the type of stimulus that elicits the sensation. Static hyperalgesia, the less commonly recognized form, is typically restricted to the affected area and can be tested using superficial and deep pressure. Punctate hyperalgesia, mediated by Aδ-fibers, most often follows a dermatomal pattern of distribution and is evaluated with von Frey monofilaments or pins 139. Therefore, ability to identify hyperalgesia and allodynia relies heavily on the appropriate selection of diagnostic test and body region evaluated. Our study exclusively looked at punctate hyperalgesia and allodynia by using VFA to test dermatomal patterns commonly affected in SM 135. The differences in location and prevalence of punctate and static hyperalgesia may explain why our findings differ from

another recent publication 19 which did not identify a difference in ST values or thermal latencies in CKCS with and without SM. Sparks et al. used hemostatic forceps to apply pressure to the subcutaneous tissues of the neck and thoracic limbs, exclusively evaluating static hyperalgesia 19. Static hyperalgesia and allodynia are less frequently reported in people with neuropathic pain, and when reported tend to affect small areas which may be difficulty to map in affected dogs 139. Additionally, several SM-affected dogs in the Sparks et al. study received analgesics immediately prior to the testing procedures. Previous studies have documented increased ST values following analgesic administration 134. Our study required a one-week washout of all medications prior to enrollment; therefore, eliminating the confounding effect of analgesics on ST values.

The presence of a neuropathic pain phenotype in SM-affected dogs, as documented by

VFA, is further supported by the identification of a significant inverse correlation between owner-reported severity of clinical signs and ST values, where SM-affected dogs with lower pelvic limb ST values displayed a higher frequency and severity of clinical behaviors commonly associated with neuropathic pain. Interestingly, this relationship did not hold true for the thoracic limbs. Of consideration here may be the difference between at-level and below-level pain. Immediately following an injury affecting the nerve roots or spinal cord, at-level pain develops in the dermatomes of the affected spinal cord segment which may resolve over time 122. Below-level pain develops later from central sensitization, affecting dermatomes caudal to the site of injury124, and has been associated

with higher pain scores than at-level neuropathic pain in people 125. Below-level pain can persist longer and is often more difficult to treat 126, mirroring the clinical course for some SM-affected dogs who can be quite refractory to analgesics. It is possible that many of the pain-related questions, asked of owners to derive a clinical signs score, focus more on manifestations of below-level pain (for example, hypersensitivity to light touch, where some of our dogs scored the highest). Alternatively, our small sample size may have created difficulty in identifying a statistical relationship given the higher variability in the thoracic limb ST dataset for SM-affected dogs.

An unexpected finding in the present study was the lack of relationship between ST values in SM-affected dogs and syrinx height observed on MRI or between SM grades using the BVA scheme. While some studies suggest that syrinx size predicts severity of neuropathic pain-related clinical signs, there is conflicting evidence regarding this relationship in the human and veterinary literature 19,32,59,105,206. Recently, the focus in human and veterinary medicine has shifted from assessing the size of the syrinx to evaluating the symmetry, with asymmetrical syringes more commonly associated with neuropathic pain 32,92,97,105. Due to the small number of dogs included in this pilot study, we were unable to assess the effect of asymmetry on ST values; however, this represents an important avenue for future studies. An additional consideration in the present study is that most SM-affected dogs were graded SM2, with only five SM1 dogs included. The

small number of SM1 dogs may have confounded our ability to identify a statistically significant difference in ST values between mild and more severely SM-affected dogs.

An important limitation in our study, and one that begs discussion for all CM/SM studies in CKCS, is the difficulty in identifying a true “normal” population for comparison within the breed. While the importance of CM as a contributor to pain has been historically minimized in dogs with SM, it has been documented in both people and dogs to cause pain in the absence of SM 19,26,207,208, with or without variable contribution from concurrent craniocervical junction anomalies 42,48,56. Selection of CKCS without CM or other minor occipital or skull-based malformations proves near impossible due to the ubiquitous nature of these malformations within the breed 1,32. Indeed 100% of the control population in the present study had CM apparent on MRI. Therefore, it is impossible to determine from the present study the degree to which CM alone influences

ST values; however, its universal presence in both the control and SM-affected groups suggests that SM is likely the driver of significant differences between the two groups.

The commonality of primary secretory otitis media (PSOM) 1 in the breed provides another potential confounder, although PSOM-affected dogs do not always display signs indicative of discomfort 209,210. In this study, we attempted to exclude patients with

PSOM by requiring otoscopic examination by a board-certified veterinary dermatologist showing a normal appearance of the tympanic member prior to enrolling. However, the

sensitivity of otoscopic examination for a diagnosis of PSOM is low210 and several SM- affected dogs had evidence of PSOM on MRI.

3.5. Conclusion

VFA is a non-invasive test that can document the presence of neuropathic pain in CKCS with SM. In the current study, ST values were lower in SM-affected dogs compared to controls, suggestive of a neuropathic pain phenotype. Additionally, ST pelvic limb values were inversely correlated with owner-reported severity of clinical signs suggesting that hyperesthetic dogs displayed more frequent and severe clinical signs of SM. While ST values did not correlate with imaging severity of SM as assess using syrinx size, additional studies are needed to assess the relationship between other imaging parameters such as asymmetry of syringes and results of mechanical QST. Future studies should focus on the effect of medical and surgical interventions on ST values in SM-affected dogs and should work to define the influence on ST values of other common comorbidities of the head and cervical spine in CKCS.

3.6. Materials and Methods

CKCS enrolled in the current study were presented to OSU Veterinary Medical Center between June 2010 and June 2018. In total, twenty-nine dogs were enrolled; 10 cavalier 74

King Charles spaniels (CKCS) with an MRI free of imaging characteristics of SM and 19

CKCS with SM documented on MRI. Dogs were included if they were greater than 1 year of age, were cardiovascularly stable for general anesthesia, and were free of significant dermatologic or otologic disease as determined by a board-certified veterinary dermatologist (LKC) on physical and dermatologic examination. Dogs less than 2 years of age had a PCV/TS, while the remainder had complete blood counts and biochemistry profiles suggesting general good health.

Control dogs (n=10)

Ten apparently healthy CKCS were recruited from a population of 29 dogs presenting to

The Ohio State University Veterinary Medical Center Dermatology Service for an unrelated study. Dogs were free of significant abnormalities on a complete neurological examination performed by one of the investigators (SAM), and free of dermatologic and otologic disease as assessed by a board-certified veterinary dermatologist (LKC) and had not received any medications in the 7 days prior to study enrollment. All dogs were assigned a CM grade and classified by a single radiologist (ETH) as having no evidence of SM based on MRI of the brain and cervical spine (SM0 using the British Veterinary

Association (BVA) scale; Table 7).

Affected dogs (n=19)

SM-affected CKCS were recruited from a population of 46 dogs presenting to the OSU

Veterinary Medical Center Dermatology and Neurology services. All dogs underwent a complete dermatologic and neurologic examination performed by one of the investigators

(SAM, LKC, ACH). They received a CM grade and were classified as “SM-affected” based on the presence of imaging characteristics consistent with SM (BVA SM grade 1 or

2) on MRI of the brain and cervical spine (Table 4) 114. Dogs were not administered any medications in the 7 days prior to study enrollment.

Table 7. Chiari malformation (CM) and syringomyelia (SM) severity scoring strategy, as proposed by the British Veterinary Association (BVA) for use in stratifying dogs with CM/SM 114.

Grade CM SM 0 Cerebellum is rounded with CSF between No syrinx or central canal dilation vermis and foramen magnum 1 Indentation of cerebellum by Central canal dilation with a supraoccipital bone; CSF present between diameter <2mm vermis and foramen magnum 2 Cerebellar vermis is impacted into or Central canal dilation >2mm, through the foramen magnum separate syrinx, or pre-syrinx with or without central canal dilation CSF: Cerebrospinal fluid

Questionnaire (n=10)

Owners of SM-affected dogs completed a clinical signs questionnaire previously validated for use in the CKCS (n=10) 12,211. The questionnaire asked owners to assign a severity score for 10 questions addressing clinical signs based on a 5-point Likert-type rating scale (behavior happens 0 = never, 1 = seldom, 2 = sometimes, 3 = usually, 4 = always) 12,211. Frequency scores for each clinical behavior were averaged across categories to provide a single clinical sign score for each dog, resulting in a maximum potential score of 5.

Von Frey Anesthesiometry

Mechanical QST of all four limbs was performed using an electronic von Frey anesthesiometer (VFA- IITC Life Science; Woodlands, CA) in a method previously described by our laboratory 15,16. Limb test order was determined at random after dogs were allowed 15 minutes to acclimate to the investigators. Dogs were prevented from visualizing the device during application to ensure behavioral responses were due to tactile stimulation 212. The minimum force, in grams, required to elicit a behavioral response (attempt to escape, eye movements, lip licking, vocalization) was recorded five times per leg, with a 1-minute break between testing to avoid windup, ST decay, and hypersensitization 133,146,212. The highest and lowest ST values obtained from each limb were discarded and the middle three values were averaged to produce a mean ST value per limb for each dog. Immediate withdrawal of the limb without pressure or conscious

response was determined to be a reflexive movement and was discarded and the stimulus was repeated in one minute 134,212,213. All measurements were made by the same investigator (SAM), who was unaware of SM status at the time of examination and was blinded to owner-reported behavior history.

Magnetic resonance imaging

An MRI of the brain and cervical spine was performed under general anesthesia in all dogs (n=29). Dogs were imaged in dorsal recumbency, with the head and neck extended, using a 3.0 Tesla Philips Achieva magnet or 3.0 Tesla Philips Ingenia model magnet

(Highland Heights, OH 44143). At minimum, T1-weighted (TR = 450-700 milliseconds;

TE = 8 milliseconds) and T2-weighted (TR = 3500-5000 milliseconds; TE = 110 milliseconds) images were obtained in the sagittal plane for review, with additional sequences and transverse images obtained on a case-by-case basis as medically indicated and dependent on presence and location of syringes.

Image interpretation was performed by a single board-certified veterinary radiologist

(ETH) blinded to patient clinical history and QST results. A commercially available

DICOM viewing software program (Horos2k v. 2.0.2, https://www.horosproject.org) was used to measure syrinx height (in millimeters), at the location of maximum apparent height, and grade the CM in the sagittal plane. CM and SM grades were assigned using the BVA grading scheme depicted in table 7114. Concurrent presence or absence of

imaging characteristics consistent with PSOM were recorded for each dog as normal, unilateral (denoted as left or right based on affected side), or bilateral 214.

Statistical analysis –

Age and sex were compared between groups using the Wilcoxin rank-sum test. To facilitate statistical comparisons, ST values for the limbs of each individual dog were averaged to produce a single mean ST value for the thoracic limbs and for the pelvic limbs. Summary data for ST in thoracic and pelvic limbs (g), syrinx height (mm), CM and SM grade, and owner-derived clinical sign scores were reported using descriptive statistics. Data were evaluated for normality using the Shapiro-Wilks test and were reported as median (range) because the data were not normally distributed. The

Wilcoxon rank-sum test was used to compare mean ST values between control and SM- affected dogs. Relationships between owner-derived composite clinical sign scores, imaging findings, and ST values were evaluated using Spearman correlations. Statistical analyses were performed using GraphPad Prism and P< 0.05 was considered significant for all tests.

3.7. Ethics approval and consent to participate

This study was reviewed and approved by the Institutional Care and Use Committee and

Clinical Research Advisory Committee of The Ohio State University (2010A00000140 and 2017V15). Signed owner consent was obtained prior to enrollment of all cases.

3.8. Funding

This study was funded in portion by the Gray Lady Foundation, The Ohio State

University Canine Research Funds, NCATS UL1TR002733, The Ohio State University

Canine Research Funds (Paladin) and American Cavalier King Charles Spaniel

Charitable Fund.

3.9. Acknowledgements

The authors gratefully acknowledge Ms. Amanda Disher, Ms. Heather Anderson, Ms.

Lane Bookenberger, and Ms. Josey Sobolewski for their assistance with patient care and data collection.

Chapter 4. Conclusions and Future Directions

The primary goal of this study was to utilize von Frey anesthesiometry (VFA) for detection of neuropathic pain (NP) in cavalier King Charles spaniels (CKCS) with syringomyelia (SM). VFA is a non-invasive test that can document the presence of NP by objectively measuring the sensory threshold (ST) value in each limb. The identification of a lower ST value in dogs with SM would indicate the presence of neuropathic pain secondary to disruption in the somatosensory system in affected dogs.

The clinical use of VFA would allow early diagnosis of neuropathic pain and ultimately guide treatment strategies in affected dogs.

This was the first study to use VFA to objectively measure ST values in control and SM- affected dogs. The results identified lower ST values in SM-affected dogs than in the control population, suggesting the presence of neuropathic pain. This confirms the suspicion of hyperesthesia in SM-affected dogs. The results also support the utility of

VFA as a quantitative sensory test to objectively measure ST values in dogs with SM- associated neuropathic pain. The clinical use of VFA will allow early detection of NP in those with signs of SM and in the future may serve as an objective test to monitor the response to treatment. 81

The severity of neuropathic pain, as documented by a low ST did not correlate with the imaging severity of the SM, although it did inversely correlate with owner reported clinical signs. The behaviors demonstrated by SM-affected dogs have long been attributed to neuropathic pain, and this belief is supported by the strong correlation between owner reported symptoms and ST values. On the other hand, the correlation between ST values and syrinx size have long been controversial in both human and veterinary medicine 19,32,59,105,206. The use of syrinx size as a predictor severity of neuropathic pain has fallen out of favor, with more emphasis being placed on the asymmetry of syringes.

Currently, a randomized, blinded prospective clinical trial has begun to evaluate the efficacy of gabapentin and amitriptyline for the treatment of SM-associated neuropathic pain (SM-NP). Gabapentin remains the first-line treatment for CKCS with SM-NP despite the lack of controlled clinical trials and controversial evidence of efficacy in the literature 13,149,154-156,215. In people, the tricyclic antidepressant (TCA) amitriptyline is preferred to treat NP based on documented efficacy and the low side effect profile 216,217.

Limited evidence exists in the veterinary literature for the efficacy of TCAs in the treatment of NP 163, but the utility in the people supports the translational investigation in dogs. The goal of this ongoing study is to investigate the effects of amitriptyline and gabapentin on quantitative sensory testing by following the change in ST values before

and after treatment. A secondary goal is to compare the change in ST values between amitriptyline and gabapentin to determine which drug is superior at treating neuropathic pain in SM-affected dogs. This information would guide treatment recommendations for dogs with SM as well as other neurologic conditions that have resulted in neuropathic pain.

The use of VFA in clinical and research settings is largely unexplored and could prove to be useful in numerous neurologic disorders. This pilot data in SM-associated NP further supports the use of VFA to investigate the pathogenesis of NP in dogs with CM/SM.

One potential avenue could be the role of syrinx asymmetry in the severity of neuropathic pain. If the ST values are lower in the asymmetrical groups, this would support the use of asymmetry as a predictor of neuropathic pain in dogs with SM.

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