Roaring in Clydesdales

Pathological classification of equine recurrent laryngeal neuropathy

Recurrent Laryngeal Neuropathy (RLN) is a highly prevalent and predominantly left-sided, degenerative disorder of the recurrent laryngeal nerves (RLn) of tall horses, that causes inspiratory stridor at exercise because of intrinsic laryngeal muscle paresis. The associated laryngeal dysfunction and exercise intolerance in athletic horses commonly leads to surgical intervention, retirement or euthanasia with associated financial and welfare implications. Despite speculation, there is a lack of consensus and conflicting evidence supporting the primary classification of RLN, as either a distal ("dying back") axonopathy or as a primary myelinopathy and as either a (bilateral) mononeuropathy or a polyneuropathy; this uncertainty hinders etiological and pathophysiological research. In this review, we discuss the neuropathological changes and electrophysiological deficits reported in the RLn of affected horses, and the evidence for correct classification of the disorder. In so doing, we summarize and reveal the limitations of much historical research on RLN and propose future directions that might best help identify the etiology and pathophysiology of this enigmatic disorder.

Genomic analysis establishes correlation between growth and laryngeal neuropathy in Thoroughbreds

Background

Equine recurrent laryngeal neuropathy (RLN) is a bilateral mononeuropathy with an unknown pathogenesis that significantly affects performance in Thoroughbreds. A genetic contribution to the pathogenesis of RLN is suggested by the higher prevalence of the condition in offspring of RLN-affected than unaffected stallions. To better understand RLN pathogenesis and its genetic basis, we performed a genome-wide association (GWAS) of 282 RLN-affected and 268 control Thoroughbreds.

Results

We found a significant association of RLN with the LCORL/NCAPG locus on ECA3 previously shown to affect body size in horses. Using height at the withers of 505 of these horses, we confirmed the strong association of this locus with body size, and demonstrated a significant phenotypic and genetic correlation between height and RLN grade in this cohort. Secondary genetic associations for RLN on ECA18 and X did not correlate with withers height in our cohort, but did contain candidate genes likely influencing muscle physiology and growth: myostatin (MSTN) and integral membrane protein 2A (ITM2A).

Conclusions

This linkage between body size and RLN suggests that selective breeding to reduce RLN prevalence would likely reduce adult size in this population. However, our results do not preclude the possibility of modifier loci that attenuate RLN risk without reducing size or performance, or that the RLN risk allele is distinct but tightly linked to the body size locus on ECA3. This study is both the largest body size GWAS and the largest RLN GWAS within Thoroughbred horses to date, and suggests that improved understanding of the relationship between genetics, equine growth rate, and RLN prevalence may significantly advance our understanding and management of this disease.

Ventroaxial luxation of the apex of the corniculate process of the arytenoid cartilage in resting horses during induced swallowing or nasal occlusion

OBJECTIVE

To report ventroaxial luxation of the apex of the left or right corniculate process of the arytenoid cartilage under the contralateral corniculate process during resting endoscopic examination, and morphologic features of the larynx of 1 affected horse.

STUDY DESIGN

Retrospective study.

ANIMALS

Horses (n=8).

METHODS

Horses had endoscopic examination as part of a survey of Clydesdale horses (n=7), or investigation of poor performance in Thoroughbred horses (1). One Clydesdale was euthanatized and the larynx examined; 4 cadaver larynges from normal horses were also examined.

RESULTS

Ventroaxial luxation of the apex of the left or right corniculate process of the arytenoid cartilage was not detected during quiet breathing but was induced by swallowing or nasal occlusion. Prevalence in Clydesdales was 5.2% (7/133). A Thoroughbred with identical endoscopic appearance of the larynx at rest had progressive ventroaxial luxation of the apex of the arytenoid cartilage during high-speed treadmill endoscopy, associated with abnormal respiratory noise. Necropsy examination of an affected Clydesdale larynx revealed an excessively wide (10 mm) transverse arytenoid ligament that allowed easy separation of the apices of the corniculate processes. In normal cadaver larynges, the apices could not be separated with abaxial traction.

CLINICAL RELEVANCE

The clinical relevance of this laryngeal observation in resting horses is unclear. Ventroaxial luxation of the corniculate process of the arytenoid cartilage during induced swallowing or nasal occlusion in resting horses or during high-speed treadmill exercise may be caused by an abnormally wide transverse arytenoid ligament.

The Prevalence of Laryngeal Disease in a Large Population of Competition Draft Horses

OBJECTIVES

(1) To determine the effect of age, height, weight, breed, sex, and specific use on the prevalence of idiopathic left laryngeal hemiplegia (ILH) in a population of draft performance horses; (2) to determine the association between tracheal mucus and laryngeal dysfunction, and the prevalence of exercise-induced pulmonary hemorrhage (EIPH) in a population of draft performance horses.

STUDY DESIGN

Cross-sectional.

ANIMALS

Draft horses competing at the 2005 Michigan Great Lakes Draft Horse Show.

METHODS

Endoscopic examinations were performed on horses competing at the 2005 Michigan Great Lakes Draft Horse Show. Signalment, height, and weight were obtained from the owners and trainers.

RESULTS

Belgian, Percheron, and Clydesdale horses (n=183) were studied. Prevalence of ILH was 35%. Horse height was significantly associated with the risk of ILH in Belgian and Percheron horses but not Clydesdales. There was a significantly different prevalence of ILH among the breeds such that 42% Belgians, 31% Percherons, and 17% Clydesdales were affected. Laryngeal disease was a risk factor for increased tracheal mucus. None of the horses had acute evidence of EIPH.

CONCLUSIONS

The prevalence of ILH in draft horses has increased or is higher in competition horses compared with previously studied groups. Tracheal and/or pulmonary inflammation may be more common in draft horses with ILH based on our findings that horses with ILH have more tracheal mucus than horses with normal laryngeal function.

CLINICAL RELEVANCE

Selection pressure for large, taller, longer-necked horses may be responsible for a seemingly increased incidence of ILH in competitive draft horses.

A study of an electrodiagnostic technique for the evaluation of equine recurrent laryngeal neuropathy

The electrodiagnostic measurement of the thoracolaryngeal reflex (TLR) ('slap test') latency was compared to 5 other diagnostic techniques used for evaluation of laryngeal function, namely laryngeal muscle palpation, resting and immediately postexercise endoscopic examinations, and palpable and endoscopic responses to the TLR. Compared to resting endoscopy, the electrodiagnostic measurement of TLR latency was not found to be an accurate test for the evaluation of recurrent laryngeal neuropathy (RLN), nor was laryngeal muscle palpation or the endoscopic response to the TLR. Twenty-five (71%) of 35 Clydesdale horses examined were affected by RLN; 16 (46%) had mild, 5 (14%) moderate, 3 (9%) severe hemiparesis and 1 (3%) had total laryngeal paralysis. Within these 35 horses, physical traits such as height or neck length did not correlate with the incidence or severity of RLN. Ten control ponies showed no evidence of abnormal laryngeal function.

Examination of the laryngeal musculature and recurrent laryngeal nerves of zebra (Equus burchelli) for evidence of idiopathic laryngeal hemiplegia

AIMS

This study was undertaken in order to determine whether wild Burchell's Zebra (Equus burchelli) are affected by Idiopathic Laryngeal Hemiplegia (ILH). ILH has a high prevalence in the domestic horse and a negative control population has not been identified.

METHODS

The work was conducted in two phases on two large farms in central Kenya during routine culls. The larynges of 42 zebra were removed and the left and right intrinsic laryngeal adductor and abductor muscles and recurrent laryngeal nerves were isolated. The muscles were weighed and biopsy samples of left and right recurrent laryngeal nerve and cricoarytenoideus lateralis muscles from 16 animals were taken for histological examination.

RESULTS

Left adductor muscles were observed to be significantly lighter than right adductor muscles (p = 0.001). There was no significant difference between the weights of left and right laryngeal abductors (p = 0.260). There was no light microscopic evidence of pathological changes consistent with ILH in any of the muscle or nerve samples.

CONCLUSIONS

Zebra do not appear to suffer from ILH and equids unaffected by ILH can have significantly lighter muscles on the left of the larynx.

CLINICAL RELEVANCE

Mild asymmetries of the larynx on palpation may not be a reliable indicator of ILH. Zebra may be a potentially useful negative control animal for observational ILH studies.

Endoscopic observations on laryngeal symmetry and movements in young racing horses

An endoscopic survey of young race horses was performed to examine the prevalence and character of laryngeal movements during quiet respiration. The main aim was to determine whether those arytenoid movements which could possibly reflect the efficiency of left dorsal cricoarytenoid muscle function changed over a period of time. Of the 452 horses examined, 439 were Thoroughbreds and 23 were Standardbreds, 250 were less than 2 years of age (6-21 months), and 202 were 2 years old. One hundred and nine of these horses were examined again 16 months later. Arytenoid movements were given one of four grades. Grades 1 and 2 were considered normal and unlikely to be the result of abnormal left dorsal cricoarytenoid muscle function, whilst grades 3 and 4 were considered likely, or almost certainly, the result of abnormal left dorsal cricoarytenoid muscle function. The percutaneous prominence of the muscular process of left and right arytenoid cartilages, endoscopic arytenoid movement on left and right sides, age, sex and breed was recorded. Chi squared analysis was used to determine the association between age, breed, sex and the other recorded variables, and the presence or absence of abnormal laryngeal movements. At the first examination, 48% of the horses had grade 1, 37% grade 2, 15% grade 3 and 0.2% grade 4 left laryngeal movements. Of the horses examined I6 months later, 52% had grade 1, 33% grade 2, 14% grade 3 and 1% grade 4 left laryngeal movements. Fifteen percent of horses with grade 1 and 9% with grade 2 initially were found to be grade 3 at the subsequent examination. Conversely, 53% of horses with grade 3 initially were found to be grade 1 and 21% grade 2 at the subsequent examination. One horse that was grade 3 at the initial examination was grade 4 at the subsequent examination. Overall, 43% of horses were graded the same, 29% were given a better grade and 28% were given a worse grade. Age and sex were not associated with abnormal left laryngeal movements. The presence of abnormal arytenoid movements was significantly less in Standardbreds, but significantly higher in those horses that had a more prominent muscular process of the left arytenoid cartilage. The number of grade 2 and 3 laryngeal movements recorded on the left side was significantly higher than the right. It was concluded that asymmetrical laryngeal movements are common in young race horses; at this age laryngeal movements may interchange between what is considered normal and abnormal; the proportion of young horses with normal or minor variations in their left arytenoid movements that develop more obvious degrees of asynchrony is low (12%); and the proportion of horses considered to have endoscopic evidence of deficient left abductor muscle function that eventually develop laryngeal hemiplegia is also low (5%).

Determination of the early age of onset of equine recurrent laryngeal neuropathy. 1. Muscle pathology

The age of onset of equine recurrent laryngeal neuropathy has not been ascertained, although the clinical condition of left laryngeal hemiplegia ("roaring") has been recognized for centuries. The purpose of this study was to evaluate the laryngeal muscles of draft horse foals for the presence of fiber-type grouping, indicating denervation and reinnervation, and to determine if histological evidence of recurrent laryngeal neuropathy was present. Abductor and adductor laryngeal muscles from the left and right sides were collected immediately after euthanasia from male draft horse foals, six less than 2 weeks and four 6 months of age, and stained for myosin ATPase. A morphometric test was used to objectively evaluate several areas from each muscle for fiber-type grouping. Extensive fiber-type grouping which was characteristic of recurrent laryngeal neuropathy was found in one of the young foals and all of the older foals. Four of the young foals had some areas of fiber-type grouping suggestive of mild, early changes associated with recurrent laryngeal neuropathy. One of the young foals had no fiber-type grouping present in any of the laryngeal muscles evaluated. These findings suggest an early age of onset of recurrent laryngeal neuropathy.

The reliability of endoscopic examination in assessment of arytenoid cartilage movement in horses. Part I: Subjective and objective laryngeal evaluation

Videorecordings of the laryngeal activity of 108 unsedated horses were obtained at rest by passing a flexible videoendoscope into the nasopharynx through the right ventral meatus. All videotaped images were reviewed once, and 72 were reviewed twice, by three veterinarians. Laryngeal cartilage movement was assessed subjectively with a five-tier grading system. The mean intraobserver agreement was 83.3% (range, 75.0%-90.2%) with a kappa statistic of .65 to .98. The mean interobserver agreement was 79.0% (range, 70.4%-80.6%) with a kappa statistic of .51 to .90. A computer program was developed to measure the left:right ratio of the rima glottidis. The mean left:right ratio for horses assigned a median laryngeal grade of I was 0.84 (range, 0.55-1.03); for grade II, 0.82 (0.50-1.12); for grade III, 0.59 (0.39-0.91); and for grade IV, 0.24 (0.07-0.35).

Preferential denervation of the adductor muscles of the equine larynx. I: Muscle pathology

The laryngeal muscles of 18 horses were examined histologically. The neurogenic changes found in each muscle were scored by four reviewers and the results evaluated statistically. Fifteen of these horses had endoscopic evidence of abnormal laryngeal function, three of which were defined as having adductor paralysis. Measurement of muscle fibre area in two horses with idiopathic laryngeal hemiplegia (ILH) was performed. In the quantitative study of neurogenic change, the adductor muscles were more significantly affected than the abductor muscle. This was also true in the clinical cases of ILH where measurement of muscle fibre area demonstrated that the lateral cricoarytenoid (adductor) muscles showed a wider range of pathological changes than the dorsal cricoarytenoid muscle (abductor). Those horses with the most severe muscle pathology also had the most abnormal endoscopic findings. The propensity for denervation of the adductor muscles should provide clues as to the pathogenesis and natural history of horses with sub-clinical laryngeal disease and ILH.

A comparison of techniques to enhance the evaluation of equine laryngeal function

This study was designed to define a simple, unequivocal test for the evaluation of laryngeal function and the diagnosis of idiopathic laryngeal hemiplegia (ILH). ILH is a disorder that results from left recurrent laryngeal neuropathy and in which there is no movement of the left arytenoid cartilage and vocal fold. Laryngeal function was evaluated in seven horses using four techniques designed to stimulate laryngeal movements:-nasal occlusion, exercise, swallowing and administration of a respiratory stimulant. In addition, the effects of sedation and twitching on the endoscopic examination were also examined. The cross-sectional area of the rima glottidis was measured in each horse at rest and after each technique was performed. There was no statistically significant difference in the increase in area seen after nasal occlusion or exercise. Doxapram hydrochloride increased the cross-sectional area of the rima glottidis, whereas xylazine caused a decrease. Neither of these pharmacological agents exaggerated or decreased the amount of asynchronous movement or tremoring of the arytenoid cartilages. Manual occlusion of the external nares during endoscopy is a simple, yet effective method of stimulating arytenoid function and hence diagnosing ILH.

Laryngeal hemiplegia in draft horses. A review of 27 cases

Case records of 27 draft horses with laryngeal hemiplegia were reviewed. Twenty-one horses were treated by ventriculectomy with or without prosthetic laryngoplasty, and 17 owners were contacted to determine the results. Fifteen horses improved after surgery and were able to perform to the owners' expectations. Performance improved significantly and hospitalization was shorter after ventriculectomy alone. Results of this study indicate that the clinical signs of exercise intolerance and excessive inspiratory noise associated with left laryngeal hemiplegia in draft horses can be treated successfully by ventriculectomy without prosthetic laryngoplasty.

The pathogenesis of equine laryngeal hemiplegia — a review

Recent research on the muscular and nervous changes which occur in idiopathic equine laryngeal hemiplegia has indicated that many of the traditional concepts of the aetiology of this disease are erroneous. In light of the new knowledge gained, the various predispositions and possible causes of laryngeal hemiplegia are discussed, and it is suggested that the underlying mechanism of axonal damage in this neuropathy of horses may be related to abnormal energy metabolism in the axon.