Laryngeal reinnervation using the spinal accessory nerve: Electromyographic study of the sternomandibularis muscle

BACKGROUND

Selective laryngeal reinnervation using the first and second cervical nerve (C1C2) is a treatment option for recurrent laryngeal neuropathy that aims to restore the function of the cricoarytenoideus dorsalis muscle (CAD). Despite the technique's satisfying success rate, it has several limitations. These triggered the search for another potential donor nerve that could reduce CAD muscle fatigue and shorten rehabilitation. The ventral branch of the spinal accessory nerve, providing motor innervation to the sternomandibularis (SM) muscle, was identified as a potentially well adapted nerve.

OBJECTIVES

To gather normative data from SM muscle activity induced by physiological spinal accessory nerve stimulation at rest and during exercise, and to determine SM muscle activity and fibre type recruitment relative to posture, gait, and respiratory cycle.

STUDY DESIGN

Clinical observational study.

METHODS

Surface electromyography of the SM muscle was performed in 9 horses: trained warmbloods, Thoroughbreds and Standardbreds. Signals were recorded in different feeding postures and at exercise, which included standardised treadmill exercise tests, lungeing and ridden work.

RESULTS

Timing of the SM muscle contraction coincided with inspiration at gallop. Intra-individual mean SM muscle activity increased with exercising speed (4 times higher in Thoroughbreds at gallop compared to walk and 7 times higher in Standardbreds at high-speed trot compared to walk). Moreover, the SM muscle was strongly activated at rest when the horse was grazing (7 times more compared to when it was eating out of a hay net or stable feeder). Frequency domain analysis revealed a predominant type I muscle fibre recruitment during feeding and at exercise (Type I muscle fibre activity was at least 2.5 times higher than type II muscle fibre activity).

MAIN LIMITATIONS

Lack of reference data on equine electromyography.

CONCLUSIONS

This study confirmed the potentially advantageous properties of the spinal accessory nerve for laryngeal reinnervation.

Influence of unilateral and bilateral vocal cordectomy on airflow across cadaveric equine larynges at different Rakestraw grades of arytenoid abduction

Objective

To assess the effect of vocal cordectomy on airflow across equine larynges at different Rakestraw grades of arytenoid abduction using a unidirectional airflow model.

Study design

Ex vivo, repeated measures.

Sample population

Twenty cadaveric equine larynges.

Methods

The right arytenoid cartilage was maximally abducted in all larynges. Each larynx was assigned a Rakestraw grade A or B, and the left arytenoid was abducted accordingly. Each larynx was tested under 3 conditions: intact, left vocal cordectomy (LVC), and bilateral vocal cordectomy (BVC). Translaryngeal pressure and airflow were measured, and digital video footage was obtained. Translaryngeal impedance (TLI) was calculated, and the arytenoid left‐to‐right quotient angle (LRQ) and rima glottis cross‐sectional area (CSA) were measured from standardized still images.

Results

Vocal cordectomy reduced TLI by 14.5% in LVC in comparison with intact larynges at Rakestraw grade B (P = .014). In Rakestraw grade A position, neither unilateral nor bilateral vocal cordectomy had any effect on TLI. Regardless of Rakestraw allocation, both LVC and BVC increased CSA in comparison with intact larynges (P < .005), with BVC larynges experiencing a greater effect than LVC (P < .0001).

Conclusion

Using a unilateral airflow model, LVC improved TLI in larynges where arytenoid position approximated Rakestraw grade B. However, when the arytenoid position approximated Rakestraw grade A, there was no effect on TLI following LVC or BVC.

Clinical significance

Surgeons considering a vocal cordectomy should take into account the degree of arytenoid abduction before performing the procedure, as it may not be warranted from a TLI point of view.

Optimisation and validation of immunohistochemical axonal markers for morphological and functional characterisation of equine peripheral nerves

BACKGROUND

Horses are affected by various peripheral nerve disorders but defining their aetiology and pathophysiology is hampered by limited understanding of associated morphological and pathological changes and involvement of specific axonal types.

OBJECTIVES

To investigate the hypothesis that selected antibody markers, used in conjunction with various tissue processing methods, would enable identification of axons with different functional modalities within a range of equine peripheral nerves.

STUDY DESIGN

Optimisation and validation study.

METHODS

A range of antibodies were evaluated immunohistochemically via fluorescence confocal microscopy in cadaver equine nerve samples of primary motor, mixed or primary sensory functions (recurrent laryngeal, phrenic and plantar digital) within formalin-fixed paraffin-embedded (FFPE) and formalin-fixed frozen (FFF) tissues subjected to different antigen retrieval protocols.

RESULTS

Immunohistochemistry of FFPE-derived nerve samples with selected antibodies and specific antigen retrieval methods enabled identification of myelinated and unmyelinated axons, cholinergic, sympathetic and peptidergic axons. The recurrent laryngeal and phrenic nerves are composed of myelinated cholinergic (motor), myelinated sensory fibres, unmyelinated adrenergic (sympathetic) axons and unmyelinated peptidergic (sensory) axons. In contrast, as expected, the plantar digital nerve had no myelinated motor fibres being mainly composed of myelinated sensory fibres, unmyelinated sympathetic and unmyelinated peptidergic sensory axons.

MAIN LIMITATION

Attempts specifically to label parasympathetic fibres were unsuccessful in any nerve examined in both FFPE and FFF tissues.

CONCLUSIONS

A panel of antibody markers can be used to reveal morphological and functional properties of equine nerves. Future work should enable better characterisation of morphological changes in equine neuropathies at various stages of disease development.

Immunohistochemical analysis of laryngeal muscle of horses clinically affected with recurrent laryngeal neuropathy

BACKGROUND

As myosin heavy chain (MyHC) profile of muscle fibres is heavily influenced by neural input, changes in MyHC expression are expected in horses clinically affected with recurrent laryngeal neuropathy (RLN) yet, this has not been thoroughly investigated.

OBJECTIVES

To describe the changes in MyHC and fibre diameter in left cricoarytenoideus dorsalis (L-CAD) muscle of horses with clinical signs of RLN.

STUDY DESIGN

Observational cohort study.

METHODS

Immunohistochemistry was used to assess the MyHC-based fibre-type proportion, size and grouping in the L-CAD of 10 Thoroughbred horses, five clinically affected with RLN and five unaffected controls based on resting endoscopic examination. The Mann-Whitney U test was used to compare the two groups.

RESULTS

Compared to controls (of mean age 3.0 ± 1.7 years) which only expressed type I, IIA and IIX MyHC, the L-CAD of affected horses (of mean age 2.8 ± 0.8 years) had obvious fibre-type grouping, and despite apparent compensatory hypertrophy of a small number of fibres, a decrease in overall fibre diameter (median difference -35.2 µm, 95% CI -47.4 to -7.9, P = .02) and diameter of type IIA fibres (median difference -46.8 µm, 95% CI -52.1 to -5.0, P = .03) was observed. Anti-fast MyHC (MY32) cross-immunoreacted with embryonic-MyHC. Whereas MY32-positive fibres were identified as type IIX in controls, in affected horses these fibres were less than 50 µm diameter with internal nuclei and were MYH3-positive for embryonic myosin indicating depletion of type IIX fibres, yet active regeneration and fibre renewal.

MAIN LIMITATIONS

Small sample size that did not include subclinical cases. Fibre size and appearance rather than staining colour were relied upon to differentiate embryonic from type IIX MyHC.

CONCLUSIONS

Horses clinically affected with RLN have overall atrophy of fibres, loss of IIX fibres and expression of embryonic myosin indicating regenerative capacity. Despite hypertrophy of some remaining fibres, the overall decline in the bulk of fibres, including those most fatigue-resistant, may be the critical change that results in failure to maintain arytenoid abduction during exercise although direct comparison to subclinical cases is needed to confirm this.

Assessing pathological changes within the nucleus ambiguus of horses with recurrent laryngeal neuropathy: An extreme, length-dependent axonopathy

INTRODUCTION

Equine recurrent laryngeal neuropathy (RLN) is a naturally occurring model of length-dependent axonopathy characterized by asymmetrical degeneration of recurrent laryngeal nerve axons (RLn). Distal RLn degeneration is marked, but it is unclear whether degeneration extends to include cell bodies (consistent with a neuronopathy).

METHODS

With examiners blinded to RLN severity, brainstem location, and side, we examined correlations between RLN severity (assessed using left distal RLn myelinated axon count) and histopathological features (including chromatolysis and glial responses) in the nucleus ambiguus cell bodies, and myelinated axon count of the right distal RLn of 16 horses.

RESULTS

RLN severity was not associated with RLn cell body number (P > .05), or degeneration. A positive correlation between the left and right distal RLn myelinated axon counts was identified (R² = 0.57, P < .05).

DISCUSSION

We confirm that RLN, a length-dependent distal axonopathy, occurs in the absence of detectable neuronopathy.

Asymmetric recurrent laryngeal nerve conduction velocities and dorsal cricoarytenoid muscle electromyographic characteristics in clinically normal horses

The dorsal cricoarytenoid (DCA) muscles, are a fundamental component of the athletic horse's respiratory system: as the sole abductors of the airways, they maintain the size of the rima glottis which is essential for enabling maximal air intake during intense exercise. Dysfunction of the DCA muscle leads to arytenoid collapse during exercise, resulting in poor performance. An electrodiagnostic study including electromyography of the dorsal cricoarytenoid muscles and conduction velocity testing of the innervating recurrent laryngeal nerves (RLn) was conducted in horses with normal laryngeal function. We detected reduced nerve conduction velocity of the left RLn, compared to the right, and pathologic spontaneous activity (PSA) of myoelectrical activity within the left DCA muscle in half of this horse population and the horses with the slowest nerve conduction velocities. The findings in this group of horses are consistent with left sided demyelination and axonal loss, consistent with Recurrent Laryngeal Neuropathy (RLN), a highly prevalent degenerative disorder of the RLn in horses that predominantly affects the left side. The detection of electromyographic changes compatible with RLN in clinically unaffected horses is consistent with previous studies that identified "subclinical" subjects, presenting normal laryngeal function despite neuropathologic changes within nerve and muscle confirmed histologically.

A retrospective cohort study of racing performance in Quarter Horses undergoing prosthetic laryngoplasty for treatment of recurrent laryngeal neuropathy

OBJECTIVE To determine effects of prosthetic laryngoplasty on return to racing, performance index, and career longevity in racing Quarter Horses with recurrent laryngeal neuropathy (RLN) and to evaluate performance variables for horses with RLN undergoing prosthetic laryngoplasty, compared with a control horse population. DESIGN Multicenter, retrospective cohort study. ANIMALS 162 racing Quarter Horses with RLN treated with prosthetic laryngoplasty (case horses) and 324 racing Quarter Horse without RLN (control horses). PROCEDURES Medical and race records of case and control horses examined at 5 referral centers between January 2000 and December 2015 were reviewed retrospectively. Two control horses were matched with each case horse. Return to racing, earnings, number of racing starts, performance index, and career longevity were evaluated. RESULTS The odds of returning to racing did not differ significantly between case and control horses but decreased with increasing age. Neither racing starts nor career longevity were affected by prosthetic laryngoplasty or by RLN grade. In fact, horses undergoing laryngoplasty for treatment of RLN and horses with the lowest RLN grade before surgery had higher performance indices after the surgery, compared with indices for control horses. CONCLUSIONS AND CLINICAL RELEVANCE The faster speeds and shorter distances raced with Quarter Horses could alter how RLN impacts respiratory variables and performance in Quarter Horses, compared with other racehorse breeds. Further study is needed to understand the impacts of RLN and surgical treatments for RLN in racing Quarter Horses.

Identification of loci affecting sexually dimorphic patterns for height and recurrent laryngeal neuropathy risk in American Belgian Draft Horses

Equine recurrent laryngeal neuropathy (RLN) is a bilateral mononeuropathy with an unknown etiology. In Thoroughbreds (TB), we previously demonstrated that the haplotype association for height (LCORL/NCAPG locus on ECA3, which affects body size) and RLN was coincident. In the present study, we performed a genome-wide association scan (GWAS) for RLN in 458 American Belgian Draft Horses, a breed fixed for the LCORL/NCAPG risk alelle. In this breed, RLN risk is associated with sexually dimorphic differences in height, and we identified a novel locus contributing to height in a sex-specific manner: MYPN (ECA1). Yet this specific locus contributes little to RLN risk, suggesting that other growth traits correlated to height may underlie the correlation to this disease. Controlling for height, we identified a locus on ECA15 contributing to RLN risk specifically in males. These results suggest that loci with sex-specific gene expression play an important role in altering growth traits impacting RLN etiology, but not necessarily adult height. These newly identified genes are promising targets for novel preventative and treatment strategies.

Nerve Stimulator-guided Injection of Autologous Stem Cells Near the Equine Left Recurrent Laryngeal Nerve

Recurrent laryngeal neuropathy (RLN) commonly affects horses and is characterized by abnormal respiratory sounds and exercise intolerance. The recurrent laryngeal nerve shows lesions of demyelination. The benefit of applying stem cells to demyelinated nerves has been demonstrated in various animal models. The aim of the study was to test the feasibility and safety of a peri-neuronal injection of autologous muscle-derived mesenchymal stem cells to the left recurrent laryngeal nerve in healthy horses by using an electrical nerve stimulator.

Muscle-derived stems cell are obtained from five healthy Standardbred horses by sampling 20 mg of muscle tissue with a semi-automatic 14 G biopsy needle from the triceps muscle. Movements of the larynx are monitored via upper-airway video endoscopy. The left recurrent laryngeal nerve is approached with an insulated nerve block needle. Nerve stimulation is applied, starting at 2 mA, and the successful abduction of the left arytenoid is monitored. The stimulation intensity is reduced progressively. When a loss of the motor response is observed at 0.5 mA, 10⁷ autologous muscle-derived stem cells are injected. Two examiners, who are blinded to the time point, score the laryngeal function of the horses prior to the treatment and at day 1, day 7, and day 28 after the injection of the cells. In a sixth horse, 1 mL of 2% lidocaine is injected to further confirm the correct positioning of the needle. This leads to a temporary paralysis of the left arytenoid cartilage.

This study proves that the recurrent laryngeal nerve can be approached with the help of an electrical nerve stimulator and that the electrical stimulation of the nerve is well tolerated by the horses. No modification of the laryngeal function was observed in any of the horses after the injection of the stem cells. Further studies should be conducted to describe the effects of a peri-neuronal injection of autologous muscle-derived mesenchymal stem cells to horses suffering from RLN.

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.

Prevalence of upper respiratory diseases and associated factors in Colombian Creole horses

ABSTRACT To determine the prevalence of diseases of the upper respiratory tract and associated factors in the Colombian Creole Horse Valley Aburrá in 2015, a random sampling of 15 farms in southern Aburrá Valley under different management conditions was performed. A total of 105 CCC underwent general clinical examination, respiratory endoscopic evaluation and a detailed inspection of housing conditions and management. A low prevalence of upper respiratory tract diseases was observed, including Pharyngeal Lymphoid Hyperplasia 38.1%, Recurrent Laryngeal Neurophaty 12.38%, Dorsal Displacement of the Soft Palate 7.62%, and presence of tracheal secretions not associated with a specific pathology. A bivariate analysis showed association between tracheal secretions and frequency of training per week (OR = 9.86), duration of the training sessions (OR = 5.55) and inadequate ventilation of the stable (OR = 10.52). Association between HLR and inadequately ventilated barns was also observed (OR = 14.9). Logistic regression showed association between tracheal secretions and inadequate ventilation (OR = 7.18). We conclude that inadequate ventilation was the most important factor for the incidence of upper respiratory tract diseases in horses at the southern area of the Aburrá Valley.

Fifty years of recurring struggles with recurrent laryngeal neuropathy

Recurrent laryngeal neuropathy appears to be a simple problem that should have a simple solution, yet the complexity and dynamic nature of laryngeal function is underappreciated. This review highlights the challenges and accomplishments that work towards that greater understanding of what is necessary to find a successful solution.

Histopathological assessment of intrinsic laryngeal musculature in horses with dynamic laryngeal collapse

REASONS FOR PERFORMING STUDY

The pathogenesis of bilateral dynamic laryngeal collapse associated with poll flexion (DLC) of horses is unknown but might be associated with intrinsic laryngeal muscle weakness.

OBJECTIVES

To investigate histopathological characteristics of the cricoarytenoideus dorsalis, the cricothyroid (CT) and the cricoarytenoideus lateralis muscles in DLC-affected horses and compare these with unaffected controls. Our hypotheses were that evidence of neurogenic atrophy of the CT or cricoarytenoideus dorsalis muscles would be found in DLC-affected horses and that observed changes would be symmetrically (left/right) distributed, or that muscle fibre diameter would be significantly reduced in DLC-affected horses compared to unaffected controls, reflecting an underlying paresis.

STUDY DESIGN

Case-control study.

METHODS

Five DLC horses and 8 controls were included. Muscle samples were harvested immediately following euthanasia. Fibre type proportions and size were evaluated by multiple immunofluorescence labelling of cryosections, and compared between sides (left/right) and groups (DLC-affected cases/ unaffected controls). Subjective and objective assessments of fibre type grouping were compared between sides and groups.

RESULTS

Fibre type proportions, fibre size and the subjective assessment of fibre type grouping did not reveal any statistically significant differences between the groups. Objective assessment of fibre type grouping revealed significantly more large clusters of T1 fibres within the left cricoarytenoideus lateralis muscle of DLC-affected cases versus controls, and within the right CT muscle of control horses compared to the DLC-cases.

CONCLUSIONS

The absence of bilateral symmetric fibre type grouping, fibre type loss and fibre atrophy in the DLC-affected cases do not support a neuromuscular component within the pathogenesis of DLC. The objective assessment of fibre type grouping revealed some statistical differences between the DLC-affected cases and the unaffected controls; however, these findings were inconsistent with regard to DLC. An alternative aetiology of DLC seems likely.

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.

Ultrasound-guided biopsy of the cricoarytenoideus lateralis muscle: technique and safety in horses

REASONS FOR PERFORMING STUDY

Current diagnosis of recurrent laryngeal neuropathy (RLN) depends upon disease recognition in the clinically affected horse. Biopsy of the intrinsic laryngeal muscles may provide a method to identify the changes in fibre-type composition that occur in RLN before clinical signs become apparent.

OBJECTIVE

To develop an ultrasound-guided biopsy technique of the left cricoarytenoideus lateralis muscle (CALM) and evaluate its efficacy and safety in vivo.

STUDY DESIGN

A longitudinal descriptive study.

METHODS

Six standing horses underwent ultrasound-guided biopsy of the left CALM. Frozen muscle cores were obtained with a breast biopsy tool. Serial endoscopic, ultrasonographic and physical examinations before and for 8 weeks after the biopsy were assessed for iatrogenic trauma. Histologies of representative muscle core cross-sections were analysed for the total number of muscle fibres obtained with each biopsy.

RESULTS

There were no immediate complications of the procedure and the left CALM was harvested in all instances. Biopsy samples had an average weight of 0.043 g (range = 0.023-0.077 g) and contained 3418 fibres in cross-section (range = 711-7143). Laryngeal endoscopic grade did not change significantly between prebiopsy and the end of the 8 week follow-up. The left CALM had significantly greater echogenicity than the right throughout the study (P<0.001), but there was no difference between the prebiopsy CALM echogenicity and that at completion of the study.

CONCLUSIONS

Ultrasound-guided biopsy of the left CALM is safe and well tolerated, providing a minimally invasive method to obtain muscle from healthy horses. This new technique may be applicable in research and clinical settings.

Miscellaneous neurologic or neuromuscular disorders in horses

NMD is an important cause of morbidity in horses. Signs of dysfunction could be variable depending on the specific area affected. NM disease can go unrecognized if a thorough evaluation is not performed in diseased horses. Electrodiagnostic testing is an area that has the potential to document and improve our understanding of NM disease yet is uncommonly performed. Keeping an open and observant mind will enhance our ability to search and find answers.

Immunohistochemical analysis of laryngeal muscles in normal horses and horses with subclinical recurrent laryngeal neuropathy

We used immunohistochemistry to examine myosin heavy-chain (MyHC)-based fiber-type profiles of the right and left cricoarytenoideus dorsalis (CAD) and arytenoideus transversus (TrA) muscles of six horses without laryngoscopic evidence of recurrent laryngeal neuropathy (RLN). Results showed that CAD and TrA muscles have the same slow, 2a, and 2x fibers as equine limb muscles, but not the faster contracting fibers expressing extraocular and 2B MyHCs found in laryngeal muscles of small mammals. Muscles from three horses showed fiber-type grouping bilaterally in the TrA muscles, but only in the left CAD. Fiber-type grouping suggests that denervation and reinnervation of fibers had occurred, and that these horses had subclinical RLN. There was a virtual elimination of 2x fibers in these muscles, accompanied by a significant increase in the percentage of 2a and slow fibers, and hypertrophy of these fiber types. The results suggest that multiple pathophysiological mechanisms are at work in early RLN, including selective denervation and reinnervation of 2x muscle fibers, corruption of neural impulse traffic that regulates 2x and slow muscle fiber types, and compensatory hypertrophy of remaining fibers. We conclude that horses afflicted with mild RLN are able to remain subclinical by compensatory hypertrophy of surviving muscle fibers.