Discussion

Tulloch, L.K., Perkins, J.D., Massey, C.A., Barakzai, S.Z., Piercy, R.J.
Aims: Recurrent laryngeal neuropathy (RLN) is a common equine distal axonopathy associated with neurogenic atrophy of the intrinsic laryngeal muscles (particularly on the left). Affected horses develop laryngeal paralysis and poor performance. Although the aetiology is unknown, a genetic cause is suspected. The aim of this study was to investigate the hypothesis that severity of laryngeal dysfunction, determined by videolaryngoscopy, would be correlated with severity of laryngeal muscle histopathology. If so, laryngoscopy could be used to identify unaffected animals and to grade disease severity for genetic studies and quantitative trait analyses. Methods: Laryngeal function was graded from videolaryngoscopy recordings in 29 adult Thoroughbred horses. Subsequently, intrinsic laryngeal muscles were collected and analysed for fibre types, fibre type grouping, minimum fibre diameter, % collagen and % fat using immunofluorescent and histological techniques. Results: The left cricoarytenoideus dorsalis muscle had significantly more collagen and fat than the right in horses with complete laryngeal paralysis. Discriminant analysis revealed significant overlap in muscle pathology between horses with different degrees of laryngeal function, with the exception of horses with complete left sided abductor paralysis (grade 4). Fibre type grouping was observed frequently, particularly on the left side in horses of all laryngoscopic grades. Conclusions and practical significance: Laryngeal function, as determined by resting laryngoscopy, does not correlate with severity of histopathology and horses with normal laryngeal function may have subclinical RLN. Resting laryngoscopy may lead to inaccurate phenotyping in genome-wide association studies and quantitative trait analyses. This study provides valuable information that will facilitate use of muscle histopathology as an outcome measure in the future investigation of novel treatments for RLN. Acknowledgements: This work was funded by a grant from Med-El Elektro-medizinische Gerate GmbH, Innsbruck, Austria.