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

Catherine M Steel, Elizabeth A Walmsley, Garry A Anderson, Chantal A Coles, Benjamin Ahern, Jason D White

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    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.
    Original languageEnglish
    Pages (from-to)710-717
    Number of pages8
    JournalEquine Veterinary Journal
    Issue number4
    Early online date01 Oct 2020
    Publication statusPublished - Jul 2021


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