Immunohistochemical analysis of laryngeal muscle of horses clinically affected with recurrent laryngeal neuropathy.
Authors: Steel Catherine M, Walmsley Elizabeth A, Anderson Garry A, Coles Chantal A, Ahern Benjamin, White Jason D
Journal: Equine veterinary journal
Summary
Recurrent laryngeal neuropathy (RLN) causes progressive denervation of the cricoarytenoid muscle responsible for opening the airway during exercise, yet the precise muscular changes underlying clinical signs remain poorly characterised. Steel and colleagues used immunohistochemistry to examine myosin heavy chain (MyHC) fibre-type composition and diameter in the left cricoarytenoid dorsalis muscle of five clinically affected Thoroughbreds and five controls, finding that affected horses displayed significant overall fibre atrophy (median reduction of 35.2 µm), particularly in type IIA fibres (46.8 µm reduction), alongside striking fibre-type grouping and loss of type IIX fibres—the muscle's fatigue-resistant population. Evidence of active regeneration was apparent through expression of embryonic myosin in small fibres with internal nuclei, suggesting the muscle was attempting to renew itself despite denervation. The authors propose that wholesale loss of bulk in the remaining fibres, rather than complete fibre loss, may be the critical functional deficit preventing adequate arytenoid abduction during exercise, though the study's small sample size and lack of subclinical cases limits definitive conclusions about disease progression. For practitioners, these findings underscore that RLN represents an active degenerative-regenerative process; understanding this dynamic may inform timing of surgical intervention and inform expectations around any potential spontaneous recovery in individual cases.
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Practical Takeaways
- •RLN involves progressive muscle atrophy and fibre loss in the laryngeal abductor muscle; understanding this pathophysiology helps explain why clinical signs worsen with exercise demand
- •The presence of embryonic myosin indicates ongoing regenerative attempts, suggesting the muscle maintains some capacity for adaptation that may be relevant to timing and type of interventions
- •Loss of fatigue-resistant fibres is the key functional deficit in RLN, not merely size reduction, which may inform prognosis and management expectations in affected horses
Key Findings
- •Horses with RLN showed overall fibre diameter decrease of 35.2 µm compared to controls (P = 0.02), with type IIA fibres decreasing by 46.8 µm (P = 0.03)
- •RLN-affected horses demonstrated obvious fibre-type grouping and depletion of type IIX fibres despite compensatory hypertrophy in some remaining fibres
- •Expression of embryonic myosin (MYH3-positive) in small fibres with internal nuclei indicates active regeneration and fibre renewal in affected horses
- •Loss of fatigue-resistant type IIX fibres combined with overall fibre atrophy may be the critical mechanism underlying failure to maintain arytenoid abduction during exercise