Masseter myodegeneration as a cause of trismus or dysphagia in adult horses.
Authors: Pearson E G, Snyder S P, Saulez M N
Journal: The Veterinary record
Summary
# Editorial Summary: Masseter Myodegeneration in Adult Horses Masseter muscle degeneration presents a clinically significant but potentially under-recognised cause of dysphagia and trismus in adult horses, warranting consideration during diagnostic workup of eating difficulties and weight loss. Pearson and colleagues examined eight cases with histologically confirmed masseter myodegeneration, characterising the clinical and pathological progression of the condition across different disease stages. Clinical presentations included difficulty prehending or mastication food, reduced mouth opening, noticeable masseter atrophy, weight loss, and locomotor difficulties; all affected horses demonstrated elevated serum muscle enzyme activity, whilst some showed suboptimal serum selenium and vitamin E concentrations below reference ranges. Histopathological findings ranged from acute to chronic changes, with some horses exhibiting multifocal skeletal muscle involvement and occasional myocardial lesions alongside the characteristic masseter degeneration and fibrotic replacement. Practitioners should consider nutritional deficiency—particularly selenium and vitamin E insufficiency—as a potential aetiological factor and assess for systemic muscle involvement when encountering horses with trismus or dysphagia accompanied by muscle enzyme elevations and masseter atrophy, as early nutritional intervention may influence disease progression and clinical outcomes.
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Practical Takeaways
- •Masseter myodegeneration should be considered in horses presenting with trismus, dysphagia, or difficulty eating combined with visible jaw muscle atrophy
- •Serum muscle enzyme panels and selenium/vitamin E status should be evaluated in suspected cases, as deficiencies of these nutrients may be contributing factors
- •Histopathology of affected muscle may be necessary for definitive diagnosis and staging of disease, and systemic involvement should be ruled out
Key Findings
- •Eight horses with masseter myodegeneration presented with difficulty eating, mouth opening difficulty, weight loss, and visible masseter atrophy
- •All eight horses had abnormally elevated serum muscle enzyme activities
- •Some horses had suboptimal whole blood and/or liver selenium and vitamin E concentrations below reference ranges
- •Histological findings ranged from acute to chronic degeneration with fibrosis, and some horses had widespread skeletal and myocardial lesions beyond the masseter