Recurrent exertional rhabdomyolysis, seen most often in Thoroughbreds, is suspected to be due to abnormal intracellular calcium regulation. A validated genetic test is not yet available so Dx relies on clinical suspicion, ruling out other causes, and muscle biopsy.
Equine myopathies are divided into exertional vs. nonexertional, and sporadic vs. chronic. Exertional sporadic myopathy can be due to overexertion/undertraining or dietary/electrolyte imbalance.
DDx for exertional chronic myopathy include:
– Recurrent exertional rhabdomyolysis (Thoroughbreds, Standardbreds, Arabians)
– Polysaccharide storage myopathy type 1 (Quarterhorse-type breeds): due to abnormal glycogen storage; genetic test available
– Polysaccharide storage myopathy type 2 (Warmbloods, drafts, Arabians, Thoroughbreds): due to abnormal glycogen storage
– Myofibrillar myopathy (Arabians, Warmbloods): due to abnormal desmin in myofibrils
– Malignant hyperthermia (Quarterhorses): due to point mutation in the ryanodine receptor associated w/ intracellular calcium regulation; genetic test available
Nonexertional causes of myopathy are myriad, including infectious, inflammatory, toxins/plants, nutritional, and hereditary.
The Kentucky Equine Research group contains a wealth of information on the topic.
