Overview of Myopathies in Horses

Muscle disorders in horses are associated with a variety of clinical signs ranging from muscle stiffness and pain to muscle atrophy, weakness, exercise intolerance, and muscle fasciculations. The most common clinical signs are muscle pain, stiffness, and reluctance to move due to rhabdomyolysis. Rhabdomyolysis (ie, disruption of striated skeletal muscle) can broadly be grouped into causes associated with exercise (exertional rhabdomyolysis) and causes unrelated to exercise.

Differential diagnoses for reluctance to move, acute recumbency, and discolored urine include lameness, colic, laminitis, fracture, pleuropneumonia, tetanus, aortoiliac thrombosis, neurologic diseases resulting in recumbency or reluctance to move, intravascular hemolysis, and bilirubinuria. Causes of non–exercise-associated rhabdomyolysis include infectious (eg, Clostridium sp, influenza, Streptococcus equi, and Sarcocystis) and immune-mediated myopathies, nutritional myodegeneration (vitamin E or selenium deficiency), and traumatic or compressive myopathy. Toxic muscle damage from the ingestion of plants, including box elder, European sycamore maple, white snake root, and vitamin D–stimulating species as well as ionophores (eg, monensin, lasalocid, or rumensin) should also be considered (). Genetic causes of nonexertional rhabdomyolysis include glycogen branching enzyme deficiency (foals), malignant hyperthermia (Quarter Horses), myosin heavy chain myopathy, and polysaccharide storage myopathy.