Muscle disorders in horses present 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 presentation is muscle pain, stiffness, and reluctance to move due to rhabdomyolysis. Rhabdomyolysis, defined as 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, aorto-iliac 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, Sarcocystis) and immune-mediated myopathies, nutritional myodegeneration (vitamin E or selenium deficiency), traumatic or compressive myopathy, idiopathic pasture myopathy, and toxic muscle damage from the ingestion of ionophores (eg, monensin, lasalocid, rumensin). Plants, including white snake root and vitamin D–stimulating species, should also be considered (see Differential Diagnoses of Equine Myopathies). Genetic causes of nonexertional rhabdomyolysis include glycogen branching enzyme deficiency (foals), malignant hyperthermia (Quarter horses), and polysaccharide storage myopathy.