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 Overview of Lameness in Horses Lameness is defined as an abnormal stance or gait caused by either a structural or a functional disorder of the locomotor system. The horse is either unwilling or unable to stand or move normally... read more , colic Overview of Colic in Horses Depiction of a horse’s GI tract as viewed from the right side. In its strictest definition, the term “colic” means abdominal pain. Throughout the years, it has become a broad term for a variety... read more , laminitis Laminitis in Horses The horse hoof. Median section through the horse digit. Equine laminitis is a crippling disease in which there is a failure of attachment of the epidermal laminae connected to the hoof wall... read more , fracture, pleuropneumonia, tetanus Tetanus in Animals Tetanus is caused by the neurotoxin produced by Clostridium tetani , which is found in soil and intestinal tracts and usually introduced into tissues through deep puncture wounds. The... read more , 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 Malignant Hyperthermia in Animals Malignant hyperthermia (MH) is a potentially fatal pharmacogenetic disorder of skeletal muscle calcium regulation. Triggered by exposure to certain drugs or stressors, clinical signs include... read more (Quarter Horses), myosin heavy chain myopathy, and polysaccharide storage myopathy.