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 Myopathies in Horses: Differential Diagnoses of Equine Myopathies). Genetic causes of non-exertional rhabdomyolysis include glycogen branching enzyme deficiency (foals), malignant hyperthermia (Quarter horses), and polysaccharide storage myopathy.

Table 1
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Differential Diagnoses of Equine Myopathies Non-exercise-associated Rhabdomyolysis Inflammatory myopathies Clostridial myositis Influenza myositis Sarcocystis myositis Immune-mediated myopathy Nutritional myopathy Vitamin E and selenium deficiency Toxic myopathy Ionophore toxicity Pasture myopathies Rayless goldenrod/white snakeroot Cassia occidentalis Atypical myoglobinuria Traumatic myopathy Compressive anesthetic myopathy Trauma Genetic myopathies Glycogen branching enzyme deficiency in Quarter horses Polysaccharide storage myopathy types 1 and 2 Malignant hyperthermia in Quarter horses Exertional Rhabdomyolysis Focal muscle strain Sporadic tying-up (overexertion) Chronic tying-up Dietary imbalances, vitamins, minerals, electrolytes Polysaccharide storage myopathy type 1 and 2 Malignant hyperthermia Recurrent exertional rhabdomyolysis Idiopathic chronic exertional rhabdomyolysis Exertional Myopathy with Normal CK Mitochondrial myopathy Muscle Atrophy Myogenic atrophy Severe rhabdomyolysis Disuse Cushing's disease Immune-mediated myositis (rapid atrophy) Polysaccharide storage myopathy Neurogenic atrophy Equine protozoal myelitis Local nerve trauma Equine motor neuron disease Muscle Fasciculations Pain, fear Weakness (botulism, chronic debilitation) Electrolyte abnormalities Equine motor neuron disease Hyperkalemic periodic paralysis Hypokalemia Otobius megnini (ear tick) infestation Myotonic dystrophy Stiff horse syndrome Shivers Differential Diagnoses of Equine Myopathies Non-exercise-associated Rhabdomyolysis Inflammatory myopathies Clostridial myositis Influenza myositis Sarcocystis myositis Immune-mediated myopathy Nutritional myopathy Vitamin E and selenium deficiency Toxic myopathy Ionophore toxicity Pasture myopathies Rayless goldenrod/white snakeroot Cassia occidentalis Atypical myoglobinuria Traumatic myopathy Compressive anesthetic myopathy Trauma Genetic myopathies Glycogen branching enzyme deficiency in Quarter horses Polysaccharide storage myopathy types 1 and 2 Malignant hyperthermia in Quarter horses Exertional Rhabdomyolysis Focal muscle strain Sporadic tying-up (overexertion) Chronic tying-up Dietary imbalances, vitamins, minerals, electrolytes Polysaccharide storage myopathy type 1 and 2 Malignant hyperthermia Recurrent exertional rhabdomyolysis Idiopathic chronic exertional rhabdomyolysis Exertional Myopathy with Normal CK Mitochondrial myopathy Muscle Atrophy Myogenic atrophy Severe rhabdomyolysis Disuse Cushing's disease Immune-mediated myositis (rapid atrophy) Polysaccharide storage myopathy Neurogenic atrophy Equine protozoal myelitis Local nerve trauma Equine motor neuron disease Muscle Fasciculations Pain, fear Weakness (botulism, chronic debilitation) Electrolyte abnormalities Equine motor neuron disease Hyperkalemic periodic paralysis Hypokalemia Otobius megnini (ear tick) infestation Myotonic dystrophy Stiff horse syndrome Shivers

Last full review/revision March 2012 by Stephanie J. Valberg, DVM, PhD, DACVIM