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 of Equine Myopathies
Myopathy Type | Differential Diagnoses |
|---|---|
Non-exercise-associated Rhabdomyolysis | |
Inflammatory | Influenza myositis Immune-mediated myositis |
Toxic | Ionophore toxicity Senna occidentalis Rayless goldenrod/white snakeroot Hypoglycin A in box elder and European sycamore trees |
Traumatic | Compressive anesthetic myopathy Trauma |
Genetic | Glycogen branching enzyme deficiency in Quarter Horse foals Polysaccharide storage myopathy types 1 and 2 Malignant hyperthermia in Quarter Horses Myosin heavy chain myopathy (Quarter Horse-related breeds) |
Exertional Rhabdomyolysis | |
Focal muscle strain Overexertion, dietary imbalances, vitamins, minerals, electrolytes | |
Polysaccharide storage myopathy types 1 and 2 Recurrent exertional rhabdomyolysis Idiopathic chronic exertional rhabdomyolysis | |
Exertional Myopathy with Normal CK activity | |
Polysaccharide storage myopathy type 2 Myofibrillar myopathy | |
Muscle Atrophy | |
Myogenic atrophy | Severe rhabdomyolysis Disuse Immune-mediated myositis (rapid atrophy) Vitamin E–responsive myopathy Homozygous polysaccharide storage myopathy type 1 Myofibrillar myopathy |
Neurogenic atrophy | Equine protozoal myelitis Local nerve trauma Idiopathic peripheral neuropathy Toxic peripheral neuropathies |
Muscle Fasciculations | |
Pain, fear Weakness (botulism, chronic debilitation) Electrolyte abnormalities Hyperkalemic periodic paralysis Hypokalemia Otobius megnini (ear tick) infestation Stiff horse syndrome |
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.
For More Information
Also see pet health content regarding muscle disorders in horses.
