Table: Differential Diagnoses of Equine Myopathies - Merck Veterinary Manual

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Differential Diagnoses of Equine Myopathies

Differential Diagnoses of Equine Myopathies

Myopathy Type	Differential Diagnoses
*Non-exercise-associated Rhabdomyolysis*
Inflammatory	Clostridial myositis Clostridial Myositis Necrosis of skeletal and cardiac muscle may occur in association with viral diseases such as equine influenza and equine infectious anemia. In most situations, viral-induced muscle damage represents... read more Influenza myositis Sarcocystis myositis Sarcocystis Myositis Necrosis of skeletal and cardiac muscle may occur in association with viral diseases such as equine influenza and equine infectious anemia. In most situations, viral-induced muscle damage represents... read more Anaplasmosis Anaplasmosis in Ruminants Anaplasmosis is a tickborne disease of ruminants caused by intracellular bacteria that infect red blood cells, causing fever and anemia. Diagnosis relies upon Giemsa-stained blood smears and... read more 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 Glycogen Branching Enzyme Deficiency Few mitochondrial myopathies have been described in horses. In a reported case, a deficiency of mitochondrial complex I (NADH coenzyme Q oxidoreductase)—an enzyme catalyzing the first step in... read more in Quarter Horse foals Polysaccharide storage myopathy types 1 Chronic Exertional Rhabdomyolysis Exertional myopathy in horses is a syndrome of muscle fatigue, pain, or cramping associated with exercise. Less common exertional myopathies that cause exercise intolerance without muscle necrosis... read more and 2 Chronic Exertional Rhabdomyolysis Malignant hyperthermia Chronic Exertional Rhabdomyolysis in Quarter Horses Myosin heavy chain myopathy (Quarter Horse-related breeds)
*Exertional Rhabdomyolysis*
Sporadic exertional rhabdomyolysis Sporadic Exertional Rhabdomyolysis Exertional myopathy in horses is a syndrome of muscle fatigue, pain, or cramping associated with exercise. Less common exertional myopathies that cause exercise intolerance without muscle necrosis... read more	Focal muscle strain Overexertion, dietary imbalances, vitamins, minerals, electrolytes
Chronic exertional rhabdomyolysis Chronic Exertional Rhabdomyolysis Exertional myopathy in horses is a syndrome of muscle fatigue, pain, or cramping associated with exercise. Less common exertional myopathies that cause exercise intolerance without muscle necrosis... read more	Polysaccharide storage myopathy types 1 Chronic Exertional Rhabdomyolysis and 2 Chronic Exertional Rhabdomyolysis Malignant hyperthermia Chronic Exertional Rhabdomyolysis Recurrent exertional rhabdomyolysis Chronic Exertional Rhabdomyolysis Idiopathic chronic exertional rhabdomyolysis
*Exertional Myopathy with Normal CK activity*
Mitochondrial myopathy Mitochondrial Myopathy Few mitochondrial myopathies have been described in horses. In a reported case, a deficiency of mitochondrial complex I (NADH coenzyme Q oxidoreductase)—an enzyme catalyzing the first step in... read more	Polysaccharide storage myopathy type 2 Chronic Exertional Rhabdomyolysis Myofibrillar myopathy
*Muscle Atrophy*
Myogenic atrophy	Severe rhabdomyolysis Disuse Cushing disease Cushing Syndrome (Hyperadrenocorticism) Cushing syndrome refers to any cause of elevated cortisol concentrations. Pituitary-dependent hyperadrenocorticism (PDH; Cushing disease) is the most common form of hyperadrenocorticism and... read more Immune-mediated myositis Immune-mediated myositis Purpura hemorrhagica is an acute, immune-mediated, necrotizing vasculitis (type III hypersensitivity) associated with sensitization and cross-reactivity to proteins in the cell wall of Streptococcus... read more (rapid atrophy) Vitamin E–responsive myopathy Homozygous polysaccharide storage myopathy type 1 Chronic Exertional Rhabdomyolysis Myofibrillar myopathy
Neurogenic atrophy	Equine protozoal myelitis Local nerve trauma Equine motor neuron disease Overview of Central Nervous System Diseases in Animals Caused by Helminths and Arthropods A number of metazoan parasites (helminths and arthropods) can cause central nervous system diseases in animals and may be categorized as described as follows. Zoonotic conditions are also noted... read more Idiopathic peripheral neuropathy Toxic peripheral neuropathies
*Muscle Fasciculations*
	Pain, fear Weakness ( botulism Botulism in Animals Botulism most commonly results from ingestion of toxin in food. The usual source of the toxin is decaying carcasses or vegetable material. Clinical signs are caused by flaccid muscle paralysis... read more , chronic debilitation) Electrolyte abnormalities Equine motor neuron disease Overview of Central Nervous System Diseases in Animals Caused by Helminths and Arthropods A number of metazoan parasites (helminths and arthropods) can cause central nervous system diseases in animals and may be categorized as described as follows. Zoonotic conditions are also noted... read more Vitamin E–responsive myopathy Vitamin E–Responsive Myopathy Young, rapidly growing foals born to dams that consumed selenium-deficient diets during gestation can develop nutritional myodegeneration. Selenium deficiency has also been implicated in masseter... read more Hyperkalemic periodic paralysis Hyperkalemic Periodic Paralysis Myotonic muscle disorders share the feature of delayed relaxation of muscle after mechanical stimulation or voluntary contraction due to abnormal muscle membrane conduction. Horses have three... read more Hypokalemia Otobius megnini (ear tick) infestation Myotonic dystrophy Myotonia Congenita and Dystrophica Myotonic muscle disorders share the feature of delayed relaxation of muscle after mechanical stimulation or voluntary contraction due to abnormal muscle membrane conduction. Horses have three... read more Stiff horse syndrome Shivers Shivers Muscle cramping is a painful condition that arises from hyperactivity of motor units caused by repetitive firing of the peripheral nervous system or CNS. The origin of the cramp in most cases... read more

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