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Veterinary Manual

Find information on animal health topics, written for the veterinary professional.

Polymyositis in Small Animals

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Polymyositis in Small Animals

By Joseph Harari, MS, DVM, DACVS, Veterinary Surgeon, Veterinary Surgical Specialists, Spokane, WA

Polymyositis is a systemic, noninfectious, possibly immune-mediated, inflammatory muscle disorder in adult dogs. It may be acute or chronic and progressive. Clinical signs include depression, lethargy, weakness, weight loss, lameness, myalgia, and muscle atrophy. CK may be increased, and electromyography reveals abnormal spontaneous muscle activity. Muscle biopsy reveals myonecrosis, lymphocytic-plasmacytic perimuscular infiltration, phagocytosis, and fiber regeneration. Polymyositis may be associated with megaesophagus and immune-mediated disorders (myasthenia gravis, lupus erythematosus, polyarthritis). Oral corticosteroids (1–2 mg/kg, bid for 3–4 wk) are the treatment of choice; other immunosuppressive agents such as azathioprine or cyclophosphamide can also be used. Prognosis is favorable, although relapses are not uncommon.

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In all animals, the motor unit of skeletal muscle consists of the motor neuron, the neuromuscular junction, and muscle fibers. Muscle dysfunction—such as ataxia, paresis, or paralysis—most commonly originates in which of the following locations?