Polymyositis in dogs.

Polymyositis was diagnosed in nine dogs. Factors utilized in making the diagnosis included (1) muscle pain or weakness, (2) high concentrations of serum muscle enzymes, (3) electromyographic abnormalities, and (4) histopathologic evidence of muscle necrosis and inflammation. Clinical signs included muscle pain, weakness, stilted gait, and pyrexia. Serum muscle enzyme concentrations were high in only three dogs. There was no apparent correlation between enzyme concentrations and severity of clinical involvement or degree of muscle necrosis on biopsy. Electromyographic changes included polyphasic motor unit potentials, fibrillation potentials, and positive waves. Variable degrees of muscle regeneration, degeneration, and inflammation were seen. Prednisone (2.2 mg/kg, OD, per os) was used effectively to treat four dogs. One dog improved initially but was euthanatized later when clinical signs became more pronounced. Three other dogs developed aspiration pneumonia secondary to megaesophagus and either died or were euthanatized.