Masticatory myositis is an immune-mediated, inflammatory condition that affects the muscles of mastication. The exact cause is unknown. Specific autoantibodies directed against type II muscle fibers have been detected in affected animals. In acute cases, muscles are swollen, and there is difficulty in opening the jaw. In chronic cases, there is anorexia, weight loss, difficulty in opening the jaw, and muscle atrophy.
Diagnostic hematologic values include eosinophilia and increased levels of globulin and muscle enzymes. Electromyography reveals abnormal spontaneous electrical activity in affected muscles. A biopsy sample of the temporalis muscle is usually taken; histologic changes include lymphocytic-plasmacytic cellular infiltrates, muscle atrophy, and fibrosis. Although spontaneous regression can occur, prednisone treatment, on a decreasing dosage schedule (2 mg/kg/day for 2 weeks, then 1 mg/kg/day for 2 weeks, then 0.5 mg/kg/day for 1 month) is usually effective. Relapses are common, and long-term (6–8 months) medication may be required.