Denervating changes in focal myositis, a benign inflammatory pseudotumor.

Focal myositis is a benign pseudotumor of skeletal muscle, the cause of which is obscure. It usually appears as a steadily enlarging mass within the muscle of an extremity. The pathologic features are those of an inflammatory myopathy in which lymphocytic infiltrates, muscle cell necrosis, and regeneration are seen. Four cases of focal myositis were studied using enzyme histochemistry and electron microscopy. The changes of denervation atrophy, reinnervation, and fiber hypertrophy were found. Intramuscular nerve damage manifested by axonal swelling, demyelination, and endoneurial fibrosis was evident. The findings suggest that a denervating process plays an important role in the pathogenesis of focal myositis.