Polymyositis beginning as a focal process.

Six patients with polymyositis initially complained of a single, localized, painful mass involving an extremity. The mass enlarged during the course of a two- to six-week period. Biopsy of the lesion revealed myopathic changes with inflammation, and the diagnosis of benign inflammatory pseudotumor was considered. The mass regressed, but during the next three to six months, a rapidly progressive generalized myopathy developed that caused weakness of the trunk and extremities in association with malaise and weight loss. Subsequent muscle biopsy specimens obtained from a site remote from the original mass were again characterized by lymphocytic infiltration, fiber necrosis, and regenerative activity. Our experience indicates that polymyositis may begin as a focal process that mimics a localized inflammatory pseudoneoplastic reaction. An essential clue to the diagnosis of polymyositis at this early stage is the elevation of ESR and serum creatine phosphokinase levels that does not occur in pseudotumor. The muscle biopsy further serves to distinguish polymyositis from pseudotumor.