Focal myositis.

Focal myositis, a new distinct clinicopathologic entity, is a benign inflammatory pseudotumor of skeletal muscle. Based on a study of 16 cases, the disease which affects both children and adults typically evolves over a period of several weeks as a localized painful swelling within the soft tissue of an extremity. At surgery the lesion is frequently considered a neoplasm, appearing pale in color and poorly demarcated from the surrounding muscle. Histologically there is lymphocytic infiltration of the perimysial and endomysial spaces, scattered muscle fiber necrosis and regeneration, and interstitial fibrosis. The etiology is unknown, but a history of trauma or family background of a similar disease was specifically excluded. Although polymyositis may be suspected initially because of muscle pain and inflammation, the process remains confined to a single area and signs of systemic disease are not encountered. Follow-up information obtained 2--6 years after surgery indicates no recurrence of the lesion in any of 16 cases.