Localized nodular myositis and the diagnosis of the localized muscle mass.

Localized nodular myositis (LNM) is a rare variant of polymyositis beginning with inflammatory nodules within muscles. Only seven cases have so far been reported in the literature. We describe a probable further case of LNM in a 67-year-old man with ischemic claudication of the left leg for three years who presented with painful nodules in the left gastrocnemius muscle and signs of systemic disease; a complete follow-up was not possible, because the patient died after only two months and autopsy was not performed. Muscle biopsy showed localized areas of necrotic and inflammatory pleomorphic changes, in keeping with the features of the other known cases. The ultrastructural findings (not previously reported in this disease) were characterized by marked changes of endomysial capillaries, with fibroblastic metamorphosis of the endothelial cells, and by the presence of filamentous inclusions in the myonuclei. The differential diagnosis of LNM from other localized muscle masses, chiefly from muscle infarct, is discussed.