Clinical and pathologic features of focal myositis.

[focal myositis]

To clarify the nosology of focal myositis (FM ), we report the clinical and pathologic features of eight patients presenting with focal enlargement of one muscle . Most patients improved without immunosuppressive therapy , and none developed polymyositis . Pathologic features were those of an inflammatory myopathy , with muscle fiber hypertrophy and moderate to severe inflammation . In most cases , a clustering of tightly packed muscle fibers , enveloped by a thick bundle of fibrosis , was associated with the diagnosis of FM . Immunohistochemistry showed T cell predominance within the interstitial infiltrates in all cases . No evidence of vasculitis was present . Our findings suggest that FM is a benign condition that has certain clinical features separating it from other inflammatory myopathies . Pathologic changes , such as large clusters of nesting muscle fibers surrounded by thick fibrosis , are more characteristic of FM than polymyositis .