Focal myositis: a polymerase chain reaction analysis for a viral etiology.

Focal myositis (FM) is a benign inflammatory condition that may clinically simulate a soft tissue sarcoma. It was first described in 1977, and only approximately 30 cases have been reported to date, yet this entity is probably more frequent. The pathogenesis of FM is totally unknown. It has been proposed that it represents a nodular form of myositis, which can evolve into polymyositis, but this hypothesis has not been confirmed by follow-up studies. We describe seven cases of FM, five of which have never been reported before. Histology of the lesions was very similar, showing a destructive inflammatory myopathy with evidence of regeneration. Our study attempted to better understand the pathogenesis of this focal inflammatory myopathy. We performed a polymerase chain reaction study to explore the presence of a number of viral infectious agents in the inflammed tissue. The present study failed to show the presence of a known viral agent with a recognized tropism for myofibers. With a panel of lymphoid cell markers, we also characterized the phenotype of the inflammatory infiltrate that was composed of many T-lymphocytes with few CD4+ cells. Lastly, we reviewed the published cases and discuss the possible pathogenesis.