How to diagnose and treat the inflammatory myopathies.

The inflammatory myopathies include 3 distinct entities, PM, DM, and IBM. These diseases differ clinically, immunopathologically, and in their response to therapies. Although DM and IBM are easy to diagnose on the basis of characteristic clinicopathologic findings, PM still remains a diagnosis of exclusion. A T cell-mediated cytotoxic process in PM and IBM and a complement-mediated microangiopathy in DM, along with the various serologic markers of autoimmunity, are the hallmarks of the underlying autoimmune processes in these groups. Although in uncontrolled studies PM and DM appear to respond to prednisone and immunosuppressive drugs to some degree and for some period of time, IBM is resistant to all therapies. Currently, high-dose intravenous immunoglobulin (IVIG) appears to be an encouraging and safe new modality of treatment for some of these conditions when other therapies have failed. In a controlled study, IVIG has been shown to be effective in DM and, in uncontrolled studies, in some patients with PM or IBM.