The inflammatory myopathies encompass a group of heterogenous muscle diseases which have in common an acquired myopathy with histological signs of endomysial inflammation. We present evidence based on recently emerged clinical, histologic, immunopathologic, demographic and therapeutic observations that these myopathies comprise three major and distinct groups: polymyositis (PM), dermatomyositis (DM), and inclusion-body myositis (IBM). Immune-mediated mechanisms characteristic for each group appear to play a primary role in the pathogenesis of these diseases. In DM there is an intramuscular microangiopathy mediated by the C5b-9 membranolytic attack complex, leading sequentially to loss of capillaries, muscle ischemia, muscle fiber necrosis and perifascicular atrophy. In contrast, in PM and IBM the muscle fiber injury is initiated by sensitized CD8+ cytotoxic T cells that recognize MHC-I restricted muscle antigens, leading to phagocytosis and fiber necrosis. Among the viruses implicated in the cause of inflammatory myopathies, only the retroviruses, HIV, HTLV-1 and simian retroviruses, have been convincingly associated with PM. Retroviruses, therefore, appear to be the leading group of viruses capable of triggering these diseases. The treatment of inflammatory myopathies has been largely empirical. A detailed therapeutic plan based on our experience with a large number of patients is presented. Patients with bona fide PM or DM respond to steroids to some degree and for some period of time. In contrast, patients with IBM do not respond to any therapy and the disease should be suspected when a patient with presumed PM has failed treatment. Methotrexate and cyclophosphamide are disappointing. Cyclosporine and Azathioprine are commonly used but they are of uncertain benefit. Plasmapheresis is ineffective. High-dose intravenous immunoglobulin is a promising new therapeutic modality.
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