Inflammatory myopathies

The inflammatory myopathies are a heterogeneous group of disorders with recent evidence demonstrating differences in clinical features, pathologic changes, pathogenesis, and response to therapy. The inflammatory myopathies generally produce predominantly proximal, symmetric muscle weakness and wasting. Additional criteria for diagnosis include elevated serum muscle enzymes, myopathic features on EMG, and muscle biopsy abnormalities, including muscle fiber necrosis, degeneration, and inflammatory infiltrates. Inclusion body myositis is distinctive in that distal weakness is most commonly equal to or greater than proximal weakness and muscle biopsy reveals rimmed, cytoplasmic vacuoles, eosinophilic inclusions in the cytoplasm, and nucleus and abnormal filamentous structures. Autoimmune mechanisms seem likely to be involved in the pathogenesis of these disorders and viral infection may be etiologically involved in some of these diseases. The differences in the site of immune-mediated damages suggest an angiography in dermatomyositis while direct muscle fiber involvement is more likely in polymyositis and inclusion body myositis. Therapy of these disorders is similar although some, particularly inclusion body myositis, may be particularly resistant to therapy. Prednisone is currently recommended as the first treatment with azathioprine or methotrexate added after 3 months if steroids are ineffective.