Dermatomyositis and Polymyositis

Dermatomyositis (DM) and polymyositis (PM) are idiopathic inflammatory myopathies characterized by proximal greater than distal muscle weakness, elevated serum creatine kinase levels, electrophysiologic abnormalities, and inflammation on muscle biopsy. Clinically and electrophysiologically, DM and PM appear very similar, and muscle biopsy is the gold standard for diagnosis. Much of the PM literature based the diagnosis on Bohan and Peter's criteria, which is now obsolete given the advances of immunopathology. As diagnostic criteria for the inflammatory myopathies have been refined, it has become apparent that PM is much less common than previously thought, and, in fact, is probably quite rare. More recent literature, using strict histopathologic criteria for diagnosis of PM, has brought into question previously reported associations. Because of this, the clinical entity of PM is poorly defined. The exact incidence of each is unknown because previous epidemiologic studies often grouped them together, but overall the annual incidence of the inflammatory myopathies is approximately one in 100,000. DM and PM respond to immunomodulating therapies. High-dose oral prednisone is generally accepted first-line therapy. In patients who do not respond adequately to prednisone alone, or in whom prednisone cannot be weaned, methotrexate or azathioprine can be added. In the authors' experience, methotrexate works faster and is more effective than azathioprine. However, because of the increased risk of interstitial lung disease with methotrexate, the authors avoid this in patients with anti-Jo-1 antibodies and, obviously, in patients who already have pulmonary disease. If patients do not respond adequately to the combination of prednisone and methotrexate or azathioprine, a trial of intravenous immunoglobulin is administered.