Since the description of the first case of dermatomyositis over a century ago, our understanding of myositis has evolved. Bohan and Peter in 1975 established diagnostic criteria for polymyositis and dermatomyositis. Subsequent investigations by Arahata and Engel delineated differences in the lymphocyte subsets on muscle histopathology distinguishing polymyositis and dermatomyositis. Following that, myositis-specific antibodies have been reported in association with various myositis subtypes and with interstitial lung disease. Polymyositis and dermatomyositis are in general responsive to immunosuppressive therapy. Inclusion body myositis (IBM) became recognized as a distinct entity nearly half a century ago. IBM is clinically and pathologically distinct from the other inflammatory myopathies. The weakness in IBM is characteristic, involving both the proximal and distal muscle groups, such as finger flexion, knee extension and ankle dorsiflexion. Vacuolated fibers, amyloid deposition, and filaments on electron microscopy are pathologic hallmarks of IBM. IBM is refractory to corticosteroids and intravenous gamma globulins. This clinical observation and the pathologic features support the hypothesis that IBM is a muscle-degenerative disease. Most recently, a fourth inflammatory myopathy subtype called necrotizing myopathy was described. Necrotizing myopathy may be related to malignancy, other autoimmune diseases, toxic exposure or can be idiopathic. The key histopathologic findings of this entity are necrotic fibers undergoing phagocytosis. Though patients ultimately respond to immunosuppressive therapy, they tend to be more refractory and therefore often require a more aggressive treatment approach.
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