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Necrotizing autoimmune myopathy (NAM) is a recently defined subcategory of the acquired idiopathic myopathies, characterized clinically by the subacute onset of proximal weakness, elevated creatine kinase levels, and electromyography revealing an irritable myopathy.1 It is distinguished from other acquired inflammatory myopathies (eg, polymyositis and dermatomyositis) by the histopathological findings of marked muscle fiber necrosis and regeneration with little or no accompanying inflammation. Immunostaining of muscle biopsies reveals macrophages around necrotic muscle fibers without the presence of lymphocytes.2,3 Necrotizing autoimmune myopathy has existed as a distinct nosological entity classified under the acquired idiopathic myopathies since 2004.4 Prior to this, it is likely that many cases were diagnosed as severe cases of polymyositis without inflammation, based on the results of a biopsy, but nevertheless were treated with immunosuppressive drugs. It is important to point out that the histological features of NAM may be seen in either acquired or genetically determined myopathies and that the subacute onset of symptoms and the response to immunotherapy are among the clinical features that help to distinguish it from an inherited or toxic myopathy.
Meriggioli MN. The Clinical Spectrum of Necrotizing Autoimmune MyopathyA Mixed Bag With Blurred Lines. JAMA Neurol. 2015;72(9):977-979. doi:10.1001/jamaneurol.2015.1517