Immune and inflammatory myopathies

Immune and inflammatory myopathies (IIM) are characterized by weakness with muscle inflammation and are frequently accompanied by extra-muscular manifestations. Previously, patients were diagnosed with dermatomyositis if a characteristic rash accompanied muscle involvement or polymyositis if no rash was present. The discovery of unique clinical features, autoantibodies, and histopathological patterns that associate with specific clinical phenotypes has been instrumental in understanding IIM and predicting treatment response and prognosis. While numerous classification systems exist, the most commonly agreed-upon criteria recognize four main categories: dermatomyositis, sporadic inclusion body myositis, antisynthetase syndrome, and immune-mediated necrotizing myopathy. Treatment still largely relies on expert opinion and empirical use of corticosteroids and steroid-sparing agents. A deeper understanding of the molecular pathways that drive pathogenesis and accurate disease classification will aid clinical research and the development of more precision therapies. This chapter discusses current knowledge of the epidemiology, clinical characteristics, diagnostic evaluation, classification, pathogenesis, treatment, and prognosis of IIM.