Definition:

Polymyositis (PM) and dermatomyositis (DM) are the two major forms of inflammatory muscle diseases. PM and DM, along with sporadic inclusion-body myositis (sIBM), belong to the heterogeneous group of the idiopathic inflammatory myopathies (IIMs), which are characterized by weakness and chronic inflammation of skeletal muscle. PM and DM differ in their clinical features, histopathology, response to treatment, and prognosis. Although their clinical pictures differ, they both present with symmetrical, proximal muscle weakness. PM is a term that was used traditionally to denote all IIMs that were not DM or sIBM, but it is now a controversial entity with questionable specificity. Traditionally, PM is described as presenting with weakness of the proximal muscles that evolves over weeks to months and affects adults, but rarely children. DM typically includes subacute progressive proximal muscle weakness and a skin rash. The disease mechanisms of PM and DM that cause muscle damage and dysfunction are not fully understood. However, because of the association with other autoimmune diseases, the presence of autoantibodies, and response to immunosuppressive medication, they are believed to be autoimmune in origin. Recent studies have highlighted the importance of the innate immune system and non-immune mechanisms and described novel adaptive immune-based pathways in the pathogenesis of PM and DM. Treatment of inflammatory myopathies is generally empirical. Corticosteroids still remain the agents of choice for the initial treatment, but their use is limited by the high frequency of side effects. In addition, as a substantial number of patients do not respond to glucocorticoids alone, additional agents such as immunosuppressants, immunomodulators, and more recently, biologics are commonly used in clinical practice.