Polymyositis

Polymyositis represents one of the classical major forms of idiopathic inflammatory myopathies (IIM). Its prevalence is not known because of overdiagnosed cases, especially muscular dystrophies and inclusion body myositis (IBM). PM usually presents in middle age, particularly in women. Pathogenic mechanisms include the abnormal expression of major histocompatibility complex (MHC)-class I antigens and the activation of CD8 cells. The clinical pattern consists in a subacute or chronic proximal muscle weakness, with an increment of serum CK values. In contrast to dermatomyositis, cancer risk is very low. PM may be associated with other autoimmune and infectious diseases. In most cases, a satisfactory clinical response can be obtained with corticosteroids and immunosuppressive agents.