Polymyositis and Dermatomyositis

This chapter discusses polymyositis (PM) and dermatomyositis (DM), which are often categorized as “dysimmune myopathies” or “idiopathic inflammatory myopathies.” It is presently thought that PM is a T-cell mediated, presumably autoimmune disorder, whereas DM is an antibody-mediated vascular disorder. In both PM and DM, proximal muscles are usually symmetrically affected. Respiratory, pharyngeal and neck muscles may be involved during later stages. The history, clinical symptoms and signs, elevated serum levels of muscle enzymes, electrophysiological changes, and histologic findings together provide the basis for the diagnosis. The main diagnostic criteria and features of the different inflammatory myopathies are compared and summarized in this chapter. Some characteristic histologic features of DM and PM are also described. Furthermore, different effector mechanisms have been attributed to PM, which is mainly T-cell- mediated and DM, which mainly is antibody-mediated, and a unique γδ T-cell-mediated subtype of PM has been described. Challenges for future research include the identification of the elusive antigens and the triggering mechanisms of the immune reactions against muscle fibers.