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INTRODUCTION
Along with polymyositis and inclusion body myositis, dermatomyositis is classified as one of the idiopathic inflammatory myopathies. Dermatomyositis is a disease of presumed autoimmune pathogenesis that presents with a symmetric, proximal, extensor inflammatory myopathy and a characteristic cutaneous eruption. Previously, polymyositis was thought to represent the same disease process but limited to the muscles; however, there is robust evidence that the pathogenetic mechanisms of polymyositis and dermatomyositis differ significantly. In polymyositis, clonally expanded, autoreactive CD8+ T cells invade myocytes expressing MHC class I antigens and cause necrosis via the perforin pathway. In contrast, there are data to support the theory that autoantigens activate a humoral immune process in dermatomyositis, in which complement is deposited in capillaries causing capillary necrosis and ischemia. Because an appreciation of these pathogenetic differences occurred relatively recently, previous studies often pooled cases of dermatomyositis and polymyositis, making interpretation of their data challenging.
Dermatomyositis is characterized by a bimodal age distribution, with both adult and juvenile forms, and up to one-quarter of patients in the adult group having an associated malignancy. Patients with juvenile dermatomyositis do not have an increased risk of malignancy, but do have an increased incidence of calcinosis cutis and associated small vessel vasculitis. There is a subset of patients in whom the cutaneous manifestations of dermatomyositis exist without objective evidence of muscle inflammation, referred to as amyopathic dermatomyositis, and formerly known as dermatomyositis sine myositis. For dermatologists, establishing the diagnosis of dermatomyositis is important because it is a serious, treatable, multisystem disease that has a different prognosis and treatment approach from lupus erythematosus (LE). The malignancy association in adults provides the dermatologist with an opportunity to assist medical colleagues in detecting the tumor at an early stage because the skin disease often precedes the onset of symptoms related to the malignancy.
Ruth Ann Vleugels and Joseph L. Jorizzo Dermatomyositis 42