Juvenile Dermatomyositis
• Juvenile dermatomyositis (JDM) is the most common idiopathic inflammatory myopathy (IIM) of childhood. Although JDM shares some risk factors, clinical and laboratory features with its adult counterpart, a number of features distinguish JDM from adult DM. • The incidence of JDM is approximately 3.2 cases/million children/year with a gender ratio of 2 girls/1 boy. • The mean age at diagnosis of JDM is approximately 7 years of age, but more than 25% of patients are 4 years old or younger. • The duration of active, untreated disease before diagnosis strongly influences patients’ features at presentation and clinical outcomes. • The occurrence of malignancy in association with JDM is extremely uncommon. This contrasts significantly with adult DM, in which paraneoplastic disease is common. In adult DM, anti-TIF-1γ and anti-NXP2 (MJ) autoantibodies often signal the presence of a concomitant malignancy. • Environmental and genetic factors, such as the composition of their HLA alleles, impact both the child’s susceptibility to myositis and the inflammatory response, adding heterogeneity to disease pathophysiology.
