Juvenile dermatomyositis (JDM) and juvenile polymyositis (JPM) are rare autoimmune myopathies in childhood. JDM is primarily a capillary vasculopathy, whereas JPM involves direct T-cell invasion of muscle fibers similar to that seen in adult polymyositis [1,2]. (See "Clinical manifestations and diagnosis of adult dermatomyositis and polymyositis".)
The epidemiology, pathogenesis, and clinical manifestations of JDM and JPM will be reviewed here. Diagnosis and treatment of these disorders are discussed elsewhere. (See "Diagnosis of juvenile dermatomyositis and polymyositis" and "Treatment and prognosis of juvenile dermatomyositis and polymyositis".)
JDM is the most common idiopathic inflammatory myopathy of childhood accounting for approximately 85 percent of cases [3,4]. In population-based studies, JDM has a reported annual incidence that ranges from two to four cases per one million children [5-9]. The peak incidence is from 5 to 10 years of age [8,9]. Girls are affected more often than boys with a two- to five-fold greater rate [7,8,10].
JPM occurs less frequently and accounts for only 3 to 6 percent of childhood idiopathic inflammatory myopathies [3,7].
Although the etiology remains unclear, it has been proposed that JDM and JPM are caused by an autoimmune reaction in genetically susceptible individuals, possibly in response to environmental triggers.