Juvenile dermatomyositis: advances in pathogenesis, evaluation, and treatment.

Juvenile dermatomyositis (JDM) is a rare, presumably autoimmune illness that causes proximal muscle weakness and a variety of typical cutaneous features. The study of this illness has been hampered by its rarity but, in recent years, important developments have increased our understanding of JDM. Genetic factors are likely important in the pathogenesis of JDM. These include several Human Leukocyte Antigen alleles, in particular those associated with the 8.1 ancestral haplotype and the tumor necrosis factor-alpha gene 308 polymorphism. Microchimerism, activation of plasmacytoid dendritic cells, and upregulation of type-1 interferon inducible genes also appear to play an important role in the pathogenesis of JDM. The study of JDM has also been limited by a lack of validated assessment tools. Recent work has validated the Childhood Myositis Assessment Scale and the Childhood Health Assessment Questionnaire as measures of muscle strength and function, and the Cutaneous Assessment Tool as a measure of skin disease activity and damage. Development of core sets of tools that should be used in all JDM studies has also been an important step. The use of magnetic resonance imaging and novel laboratory assessments (such as type-1 interferon inducible gene products, peripheral blood B cell and natural killer cell numbers, and myositis-associated and myositis-specific autoantibodies) are also playing an increasing role in the diagnosis and assessment of JDM. Current treatment is with corticosteroids, frequently in combination with other medications such as methotrexate or intravenous gammaglobulin. Newer therapies, such as anti-tumor necrosis factor agents and rituximab are currently being evaluated; it is not clear what role these medications will have in the future.