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Juvenile dermatomyositis (JDM) and juvenile polymyositis (JPM) are inflammatory myopathies in which inflammation of capillary endothelium of muscle, skin, and other tissues causes vascular and tissue damage. JDM patients present with characteristic rashes and muscle weakness. JPM patients have inflammatory myopathy but lack skin findings. Both disorders have a wide range of severity and presenting findings.
- The average age of onset is 7 years, but 25% of cases are diagnosed by 4 years of age.
- Male-to-female ratio in the United States is 1:2.3.
- JDM incidence is 3.2 new cases per 1 million children per year; JPM is extremely rare.
- Underlying genetic susceptibility
- Environmental triggers
- Ultraviolet light exposure
- Infectious triggers are inconsistently reported, including group A β-hemolytic streptococci, coxsackievirus B, toxoplasma, enterovirus, and parvovirus.
- Reports of drug exposure, vaccination, and psychological stress prior to diagnosis, but no causation has been found
- Genetic factors, including the following:
- HLA alleles: B8, DRB1*0301, DQA1*0501, DQA1*0301
- Cytokine polymorphisms: TNFα-308A promoter, various IL1 genes, interferon regulatory factor 5, and others, all resulting in upregulated inflammation
- Polymorphisms of immunoglobulin constant regions
- Epigenetic factors likely exist: Monozygotic twin studies show low concordance.
- Vasculopathy in patients with underlying inflammatory genetic susceptibility, triggered by environmental factors
- JDM: immune attack on muscle capillary endothelium with infiltration of plasmacytoid dendritic cells causing a type I interferon response and upregulation of myofiber MHC class I expression
- Immune complex deposition and complement activation drive vasculopathy
- Upregulation of ICAM-1 and von Willebrand factor antigen indicates endothelial injury.
- After vasculopathy and MHC class I upregulation, plasmacytoid dendritic and other immune cells infiltrate perivascular and perimysial tissue, resulting in upregulated type I interferon response which perpetuates inflammatory processes including increased production of proinflammatory cytokines
- JPM: CD8 T cell and myeloid dendritic cell-mediated attack on myofibers causing myonecrosis; no increased interferon response
- Myositis-specific and associated autoantibodies directed against vascular and muscle antigens implicated in pathogenesis of JDM and JPM
- Maternal cell chimerism reported in peripheral blood T cells and muscle tissue of JDM patients may be autoreactive toward host cells.
Commonly Associated Conditions
- Dermatomyositis in children is not associated with presence of malignancy as seen in adults.
- Celiac disease is rarely associated with JDM.