DESCRIPTION

Juvenile dermatomyositis (JDM) and juvenile polymyositis (JPM) are inflammatory myopathies in which inflammation within tissues and capillary endothelium of muscle, skin, and other organs causes vascular and tissue damage. JDM patients present with characteristic rashes and muscle weakness. JPM patients have inflammatory myopathy without skin findings. Both disorders have a range of severity and presenting findings.

EPIDEMIOLOGY

• 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.

RISK FACTORS

• Underlying genetic susceptibility
• Environmental triggers
  • Ultraviolet light exposure
  • Infectious triggers are inconsistently reported, including group A β-hemolytic streptococci, coxsackievirus B, toxoplasma, enterovirus, parvovirus, Borrelia burgdorferi, hepatitis, and influenza. There are case reports of possible SARS-CoV-2–triggered JDM.
  • Reports of drug exposure, vaccination, air pollution, and psychological stress prior to diagnosis, but no causality has been established

PATHOPHYSIOLOGY

• Immune attack on vasculature (vasculopathy), muscle, skin, and other organ tissues, 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 major histocompatibility complex (MHC) class I expression
• 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 are implicated in pathogenesis of JDM and JPM.

COMMONLY ASSOCIATED CONDITIONS

• Dermatomyositis in children is not associated with presence of malignancy as seen in adults.
• Celiac disease is rarely associated with JDM, but screening may be reasonable.