Description

• Dermatomyositis (DM) and polymyositis (PM) are systemic inflammatory myopathies, which represent the largest group of acquired and potentially treatable causes of skeletal muscle weakness
• Patients experience a marked progression of muscle weakness over weeks to months
• Can lead to respiratory insufficiency from respiratory muscle weakness
• Aspiration pneumonia can occur owing to a weak cough mechanism, pharyngeal muscle dysfunction, and esophageal dysmotility
• Cardiac manifestations include myocarditis, conduction defects, cardiomyopathy, and congestive heart failure (CHF)
• Arthralgias of the hands, wrists, knees, and shoulders with distinct associated skin findings for DM
• Ocular muscles are not involved but facial muscle weakness may be seen in advanced cases

Etiology

• The exact cause is unknown, although autoimmune mechanisms are thought to be largely responsible
• Incidence ∼2:100,000 with a female preponderance
• There may be an association between PM and certain viral, bacterial, and parasitic infections
• DM/PM occurs with collagen vascular disease about 20% of the time
• In DM, humoral immune mechanisms are implicated, resulting in a microangiopathy and muscle ischemia
• In PM, a mechanism of T-cell–mediated cytotoxicity is posited
  • CD8 T-cells, along with macrophages, surround and destroy healthy, nonnecrotic muscle fibers that aberrantly express class I major histocompatibility complex (MHC) molecules
• Deposition of complement is the earliest and most specific lesion, followed by inflammation, ischemia, microinfarcts, necrosis, and destruction of the muscle fibers

• Although DM is seen in both children and adults, PM is rare in children
• Similar to adult DM, juvenile DM (JDM) primarily affects the skin and skeletal muscles
• Juvenile form may include vasculitis, ectopic calcifications (calcinosis cutis), and lipodystrophy
• The juvenile form may be associated with coxsackievirus