Dermatomyositis/Polymyositis

Basics

Description

  • Dermatomyositis (DM) and polymyositis (PM) are immune-mediated myopathies and the largest group of acquired and treatable causes of skeletal muscle weakness
  • Patients experience an indolent 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
  • Clinically amyopathic dermatomyositis (CADM) is a condition in which patients have the characteristics cutaneous findings of DM but no muscle weakness
  • DM has also been observed in patients exposed to high-intensity ultraviolet radiation

Etiology

  • Exact cause unknown, although autoimmune mechanisms are thought to be largely responsible
  • Incidence ∼2:100,000 with a female preponderance
  • Possible association between PM and certain viral, bacterial, and parasitic infections
  • DM/PM coexists with collagen vascular disease in about 20% of cases
  • 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
  • Several medications can also trigger DM
    • Antineoplastic (hydroxyurea, cyclophosphamide)
    • Anti-infectious agents (penicillin, sulfonamides, isoniazid)
    • NSAIDs (diclofenac, phenylbutazone)
    • D-penicillamine, statins, and certain vaccines

Pediatric Considerations

  • 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 and echovirus, causing chronic JDM in particular with patient with agammaglobulinemia

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