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At a glance

Both diseases belong to a group of connective tissue disorders known as idiopathic inflammatory myopathies. They are a multisystem disease characterized by necrotizing inflammatory myopathy of striated muscles and a skin rash, both of unknown etiology. Dermatomyositis is a form of polymyositis accompanied by distinctive skin inflammation and rashes, muscle weakness, and inflammatory myopathy. Polymyositis is characterized by inflammation and degeneration of muscles, especially the proximal musculature, flexion of the neck and torso.


For Dermatomyositis—Adult Dermatomyositis; ADM; Childhood Dermatomyositis; Idiopathic Inflammatory Myopathy Syndrome. For Polymyositis—PM; Inclusion Body Myositis Syndrome.


This medical condition was first described in the 1800s.


  • Primary Idiopathic Polymyositis (Adult): Accounts for 33% of all cases and is progressive over weeks but most usually years. It affects women twice as often as men. The clinical features include proximal limb weakness (gait and shoulder).

  • Primary Idiopathic Dermatomyositis (Adult): Considered a progressive muscular inflammatory disorder associated with skin rash (often on the hands) and severe muscle weakness over time. It may occur suddenly or develop over months. Severe pain is associated also with the presence of calcium deposits within the muscles and skin.

  • Childhood Dermatomyositis or Polymyositis with Necrotizing Vasculitis: Characterized by heart failure due to cardiac involvement in more than 77% of affected individuals. Also, interstitial lung disease (ILD) is present in up to 65% of patients with polymyositis, as defined by high-resolution computed tomography (HRCT) or restrictive ventilatory defects compatible with ILD.

  • Polymyositis Associated with Other Connective Tissue Disorders: Characterized by an Overlap Syndrome with mixed connective tissue manifestations.

  • Polymyositis or Dermatomyositis Associated with Malignant tumors: Associated with an increased risk of cancer, especially in patient older than 45 years of age, male, presenting dysphagia, cutaneous necrosis, cutaneous vasculitis, rapid onset of myositis (<4 weeks), elevated creatine kinase, higher erythrocyte sedimentation rate and higher C-reactive protein levels.

  • Sporadic Inclusion Body Myositis: Characterized by muscle weakness and wasting especially affecting the wrists and fingers as well as the quadriceps. Affected individuals frequently stumble while walking and find it difficult to grasp items. As in dermatomyositis and polymyositis, swallowing can be difficult. The onset usually occurs at a later age. Muscle biopsy reveals myoplastic “rimmed” vacuoles associated with necrosis, and regenerative processes. Sporadic inclusion body myositis may also result from autoimmune dysfunction although it is not as convincing as it appears for polymyositis.

  • Amyopathic Dermatomyositis: Represents an estimated 20% of all dermatomyositis cases.


The exact incidence is unknown; however, a population study reported an estimated overall age- and sex-adjusted incidence for dermatomyositis (including all subtypes) at 9.63 (95% confidence interval [CI], 6.09-13.17) per 1 million persons and 2.08 (95% CI, 0.39-3.77) per 1 million persons for clinically amyopathic dermatomyositis. Age- and sex-adjusted prevalence for all subtypes of dermatomyositis was 21.42 (95% CI, 13.07-29.77) ...

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