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Inclusion body myositis, because of its tendency to mimic polymyositis, is a common cause of “treatment-resistant polymyositis.”

  • – In contrast to polymyositis, the typical patient with inclusion body myositis is White, male, and over the age of 50 years.

  • – The onset of inclusion body myositis is more insidious than that of polymyositis or dermatomyositis (eg, occurring over years rather than months), and the distal motor weakness is commonly asymmetric.

  • – Creatine kinase levels are often minimally elevated and are normal in 25%.

  • – Electromyography may show a mixed picture of myopathic and neurogenic abnormalities.

  • – The disease is associated with antibodies to cytoplasmic 5'-nucleotidase 1A (cN1A).

  • – Inclusion body myositis is less likely to respond to therapy.

Immune-mediated necrotizing myopathy, although similar to polymyositis, is distinct because of the presence of muscle necrosis.

  • – Autoantibodies aid in diagnosis; anti-SRP antibodies are associated with severe muscle weakness, pain, and cardiac involvement.

  • – Anti-HMGCR antibodies occur in the setting of statin use and are associated with proximal muscle weakness and marked creatine kinase elevations.

  • – Unlike statin-induced myopathy, anti-HMGCR myositis does not resolve when statins are stopped.

  • – Instead, many patients have a severe and unrelenting disease course with persistent weakness.


  • Progressive muscle weakness.

  • Dermatomyositis: characteristic cutaneous manifestations (Gottron papules, heliotrope rash); increased risk of malignancy.

  • Elevated creatine kinase, myositis-specific antibodies, diagnostic muscle biopsy.

  • Mimics include infectious, metabolic, or drug-induced myopathies.


Idiopathic inflammatory myopathies include polymyositis, dermatomyositis, myositis resulting from a rheumatic disease or overlap syndrome, inclusion body myositis (IBM), and immune-mediated necrotizing myopathy. These disorders are characterized by progressive muscle weakness, and all but IBM demonstrate an inflammatory infiltrate in muscle tissue.

Polymyositis and dermatomyositis are systemic disorders of unknown cause whose principal manifestation is muscle weakness. Although their clinical presentations (aside from the presence of certain skin findings in dermatomyositis, some of which are pathognomonic) and treatments are similar, the two diseases are pathologically quite distinct. They affect persons of any age group, but the peak incidence is in the fifth and sixth decades of life. Women are affected twice as commonly as men, and the diseases (particularly polymyositis) also occur more often among Blacks than Whites. There is an increased risk of malignancy, especially in dermatomyositis. Indeed, up to one patient in four with dermatomyositis has an occult malignancy. Malignancies may be evident at the time of presentation with the muscle disease but may not be detected until months afterward in some cases. The malignancies most commonly associated with dermatomyositis are lung, ovarian, breast, colorectal, cervical, bladder, nasopharyngeal, esophageal, pancreatic, and renal cancer. Patients may have skin disease without overt muscle involvement, a condition termed dermatomyositis sine myositis; these patients can have aggressive interstitial lung disease. Myositis may also overlap with other connective tissue diseases, especially systemic sclerosis, systemic lupus erythematosus, mixed connective tissue disease, and Sjögren ...

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