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Contributors: Michael W. Winter MD, Mary Anne Morgan MD, Paritosh Prasad MD
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Polymyositis (PM) is an idiopathic inflammatory myopathy. It presents with symmetric proximal muscle weakness and elevated skeletal muscle enzymes. PM most often onsets in the fourth to fifth decade of life and rarely in patients younger than 20 years, with a female predominance of approximately 65%. PM is a rare disease with an estimated incidence of about 1:100 000. This summary addresses PM in adults.

Muscle weakness, usually proximal and symmetric, is the most common feature of PM and is seen in over 90% of patients. The onset of muscle weakness from PM is typically subacute, with patients reporting symptoms developing over a several-month time period. Common concerns include progressive difficulty lifting, kneeling, using the stairs, and raising arms. Other nonspecific symptoms include fatigue, weight loss, myalgia, arthralgia, dyspepsia, and dysphagia.

PM can manifest in a variety of organ systems. In addition to characteristic skeletal muscle weakness, many patients develop interstitial lung disease, dysphagia (due to involvement of skeletal muscle that makes up the upper third of the esophagus), and polyarthritis. Patients may also have myocarditis. It can be difficult to distinguish PM-related symptoms from those associated with PM and an autoimmune disease overlap syndrome, which presents diagnostic challenges.

There is no cure for PM, and patients' clinical courses can vary considerably. Patients with PM are at increased risk for myocardial infarction and ischemic stroke.

There are rare cases of juvenile PM.


M33.20 – Polymyositis, organ involvement unspecified

31384009 – Polymyositis

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Diagnostic Pearls

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Differential Diagnosis & Pitfalls

  • Dermatomyositis
  • Inclusion body myositis
  • Infectious myositis (Viral myositis or bacterial)
  • Rhabdomyolysis
  • Obstructive sleep apnea
  • Anemia (see, eg, Iron deficiency anemia)
  • Malignancy – leukemia, Lymphoma
  • Amyotrophic lateral sclerosis
  • Hypothyroidism
  • Fibromyalgia
  • Inflammatory bowel disease (eg, Crohn disease, Ulcerative colitis)
  • Polymyalgia rheumatica
  • Muscular dystrophy (see, eg, Duchenne muscular dystrophy)
  • Myotonic dystrophy
  • Drug-induced myopathies
  • Myasthenia gravis
  • Lambert-Eaton myasthenic syndrome
  • Human immunodeficiency virus disease-associated myopathy
  • Metabolic disorders of carbohydrate and lipid metabolism
  • AL amyloidosis
  • Sarcoidosis
  • Diabetic amyotrophy (see Diabetic neuropathy)
  • Systemic lupus erythematosus

Best Tests

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Management Pearls

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Drug Reaction Data

Below is a list of drugs with literature evidence indicating an adverse association with this diagnosis. The list is continually updated through ongoing research and new medication approvals. Click on Citations to sort by number of citations or click on Medication to sort the medications alphabetically.

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Last Reviewed:02/03/2019
Last Updated:12/07/2021
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A medical illustration showing key findings of Polymyositis : Aldolase elevated, LDH elevated, Proximal muscle weakness, Arthralgia, Creatine kinase elevated, Dysphagia, Myalgia, Dyspepsia
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