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.