PM is a subacute or chronic myopathy characterized by proximal limb muscle weakness. The disease is due to cytotoxicity mediated by CD8+ T cells invading muscle fibres that express MHC-I antigen.

PM is a rare disease and is considered the least common among IIMs, with an incidence of around 4 cases/million population/year. Patients with PM have higher risk for cancer (6.2 %) if compared to general population [6]. Cancer may precede, accompany, or follow the onset of polymyositis. Ovarian cancer is the most frequently associated in Europe and North America; gastric, nasopharyngeal carcinoma, and breast cancers are the most often associated in Asia. Therefore screening for cancer is highly recommended in PM patients [7].

Subacute, symmetric, proximal limb weakness with difficulty in raising arms, walking, and climbing stairs are usually the presenting symptoms. Distal muscles may be involved in later stages of the disease, facial muscles being spared.

Arthralgia, cardiac manifestations (arrhythmia, congestive failure), probably secondary to autoimmune myocarditis, and respiratory muscle involvement may ensue over the course of the disease. Interstitial lung disease may be observed, particularly in patients with serum anti Jo-1 antibodies.

Serum autoantibodies are a diagnostic tool for the diagnosis and the definition of the disease subset. The most frequent MSAs antibodies observed in polymyositis are Ab anti-aminoacyl-tRNA-synthetase (anti ARS), overall found in 25–35 % of PM patients. Among these, anti-Jo-1 is the most common in PM (20–30 %), while other anti ARS autoantibodies are much less frequent (no more than 5 %). Anti-signal recognition particles (SRP) antibodies and cytoplasmic proteins involved in the recognition and targeting of signals to the endoplasmic reticulum, are described in 4–6 % of adult PM patients. Among MAAs, antibody against the cytoplasmic Ro ribonucleoprotein (Ro52) is found in more than 30 % of myositis, and is frequently associated with anti-ARS antibodies [3].

See therapy of Myasthenia Gravis – Chap. 25

Treatment of PM is not evidence-based but empirical. Corticosteroids (oral prednisone 1 mg/kg per day, in a single daily morning dose, after breakfast) are the principal treatment. Daily dose is slowly reduced after 3–4 weeks according to efficacy and adverse effects. In patients with severe disease, treatment may start with intravenous methyl-prednisolone at a dose of 1 g per day for 3–5 days and then be switched to the oral prednisone regimen. Azathioprine, methotrexate, mycophenolate mofetil, and ciclosporin can be coadministered. High dose immunoglobulin infusion may be considered.

Rituximab is a further potential therapeutic agent for inflammatory myopathies [8, 9].

There is a worldwide increase in survival of patients with IIM, due to earlier diagnosis and immuno-suppressants. Survival rate of PM patients ranges between 77–92 % and 95 % at 5 and 10 years, respectively. Predictors of poor outcome are higher age, male sex, longer duration of symptoms, presence of cancer, pulmonary disease (particularly interstitial lung disease-ILD), cardiac involvement, dysphagia, and infections.