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The estimated annual incidence has been reported to be between 1/250,000 and 1/130,000 new cases/year and prevalence 1/14,000. It is a rare disorder that may be over-diagnosed because of overlap with other myopathies. PM is more common in women than in men (2:1). In the United States, a Black-to-Caucasian ratio of 5:1 has been reported.

Polymyositis has non-specific clinical features including proximal muscle weakness and elevated serum creatine kinase. The age of onset is over 20 years, with most patients aged 45-60 years. The disease develops gradually over a period of 3 to 6 months with a variable impact on physical capacities: difficulty lifting objects, raising arms, kneeling, and climbing or descending stairs. Arthralgia may be found and myalgia is reported in less than 30% of patients. Other signs include neck flexor weakness, fatigue, stiffness, weight loss anorexia, dysphagia and less commonly dysphonia. Extramuscular organ involvement is described including pulmonary disorders (exertional dyspnea, aspiration, interstitial lung disease (see this term)), and less commonly cardiac involvement (arrhythmia, pericarditis, myocarditis, congestive heart failure). 10-15% of patients develop malignancy: breast and ovarian cancers in women (see this term), and lung and prostate cancer in men. Other less frequently reported neoplasms include colorectal cancer, and non-Hodgkin lymphoma, pancreatic, gastric, and bladder cancer (see these terms). PM does not involve the characteristic cutaneous signs found in dermatomyositis (DM, see this term).

The exact mechanisms underlying PM have not been clearly elucidated to date. PM appears to be caused by inflammation-mediated muscle fiber necrosis and regeneration. A combination of genetic (HLA-DQA1*0501 and HLA-DRB1*0301) polymorphism of TNF-a allele and environmental factors (virus coxackie B, HTLV-1, and HIV) has been suggested.

The diagnosis is often delayed in the absence of characteristic findings. It is based on the presence of the following four criteria: proximal muscle weakness, elevated muscle enzymes (creatine kinase, aldolasa), myopathic findings on electromyography (EMG), and muscle biopsy showing scattered and regenerating fibers and a perivascular and endomysial inflammatory infiltrat (lymphocytes T CD8 and macrophages). Antinuclear and antisynthetase antibodies (such as anti-Jo-1 antibodies) may reach 20-25%.

The differential diagnoses include muscular dystrophies of late onset, as well as adult-onset nemaline myopathy or proximal myotonic myopathies (see these terms). Distinction between overlap myositis (see this term), necrotizing myopathy (see this term) and pure PM is still under debate.

The aim of treatment is to eliminate inflammation and restore muscle performance. Initial treatment includes high-dose corticosteroids. Dosage is then tapered to reach an appropriate maintenance dose. Immunosuppressive agents are also frequently used in combination, typically methotrexate, azathioprine, or mycophenolate mofetil. Intravenous immunoglobulin (IVIg) or intravenous methylprednisolone (IVMP) may be used in severe cases. Physical therapy is also recommended. Monitoring for extramuscular involvement should include chest X-ray and pulmonary function testing. If cardiac involvement is suspected, echocardiography is recommended. Age-appropriate cancer screening is also recommended.

Prognosis depends on patient response to treatment (good in most cases), severity of disease manifestations, age at diagnosis, presence of cardiac or pulmonary involvement, and associated malignancy. Long-term corticosteroids may be a major source of morbidity.