This information is provided by the National Institutes of Health (NIH) Genetic and Rare Diseases Information Center (GARD).
Polymyositis is a type of inflammatory myopathy, which refers to a group of muscle diseases characterized by chronic muscle inflammation and weakness. The muscles affected by polymyositis are the skeletal muscles (those involved with making movements) on both sides of the body. Although the disease can affect people of all ages, most cases are seen in adults between the ages of 31 and 60 years. The disease is more common among women and among black individuals.
The exact cause of polymyositis is unknown. The disease shares many characteristics with autoimmune disorders, which occur when the immune system mistakenly attacks healthy body tissues. In some cases, the disease may be associated with viral infections, connective tissue disorders, or an increased risk for malignancies (cancer). Diagnosis is based on a clinical examination that may include laboratory tests, imaging studies, electromyography, and a muscle biopsy. Although there is no cure for polymyositis, treatment with corticosteroids or immunosuppressants can improve muscle strength and function.
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