The cause of polymyositis is unknown. It is suspected that a number of factors play a role in the development of the condition's symptoms. It is thought that polymyositis may be an autoimmune disease, caused by the body’s natural immune defense mechanisms attacking its own tissue (autoimmune reaction). Viruses are also suspected of playing a role in its development, and there may be a genetic component to the condition. Polymyositis can occur at any time of life, althought it is uncommon under the age of 18 years and about 60% of cases occur between the ages of 30 and 60 years. Women are more commonly affected by the condition than men.
Polymyositis is one of several different types of inflammatory muscle diseases (myopathies). Other types include dermatomyositis and inclusion body myositis.
Signs and symptoms
As the condition progresses the muscles affected become wasted. Eventually people may have difficulty walking, standing, climbing stairs and lifting things. Their neck and throat muscles may also be affected, making it difficult for them to swallow and changing their voice. The muscles involved with breathing (including the diaphragm) may be affected in severe cases. In rare cases the lungs and heart may be affected leading to symptoms such as heart rhythm disturbances.
Two tests most commonly performed to provide a definitive diagnosis are an electromyogram (EMG) and a muscle biopsy:
- An EMG measures the electrical activity in the muscles. The pattern of the electrical activity in the muscles can demonstrate if the muscle weakness is due to polymyositis.
- A biopsy is where a sample of muscle tissue is collected during a minor surgical procedure under a local anesthetic. The muscle tissue sample is sent to a laboratory for further investigation. Characteristic changes in the muscle tissue can be a strong indicator of polymyositis.
Treatment may take two to three months to produce significant improvement in symptoms. Once symptoms improve the medication doses may be reduced. Physical therapy and exercise, in conjunction with a physiotherapist and/or occupational therapist, is important to maintain muscle and joint movement and help restore muscle strength. Alternative treatments such as nutritional supplements, elimination diets, homeopathy and acupuncture may help to relieve symptoms in some people.
The course of the disease varies from person to person. Most people affected will improve, however not all will make a full recovery. Those who experience severe symptoms that do not respond adequately to treatment may be left with significant disability.
Further information and support
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Ph: 0800 800 337 or (09) 815 0247
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National Institute of Neurological Disorders (2011) Polymyositis. National Institutes of Health. http://www.ninds.nih.gov/disorders/polymyositis/polymyositis.htm
Robinson, R. (2006) Polymyositis. The Gale Encyclopaedia of Medicine, Third Edition. Jacqueline L. Longe, Editor. Farmington Hills, MI. Thomson Gale.
Shiel, W. C. (2013) Polymyositis and Dermatomyositis. MedicineNet, Inc. http://www.medicinenet.com/script/main/art.asp?articlekey=454&pf=3&page=1
Last Reviewed – 20 August 2013