What is Polymyositis

Polymyositis is an inflammatory muscle disease of unknown cause. It is often grouped with dermatomyositis, a disease that causes similar symptoms with the addition of skin manifestations.

Statistics on Polymyositis

Polymyositis (PM) is rare. Incidence is estimated to be somewhere between 1-8 cases per million people. Women are twice more likely to be affected than men. PM typically occurs during middle age and is rarely seen in people younger than 30 years.

Risk Factors for Polymyositis

The causes of PM are still poorly understood. It is thought to be an autoimmune disease triggered by environmental agents such as viruses. PM is associated with autoimmune diseases such as rheumatoid arthritis and lupus erythematosus.

Progression of Polymyositis

Onset of PM is gradual over several months. Initially, patients may experience fevers, weight loss and a general feeling of unwellness. This is followed by wasting of the shoulder and hip muscles, leading to weakness in these areas. Severity of PM is variable,from a mild condition to severe disability. In progressive disease, widespread weakness and wasting may develop, with heart and lung involvement.

How is Polymyositis Diagnosed?

Diagnosis is made on the basis of clinical and physical findings, and aided by laboratory tests performed on blood samples. A MRI scan may be performed to look for muscle changes. Electromyography is often performed – this tests the ability of nerves to conduct impulses and the ability of the muscles to respond. In PM the results are often abnormal. Muscle biopsy may also be performed (taking a sample of affected muscle) to look for the inflammation that characterises PM.

Prognosis of Polymyositis

Most patients experience an improvement of their symptoms with treatment, although there may be some long term muscle weakness. PM rarely causes death, but is associated with lung, and heart disease, as well as an increased risk of certain cancers, such as bladder cancer and lymphoma.

How is Polymyositis Treated?

Many different health professionals may be involved in the care of a patient with PM. These include rheumatologists, neurologists, speech therapists and physiotherapists. Patients are encouraged to follow an exercise program to help maintain muscle strength and keep full range of movements.
Treatment is normally with corticosteroid medications. Patients are monitored for improvement and side effects to the corticosteroids such as weight gain, skin changes and reduced bone density. For cases of PM which are difficult to treat, medications to suppress the immune system may be added.

Polymyositis References

[3] Danieli, Malcangi, Palmieri, et al. Cyclosporin A and intravenous immunoglobulin treatment in polymyositis/dermatomyositis. Annals of the Rheumatic Diseases 2002;61:37-41.
[5] Hankey G., Wardlaw J. Clinical Neurology. Demos Medical Publishing, United Kingdom, 2002.
[1] Kumar and Clark. Clinical Medicine, 5th ed. WB Saunders, Toronto (2002).
[2] Lundberg, Chung. Treatment and investigation of idiopathic inflammatory myopathies. Rheumatology 2000; 39: 7-17.
[4] Pappu R. Polymyositis. eMedicine2004.
[available online @ http://www.emedicine.com/MED/topic3441.htm]

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