What is polymyositis?Polymyositis is a rare inflammatory muscle disease that causes varying degrees of muscle weakness. The disease has a gradual onset and may occur in childhood or later adult life. The childhood form of the condition is rare.
The most common symptom is muscle weakness, usually affecting those muscles that are closest to the trunk of the body (proximal). Patients typically have difficulty rising from a sitting position, climbing stairs, lifting objects, or reaching overhead, for example combing hair or packing shelves.
Rarely, distal muscles (those not close to the trunk of the body) may also be affected later in the course of the disease. Trouble with swallowing (dysphagia) may occur. Occasionally, the muscles ache and are tender to touch. Patients may also feel fatigue and discomfort and have weight loss or a low-grade fever.
There are several types that can occur:
- Idiopathic primary muscle inflammation – polymyositis
- Related or secondary to an underlying disease such as scleroderma or lupus
- Associated with rash – dermatomyositis
- Associated with malignancy
- Associated with peripheral involvement and histology changes of inclusion - body myositis
DiagnosisThis requires demonstration of clinical weakness, plus changes of the muscles on electrical testing and changes of the blood measurement of muscle enzymes – especially CPK levels. The biopsy of the main affected muscles is then desirable to confirm the diagnosis, and establish how much scarring is present in the muscle tissue.
PrognosisThe prognosis for polymyositis varies. Many cases respond to therapy. The disease is more severe and resistant to therapy in patients with heart or lung involvement. Death is rare but may occur in patients with severe and progressive muscle weakness, dysphagia or respiratory failure.
How is it treated?Treatment for polymyositis generally consists of prednisone (a form of cortisone). The cortisone is used at high doses, aimed at reducing the blood CPK levels to normal. The patient should be using preventative measures for steroid side-effects in the time that high doses are used. The patient may be on high doses for several months. For patients in whom prednisone is not effective, other immunosuppressants such as azathioprine and methotrexate may be prescribed.
Physical therapy is usually recommended to preserve muscle function and avoid muscle atrophy.
- Reviewed by Dr David Gotlieb, rheumatologist, MBChB FCP(SA)