Polymyositis is an inflammatory disease that causes muscle weakness primarily affecting the shoulders, upper arms, pelvis and thighs. The hands, feet, and face are not usually affected. Although there is no cure for polymyositis, treatment can improve muscle strength and function.
Polymyositis can occur at any time of life, although it is uncommon under the age of 18 years and generally occurs between the ages of 30 and 60 years. Women are more commonly affected by the condition than men.
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 defence 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.
Signs and symptoms
The onset of symptoms usually occurs gradually over a period of months. Occasionally, however, symptoms can develop rapidly over a period of days. Symptoms may also come and go for no apparent reason.
The main symptom associated with polymyositis is muscle weakness. The muscle weakness usually first occurs in the hip and shoulder areas and is slowly progressive. The weakness is usually painless but sometimes aching, discomfort, or mild tenderness of the affected muscles is experienced. 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.
Because polymyositis is a rare disease and its symptoms can be mistaken for other conditions, it can be difficult to diagnose. For an accurate diagnosis to be made a referral to a neurologist or musculoskeletal specialist may be recommended.
The doctor will perform a physical and neurological examination and will assess the history and nature of the symptoms. Blood tests may be taken to determine if elevated levels of muscle enzymes are present (eg: creatine kinase and lactate dehydrogenase), which indicate muscle damage. Other blood tests may be done to detect the presence of rheumatoid factor and other autoantibodies (antibodies directed against the body's own tissue), which can help to determine the approach to treatment.
Two tests most commonly performed to provide a definitive diagnosis are an electromyogram (EMG) and 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 anaesthetic. 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.
Other tests that may also be performed include magnetic resonance imaging (MRI) to help determine the extent of muscle involvement, electrocardiogram (ECG) to trace the heart’s electrical activity, and tests to determine lung function.
The main treatment for polymyositis is medication.
Corticosteroids such as prednisone are most commonly used. These are usually given in high doses and have a powerful anti-inflammatory effect. However, long-term use of corticosteroids is associated with some serious short- and longer-term side effects, including high blood pressure and osteoporosis. Corticosteroid treatment may take two to three months to produce a significant improvement in symptoms. Once symptoms improve the dose of corticosteroid may be reduced to lessen the risk of side effects.
Medications that suppress the immune system are often used in combination with the corticosteroids because they allow a lower dose of corticosteroid to be used and hence reduce the risk of side effects related to corticosteroid treatment.
Biologic medicines commonly used to treat rheumatoid arthritis have been used to treat patients with severe polymyositis that does not respond adequately to corticosteroid treatment.
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.
The course of polymyositis varies from person to person. Most people affected will improve with treatment but 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
The Muscular Dystrophy Association of New Zealand aims to improve the quality of life for all people with neuromuscular conditions and their families.
Muscular Dystrophy New Zealand
Freephone: 0800 800 337
Email: [email protected]
National Institute of Neurological Disorders and Stroke (2020). Inflammatory myopathies fact sheet (Web Page). Bethesda, MD: National Institutes of Health. https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Fact-Sheets/Inflammatory-Myopathies-Fact-Sheet [Accessed: 30/11/20]
Mayo Clinic (2020). Polymyositis (Web Page). Rochester, MN: Mayo Foundation for Medical Education and Research. https://www.mayoclinic.org/diseases-conditions/polymyositis/symptoms-causes/syc-20353208 [Accessed: 30/11/20]
Seetharaman, M. (2020). Polymyositis (Web Page). Medscape Drugs and Diseases. New York, NY: WebMD LLC. https://emedicine.medscape.com/article/335925-overview [Accessed: 30/11/20]
Last Reviewed: November 2020
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