Polymyositis is characterized by inflammation and degeneration of the muscles. Dermatomyositis is polymyositis accompanied by skin inflammation.

• Muscle damage may cause muscle pain and difficulty lifting the arms above the shoulders, climbing stairs, or arising from a sitting position.
• Doctors check muscle enzymes in the blood and sometimes test electrical activity of muscles, perform magnetic resonance imaging (MRI) on muscles, check levels of muscle enzymes in the blood, examine a piece of muscle tissue (biopsy), or a combination.
• Oral corticosteroids are usually helpful.

These disorders result in disabling muscle weakness. The weakness typically occurs in the shoulders and hips but can affect muscles symmetrically throughout the body.

Polymyositis and dermatomyositis usually occur in adults from ages 40 to 60 or in children from ages 5 to 15 years. Women are twice as likely as men to develop either disorder. In adults, these disorders may occur alone or as part of other connective tissue disorders, such as mixed connective tissue disease.

The cause of polymyositis and dermatomyositis is unknown. Viruses or autoimmune reactions may play a role. Cancer may also trigger polymyositis and dermatomyositis. It is possible that an immune reaction against cancer may be directed against a substance in the muscles.

Symptoms

Polymyositis: In polymyositis, the symptoms are similar for people of all ages, but the disorder usually develops more abruptly in children than in adults. Symptoms, which may begin during or just after an infection, include symmetrical muscle weakness (particularly in the upper arms, hips, and thighs), joint pain (but often little muscle pain), difficulty in swallowing, fever, fatigue, and weight loss. Raynaud's syndrome (in which the fingers suddenly become very pale and tingle or become numb in response to cold or emotional upset—see Peripheral Arterial Disease: Raynaud's Syndrome) occurs more commonly among people who have polymyositis along with other connective tissue disorders.

Muscle weakness may start slowly or suddenly and may worsen for weeks or months. Because muscles close to the center of the body are affected most, tasks such as lifting the arms above the shoulders, climbing stairs, and getting out of a chair can become very difficult. If the neck muscles are affected, even raising the head from a pillow may be impossible. Weakness in the shoulders or hips can confine a person to a wheelchair or bed. Muscle damage in the upper part of the esophagus can cause swallowing difficulties and regurgitation of food. The muscles of the hands, feet, and face, however, are not affected.

Joint aches and inflammation occur in about 30% of people. The pain and swelling tend to be mild.

Polymyositis usually does not affect internal organs other than the throat and esophagus. However, the lungs and heart may be affected, causing shortness of breath and a cough.

Dermatomyositis: In dermatomyositis, all the symptoms of polymyositis occur. In addition, rashes tend to appear at the same time as muscle weakness and other symptoms. A shadowy-red or purplish rash (heliotrope rash) can appear on the face with reddish purple swelling around the eyes. Another rash, which may be scaly, smooth, or raised, may appear almost anywhere on the body but is especially common on the knuckles and sides of the hands. The nail beds may redden. When the rashes fade, brownish pigmentation, scarring, shriveling, or pale depigmented patches may develop on the skin.

Diagnosis

Doctors use the following criteria to make the diagnosis of polymyositis or dermatomyositis:

• Muscle weakness at the shoulders or hips
• A characteristic rash
• Increased blood levels of certain muscle enzymes (especially creatine kinase) in the blood, indicating muscle damage
• Abnormalities in muscle electrical activity as measured by electromyography (see Diagnosis of Brain, Spinal Cord, and Nerve Disorders: Electromyography and Nerve Conduction Studies), or on appearance on a magnetic resonance imaging (MRI) scan
• Characteristic changes in muscle tissue obtained by biopsy and observed under a microscope (the most conclusive evidence)

Laboratory tests are helpful but cannot specifically identify polymyositis or dermatomyositis. Muscle enzymes are measured repeatedly in blood samples to monitor the disorder; the levels usually fall to normal or near normal with effective treatment. Magnetic resonance imaging (MRI) may also show areas of inflammation and help the doctor select a site for biopsy. Special tests performed on muscle tissue samples may be needed to rule out other muscle disorders.

Prognosis

Within 5 years, up to 50% of people (especially children) who have received treatment experience a long remission (even apparent recovery). However, the disorder may still return at any time. About 75% of people survive at least 5 years after the diagnosis is made. This percentage is even higher among children. Adults are at risk of death from severe and progressive muscle weakness, difficulty swallowing, undernutrition, inhaling food that causes pneumonia (aspiration pneumonia), and respiratory failure, which often occurs at the same time as pneumonia. Polymyositis tends to be more severe and resistant to treatment in people whose heart or lungs are affected. In people who have cancer, it is the cancer, rather than the polymyositis, that is the cause of death.

Treatment

Modest restriction of activities when the inflammation is most intense often helps. Generally, a corticosteroid, usually prednisone, given by mouth in high doses slowly improves strength and relieves pain and swelling, controlling the disease. After about 6 to 12 weeks, when the muscle enzyme levels have returned to normal and muscle strength has returned, the dose is gradually decreased. Many adults must continue taking a low dose of prednisone or an alternative drug for many years or even indefinitely to prevent a relapse. After about a year, children may be able to stop taking the drug and stay symptom-free.

In some people, corticosteroids are not very effective or must be taken at very high doses to be effective. In some people, corticosteroids may cause muscle damage and weakness (see Diagnosis of Brain, Spinal Cord, and Nerve Disorders: Motor and Sensory Nerves). In such situations, an immunosuppressive drug (methotrexate, azathioprine, or cyclosporine) is used instead of or in addition to prednisone. When other drugs are ineffective, gamma globulin (a substance that contains large quantities of many antibodies) may be given by vein (intravenously). Other new remedies that may be effective in treating polymyositis and dermatomyositis include rituximab and a class of drugs (such as infliximab and etanercept) that inhibit a chemical called tumor necrosis factor (tumor necrosis factor inhibitors).

When polymyositis is associated with cancer, it usually does not respond well to prednisone. However, the condition usually lessens in severity if the cancer can be successfully treated.

Because people who take corticosteroids are at risk of fractures related to osteoporosis, they should be closely screened and treated for osteoporosis. Preventive measures (such as treatment of high blood pressure, high blood cholesterol levels, and osteoporosis) should be taken.

Last full review/revision February 2008 by Rula A. Hajj-Ali, MD