Polymyositis is characterized by inflammation and degeneration of the muscles. Dermatomyositis is polymyositis accompanied by skin inflammation.
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 aged 40 to 60 or in children aged 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 (see Mixed Connective Tissue Disease (MCTD)).
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. The disorders tend to run in families.
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 phenomenon (in which the fingers suddenly become very pale and tingle or become numb in response to cold or emotional upset—see see 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 or off of a toilet seat 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 usually affected very much.
Joint aches and inflammation occur in about 30% of people. The pain and swelling tend to be mild.
Polymyositis usually does not affect most internal organs other than the throat and esophagus. However, the lungs and heart may be affected, causing abnormal heart rhythms (arrhythmias), shortness of breath, and a cough. Gastrointestinal symptoms, which can occur in children but typically not in adults, are caused by inflammation of the blood vessels (vasculitis) and may include vomiting of blood, black and tarry stools, and severe abdominal pain, sometimes with a tear in the bowel.
Occasionally, people have dermatomyositis without polymyositis. However, in most people who have dermatomyositis, all the symptoms of polymyositis occur. In addition, rashes tend to appear at the same time as muscle weakness and other symptoms. A dusky or purplish rash (called a heliotrope rash) can appear on the face with reddish purple swelling around the eyes. The rash may also be raised and scaly and may appear almost anywhere on the body but is especially common on the knuckles, elbows, knees, and parts of the hands and feet. The area around the nails may redden or thicken. When the rash fades, brownish pigmentation, scarring, shriveling, or pale depigmented patches may develop on the skin. Rash on the scalp may look like psoriasis and be intensely itchy. Bumps composed of calcium may develop under the skin or in muscle, particularly in children.
Doctors use the following criteria to make the diagnosis of polymyositis or dermatomyositis:
Laboratory tests are helpful but cannot specifically identify polymyositis or dermatomyositis.
To monitor how the disorder is responding to treatment, doctors periodically do a blood test to measure the levels of muscle enzymes. 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 determine how the disorder is responding to treatment or select a site for biopsy. Special tests performed on muscle tissue samples may be needed to rule out other muscle disorders.
Doctors may do cancer screening for people age 40 or older who have dermatomyositis or for people age 60 or older who have polymyositis because these people may have unsuspected cancers.
Within 5 years, up to 50% of people (especially children) who have received treatment have 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 resulting from severe and progressive muscle weakness, difficulty swallowing, undernutrition, inhalation of food that causes pneumonia (aspiration pneumonia), and respiratory failure, which often occurs at the same time as pneumonia. Children may die from inflammation of the blood vessels (vasculitis) that supply the bowel. 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.
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. If children are symptom-free after about 1 year, they may be able to stop taking the drug.
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 see Motor nerves). In such situations, an immunosuppressive drug (such as methotrexate, azathioprine, mycophenolate mofetil, rituximab, or cyclosporine) is used instead of or in addition to prednisone. When other drugs are ineffective, immune globulin (a substance that contains large quantities of many antibodies) may be given by vein (intravenously).
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.
People who take corticosteroids are at risk of fractures related to osteoporosis. To prevent osteoporosis, these people are given the drugs used to treat osteoporosis, such as bisphosphonates and supplemental vitamin D and calcium. People who have polymyositis or dermatomyositis are at increased risk of atherosclerosis (see Atherosclerosis) and are closely monitored by doctors. People who are receiving immunosuppressants are also given drugs to prevent infections such as by Pneumocystis jirovecii.
Last full review/revision August 2013 by Rula A. Hajj-ali, MD