Polymyositis and dermatomyositis can occur at any age but are most common in children and adults over age 50. More adults than children get these diseases. One to five new cases per 100,000 are seen each year. Women are affected by polymyositis and dermatomyositis twice as often as men.

Muscle Weakness and Pain

Usually, the main symptom of polymyositis is muscle weakness. At the onset of the disease it generally affects the thighs and hips. If you have polymyositis, you may have difficulty climbing stairs, getting off buses with high steps or rising from low surfaces. The distance you are able to walk may eventually be limited because your muscles get too tired.

If your shoulders are involved, you may have trouble getting dressed, combing your hair or lifting heavy objects. If polymyositis affects your neck, chest and abdomen muscles it may be hard to lift your head from a pillow or get out of bed. Polymyositis can also cause you to have trouble swallowing or talking, though these are less common results of the disease.

At the start of the disease your muscles may feel sore and tender. Many people do not experience muscle pain; it occurs particularly in people with rapidly progressing disease. The muscle pain and tenderness usually goes away with treatment. After having the disease for a long period of time you may lose muscle bulk overall and the affected muscles may appear thin and ‘wasted.’

Skin Rash

When a skin rash accompanies polymyositis, the term dermatomyositis is used to describe the disease. If you have dermatomyositis you will have muscle weakness and will develop a rash, most often on your face, scalp, neck and chest. The rash can also appear on your hands and fingers (often the knuckles), elbows, knees, ankles, upper arms and thighs. The rash is often deep red in colour (almost purple) and in some areas may be slightly raised.

Joint Pain

Joint pain commonly occurs during periods when the disease is active, but the joints are not usually warm or swollen, as often happens with other forms of arthritis.

Lung Involvement

Polymyositis and dermatomyositis can cause weakness of the muscles required for breathing. They may also cause fibrosis (build up of excessive fibrous tissue) of the lungs. If you have one of the diseases and your lungs are involved you may experience coughing and shortness of breath.

Heart Involvement

In rare cases myocarditis (inflammation of the muscular walls of the heart) and congestive heart failure (heart disease accompanied by breathlessness and excessive retention of sodium and water) can occur as a result of polymyositis or dermatomyositis.

Calcium Deposits

Calcification (pronounced cal-si-fi-kay-shun) is hardening of skin and muscles as a result of calcium salt deposits. Calcification doesn’t often occur in adults with the disease, but children with dermatomyositis may develop calcium deposits years after the disease starts. The deposits generally develop in the shoulder, pelvis, hip, calf and thigh and may severely limit motion. The masses that develop under the skin can rupture and the calcium salts may drain.

Polymyositis and dermatomyositis are autoimmune diseases. This means that they begin with the immune system (which normally protects the body from germs, viruses, and bacteria) malfunctioning. It generates antibodies that attack healthy tissue in different parts of the body. The cause of polymyositis and dermatomyositis is unknown, though researchers suspect that environmental factors (such as viral infections) may play a role, as may genetic factors (meaning risk of getting the diseases may be inherited).

There is no cure for polymyositis and dermatomyositis. The goal of treatment is to help in the management of symptoms. Establishing the correct diagnosis early is important because something can be done to manage most forms of arthritis.

To be able to diagnose whether you have polymyositis or dermatomyositis, your doctor will perform a physical examination and probably order laboratory tests, such as blood tests. One test, called a biopsy, involves cutting away a very small segment of muscle tissue for analysis. Your doctor may also perform a test called an EMG. With this test electrodes are taped to your skin and the electric currents running through your muscles are recorded. This shows whether your muscles are working properly.

A test called an MRI (magnetic resonance imaging) may also be done. This test is somewhat like an X-ray, in that it creates a picture of the inside of your body. X-rays are only used for seeing bones though, and with an MRI a picture can also be taken of your muscles.

If you are diagnosed with polymyositis or dermatomyositis your active involvement in developing your prescribed treatment plan is essential.

Medicine

Cortisone is a steroid that reduces inflammation and swelling and that can influence regulation of the immune system. It is a hormone naturally produced by the body. Corticosteroids are man-made drugs that closely resemble cortisone.

The most common form of corticosteroid is called prednisone, taken in pill form. Prednisone use needs to be carefully monitored because of its many side effects, and the drug must never be stopped abruptly. Some of the side effects from long-term use include cataracts, high blood pressure, sleep problems, muscle loss, bruising, thinning of the bones (osteoporosis), weight gain and increased risk of infections. The goal with this and most drugs is to find the lowest effective dose that will avoid as many of the side effects as possible.

DMARDs target the immune system and the processes causing the symptoms, but do not reverse permanent damage. The most common of them are gold salts, methotrexate, sulfasalazine, hydroxychloroquinine, chloroquinine and azathioprine. DMARDs are usually given in addition to other medications. They usually take a few months to make a difference in the inflammation. Side effects may include mouth sores, diarrhea and nausea. More serious side effects, monitored through regular blood and urine tests, include liver damage, and excessive lowering of the white blood cell count (increasing susceptibility to certain infections) and platelet count (affecting blood clotting).

Intravenous gamma globulin infusion has been used with some success in children with polymyositis and dermatomyositis, and in some adults with severe dermatomyositis.

Exercise

Rest is generally recommended during periods of active disease. Exercise should be done when symptoms are under control to keep muscles from becoming too weak. Always consult your doctor before beginning an exercise program. He or she may also be able to refer you to a physical therapist who can advise you of the forms of exercise that are likely to be helpful, and those that could be harmful. If the disease has impaired your breathing, your therapist can instruct you in breathing exercises.

Outcomes

Many people with polymyositis and dermatomyositis respond well to treatment. Some people only have a single attack of the disease and are able to stop taking medications after about a year.

Other people have recurrent attacks or active periods of the disease that respond to medication. These people are often given regular low doses of medication to keep the disease under control. While it is less common, some people experience chronically active polymyositis or dermatomyositis and must receive continuous immunosuppressive treatments. Infections, such as pneumonia, can add to the severity of the disease.

Early diagnosis and treatment, prompt treatment of infections and careful monitoring of medications may reduce the severity of complications and improve outcomes.