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Polymyositis is an uncommon disease that causes inflammation in your muscles. It's a type of connective tissue disease. The most noticeable characteristic of this disease is muscle weakness, especially in the muscles closest to your trunk, such as your shoulder and hip muscles. As a result, you may find it difficult to get out of chairs, climb stairs, brush your hair or work with your arms over your head.

It can occur at any age, but it mostly affects adults in their 40s and 50s. It's more common in blacks than in whites, and women have it more often than men do. This disease usually develops gradually over weeks or months.

Periods of remission, during which symptoms improve spontaneously, rarely occur in this. However, treatment can improve your muscle strength and function.

Symptoms usually appear gradually, so it may be difficult to pinpoint when they first started. They may also fluctuate from week to week or month to month. The most common symptoms include:

  • Progressive muscle weakness, particularly in the muscles closest to the trunk, such as those in your hips, thighs, shoulders, upper arms and neck. This weakness is symmetrical, affecting both the left and right sides of your body.
  • Difficulty swallowing
  • Mild joint or muscle tenderness
  • Fatigue

  • Causes:
    Polymyositis belongs to a group of conditions called inflammatory myopathies. Myopathies are diseases or abnormal conditions of the muscles. The cause of most inflammatory myopathies is unknown. Infections caused by bacteria, parasites or viruses can cause inflammatory myopathies, but in most cases, doctors aren't able to identify a preceding infection in this disease.

    A similar disease is dermatomyositis. Dermatomyositis leads to many of the same symptoms, but it causes skin inflammation or a rash as well. Some other inflammatory muscle diseases are: inclusion body myositis, which progresses more slowly than other forms; myositis associated with other connective tissue diseases, such as lupus or scleroderma; and myositis associated with cancer.

    . . . Immune system plays role
    Doctors suspect that myopathies are autoimmune disorders, in which your body's immune system mistakes normal components of your body for foreign substances and attacks them. If you have this disease, an unknown cause seems to trigger your immune system to begin producing autoimmune antibodies that may damage healthy body tissues. Many people with polymyositis show a detectable level of autoantibodies in their blood. It's still unclear, however, whether these autoantibodies are indeed involved in causing polymyositis.

    Polymyositis has also been associated with several viral diseases, including HIV. Some researchers speculate that, in some form, polymyositis may be caused by a viral infection of the muscle. This theory isn't well supported, however.

    Although there's no cure for polymyositis, treatment can improve your muscle strength and function. Treatment begun early in the disease process tends to be more effective, often because there are fewer complications. Therapies include the following:
      Corticosteroids These medications suppress your immune system, limiting the production of antibodies and reducing muscle inflammation. Corticosteroids, especially prednisone, are usually the first choice in treating inflammatory myopathies, such as polymyositis.
      Your doctor may begin with a very high dose then decrease it as your symptoms improve. Improvement generally takes about two to four weeks. Significant results are usually evident within three to six months, but therapy is often needed for years.
      Prolonged use of corticosteroids can have serious side effects including osteoporosis, weight gain, diabetes, increased risk of some infections, mood swings, cataracts, high blood pressure, a redistribution of body fat and muscle weakness. As a result, your doctor may also recommend supplements, such as calcium and vitamin D, and may prescribe bisphosphonates, such as alendronate (Fosamax) or risedronate (Actonel), to counteract loss of bone density.
      Other immunosuppressants If your body doesn't respond adequately to corticosteroids, your doctor may recommend other immunosuppressive drugs, such as azathioprine (Imuran) or methotrexate (Rheumatrex). Your doctor may prescribe these alone or in combination with corticosteroids.
      When in combination, these additional immunosuppressants can be used to lessen the dose and potential side effects of the corticosteroid. Immunosuppressants, such as cyclophosphamide (Cytoxan) and cyclosporine (Neoral, Sandimmune), may improve symptoms of polymyositis and interstitial lung disease.
      Physical therapy A physical therapist can show you various exercises to maintain and improve your strength and flexibility and advise an appropriate level of activity. Your exercise program is likely to change during the course of the disease and treatment period. Staying active will help maintain muscle strength.
    Other treatments that are still under investigation include:

  • Plasmapheresis This treatment, also called plasma exchange, is a type of blood cleansing in which damaging antibodies are removed from your blood.
  • Radiation therapy This involves irradiation of the lymph nodes to suppress your immune system.
  • Intravenous immunoglobulin (IVIg) This involves receiving intravenous infusions of antibodies from a group of donors over two to five days. This treatment is usually expensive. It may be an option for you if your dermatomyositis is severe or resistant to other forms of therapy.
  • Fludarabine (Fludara) This agent prevents the development and growth of malignant cells.
  • Tacrolimus (Prograf) This transplant-rejection drug may work to inhibit the immune system. Tacrolimus is often used topically to treat dermatomyositis and other skin problems.
  • Monoclonal antibodies These man-made antibodies are designed to target and destroy specific types of cells. Clinical trials are studying the effects of infliximab (Remicade) and rituximab (Rituxan) on both polymyositis and dermatomyositis.

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