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Polymyositis

Definition

Polymyositis is a condition that causes inflammation and weakness in many different muscles of the body.

What is going on in the body?

This condition is thought to be an autoimmune disorder, one in which a person's immune system attacks his or her own body tissues. When a person has polymyositis, the immune system attacks the muscles, causing muscular inflammation and weakness. Other organs in the body can also be affected.

Risks

What are the causes and risks of the condition?

The exact cause of this condition is unknown. Though it occurs most often among women between 40 to 60 years old, men and children can also be affected.
A person has a higher than normal risk of developing polymyositis if he or she has another autoimmune disorder, such as:
  • systemic lupus erythematosus (SLE). This disorder causes problems with many body organs, including the joints, lungs, kidneys, skin, and heart.
  • systemic sclerosis, or scleroderma, which causes skin hardening and tightness
Sometimes, for unknown reasons, cancer can cause polymyositis.

Prevention

What can be done to prevent the condition?

There is no known way to prevent this condition.

Diagnosed

How is the condition diagnosed?

Polymyositis is often suspected from the history of symptoms and from muscle tenderness on the physical exam. Blood tests are commonly done to help make the diagnosis. In this condition, the immune system often makes certain antibodies which can be detected in the blood.
In addition, certain muscle proteins often increase in the blood due to muscle damage. For example, creatine phosphokinase and aldolase levels commonly rise. Another blood test, called an erythrocyte sedimentation rate (ESR or sed rate), is also usually abnormal.
When these blood tests are abnormal, two other tests are commonly employed to confirm the diagnosis:
  • Electromyography (EMG) involves attaching wires to the skin and measuring the electrical activity in the muscles. The response of the muscles to small shocks is also measured.
  • A muscle biopsy involves using a special tool to remove a small piece of muscle. The muscle tissue can then be sent to the lab for examination and testing.

Long Term Effects

What are the long-term effects of the condition?

The muscles can shrink and become very weak in this condition. If the muscles become too weak, the person can die from breathing or swallowing troubles. Heart or lung damage can also occur and may be severe enough to cause permanent disability or death.
In other cases, a person, especially a child, may get better with treatment or the disease may go away completely. If polymyositis is due to another autoimmune disorder or cancer, the underlying condition may cause disability or death.

Other Risks

What are the risks to others?

This condition is not contagious.

Treatments

What are the treatments for the condition?

Medications called corticosteroids, such as prednisone, are the most common treatment. These medications suppress the immune system and reduce the inflammation.
Azathioprine (i.e., Azasan, Imuran) and methotrexate (i.e., Rheumatrex, Trexall) may also be used to suppress the immune system. If a tumor or cancer is the cause of this condition, removing or treating the tumor may improve or stop symptoms in the muscles.

Side Effects

What are the side effects of the treatments?

Medications that suppress the immune system increase a person's risk of serious infections. Corticosteroids may cause emotional problems, weight gain, and bone thinning. These medications may even cause muscle weakness, one of the symptoms they are being used to treat. If muscle weakness occurs, other medications must be used.

After Treatment

What happens after treatment for the condition?

Physical therapy can help strengthen muscles after treatment, especially in children.

Monitor

How is the condition monitored?

Repeated blood tests are sometimes used to measure the level of muscle proteins in the blood. When the levels are high, the treatment medication dose is increased because the muscles are still being damaged. Any new or worsening symptoms should be reported to the healthcare professional.

Sources

The Merck Manual, 1995, Berkow et al.

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