Patient Articles


What Is Myositis?

Myositis is an inflammation in your muscles. You can have one of several different types of myositis.

  Polymyositis (PM) refers to inflammation in a number of muscles.
  Dermatomyositis (DM) refers to inflammation in muscles and skin.
  Inclusion Body Myositis (IBM) refers to inflammation, protein deposits, and holes in the muscles.

What Causes Myositis?

The cause of myositis is not known. But it is probably an autoimmune disease. In autoimmune diseases, your body’s immune system thinks that some of your body cells are invading cells. Therefore, it attacks these cells by mistake. This causes inflammation and pain.

Myositis can affect adults and children alike. In children, it often begins between the ages of five and 15. Adults usually get myositis between the ages of 30 and 60. Two types of myositis—PM and DM—occur more often in women. But IBM is seen more often in men, and also in people who are over 50 years old.

What Are the Symptoms?

People with myositis have muscle weakness in their arms, hands, legs, and feet. Usually, the weakness is seen at the same places on both sides of the body. Myositis can make it hard to get up from a chair or to walk upstairs. It also can cause problems with swallowing. Muscle aches and pain are common.

People with DM often have a rash on their eyelids and knuckles. In those with IBM, the weakness is very different from one side of the body to the other. Having IBM often leads to falls or a loss of grip strength.

How Is Myositis Diagnosed?

A number of tests may be used to diagnose myositis and to rule out other diseases. Blood tests measure the health of your muscles and of your liver. Your doctor may also check how strong your muscles are during your checkups. An electromyogram (EMG) and nerve conduction studies can show how well your muscles and nerves function.

Another common test is a muscle biopsy. The biopsy shows exactly which part of your muscle is inflamed. In PM and IBM, inflammation is seen inside the muscle bundles (fascicles). In DM, inflammation occurs in the connective tissue around the muscle bundle, and in only the outer layers of the muscle bundles. Your doctor may order an MRI first, to find out which muscles to biopsy. An MRI is also used to monitor any changes in your myositis.

Blood Tests

Blood tests show the amount of muscle enzymes in your blood. Muscle enzymes are chemicals in muscles that make muscles do the work they do. Healthy muscles keep enzymes inside. Damaged muscles allow enzymes to leak into the blood. The most common blood test is the CPK test. Normal CPK values are between 20 IU/L and 200 IU/L. Some people with myositis have blood CPK levels that are over 1000 IU/L!  Other blood tests your doctor may order include the aldolase, and sometimes, blood tests that are more often associated with the liver may reveal muscle inflammation as well.

Manual Muscle Testing

This is a simple test to determine how well your muscles work. Your doctor pushes against your arm or leg, and you try to resist. This test is usually done each time you see your doctor. It shows whether your myositis is getting better or worse.

Electromyogram (EMG) and Nerve Conduction Studies

For these tests, small needles are placed into the muscle. The doctor then can see how well your muscles and nerves function. These tests are a little uncomfortable but they are not very painful, and may be very helpful to your doctor.

Muscle Biopsy

A muscle biopsy is minor surgery. It is performed under local anesthesia. During the biopsy procedure, a small piece of your muscle is removed. You will be able to go home later on in the day of the biopsy. A muscle biopsy may be important in helping your doctor to know what type of myositis is affecting you.


Your doctor will probably prescribe corticosteroids to treat the muscle inflammation. Sometimes, fairly high doses may be needed. If corticosteroids do not work well enough, other medications may be added. These may include azathioprine, methotrexate, or the biologic medications. For more information about these medications, please see the Rheumatology Information articles about these topics.

Exercise and rest may also be advised. Particularly after inflammation is controlled, exercise may be important in rebuilding muscle strength.


Myositis Resources

Find out more about myositis from the following organizations:

The Myositis Association -  800-821-7356
The Myositis Support Group -  903-675-6825
American Autoimmune Related Diseases Foundation -  586-776-3900
National Organization of Rare Diseases -  203-744-0100
Autoimmune Information Network, Inc. -  732-664-9259