Muscle biopsy may be ordered, depending on the results of other lab tests. The biopsy is one of the best ways to diagnose myositis and to distinguish it from other muscle disorders. For this test, a doctor will remove a piece of muscle tissue and study it under a microscope for abnormalities.

A muscle biopsy can usually be done under local anesthesia (numbing the area to be sampled). The procedure may be done using a very large needle to remove a piece of muscle tissue (needle biopsy). Most times, however, an open biopsy is needed to sample a wider area. This requires making a small incision (usually an inch or so) on the skin to get to the muscle tissue.

Your doctor will choose which muscle to biopsy depending on where you are feeling the pain and weakness. Most likely, he or she will avoid muscles already tested by a recent EMG or muscle biopsy. MRIs are often used to locate a specific area where the biopsy is more likely to find the abnormalities.

There is usually little or no pain during this test, but you may feel an uncomfortable tugging. Some people, however, report more pain depending on the size of the muscle sample taken. If you have an open biopsy, you may feel more pain than with a needle biopsy because of the amount of muscle tissue removed. After the procedure, there may be a little bleeding at the site, and you may have some stitches in the area. You may feel sore for up to a week after the procedure, and you may see bruising. Ask the doctor about medication to control the pain.

If the doctor sees something unusual about the muscle that was tested, he or she will run as many tests on that muscle as necessary to decide what type of muscle condition you have. The results can show conditions such as inflammation, swelling, muscle death, and loss of muscle mass.

Skin biopsy is done to better understand the skin symptoms related to dermatomyositis. The doctor will remove a small piece of affected skin by first numbing the area with an injection of local anesthetic and then removing a small piece of the tissue. A special doctor called a pathologist will then study the skin under a microscope, looking for specific features that will help diagnose the different skin manifestations of the disease. Understanding these specific features can also help guide treatment.

The biopsied area should heal within a week or two.

You can watch a video of a skin biopsy here.

Whether it is muscle or skin, tissue from a biopsy will be examined by a physician who has special training in interpreting biopsies under the microscope. The processing can take a week or more, so the final diagnosis from a biopsy can be delayed. Occasionally, the information obtained on the biopsy is inconclusive, and a second biopsy is sometimes necessary.