Polymyositis and Dermatomyositis

  • Medical Author:
    William C. Shiel Jr., MD, FACP, FACR

    Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

  • Medical Editor: Catherine Burt Driver, MD
    Catherine Burt Driver, MD

    Catherine Burt Driver, MD

    Catherine Burt Driver, MD, is board certified in internal medicine and rheumatology by the American Board of Internal Medicine. Dr. Driver is a member of the American College of Rheumatology. She currently is in active practice in the field of rheumatology in Mission Viejo, Calif., where she is a partner in Mission Internal Medical Group.

What tests do doctors use to diagnose polymyositis or dermatomyositis?

When a patient first sees the doctor, the recent symptoms especially concerning weakness will be discussed. The condition of many other body areas might be reviewed, for example, the skin, heart, lungs, and joints. An examination will further focus on these and other systems. Various measures of strength might be noted. The characteristic features of polymyositis include weakness of the muscles closest to the trunk of the body, abnormal elevation of muscle enzymes, electromyograph (EMG) findings, magnetic resonance imaging (MRI) findings, and certain abnormalities detected with muscle biopsy.

Blood testing usually (but not always) reveals abnormally high levels of muscle enzymes, CPK or creatinine phosphokinase, aldolase, SGOT, SGPT, and LDH. These enzymes are released into the blood by muscle that is being damaged by inflammation. They can also be used as measures of the activity of the inflammation. Other routine blood and urine tests can also look for internal organ abnormalities. Chest X-rays, mammograms, PAP smears, and other screening tests might be considered. Autoantibodies can often be found in the blood of people with polymyositis. These include antinuclear antibodies (ANAs) and myositis-specific antibodies (such as Jo-1 antibody).

An electromyograph (EMG) and nerve conduction velocity are electrical tests of muscle and nerves that can show abnormal findings typical of polymyositis as well as exclude other nerve-muscle diseases.

Imaging of the muscles using radiology tests, such as magnetic resonance imaging (MRI scanning), can show areas of inflammation of muscle. This sometimes can be used to determine optimal muscle biopsy sites.

A muscle biopsy is used to confirm the presence of muscle inflammation typical only of polymyositis. This is a surgical procedure whereby muscle tissue is removed for analysis by a pathologist, a specialist in examining tissue under a microscope. Muscles often used for biopsy include the quadriceps muscle of the front of the thigh, the biceps muscle of the arm, and the deltoid muscle of the shoulder.

Medically Reviewed by a Doctor on 5/18/2016

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