Dermatomyositis (DM), polymyositis (PM), and inclusion body myositis (IBM) are diseases that affect the muscles. Diagnosis of these diseases can be difficult. A medical history and physical exam and laboratory tests are needed to help the doctor reach a diagnosis.

The patient's medical and family history provides important clues about the possible cause of muscle weakness. The doctor will want to know if other family members have had similar problems. The patient's history of drug use will also be important since certain drugs can cause symptoms similar to those seen in DM, PM, and IBM. Identifying how long symptoms have been present helps the doctor diagnose the disease. Information about factors that might cause the symptoms to improve or worsen provides helpful diagnostic clues as well.

During the exam, the doctor will test muscle strength. In the early phases of the disease, weakness affects the muscles closest to the trunk. As PM and IBM progress, other muscles can be affected.

Dermatomyositis causes changes in the skin. The characteristics of any skin changes along with other symptoms such as muscle weakness can help the doctor reach a diagnosis.

Blood Tests

Creatine kinase (also known as CK or CPK) and aldolase are enzymes that play a role in muscle activity. High levels of these enzymes in the blood serum indicate muscle damage. Not surprisingly, most patients with DM, PM or IBM have high levels of serum creatine kinase and serum aldolase. Although these tests indicate muscle damage, they do not provide information about the severity of the disease.

Other blood tests that may be performed include a CBC or complete blood count, which measures the number of cells in the blood. The erythrocyte sedimentation rate (ESR or sed rate) indicates the presence certain forms of inflammatory diseases. However, the sed rate may be normal with DM, PM, and IBM. Your doctor may do a test to screen for certain types of antibodies, or to measure thyroid function.

Electrodiagnostic (EMG) Studies

Electrodiagnostic tests are used to diagnose problems with muscles and nerves. These tests may be performed in the doctor's office or in a special clinic.

An electromyography, or EMG test, measures the activity of muscles and helps distinguish nerve disorders from muscle disorders. In this test, a thin needle is inserted into a muscle and connected to a computer that records the muscle's electrical activity. Doctors will look at the pattern of activity to diagnose problems that may be occurring.

Nerve conduction studies (NCS) measure how quickly an electrical impulse travels through a nerve. Small electrodes are placed on the skin and connected to a computer that applies a small electrical current to one electrode. The computer then records the speed at which the current travels to the next electrode. Your doctor will use this information to determine if a nerve problem is causing muscle weakness.

Muscle Biopsy

A muscle biopsy is a procedure in which a sample of muscle tissue is removed for further study under the microscope. The procedure is usually performed using local anesthesia. This means that the skin around the biopsy site is numbed with a special drug. An incision is then made through the skin down to the muscle. A small section of muscle, about a half-inch long and about ¼ inch wide is removed. Sutures are then placed to keep the wound closed while the skin heals.

The muscle biopsy is the most important study to identify inflammatory muscle diseases from other causes of muscle weakness. Most importantly, the muscle biopsy can help distinguish between PM, DM, and IBM.

References

First published April 1, 2000
Last updated August 1, 2003
Copyright © 2000 Accordant Health Services, Inc. All Rights Reserved.

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