As with other muscle diseases, a doctor diagnoses inclusion-body myositis (IBM) by considering the individual’s personal history, family medical history and the results of a careful physical examination. This may be followed by some lab tests, perhaps of the electrical activity inside the muscles. Usually, a muscle biopsy is ordered.
After a careful history and physical exam to document the pattern of weakness in the patient’s muscles, a doctor who suspects myositis likely will order a blood test to check the level of creatine kinase (CK), an enzyme that leaks out of muscle fibers when the fibers are being damaged. Although CK levels are usually very high in other inflammatory myopathies, in IBM it may be only mildly elevated, or even normal.
In some cases, the doctor may ask for a blood test for specific antibodies, proteins produced by the immune system in myositis and other autoimmune diseases. Some of these antibodies appear to be specific to autoimmune muscle disease.
The next step is sometimes an electromyogram, a test in which tiny needles are inserted into the muscles to test their electrical activity both at rest and when the person tries to contract the muscle.
Inflammatory myopathies show a distinctive pattern of electrical activity that can help differentiate them from other types of muscle disease.
A nerve conduction velocity test is sometimes performed. This test measures how fast a nerve impulse travels and how strong it is.
Sometimes these tests are used to rule out disorders that may mimic the symptoms of IBM.
A person with a suspected inflammatory myopathy is often asked to undergo a muscle biopsy, a procedure in which a small piece of muscle is removed for examination. This biopsy can enable the physician to pinpoint the diagnosis to a type of myositis.
The biopsy sample from a person with IBM is unique because of its inclusion bodies, for which the disease is named.
These “bodies” (which don’t appear in normal cells) contain clumps of discarded cellular material. Inflammatory cells can be seen invading muscle tissue, although some researchers believe this invasion is secondary to the primary events in the muscle tissue, presumably those that cause the inclusion bodies to appear.