Inclusion Body Myositis – Electromyogram (EMG) and Nerve Conduction Studies

The electromyogram (EMG) and nerve conduction studies are tests that can help diagnose inclusion body myositis. They are frequently performed together, and are often referred to as electrodiagnostic tests.

Muscle weakness can be due to disease affecting the muscles themselves, or due to nerve damage. Muscles move because nerves tell them to move. If the nerve impulses are not normal, the muscle function will not be normal, even if the muscle itself is healthy. The EMG and nerve conduction studies help doctors know if the problem is in the muscles or in the nerves.

Electromyograms (EMG) study muscles. Active muscles produce electrical activity, and this can be detected by an electromyogram. A needle is inserted through the skin into the muscle, and the electrical activity is displayed on a special screen and converted into sound. The activity is measured with the muscle at rest and while the muscle is contracting.

Nerve conduction studies study the health of nerves by analyzing the speed at which an electrical impulse travels through a nerve. Two electrodes are placed on the skin, and a mild electric shock is applied at one electrode. The time needed for the electrical impulse to reach the second electrode is measured, and the speed can then be calculated.

What to Expect: EMG/Nerve Conduction Study


Are these tests painful?

Most patients agree that nerve conduction studies are not very painful, causing no more than slightly irritating mild electric shocks.
As for the EMG, it varies. Most people report very little pain, while others do experience significant pain.


What do the electromyogram (EMG) and nerve conduction studies show in inclusion body myositis?

The electromyogram (EMG) is abnormal, showing signs of muscle damage. Certain characteristic findings are typical of inclusion body myositis.

EMG frequently shows spontaneous activity in muscles affected by inclusion body myositis. There is often a mixed myopathic and neurogenic pattern of motor unit potentials. Large motor unit potentials are presumably due to changes in the motor units following muscle fiber necrosis and subsequent regeneration and reinnervation. These findings are sometimes misinterpreted as indicating a neurogenic process, leading to a mistaken diagnosis of ALS (amyotrophic lateral sclerosis). EMG of the flexor digitorum profundus, especially for the fourth and fifth fingers, is often useful. Interpretation can be difficult, and should be done by an experienced physician.

The nerve conduction study is usually normal, but sometimes it does show mild nerve damage (asymptomatic sensory peripheral neuropathy).

by Kevin Dooley, MD

Revised May 1, 2024

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