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Tuesday, March 11, 2014
Nerve testing confusion
What is the difference between a Repetitive Nerve Stimulation test, an EMG, and a Single Fiber EMG. Would all of these tests be equally helpful in diagnosing Myasthenia gravis? If not, which one is the most reliable?
Electromyography (EMG) is a general term for studies that involve testing nerve conduction and muscle function. Nerve conduction studies are part of the EMG and involve electrical stimulation of a nerve and recording a response. The muscle test is referred to as the `needle EMG` and involves placing a recording electrode (looks like a needle) into the muscle. These tests can be very helpful in the diagnosis of a number of nerve and muscle problems.
Now to your questions... Repetitive nerve stimulation is part of an EMG test and is designed to look for problems in nerve-muscle communication, in particular myasthenia gravis. About 80-90% of patients with myasthenia will have an abnormal repetitive stimulation.
A single-fiber EMG is a more technically difficult procedure. This involves a special needle electrode being placed near the nerve-muscle communication points (the neuromuscular junctions) and recording abnormal signals. Studies of this method suggest that it may detect abnormalities in over 95% of myasthenics. HOWEVER, other diseases may produce similar abnormalities. The test should only be done by someone with a lot of experience, which generally means someone at a University. Also, the test may take 1-2 hours to do. It is uncomfortable.
Henry J Kaminski, MD
Formerly, Professor of Neurology
School of Medicine
Case Western Reserve University