Numbness and weakness are some of the most common problems that bring people to see a neurologist. Determining the location and cause of the problem can lead to effective treatments.
Why Are EMG and NCSs Performed?
Nerve conduction studies (NCSs) and electromyography (EMG) are commonly used to investigate nerve and muscle disorders like large-fiber peripheral neuropathy, radiculopathy, Guillain-Barre syndrome, myasthenia gravis, and amyotrophic lateral sclerosis. An EMG evaluates abnormal electrical activity in muscles, and a nerve conduction study investigates how electricity flows through a nerve. These tests enhance the physician's ability to determine the exact location of a nerve or muscle disorder, the nerve functions that are compromised, and whether a neuropathy is affecting the axon of the nerve or the protective myelin sheath.
Because different disease categories cause different electrical abnormalities in nerves and muscles, an EMG and NCS can help physicians determine the cause of numbness or weakness. Other benefits of these tests include the ability to follow disease progression over time.
How Are These Tests Done?
While an EMG and NCS are different procedures, they are used to address similar diagnostic questions, and so are often used together. The information gained from one test often adds to that gained from the other.
An electromyogram involves inserting a thin needle, about the size of an acupuncture needle, into a muscle in order to measure electrical activity. This electrical activity is represented as waves on a computer monitor, and also as static-like sounds. Someone performing the test must both look and listen in order to detect abnormal patterns. In addition to measuring spontaneous signals from a muscle at rest, the person doing the procedure will also ask you to flex the muscle so that he can detect abnormalities in contraction.
During a nerve conduction study, small electrical shocks are delivered to a nerve at set locations. Electrical leads attached elsewhere on the body pick up that electrical signal, and a computer then calculates the speed, amplitude and waveform of electrical transmission.
Do The Tests Hurt?
The amount of discomfort experienced by people who undergo EMG and NCSs varies a lot from person to person. While the tests are usually described as uncomfortable, they're generally not described as painful.
The needle used during an EMG is very thin, somewhat like an acupuncture needle. Inserting the needle is usually the least pleasant part, but afterwards the procedure is normally painless. Some people describe some muscle aches for a day or two afterwards, and there may be very mild bleeding, but even these complications are uncommon.
The shocks applied during an NCS are unpleasant but brief. Some people describe them as painful, and others do not. The electricity can cause muscles to twitch and jerk, which is an unpleasant sensation for many, though it's not painful.
I've had these tests done myself, as do most people who are learning how to do the procedures. This allows us to practice technique and appreciate what patients go through. Having been through the procedures, I can say that — while they are not pleasant — I did not find them very painful. I would willingly undergo the tests again, even if just to demonstrate how the test was done.
Special EMG and NCS Protocols
Some situations call for specialized techniques using EMG and NCS. The best example is myasthenia gravis, a disease in which people get progressively weaker as they use a muscle repeatedly. Repetitive stimulation during the NCS allows doctors to see if the signal declines over time, which is suggestive of myasthenia. A technique called single-fiber EMG can also be very useful in diagnosing myasthenia gravis.
While these tests are not pleasant, they are temporary, and the discomfort does not last. The knowledge gained by these tests, on the other hand, may offer more lasting relief from the symptoms of peripheral neuropathy and other diseases.
Alport AR, Sander HW, Clinical Approach to Peripheral Neuropathy: Anatomic Localization and Diagnostic Testing. Continuum; Volume 18, No 1, February 2012
Blumenfeld H, Neuroanatomy through Clinical Cases. Sunderland: Sinauer Associates Publishers 2002