NCS VS EMG: What’s Needed for a CMT Diagnosis

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I’m confused about the difference between electromyograms (EMG) and nerve conduction studies (NCS)—can you please explain?

Dr. Richard A. Lewis* answers:

EMG, which stands for electromyogram (“myo”= muscle), is the term used for electrodiagnostic tests (EDX) for neuromuscular disorders. The total EDX includes both nerve conduction studies (NCS) and needle EMG. Depending on the clinical question, one or both parts of the test may be conducted.

For CMTers, the most important component is the NCS, which determines whether one has a neuropathy and whether it involves sensory nerves, motor nerves or both. If the disorder just involves motor nerves, it’s Hereditary Motor Neuropathy (HMN); if sensory, Hereditary Sensory Neuropathy (HSN) and if both Hereditary Motor/Sensory Neuropathy (HMSN). HMSN is the primary disorder that comprises CMT.

Sensory nerves usually only require one stimulation point—the wrist (sometimes the finger) or ankle. The motor nerve conduction study requires stimulation of the nerve: and recording electrodes are put on the surface of the skin overlying the belly of the muscles being studied.

To determine motor nerve conduction velocity, it is necessary to stimulate the nerve at two locations: In the arm, the two stimulation sites are the wrist and the elbow. In the leg, they are the ankle and knee. The motor nerve velocity in the arm is determined by taking the time it takes (latency) for the signal to go from the wrist to the muscle and subtracting that from the latency from the elbow to the muscle. Dividing that nerve latency (subtracting out the time it takes for the signal to get from the nerve to the muscle) into the distance from wrist to elbow determines the motor nerve conduction velocity (see below).

The amplitude of the response indicates whether the nerve fibers are functioning: A low amplitude suggests that many have stopped. The velocity determines whether the disorder primarily affects the myelin and Schwann cells or the axon. The disorders that comprise CMT1 are characterized by very slow velocities and are due to mutations of genes that form myelin. CMT2 has more normal velocities but very low amplitudes and are disorders of the axon. Normal nerves conduct at ~ 50 meters/second. CMT1A usually has nerve velocities around 20 meters/second. There is a somewhat arbitrary cut-off of 38 meters/second (m/sec) in the median or ulnar nerve of the forearm that determines CMT1 or CMT2. CMTX is considered intermediate with velocities between 30 and 40 meters/second.

The EMG portion of the test, which involves the insertion of fine needles in the muscle, can determine if there is nerve damage to the muscle not identified by the nerve conduction tests. This can be particularly helpful in HMN and can determine if there is any muscle involvement in HSN. EMG can evaluate muscles that are more proximal—above the knees and elbows—which are not easily tested with NCS. This can be helpful, but rarely allows a diagnosis of CMT. Because it is not always necessary for the EMG portion of the EDX to be done, the decision should be discussed with the doctor and electromyographer.

Electrodiagnostic studies are not risky or dangerous and do not cause problems afterward, but they can be uncomfortable. The NCS requires electrical stimulation, which is very brief but can be painful. Some CMTers have nerves that are difficult to stimulate, which can require higher amounts of stimulation. This may be painful, but the pain lasts a fraction of a second. It’s best if the patient can allow testing of at least one motor and one sensory nerve even if it’s uncomfortable. Sensory nerves need less stimulation than motor nerves and are less painful. For CMTers, studies of the arms may provide more information than the legs, but each case is different. Relaxation techniques can help reduce anxiety and pain.

The needles used in EMG are very thin and sharp. They are disposable, so there is virtually no risk of infection. They are thinner than the needles used for drawing blood and there is minimal risk of bleeding even if one is on aspirin. Anyone on a blood thinner should bring it to the electromyographer’s attention, but most muscles can be tested even if the patient is taking Coumadin or other major blood thinners. For most patients, the needle examination is only mildly uncomfortable, but for some, particularly patients with aversion to any needles, the needle examination can be painful. The good news is that if needle studies are done, there shouldn’t be the need to study many muscles.

NCS and EMG can be performed at any age, including infancy, but with children, the examination has to be modified to account for their size and inability to fully cooperate during the study. The 38 m/sec velocity that distinguishes CMT 1 and 2 cannot be used under the age of 2. The examination in young children is usually brief and sedation is not normally necessary. If a child is from a family with known CMT that has been diagnosed genetically, then EDX may not be needed. If the child is symptomatic, there may not be a need for any testing.

*Dr. Lewis is the co-director of the Inherited Neuropathy Clinic and the director of the EMG Laboratory and of the Clinical Specialty Clinic at Cedars-Sinai Medical Center in Los Angeles. He moved to Los Angeles in November 2012 after 19 years at Wayne State University in Detroit, Michigan, where he helped develop the CMT clinic. He was the principal investigator of the Vitamin C trial for CMT1A. He has served on the Board of Directors of the Peripheral Nerve Society and is currently on the Steering Committee of the Inflammatory Neuropathy Consortium.