Physicians will generally carry out a nerve conduction study (NCS) — also called a nerve conduction velocity (NCV) test — prior to ordering an electromyography (EMG) to diagnose amyotrophic lateral sclerosis (ALS).
Electrical signals are passed along nerve cells throughout the body to relay messages, for example, to tell a muscle to contract. An NCS assesses the conduction velocity, or speed of the electrical signal, through the nerve. This can determine if and where nerve damage exists.
Preparing for the test
Prior to the test, the patient will be asked to avoid moisturizers, creams or lotions, all of which can interfere with the procedure. Depending on what sites are being tested, the patient also be required to remove clothing (as well as jewelry, hairpins, eyeglasses, hearing aids or other metal objects) and will be given a gown to wear.
What happens during the test?
To diagnose ALS, at least two sites are tested during the exam. This includes one upper limb and one lower limb, starting with the site that shows the most severe symptoms.
For the NCS, flat metal disc electrodes are attached to the skin with tape or a special paste. A shock-delivering electrode is placed over the nerve being tested, while a recording electrode is placed at a site the nerve would normally stimulate, such as a muscle. One electrode stimulates the nerve by delivering a small electrical shock, while another is placed over the muscle directly supplied by that nerve to record the electrical signal.
Several mild electrical shocks are delivered to stimulate the nerve, and the second electrode records the resulting electrical activity. The time it takes for the response to register over the distance between those two points determines the conduction velocity.
This process is repeated for each site to be tested. Depending on the number of nerves being tested, the procedure can take 30 to 60 minutes.
What do the results mean?
Different nerves have different associated conduction velocities, but a damaged nerve will produce a weaker and slower signal compared to a healthy one.
Patients with ALS tend to show mostly normal results, especially for sensory nerves, though abnormal results can lead to a different diagnosis than ALS such as a myopathy, or muscle disease. However, ALS patients commonly show abnormally low compound muscle action potential (CMAP) amplitudes, meaning the combined level of signal produced by the muscle fibers supplied by the nerve.
The results of the NCS can help direct where to carry out an EMG.
Risks associated with the test
The procedure causes no side effects. Patients may feel minor discomfort from the brief low-voltage electrical pulse, but this does not cause any damage. Some patients report a brief burning and tingling sensation, and a twitching of the muscle.
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