Definition
A nerve conduction study is a test that measures the movement of an impulse through a nerve after the deliberate stimulation of the nerve.
Purpose
The ability of a nerve to swiftly and properly transmit an impulse down its length, and to pass on the impulse to the adjacent nerve or to a connection muscle in which it is embedded, is vital to the performance of many activities in the body.
When proper functioning of nerves does not occur, as can happen due to accidents, infections, or progressive and genetically based diseases, the proper treatment depends on an understanding of the nature of the problem. The nerve conduction study is one tool that a clinician can use to assess nerve function. Often, the nerve conduction study is performed in concert with a test called an electromyogram. Together, these tests, along with other procedures that comprise what is known as electrodiagnostic testing, provide vital information on the functioning of nerves and muscles.
Description
Nerve cells consist of a body, with branches at one end. The branches are called axons. The axons are positioned near an adjacent nerve or a muscle. Nerve impulses pass from the axons of one nerve to the next nerve or muscle. The impulse transmission speed can be reduced in damaged nerves.
Surrounding a nerve is a tough protective coat of a material called myelin. Nerve damage can involve damage or loss of myelin, damage to the nerve body, or damage to the axon region. The nerve conduction study, which was devised in the 1960s, can detect the loss of nerve function due to these injuries, and, from the nature of the nerve signal pattern that is produced, offer clues as to the nature of the problem.
Depending on the nature of the nerve damage, the pattern of signal transmission can be different. For example, in a normal nerve cell, sensors placed at either end of the cell will register the same signal pattern. But, in a nerve cell that is blocked somewhere along its length, these sensors will register different signal patterns. In another example, in a nerve cell in which transmission is not completely blocked, the signal pattern at the axon may be similar in shape, but reduced in intensity, to that of the originating signal, because not as much of the signal is completing the journey down the nerve cell.
Diseases of the nerve itself mainly affect the size of the responses (amplitudes); diseases of the myelin mainly affect the speed of the responses.
Nerve conduction studies are now routine, and can be done in virtually any hospital equipped with the appropriate machine and staffed with a qualified examiner. The nerve conduction study utilizes a computer, computer monitor, amplifier, loudspeaker, electrical stimulator, and filters. These filters are mathematical filters that can distinguish random, background electrical signals from the signal produced by an activated nerve. When the study is done, small electrodes are placed on the skin over the muscles being tested. Generally, these muscles are located in the arms or legs. Some of the electrodes are designed to record the electrical signal that passes by them. Other electrodes (reference electrodes) are designed to monitor the quality of the signals to make sure that the test is operating properly. If monitoring of the test is not done, then the results obtained are meaningless.
After the electrodes are in place, a small electrical current can be applied to the skin. The electrical stimulation is usually done at several points along the nerve, not just at a single point. This is done because conduction of an impulse through a nerve is not uniform. Some regions of a nerve conduct more slowly than other regions. By positioning the stimulating electrodes at several sites, a more accurate overall measurement of conduction velocity is obtained.
The electrical current activates nerves in the vicinity, including those associated with the particular muscle. The nerves are stimulated to produce a signal. This is known as the "firing" of the nerve. The nerve signal, which it also electric, can be detected by some of the electrodes and conveyed to the computer for analysis.
The analysis of the nerve signal involves the study of the movement of the signal through the nerve and from the nerve to the adjacent muscle. Using characteristics such as the speed of the impulse, and the shape, wavelength, and height of the signal wave, an examiner can assess whether the nerve is functional or defective.
Risks
A nerve conduction study can be done quite quickly. A person will experience some discomfort from the series of small electrical shocks that are felt. Otherwise, no damage or residual effects occur.
Normal results
Analysis of the results of a nerve conduction study
Under normal circumstances, the movement of the electrical impulse down the length of a nerve is very fast, on the order of 115–197 ft/sec (35–60 m/sec).
A number of aspects of the nerve impulse are measured in nerve conduction studies. The first aspect (or parameter) is known as latency. Latency is the time between the stimulus (the applied electrical current) and the response (the firing of the nerve). In damaged nerves, latency is typically increased.
Another parameter is known as the amplitude. Electrical signals are waves. The distance from the crest of one wave to the bottom of the trough of the adjacent wave is the amplitude. Impulses in damaged nerves can have an abnormal amplitude, or may show different amplitudes in the undamaged and damaged sections of the same nerve.
The area under a wave can also vary if not all muscle fibers are being stimulated by a nerve or if the muscle fibers are not all reacting to a nerve impulse at the same time. The speed of a nerve impulse (the conduction velocity) can be also be determined and compared to data produced by a normally functioning nerve.
A number of other, more technically complex parameters can also be recorded and analyzed. A skilled examiner can tell from the appearance of the impulse waves on the computer monitor whether or not a nerve or muscle is functioning normally, and can even begin to gauge the nature of a problem. Examples of maladies that can be partially diagnosed using the nerve conduction study include Guillain-Barré syndrome, amyotrophic lateral sclerosis (ALS, or Lou Gehrig's disease, Charcot's disease), and multifocal motor neuropathy.
Conditions affecting the nerve conduction study
The nerve conduction study does not produce uniform results from person to person. Various factors affect the transmission of a nerve impulse and the detected signal.
Temperature affects the speed of impulse movement. Signals move more slowly at lower temperatures, due to the tighter packing of the molecules of the nerve. This variable can be minimized during the nerve conduction study by maintaining the skin temperature at 80–85° Fahrenheit (27–29° Celcius). Use of a controlled temperature also allows study runs done at different times to be more comparable, which can be very useful in evaluating whether muscle or nerve problems are worsening or getting better.
The speed of nerve impulse transmission changes as the body ages. In infants, the transmission speed is only about half that seen in adults. By age five, most people have attained the adult velocity. A gradual decline in conduction velocity begins as people reach their 20s, and continues for the remainder of life. Another factor that influences conduction velocity is the length of the nerve itself. An impulse that has to travel a longer distance will take longer. Some nerves are naturally longer than others. Measurement of nerve conduction takes into account the length of the target nerve.
Resources
OTHER
"Electromyography (EMG) and Nerve Conduction Studies." WebMD. May 1, 2004 (June 2, 2004). http://my.webmd.com/hw/health_guide_atoz/hw213852.asp.
"Nerve conduction velocity." Medline Plus. National Library of Medicine. May 3, 2004 (June 2, 2004). http://www.nlm.nih.gov/medlineplus/print/ency/article/003927.htm.
"Nerve Conduction Velocity Test." MedicineNet.com. May 1, 2004 (June 2, 2004). http://www.medicinenet.com/Nerve_Conduction_Velocity_Test/article.htm.
Brian Douglas Hoyle, PhD
