Electroneurography

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Electroneurography

Definition

Electroneurography is the measurement of the speed of conduction of impulses down a peripheral nerve. The test is done to detect and roughly quantify the extent of nerve damage.

Purpose

Electroneurography, also known as nerve conduction studies (NCS), nerve conduction velocity (NCV), or stimulation myelographic study (SMS), is used to detect the presence of a neuropathy in a particular nerve. Anatomically, there are three conditions that significantly decrease nerve conduction velocities:

demyelination (loss of myelin covering of the nerve)
conduction blocks (damage that stops continued movement of nerve impulse)
axonal loss (nerve cell death)

Electroneurography is used to detect and evaluate a wide variety of diseases or conditions involving nerve damage. It is a routine test after traumatic nerve damage such as carpal tunnel syndrome or to investigate suspected peripheral nerve dysfunctions or neuropathies. Nerve problems caused by viral infections such as HIV-1 or HSV-1 are also common indications for this procedure. Electroneurography can detect nerve damage that occurs as a side effect of systemic problems, including diabetes mellitus, B vitamin deficiency, multiple nutrient deficiency due to malabsorption of digested food, kidney failure, amyloidosis, and alcoholism.

Electroneurography can also evaluate nerve damage caused by several bacterial infections or toxicities such as diphtheria, leprosy, and botulism poisoning. This test is also used to diagnose and follow the progression of many diseases of the nervous and muscular systems such as amyotrophic lateral sclerosis (ALS), myasthenia gravis, muscular dystrophies, and multiple sclerosis (MS).

Precautions

There are no contraindications for this test. It is noninvasive and very low risk.

Description

Electroneurography is based on the observation that when a nerve is electrically stimulated, a reaction will occur somewhere down the nerve or in the muscle served by the nerve. By using appropriate electrode placement, the reaction to the electrical stimulus is recorded. Examining the characteristics of the reaction and the timing of the reaction reveals both the velocity of conduction and the latency (time between stimulus and response) of the tested nerve.

This test requires that the nerve being tested is relatively close to the skin surface, although needle electrodes can be used to test deep nerves. Two sets of electrodes are used to perform the test, stimulating and recording. Normally, the stimulating electrodes are metal or felt pads placed on the surface of the skin, about 0.6-1.1 inches (1.5-3 cm) apart. Correct placement requires a strong understanding of neurological anatomy and varies from nerve to nerve. Conduction cream can be applied to maximize the effectiveness of the connection. Usually, the cathode (typically the black-colored electrode) is placed down the nerve from the anode (typically the red-colored electrode) in the direction of conduction.

The test works most effectively if maximal stimulation of the nerve is achieved. This is determined through step-wise increases in the stimulus output, and setting the stimulus 25-50% above this level. However, the greater the stimulation, the greater the chance of the stimulus being perceived as painful by the patient. Nevertheless, the duration of the discomfort is relatively short, and less than maximal stimulation produces results that cannot be interpreted and are therefore not medically useful. Stimulation is most difficult in patients who are obese, edemic (retaining water), or have unusually thick or calloused skin. Increasing stimulus duration, altering the placement of the cathode, or using needle electrodes can overcome stimulation problems.

Recording electrodes are placed according to the type of response that is being sought. If muscular reaction is the goal, the active recording electrode is placed over the belly (thickest area) of the muscle being tested, while the second recording electrode, called a reference electrode, is place on a tendon. Placement is correct if the graphic representation of the response shows an initial negative deflection (upwards) in the graph of the response. If a nerve is being tested, the active electrode is placed directly over the nerve. The reference electrode is placed distally (pointing away from the electrode). Recording electrodes that test motor response are often metal, circular discs, while sensory recording electrodes come in many shapes such as buttons, rings, clips, discs, or bare wire.

The test will run from about 20 minutes to about two hours, depending on the number of nerves being tested and if electromyography, a test commonly performed in conjunction with electroneurography, is being done. The cost of the test is about $500 and is usually covered by insurance.

Preparation

Low body temperature can greatly distort the results of electroneurography. Particularly in cold weather, it is important to warm the muscles being tested and to maintain normal body temperature throughout the testing procedure.

Aftercare

There are no aftercare procedures for this test. Patients may immediately resume normal activities.

Complications

There are no complications resulting from this test.

Results

Among the possible results from this test are measurements of motor response, sensory nerve response, and nerve conduction velocities.

Motor response

The motor (muscular) response is characterized by its waveform, amplitude, duration, and distal latency. The waveform is relatively simple, with a large negative deflection (upwards) followed by a large positive deflection (downward), producing a peak, although the exact shape depends on the placement and type of electrode. The amplitude (expressed in millivolts) is the value from the baseline to the peak of the negative response. Amplitude value depends on the number and synchronization of the muscle fibers that are being stimulated. The use of a maximal stimulation of the nerve ensures all possible fibers will be recruited.

The duration of the motor response is the time from the beginning of the negative deflection to the completion of the positive deflection. In disease states, along with decreased amplitude, the duration of the response will be increased if the muscle fibers do not fire together. The distal latency is the time it takes from stimulus to the beginning of the negative deflection, and is measured in milliseconds. If bilateral measurements are taken, this value will be increased on the side having damage to the nerve or neuromuscular junction. Each motor response characteristic is more difficult to read if electrode placement covers more than one muscle, emphasizing the need to isolate one muscle whenever possible.

Sensory nerve response

Like motor response, sensory nerve responses, or action potentials, are characterized by a particular waveform, amplitude, duration, and distal latency. The normal waveform includes a negative deflection (upward), a larger positive deflection (downward), followed by a negative rebound back to baseline, forming an S-shaped wave, although the exact shape differs with the type of electrodes, placement, or the use of three electrodes. Amplitude is measured from the peak of the negative response to baseline and is measured in microvolts.

Like motor responses, high amplitude and short duration in a sensory nerve response indicate large numbers of axons firing simultaneously. Disease states can reduce amplitude and increase duration. Distal latency is measured from stimulus to negative peak and is expressed in milliseconds. When taking sensory nerve responses, it is important to isolate the recording electrodes away from innervated muscles, as the motor response will swamp the much smaller sensory nerve response.

Nerve conduction velocity

Nerve conduction velocities to a muscle can be calculated if it is possible to stimulate a nerve in two places along its length. Two latency measurements are made, a distal latency and a proximal latency (in milliseconds, msec). The distance between the two stimulation points is then measured (in millimeters, mm) and divided by the difference between the two latency values. This value is the conduction velocity of the nerve in meters per second. For sensory nerves, only one stimulation point is used, and the velocity is calculated by dividing the distance between the active and reference electrodes (in mm) by the latency (in msec).

Once results have been calculated, these are compared to a table of standard values. Tables have been devised that sort results by different characteristics such as the patient's age, sex, height, nerve length, or a combination of these factors. An example of a commonly used table is one published by the Cleveland Clinic Foundation. This table sorts results by patient age and is based on standard electrode distances for the measurement of different nerve velocities. In general, demyelination is indicated if conduction velocities have fallen below 50% of normal. Even significant loss of axons commonly reduces conduction velocities by only about 30%, based on a loss of the fastest conducting fibers.

When analyzing the results of this test, it should be taken into consideration that electroneurography tests the best surviving nerve tissue. This characteristic means that results can be normal despite extensive nerve damage. Nevertheless, abnormal results can provide extremely useful information, including distinguishing between demyelination and loss of axons and pinpointing the exact location of a nerve injury.

Health care team roles

Electroneurography is often performed by specially trained electrodiagnostic technologists. Training for such a position can be on the job but often involves study at a one- to two-year college or vocational program. A typical program would include:

human anatomy and physiology
neurology and neuroanatomy
neurophysiology
medical terminology
computer technology and instrumentation

Certification of electrodiagnostic technologists specializing in electroneurography and the related area of evoked potentials is available through the American Board of Registration of Electroencephalographic and Evoked Potential Technologists.

A physician such as a neurologist, neurosurgeon, or internist does the final review and diagnosis based on the results of electroneurography. The physician can be present for the testing or may review saved tracings. Other health care professionals such as nurses aid in patient education concerning this procedure.

KEY TERMS

Amplitude— The distance from the baseline to the peak of the motor or sensory response represents the approximate number of healthy muscle fibers or nerves available.

Demyelination— Loss of the insulating cover of the nerve cell. Demyelination significantly reduces measured nerve conduction velocity.

Latency— The amount of time between stimulus and motor or sensory nerve response.

Waveform— The shape of the electrical response recorded by the active recording electrode.

Resources

BOOKS

Kimura, Jun and Nobuo Kohara. "Electrodiagnosis of Neuromuscular Disorders." In Neurology in Clinical Practice, ed. Walter G. Bradley et al. Boston: Butterworth Heinemann, 2003.

ORGANIZATIONS

American Board of Registration of Electroencephalographic and Evoked Potential Technologists. P.O. Box 916633, Longwood, FL, 32791-6633. (407) 788-6308. 〈http://www.abret.org〉.

The American Society of Electroneurodiagnostic Technologists. 204 West 7th Street, Carroll, IA 51401-2317. (712) 792-2978. 〈http://www.aset.org〉.