Evoked potential studies

Definition

Sensory evoked potential (EP) studies are the measurement of the electrical response of nervous tissue to auditory, visual, and somatosensory stimulation.

Purpose

EP studies, also known as evoked responses, measure the very faint electrical response of the brain , brainstem, or peripheral nerves to a mild stimulus, displaying it as a wave on a paper strip or computer monitor. Because the response is of such low amplitude, the responses to many stimuli must be averaged to distinguish the potentials from the background brain activity. The general purpose of this group of tests is to diagnose nerve disorders and to distinguish nonorganic loss of sensation from nerve damage. The tests can locate the site of the lesion and serve to evaluate the condition of a patient's nervous response after treatment or during surgery.

Auditory EP studies are often used to assess the cause of a hearing loss or balance problems, especially in children. They are the most common way hearing loss is evaluated in premature infants . Most auditory EPs record activity from the brainstem, and are therefore called brainstem auditory evoked potentials. This test is also particularly useful in screening for acoustic neuroma (a benign tumor of the nerves of the auditory canal).

Visual EP studies are used to assess eyesight in infants and children, as well as to diagnose disorders of the optic nerve and muscles. They can also distinguish hysterical blindness, a psychiatric disorder, from blindness caused by nerve damage.

Somatosensory EP studies are commonly used in the diagnosis of multiple sclerosis and transverse myelitis, although magnetic resonance imaging (MRI) has made this test less useful for these purposes. After trauma, somatosensory EP can determine whether loss of sensation in an arm or leg is due to injury in the brain or spinal cord .

Auditory, visual, or somatosensory testing can detect tumors and other abnormalities affecting the brain and spinal cord, assess brain stem function in coma , and assist in the determination of brain death. Perhaps the most prominent future role of this group of tests is the monitoring of brain activity and signals from the nerves during surgery on the brain, spine, or carotid region, and during general anesthesia .

Precautions

There are several benign conditions that can affect the results of evoked potential studies. Visual EP tests should not be performed on persons having severe nearsightedness. Auditory EP studies are contradicted for persons having excessive earwax or severe inflammation of the middle ear. All three types of tests can be adversely affected by muscle spasms in the head or neck of the patient.

KEY TERMS

Amplitude —The distance from the baseline to the peak of the sensory response; represents the approximate number of healthy nerves available.

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

Waveform —The shape of the electrical response of the brain, brainstem, or peripheral nerve as recorded by the active electrode.

Description

The test is often performed by an electrodiagnostic technologist, a nurse, or another allied health professional. It begins with the location and marking of specific spots on the patient's head for placement of electrodes. These spots are cleaned, and an adhesive conducting paste is applied. Cup electrodes are attached. For somatosensory EP, spots on the arm or leg are also marked and cleaned; electrodes may be taped in place. The patient sits or reclines in a chair throughout the tests.

For a visual EP, electrodes are attached to the scalp over the brain region responsible for vision . The patient focuses on a TV screen that displays a checkerboard pattern. The eye not being tested is covered with a patch. For children or others whose attention may wander, goggles are used which show the pattern to one eye at a time. Each eye is usually put through two series of tests where hundreds of sample recordings are collected for averaging. The entire procedure takes approximately one hour.

For auditory EP, electrodes are placed on the top of the head and on the earlobe of the ear being tested. Headphones are used to deliver a series of clicks to one ear at a time. A masking or static sound is played into the other ear. Each ear is usually put through two trials, and the entire procedure takes approximately one hour.

For somatosensory EP, electrodes for recording are placed on the scalp and along the spinal cord, while electrodes for delivering a mild electrical shocks are placed on either the arm or the leg. The shock is not painful, but may cause some twitching and tingling. Multiple trials are done for each nerve tested, and the entire procedure can take up to three hours, if both arm and leg nerves are evaluated.

Preparation

The patient's hair should be clean, with no gels, sprays, or other preparations applied. Jewelry should be removed.

Aftercare

After the tests, the electrodes are removed with acetone and the scalp is cleaned. The patient can resume normal activities.

Complications

There are no complications that result from this group of tests.

Results

Results are recorded as waveforms, either on paper or a digital display. For somatosensory or visual EPs, the traces are displayed so a negative potential at the active electrode produces an upward deflection in the wave. Brainstem auditory EPs, in contrast, are recorded such that a positive potential at the active electrode produces an upward deflection. By using these conventions, the waves of interest are always recorded as upward deflections. In general, EP tests produce both latency (speed of conduction) and amplitude (electrical strength of response) results, with the latency measurements more often clinically significant. Results are often compared to age-specific normal values that are established at each laboratory, with a value of 2.5 to 3.0 standard deviations from the mean interpreted as abnormal. Comparisons between data collected from the left and right sides of an individual patient are also informative.

Health care team roles

Evoked potential studies are 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 evoked potentials and the related area of electroencephalography is available through the American Board of Registration of Electroencephalographic and Evoked Potential Technologists.

A physician such as neurologist, neurosurgeon, or internist does the final review and diagnosis based on the results of EP studies. The doctor 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.

Resources

BOOKS

Emerson, Ronald G. "Somatosensory Evoked Potentials." In Comprehensive Clinical Neurophysiology, Eds. Levin, Kerry H. & Hans O. Lüders. Philadelphia: W. B. Saunders Company, 2000.

Epstein, Charles M. "Visual Evoked Potentials." In Comprehensive Clinical Neurophysiology, Eds. Levin, Kerry H. & Hans O. Lüders. Philadelphia: W. B. Saunders Company, 2000.

Lüders, Hans O., and Kiyohito Terada. "Auditory Evoked Potentials" In Comprehensive Clincial Neurophysiology, Eds. Levin, Kerry H. & Hans O. Lüders. Philadelphia:W.B. Saunders Company, 2000.

Misulis, Karl E. "Evoked Potential Basics." In Essentials of Clinical Neurophysiology. Boston: Butterworth-Heinemann, 1997.

PERIODICALS

Shpritz, D.W. "Neurodiagnostic Studies." Nursing Clinics of North America 34 (September 1999): 593-606.

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>.

Michelle L. Johnson, M.S., J.D.