Evoked Responses

Evoked Potentials Studies

(Evoked Brain Potentials, Evoked Responses, Visual Evoked Response [VER], Brainstem Auditory Evoked Response [BAER], Auditory Brainstem Evoked Potentials [ABEP] or alternatively termed Brainstem Auditory Evoked Potentials [BAEP], Somatosensory Evoked Response [SER or SSEP])

Procedure Overview

Evoked potentials studies measure electrical activity in the brain in response to stimulation of sight, sound, or touch. Stimuli delivered to the brain through each of these senses evoke minute electrical signals. These signals travel along the nerves and through the spinal cord to specific regions of the brain and are picked up by electrodes, amplified, and displayed for a physician to interpret.

Evoked potentials studies involve three major tests that measure response to visual, auditory, and electrical stimuli.

• Visual evoked response (VER) test - can diagnose problems with the optic nerves that affect sight. Electrodes are placed along the scalp. The patient is asked to watch a checkerboard pattern flash for several minutes on a screen, and the electrical responses in the brain are recorded.

• Brainstem auditory evoked response (BAER) test - can diagnose hearing ability and can indicate the presence of brain stem tumors and multiple sclerosis. Electrodes are placed on the scalp and earlobes. Auditory stimuli, such as clicking noises and tones, are delivered to one ear.

• Somatosensory evoked response (SSER) test - can detect problems with the spinal cord as well as numbness and weakness of the extremities. For this test, electrodes are attached to the wrist, the back of the knee, or other locations. A mild electrical stimulus is applied through the electrodes. Electrodes on the scalp then determine the amount of time it takes for the current to travel along the nerve to the brain.

A related procedure that may be performed is an electroencephalogram (EEG). An EEG measures spontaneous electrical activity of the brain. Please see this procedure for additional information.

Evoked potential studies may be used to assess hearing or sight, especially in infants and children, to diagnose disorders of the optic nerve, and to detect tumors or other problems affecting the brain and spinal cord. The tests may also be performed to assess brain function during a coma.

A disadvantage of these tests is that they detect abnormalities in sensory function, but usually do not produce a specific diagnosis about what is causing the abnormality. However, the evoked potentials test can confirm sometimes a diagnosis of multiple sclerosis.

There may be other reasons for your physician to recommend an evoked potentials test.

The evoked potential studies are considered safe procedures. The tests cause little discomfort. The electrodes only record activity and do not produce any sensation.

There may be risks depending upon your specific medical condition. Be sure to discuss any concerns with your physician prior to the procedure.

Certain factors or conditions may interfere with the results of the test. These include, but are not limited to, the following:

• Severe nearsightedness

• Presence of earwax or inflammation of the middle ear

• Severe hearing impairment

• Muscle spasms in the head or neck

• Your physician will explain the procedure to you and offer you the opportunity to ask any questions that you might have about the procedure.

• You will be asked to sign a consent form that gives your permission to do the procedure. Read the form carefully and ask questions if something is not clear.

• Generally, no prior preparation, such as fasting or sedation, is required.

• Wash your hair the night before the test, but do not use conditioner or apply any hairspray or other hair products.

• Based upon your medical condition, your physician may request other specific preparation.

An evoked potentials test may be performed on an outpatient basis or as part of your stay in a hospital. Procedures may vary depending on your condition and your physician's practices.

Generally, the evoked potentials test follows this process:

1. You will be asked to remove any clothing, jewelry, hairpins, eyeglasses, hearing aids, or other metal objects that may interfere with the procedure.

2. If you are asked to remove clothing, you will be given a gown to wear.

3. You will be asked to relax in a reclining chair or lie on a bed.

4. A paste will be used to attach the electrodes. The electrodes will be positioned depending on which type of evoked potentials test is being performed.

5. The test will generally proceed as follows:

   Visual evoked response:

   You will be seated about three feet away from a screen.

   Electrodes will be placed on your scalp over the areas of the brain responsible for interpreting visual stimuli.

   You will be asked to focus your gaze on the center of the screen.

   You will then be asked to close one eye at a time while the screen displays a checkerboard pattern. The squares of the checkerboard reverse color once or twice a second.

   Brain stem auditory evoked response:

   You will sit in a soundproof room and be asked to wear earphones.

   Electrodes will be placed on top of your head and on one earlobe and then the other.

   A clicking sound or another auditory stimulus will be delivered through the earphones to the ear being tested while a "masking" noise will be delivered to the other ear to shield it from the stimulus.

   Somatosensory evoked response:

   Electrodes will be placed on the scalp and at one or more locations on your body, such as the wrist, back of the knee, or the lower back.

   Minute, painless electrical shocks will be delivered through the electrodes placed on the body.

6. For each of the tests, the electrical activity detected by the electrodes on the scalp will be fed into a recorder, which amplifies the signal and charts it so that your physician can interpret the results.

Once the test is complete, the electrodes will be removed and the electrode paste washed off. In some cases, you may need to wash your hair again at home.

Your physician will inform you as to when to resume any medications you may have stopped taking before the test.

Your physician may give you additional or alternate instructions after the procedure, depending on your particular situation.