An evoked potential test measures the time it takes for nerves to respond to stimulation. The size of the response is also measured. Nerves from different areas of the body may be tested. Types of responses are:

Visual evoked response or potential (VER or VEP), which is when the eyes are stimulated by looking at a test pattern.

Auditory brain stem evoked response or potential (ABER or ABEP), which is when hearing is stimulated by listening to a test tone.

Somatosensory evoked response or potential (SSER or SSEP), which is when the nerves of the arms and legs are stimulated by an electrical pulse.

Reasons for the procedure

žEvoked 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.

žA 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.

žThere may be other reasons for your doctor to recommend an evoked potentials test.

Risks of the procedure

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

žThere may be risks depending on your specific medical condition. Be sure to discuss any concerns with your doctor prior to the procedure.

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

• žSevere nearsightedness
• žPresence of earwax or inflammation of the middle ear
• žSevere hearing impairment
• žMuscle spasms in the head or neck

The Procedure

žBefore the procedure

žYour doctor will explain the procedure to you and offer you the opportunity to ask any questions that you might have about the procedure.

žYou 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.

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

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

žBased on your medical condition, your doctor may request other specific preparation.

žDuring the procedure

žAn 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 doctor’s practices.

žGenerally, the evoked potentials test follows this process:

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

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

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

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

žThe test will generally proceed as follows:

žVisual evoked response:

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

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

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

žYou 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.

žBrain stem auditory evoked response:

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

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

žA 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.

žSomatosensory evoked response:

žElectrodes 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.

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

žFor 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 doctor can interpret the results.

After the procedure

žOnce 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.

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

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