Evoked Potential (EP) Testing
Before your evoked potential (EP) test, you may have questions about how to prepare and what will happen. The information below will help answer your questions and provide you with instructions to follow before you arrive for your test. Should you need additional information, please contact Mischer Neuroscience Institute at (713) 704-2144.
What is an EP Test?
An evoked potential (EP) 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 potential (VEP), which is when the eyes are stimulated by looking at a test pattern
- Brainstem auditory evoked response (BAER) or Brainstem auditory evoked potential (BAEP), 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
Each type of response is recorded from brain waves by using electrodes taped to the head.
Why are EP Tests done?
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 done to assess brain function during a coma.
How do I prepare for an EP?
Below is a list of common steps that you may be asked to do:
- You will 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.
- Tell your healthcare provider of all medications (prescribed and over the counter) and herbal supplements you are taking. He or she will let you know if you can take your medications before your test.
- Wash your hair the night before the test, but do not use conditioner or apply any hairspray or other hair products.
- Based on your medical condition, your healthcare provider may request other specific preparations.
What should I expect?
Generally, the evoked potentials test follows this process:
- You will be asked to remove any clothing, jewelry, hairpins, eyeglasses, hearing aids, or other metal objects that may interfere with the procedure.
- If you are asked to remove clothing, you will be given a gown to wear.
- You will be asked to relax in a reclining chair or lie on a bed.
- A paste will be used to attach the electrodes. The electrodes will be positioned according to which type of evoked potentials test is being performed.
The test will generally proceed as follows:
Visual Evoked Potential (VEP)
- You will be seated a few 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.
Brainstem Auditory Evoked Response (BAER)
- You will sit in a soundproof room wearing 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 Potential (SSEP)
- 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.
- Small, painless electrical shocks will be delivered through the electrodes placed on the body.
- 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 so that your doctor 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 healthcare provider will inform you as to when to resume any medications you may have stopped taking before the test.
He or she may give you additional or alternate instructions after the procedure, depending on your particular situation. Should you have any questions about your test, please contact Mischer Neuroscience Institute at (713) 704-2144.