Evoked Potentials Studies

What is an evoked potentials study?

An evoked potentials study measures electrical activity in the brain in response to stimulation of sight, sound, or touch. When the brain is stimulated by sight, sound, or touch, signals travel along the nerves to the brain. There, electrodes find the signals and display them for your healthcare provider to interpret. Evoked potentials studies involve 3 tests that measure response to visual, auditory, and electrical stimuli.

• Visual evoked response test. This test can diagnose problems with the optic nerves and other visual pathways in the brain that affect sight. A healthcare provider places electrodes along your scalp. They record the electrical signals as you watch a checkerboard pattern flash for several minutes on a screen.

• Brainstem auditory evoked response test. This test can diagnose hearing ability and can point to possible brainstem tumors or multiple sclerosis. A provider places electrodes on your scalp and earlobes and delivers auditory stimuli, such as clicking noises and tones, to each ear.

• Somatosensory evoked response (SSER) test. This test can find problems with the spinal cord that cause numbness of the arms and legs. For this test, a provider attaches electrodes to your wrist, ankles, the back of your knee, neck, or other locations. They will apply a mild electrical stimulus through the electrodes. Electrodes on your scalp then determine the amount of time it takes for the current to travel along the nerves to the brain.

Why might I need an evoked potentials study?

Your healthcare provider may order these tests to evaluate hearing or sight, especially in infants and children. They are also done to diagnose disorders of the optic nerve, and to find tumors or other problems affecting the brain and spinal cord. They are also used to assess brain function during a coma and help predict the chance of recovery. They may also be used to keep track of nerve function during some surgeries.

These tests usually don't lead to a specific diagnosis about what is causing the abnormality. But the evoked potentials test can sometimes support a diagnosis of multiple sclerosis.

There may be other reasons for your healthcare provider to advise an evoked potentials test.

What are the risks of an evoked potentials study?

Evoked potentials studies are considered safe procedures. The tests can cause a little discomfort. There may be risks depending on your specific medical condition. Talk about any concerns with your healthcare provider before the procedure.

Certain factors or conditions may interfere with the results of the test. These include:

• Severe nearsightedness

• Presence of earwax or inflammation of the middle ear

• Severe hearing impairment

• Muscle spasms in the head or neck

• Any implantable devices, such as a spinal cord stimulator, pacemaker, or cardiac defibrillator. Tell the technician if you have one of these devices as the tests may need to be changed.

How do I get ready for an evoked potentials study?

Ask your healthcare provider to tell you what you should do before your test. Below is a list of common steps that you may be asked to do, if you are getting the test done as an outpatient procedure:

• You will sign a consent form that gives your permission to do the test. Read the form carefully and ask questions if something is unclear.

• In most cases, sedation isn't needed and you won't need to fast for this test. But children may need sedation to get accurate results. Check with your healthcare provider about any specific instructions if sedation is needed.

• Tell your healthcare provider of all medicines (prescribed and over-the-counter), vitamins, and herbal supplements that you're taking.

• Wash your hair the night before the test, but don't use conditioner or apply any hairspray or other hair products.

• Based on your medical condition, your healthcare provider may request other specific preparations.

• Don't put lotions or cream on your skin if you are getting SSER done.

What happens during an evoked potentials study?

An evoked potentials test may be done on an outpatient basis. This means you go home the same day. Or it may be done as part of a hospital stay. Procedures may vary depending on your condition and your healthcare provider's practices. Talk with your healthcare provider about what to expect during your test. Evoked potentials can also be done in the operating room during certain surgeries. If done during an operation, you will likely not be aware of the test being done.

Generally, the 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 healthcare provider will attach the electrodes with a paste. The electrodes will be positioned depending on which type of evoked potentials test is being done.

The test will generally proceed as follows.

Visual evoked response

• You will be seated a few feet away from a screen.

• A healthcare provider will place electrodes 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 focus on a screen that displays a checkerboard pattern. The squares of the checkerboard reverse color once or twice a second. You will often wear an eye patch to test 1 eye at a time.

Brainstem auditory evoked response

• You will sit in a soundproof room wearing earphones.

• A healthcare provider will place electrodes on top of your head and on 1 earlobe and then the other.

• A clicking sound or another auditory stimulus will be delivered through the earphones to the ear being tested . At the same time, a masking noise will be sent to the other ear to shield it from the stimulus.

Somatosensory evoked response

• A healthcare provider will place electrodes on the scalp and on 1 or more parts of your body. These may include the wrist, back of the knee, or the skin over your spine.

• Small, painless electrical shocks will be sent through the electrodes placed on the body.

• For each of the tests, the electrical activity found by the electrodes on the scalp will be fed into a recorder. The recorder amplifies the signal and charts it. This helps your provider interpret the results.

What happens after an evoked potentials study?

Once the test is complete, a healthcare provider will remove the electrodes and wash off the electrode paste. In some cases, you may need to wash your hair again at home.

Your provider will tell you when you will get the test results. They will also tell you when to resume any medicines you may have stopped taking before the test.

Your provider may give you other instructions after the procedure, depending on your particular situation.

Next steps

Before you agree to the test or procedure make sure you know:

• The name of the test or procedure

• The reason you are having the test or procedure

• What results to expect and what they mean

• The risks and benefits of the test or procedure

• What the possible side effects or complications are

• When and where you are to have the test or procedure

• Who will do the test or procedure and what that person’s qualifications are

• What would happen if you did not have the test or procedure

• Any alternative tests or procedures to think about

• When and how you will get the results

• Who to call after the test or procedure if you have questions or problems

• How much you will have to pay for the test or procedure