What is an evoked potentials study?
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 doctor to interpret.
Evoked potentials (EPs), or evoked responses, measure the electrical and physiologic responses of the nervous system to a variety of stimulation.
Theoretically, in broadband terms almost any mode of functionality in a person can be tested, in clinical practice, only a few are used routinely.
Evoked potentials reflect changes in the electrical activity that occurs within the Central Nervous System (CNS) due to sensory input stimulation.
Types Of Evoked Potentials Tests:
The Evoked Potentials most frequently utilized are:
Visual evoked potentials (VEPs)
- Short-latency brainstem auditory evoked potentials (BAEPs)
Short-latency somatosensory evoked potentials (SEPs)
Visual evoked potentials (VEPs)
Visual evoked potentials (VEPs) are via stimulating an eye with a checkerboard pattern of light and dark squares which are alternated on a television monitor.
Visual Evoked Potentials test function of the visual pathway from the retina to the brain. Conduction of the visual pathways from the optic nerve to the brains occipital cortex is measured. VEP’s are useful in detecting potential diagnosis of Multiple Sclerosis but conduction can also be affected by many other conditions such as a tumor completely unrelated to MS.
Short-latency brainstem auditory evoked potentials (BAEPs)
The brainstem auditory evoked potential (BAEP), or brainstem auditory evoked response (BAER), measures the functioning of the auditory nerve and auditory pathways in the brainstem. BAEPs are useful in estimating or aiding in the assessment of hearing loss. The most common uses of the BAEP are in multiple sclerosis (MS) and in acoustic neuroma. As a screening test, it has some limitations; magnetic resonance imaging (MRI) may be preferable when a small lesion is being considered.
In various studies that have taken place some 20% of the patients tested for a second lesion have an abnormal BAEP, about half of these go on to develop MS in the next 1-3 years.
Short-latency somatosensory evoked potentials (SEPs)
Somatosensory evoked potentials are produced by electrical stimulation of nerves in the hands, feet or both.
A SEP test studies the relay of body sensations to your brain and how the brain receives those sensations. A stimulating electrode is placed on your arm or leg, and it generates an electrical signal. Recording electrodes are placed on your head and/or spine. The information received from these electrodes can help to diagnose your problem.
The test evaluates the health of peripheral nerves and your spinal cord. It also tests how your spinal cord and/or brain transmits information about body sensations through your peripheral nerves. It can localize a “signal blockage” either in the relay system (peripheral nerves act like telephone wires) or in the interpretive center (the brain and spinal cord act like a telephone receiver).
How Evoked Potentials Work:
The time between applied stimulation and response or evoked potential is a measure of the nerve’s ability to conduct electrical impulses from one point to another.
In Multiple Sclerosis if the response time is slowed, the nerve pathway may not be functioning properly as a result of demyelination. Evoke Potentials tests are abnormal in 70-90% of people afflicted with MS and often are not detected as abnormalities in a standard or common neurological examination. Due to EP’s being a measure of function within the brain or spinal cord, they can complement the information about brain structure provided by the MRI.
Why might I need an evoked potentials study?
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.
A disadvantage of these tests is that they detect abnormalities in sensory function, but usually do not lead to a specific diagnosis about what is causing the abnormality. However, the evoked potentials test can sometimes confirm a diagnosis of multiple sclerosis.
There may be other reasons for your healthcare provider to recommend an evoked potentials test.
What are the risks of an evoked potentials study?
The evoked potential studies are considered safe procedures. The tests can cause a little discomfort. The electrodes only record activity and do not produce any sensation.
There may be risks depending on your specific medical condition. Be sure to discuss any concerns with your healthcare provider prior to the procedure.
Certain factors or conditions may interfere with the results of the test:
- Severe nearsightedness
- Presence of earwax or inflammation of the middle ear
- Severe hearing impairment
- Muscle spasms in the head or neck
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:
- 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 that you are taking.
- 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 happens during an evoked potentials study?
An evoked potentials test may be done on an outpatient basis or as part of your stay in a hospital. Procedures may vary depending on your condition and your healthcare provider’s practices. Talk with your healthcare provider about what you will experience during your test.
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 depending on which type of evoked potentials test is being performed.
The test will generally proceed as follows.
Visual evoked response:
- 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:
- 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 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.
- 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 it so that your doctor can interpret the results.
What happens after an evoked potentials study?
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.
Your healthcare provider may give you additional or alternate instructions after the procedure, depending on your particular situation.