This page explains about evoked potential (EP) tests and what to expect when your child comes to Great Ormond Street Hospital (GOSH) to have this procedure.
What is an evoked potential test?
The brain works by a series of nerve impulses, which cause electrical signals within the brain. These signals (also called brainwaves) can be recorded through the scalp.
When a part of the body is stimulated, for instance, the eyes by a flashing light, or the ears by a clicking sound, the brain responds to this stimulation. This response is called an ‘evoked potential’.
The response is often very small, but it can be recorded using special techniques. Usually, we repeat each stimulus a number of times so that plenty of responses are recorded. The computer then averages these to show how the nerve pathways are working.
There are various types of evoked potential, each with a different method of stimulation:
- A visual evoked potential (VEP) test looks at the pathway from the optic nerve to the part of the brain where images are interpreted and turned into pictures.
- A brainstem auditory evoked potential (BAEP) test looks at the pathway from the ear to the part of brain that interprets sound.
- A somatosensory evoked potential (SEP) test looks at the pathway from the peripheral nerves in the arms and legs to the sensory part of the brain.
Evoked potential tests are a safe way of checking a variety of nerve pathways in the body without the need for sedation or an anaesthetic. All three tests are quick and pain-free but give good results before further testing or surgery.
Children often have these tests at the same appointment as an electroencephalogram (EEG) or other neurophysiology tests.
What happens before the test?
If your child is having an evoked potential test as an outpatient, the department of Clinical Neurophysiology will send you an appointment letter. If your child is having an evoked potential test during their hospital stay as an inpatient or day case patient, the ward will arrange the test and let you know when it will happen.
It is helpful if you could make sure that your child’s hair is clean before the test, with no mousse, gel, oil or hairspray. If your child is taking medicines, you should continue to give them as normal.
What does the test involve?
Depending on the information needed by the doctor, your child may have all three types of EP test or just one or two. This will be explained fully before testing starts.
The clinical physiologist will attach a few small silver discs (electrodes) to certain points on your child’s scalp and face using a soft paste. Sometimes, he or she will measure your child’s head beforehand and mark the points with a soft pencil before attaching the electrodes.
As each electrode is attached, he or she will clean the area with a cotton bud and some cream. This does not hurt but some children do not like it. While the electrodes are applied and for most of the tests, your child can sit on a chair, the bed or your lap.
Visual evoked potential (VEP)
For this test, your child will need to look at a flashing light. Sometimes, a television screen displaying a checkerboard pattern that moves from side to side might be used instead. As light flashes or the pattern on the television screen moves, the brain responds to the stimulation and this is recorded as brainwaves.
Brainstem auditory evoked potential (BAEP)
Your child will usually need to wear headphones for this test. Loud clicks will be played through the headphones and the brain’s response to these clicks is recorded as brainwaves.
Somatosensory evoked potential (SEP)
Your child will have another electrode or two attached to his or her hand or foot through which a very small current will pass. This will make his or her fingers or toes tingle and wiggle but does not hurt. The brain’s response to this stimulation is recorded as brain waves.
How long does the test last?
The length of all three tests depends on how much a child cooperates and how much information is needed. We usually allow an hour for all three tests and individual tests will be much shorter.
Are there any risks or complications with the test?
What happens after the test?
The clinical physiologist will remove the electrodes. This will not hurt as the paste stays soft and so is easy to remove. Once all the electrodes have been removed, you will be free to go home if no other tests or appointments are planned.
How long until we get the results?
You will not be given the results during the appointment. The doctors in the EEG department will write a detailed report of the test results and send it to your child’s doctor. The results will usually be ready in time for your child’s next appointment.
Last reviewed by Great Ormond Street Hospital: February 2008
Ref: F070101 © GOSH Trust February 2008
Compiled by the Department of Clinical Neurophysiology in collaboration with the Child and Family Information Group.
This information does not constitute health or medical advice and will not necessarily reflect treatment at other hospitals. If you have any questions, please ask your doctor. No liability can be taken as a result of using this information.