Evoked potential tests are used to evaluate responses to sensory stimulation.
Used as a diagnostic tool, evoked potential tests can detect abnormal responses to stimulation that may indicate a medical condition. Evoked potential tests are commonly used to help diagnose multiple sclerosis (MS).
This article will give an overview of what evoked potential tests are, how they are used, and what they mean.
What are evoked potential tests?
The nervous system connects the body through a series of nerve cells that communicate with electrical signals.
When the body receives stimulation through the eyes (visual stimulation), ears (auditory stimulation) or skin (somatosensory stimulation or touch), electrical signals are sent along nerves to the brain.
For example, the light bouncing off an object would stimulate sensory receptors in the eye, which sends electrical signals to the brain for processing. This signal will take longer to arrive at the brain than a signal stimulated by touch. This is because when a person sees something, the body must first convert light into an electrical signal before sending it to the brain.
It is also possible for this signaling time to be affected by a medical condition that causes an abnormally slow response time.
Evoked potential tests measure the time it takes for the brain to respond to sensory stimuli as a way of detecting and monitoring problems or irregularities with how the nervous system is functioning.
How are they used?
Evoked potential tests are often used to confirm a diagnosis or monitor the nervous system, rather than determining the cause of an abnormality.
An evoked potential test can be used to supplement or confirm a diagnosis of MS through demonstrating this slower signal transmission.
MS occurs when the fatty layer (myelin) that insulates nerve cells gets damaged. When the myelin is damaged, it affects the speed that electrical signals can travel across nerve cells.
The evoked potential test can also be used to:
- assess hearing or sight
- detect lesions and tumors
- detect nerve damage, such as to the optic nerve
- assess brain activity in coma patients
- diagnose and monitor diseases that damage nerves
More advanced diagnostic tools, such as magnetic resonance imaging (MRI) have meant that evoked potential tests are becoming less widespread, but they are still considered to be a valuable medical tool.
What to expect
Evoked potential tests use electrodes placed on the scalp to record electrical signals sent to the brain. The three primary types of test are:
Electrodes will be placed on the scalp during the evoked potential tests, in order to record the brain's reactions to sensory stimulation.
- Visual evoked response (VER) test, where the eye is exposed to checkerboard patterns or flashes of light.
- Brainstem auditory evoked response (BAER) test, where the ear is exposed to clicking sounds or tones. The signal response time can determine whether there is damage to the auditory pathway within the brain or the acoustic nerve connecting the ear to the brain.
- Somatosensory evoked response (SSER) test, where a low-intensity electric shock is administered to a certain location on the body and the signal response time is recorded in the brain. It is used in the region of the nervous system that might be damaged. For example, it could be used to detect a spinal cord lesion.
Evoked potential tests are very low intensity and pose little risk to the person beyond minor discomfort during the test.
In extremely rare circumstances, it is possible for a visually evoked response test to cause a seizure.
Certain factors, such as severe visual or hearing impairments or muscle spasms, can also affect the accuracy of the test.
The results could show an abnormally long signal transmission time, which would indicate damage to a nerve pathway. This can occur regardless of whether the person is experiencing any symptoms relating to the area of potential damage.
Typically, follow-up imaging is used to investigate the problem in more detail, such as MRI or a computed tomography scan (CT).