Evoked potential tests measure the time it takes for the brain to respond to sensory stimulation either through sight, sound, or touch.
Doctors use the test to help diagnose multiple sclerosis (MS) and other conditions that can cause a person's reactions to slow. The test can detect unusual responses to stimulation.
The popularity of the evoked potential test has fallen in recent years, as MRI scans have a good record of accurately showing lesions that occur in MS. However, a study from 2016 suggests the test may still have its uses.
With this article, we give an overview of what evoked potential tests are, what to expect, and what the results can mean.
The nervous system connects the body through a series of cells that communicate with electrical signals.
When the body receives stimulation, electrical signals pass through the nervous system to the brain.
Stimulation can be:
- visual, entering the body through the eyes
- auditory, entering through the ears
- somatosensory stimulation or touch, when sensations enter via the skin
For example, when light bounces off an object, it stimulates sensory receptors in the eye. The receptors send electrical signals to the brain for processing.
A signal that light stimulates arrives at the brain more slowly than one that touch stimulates. This is because when a person sees something, the body must first convert light into an electrical signal before sending it to the brain.
Some medical conditions can also affect this signaling time, resulting in an unusually slow response time. This can happen when a person has MS, for example.
MS occurs when a fault in the immune system results in damage to the fatty layer of myelin that protects nerve cells. This damage affects the speed that electrical signals can travel across nerve cells.
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.
Doctors often use evoked potential tests to confirm a diagnosis or monitor the nervous system, rather than to determine the cause of a slow reaction.
MRI scans and tests of cerebrospinal fluid are the main ways of diagnosing MS nowadays.
However, an evoked potential test can still supplement or confirm a diagnosis of MS. It can be a valuable tool for demonstrating a slow signal transmission.
A study that researchers published in 2016 notes that an evoked potential test:
- is an economical method of diagnosis
- can complement results of other tests
- is the only way to assess how well myelin, axons, and synapses are working in specific sensorimotor pathways
As well as playing a role in the diagnosis of MS, an evoked potential test can:
- 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
What is MS, and how does it affect a person? Learn more here.
A doctor will advise a person on what to do before the test and what to expect.
The individual will need to:
- sign a consent form
- tell their doctor about any health issues, allergies, and medications they are using
- bring their glasses if they are doing a visual test
They are unlikely to need to fast or stop using medications before the test.
The healthcare professional conducting the test will use a special paste, jelly, or tape to attach the electrodes.
The location of the electrodes will depend on the type of test. Then, the test can begin.
In an evoked potential test, the person will sit in a chair, and a healthcare provider will place electrodes on the relevant part of the body. The electrodes will record electrical signals that travel to the brain.
There are three main types of test.
Visual evoked response test
The healthcare provider will place electrodes on the scalp.
The person will sit and focus on the center of a screen a few feet away. They will need to close one eye at a time and gaze at a checkerboard pattern on the screen. The colors of the squares will alternate once or twice each second.
The test will record how the eye responds to the changing patterns.
Brainstem auditory evoked response test
The person will sit in a soundproof room, wearing earphones. The electrodes will be on the top of the head and first one earlobe, then the other.
The person will hear clicking sounds or tones in one ear, while a masking sound prevents the other ear from picking up the signal. Then the other ear will be tested.
The time it takes to respond to the signal can show whether there is damage to the auditory pathway within the brain or the acoustic nerve connecting the ear to the brain.
Somatosensory evoked response test
The individual will sit or lie in a comfortable position.
The health professional will place electrodes on the scalp and relevant areas of the body, such as the arm, leg, or lower back.
They will then deliver a low-intensity electric shock through the electrodes and record the time it takes for the brain to respond to the signal. The shock should not be painful, but it may be uncomfortable for a short while.
The professional will apply the stimulus to the part of the nervous system where damage might be present, for example, the spinal cord.
If the results show an unusually long signal transmission time, this could indicate damage to a nerve pathway, even in "silent" cases when a person is not experiencing symptoms.
A doctor will usually use an evoked potential test alongside an imaging test to investigate the problem in more detail. This may be an MRI or CT scan.
Evoked potential tests are a low-intensity procedure and, typically, pose little risk to the person beyond minor discomfort during the test.
The test may be less effective in a person with advanced symptoms. Muscle spasms or severe visual or hearing impairments might affect the accuracy of the test.
If the test indicates that MS might be present, a doctor will usually do further tests to confirm the diagnosis and rule out other possible causes of symptoms.
If the results of all the tests indicate MS, a doctor will work with the individual to provide appropriate treatment and monitoring.
In the past, doctors considered MS untreatable, but scientists are making rapid progress in understanding MS and developing new treatments.
A type of medication known as a disease-modifying therapy (DMT) can reduce the number of flares in a kind of MS doctors call remitting relapsing MS (RRMS). When a person has RRMS, symptoms can temporarily worsen and then get better again.
DMT may also slow the progression of MS and reduce the risk of more severe symptoms appearing in time.
For people who experience severe symptoms, various treatment options are available to help them manage their condition.
MS is rarely fatal, and most people do not develop severe symptoms. According to the National Multiple Sclerosis Society, two in every three people with MS will continue to be able to walk, although they may need an aid, such as a walking cane.
Thanks to current and improving treatment, a person will MS can expect to enjoy the same lifespan as a person without the condition, according to the National Institute for Neurological Diseases and Stroke.
MS affects each person differently. If someone has a diagnosis of MS, their doctor will work with them to establish a treatment plan to suit their needs.
What shall I do about my clothes, jewelry, and hair for the test?
You should wear comfortable clothes, avoid jewelry, and be ready to move your hair away from your eyes and ears.Heidi Moawad, MD Answers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.