An evoked potential test measures the time it takes for nerves to respond to stimulation. The size of the response is also measured. Nerves from different areas of the body may be tested. Types of responses are:
Each type of response is recorded from brain waves by using electrodes taped to the head. The visual evoked response (VER) is the most commonly used evoked potential test in the diagnosis of multiple sclerosis (MS).
Conducting gel and electrodes are applied to the scalp. The location will depend on the type of response being recorded. For example, when VERs are recorded, the electrodes are applied to the rear (occipital region) of the scalp over the brain areas that register visual stimuli.
Stimuli are delivered:
Responses from the electrodes are recorded. The time between the stimulation and the response is called the latency, which indicates the speed at which the nerves pass a signal.
This test may be used when MS is suspected and a neurological examination alone does not provide enough evidence.
For a clear diagnosis of MS, the doctor has to find evidence that multiple parts of the central nervous system are affected. When there are symptoms clearly caused by MS lesions of the spine but no visual symptoms, the visual response may be tested anyway. Abnormal results in such cases mean that there are also areas of damage (MS lesions) on the brain.
Findings of this test may include the following.
The time between the stimulation and the nerve's response is within the normal range.
Some people who are free from symptoms in the nerve area tested will still have abnormal responses in that area.
Abnormal response times can also be associated with other neurological diseases or with damaged optic nerves and eyes.
An evoked potential test typically takes half an hour or longer to do.
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