Evoked potentials(EP) tests are used to record and measure electrical impulses which travel through specific sensory pathways of the nervous system in response to stimuli. When you hear a loud noise, your nervous system sends an electrical signal or impulse through the auditory (hearing) pathway of your nervous system. Similarly, when you see an image, your nervous system sends an impulse through the visual pathway.
The damage caused by MS (demyelination) leaves scars (lesions) in places where the protective coating on nerves (myelin) has been stripped. Myelin not only protects the nerve cell, it also helps to transmit nerve impulses. Where myelin is damaged, nerve signals may travel more slowly or not at all. Results of EP testing can provide evidence of damage to nerve pathways that may be subclinical (not detected during neurologic exam). Different EP tests are used, including somatosensory evoked response/potential (SSEP), auditory brain stem evoked response/potential (ABEP), and visual evoked response/potentials (VEP). Results in VEP testing are abnormal in about 90% or 9 out of 10 people with MS making it the most useful EP in confirming a diagnosis of MS.
The procedure, which takes approximately 45 minutes per test, is non-invasive and completely painless except for the slight (and we mean slight) discomfort caused by the mild shock given for the sensory evoked potential test. Two sets of needle electrodes are used to test for nerve damage. One set of electrodes, which will be used to measure the electrophysiological response to stimuli, is attached to the patient’s scalp using conducting paste. The second set of electrodes is attached to the part of the body to be tested. Conductive gel is used to attach electrodes to the skin. The electrodes (or leads) pick up electrical signals traveling through the nervous system. The location of electrode placement will vary according to the nerve responses or pathway being tested.
Three types of evoked potential testing have been used in the past to help confirm a diagnosis of MS:
The current diagnostic criteria for MS consider only VEP findings because this particular EP study has been shown to be the most useful.