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 Revised McDonald Criteria for diagnosis of MS includes guidelines for the use of VEP testing in confirming a diagnosis of MS.
How is an EP test done?
Evoked potential tests, like MRI scans, are non-invasive and primarily painless. There are several types of evoked potential tests of which three are commonly used in evaluating nerve damage associated with MS: somatosensory evoked response/potential (SSEP), auditory brain stem evoked response/potential (ABEP) and visual evoked response/potential (VEP). Each test takes about 45 minutes to complete.
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.
For the VEP test, as an example, electrodes are applied to the scalp over areas of the brain that receive electrical signals in response to stimuli, a flashing checkerboard pattern on a computer screen, along the visual pathway. The speed (or delay) of signals is recorded to be later analyzed. In auditory and brain stem EP testing, the stimuli will be sounds, such as tones or beeps, transmitted through headphones. In sensory EP testing, a mild electrical shock or pulse is given in areas of the knee, wrist, or ankle.