Evoke potential

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.

Procedure for the Evoked Potential

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:

1. Auditory evoked potentials: also called brain stem auditory evoked potential (BAEP) are used to assess high-frequency hearing loss, diagnose any damage to the acoustic nerve and auditory pathways in the brainstem, and detect acoustic neuromas.  The patient sits in a soundproof room and wears headphones.  Clicking sounds are delivered one at a time to one ear while a masking sound is sent to the other ear.  Each ear is usually tested twice, and the entire procedure takes about 45 minutes.
2. Visual evoked potentials : (VEP) detect loss of vision from optic nerve damage (in particular, damage caused by multiple sclerosis).  The patient sits close to a screen and is asked to focus on the center of a shifting checkerboard pattern.  Only one eye is tested at a time; the other eye is either kept closed or covered with a patch.  Each eye is usually tested twice.  Testing takes 30-45 minutes.
3. Somatosensory evoked potentials: (SEP) measure response from stimuli to the peripheral nerves and can detect nerve or spinal cord damage or nerve degeneration from multiple sclerosis and other degenerating diseases.  Tiny electrical shocks are delivered by electrode to a nerve in an arm or leg.  Responses to the shocks, which may be delivered for more than a minute at a time, are recorded.  This test usually lasts less than an hour.

The current diagnostic criteria for MS consider only VEP findings because this particular EP study has been shown to be the most useful.

• The VEP is used to identify impaired transmission along the optic nerve pathways, which is a fairly common early finding in MS, even in someone who has never been aware of any visual symptoms.
• The results of VEP testing are interpreted by a neurologist or neurophysiologist with specialized training in the use of these tests.