From The MS Information Sourcebook, produced by the National MS Society.
Evoked potential (EP) tests measure the electrical activity of the brain in response to stimulation of specific sensory nerve pathways. They are able to detect the slowing of electrical conduction caused by demyelination along these pathways even when the change is too subtle to be noticed by the person or to show up on neurologic examination. Because the diagnosis of MS requires evidence of demyelination in two distinct areas of the central nervous system, EP testing can help confirm the diagnosis by enabling the physician to identify a second demyelinating event that wasn't otherwise apparent.
In order to measure evoked potentials, wires are placed on the scalp overlying the areas of the brain being stimulated. The examiner then provides specific types of sensory input (e.g., sound, light, or sensation), and records the responses of the person's brain. Evoked potential testing is harmless, generally painless, and is a very sensitive technique for detecting lesions—or damaged areas.
Types of Evoked Potential Tests
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.
Despite the fact that VEPs are used to help make a diagnosis of MS, other conditions can also produce abnormal results, so this test is not specific for MS. The information the tests provide needs to be considered along with other laboratory and clinical information before a diagnosis of MS can be made.
For Healthcare Professionals
Kalb R. (ed.). Multiple Sclerosis: A Guide for Families (3rd ed.). New York: Demos Medical Publishing, 2005.
Last updated December 2005