Curious to learn more about the different evoked potentials tests and when they were used in diagnosing MS, I checked out the article about diagnosing multiple sclerosis on UpToDate -- an electronic reference used by many physicians who encounter patients with suspected MS or other neurological disorders.
See what UpToDate has to say, then read on for answers to questions you may have about what all of this means for you.
Evoked Potentials for the Diagnosis of Multiple Sclerosis: An Explanation from UpToDate
“Evoked potentials (EPs) are the CNS electrical events generated by peripheral stimulation of a sensory organ. EPs are used to detect abnormal CNS function that may be clinically undetectable. Detection of a subclinical lesion in a site remote from the region of clinical dysfunction supports a diagnosis of multifocal MS. EPs also may help define the anatomical site of the lesion in tracts not easily visualized by imaging (eg, optic nerves, dorsal columns).”Your Questions: Answered
Fully understanding what this means not only give you a better sense of what to expect from your test, but it will help you be better able to discuss your situation with your doctor.What Are Evoked Potentials?
Evoked potential tests measure how long it takes for stimulation of different nerves to reach the brain and how big the response is. The speed of the impulses along neurons can indicate if there has been damage (like demyelination caused by MS) to the nerves, as the impulses will travel more slowly to the brain.What Do the Tests Feel Like?
For all of the tests, electrodes are applied to the scalp with some sort of conducting gel. They are placed in different locations on the head according to which type of evoked potential they are recording.The stimulations are different for each test:
- Visual Evoked Response (VER): The eyes are stimulated by looking at a computer screen that is flashing checkerboard patterns of differing sizes or a strobe-type light. Usually, one eye is covered with a patch or a hand-held shield while the other eye is tested, then the process is repeated with the other eye. Some people report feeling slightly nauseated during the test, a feeling much like mild motion sickness.
- Brainstem Auditory Evoked Potentials (BAEP): The hearing is stimulated by listening to test tones, beeps or clicks through headphones, usually in a dark room.
- Somatosensory Evoked Potential (SSEP): The nerves of the arms and legs are stimulated by an electrical pulse delivered through electrodes stuck onto the skin, usually at the wrist or knee, but occasionally near an ankle or elbow. It feels like a small electric shock. Most people say this is completely painless, but some people find the stimulation bothersome.
Each of these tests usually takes between 30 and 60 minutes.
What Do the Results Mean?
An “abnormal” result basically means that the signal took longer to reach the brain that it normally would in a person with healthy nerves, or that it was less intense. In terms of MS, the tests play out as follows:- VER: 85% of people with MS have abnormal results to the visual evoked response test. This is the most useful of the evoked potential test in helping to diagnose MS, especially for things like progressive MS that do not show up so well on MRI scans. However, people with vision disorders not necessarily related to MS can also have abnormal results on their VER.
- SSEP: Results to this test are abnormal in 77% of people with MS.
- BAEP: Abnormal results to this test occur in only about 67% of people with MS.
So, When Are These Tests Useful?
Evoked potential tests may be used when MS is suspected, but a neurological examination alone does not provide enough evidence. They are becoming less popular and important with the widespread availability of MRI scans, but are still used in some cases, especially if symptoms indicate that there might be damage to places that are hard to capture on an MRI scan, like the optic nerve. Also, for a diagnosis of MS, it must be determined that there are lesions on multiple parts of the spine, brain or optic nerves. Sometimes there might be damage that does not result in a symptom, but can still be detected using these tests.In terms of which of the evoked potential tests is the most useful, The American Academy of Neurology says that “VERs are probably useful for identifying patients with clinically definite MS, SSEPs are possibly useful, and there is insufficient evidence at this time to recommend BAEP as a useful test for diagnostic purposes.”
Want to learn more? See UpToDate’s topic, "Diagnosis of multiple sclerosis in adults," for additional in-depth, current and unbiased medical information on multiple sclerosis in adults, including expert physician recommendations.
Source:
Olek, Michael J. "Diagnosis of multiple sclerosis in adults." UpToDate. Accessed: January 2009.
