Evoked Potential (EP) tests
Evoked Potential tests are procedures for measuring the speed of impulses along neurons. Responses can be measured using EEG readings from electrodes attached to the scalp and occasionally other areas of the skin. Although this may sound like something from Frankenstein, they are in fact completely painless and entirely harmless. Based on input signals to the particular sense being measured, the time taken for that response to register can be accurately measured and compared to normal readings. The results are then analysed on a computer and average speeds recorded.
Demyelinated neurons transmit nerve signals slower than non-demyelinated ones and this can be detected with EP tests. Although they may appear to function perfectly, even remyelinated neurons are slower than normal nerves and so historical lesions can be detected in this way.
There are three main types of evoked potential test:
Visually Evoked Potential (VEP)
This test measures the speed of the optic nerve. The patient has to focus on the centre of a “TV” screen on which there is a black and white chequered pattern. Each square in the pattern alternates between black and white at measured intervals. The patient wears a patch on one eye for a while and then on the other, so that the speed of both optic nerves can be measured.
85-90% of people with definite MS and 58% of people with probable MS will have abnormal VEP test results.
Brainstem Auditory Evoked Response (BAER)
The BAER test measures the speed of impulses along the auditory portion of Cranial Nerve VIII. This nerve arises in the Pons area of the Brainstem and therefore this test may be indicative of lesions in that area. The patient lies down in a darkened room to prevent visual signals from interfering with measurements. A series of clicks and beeps are played back to the patient.
67% of people with definite MS and 41% of people with probable MS will have abnormal BAER test results.
SomatoSensory Evoked Potential (SSEP)
The SSEP test involves strapping an electrical stimulus around an arm or leg. The current is switched on for 5 seconds and electrodes on the back and skull measure the response at particular junctions. The current is very low indeed and completely painless. The speed of various nerves can be measured in this way and the points of slow-down (i.e. demyelinated lesions) approximated to because of the sampling at several places.
77% of people with definite MS and 67% of people with probable MS will have abnormal SSEP test results.
Slow nerve responses in any of these tests are not necessarily indicative of MS but can be used in conjunction with a neurological examination, medical history, an MRI and a spinal tap to deduce some kind of diagnosis.