Responses to paired onset stimuli: implications for the delayed evoked potentials in multiple sclerosis

In multiple sclerosis (MS), demyelination is reported to be patchy and randomly distributed across the optic nerve, and a double peaked or broadened shape of the peaks of the pattern evoked potential would be expected. We tested quantitatively this hypothesis for our routinely recorded responses of MS patients. There was no significant broadening of the peak shape in the 75 prolonged latency responses. In order to simulate the patchy character of the affection of the optic nerve in MS, we tested 2 healthy subjects with a stimulus consisting of 2 presentations of a checkerboard pattern to different parts of the visual field, but with an adjustable onset asynchrony. It was found that, when the asynchrony was small (less than 40 msec) the second of the two stimuli did not contribute to the response. This suppression of the second response could be observed even when the second stimulus, by increasing its contrast, was made to evoke a much larger response than the first stimulus, when presented separately. Inhibition of the second response seems to explain the lack of wave shape alteration in the responses of MS patients, which would be expected on the basis of the patchy character of the demyelination. The responses from the fastest conducting fibres seem to determine the response, and inhibit the responses of the demyelinated fibres. From this, it can be concluded that a delayed response would be expected only in patients in whom a considerable number of the fibres of the optic nerve is affected by the demyelination. This seriously limits the sensitivity of the method for early diagnosis.