The refractory period of the visual evoked potential produced by pattern reversal in multiple sclerosis

Refractoriness of the visual evoked potential to pattern-reversal stimulation has been studied using a light-emitting diode stimulator, the effect on a test P100 of a conditioning pattern reversal being determined for conditioning-test intervals of 20 ms, 50 ms and 100 ms. In 20 normal subjects, the latency of the test P100 was increased by a mean of 9.8% (SD 5.0%) with the 20-ms interval, 5.8% (SD 6.6%) with the 50-ms interval, and 2.8% (SD 4.7%) with the 100-ms interval. Patients with clinically definite multiple sclerosis were divided into 3 groups based on the latency of P100 in the conventional VEP: within 2 SD of the normal mean (7 patients), plus 3-6 SD (7 patients), and greater than 10 SD (7 patients), representing normal, mildly abnormal and grossly abnormal latencies, respectively. Only 4 patients had "abnormal" results in the conditioning-test paradigm, 2 of whom were in the "grossly abnormal" group and 1 in each of the other two groups. The prolongation in latency tended to be greater the longer the control P100. The conditioning-test paradigm did not reveal evidence of a significantly prolonged refractory period of demyelinated axons in the optic nerve, and did not assist in the identification of changes "typical of demyelination" in patients with normal or mildly abnormal control P100s. It is concluded that testing the refractoriness of the VEP may not enhance the diagnostic yield of the VEP in multiple sclerosis.