The clinical significance of the bifid or "W" pattern reversal visual evoked potential

Although the visual evoked potential (VEP) to an alternating checkerboard stimulus is usually recorded from the occipital midline as an N-P-N complex with a major positive deflection at 100 ms (P100), a wave form exhibiting a P-N-P or "W" morphology is occasionally encountered and its interpretation is the source of some controversy. A retrospective chart review identified 15 patients exhibiting the "W" VEP. This represented 7.6 percent of 197 VEP studies and 5.1 percent of 394 eyes. The response was encountered in 1.7 percent of 57 normal patients and 21.4 percent of 56 patients with definite, probable or possible multiple sclerosis (P less than .001). The "W" response was considered normal in only one patient. Of the remaining 14 cases, 13 had definite, probable or possible MS and one had ischemic optic neuropathy. It is concluded that the "W" VEP is an aberrant response that is rarely seen in normals and may have the same significance as a delayed P100 latency.