Central fiber contribution to W-shaped visual evoked potentials in patients with optic neuritis

We studied W-shaped waveforms that occurred in full-field responses to pattern large-field stimulation in patients who had optic neuritis. Affected eyes showed no absolute scotomata; visual acuity was normal at the time of recording. To evaluate the contribution of macular- and paramacular-derived components to the development of the W-shaped waveforms in the patients, half-field and central full-field stimulation was used. The responses were compared with those obtained with the use of experimental scotomata in healthy subjects. The W-shaped waveforms recorded in the patients closely resembled the responses observed in healthy subjects after the introduction of experimental scotomata. In all affected eyes, half-field stimulation showed absence of the ipsilateral P100 component or its interaction with the P135 component. Enhanced paramacular N105 and P135 components were seen over the contralateral hemisphere. Responses to central full-field stimulation were an attenuated and prolonged P100 in the majority of affected eyes. Results of our study showed that W-shaped waveforms in response to large full-field stimulation may reflect impaired function of macular fibers. These electrophysiologic findings, however, were not always associated with evidence of a central field defect demonstrated by Friedmann perimetry.