An interocular comparison of the multifocal VEP: a possible technique for detecting local damage to the optic nerve.

PURPOSE: To develop a quantitative measure of local damage to the ganglion cells/optic nerve based on an interocular comparison of multifocal visual evoked potentials (mVEP). METHODS: Multifocal VEPs were recorded from both eyes of six normal subjects and four patients; each eye was stimulated separately. Two of the patients had glaucoma, one had ischemic optic neuropathy, and one had unilateral optic neuritis. All four patients had considerably more damage in one eye than in the other, as indicated by their Humphrey visual fields. The multi-input procedure of Sutter was used to obtain 60 VEP responses to a scaled checkerboard pattern. The amplitude in each response was obtained using a root mean square measure of response magnitude. For each of the 60 pairs of responses, a ratio between the amplitude of the responses from the two eyes was obtained as a measure of the relative health of one eye compared with the other. The mean and SD of this ratio measure for the control group were used to specify confidence intervals for each of the 60 locations. All patients had Humphrey 24-2 visual fields performed. To allow a comparison of the mVEPs to the visual fields, a procedure was developed for displaying the results of both tests on a common set of coordinates. RESULTS: Except for a small interocular difference in timing attributable to nasotemporal retinal differences, the pairs of mVEP responses from the two eyes of the control subjects were essentially identical. Many of the pairs of responses from the patients were significantly different. In general, there was reasonably good agreement with the Humphrey 24-2 visual field data. Although some regions with visual field defects were not detected in the mVEP due to small responses from the better eye, other abnormalities were detected that were hard to discern in the visual fields. CONCLUSIONS: Local monocular damage to the ganglion cell/optic nerve can be quantitatively measured by an interocular comparison of the mVEP.