Mapping of glaucomatous visual field defects by multifocal VEPs.

PURPOSE

To objectively measure a visual field and to sensitively detect glaucomatous visual field defects by measuring the contrast sensitivity function (CSF), using multifocal visual evoked cortical potentials (MVEPs).

METHODS

MVEPs of normal subjects (n = 28) and of patients with glaucoma (n = 12) or ocular hypertension (OH, n = 1) were recorded. A multi-input procedure was used to obtain 37 local VEP responses to each scaled hexagon, composed of 24 triangular patterns, reversing in a counterphase manner. Two pattern contrasts of 32% and 8% were used for measuring the contrast threshold. To improve the signal-to-noise ratio, 37 MVEPs were averaged into 20 groups. The root-mean-square (RMS) measures at both contrasts were calculated. Contrast thresholds were estimated by extrapolating the regression line of the amplitude versus contrast to the mean noise levels.

RESULTS

RMS amplitudes of each local MVEP decreased as the eccentricity increased and as the pattern contrast decreased in normal subjects. It was also revealed that the amplitudes were smaller in the upper-half field than those in the lower-half field. Compared with the RMS amplitudes, CSFs estimated by MVEPs were relatively constant without being strongly influenced by retinal eccentricity. In patients with glaucoma, the CSFs, even from the locations where the mean perimetric sensitivities ranged to more than 30 dB, were significantly smaller than those in the normal control subjects (P < 0.001, Mann-Whitney test). CSFs in the 20- to 30-dB and 100 to 20-dB groups were also significantly smaller than those in the more-than-30-dB or 20- to 30-dB groups, respectively (P < 0.001, Mann-Whitney test). A significant correlation was found between the logarithmic function of the CSF and perimetric sensitivity (r = 0.57, P < 0.001, n = 216). The CSFs were evaluated on the basis of normal SD. Mappings of the CSFs agreed well with those of the perimetric sensitivity in all patients with glaucoma and was even more sensitive in detecting slight optic nerve damage by glaucoma than was perimetry.

CONCLUSIONS

Local optic nerve damage caused by glaucoma can be sensitively detected by measuring contrast sensitivity using the MVEP.