High resolution multi-focal VEP: towards better denfinition of scotomas

Purpose: The mfVEP standard stimulus includes up to 60 areas of the visual field incorporated into 5 eccentric rings. High amplitude and waveform variability exists between various sectors within the tested field. This is largely due to cortical anatomy and possible cancellation effects since each stimulating sector projects to a large enough area of the striate cortex that it may include oppositely or variously oriented dipoles. The purpose of this study was to determine if increasing the number of stimulating areas reduces field variability and increases resolution to detect focal scotomas. Methods: A multichannel mfVEP cortically scaled checkerboard stimulus was modified to present 120 test zones using the same type of pseudorandom sequence (with 5 rings increased to 8 rings). The number of checks in each segment was reduced to nine. The pattern was presented as blue on yellow sparse pattern onset. Normal individuals (27) were tested on standard 56 segment and 120 segments sequentially in random order (electrodes in same position) and a comparison of intra field variability of amplitudes performed. Artificial scotomas were created to test spatial resolution. 20 glaucoma patients with very early to moderate glaucomatous defects were also tested (MD <6DB). Results: The high resolution required a longer test time but usually <10 mins per eye. Intra-field (within individual) amplitude variability decreased, but only slightly (COV 0.46 vs 0.42, p <0.001) suggesting more uniform signals throughout the field, and thus reducing some of the effects of cortical convolutions. Between individual variation was not significantly different for the two tests. In glaucoma both tests detected all defects with good topographic correlation except for one that was missed on standard grid, but they were better defined and in the majority of cases larger on high resolution plots. In 4/20 early cases, a defect was detected in the fellow eye with a still normal visual field. Conclusions: High resolution 120 point mfVEP is a clinically viable technique that may improve objective assessment of localized scotomas by increasing spatial resolution. The inclusion of smaller test patches may aid in the definition, but probably not detection, of abnormality.