Abstract
Purpose :
The ERG photopic negative response (PhNR) is reduced in glaucoma; generally detected best using a red stimulus flash on a blue background (R/B). Longer duration flashes separate the ERG "on" (increment) and "off" (decrement) responses, which also combine to shape flicker responses in a manner that depends on stimulus frequency, and possibly also sensitivity to glaucoma. Here we compare brief (4.5 ms) R/B flashes, long (200 ms) R/B and white-on-white (W/W) flashes, and varied W/W flicker frequencies for their ability to detect early functional loss in a non-human primate (NHP) model of experimental glaucoma (EG).
Methods :
Photopic ERGs were recorded (UTAS BigShot, LKC, Gaithersburg, MD) at an early stage of unilateral EG (defined by OCT structural changes, PMID: 27564522) in 6 anesthetized adult rhesus monkeys (Macaca mulatta, 5F/1M ages 4.7-16.5 y) with dilated pupils after 10 min adaptation to 30 cd/m2 blue light, then scotopic ERGs after 20 min dark adaptation. All photopic stimuli were 560 cd/m2, including 4.5 ms R/B, 200 ms R/B and W/W flashes, and 4.5 ms W/W flicker at 5, 10, 20, 30.3, 40, and 50 Hz; scotopic W flashes ranged -3.6 to 2.4 log cd-s/m2. Amplitude and implicit time (IT) of a-wave, b-wave and d-wave peaks were measured, along with PhNR amplitude 65 ms after flash onset and 80 ms after long flash offset. Flicker amplitude and IT were measured by peak-trough and harmonics by Fourier analysis (latter not shown). ANOVA and paired t-tests were used for statistical analysis.
Results :
Peripapillary retinal nerve fiber layer thickness in EG and fellow control (FC) eyes was 86.0 ± 11.3 and 101.2 ± 7.4 µm, respectively. Significant ERG differences between EG and FC eyes were found only for the PhNR amplitude of the R/B brief flash (p = 0.031) and the “on” PhNR of the R/B long flash (p = 0.042, Table/Fig). D-wave latency (p < 0.0001) and IT (p = 0.003) were delayed by 10-15 ms for R/B relative to W/W long flashes, but did not differ between EG and FC eyes.
Conclusions :
At this early stage of NHP EG, full-field ERG changes were limited to reduction of the PhNR for brief R/B and the “on” PhNR of long R/B flashes. Thus, R/B stimuli were better than W/W for detecting early functional loss and separating “off” responses offered no benefit. The chromaticity-dependent timing of the d-wave has some precedent but requires further investigation.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.