Utility of Light-Adapted Full-Field Electroretinogram ON and OFF Responses for Detecting Glaucomatous Functional Damage

Abstract

Purpose: To compare parameters of electroretinogram (ERG) responses for their ability to detect functional loss in early stages of nonhuman primate (NHP) experimental glaucoma (EG), including photopic negative responses (PhNR) to a standard brief red flash on a blue background (R/B) and 200-ms-long R/B and white-on-white (W/W) flashes, to W/W flicker stimuli (5-50 Hz), and to a dark-adapted intensity series.

Methods: Light-adapted ERGs were recorded in 12 anesthetized monkeys with unilateral EG. Amplitudes and implicit times of the a-wave, b-wave, and d-wave were measured, as well as amplitudes of PhNRs and oscillatory potentials for flash onset and offset. Flicker ERGs were measured using peak-trough and fundamental frequency analyses. Dark-adapted ERG parameters were modeled by Naka-Rushton relationships.

Results: Only PhNR amplitudes were significantly reduced in EG eyes compared to fellow control (FC) eyes. The d-wave implicit time was delayed in EG versus FC eyes only for the W/W long flash, but in all eyes it was 10 to 20 ms slower for R/B versus the W/W condition. Flicker ERGs were <0.5 ms delayed in EG versus FC overall, but amplitudes were affected only at 5 Hz. The brief R/B PhNR amplitude had the highest sensitivity to detect EG and strongest correlation to parameters of structural damage.

Conclusions: The PhNR to the standard brief R/B stimulus was best for detecting and following early-stage functional loss in NHP EG.

Translational relevance: These results suggest that there would be no benefit in using longer duration flashes to separate onset and offset responses for clinical management of glaucoma.

Figure 1.

Example of ERG responses to each stimulus type from an eye with experimental glaucoma (EG, red traces) and control eye (blue traces) of NHP1. (A) Light-adapted (photopic) single-flash ERG responses are shown in the left, middle, and right panels, respectively, for the brief 4.5-ms R/B stimulus, the long-duration 200-ms R/B stimulus, and the long-duration 200-ms W/W stimulus. (B) Light-adapted flicker ERG responses are shown for each flicker rate. (C) Dark-adapted (scotopic) single-flash ERG responses are shown for the complete range of stimulus strength. See Methods for details.

Figure 2.

Characterization of experimental glaucoma (EG) stage at the time the ERG study was conducted. (A) Each box plot represents the median, interquartile range, and extremes of the distribution of parameter values among the 12 eyes in each group. Checkered boxes indicate baseline average values; unfilled boxes, values at the time of the ERG study. IOP was measured at the start of the ERG study session. (B) Optic nerve head (ONH) rim tissue thickness MRW parameter measured by OCT (^†7.3 ± 2.3 days earlier). (C) Circumpapillary retinal nerve fiber layer thickness (RNFLT) measured by OCT (^†7.3 ± 2.3 days earlier). (D) Circumpapillary RNFLT derived by scanning laser polarimetry (SLP; ^†7.3 ± 2.3 days earlier). (E) ONH tissue blood flow measured by the laser speckle flowgraphy (LSFG) mean blur rate (MBR) parameter (^†7.3 ± 2.3 days earlier). (F) Average amplitude of the multifocal ERG high-frequency component (mfERG HFC), measured during the same session just prior to the full-field flash ERGs. ^*P < 0.05; ^**P < 0.01; ^***P < 0.001; ^****P < 0.0001 (Šídák's multiple-comparisons post hoc test). ns, not significant.

Figure 3.

Light-adapted (photopic) full-field flash ERG results. Bars indicate group mean (±SEM) for EG eyes (red bars, n = 12) and control eyes (blue bars, n = 12). (A) PhNR amplitude for the brief R/B stimulus condition. ^**P < 0.01 (paired t-test). (B, C) ON PhNR amplitude (B) and OFF PhNR amplitude (C) for the long-duration R/B and W/W stimulus conditions. ^*P < 0.05; ^**P < 0.01, Šídák's multiple comparisons post-hoc test. (D–F) The d-wave amplitude (D), d-wave latency (E), and d-wave implicit time (F) for long-duration R/B and W/W stimulus conditions. ^*P < 0.05 (Šídák's test). (G) Oscillatory potential (OP) root mean square (RMS) amplitude for the brief R/B stimulus condition. (H, I) ON OP RMS amplitude (H) and OFF OP RMS amplitude (I) for the long-duration R/B and W/W stimulus conditions. ns, not significant.

Figure 4.

Light-adapted flicker ERG results. Symbols indicate group mean (±SEM, hashmarks) for EG eyes (red circles, n = 12) and control eyes (blue circles, n = 12); hashmarks are not shown if the SEM was smaller than the size of the symbol. (A, B) Peak-to-trough amplitude (A) and implicit time (B) of the ERG flicker response to each frequency tested. (C) Amplitude of the ERG response fundamental frequency versus stimulus frequency. ^*P < 0.05, ^***P < 0.001 (Šídák's multiple-comparisons post hoc test); all other pairwise differences were not significant.

Figure 5.

Dark-adapted (scotopic) ERG results for response amplitude versus stimulus strength. Symbols indicate group mean (±SEM, hashmarks) for EG eyes (red circles, n = 12) and control eyes (blue circles, n = 12). Curves represent the best fit of the Naka–Rushton intensity–response function to the aggregate data for each group. (A) a-Wave amplitude. (B) b-Wave amplitude. (C) OP amplitude.

Figure 6.

Structure–function relationships. Scatterplots show relationship between PhNR amplitude and the ONH rim tissue thickness parameter MRW (top row) or the circumpapillary RNFL thickness parameter (bottom row), expressed as the percent change from their baseline average values (N = 24 eyes total, 12 EG and 12 control eyes). The solid line in each plot represents the best fit of ordinary least squares linear regression, and dashed curves represent the 95% confidence intervals. Regression results and equations are shown below each panel. The horizontal dotted line represents the lower limit or normal amplitude (least negative value of control eye group).