Adapting background illuminance modulates visual thresholds in patients with retinitis pigmentosa throughout the macula

Abstract

Purpose : To quantify patterns of threshold elevation under different light adaptation conditions throughout the macula in patients with retinitis pigmentosa (RP).

Methods : Nine subjects with RP (24 to 59 years old) and 10 visually-normal individuals (18 to 49 years old) participated. Threshold was measured at 7 locations along the horizontal meridian of the visual field (10^o nasal to 10^o temporal) using Goldmann size III targets. The field locations were grouped into three regions based on eccentricity from fixation: perifoveal (5^o-10^o), parafoveal (2^o) and foveal. Threshold was measured under DA conditions using long-wavelength stimuli and across a broad range of adaptation levels (DA to 50 cd/m²) using short-wavelength stimuli. The relationship between threshold and adaptation level across the field was described using a standard “threshold-vs-illuminance” (TvI) model to derive absolute threshold (T_o), slope (n), and eigengrau (A_o). Repeated measures analysis of variance (ANOVA) was used to compare the three fit parameters between the two subject groups.

Results : Under DA conditions, thresholds for the short- and long-wavelength stimuli were similar throughout the visual field for 8 of 9 subjects, consistent with the cone pathway mediating threshold at all locations. Threshold elevation was greater for the RP subjects under DA and low adaptation conditions, compared to LA conditions. ANOVA indicated significantly elevated T_o and A_o for the RP subjects for each field location. The value of n was approximately independent of field location for the RP subjects and increased with eccentricity for the controls. This pattern resulted in significantly increased n in the fovea and significantly decreased n in the perifovea (both p < 0.05), with no significant difference in the parafovea (p = 0.17).

Conclusions : The approximately similar elevations in T_o and A_o for the RP subjects suggests that reduced quantal absorption may underlie threshold elevation throughout the macula in this cohort of RP subjects. Alternatively, the results may be attributed to the lack of rod function in these RP subjects, which would elevate DA threshold more than LA threshold.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.