Contrast sensitivity is elevated for stimuli outside cone-driven complete spatial summation in age-related macular degeneration

Abstract

Purpose : Contrast sensitivity (CS) is known to decrease in certain ocular diseases, and the extent of this change has been shown to be dependent on the stimulus size. In this study, we characterised the relationship between CS and stimulus size in the central visual field (VF) in patients with age-related macular degeneration (AMD) using standard automated perimetry (SAP) to determine CS changes for different test sizes.

Methods : Contrast sensitivities across the Humphrey Field Analyzer (HFA) 10-2 VF were obtained from one eye from patients with intermediate AMD (n=22) and early AMD (n=1) (range: 61-80 years; mean: 71±5 years) using Goldmann stimulus (G) I to V in full threshold mode on a 10 cd/m² background. Sensitivities were corrected to a 50 year-old equivalent using correction factors based on previously published work and compared to our normal group.

Results : When CS values along the horizontal meridian were plotted (Figure 1A), the Hill of Vision (HoV) for the AMD patients showed little difference with GI to GIII when compared to that of 50 year-old equivalent normals described in our previous study (Choi et al 2016). However, CS measured with GIV and GV were found to be significantly higher in AMD patients compared to the normal group with a mean difference of ≈ 0.5 to 1 dB implying Ac has increased.

Conclusions : We showed higher CS of ≈ 0.5 to 1 dB on average when using stimuli larger than the size of cone-driven complete spatial summation (i.e. GIV or GV) within the 10-2 VF in AMD patients. The established elevation of rod thresholds in AMD may contribute to stimuli detection under these conditions leading to improved sensitivity due to favouring of rod detection and the established superior retinal pooling of the rod pathway. Further research is required to explore this hypothesis. Using a large stimulus in SAP may be an alternative way to detect earlier visual dysfunction in AMD across the various locations in VF rather than localized dysfunction in dark adaptation test.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.

 

(A) Contrast sensitivities along the horizontal meridian for GI, GIII and GV. All data points represent the mean and error bars represent 95% confidence intervals.

 

(B) Schematic diagram of proposed spatial summation model for AMD showing enlarged Ricco’s area (critical area, Ac) and increased sensitivity when stimuli at or larger than Ac of AMD group (i.e. GIV and GV) are used.