Photoreceptor Density–Dependent Kinetics of Geographic Atrophy Progression

Purpose: To investigate whether the spatial distribution of photoreceptor density governs the rate and directionality of geographic atrophy (GA) progression and to propose a photoreceptor density-based metric for biologically informed assessment of GA progression. Design: A retrospective longitudinal analysis of untreated fellow eyes from a multicenter, randomized, sham-controlled clinical trial (MAHALO). Participants: A total of 103 untreated fellow eyes from 143 eyes enrolled in the MAHALO study, all with GA secondary to age-related macular degeneration. Methods: Fundus autofluorescence images obtained at baseline and 18 months were analyzed. Geographic atrophy lesion area and boundary progression distance ("front") were quantified using rigidly registered images. The retina was divided into concentric eccentricity rings centered on the fovea. Theoretical cone and rod density models derived from histologic data were applied to generate photoreceptor density maps. Photoreceptor loss within GA lesions was estimated, and a photoreceptor-adjusted front was calculated by weighting boundary progression by local photoreceptor density. Directional progression was further evaluated using clock-hour-based analysis and a cosine-based directional metric (cosθ). Main Outcome Measures: Geographic atrophy area enlargement, front progression distance, photoreceptor-adjusted front, ring-wise photoreceptor loss, and directional variability of lesion expansion. Results: < 0.01). In extrafoveal lesions, conventional front measurements demonstrated more rapid peripheral versus foveal progression, whereas this directional asymmetry was attenuated after photoreceptor density adjustment. Conclusions: Geographic atrophy progression may be partly explained by the spatial distribution of photoreceptor density, among other contributing biological factors. Apparent directional and regional differences in GA expansion may reflect underlying gradients in photoreceptor density in addition to geometric and other biological influences. Photoreceptor density-based metrics provide a biologically informed framework for interpreting GA progression and may serve as complementary measures for natural history studies and clinical trials. Financial Disclosures: Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.