Impaired Rod-Mediated Vision is the Functional Hallmark of Ageing and Early and Intermediate Age-Related Macular Degeneration.

Laboratory and patient-oriented research on photoreceptors and vision in ageing and early and intermediate age-related macular degeneration (AMD) were conducted in parallel starting in the 1990s by Christine Curcio and Cynthia Owsley, respectively. They joined forces in two longitudinal observation studies, the Alabama Study on Age-related Macular Degeneration (ALSTAR) in 2009 and in ALSTAR2 in 2019, together involving >1100 participants. These studies established rod-mediated dark adaptation (RMDA) measured close to the fovea as the first functional biomarker for incident early AMD and the progression of AMD. These studies used standardised grading of colour fundus photography and prioritised large samples for statistical power. RMDA is a global measure of dysregulated transport between the circulation and photoreceptors, involving at least seven different steps in a retinoid re-supply route especially needed by rods, and a surrogate for the delivery of other essentials across this route. The choice of functional and imaging outcome measures was informed by a model of AMD pathophysiology based on drusen biology, as discovered in the laboratory using high-quality human donor eyes. Specifically, high-risk drusen in the central retina were thought to result from large lipoproteins constitutively made by the retinal pigment epithelium and impaired in transit to circulation by ageing changes in Bruch's membrane and choriocapillaris. Imaging studies in ALSTAR/2 thus included optical coherence tomography (OCT) angiography assessments of choriocapillaris flow signal and OCT assessments of outer bands involved in intercellular transfer (interdigitation zone). Of seven vision tests utilised in ALSTAR2, only RMDA achieved a Minimum Clinically Important Difference at 3 years follow-up. This research highlights the importance of developing functionally valid structural endpoints for use in early and intermediate AMD intervention trials. Research to date supports the idea that functional changes emerge earlier than structural changes in early and intermediate AMD. Clinicaltrials.gov # NCT04112667 (registration date October 7, 2019).