To explore light-dependent changes in retinal reflectivity with optical coherence tomography (OCT) as a biomarker for Alzheimer’s disease.

There are subtle stimulation-induced changes in how neural tissues reflect and absorb infrared light, thereby altering the tissues’ appearance in OCT images. Previously-documented changes of retinal photoreceptors may be driven by local changes in tissue water content. We therefore examined whether layers occupied by Müller glia – which participate in retinal water management – show stimulation-dependent changes in reflectivity.

In this two-part study, we assessed healthy young adults, and then compared patients with early-onset Alzheimer’s disease to age-matched controls. Para-foveal OCT scans were obtained of the right eye, alternating bright light exposure and dark adaptation. Retinas were spatially normalized to a common template, and reflectivities were normalized to the retinal pigment epithelium. At each depth into the retina, reflectivity in light versus darkness was compared using generalized estimating equations, with significant results falling below a standard false discovery rate threshold (q=0.05). OCT beam angle was assessed as a covariate.

In healthy adults, virtually all layers of the retina showed significant stimulation-dependent changes in reflectivity. With bright light, all layers with Müller glia showed a significant ~2% reduction in reflectivity. The same stimulus caused a ~6% increase in reflectivity in avascular and glia-free photoreceptor layers. Light-induced reflectivity changes at the outer nuclear layer were exaggerated in patients with Alzheimer’s disease. These patterns remained after accounting for OCT beam angle, which independently influences reflectivity in some retinal layers.