Hypothesized modulators of outer retinal autofluorescence signal associated with SDD. Schematic is adapted from Chen et al.
37 to demonstrate a model of anatomical and physical factors influencing fluorescence lifetimes. Drawing is not to scale. Stages 1-3 of Zweifel et al.
13 are shown as dark gray to represent lack of FAF signal. A Stage 3 AF has been added as one possible explanation for the increase of FAF associated with SDD over time. As elaborated in the text, appearance of net greater FAF could result from the appearance of new fluorophores in SDD, the decrease of existing absorbers in adjoining photoreceptors, increased RPE fluorescence, or a combination. ELM, external limiting membrane; IS, OS, inner and outer segment of photoreceptor, RPE, retinal pigment epithelium; BLamD, basal laminar deposit; pre-BLinD, pre-basal linear deposit.
(A) Normal aged RPE has outer segments embedded in melanosome-containing apical processes and autofluorescent lipofuscin and melanolipofuscin in the cell body.
(B) Aged RPE with small SDD thought to correspond to stage 1 of OCT is associated with shorter, deflected outer segments.
(C) At Stage 2 of OCT, in which the EZ line is elevated, inner segments are now uneven in length, and some OS are deflected.
(D) At Stage 3 of OCT, overlying photoreceptors retract their inner segments or die, leading to the loss of EZ visibility in OCT. Some large deposits have evidence of outer segment fragments with disks still visible.
37 (E) Signal in Stage 3 AF may result from new fluorophores appearing in SDD, depicted as a change in color.