Bisretinoid lipofuscin, fundus autofluorescence and retinal disease

Abstract

Retinal pigment epithelium emits an inherent autofluorescence that originates from naturally occurring fluorophores when excited by short-wavelength light (SW-AF) in the spectral range between 400 and 590 nm. Peak excitation is 490 nm. The autofluorescence emission occurs at wavelengths between 520 and 800 nm with a peak of approximately 600 nm. For clinical purposes this emission is recorded as fundus autofluorescence either using a confocal scanning laser ophthalmoscope (cSLO; 488 nm excitation); a modified fundus camera or by ultra-wide-field ophthalmoscopic technology. The topographic distribution and intensities of fundus autofluorescence are modulated by superior-inferior differences in retinal illuminance. The autofluorescence distribution also departs from normal in the presence of retinal disease; accordingly these changing patterns assist in the diagnosis and monitoring of the disorders. The cellular source of SW-AF is consistent with an origin from a group of di-retinaldehyde (bisretinoid fluorophores) compounds that are produced randomly in photoreceptor cells and constitute the lipofuscin of the retinal pigment epithelium. Bisretinoids also contribute to retinal disease processes. Here we will primarily address this family of bisretinoid fluorophores since they account for the spectral, age- and disease-related properties of retina lipofuscin and SW-AF. Moreover, the differing absorbances exhibited by the members of this group of fluorophores accounts for the range of excitation wavelengths that elicit fluorescence emission from RPE lipofuscin and from the fundus. That range is consistent with emission from a family of fluorophores, not a single fluorophore.

Keywords: Bisretinoid; Fundus autofluorescence; Photoreceptor cell; Retina; Retinal pigment epithelial cell; Visual cycle.