Visualization of the Optic Fissure in Short-Wavelength Autofluorescence Images of the Fundus

Abstract

To document and explain the presence, inferior to the optic disc, of a distinct vertical boundary between two retinal areas of different short-wavelength autofluorescence (SW-AF) intensities. SW-AF images of the inferonasal region were acquired from 32 healthy subjects. Additionally, color, 488-nm reflectance (488-R), near-infrared reflectance (NIR-R), NIR autofluorescence (NIR-AF) images, and a spectral domain optical coherence tomography (SD-OCT) image were obtained in selected subjects. Gray levels (GL) on both sides of the demarcation line were measured in SW-AF and 488-R at fixed distances from the disc center. A curved demarcation line inferior to the optic disc was observed on SW-AF images in 31/32 subjects. AF levels on the nasal side were 13% (±6%) lower than on the temporal side at 20° inferior to the disc center. The contrast between the nasal and the temporal areas was not significantly affected by age, sex, refractive error, race, or iris color. The demarcation line visible in SW-AF was also seen, though with reduced contrast, in approximately 80% of the 488-R images (lower reflectance on the nasal side) and 50% of color images. The boundary was not detected by NIR-R, NIR-AF, or by SD-OCT imaging. The location and the distinctness of the demarcation line may indicate a relationship to the closed embryonic optic fissure. The reduced SW-AF intensity and 488-R reflectance observed on the nasal side of this line may be attributable to lower lipofuscin and melanin content per unit area, possibly resulting from a difference in RPE cell shape.