Differences In Foveal Pit Morphology, Cone Density And Axial Length Between Fellow Eyes Of Normal Subjects

Abstract

Purpose: : While large variations in foveal pit morphology and cone photoreceptor density are known to exist across normal individuals, less is known about the relationship of these parameters between fellow eyes of the same subjects. We measured and compared foveal pit shapes and cone densities between fellow eyes of normal subjects with differing axial lengths.

Methods: : Axial length was measured in both eyes of 15 normal, Caucasian subjects (mean age = 29.2 ± 10.2 years) using an IOLMaster (Carl Zeiss Meditec). Volume scans of the central macula (6 x 6 mm, 128 B-scans) were acquired using spectral domain optical coherence tomography (Cirrus HD-OCT, Carl Zeiss Meditec) and analyzed using custom MATLAB software to determine foveal pit depth, diameter and maximum slope (Dubis et al. Br J Ophthalmol. 2009;93:1223-1227). In vivo images of the cone mosaic were acquired over the central 3 degrees of the retina in both eyes of 4 subjects using an adaptive optics scanning laser ophthalmoscope. Mean cone density was calculated in each eye at a retinal eccentricity of 0.3 mm.

Results: : Mean axial length across eyes was 24.05 ± 1.56 mm (range: 21.64 - 27.14 mm). The mean difference in axial length between fellow eyes was 0.19 ± 0.12 mm (range: 0.01 - 0.36 mm). Across subjects, foveal pit depth and maximum slope decreased with increasing axial length (R²=0.29 [p=0.04] and R²=0.65 [p<0.001], respectively) while foveal pit diameter increased with increasing axial length (R²=0.45, p<0.01). Consistent with previous reports, cone density decreased with increasing axial length across subjects (R²=0.82, p<0.04). Between fellow eyes, however, there was no correlation between differences in any foveal pit parameter and corresponding differences in axial length. Within an individual, there was a weak tendency for the subject's longer eye to have a decreased cone density (R²=0.79, p=0.11).

Conclusions: : Across normal subjects, eyes with longer axial lengths have wider, shallower and less steep foveal pits and decreased cone densities. Between fellow eyes of normal subjects, inter-eye differences in foveal pit shape and cone density cannot be solely explained by inter-eye differences in axial length. Examining additional features (e.g., size of the foveal avascular zone, peak cone density) may improve understanding of the relationship between these foveal parameters.

Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • macula/fovea • photoreceptors