Abstract

Purpose: Optical coherence tomography (OCT) determined thickness measures of the macula are often used for the diagnosis and management of retinal and optic nerve pathology. Current methods use a fixed angular projection volume scan to derive thickness measures. The purpose of this study was to assess the relationship between axial length and the spectral domain OCT-measured thickness of seven retinal layers in normal human subjects after incorporating transverse scaling.

Methods: For each of 98 normal subjects recruited, one randomized eye was enrolled in the study. After pupil dilation with 1% tropicamide, a 49 line, 20 × 20 degree OCT raster scans, centered on the fovea, were acquired on each subject with averaging of 16 frames. Corneal curvatures and axial length parameters were measured with an optical biometer. Using an automated retinal segmentation algorithm (Orion, Voxeleron LLC, Pleasanton, CA), average thickness, ETDRS measures were exported for seven retinal layers. The average thicknesses of these layers at the three annulus diameters of the macular grid (2mm, 3mm and 5mm annulus rings) were correlated with axial length measurements using linear regression analysis.

Results: The average age of subjects recruited was 26.14 years (range 21 to 42 years). All subjects had healthy retinas and optic nerve heads as observed with fundus photography. The mean axial length was 24.60 mm (range 22.02 to 27.72mm). There was a significant relationship between scan focus and axial length (slope = -0.35D/mm, r²=0.5067, p<0.01). Based on instrument scaling that incorporates scan focus and corneal curvature, there was a statistically significant relationship between axial length and the thickness measures of the inner nuclear layer (p<0.01) of the outer annulus. However, only a small percentage of the variance (r² < 0.108) could be accounted for by the linear regression. No significant correlations were found between axial length and average thickness measures from annuli for the other six retinal layers (Bonferroni correction, p > 0.017).

Conclusions: When scans are scaled, there was a weak relationship between axial length and inner nuclear layer thickness in the parafoveal region (3-5mm annulus diameter). This relationship might not be clinically significant based on the variance. Hence, when scaling is incorporated into thickness analysis of the macula region, the effects of ocular magnification are minimal.