Refractive error and ocular parameters: comparison of two SD-OCT systems

Abstract

Purpose: Spectral domain optical coherence tomography (OCT) was used to examine the influence of refractive error (RE) on foveal retinal and choroidal thicknesses and scleral canal width (SCW). The performance of the Cirrus and Bioptigen spectral domain OCT instruments was compared in the same eyes.

Methods: Both eyes of 40 healthy human subjects, aged 22 to 38 years, were dilated and imaged, with the Cirrus OCT, using 6-mm five-line rasters collapsed into one line, one centered on the fovea and one bisecting the optic nerve head. Seventy-two of the same eyes were imaged with the Bioptigen OCT, using 6- by 6-mm scans, one centered on the fovea and one on the optic nerve head. Subfoveal retinal and choroidal thicknesses and SCW were measured. Axial lengths (ALs) and REs were obtained using an IOLMaster and a Grand Seiko autorefractor, respectively.

Results: Only right eyes were included in analyses. Spherical equivalent REs ranged from -12.18 to +8.12 diopters (mean [±SD], -3.44 [±4.06] diopters), and ALs ranged from 20.56 to 29.17 mm (mean [±SD], 24.86 [±1.91] mm). Myopia was associated with relatively thin choroids at the fovea (p < 0.05) but normal retinal thickness. Scleral canal width was significantly correlated with AL as measured with the Bioptigen OCT (p < 0.05). Retinal and choroidal thicknesses recorded with the Bioptigen OCT tended to be smaller than values obtained with the Cirrus OCT (mean difference, 5.63 and 24.76 μm, respectively), whereas the converse was true for the SCW (mean difference, 25.45 μm).

Conclusions: The finding that high myopes tend to have a thinner subfoveal choroid is consistent with previous studies. That high myopia was linked to enlarged scleral canals may help to explain the increased risk of glaucoma in myopia. Observed differences obtained with the Cirrus and Bioptigen instruments urge caution in comparing results collected with different instruments.

Figure 1

Plots of (A) spherical equivalent refractive error (RE), (B) anterior chamber depth (ACD), (C) average corneal curvature (K) and (D) white-to-white corneal diameter (WWD), against axial length (AL) for right eyes only; results of regression analyses added as inserts. Shaded areas represent the 95% confidence intervals.

Figure 2

Horizontal line scans (6 mm) captured using a Cirrus OCT of (A) the central retinal (foveal) region, and (B) optic nerve from one subject; C) a 6×6 mm rectangular cube scan from the Bioptigen OCT spanning the foveal region and D) corresponding en face image from another subject. Horizontal line in D shows the location of the scan in C. arrows: fovea; arrowheads: borders of scleral canal.

Figure 3

Foveal retinal thickness (RT) (A) and subfoveal choroidal thickness (CT) (B) plotted against axial length (AL) for the Cirrus OCT (open squares, dashed line) and Bioptigen OCT (filled circles, solid line); RT (C) and CT (D) for Cirrus OCT plotted against equivalent data from Bioptigen OCT; dashed line is 1:1; Bland Altman analysis of differences in RT (E) and CT (F) recorded with Cirrus and Bioptigen OCTs (dashed lines are mean ± 1.96*standard deviation)

Figure 4

(A) Scleral canal widths (SCWs) corrected for lateral magnification, plotted against axial length (AL), for both the Cirrus OCT (open squares, dashed line) and Bioptigen OCT (filled circles, solid line); (B) SCW measured with Cirrus OCT plotted against equivalent data from Bioptigen OCT; dashed line is 1:1; (C) Bland Altman analysis of differences in corrected SCW recorded with Cirrus and Bioptigen OCTs (dashed lines are mean ± 1.96*standard deviation).