Vesta CK Chan, Mary Ho, David TL Liu, Alvin L Young, Dennis SC Lam

1. Choroidal thickness measurement in myopic eyes by enhanced depth optical coherence tomography Vesta CK Chan, Mary Ho, David TL Liu, Alvin L Young, Dennis SC Lam Department of Ophthalmology & Visual Sciences, Prince of Wales Hospital, Hong Kong SAR Introduction The axial elongation of the globe in pathological myopia leads to stretching of retinal tissue and reduced retinal function. Choroid is important to maintain retinal function as it supplies nutrients to retinal pigment epithelial cells and the outer retina. Compromised choroidal circulation may account for the retinal dysfunction and vision loss in high myopic. Direct choroidal layer visualization and measurement could be obtained through enhanced depth imaging spectral domain optical coherence tomography (OCT). This study aims at delineating cause for loss in vision in high myopic eyes without attributable myopic macular changes. Design To measure choroidal thickness in myopic eyes using enhanced depth imaging in a cross-sectional study. Methods Fifty-six consecutive patients with spherical equivalent refractive error of at least 6 diopters were evaluated using enhanced depth imaging optical coherence tomography images that were obtained by positioning the spectral-domain optical coherence tomography (OCT) device close enough to the eye to acquire an enhanced signal of the choroidal layer. The choroidal depth was measured as the distance between the outer reflective retinal pigment epithelium layer and the inner sclera border. Measurements were made at horizontal fashion across the fovea at 500µm intervals of sections. Choroidal thickness (CT) were measured at the subfoveal region, 3mm temporal to fovea and 3 mm nasal to fovea respectively in a horizontal fashion. Results Mean age of the 56 patients was 50.4 years ( +/- 2.03 years standard deviation SD; interquatile range IQR: 42 years to 62 years) , and the mean refractive error was – 8.7 D (IQR; -6.1D to -11D) . The mean subfoveal CT was 118µm ( +/- 68μm ) and was correlated negatively with age (P=0.032) and refractive error (P= 0.011). Regression analysis suggested that subfoveal CT decreased by 11.9µm per each decades of life and by 6.205µm for each diopter of myopia. The subfoveal choroidal thickness was inversely correlated with logarithm of the minimum angle of resolution visual acuity (P= 0.008) and visual acuity improved by 0.02 (log MAR) by 10µm increase in choroidal thickness. Conclusion Choroidal thickness decreases with age and severity of myopia. Visual acuity drops in line with decreasing subfoveal choroidal thickness. Reduction in choroidal thickness is related to aging and severity of myopia, while visual acuity depends on subfoveal choroidal thickness. Our study supports the theory that choroidal abnormality may play a key role in pathogenesis of myopic degeneration. The axial elongation of the globe in pathological myopia leads to stretching of retinal tissue and reduced retinal function. Choroid is important to maintain retinal function as it supplies nutrients to retinal pigment epithelial cells and the outer retina. Compromised choroidal circulation may account for the retinal dysfunction and vision loss in high myopic. Direct choroidal layer visualization and measurement could be obtained through enhanced depth imaging spectral domain optical coherence tomography (OCT). Figure 1: Figure illustration of obtaining choroidal thickness of myopic eyes. The choroid is seen in cross-section on OCT scan. Fig 1A: Showing a sample scan from a 28-year-old male with 6-diopter of myopia, the choroidal thickness was measured vertically from the outer border of retinal pigment epithelium to the outer border of choroid (bounded by two lines). Fig 1B: An OCT scan of a 67-year-old male with reduced choroidal thickness with increasing age. The Association Between the Visual Acuity in logMAR with the Subfoveal Thickness. Univariate Linear Mixed Model: (dependent variable: log MAR, independent variable: choroidal thickness) Figure 2: Scatterplot showing negative correlation between subfoveal choroidal thickness with severity of myopia. (CT: choroidal thickness, D: diopter) The visual acuity in logMAR was fitted with fixed coefficients (fixed effects) on subfoveal thickness, with random intercepts (random effects) at the subject level to adjust correlation between fellow eyes. • Lam CS, Goldschmidt E, Edwards MH. Prevalence of myopia in local and international schools in Hong Kong. Optom Vis Sci 2004;81:317-22. • Curtin BJ, Karlin DB. Axial length measurements and fundus changes of the myopic eye. Am J Ophthalmol 1971;71:42-53. • Povazay B, Hermann B, Unterhuber A, et al. Three-dimensional optical coherence tomography at 1050 nm versus 800 nm in retinal pathologies: enhanced performance and choroidal penetration in cataract patients. J Biomed Opt 2007;12:041211. • Fujiwara T, Imamura Y, Margolis R, et al. Enhanced depth imaging optical coherence tomography of the choroid in highly myopic eyes. Am J Ophthalmol 2009;148:445-50. References