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Qiang Wang, Barry Cense, Omer Kocaoglu, Zhuolin Liu, Donald Miller; Phase retardation changes of Henle’s fiber layer associated with aging. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1467.
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Henle’s fiber layer (HFL) is composed of photoreceptor axons whose structured organization is well known to exhibit birefringence, a difference in refractive index experienced by light polarized perpendicular and parallel to the axon. While reduced birefringence has been suggested as an early indicator of disease onset, little is known about its distribution in the normal population and dependence on age. To address this issue, we compared phase retardation of HFL in two age groups using a research-grade polarization-sensitive optical coherence tomography (PS-OCT) system. PS-OCT depth resolves birefringence, permitting separation of the phase retardation contribution of HFL from that of other layers in the eye.
The PS-OCT system - previously developed at Indiana - simultaneously records depth-resolved intensity and birefringence by modulating the input polarization between two states. For this study, PS-OCT was configured to acquire volumes (15°x15°; 100 B-scans x 1000 A-scans) centered on the subject’s fovea at an A-scan rate of 25 kHz. Double pass phase retardation (DPPR) measurements were recorded on 10 young (20-35 yrs) and 10 older (50-65 yrs) gender matched subjects, with no signs of ocular pathology based on ophthalmologic examination. All subjects had refractive error less than ±4D. DPPR induced by HFL was extracted by comparing Stokes vectors at the inner limiting membrane to that at the inner-and-outer segment junction of the photoreceptor layer, which is the first strongly reflecting surface below HFL. To increase signal-to-noise, radial averages of DPPR about the foveal center were computed. Aging was tested by linear regression on the maximum DPPR value and corresponding retinal eccentricity.
To date, 12 of the 20 subjects have been processed. For these 12, Fig. 1 shows traces of the mean and standard deviation of the radially-averaged DPPR. The maximum DPPR is 23.0+/-3.9° (avg+/-2σ) and occurs at 2.0° retinal eccentricity. Linear regression reveals an overall decrease of 0.07°/yr (R^2=0.25) for maximum DPPR, 0.06°/yr (R^2=0.30) for average DPPR from 0.5° to 5° retinal eccentricity, and no age correlation for retinal eccentricity.
Phase retardation of HFL varies significantly across eyes and decreases with age.
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