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Nimesh B. Patel, Brenda Garcia, Ronald S. Harwerth; The Influence of Anterior Segment Power on Ocular Magnification and RNFL Thickness measures from Spectral Domain Optical Coherence Tomography. Invest. Ophthalmol. Vis. Sci. 2012;53(14):681.
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© ARVO (1962-2015); The Authors (2016-present)
Optical imaging by SD OCT provides high resolution measurements of RNFL thickness for diagnosis and management of glaucomatous neuropathy. While the optical principles of the instrument are sound, the influences of the optics of the patient's eye on the accuracy of the measurement are less understood. Effects of one component, axial length, have been reported, but the effects of the optical power of the anterior segment have not been fully described. The purpose of this study was to determine the influence of the optical power of the anterior segment, using contact lenses, on the location of the scan path and measurements of the thickness of RNFL in normal healthy eyes.
15 normal subjects with less than 4D of ammetropia and no ocular pathology were recruited. One eye of each subject was randomly selected for scaning, including a raster cube centered on the optic nerve and a standard 120 diameter RNFL scan (Spectralis HRA+OCT). The scans were repeated with 10 separate contact lenses, (Proclear daily, CooperVision omafilcon A/60%) ranging from +8 to -12D in 2D steps. The extent of the retinal scan and RNFL thickness and area measures were quantified using custom MATLAB programs that included ocular biometry measures (IOL Master).
RNFL thickness decreased (0.52µm/D, r=-.33, p<0.01) and the retinal region scanned increased (0.52%/D, r=0.97, p<0.01) with increase in contact lens power (-12D to +8D). The normalized rates of change for RNFL thickness (-0.11/mm, r=-0.67, p<0.01) and image size (0.11/mm, r=0.96, p<0.01) were related to axial length. Changes in transverse scaling, computed with a three surface schematic eye, were in good agreement with predictions from image registration (mean=-0.00012µm/pixel, 95%LOA=-0.1, 0.1). RNFL area measures (calculated scan circumference X RNFL thickness) were not significantly related to contact lens power (863µm2/D, r=0.05, p=0.45).
Measurements of RNFL thickness by SD-OCT are dependent the optics of the eye, including both anterior segment power and axial length. The relationships between RNFL thickness measures and optical power are a direct reflection of the location of scan path with respect to rim of the optic nerve head caused by relative magnification. An incorporation of transverse scaling based on ocular biometry eliminated the effects of both axial length and the power of the anterior segment.
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