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N. B. Patel, X. Luo, J. L. Wheat, R. S. Harwerth; Retinal Nerve Fiber Layer Assessment: Area vs. Height Measurements From Elliptical Scans Centered on the Optic Nerve. Invest. Ophthalmol. Vis. Sci. 2010;51(13):2724.
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An evaluation of the retinal nerve fiber layer (RNFL) provides important information for evaluation of the health of an optic nerve. Clinical measurements of RNFL consider only thickness, but an accurate assessment requires consideration of, axial length, size of the optic nerve head (ONH), blood vessel contribution, and distance of the scan from the ONH margin. In addition, although most ONHs are elliptical in shape, circular scans centered on the ONH are the mainstay for the analyses. The purpose of this study was to evaluate the thickness and area of RNFL cross sections when axial length and ONH shape are included.
Raster and radial scans of the left optic nerve were acquired from 23 normal M. mulatta subjects using the Spectralis HRA+OCT. The disc margin was marked by manually selecting the choroid/optic nerve junction when visible and a best-fit ellipse was determined. With conversion of pixels to microns from each subject’s axial length, RNFL scans were extrapolated at 300 microns to 600 microns (50 micron increments) from the edge of the ONH. The thickness and area of the RNFL at each distance were obtained by custom MATLAB programs. Blood vessels in the RNFL were selected, and removed from the overall RNFL measures.
The average RNFL thickness decreased systematically from 151 ± 10.5µm for scans 300 microns from the disc margin to 114 ± 6.3 µm measured at 600 microns (p < 0.05). In contrast, the cross section area of the RNFL did not vary with scan location from the disc margin (0.85 ± 0.06 mm2 at 300 microns compared to 0.86 ± 0.05 mm2 at 600 microns). Blood vessels accounted for 9.6% of total RNFL thickness or area, but varied with retinal location. 17.4% of the superior and 14.5% of the inferior RNFL area was from blood vessel area, while blood vessels accounted for only 3.4% of the areas of the temporal and nasal RNFL regions.
With appropriate transverse scaling and elliptical OHN shape analysis, the cross- sectional area of the RNFL is independent of scan eccentricity, up to 600 microns from the rim margin, indicating that the axonal composition changes little over this range. The results suggest that the use of RNFL cross-section area, rather than RNFL thickness, can provide a more accurate assessment of the axonal losses associated with optic neuropathies such as glaucoma.
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