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Jean-Claude Mwanza, Aaron D Webel, Joshua L Warren, Donald L Budenz; Retinal Nerve Fiber Layer Thickness Floor and Corresponding Functional Loss in Glaucoma. Invest. Ophthalmol. Vis. Sci. 2014;55(13):976.
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© ARVO (1962-2015); The Authors (2016-present)
To estimate the floor of global and quadrant retinal nerve fiber layer (RNFL) thicknesses and the corresponding visual field loss in glaucoma.
Spectral domain OCT RNFL and visual fields (VF) were obtained in 83 glaucomatous (41 moderate, 42 severe) and 37 normal eyes. The RNFL thickness floor or residual layer thickness was estimated with simple linear model (SLM) and direct OCT measurement in subjects with VF loss greater than the VF loss at which RNFL thinning reaches asymptotic and change points. The asymptotic point was obtained by comparing pairs of regression lines. The pair with the steepest slope in the first part and the least slope in the second part of the RNFL-VF relationship was chosen as best fitting the data. The intersection between the steep and plateau regression lines was identified as the asymptotic point and its coordinate on the VF loss axis was determined. The change point (VF loss at which RNFL stops thinning) was estimated with a Bayesian change point statistical model.
The SLM-predicted residual thickness was 49.9 µm for global, 57.1 µm for superior, 55.2 µm for inferior, 43.1 µm for nasal, and 40.1 µm for temporal quadrant. RNFL thicknesses decreased exponentially and reached asymptotic points at VF loss of ranging between -14.6 dB and -15.7 dB. The change point model estimated change points between -21.7 dB and -25.1 dB. The residual thickness from OCT measurement was 44.7 µm globally, 51.9 µm superiorly, 50.5 µm inferiorly, 38.6 µm nasally, and 37.0 µm temporally, with VF losses ranging between -26.4 dB and 28.6 dB in subjects with VF loss greater than the change points, and 47.6 µm globally, 52.8 µm superiorly, 52.3 µm inferiorly, 38.1 µm temporally, and 39.6 µm nasally with VF losses between -20.9 dB and -24.4 dB. There were no significant differences between residual thicknesses from the two approaches despite significant differences in VF losses. In each scenario, residual layers in the temporal and nasal quadrants were thinner than residuals in the superior and inferior quadrants.
RNFL thinning reaches the asymptotic point at VF losses of about -15 dB globally and in quadrants and remains stable beyond that. The residual layers in the temporal and quadrants are significantly thinner than global, superior and inferior quadrant residuals. Once VF loss reaches -15 dB, VF but not RNFL thickness should be used to monitor glaucoma progression.
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