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J. A. Salant, X. Zhang, S. N. Arthur, D. Xin, A. S. Raza, J. M. Liebmann, R. Ritch, D. C. Hood; A Test of a Linear Model Relating Visual Field Loss to Retinal Nerve Fiber Thickness Measured With Scanning Laser Polarimetry (SLP). Invest. Ophthalmol. Vis. Sci. 2008;49(13):728.
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© ARVO (1962-2015); The Authors (2016-present)
To test a linear model, which relates structural loss in retinal nerve fiber (RNFL) thickness to functional loss in visual fields, RNFL thickness was measured with scanning laser polarimetry (SLP). The model was previously shown to fit RNFL thickness measured with optical coherence tomography (OCT),[1,2] where blood vessels contribute to the measurements.
One eye from 50 controls and 45 glaucoma patients was tested with OCT (fast circular RNFL scan, OCT3, Zeiss), scanning laser polarimetry (SLP) (GDx-VCC, Zeiss) and 24-2 Humphrey visual fields. Glaucoma patients had abnormal visual fields (GHT or PSD) and optic discs. For each eye, the SLP RNFL thickness was exported and the RNFL profile determined for a circle of 3.4 mm dia; this circle is comparable to the OCT circular scan. The mean visual field loss was calculated for an upper and lower arcuate field region  by averaging the loss in relative sensitivity on a linear scale. The average OCT and SLP RNFL thicknesses for sectors of the lower and upper optic disc corresponding to the arcuate regions  were obtained. A linear model was fitted to the plots of RNFL thickness vs. field loss. According to the model, the RNFL thickness R = sT + b, where T is the field sensitivity loss relative to age-matched normal eyes (linear scale; e.g. for -3dB, T=0.5), (s + b) is the RNFL thickness in the healthy/normal state (T=1), and b is the residual RNFL measured when sensitivity is zero and all axons are lost. The model was fitted separately to the data for the upper and lower arcuates.
The linear model fitted both OCT and SLP data, although the values of the parameters differed. The best fitting [(s+b); b] values were (OCT: 142.7um; 47.6um) and (SLP: 61.6 um; 23.1 um) for the upper arcuate field region and (OCT: 129.5 um; 43.1 um) and (SLP: 64.9 um; 22.7 um) for the lower field.
A linear model describes structural glaucomatous loss as a function of field loss, when structural loss is measured as SLP or OCT RNFL thickness. The smaller values of the SLP parameters are due to a number of factors, including a smaller contribution of blood vessels to the SLP measurements.1. Hood & Kardon, PRER (2007); 2. Hood et al, IOVS (2007); 3. Hood et al, J. Glau (in press); 4. Garway-Heath et al, Ophthal (2000).
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