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D. C. Hood, S. C. Anderson, M. Wall, R. H. Kardon; Structure versus Function in Glaucoma: A Test of a Linear Model. Invest. Ophthalmol. Vis. Sci. 2007;48(13):490.
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To test a linear model  relating the glaucomatous loss in retinal nerve fiber (RNFL) thickness measured with optical coherence tomography (OCT) to the loss in sensitivity measured with standard automated perimetry (SAP).
15 patients with asymmetrical glaucoma were tested (OAG, n=5; NTG, n=7; PG, n=2; PXG, n=1). The least affected eye was normal on SAP or had mild damage (better than a mean deviation of -3dB). 24-2 SITA standard (Zeiss Meditech) and OCT RNFL thickness (fast circular scan, OCT3, Zeiss Meditech) measures were made on 3 to 5 different occasions and the mean values obtained. For each eye, the mean SAP loss was calculated for an upper and lower arcuate field region  by averaging the loss in relative sensitivity on a linear scale. The average RNFL thickness for corresponding arcuate sectors of the lower and upper optic disc  was obtained for each eye. A linear model was fitted to the plots of RNFL thickness vs. SAP loss. According to the linear model, the RNFL thickness R = sT + b, where T is the SAP 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 all sensitivity and axons are lost. The model was fitted separately to the data for the upper and lower arcuates.
The model provided a reasonable fit to the data, although there is scatter in the data. The scatter, to some extent, is predicted. In particular, the model predicts a family of theoretical curves, which depend upon the range of values of (s + b) when the retina is normal (T=1.0). While an analysis of the results, on average, support the Garway-Heath et al  map of field regions to optic disc sections, an analysis of the outliers suggest that this map may vary among individuals and also contribute to the scatter.
A linear model that relates RNFL thickness to losses in SAP sensitivity describes the results for arcuate regions of glaucomatous damage. The RNFL thickness data from studies in the literature appear to be in agreement with this model . This model does not require that structural damage precede functional loss. Rather, it provides a framework for assessing the relative efficacy of structural and functional tests throughout the course of the disease. 1. Hood DC. J. Opt. Soc. Amer (in press); 2. Garway-Heath et al, Ophthal (2000).
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