Structure versus function in glaucoma: An application of a linear model

PURPOSE. To evaluate a linear model that relates 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). METHODS. Fifteen patients with asymmetrical glaucoma, whose better eye was normal or near normal (mean deviations better than -3 dB) on SAP, were tested. SITA 24-2 standard and OCT RNFL thickness measures were made on three to five different occasions and the mean values were 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 versus SAP loss. According to the linear model, the RNFL thickness R = s(o)T + b, where T is the SAP sensitivity loss relative to age-matched normal eyes (linear scale), (s(o) + b) is the RNFL thickness in the healthy/normal state ( T = 1), and b is the residual RNFL thickness measured when all sensitivity and all axons are lost. RESULTS. The model provided a reasonable fit to the data with best fitting values of (s(o); b) of (upper field: 80.6 mu m; 50.5 mu m) and (lower field: 67.4 mu m; 50.5 mu m) and (upper field: 78.8 mu m; 54.9 mu m; r = 0.82) and (lower field: 59.2 mu m; 61.5 mu m; r = 0.70) for two different methods of best fit. CONCLUSIONS. A linear model that relates RNFL thickness to losses in SAP sensitivity describes the results for arcuate regions of glaucomatous visual fields. The linear model provides a framework for assessing the relative efficacy of structural and functional tests throughout the course of the disease.