Assessment of linear-scale indices for perimetry in terms of progression in early glaucoma

Abstract

Currently, global indices that summarize the visual field combine sensitivities on a logarithmic (decibel) scale. Recent structure-function models for glaucoma suggest that contrast sensitivity should be converted to a linear scale before averaging across visual field locations, to better relate sensitivity with the number of surviving retinal ganglion cells (RGCs). New indices designed to represent the number of RGCs already lost are described. At least one was found to be a significantly better predictor of subsequent rate of change than traditional Mean Deviation (p=0.014) in participants with glaucomatous optic neuropathy. Issues concerning the creation of optimal global indices are discussed.

Figure 1

The subsequent rate of change over years 2–7 of the sequence, plotted against baseline value in Year 1, for three of the global indices considered in this study; Mean Deviation (MD), Capped Linear Mean Sensitivity (LMS(cap)), and Capped RGC Loss according to the model by Hood et al (Loss_Ho(cap)). Note that while the units for all indices are decibels, the scales on the axes differ, since the indices took values over different ranges. Shading of the data points indicates whether the participant exhibited glaucomatous optic neuropathy at their initial visit.

Figure 2

Comparison of the rates of change in Mean Deviation (MD) and in the Capped RGC Loss according to the model by Hood et al (Loss_Ho(cap)). The left plot compares the actual rates of change (slope of linear regression over time); the right plot compares the ranks of the rates.