The structural parameters, such as GCC and NFL thickness, are in units of microns on a linear scale, whereas the VF results are traditionally given in units of decibels on a logarithmic scale. Thus directly correlating structural (linear) and functional (dB) parameters results in a nonlinear relationship that can lead to the appearance of more structural loss early in the disease and more functional loss in late disease.
6,8,32,34 By converting both structural and functional measurements to units of percent loss, we found that they have a more linear relationship, although there is still relatively more severe VF damage in late disease (
Fig. 5). Another approach to linearize VF data is to use the 1/Luminance (in lamberts) unit, which has the advantage of having a simple physical meaning.
6,11,32 However, we prefer to use the percent loss unit because this allows us to look at the relationship between the structure and function at various disease stages. In particular, this allowed us to determine the residual GCC and NFL thickness for end-stage glaucoma (at or near 100% VF loss), representing glial tissue and blood vessels. Using the linear regression slope extrapolated to 100% VF loss, our estimate of end-stage residual thicknesses are 26% to 50% for GCC regions, and 6% to 32% for NFL regions. Using the maximal regional loss in our dataset (
Table 2), our estimate of end-stage residual thicknesses are 50% to 58% for GCC regions, and 16% to 34% for NFL regions. In comparison, Aggarwal and colleagues
35 found average inferior and superior hemifield GCC residual thickness of 50.6 μm (53%) and 61.7 μm (64%), respectively, in subjects with nonarteritic ischemic optic neuropathy. Sihota et al.
36 reported an average residual NFL thickness of 44.9 μm (44%) in blind glaucoma eyes. Hood and colleagues
37 reported a residual NFL value of 45.5 μm (33%) in patients with ischemic optic neuropathy. Overall, our results gave estimates of residual GCC and NFL thicknesses that were slightly lower than previously published studies. Some of these differences may be due to the different OCT systems and image-processing software used. Notably, we also found that there may be superior-inferior asymmetries and arcuate-perifoveal differences in the end-stage residual thickness. These regional variations may have anatomic explanations that deserve further study.