Transforming the Visual Field Scale From Decibel to 1/Lambert: Does it Alter the Structure-Function Relationship?

Abstract

Abstract: : Purpose: Although damage to the retinal ganglion cells (RGCs) is responsible for glaucomatous visual field loss, the measurable structure-function relationship is weak. Recent evidence suggests that this relationship may be dependent on the scale used to express visual field (VF) data. Garway-Heath et al reported that the number of RGCs was logarithmically related to decibels (dB), but linearly related to 1/Lambert (1/L). They also showed a curvilinear relationship between dB VF and neuroretinal rim area, but a linear relationship after transforming to 1/L. We sought to replicate these findings and assess the dB and 1/L scales with other structural measures. Methods: 203 subjects were tested: 149 normals with normal VF (mean age: 54±15, mean deviation (MD): -0.12±1.5), and 54 glaucoma patients with repeatable abnormal VF (mean age: 69±11, MD: –5.09±5.4). Subjects had reliable 24-2 full-threshold standard automated perimetry and HRT 10 degree imaging completed within six months. The mean threshold of the central 16 VF locations was compared to the temporal neuroretinal rim area, rim-to-disc ratio and retinal nerve fiber layer (RNFL) thickness. The curvilinear relationship was the best fit (* in Table) when the p-value of the b2 coefficient was significant. Results: Unlike Garway-Heath et al, we found a linear relationship (p<.0000) between neuroretinal rim area and central dB VF. This relationship became curvilinear (p<.0000) once the dB was transformed into 1/L. These results accounted for a relatively small proportion of the variance (dB: R2=0.16; 1/L: R2=0.13). Similar relationships were observed for rim-to-disc ratio and RNFL, were confirmed when more eccentric VF locations were added, and when glaucoma patients with unconfirmed abnormal and normal VF were included (N=359). Conclusions: In our population, transforming dB VF into 1/L did not improve the measurable structure-function relationship. Further studies are needed to clarify this relationship and to determine the reason for the difference between our results of those of Garway-Heath. Structure-function relationships for the dB and 1/L VF scales  

Keywords: visual fields • optic disc