Correlation of structural retinal nerve fibre layer parameters and functional measures using Heidelberg Retinal Tomography and Spectralis spectral domain optical coherence tomography at different levels of glaucoma severity

Background: To compare the structure/function relationship in glaucoma cases at different levels of severity, and with different disc sizes, between the Heidelberg Retinal Tomography and Spectralis spectral domain optical coherence tomography. Design: Retrospective study of glaucoma patients attending a Sydney-based private practice. Participants: 169 eyes of 169 patients with a clinical diagnosis of glaucoma. Methods: Patients were divided on visual field criteria into early (mean deviation>-4dB), moderate (-4dB<mean deviation<-10dB) and severe (mean deviation<-10dB) disease. Bivariate correlation (Spearman's rho) between mean threshold scores for each area and the corresponding mean retinal nerve fibre layer thickness sectoral measurement were calculated. Main Outcome Measures: Correlation, as measured by Spearman's rho, between retinal nerve fibre layer measurements and mean threshold scores. Comparison of correlation strengths between the two scanning modalities with analysis of the effect of disease severity and disc size. Results: Both imaging techniques showed only moderate correlations at best. Spectral domain optical coherence tomography (global retinal nerve fibre layer Spearman's rho=0.670, P<0.01) had higher correlation coefficients compared with Heidelberg Retinal Tomography rim area (Spearman's rho=0.449, P<0.01) and retinal nerve fibre layer (Spearman's rho=0.421, P<0.01). Disc size did not have a significant influence on the structure/function relationship. Conclusions: Spectral domain optical coherence tomography retinal nerve fibre layer measurements demonstrated closer correlations to visual field threshold reductions using a structure/function model in varying stages of glaucoma.