Depth-resolved variations in visibility of retinal nerve fibre bundles across the retina in enface OCT images of healthy eyes.

Abstract

Recent developments in optical coherence tomography (OCT) technology enable direct enface visualisation of retinal nerve fibre bundle (RNFB) loss in glaucoma. However, the optimum depth at which to visualise RNFBs across the retina is unknown. We aimed to evaluate the range of depths and optimum depth at which RNFBs can be visualised across the retina in healthy eyes. The central±25° retina of 10 healthy eyes from 10 people aged 57-75years (median 68.5years) were imaged with spectral domain OCT. Slab images of maximum axial resolution (4μm) containing depth-resolved attenuation coefficients were extracted from 0 to 193.5μm below the inner limiting membrane (ILM). Bundle visibility within 10 regions of a superimposed grid was assessed subjectively by trained optometrists (n=8), according to written instructions. Anterior and posterior limits of RNFB visibility and depth of best visibility were identified for each grid sector. Effects of retinal location and individual eye on RNFB visibility were explored using linear mixed modelling with likelihood ratio tests. Intraclass correlation coefficient (ICC) was used to measure overall agreement and repeatability of grading. Spearman's correlation was used to measure correlation between depth range of visible RNFBs and retinal nerve fibre layer thickness (RNFLT). Retinal location and individual eye affected anterior limit of visibility (χ2(9) =58.6 and 60.5, both p<0.0001), but none of the differences exceeded instrument resolution, making anterior limit consistent across the retina and different eyes. Greater differences were observed in the posterior limit of visibility across retinal areas (χ2(9) =1671.1, p<0.0001) and different eyes (χ2(9) =88.7, p<0.0001). Optimal depth for visualisation of RNFBs was around 20µm below the ILM in most regions. It varied slightly with retinal location (χ2(9) =58.8, p<0.0001), but it was not affected by individual eye (χ2(9) =10.7, p=0.29). RNFB visibility showed good agreement between graders (ICC 0.89, 95%CI 0.87-0.91), and excellent repeatability (ICC 0.96-0.99). Depth range of visible RNFBs was highly correlated with RNFLT (ρ=0.9, 95%CI: 0.86-0.95). The range of depths with visible RNFBs varies markedly across the healthy retina, consistently with RNFLT. To extract all RNFB information consistently across the retina, slab properties should account for differences across retinal locations and between individual eyes.