Prediction of Retinal Ganglion Cell (RGC) Density from SD-OCT Thickness Measures and Correlation of RGC with Inner Plexiform Layer Thickness in Glaucoma

Abstract

Purpose : To test the hypotheses that, 1) the macular RGC density can be predicted from the RGC layer (GCL) thickness measurements with SD-OCT; and that 2) the inner plexiform layer thickness preferentially decreases in early glaucoma compared to the GCL.

Methods : 83 normal eyes (83 subjects) and 115 eyes with glaucoma (102 patients of which 10 had normal achromatic visual fields) underwent macular imaging with Spectralis SD-OCT. Raw OCT data (768x61 A-scans) and 8x8 arrays of 3-degree superpixel thickness measurements were exported after segmentation; 768x768 matrices of SD-OCT thickness measurements were created after interpolation with MATLAB. The GCL thickness vs. eccentricity was plotted along the 90- and 180-degree meridians, the RGC density data (based on Curcio and Allen) were overlaid, and the correlation of the two measures was explored with bivariate plots and regression analyses. The relationship of GCL and IPL thickness at the superpixel level was also explored in normal and glaucoma subjects as a function of glaucoma severity and eccentricity.

Results : Median visual field mean deviation in the glaucoma group was −5.4 dB (IQR −8.8 to −1.8 dB). The correlation of GCL thickness with RGC density was higher in the horizontal meridian compared to the vertical meridian (r=0.95 vs. 0.85; p=0.02). GCL thickness overestimated RGC density beyond the peak of RGC density, where the ratio of RGC density to GCL thickness linearly decreased (r²=0.83, p<0.001; Figure 1). The IPL/GCL relationship was mostly linear in normal and glaucoma subjects but varied as a function of eccentricity with the IPL to GCL ratio and increased with eccentricity. There was no evidence of preferential IPL thinning in eyes or superpixels with less damage (p>0.688 for comparison of IPL to GCL ratio, Figure 2).

Conclusions : No evidence of preferential IPL thinning was observed in early glaucoma. Our findings suggest that the relationship of GCL thickness and RGC density varies as a function of eccentricity and should be taken into account when RGC density or counts are estimated based on GCL thickness measurements.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.