Danit Saks1, Angela Schulz1, Jamie Craig2, Stuart Graham1, the PROGRESSA Study Group1,2

Purpose: To compare two spectral‐domain optical coherence tomography (OCT) systems for measurement of (i) combined ganglion cell and inner plexiform layers (GCLIPL), and (ii) retinal nerve fibre layer (RNFL) thicknesses and progression rates in glaucoma patients.

Methods: 100 eyes from 52 glaucoma suspects/patients underwent OCT imaging of the macula and optic nerve. Images were acquired using Cirrus (Carl Zeiss) and Spectralis (Heidelberg Engineering) systems at the same visit. A subset of 34 PROGRESSA study participants had 6 reviews over 3 years. Average GCLIPL and RNFL thicknesses were manually calculated based on the automated segmentation of each device for matched superior, inferior and global regions, then used to calculate rate of change.

Results: At baseline, RNFL thicknesses did not differ significantly between systems (P = 0.683), however, Cirrus GCLIPL was significantly thicker than Spectralis by ~12 μm (P < 0.001). Progression rate (μm/yr) analysis determined no significant differences between OCT systems for GCLIPL (P > 0.05), yet Spectralis percentage change from baseline was significantly faster (P = 0.006). For RNFL, Cirrus progression rates were faster than Spectralis within the inferior quadrant (μm/yr: P = 0.01, % change: P = 0.02). For both OCT systems, RNFL loss (μm/yr and % change) progressed faster than GCLIPL, particularly inferiorly.

Conclusion: Despite baseline differences in GCLIPL thicknesses, Cirrus and Spectralis provided similar progression rates for GCLIPL and RNFL in glaucoma and can be considered comparable, though not interchangeable, for progression studies. Further analysis is needed to determine if RNFL progresses faster than GCLIPL in glaucoma, particularly within the inferior region, and whether one precedes the other.