Estimating Optical Coherence Tomography Structural Measurement Floors to Improve Detection of Progression in AdvancedGlaucoma

Purpose"Floor effects" in retinal imaging are defined as the points at which no further structural loss can be detected. We estimated the measurement floors for spectral-domain optical coherence tomography (SDOCT) measurements and compared global change over time in advanced glaucoma eyes.DesignValidity study to investigate measurement floors.MethodsA longitudinal "Variability group" of 41 eyes with moderate to advanced glaucoma (standard automated perimetry mean deviation ≤-8 dB) was used to estimate measurement floors. Minimum rim width (MRW), ganglion cell-inner plexiform layer thickness (GC-IPLT), and circumpapillary retinal nerve fiber layer thickness (cpRNFLT) were determined. Floors were defined as the average image area with a loss less than first-percentile confidence interval of the variability in this group. Global rate of change and percentage of the region of interest that did not reach the measurement floor at baseline were calculated in 87 eyes with advanced glaucoma (SAP MD ≤-12 dB).ResultsGlobal change over time in longitudinal eyes was-1.51μm/year for MRW,-0.21μm/year for GC-IPL, and-0.36μm/year cpRNFL (all P ≤ .03). The percentage of region of interest that did not reach the floor at baseline was 19% for MRW, 36% for GC-IPLT, and 14% for cpRNFLT. Average (± standard deviation) floors were 105μm (± 15.9μm) for MRW, 38μm (± 3.4μm) for GC-IPLT, and 38μm (±4.2μm) for cpRNFLT.ConclusionsIn advanced glaucoma, more GC-IPL tissue remains above the measurement floor compared with other measurements, suggesting GC-IPL thickness is the better candidate for detecting progression. Progression in SDOCT measurements is observable in advanced disease.