Separation and thickness measurements of superficial and deep slabs of the retinal nerve fiber layer in healthy and glaucomatous eyes

Purpose: The authors describe a new method to analyze retinal nerve fiber layer (RNFL) thickness maps. Design: Cross-sectional study. Participants: Retinal nerve fiber layer thickness maps of healthy and glaucomatous eyes. Methods: OCT RNFL raster scans from 98 healthy and 50 glaucomatous eyes were analyzed. The RNFL thickness maps were separated into a superficial slab of the RNFL (SNFL) and deep slab of the RNFL (DNFL) through a partial-thickness plane set at the modal thickness (mode). Associations between mode and OCT signal strength (SS), age, axial length, and visual field mean deviation (VFMD) were tested (Pearson coefficient, r). Thicknesses of inferior SNFL (i-SNFL) and superior SNFL (s-SNFL) regions and inferior DNFL (i-DNFL), superior DNFL (s-DNFL), nasal DNFL (n-DNFL), and temporal DNFL (t-DNFL) regions were calculated. The region thicknesses were compared between healthy and glaucomatous eyes (t test) and between glaucomatous eyes with early, moderate, and severe disease (analysis of variance and linear regressions of thickness on VFMD). Diagnostic accuracy and correlation with VFMD of RNFL regions thicknesses were calculated as the area under the receiver operating characteristic curve (AUC) and Pearson r, respectively. P < 0.05 was considered significant. Main Outcome Measures: Thickness of regions in SNFL and DNFL slabs. Results: The mode was not associated with SS, age, axial length, or VFMD. It circumscribed the thicker RNFL around the optic disc of healthy and glaucomatous eyes and was used to separate the SNFL and DNFL slabs of RNFL thickness maps. The thickness of the SNFL slab was less in glaucomatous eyes than in healthy eyes (P < 0.001). The s-SNFL and i-SNFL thicknesses were 86.0±8.2 μm and 87.3±9.6 μm in healthy eyes versus 66.1±9.1 μm and 63.4±8.2 μm in glaucomatous eyes, respectively (P < 0.001 for both). The thickness of the DNFL slab was similar between groups (P = 0.19). The t-DNFL thickness was 37.0±5.3 μm in healthy eyes versus 33.9±5.0 μm in glaucomatous eyes (P < 0.001). The thickness of all other DNFL regions was similar. The SNFL regions only thinned with progressively worse glaucoma severity, had excellent AUCs (AUC > 0.95, P < 0.001), and correlated strongly with VFMD (r > 0.60, P < 0.001). Conclusions: Glaucomatous RNFL thinning is predominantly detected within a slab with thickness greater than the mode. Superficial slab of the RNFL thickness has great AUC and correlation with VFMD in glaucomatous eyes. The usefulness for diagnosis and monitoring of glaucoma needs further study.