Anterior Scleral Canal Opening (ASCO) offset relative to Bruch’s Membrane Opening (BMO) influences the Angular Location of Peak Retinal Nerve Fiber Layer Thickness (RNFLT) in Healthy Human Eyes

Abstract

Purpose : To determine the effects of ASCO/BMO offset direction and magnitude on the sectoral distribution of peripapillary RNFLT and peak RNFLT location in 303 healthy mixed ethnicity eyes.

Methods : One eye of each subject underwent RNFL 12° circle scan and ONH imaging (Spectralis) (24 enhanced depth ONH radial B-scans, centered on BMO centroid and aligned to the Fovea-BMO (FoBMO) axis). For each eye, in each ONH B-scan, BMO and ASCO were manually segmented and BMO and ASCO reference planes, centroids, and area were determined. The neural canal axis was defined to be the vector connecting the BMO and ASCO centroids, (Fig 1A). ASCO/BMO offset direction and magnitude were determined by projecting the axis vector onto the BMO plane and deconstructed into multiple vector component combinations. Sectoral (30°) RNFLT and the angular location of superior and inferior peak RNFLT were determined relative to the FoBMO axis. The effect of gender, age, axial length, IOP and race/ethnicity on ASCO/BMO offset was assessed by ANOVA. Correlations between the superior-inferior and nasal-temporal components of ASCO/BMO offset direction, sectoral RNFLT and the angular difference between the superior and inferior peak RNFLT were assessed.

Results : Axial length had positive correlation to ASCO/BMO offset magnitude (P<0.001) and Caucasian and Asian and Native American Ethnicity had greater ASCO/BMO offset than Hispanic and African Ethnicity (P≤0.004). The nasal-temporal component of ASCO/BMO offset demonstrated direction-specific correlations to sectoral RNFLT, (Fig 2), (all sectors significant except inferior-temporal, P≤0.009 by linear regression, R²s ranging from 0.023 to 0.175, being highest nasal-superiorly). The superior-inferior component of ASCO/BMO offset demonstrated significant correlation only to the superior sectoral RNFLT, (P=0.023, Fig 2A). The angular difference between superior and inferior peak RNFLT negatively correlated with the nasal offset of ASCO relative to BMO (P<0.001, R²=0.088).

Conclusions : Nasal offset of the ASCO relative to BMO, is associated with rotation of the superior and inferior RNFLT peaks towards the FoBMO axis. While this rotation would be expected to diminish both the superior-nasal and inferior-nasal RNFLT, the effect is greatest within the nasal-superior sector.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.