Optical Coherence Tomography (OCT) Neural Canal Direction, Obliqueness and Minimum Cross-sectional Area in a Normal Population

Abstract

Purpose : To quantify the OCT-detected, optic nerve head (ONH) anatomic components of clinical “optic disc size”, we assessed the size and relative position of the anterior scleral canal opening (ASCO) relative to Bruch’s membrane opening (BMO) so as to determine the direction, obliqueness and minimum cross-sectional area of the neural canal (i.e. the neural canal minimum (NCM)) (Fig.1). We then assessed their correlation to retinal nerve fiber layer thickness (RNFLT), in 362 normal subjects.

Methods : One eye of each subject underwent RNFL circle scan and ONH OCT imaging (Spectralis) (24 enhanced depth ONH radial B-scans, aligned to the Fovea-BMO axis and centered on the BMO centroid). In each ONH B-scan, BMO, ASCO, and the Border Tissue of Elshnig were manually segmented. 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 1B). Neural canal direction was defined by projecting the axis vector onto the BMO plane. Neural canal obliqueness was defined by the angle between the axis and a vector through BMO centroid, perpendicular to BMO plane. By convention, the angle was (+) if the axis pointed towards the chiasm, (nasally), and (-) if it pointed away from the chiasm, (temporally), (Fig.1B). To determine the NCM, BMO and ASCO points were projected onto a plane perpendicular to the axis vector, and the overlap area was identified (Fig. 1C). RNFLT was quantified within the circle scans. Correlations between BMO, ASCO, and NCM areas and RNFLT were assessed.

Results : ASCO area (mean ± standard deviation, (SD)) (2.23 ± 0.43 mm²) was greater than BMO area (1.83 ± 0.38 mm²), and both were greater than the NCM (1.34 ± 0.42 mm², P<0.001, Fig. 2A). Neural canal direction was most commonly inferior nasal (55%, Fig. 2B). Mean neural canal obliqueness was +15.2 ± 20.3° (range -47.5° to +62.4°, Fig. 2C). The correlation between RNFLT and NCM (r=0.374, P<0.001) was stronger than the correlation with BMO area (r=0.266, comparison P=0.035), but not significantly different from ASCO area (r=0.338, P=0.490).

Conclusions : Our study establishes anatomic relationships between BMO, ASCO, neural canal direction, obliqueness, and the NCM. It additionally suggests that in normal eyes, NCM and ASCO area more strongly correlate to RNFLT than BMO area.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.