Posterior Eyeball Topography based on Three-dimensional Magnetic Resonance Imaging and Optic Nerve Head Morphology

Abstract

Purpose : To investigate the three-dimensional eyeball protrusion from the best-fitted ellipsoid and its association with the offset between the Bruch’s membrane opening (BMO) and lamina cribrosa (LC).

Methods : Three-dimensional magnetic resonance imaging (MRI) scans were taken from 76 subjects (152 eyes) with a diagnosis of glaucoma or glaucoma suspect. To quantify the three-dimensional shape, an ellipsoid was fitted along the posterior 2/3 contour of every eyeball by customized MATLAB codes, and the outermost protruded part (OPP) was determined from the best-fitted ellipsoid. The eyeball was classified into group A (ellipsoid type) when ellipsoid fitting left no significant protrusion, group B (focal bulging type) when a dominant protrusion could be identified: an adjacent pair of outward protrusion and reciprocal inward depression, and group C (complex type) when a singular dominant protrusion cannot be identified. In case with a dominant protrusion (group B), the angular deviation of the OPP was measured from the nasal side of the foveo-BMO axis and the superior location as a positive value. The LC/BMO offset was evaluated by measuring the central retinal vascular trunk (CRVT) location from the BMO center: 1) the angular deviation in the same manner of OPP deviation measurement, and 2) the offset index as the ratio between the CRVT-BMO center distance and the BMO radius on the same direction.

Results : After excluding 15 eyes due to poor MRI image quality, 45 eyes (33%) were classified as group A, 67 eyes (49%) as group B, and 25 eyes (18%) as group C. The average axial length was in the order of group C (26.15±2.04), group B (25.73±1.62), and group A (24.80±1.99). In group B, the angular deviation of the OPP showed a significant correlation with that of the CRVT/BMO offset (r=-0.620, P<0.001). A generalized estimating equation analysis revealed that the angular deviation of the CRVT/BMO offset was associated with age (P=0.045) and the angular deviation of the OPP (P<0.001).

Conclusions : In the eyes having a dominant protrusion, the OPP location was closely associated with the LC/BMO offset direction. This indicates that the focal asymmetry during scleral expansion might be associated with the diverse direction of the LC/BMO offset and accompanied anatomical changes in the optic nerve head canal.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.