Posterior eyeball shape affects offset of openings in optic nerve head canal

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 93 subjects (186 eyes) with a diagnosis of glaucoma or glaucoma suspect. To quantify eyeball asymmetry, an ellipsoid was fitted along the posterior 2/3 contour of every eyeball by customized MATLAB codes. Focal bulging of the eyeball was defined when an adjacent pair of outward protrusion and reciprocal inward depression exists, and the angular deviation of the outermost protruded point (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 16 eyes due to poor MRI image quality, 79 (46%) eyes were classified as to have focal bulging. Eyes with focal bulging had more superior CRVT angle (P<0.001), and larger offset index (P=0.002). In those eyes, the angular deviation of the OPP showed a significant correlation with that of the CRVT/BMO offset (r=-0.724, P<0.001), and protrusion depth with the offset index (r=0.291, P=0.009). In the random-effects logistic regression model, the presence of focal bulging was associated with the superior CRVT/BMO offset (P=0.001) and larger offset index (P=0.008). A generalized estimating equation analysis revealed that the angular deviation of the CRVT/BMO offset was associated with age (P<0.001), axial length (P<0.001) and the angular deviation of the OPP (P<0.001).

Conclusions : In the eyes with focal bulging, the location and extent of bulging were closely associated with the direction and extent of the LC/BMO offset. The optic nerve head canal anatomy is affected by the mode of eyeball expansion.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.

 

Eyeball asymmetry and lamina cribrosa (LC) offset from the Bruch's membrane opening (BMO). (A-B) Optic nerve head morphology. (C) Three-dimensional MRI images of the eyeball. (D) Posterior eyeball topography. The LC/BMO offset direction matches with the direction of focal bulging (from the overgrowth region to the undergrowth region).