# Association between eyeball asymmetry and offset of openings in optic nerve head canal assessed by posterior polar eyeball topography

## TL;DR

This study found a link between eyeball asymmetry and the offset of openings in the optic nerve head canal, suggesting focal bulging may influence directional shifts in these structures.

## Contribution

The study introduces a novel method to assess eyeball asymmetry and its association with optic nerve head canal offset using 3D-MRI scans.

## Key findings

- Eyeball asymmetry was found in 42% of eyes and correlated with superior LC/BMO offset and larger offset index.
- Focal bulging in the eyeball was closely associated with the direction and extent of LC/BMO offset.
- Older age and shorter axial length were linked to superior LC/BMO offset.

## Abstract

We investigated three-dimensional (3D) eyeball protrusion and its association with the offset between the lamina cribrosa (LC) and Bruch’s membrane opening (BMO). 3D-MRI scans were taken from 93 subjects (186 eyes). An ellipsoid was fitted along the posterior 2/3 contour of each eyeball. Eyeball asymmetry with focal bulging was determined by the existence of an adjacent outward protrusion/reciprocal inward depression pair, and the angular deviation of the outermost protruded point (OPP) was measured from the nasal side of the fovea-BMO axis. The LC/BMO offset was evaluated by measuring the central retinal vascular trunk (CRVT) location from the BMO center: (1) the angular deviation and (2) the offset index as the ratio between the CRVT-BMO center distance and the BMO radius in the same direction. Seventy-nine eyes (42%) were classified as having eyeball asymmetry, which had a more superior LC/BMO offset (P < 0.001) and a larger offset index (P = 0.002). In those eyes, the angular deviation of the OPP showed a significant correlation with that of the LC/BMO offset (r = -0.724, P < 0.001), as did protrusion depth with the offset index (r = 0.291, P = 0.009). The presence of eyeball asymmetry was associated with superior LC/BMO offset (P = 0.004) and larger offset index (P = 0.009). Superior LC/BMO offset was associated with older age (P < 0.001), shorter axial length (P < 0.001) and inferior location of OPP (P < 0.001). The location and extent of focal bulging were closely associated with those of LC/BMO offset. This indicates that focal bulging during expansion might be associated with diverse directionality of LC/BMO offset.