Optic nerve displacements during horizontal eye movements in healthy and high myopic subjects

Abstract

Purpose : Optic nerve (ON) traction in eye movements can deform the posterior globe, which may partly contribute to axial elongation in myopia. Displacements and tortuosities of ONs may reflect the effects of ON traction. The aims of this study were to: (1) assess ON displacements in adduction and abduction using orbital MRI; (2) assess the difference in ON tortuosity and displacement during eye movements between healthy and high myopia (HM) subjects.

Methods : MRI volumes of both eyes from 18 healthy controls and 20 HM subjects in central gaze, abduction and adduction of 15° were analyzed. MRI volumes were reoriented to align with their corresponding central gaze volume through rigid translation/rotation. Images were manually-segmented in both axial and coronal views to isolate all ONs (Figure A, B). Segmented ON voxels were fitted to a 3D curve to assess ON tortuosity (Figure C), defined as the length of the ON central line segment (15 mm long from the globe-ON junction) divided by the distance between two end-points. ON displacements were evaluated in 4 quasicoronal planes which were perpendicular to the ON central line in central gaze and 3 mm apart (Figure D).

Results : Axial length measured with MRI was higher in the HM group (28.62±2.60 vs 22.84±0.89 mm; p<0.0001). ON tortuosities were larger for HM subjects in central gaze (1.043±0.041 in HM eyes vs 1.015±0.018 in controls; p=0.0002), adduction (1.039±0.039 vs 1.013±0.018; p=0.0006) and abduction (1.049±0.046 vs 1.018±0.021; p=0.0004). In all eyes, ON displacements in adduction were significantly different from those in abduction in the naso-temporal direction (p<0.0001 in all 4 planes) but not in the supero-inferior direction (p>0.27). ON moving distances in the axial plane were similar in adduction and abduction (p>0.28 in all 4 planes). ON displacements in plane 3 and 4 were smaller in the HM group in both gaze directions and were larger in plane 1 in adduction only (Figure E).

Conclusions : Horizontal eye movements induce ON displacements only in the axial plane. Therefore, 2D axial MRI scans are able to capture ON morphological changes during horizontal eye movements. In this sample, eye elongation in HM subjects resulted in a higher ON tortuosity, which may protect the posterior globe against gaze induced mechanical strains. The more ‘slack’ ONs in HM subjects allowed the ONs to move less in comparison with controls.

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