Human optic nerve sinuosity is not caused by orbital blood vessel proximity

Abstract

Purpose : Gaze-direction related changes in optic nerve (ON) path have been proposed as an intraocular pressure-independent cause of glaucomatous optic neuropathy. The orbit contains many blood vessels (BV) that may also influence ON path. This project employed magnetic resonance imaging (MRI) and histopathological orbital samples to determine if BV near the ON deflect its path in both healthy subjects and patients with primary open angle glaucoma (POAG) without elevated intraocular pressure.

Methods : 20 patients (ages 47–78 yrs) with POAG but normal intraocular pressures, and 21 normal control subjects (ages 44-71) underwent high resolution MRI in 2-mm thick quasi-coronal planes during target-controlled central gaze, ad- and abduction. ON centroids were tracked from orbital apex to globe to define ON path. Measured ON path was compared with shortest distance path. The 3 planes exhibiting greatest deviation from straight path were analyzed for nearby BV. For histology, 5 whole cadaver orbits ages (17 mos - 93 ys) were formalin fixed, paraffin embedded, stained with Masson’s trichrome, sectioned at 10-μm thickness, and analyzed for anatomical structures near the ON.

Results : On MRI, the ON exhibited a characteristic path deviation, temporally beginning ~10mm posterior to the globe-ON junction and inferotemporally more anteriorly. In both POAG and normal subjects, the ON sagged vertically in the region 4-16mm posterior to the ON-globe junction in all gaze positions. Deviations from shortest ON path did not correlate with presence of BV in any gaze position (P>0.05) for POAG or normal subjects. Histological examination showed no BV, ligament, or neural structures adjacent to the ON, but there were compartmentalized fat lobules throughout the posterior orbit.

Conclusions : As the ON courses from orbital apex to globe, its path deviates first temporally and then inferiorly. These path deviations are not caused by proximity to surrounding BV, ligaments, or nerves, but might be related to fat compartmentalization, gravity, or inherent structural characteristics of the ON.

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