Magnetic Resonance Imaging (MRI) Demonstrates Optic Nerve (ON) Traction During Adduction in Primary Open Angle Glaucoma (POAG) With Normal Intraocular Pressure (IOP)

Abstract

Purpose : ON head deformation in adduction (Chang et al. AJO 2016) due to ON traction has been proposed as an IOP-independent cause of optic neuropathy in POAG (Demer, IOVS, 57:1826-38, 2016). We used MRI to determine if ON sheath traction occurs in patients having POAG but without elevated IOP.

Methods : 15 patients with POAG (30 eyes, average mean deviation -9.1±1.3 dB, max. IOP 20 mmHg) & 30 normals underwent, surface coil, axial & quasi-coronal orbital MRI in target-controlled central gaze, & large (mean 30°), or both moderate (mean 20°) & large ab- & adduction. ON & sheath area centroids in 2 mm thick planes permitted computation of mid-orbital lengths vs. minimum paths as a straightness index, with total lengths measured in axial MRI. Strain was determined from ON elongation in moderate vs. large adduction when ON path remained straight. Globe dimensions were determined from cross sections, and translation by centroid displacement.

Results : For both subject groups, the ON was significantly straighter in adduction than central gaze & abduction, not significantly different between groups (P<0.15). Maximal adduction straightening occurred in the temporal ON sheath although ON & sheath behaviors corresponded. For 28° adduction, the ON had 102.0%±0.2% (SEM) of minimum path length versus 104.1%±0.5% in central gaze for POAG (P<10^-5), compared with 101.6%±0.1% versus 102.7%±0.3% for controls (P<10^-4). ON path length was 2.13±0.39 mm greater in adduction than central gaze in POAG, but only 1.14±0.29 mm greater in controls (P=0.06, GEE). During large adduction, globes retracted 0.73±0.10 mm in POAG, but only 0.06±0.08 mm in controls (P<10^-5), and globes elongated 0.52±0.14 mm vertically only in POAG (P<10^-4). In POAG, 9±3° incremental adduction in 6 eyes elongated the straight ON 0.9±0.2 mm, corresponding to 3.2±0.9% strain, while in 14 eyes of 7 normal subjects 8±1° incremental adduction elongated the ON 1.4±0.2 mm for 5.0±0.8% strain.

Conclusions : Although globe tethering & elongation of ON & sheath are normal in adduction, adduction is associated with significantly greater globe retraction & deformation in a subgroup of POAG patients who lack elevated IOP. Traction in adduction, particularly in context of age-related ON sheath stiffening, may cause IOP-independent, neuropathic mechanical overloading of the ON head & peripapillary sclera.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.