Stiff Optic Nerve (ON) Is Associated With Globe Retraction During Adduction Tethering in Primary Open Angle Glaucoma (POAG)

Abstract

Purpose : Abnormal globe retraction due to ON tethering in adduction occurs in most but not all eyes with POAG. We used MRI to determine if retrobulbar ON stiffening causes globe retraction.

Methods : We performed target-controlled, quasi-coronal MRI with 2mm slices in central gaze & 27.7±0.5° adduction in 24 controls (31 eyes) & 33 patients with POAG (52 eyes, mean dev. -7.3±1.0dB, SEM). Patients were treated to achieve target IOPs. Maximum untreated IOP was <21mmHg in 18 patients in the lower IOP group (POAG-LP), but ≧21mmHg in 9 patients in the higher IOP group (POAG-HP). Adduction angle was similar in all subjects. Globe retraction & ON paths were computed from centroids. In the region 2–10mm posterior to the globe, we measured partial volumes of the ON & subarachnoid space (SAS). Computation of ON length assumed volume conservation.

Results : Averaged over all groups, SAS partial volume of 48±4 µL in central gaze declined to 39±3 µL in adduction, reflecting cerebrospinal fluid (CSF) redistribution similarly in all groups. In controls who exhibited insignificant globe retraction in adduction, ON partial volume in central gaze of 65±2µL decreased to 59±2µL in adduction (P<10^-7), implying 0.8±0.1mm ON elongation (P<10^-11). In the subgroup with POAG-LP who exhibited significant globe retraction averaging 0.9±0.1mm (P<10^-6), ON partial volume in central gaze of 60±2µL was unchanged in adduction, implying insignificant ON elongation. In 8 eyes of 7 subjects with POAG-LP without retraction in adduction, mean ON partial volume in central gaze of 58±4µL decreased to 54±4µL in adduction (P<0.01), implying 0.7±0.2mm ON elongation (P<0.001) similar to controls. In POAG-HP, significant globe retraction averaged 0.7±0.1mm, & mean ON partial volume in central gaze of 58±4µL decreased to only 54±4µL in adduction (P=0.05), implying ON elongation of 0.3±0.1mm (P=0.03) similar to POAG-LP. Age did not account for POAG differences.

Conclusions : During adduction tethering, the ON normally elongates to avoid globe retraction. CSF redistributes but the ON does not elongate normally in patients with POAG in whom the globe retracts, regardless of pre-treatment IOP. This would increase peripapillary stress during adduction. Traction in adduction may cause mechanical overloading of the ON head & peripapillary sclera, & contribute by repetitive strain to glaucomatous ON damage at all levels of IOP.

This is a 2020 ARVO Annual Meeting abstract.