Optic Nerve Traction During Adduction in Open Angle Glaucoma with Normal versus Elevated Intraocular Pressure

Abstract

Purpose/Aim: We used magnetic resonance imaging (MRI) to investigate effects of intraocular pressure (IOP), race, and other factors on optic nerve (ON) traction in adduction, a phenomenon proposed as neuropathic in open angle glaucoma (OAG). Materials and Methods:Thirty-five patients with OAG (26 with maximal untreated IOP ≤21mmHg, 9 with IOP >21mmHg) and 48 controls underwent axial and quasi-coronal MRI in central gaze and large (27-33°) abduction and adduction. Some underwent MRI at smaller ductions (21-28°). Effects of presence vs. absence of OAG; within OAG whether maximum IOP level was ≤21mmHg vs. >21mmHg; adduction angle; race; age; and gender on ON path length and globe translation were analyzed using generalized estimating equations to account for possible intereye correlations of individual subjects.Results: Average visual field mean deviation (±standard error of mean, SEM) was −8.2±1.2 dB in OAG with normal IOP, and −6.1±1.4 in high IOP. In central gaze, ON path in OAG was significantly more redundant than in controls but in both groups the ON became significantly and almost equally straighter in small (~21°) or large (~27°) adduction than in central gaze. With progressive adduction only, globes retracted in OAG (P<0.005) but not in controls; this was only weakly related to globe size and not to IOP elevation. Globe retraction in adduction was significant only in OAG, and in that group was significantly greater in Asian than white patients (P<0.02). Conclusions:Although ON tethering in adduction is normal, progressive adduction is associated with abnormal globe retraction, in OAG regardless of IOP level. This phenomenon is more prominent in Asians who have OAG. Traction in adduction may cause repetitive strain injury

Finite Element Model of Ocular Adduction by Active Extraocular Muscle Contraction

“…14 When adduction exceeds about 26 degrees, 15 and in some people, even a smaller angle, 16 the redundancy in typical ON length is geometrically exhausted so that its path becomes maximally straightened, and the ON begins to act as a tether. 14,[17][18][19] In healthy people, this tethering in adduction stretches the ON 16 and translates the eyeball nasally but does not retract it posteriorly. 16,19,20 However, in patients with primary open angle glaucoma (POAG) who have either had only normal 20 or elevated levels of intraocular pressure, 19 ON traction in adduction causes significant globe retraction.…”

Orbital Fat Volume After Treatment with Topical Prostaglandin Agonists

“…There is evidence that when the globe adducts, the ON becomes tethered, pulls against the GONJ and causes medial translation and retraction of the globe. 25 , 27 , 29 , 30 Because orbits of subjects with POAG are horizontally narrower and vertically shorter than normal even up to 4 mm anterior to the globe-optic nerve junction, this may restrict mediolateral globe translation, increase globe retraction, and increase tension along the taut ON in adduction. If PGA had actually reduced posterior OFV, this might have ameliorated ON tethering in adduction by allowing the globe to shift posteriorly.…”

“…[25][26][27][28][29] Adduction may be particularly pathogenic, because ON length is insufficient to avert tethering the globe in adduction. 25,[27][28][29] Optical coherence tomography shows that when adduction exceeds 26 degrees, 30 deformations of the ON head and Bruch's membrane greatly exceed those during extreme IOP elevation, 31 or deformations recently proposed as pathological to retina. 32 The human horizontal oculomotor range is ± 55 degrees, meaning that people can maximally abduct and adduct to that angle.…”

“…37 Despite head restraint, vertical saccades of up to 30 degrees can be made from horizontal eccentricities of up to 50 degrees. 38 Magnetic resonance imaging has demonstrated significant globe retraction in adduction of only about half the oculomotor range in patients who have POAG with 29 and without 27 abnormally elevated IOP, but not in healthy controls. This globe retraction suggests that adduction tethering induces mechanical strain on the globe's suspensory tissues, including the orbital fat.…”

“…Eye movement–related deformation of the optic nerve (ON) has been suggested as another possible mechanical etiology for optic neuropathy in POAG. 25 – 29 Adduction may be particularly pathogenic, because ON length is insufficient to avert tethering the globe in adduction. 25 , 27 – 29 Optical coherence tomography shows that when adduction exceeds 26 degrees, 30 deformations of the ON head and Bruch's membrane greatly exceed those during extreme IOP elevation, 31 or deformations recently proposed as pathological to retina.…”