Magnetic Resonance Imaging of Optic Nerve Traction During Adduction in Primary Open-Angle Glaucoma With Normal Intraocular Pressure

Abstract

PurposeWe used magnetic resonance imaging (MRI) to ascertain effects of optic nerve (ON) traction in adduction, a phenomenon proposed as neuropathic in primary open-angle glaucoma (POAG).MethodsSeventeen patients with POAG and maximal IOP ≤ 20 mm Hg, and 31 controls underwent MRI in central gaze and 20° to 30° abduction and adduction. Optic nerve and sheath area centroids permitted computation of midorbital lengths versus minimum paths.ResultsAverage mean deviation (±SEM) was −8.2 ± 1.2 dB in the 15 patients with POAG having interpretable perimetry. In central gaze, ON path length in POAG was significantly more redundant (104.5 ± 0.4% of geometric minimum) than in controls (102.9 ± 0.4%, P = 2.96 × 10−4). In both groups the ON became significantly straighter in adduction (28.6 ± 0.8° in POAG, 26.8 ± 1.1° in controls) than central gaze and abduction. In adduction, the ON in POAG straightened to 102.0% ± 0.2% of minimum path length versus 104.5% ± 0.4% in central gaze (P = 5.7 × 10−7), compared with controls who straightened to 101.6% ± 0.1% from 102.9% ± 0.3% in central gaze (P = 8.7 × 10−6); and globes retracted 0.73 ± 0.09 mm in POAG, but only 0.07 ± 0.08 mm in controls (P = 8.8 × 10−7). Both effects were confirmed in age-matched controls, and remained significant after correction for significant effects of age and axial globe length (P = 0.005).ConclusionsAlthough tethering and elongation of ON and sheath are normal in adduction, adduction is associated with abnormally great globe retraction in POAG without elevated IOP. Traction in adduction may cause mechanical overloading of the ON head and peripapillary sclera, thus contributing to or resulting from the optic neuropathy of glaucoma independent of IOP.

Optic Nerve Sheath Tethering in Adduction Occurs in Esotropia and Hypertropia, But Not in Exotropia

“…Since initial positions of one or both eyes are generally more adducted than normal in esotropic subjects, greater tension on the ON during further adduction might have been expected to induce abnormally great globe retraction, similar to that recently demonstrated by MRI in patients with POAG without increased IOP. 12 However, globe retraction was not observed here in esotropic subjects, suggesting that globe retraction in adduction may be specifically related to glaucoma, consistent (albeit not demonstrative) with a hypothesized causative role of ON sheath tethering in glaucomatous optic neuropathy. Although little is known about the mechanical properties of the ON sheath, recent biomechanical studies have suggested that it is much stiffer than the peripapillary sclera 6 – 8 and thus might exert traction in adduction that could retract the globe.…”

“… 9 – 11 Most recently, MRI showed that adduction tethering is accompanied by significantly greater globe retraction in patients who have primary open angle glaucoma (POAG) without increased IOP, implying that abnormally high traction force is exerted in adduction in these patients. 12 Based on these findings, repetitive ON sheath tethering over numerous adduction eye movements has been proposed as an IOP-independent mechanism of glaucomatous ON damage.…”

Pluripotent Stem Cell-Based Approaches to Explore and Treat Optic Neuropathies

“…One hypothesis points to the impairment of neurotrophic factor delivery compromising RGC survival, as a consequence of the axonal transport blockade caused by excessive IOP ( Pease et al, 2000 ; Salinas-Navarro et al, 2010 ; Almasieh et al, 2012 ; Fahy et al, 2016 ; Kimura et al, 2016 ). However, glaucoma can take place without abnormal IOP, especially in the case of primary open-angle form and some patients display evolving RGC degeneration despite the normalization of IOP ( Sluch and Zack, 2014 ; Demer et al, 2017 ). Finally, some people display high IOP without any symptoms of the disease ( Friedman et al, 2004 ), suggesting that other mechanisms may exist and/or that the degenerative process persists despite the abolition of the initial cause of the disease.…”

Compression of the Choroid by Horizontal Duction

“…W hen the human eye rotates into a large angle adduction, we have shown by magnetic resonance imaging (MRI) that the optic nerve (ON) and its sheath tether the globe and exert tractional force on the optic disc. 1,2 We have also demonstrated by optical coherence tomography (OCT) that adduction deforms the peripapillary Bruch's membrane (ppBM) and tilts the ON head (ONH) anterposteriorly. 3 The deformation is exaggerated at a threshold of approximately 268 adduction, the angle at which the limited ON length necessitates the onset of tethering.…”