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Soh Youn Suh, Joseph L Demer; Optic nerve tethering occurs in hypertropia but not in exotropia. Invest. Ophthalmol. Vis. Sci. 2018;59(9):2030.
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© ARVO (1962-2015); The Authors (2016-present)
Optic nerve (ON) tethering in adduction causing repetitive strain to the ON has been recently proposed as an intraocular pressure (IOP) - independent mechanism of primary open-angle glaucoma (POAG). Magnetic resonance imaging (MRI) has demonstrated that adduction tethering is associated with significant globe retraction in cases of POAG without elevated IOP, a phenomenon neither observed in normal people nor in esotropic patients. We investigated ON paths and gaze-related globe movements differ in patients with exotropia (XT, 30.5±6.5Δ SEM) and hypertropia (HT,10.5±2.8Δ).
High-resolution (312μm in plane) orbital MRI was obtained in 2 mm thick axial and quasi-coronal planes using surface coils in 13 patients (18 orbits) with XT, 8 patients (16 orbits) with HT, and 23 (46 orbits) normal controls in target-controlled central gaze and in maximum attainable ab- and adduction. ON area centroids in 2 mm thick contiguous quasi-coronal image planes were analyzed to determine ON path length. ON straightness was measured by the ratio between actual and minimum ON path length, with 100% as theoretical minimum. Globe positions were analyzed from globe area centroids.
Abduction angles achieved in XT (23.3±1.5°) and HT (20.6±5.9°) did not differ significantly from normal (22.5±0.5°). While adduction angle achieved in HT (25.1±9.7°) did not differ significantly from normal (28.6±0.8°, P=0.158), significantly lesser adduction was achieved in XT (20.2±2.4°, P=0.001). In XT, ON straightness in central gaze and abduction did not differ significantly from normal, but ON was significantly less straightened in adduction (103.3±0.9% vs. 101.5±0.1%, P=0.045). In HT, ON paths did not significantly differ from normal in central gaze, abduction, or adduction. There was no significant globe retraction in either abduction or adduction in XT and HT patients.
In XT, ON tethering does not occur in adduction, possibly due to limited adduction angle achieved. Thus, therapeutic limitation of adduction can be considered as a possible option in POAG. However, ON tethering occurs in HT in adduction as it does in normal subjects and in patients with POAG.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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