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

Abstract

PurposeRepetitive strain to the optic nerve (ON) due to tethering in adduction has been recently proposed as an intraocular pressure-independent mechanism of optic neuropathy in primary open-angle glaucoma. Since strabismus may alter adduction, we investigated whether gaze-related ON straightening and associated globe translation differ in horizontal and vertical strabismus.MethodsHigh-resolution orbital magnetic resonance imaging was obtained in 2-mm thick quasi-coronal planes using surface coils in 25 subjects (49 orbits) with esotropia (ET, 19 ± 3.6Δ SEM), 11 (15 orbits) with exotropia (XT, 33.7 ± 7.3Δ), 7 (12 orbits) with hypertropia (HT, 14.6 ± 3.2Δ), and 31 normal controls (62 orbits) in target-controlled central gaze, and in maximum attainable abduction and adduction. Area centroids were used to determine ON path sinuosity and globe positions.ResultsAdduction angles achieved in ET (30.6° ± 0.9°) and HT (27.2° ± 2.3°) did not significantly differ from normal (28.3° ± 0.7°), but significantly less adduction was achieved in XT (19.0° ± 2.5°, P = 0.005). ON sheath tethering in adduction occurred in ET and HT similarly to normal, but did not in XT. The globe translated significantly less than normal, nasally in adduction in XT and temporally in abduction in ET and HT (P < 0.02, for all). Globe retraction did not occur during abduction or adduction in any group.ConclusionsSimilar to normal subjects, the ON and sheath become tethered without globe retraction in ET and HT. In XT, adduction tethering does not occur, possibly due to limited adduction angle. Thus, therapeutic limitation of adduction could be considered as a possible treatment for ON sheath tethering.

Compression of the Choroid by Horizontal Duction

“…This greater compression in adduction than abduction is consistent with optical imaging showing greater deformation of the disc and peripapillary retina in these duction directions 15 and the observation by MRI imaging of ON tethering only in adduction. 3,4,29 Compression of the temporal choroid is also consistent with observed temporal displacement of peripapillary vessels 15 and the overall temporalward compression of optic disc tissue during ON tethering in adduction. 2 The situation in abduction is quite different, being associated with only minimal choroidal compression and less ON and peripapillary deformations in young adults but none in older subjects.…”

“…Forty volunteers (19 male and 21 female; mean age, 37 6 20 [standard deviation, SD] years; range, 18-73) were recruited for the study. Subjects, who had not previously participated in research of this sort, were separated into a younger group of 25 (15 male and 10 female; mean age, 24 6 5; range, [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] and an older group of 15 volunteers (4 male and 11 female; mean age, 62 6 8 years; range, 50-73). Subjects gave written informed consent before participating according to a protocol approved by the University of California, Los Angeles, Institutional Review Board and compliant with the Declaration of Helsinki.…”

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.…”

Optic disc shape in patients with long-lasting unilateral esotropia and exotropia

“…Spectral-domain OCT studies showed that adduction displaced the nasal peripapillary Bruch’s membrane anteriorly and the temporal nasal peripapillary Bruch’s membrane posteriorly, leading to a tilting the optic nerve head around the vertical axis, with a reversal of this pattern in abduction [12, 13]. Studies using MRI revealed that the backward pull of the optic nerve on the optic nerve head in markedly axially elongated eyes could lead to a novel mechanical load on the globe at the optic nerve head in ocular adduction [10, 11]. It could explain the marked rotation of the optic nerve head around its vertical axis in highly elongated eyes, with the temporal border of the optic disc turning backward.…”

“…Such deformations could be as large as an intraocular pressure elevation to 50 mmHg [8, 9]. Magnetic resonance imaging (MRI) studies provided evidences that optic nerve straightening is present in adduction, suggesting tethering by the optic nerve sheath in adduction is a novel mechanical load on the globe [10, 11]. Deformation of the optic nerve head and peripapillary structures during horizontal duction has also been visualized using spectral-domain optical coherence tomography (OCT) [12, 13].…”