Abstract
BackgroundHorizontal eye movements have been proposed to induce biomechanical stress and strain on optic nerve head. Since strabismus may lead to sustained adduction or abduction, we investigate the effects of long lasting unilateral horizontal strabismus on the morphology of optic disc.MethodsThe observational cross-sectional study included patients with unilateral constant horizontal strabismus lasting for more than two years. The patients underwent an ophthalmological examination including refraction and morphometry of the optic nerve head. A prism cover test using right angle glass prism was performed to measure the magnitude of the ocular deviation.ResultsThe study included 70 patients with a unilateral constant strabismus (35 esotropic patients, 35 exotropic patients) with a mean age of 26 ± 19 years, mean refractive error of − 0.72 ± 3.3 diopters, mean axial length of 23.8 ± 1.7 mm, and a mean angle of deviation of 87 ± 36 prism diopters (Chinese right-angle glass method) in the esotropic group and − 97 ± 29 prism diopters in the exotropic group. In the whole study population and taken separately in the esotropic group and exotropic group, the disc ovality index (defined as ratio of minimal-to-maximal optic disc diameter) did not differ significantly between the deviating eyes and the contralateral fixating eyes (all P > 0.05). As a corollary, the disc ovality index and the prevalence of parapapillary beta/gamma zone did not differ significantly between the esotropic group and the exotropic group (all P > 0.05).ConclusionsOptic disc ovality did not differ markedly among long-lasting esotropic eyes, exotropic eyes, and non-strabismic eyes. It suggests that optic disc shape may not be markedly influenced in non-highly myopic eyes by a potential backward pull of the optic nerve on the optic disc structures in adduction or abduction.
Highlights
Horizontal eye movements have been proposed to induce biomechanical stress and strain on optic nerve head
Seventy patients with a horizontal unilateral strabismus were included into the study, with 35 patients in the esotropic group and 35 patients in the exotropic group
The results showed that a more myopic refractive error was associated with a lower optic disc ovality index (P = 0.045), and higher prevalence of beta/ gamma zone (P < 0.001) in the whole study group (P = 0.002)
Summary
Horizontal eye movements have been proposed to induce biomechanical stress and strain on optic nerve head. The shape of the optic nerve head can be described by the ratio of its minimal to maximal diameter (disc ovality index), and by its tilt/rotations around the vertical axis, the horizontal axis and the sagittal axis [1, 2]. Models of infinite element analysis predicted that eye movements (in both adduction and abduction) could generate large deformations within the optic nerve head through the pulling action of the optic nerve sheaths. 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
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