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Joseph L Demer; Magnetic Resonance Imaging (MRI) Suggests New Non-Muscular Constraints on Ocular Rotation in High Myopia. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):1329.
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© ARVO (1962-2015); The Authors (2016-present)
Traditional concepts of motility assume a spherical globe freely rotated by extraocular muscles (EOMs). This study employed MRI to investigate the hypothesis that globe irregularity and the optic nerve (ON) length may constrain ocular rotation in axial high myopia..
High resolution, surface coil axial MRI using T1 or T2 fast spin echo sequences was obtained in 8 normal adults, 12 adult esotropes (ET) with normal axial length (AL) <25.3 mm, 11 myopic esotropes with mean AL 28.7±3.6 (SD) mm, and 7 exotropes (XT). ON length was measured in eccentric horizontal gaze positions where it became straight.
ON straightening occurred only in adduction, at 23.6±9.0° in normal subjects, not significantly different from XT at 22.2±11.8°, but significantly greater in ET at 36.3±9.3°, and in myopic ET at 34.1±11.7° (P<0.003). ON length at straightening was 30.4±3.4 mm in normals, not significantly different from XT at 30.7±2.6 mm, but less in ET at 27.8±2.7 mm and 25.5±2.0 mm in myopic ET (P<0.03). ET was associated with globe retraction and elongation in adduction, suggesting ON tethering with forces concentrated at the scleral canal. Eight globes of the myopes exhibited prominent irregular nasal posterior, temporal posterior, nasal and temporal posterior, or equatorial staphylomata. These scleral ectasias were positioned to contact and elongate horizontal rectus EOM paths in some gaze positions.
Concepts of strabismus in axial high myopes should be expanded to include irregular posterior staphylomata, and tethering in adduction by the ON. Staphylomata act like "cams" affixed to the normally spherical globe, exerting no mechanical effect until ecentrically rotating against EOMs. After rotational contact, staphylomata would stretch and thus add tension to the EOM that increases non-linearly with further duction. Tethering by the ON can add a further strong nonlinear force opposing adduction, even sufficient to deform the posterior pole.
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