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Joseph Demer, Robert Clark; Magnetic Resonance Imaging (MRI) Demonstrates Differential Compartmental Contractility of Medial Rectus Muscle During Vertical Duction. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1302.
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© ARVO (1962-2015); The Authors (2016-present)
Recent anatomical studies demonstrate selective compartmental innervation of horizontal rectus extraocular muscles (EOMs), suggesting potential for differential compartmental control. We used functional MRI to investigate differential compartmental contractility during vertical duction.
Surface coil coronal MRI was obtained in target-controlled central gaze, 19° supraduction, and 19° infraduction in 24 orbits of 13 normal volunteers. Cross-sectional areas and volumes of superior vs. inferior compartments of horizontal rectus EOMs were compared in contiguous orbital image planes 2 mm thick spanning the origins to the globe equator.
There were significant contractile increases in maximum cross-section (p= 0.00008) and posterior volume (p = 0.00003) of the superior MR compartment from infraduction to supraduction, but insignificant decreases for both measures in the inferior compartment. There was no significant change in both measures for the lateral rectus (LR) superior compartment. The LR inferior compartment showed no change in maximum cross-section but a significant decrease in posterior volume in supraduction only (p = 0.05).
Differential contraction in the MR superior compartment, coupled with possible relaxation of the LR inferior compartment, may augment vertical duction and compensate for torsion created by cyclovertical EOMs. Differential horizontal rectus compartmental contractility during vertical duction extends the corresponding finding for the LR during ocular counter-rolling. In both instances, differential contraction of horizontal rectus EOM compartments could actively generate vertical/torsional forces perpendicular to their classical actions. Such vertical/torsional actions might contribute to cyclovergence, vertical vergence, and, in pathology, pattern strabismus not directly related to oblique EOM actions.
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