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JL Demer, R Kono, W Wright; Magnetic Resonance Imaging (MRI) of Human Extraocular Muscles (EOMs) During Asymmetrical Convergence . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1915.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Even when the target remains aligned on one eye, binocular convergence changes the kinematics of both eyes so that the Listing plane for each rotates temporally. We used MRI to test the prediction that this results from rectus pulley repositioning (Clark, Miller & Demer, IOVS 41:3787-97, 2000). Methods: Contiguous coronal, axial, and sagittal MRI scans were obtained at 2 mm thickness and 312 micron resolution with gadodiamide contrast and surface coils in both orbits of 5 normal young adults. Using mirror presentation, scans were repeated during binocular fixation of a target aligned to one eye presented at 15 cm and 800 cm distance. EOM paths and cross sections were determined from area centroids in a normalized coordinate system. Lens cross sections were fitted to an ellipse. Results: During convergence, the aligned eye rotated 0.5±1.2° (mean ± SD) horizontally and 0.5±1.8° vertically with 0.6±0.4 mm lens thickening. The aligned globe moved anteriorly 0.3±0.2 mm, nasally 0.7±1.2 mm, and inferiorly 0.3±0.5 mm. The only aligned eye EOM showing a change in cross section with convergence was the inferior oblique (IO), which increased by 1.0±0.6 sq mm (P<0.025). The converging eye rotated nasally 22.7±1.3° and superiorly 1.0±1.4°, with 0.5±0.2 mm lens thickening. The converging globe moved anteriorly 0.5±0.2 mm, nasally 0.3±0.8 mm, and inferiorly 0.1±0.8 mm. Interocular spacing was reduced by 1.0±0.7 mm. The converging eye medial (MR) and lateral rectus (LR) EOMs had large contractile changes in cross section, but the IO also increased by 1.8±1.1 sq mm (P<0.025). In the aligned eye, convergence was associated with excyclo-rotation of the anterior paths of the rectus EOMs, consisting of nasal displacement of the inferior rectus, superior displacement of the MR, lateral displacement of the superior rectus, and inferior displacement of the LR by 0.3 - 1.2 mm. Similar changes were suggested for the converging eye but were less evident due to the eye movement. There was no displacement of either IO but surrounding connective tissues shifted in both eyes. Conclusion: Shifts of EOM paths of the aligned eye during asymmetric convergence suggest movements of rectus EOM pulleys independent of horizontal and vertical eye position, but consistent with known extorsion during convergence. These shifts represent torsional rotation of the rectus pulley array around the orbital axis, and thus differ from earlier predictions. Rectus pulley shifts might be implemented symmetrically in both eyes by the orbital layer of the IO and the peribulbar smooth muscle. Absence of globe retraction is evidence against horizontal rectus EOM co-contraction in convergence.
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