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Avery Weiss, John Kelly, James Phillips; Globe Excyclorotation in Crouzon Syndrome: Relationship to Pattern Strabismus and Apparent Inferior Oblique Overaction. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1304. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
. The purposes of this study were to investigate the relationship of globe excycloroation to the horizontal V-pattern and divergent vertical pattern strabismus using a model of eye alignment and to compare observed alignment with predicted alignment.
Binocular eye alignment assessed with prism cover or the Krimsky tests in primary gaze and at eccentricities in 25 children. The relative horizontal position of the SR/IR and MR/LR rectus muscle pulleys were determined from coronal images normalized in a craniotopic coordinate system. The angular alignment of the SR/IR and MR/LR rectus muscle pulleys were measured in Image J. To model static eye alignment we used the Orbit 1.8 computer program after simulating vertical offset of the horizontal rectus muscle pulleys alone or combined with horizontal offset of the vertical rectus muscle pulleys.
Sixteen (64%) of the 25 patients had strabismus, 14 had a V-pattern exotropia and 2 had an A-pattern esotropia. All patients with strabismus, but none without strabismus, had CT evidence of excyclorotation of the horizontal rectus muscle pulleys, the vertical rectus muscle pulleys or both. The Hess plot generated by Orbit demonstrates a divergent horizontal strabismus that increases in magnitude going from -30 deg downgaze to +30 deg upgaze and a divergent vertical strabismus that increases in magnitude going from primary gaze to 30 deg left and right gazes. The relationship between the magnitude of the displacement for the rectus muscle pairs and eye alignment was best fit by an exponential. Observed eye alignment paralleled predicted eye alignment.
Excyclorotation of the axes aligned on the SR/IR and MR/LR muscle pulleys relative to the rotation planes of the globe changes their pulling direction. As a consequence of these angular misalignments, the direction of their active and passive pulling forces shift to oblique axes which can be decomposed into horizontal and vertical components. Each of these components leads to passive stretch of the muscle or an off-axis force vector. Stretch of the muscle and the tangent of the off-axis vector produce exponential increases in muscle tension resulting in incomitant eye misalignments. Therefore surgical correction should take into account the displacement of the SR/IR and MR/LR muscle pairs.
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