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J. S. Kung, J. L. Demer; Magnetic Resonance Imaging (MRI) of the Functional Anatomy of the Inferior Oblique (IO) Muscle in Head Tilt-Dependent Hypertropia (HTDH). Invest. Ophthalmol. Vis. Sci. 2010;51(13):6428.
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© ARVO (1962-2015); The Authors (2016-present)
While oblique extraocular muscles are believed to produce ocular counter-rolling (OCR), this physiologic torsional eye movement persists even in superior oblique (SO) palsy, a prototypical form of cyclovertical strabismus. We performed orbital MRI to investigate the role of the IO in mediating OCR.
We studied 16 normal adults and 19 with HTDH: 12 with typical unilateral SO palsy confirmed by significant SO atrophy, and 7 deemed "masquerading" unilateral SO palsy due to absent SO atrophy but positive 3-step test. Surface coil MRI was performed in 2 mm thick quasi-sagittal planes, comparing right with left decubitus positions to achieve ± 90° head tilt. IO contractility was quantified by change in cross-section from ipsi- to contralateral head tilt, at the image plane containing the center of the inferior rectus crossing.
Normal subjects displayed a +1.5 ± 0.4 contractile change in IO cross-section during OCR (mean ± SEM mm2, P < 0.002). This contractile change was similar in hypertropic (+1.0 ± 0.8) and hypotropic eyes (+1.4 ± 1.2) in SO palsy, as well as in hypotropic eyes in "masquerading" SO palsy (-0.2 ± 1.0). However, the IO in hypertropic "masquerading" eyes paradoxically relaxed when the orbit tilted to the contralateral side, showing an abnormal decrease in cross-section (-1.7 ± 0.7, P < 0.002). During ipsilateral head tilt when the IO would be expected to relax, IO cross-section in hypertropic "masquerading" eyes was larger than normal (P < 0.05), but otherwise did not differ between groups.
IO contractility during OCR remains normal despite SO atrophy, suggesting that SO palsy does not cause a compensatory change in IO contractility. In HTDH masquerading as SO palsy, the IO relaxes instead of contracting during head tilt. This result is paradoxical because the IO is traditionally thought to be "overacting" in response to its weak antagonist, the SO. In general, IO over-contraction does not explain the wide spectrum of incomitant vertical strabismus in SO palsy.
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