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J.L. Demer, R.A. Clark; Magnetic Resonance Imaging (MRI) Demonstrates Shifts in Rectus Extraocular Muscle (EOM) Pulleys in Gravity–Dependent Hypertropia (HT) . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2931.
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© ARVO (1962-2015); The Authors (2016-present)
The effect of head tilt on HT is a vestibular response to gravity commonly used to diagnose superior oblique palsy (SOP). We used MRI in lateral decubitus head tilts to determine rectus pulley positions in SOP and a clinically simulating condition.
12 subjects without prior strabismus surgery who had gravity–dependent HT conforming to the 3–step test and 10 controls underwent gadodiamide–enhanced, surface coil orbital MRI in 2 mm thick quasi–coronal planes (312 micron pixels) during 90° lateral whole body tilts. EOM paths were determined by analysis of cross–sections in normalized, oculocentric coordinates. Unilateral SOP, clinically diagnosed by 3–step testing, was confirmed with MRI demonstrating significant SO atrophy in 7 subjects, while 5 without atrophy were considered "masquerading SOP" (MSQ).
Subjects with SOP had ipsilateral HT in central gaze averaging (±SEM) 9±1.5° that varied clinically by 17±3.5° with head tilt. Subjects with MSQ had central gaze HT of 7.6±5.3° that varied by 10.6±3.7° with head tilt. Rectus pulley arrays of non–HT orbits incyclorotated when tilted down relative to up in controls (3.4±0.9°), SOP (3.2±0.9°), and MSQ (2.4±2.2°). In orbits with HT due to SOP, the medial (MR) and superior rectus (SR) pulleys intorted normally during head tilt, but the lateral (LR) and inferior rectus (IR) pulleys abnormally extorted by 2.4±0.8°. Relative extorsion of the SR and LR pulleys was ablated post–operatively in a subject also imaged after ipsilesional inferior oblique recession and SO tucking surgery. Post–operatively, this subject also had markedly increased gravity–dependent intorsion of the SR, LR, and IR pulleys in the contralateral, unoperated orbit. In HT orbits due to MSQ, the SR, LR, and IR pulleys intorted normally with head tilt (2.4 ±2.0°), but the MR pulley abnormally extorted (2.0±2.1°). In hypotropic MSQ orbits, all rectus pulleys intorted with downward head tilt by 2.4±2.2°.
Gravity–dependent ocular counter–rolling is normally associated with similarly–directed cyclotorsion of the pulleys that is reversed in direction for the IR and LR in SOP. Torsional behavior of rectus pulleys is abnormal, but different from SOP, when gravity–dependent HT is not associated with superior oblique atrophy and masquerades as SOP. Surgery for SOP can alter the gravity–dependent torsional positions of rectus pulleys. Changes in rectus EOM pulling directions in both orbits may be neural adaptations to a unilateral lesion, and probably contribute to complex motility patterns in true and masquerading SOP.
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