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J.L. Demer; Magnetic Resonance Imaging (MRI) of Human Extraocular Muscles (EOMs) During the Static Torsional Vestibulo-ocular Reflex (VOR) . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2736.
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Purpose: Gravitational stimulation of the otoliths induces counterrolling of the eyes around the visual axis of 3 – 7 deg in response to sustained 90 deg head tilt (Bockisch & Haslwanter, Vis Res 41: 2127-37, 2001). We used MRI to investigate the functional anatomy of the EOMs during such head tilt. Methods: Contiguous coronal, axial, and sagittal MRI scans were obtained at 2 mm thickness and 312 micron resolution with surface coils in both orbits of 5 normal young adults. During monocular fixation of a centered target, scans were performed in the 90 deg right ear down (RED) position, and repeated with left ear down (LED). Images were analyzed quantitatively. Results: Correct head orientation was verified from MRI images. In RED, the array of anterior rectus EOM paths incyclorotated for the right and excyclorotated for the left eye by up to 4 – 10 deg. In LED, the array of anterior rectus EOM paths incyclorotated for the left and excyclorotated for the right eye. The amount of this torsional change varied among subjects. Incyclorotation was associated with thickening of the superior oblique (SO) and thinning of the inferior oblique (IO) muscles, while excyclorotation was associated with SO thinning and IO thickening. Conclusions: Since rectus EOM paths are determined by their pulley locations, the observed shifts of rectus EOM paths during head tilt indicate re-orientation of the rectus pulleys during the VOR. This pulley reconfiguration is associated with appropriate contractile changes in oblique EOM cross sections. Rectus pulley configuration during the VOR is probably an active process mediated by known insertions of the orbital layers of the oblique EOMs on the rectus pulleys, and has broad implications for neural control of the VOR. Interindividual variation in this phenomenon may relate to known individual variability in the static torsional VOR. View OriginalDownload SlideView OriginalDownload Slide
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