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Joseph L. Demer, Robert A. Clark; Compartmental Function of Horizontal Rectus Extraocular Muscles (EOMs) During Head Tilt: Impaired Lateral Rectus (LR) Inferior Zone Contractility In Superior Oblique (SO) Palsy. Invest. Ophthalmol. Vis. Sci. 2011;52(14):2077.
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© ARVO (1962-2015); The Authors (2016-present)
Innervation to the horizontal but not vertical rectus EOMs has been recently discovered to be segregated into approximately equal superior and inferior zones, but until now functional significance of this compartmentalization has not been evident. We used magnetic resonance imaging (MRI) to seek evidence of differential zonal activation in the LR and medial rectus (MR) during ocular counter-rolling (OCR) evoked by head tilt.
Surface coil coronal MRI (312 micron resolution) in target-controlled central gaze was obtained in 90° right and left head tilts in 6 normal subjects and 9 with unilateral SO palsy manifested by significant unilateral SO atrophy (P<0.00001). Cross-sectional areas and volumes of the superior and inferior LR and MR halves were compared in 6 contiguous image planes 2 mm thick.
In orbit up (excyclotorted) position, volume and maximum cross-sectional area increased significantly in the inferior LR zone (P < 0.05), but there was no change in the superior zone for both normal subjects and contralesional orbits in SO palsy. For orbits ipsilesional to SO palsy, there were no significant changes in LR volume or maximum cross-sectional area in either LR zone during head tilt. There was no significant change in either MR zone in either normal subjects or subjects with SO palsy. Biomechanical modeling suggests that differential contraction of horizontal rectus EOM zones could generate torsional/vertical actions 15-20% of total EOM force, 6X greater than required to execute normal vertical fusional vergence.
Anatomical studies have demonstrated projections from premotor torsional/vertical centers to the abducens (CN6) nucleus in primates and lower mammals, heretofore inexplicable but perhaps commanding torsional/vertical actions via compartmental projection of CN6 within the LR. Differential compartmental LR activation is a novel physiological mechanism that apparently contributes to OCR. Impairment of LR compartmental activation in SO palsy may contribute to head tilt dependent hypertropia. While there is essentially no zonal innervation overlap of CN6 branches within the LR, a small region exists in the MR that is innervated by both zones. Overlap or a weaker compartmental effect may have precluded detection of MR compartmental activation in this experiment.
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