Functional morphometry of horizontal rectus extraocular muscles during horizontal ocular duction

Abstract

Purpose: We explored multiple quantitative measures of horizontal rectus extraocular muscle (EOM) morphology to determine the magnetic resonance imaging (MRI) measure best correlating with duction and thus contractility.

Methods: Surface coil coronal MRI was obtained in target-controlled central gaze and multiple positions of adduction and abduction in 26 orbits of 15 normal volunteers. Duction angles were determined by position changes of the globe-optic nerve junction. Cross-sectional areas, partial volumes, and location of peak cross-sections of the horizontal rectus EOMs were computed in contiguous image planes 2-mm thick spanning the EOM origins to the globe equator.

Results: All measures correlated significantly with duction angle (P < 0.0001). The best measures obtainable in single image planes were the maximum change in the cross-sectional area between equivalent image planes, with coefficients of determination R(2) = 0.92 for medial rectus (MR) and 0.91 for lateral rectus (LR), and percentage change in maximum cross-section with R(2) = 0.79 for MR and 0.78 for LR. The best partial volume measure of contractility was the change in partial volumes in four contiguous posterior planes (R(2) = 0.86 MR and for 0.89 LR), particularly when combined with the corresponding change in partial volume for the antagonist EOM (R(2) = 0.95 for MR and LR).

Conclusions: EOM morphologic changes are highly correlated with degrees of duction and thus contractility. Both changes in single-plane maximum cross-sectional areas and posterior partial volumes provide accurate, quantitative measures of EOM contractility.

Figure 1.

Quasi-coronal magnetic resonance images at 2-mm spacing contiguously from the globe-optic nerve junction (plane 0) to the image plane 14 mm posterior (plane 7) in abduction, central gaze, and adduction. The change in position of the globe-optic nerve junction in plane 0 was used to measure globe rotation in eccentric gaze. Cross-sectional areas were measured by manually outlining the EOM bellies (white lines), counting pixels within those EOMs, and converting the pixels into mm². Multiplane partial volume measurements were obtained by multiplying those cross-sectional areas by the slice thickness and summing partial volumes from adjacent image planes. The midorbital partial volume was measured using planes 0 to 5 and the posterior partial volume planes 4 to 7. Other partial volume measurements were posterior by two, three, or four image planes from the image plane containing the maximum cross-sectional area in central gaze, plane 2 for the LR and plane 3 for the MR for this subject. IR, inferior rectus; ON, optic nerve; SR, superior rectus.

Figure 2.

Scatter plots demonstrating the relationship of the percentage change in maximum cross-sectional area with degrees of horizontal duction. (A) Medial rectus. (B) Lateral rectus. These muscles had similar linear regressions and coefficients of determination R². Data from 26 orbits of 15 subjects.

Figure 3.

Scatter plot demonstrating the relationship of combined agonist/antagonist posterior partial volumes with horizontal duction. This measurement had the strongest correlation with degrees of duction for both horizontal rectus muscles. Data from 26 orbits of 15 subjects.

Figure 4.

Percent change in posterior partial volume of the MR-LR antagonist pair as a function of horizontal duction. For each of the seven numbered subjects who achieved four to seven horizontal gaze positions per orbit, linear regression demonstrated excellent correlation with the magnitude of horizontal duction, accounting for 97% to 99% of the variance.