| Purpose: To compare the different volume of superior oblique muscle (SO) by using high—resolution magnetic resonance imaging system (MRI) to measure the cross-sections of SO in orthotropic subject and patients with SO palsy, and investigate whether it occur the displacement of rectus extraocular muscle (EOMs) in patients with SO palsy
Study design: prospective, case control study.
Methods: 13 orthotropic volunteers and 5 patients clinical diagnosed of SO palsy underwent complete ophthalmologic examination. High—resolution, surface coil and contiguous MRI in planes perpendicular to the orbit axis was performed with fixation control, slice thickness of 2.5mm in multiple gaze direction in quasi-coronal, sagittal and horizontal axis spanning the orbit from the apex to the equator of the globe. Using image analysis system to measure the cross-sectional cross-sections and anterior-posterior position of EOMs relative to orbital center, for evaluation the size and contractility of SO between the ipsilateral, contralateral and controls, meanwhile, study the location of EOMs path relative to orbital center.
Result: Of all the control subject, the maximum cross-sectional cross-sections of SO muscle (mean±standard deviation) was17.80±4.26mm2 . In patient group, the maximum cross-sectional cross-sections of ipsilateral and contralateral SO muscle was 9.33±5.69mm2, 17.84± 4.54 mm2, respectively. By comparing the size of above, the ipsilateral SO was significant small than that of contralateral ( P≤0.001) and control group ( P≤0.001) . Moreover, in contrast to normal controls, contralateral SO were slightly larger but not statistically ( P≥0.05).The contractility of SO in ipsilateral, contralateral and normal controls were, 1.80±1.2 mm2, 7.80±1.50 mm2and 4.62±1.82 mm2, respectively. As to the path of muscle, It was observed that, in contrast to the normal controls and contralateral orbits, only the medial rectus (MR) showed a statistically significant 0.7mm superior displacement on the the ipsilateral ( P≤0.05) , while no significant displacement were found on other muscles. Unexpectedly, we found the absence of SO in two cases which clinically diagnosed SO palsy.
Conclusion: The location, size and contractility of SO muscle can be demonstrated sufficiently with the application of high resolution MRI in the study. The MRI findings suggested that there have a strong correlation between the muscle size and contractility of SO. The only displacement in MR may be attributed to the result of atrophy of contiguous SO belly in the posterior orbit. Consequently, it should be concerned that the displacement of rectus may perform function in SO palsy by masequerading the abnormality of oblique. From above all, MRI may play an important role in promotion of clinical diagnosis by providing a window into the functional anatomy of the orbital structures.
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