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L. Jiang, J. L. Demer; MRI Shows Relative Adaptive Size and Contractility Changes of Inferior Rectus in Superior Oblique Palsy. Invest. Ophthalmol. Vis. Sci. 2007;48(13):5275.
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© ARVO (1962-2015); The Authors (2016-present)
Biomechanical modeling consistently indicates that Superior Oblique (SO) Palsy weakness alone can not explain the large hypertropia observed in SO palsy. We used magnetic resonance imaging (MRI) to investigate if any adaptive size or contractility changes in Inferior Rectus (IR) may contribute.
We used surface coils to obtain high-resolution, quasi-coronal MRI in 2 mm thick planes at 312 µm resolution in central gaze, supraduction and infraduction in 17 subjects with unilateral SO palsy and 27 orthotropic controls. We determined maximum IR cross sections in central gaze, and contractility from cross section changes from supraduction to infraduction.
Subjects with SO palsy had 16.6±9.1 PD (mean ± SD) hypertropia in central gaze, and all exhibited ipsilesional SO atrophy and subnormal contractility. Mean ipsilesional,contralesional and normal IR cross-sections were 28.5±3.5 mm2, 31.6±3.8 mm2 and 31.8± 5.8 mm2 ,while mean contractilities were 7.0±4.0 mm2 , 8.8±4.3 mm2 , and 7.2±3.8 mm2, respectively. Both ipsilesional IR cross-sections and contractility were significantly less than contralesional IR (Paired Samples t-test, P<0.05), but neither IR cross-sections nor contractility differed significantly from normal controls (P>0.1).
In SO palsy, the ipsilesional IR exhibits reduced size and contractility relative to the contralesional IR, reflecting likely neurally mediated changes that increase substantially the relatively small hypertropia due to SO weakness. Thus, both infraducting muscles ipsilesional to SO palsy have less size and contractility than those of the fellow eye.
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