Magnetic resonance imaging of the functional anatomy of the superior oblique muscle in patients with primary superior oblique overaction

PURPOSE: To quantitatively determine the size and contractility of the superior oblique (SO) muscle in primary SO overaction (PSOOA). PATIENTS AND METHODS: A prospective, observational study was conducted on 12 patients with PSOOA, and 10 healthy, orthotropic subjects. Sets of contiguous, 2 mm slice thickness, quasi-coronal magnetic resonance imaging were obtained during different gazes, giving pixel resolution of 0.391 mm. Cross-sectional areas of the SO muscles were determined in primary position, supraduction, and infraduction to evaluate size and contractility. The cross-sectional areas of SO muscle were compared with those of controls in the primary position to detect hypertrophy or atrophy and changes in contractility could be detected during the vertical gaze. All statistical calculations were performed using PROC MIXED (SAS 9.4). RESULTS: There was no difference between the ipsilesional (affected eye), contralesional (unaffected eye), and normal SO muscle cross-sections: 0.176±0.018 cm(2), 0.175±0.005 cm(2), and 0.173±0.015 cm(2), respectively (P=0.82). The maximum contractility of SO muscle on the ipsilesional (affected) side was 0.097±0.024 cm(2), and was different than on the contralesional (unaffected) side: 0.067±0.015 cm(2) and in control subjects: 0.063±0.018 cm(2) (P=0.0002). CONCLUSIONS: In PSOOA, the ipsilesional SO is more contractile than the contralesional SO muscle and different than in controls, with no difference in SO muscle size in primary position, which suggests that excessive innervation rather than muscle hypertrophy underlies PSOOA.