Magnetic Resonance Imaging of Extra Ocular Muscles and Oculomotor Nerves in Patients with Congenital Monocular Elevation Deficit.

There is sparse literature on radiological findings in cases of monocular elevation deficit (MED). We conducted this study to report magnetic resonance imaging (MRI) characteristics of extra-ocular muscles and oculomotor nerves in patients with congenital MED and to investigate its patho-mechanisms. We included patients with congenital MED without prior strabismus surgery. The cross-sectional area (CSA) of the extra-ocular muscles in quasi-sagittal and quasi-coronal scans was measured manually on 2-mm-thick T2-weighted MRI images using Osirix MD software. Oculomotor nerves were imaged with balanced steady state free precession (bFFE) sequence. Clinical findings of Bell's phenomenon and forced duction test (FDT) for inferior rectus (IR) muscle were recorded. Eleven patients were included. The median CSA of superior rectus (SR) muscle at mid-orbit section was significantly lesser in the affected eye (8.83 mm2, Inter-quartile range (IQR): 5.39-11.36 mm2) than in the normal eye (13.63 mm2, IQR: 10.99-16.92 mm2), p = .02. From 11 patients, 10 had a thinner SR muscle on the affected side. Bell's phenomenon was intact in six patients. Forced duction test was positive for IR muscle in five cases. Oculomotor nerve was thinner on the affected side in two patients. Majority patients with congenital MED have a hypotrophic SR muscle, due to dysinnervation or primary muscle pathology. Bell's phenomenon may or may not be present in these conditions, and its presence does not warrant a supra-nuclear cause.