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Sylvia Yoo, Joseph Demer; Magnetic resonance imaging (MRI) features of partial or complete avulsion of the inferior rectus (IR) muscle. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1923.
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Rectus muscle flap tears have been described as an etiology of strabismus following orbital trauma, but evidence supporting this mechanism has been largely limited to findings at surgical exploration. This study investigates strabismus secondary to partial or complete IR avulsion, including flap tears, identified on MRI prior to surgical exploration.
Surface coil MRI (312 micron resolution) was obtained in target-controlled central gaze, supraduction, and infraduction in quasi-coronal and quasi-sagittal planes in both orbits of five patients (mean age 39±20 years) who had vertical strabismus associated with unilateral partial or complete, traumatic IR avulsion. MRI was performed an average of 8.8 months post-injury but prior to surgical exploration, and was compared with MRI in control subjects.
All five patients had a history of blunt and possibly penetrating orbital trauma resulting in strabismus prior to any orbital fracture repair. Four patients had coexisting orbital fractures, three repaired by orbital plate implantation. Partial or complete IR avulsion was identified on sagittal and coronal MRI. In all cases, the anterior IR was absent, split, or distorted at the level of the inferior oblique (IO) muscle. Distortion of the IR included thinning or irregularity and downward displacement of the lateral edge. Separation between the IR and the globe was notable in one patient at the level of the IO. Four patients had transverse and/or longitudinal splitting of the IR observable on MRI. One patient who had undergone orbital floor fracture repair had splitting of the global and orbital IR layers by the orbital implant itself. Another patient had longitudinal splitting of the global and orbital layers; one had a transverse fissure with inhomogeneity of the muscle at the area of splitting; and one had both transverse and longitudinal IR splitting. Four of the patients had strabismus surgery following evaluation with MRI. Two underwent IR recovery; one IR was found to break up into fibrous fatty scar tissue intraoperatively.
Clinical findings may not be diagnostic of the etiology of strabismus following orbital trauma, and avulsions of extraocular muscles may not be suspected without MRI or surgical exploration. Avulsions at the IR insertion may indicate deeper damage with both transverse and longitudinal intramuscular splitting, also detectable on MRI.
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