High-Resolution Magnetic Resonance Imaging (MRI) in Diagnosis and Management of Inferior Rectus (IR) Palsy
Rui Zhang; Joseph L. Demer
Department of Ophthalmology, David Geffen Medical School at University of California, Los Angeles
Los Angeles, California
Introduction: IR palsy presents as incomitant hypertropia due to failure of IR force transmission or generation. Here we present cases of IR palsy evaluated etiologically by high resolution MRI.
Methods: We studied fifteen hypertropic patients who demonstrated non-restrictive deficit of infraduction in abduction, excluding myasthenia gravis or immediately after orbital trauma. High-resolution, fixation controlled, surface-coil MRI with 234-312 micron resolution was obtained in 2mm thick coronal and sagittal planes. IR morphology was evaluated in central gaze, supraduction, and infraduction.
Results: Two categories of IR palsy were identified: direct mechanical myopathy (transection, avulsion, or disinsertion), and denervational (atrophy, infraction, focal lesion). Seven mechanical cases exhibited discontinuity between the muscle belly and globe secondary to trauma or endoscopic surgical accident; the IR showed increased deep bulk due to posterior recoil, and the posterior belly contracted in infraduction. Eight cases of denervational palsy exhibited atrophy of the deep IR belly sparing the tendon, and lacked contractile thickening in attempted infraduction. Denervational cases were associated with nuclear or peripheral lesions (metastasis, congenital fibrosis syndrome or schwanoma) of the IR motor nerve; infarction, surgery or trauma to the oculomotor nerve; or idiopathic atrophy without lesion identifiable on imaging.
Discussion: High resolution MRI may distinguish mechanical from neurological causes of IR palsy. In cases of deep muscle transection or atrophy, imaging of the deep orbit provides information unavailable from surgical exploration.
Conclusion: High resolution MRI should be considered in appropriate cases where MRI can distinguish mechanical from neurological causes of IR palsy.
MRI surface coils used in this study were investigational, not approved by the U.S. Food and Drug Administration for this purpose.
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