High-Resolution Magnetic Resonance Imaging of the Extraocular Muscles and Nerves Demonstrates Various Etiologies of Third Nerve Palsy

Abstract

The etiology of third nerve palsy is usually diagnosed by history, motility examination, and presence of lid and pupil involvement, as well as cranial and vascular imaging. We used high-resolution magnetic resonance imaging (hrMRI) of the oculomotor nerve and affected extraocular muscles (EOMs) to investigate oculomotor palsy. Prospective, noncomparative, observational case series in an academic referral setting. Twelve patients with nonaneurysmal oculomotor palsy of 0.75 to 252 months' duration were studied. In the orbit and along the intracranial oculomotor nerve, hrMRI at 1- to 2-mm thickness was performed. Coronal plane images of each orbit were obtained in multiple, controlled gaze positions. Structural abnormalities of the oculomotor nerve and associated changes in EOM volume and contractility were evaluated. Cases were categorized as tumor related, congenital, diabetic, traumatic, and idiopathic according to clinical characteristics and hrMRI findings. Reduction of volume and contractility of affected EOMs were noted in six patients; however, there was no marked EOMs atrophy in two cases of diabetic oculomotor palsy, and there were four cases of aberrant regeneration. hrMRI demonstrated the oculomotor nerve at the midbrain and at EOMs in all cases, and in two cases with previous normal neuroimaging elsewhere that demonstrated contrast-enhancing tumors on the oculomotor nerve. One patient with apparently unilateral congenital inferior division oculomotor palsy had no detectable ipsilateral and a hypoplastic contralateral oculomotor nerve exiting the midbrain. hrMRI provides valuable information in patients with oculomotor palsy, such as structural abnormalities of the orbit and oculomotor nerve, and atrophy and diminished contractility of innervated EOMs. This information could be helpful in diagnosis and management of oculomotor palsy.