Enhancement of the Third Cranial Nerve due to Microvascular Ischemia: Case Report

Third nerve palsy (3NP) commonly results from a microvascular ischemic insult. Typically, computed tomography or magnetic resonance angiography is performed to rule out a posterior communicating artery aneurysm. If this is normal and the pupil is spared, patients are often observed with the expectation of spontaneous improvement within 3 months. Oculomotor nerve enhancement on MRI with contrast in the context of microvascular 3NP is not well recognized. Here, we report third nerve enhancement in a case of a 67-year-old woman with diabetes and other vascular risk factors who presented with left eye ptosis and a limitation of extraocular eye movements consistent with 3NP. She underwent an extensive inflammatory workup that was negative and the diagnosis of a microvascular 3NP was made. A spontaneous recovery was achieved within 3 months, and she did not receive any treatment. She remained clinically well, although increased T2 signal in the oculomotor nerve persisted after 10 months. While the exact mechanism remains unknown, it is likely that microvascular ischemic insults lead to intrinsic changes of the third nerve that may result in enhancement and persistent T2 signal. Additional workup for inflammatory causes of 3NP may not be required when enhancement of the oculomotor nerve is seen in the right clinical context. Further study is required to understand why enhancement is a rarely reported finding in patients with microvascular ischemic 3NP.