Oculomotor nerve palsy in the setting of an anterior cerebral A2 segment aneurysm

A case is presented which highlights a rare cause of oculomotor nerve palsy (third nerve palsy) in the setting of subarachnoid hemorrhage secondary to an A2 segment anterior cerebral aneurysm. A third nerve palsy is most often associated with posterior communicating artery aneurysms which are explained by the anatomic proximity of nerve and artery. Third nerve palsies from remote aneurysms however are more difficult to understand.

A patient presented to the emergency department with severe headache, dizziness, nausea and vomiting. Her examination was remarkable only for a partial left third nerve palsy manifest as a non-pupil sparing mild ptosis. A CT scan and digital subtraction cerebral angiography revealed subarachnoid hemorrhage secondary to a small ruptured aneurysm at the A2 segment of the anterior cerebral artery. The aneurysm was deemed treatable by endovascular coil embolization and the patient underwent successful placement of a detachable helical coil. At 1 month follow-up, the patient had no complaints and showed complete resolution of all oculomotor symptoms.

While oculomotor nerve palsy is an incredibly rare sequelae of anterior cerebral artery aneurysm rupture, it is important that clinicians and researchers continue to report and study such cases. It has been hypothesized that mass effect, hemotoxicity and ischemia are all possible causes of third nerve injury in remote aneurysms, as in this case.