Pupil-sparing third nerve palsy with preoperative improvement from a posterior communicating artery aneurysm.

Background: Despite the plenitude of reports concerning partial or complete third nerve palsies, especially as presenting symptoms with posterior communicating artery (PCoA) aneurysms, we present a patient with an unusual variation.

Case Description: A 66-year-old woman presented with progressive right-sided headaches and diplopia and was found to have a partial, pupil-sparing third nerve palsy. A small right-sided PCoA aneurysm, nearly indistinguishable from an infundibulum, was identified on magnetic resonance angiography and subsequent digital subtraction angiography. The third nerve palsy improved before surgical repair of the aneurysm.

Results: Microsurgical exploration revealed a small PCoA aneurysm, which was tethered to the third nerve by arachnoid adhesions. Adhesions were lysed and the aneurysm was repaired sparing the PCoA and its branches. The patient's third nerve function recovered completely postoperatively.

Conclusions: Even a very small PCoA aneurysm may present with an improving, pupil-sparing partial third nerve palsy. Selection of patients for imaging studies should take this unusual variant into consideration. We describe the anatomy and potential mechanisms of this pupil-sparing third nerve palsy.