A Recurrent Large Posterior Cerebral Artery Aneurysm Successfully Treated with Parent Artery Occlusion Using Somatosensory-Evoked Potential: A Case Report.

Objective: Treatment of large posterior cerebral artery (PCA) aneurysm involving the P1-P2 segment is difficult by both neurosurgery and endovascular treatment. Balloon occlusion test (BOT) to identify precise peripheral collateral flow is difficult prior to parent artery occlusion (PAO). Besides, PAO at the aneurysm at this location can cause peripheral cortical infarction of the occipital and temporal lobes and/or perforator infarction involving the midbrain and thalamus perfused by the perforating artery arising from the P1-P2 segment. However, detection of the perforator during PAO is difficult. Case Presentation: The patient was a 49-year-old woman. At the age of 43 years, a right large PCA aneurysm was discovered in the right P1-P2 segment. A simple technique coiling was performed. As recurrence was identified 1 year later, embolization was performed using a same procedure. Since further recurrences were later found, a third round of treatment was planned. Somatosensory-evoked potential (SEP) was recorded as intraoperative electrophysiological monitoring. Tortuosity of the right PCA was observed at the aneurysm neck and the distal right PCA could not be secured. We could neither perform stent-assisted coil embolization nor BOT in the right PCA. Hence, we inflated the balloon in the basilar artery and checked the collateral circulation routes retrograde into the right PCA from the right middle cerebral artery via a leptomeningeal anastomosis. PAO was performed on the right P1-P2 segment at the aneurysm neck. The signal of the SEP was not decreased, and the aneurysm was not visualized. Another coil was added to strengthen the PAO to the right P1 segment, which decreased the SEP amplitude in the extremities by 3 minutes after. As the last coil was thought to be occluding the perforator branching from the right P1 segment, it was removed without detaching. The SEP amplitude began to improve and recovered by 9 minutes after. There was no postoperative deficit. No recurrence of aneurysm was observed on MRA 9 months postoperatively. Conclusion: During PAO at the P1 segment of large PCA aneurysm involving the P1-P2 segment, SEP may be helpful to prevent perforator infarction, even if perforating artery originating from the proximal portion of the aneurysm was not detected by angiography.