Internal Carotid Artery Aneurysm Anomalously Originating from the Posterior Communicating Artery

BACKGROUND: We report a case of an internal carotid artery (ICA) aneurysm anomalously originating from the posterior communicating artery (PComA). CASE DESCRIPTION: Preoperative radiologic findings revealed a paraclinoid carotid artery aneurysm at the level of the distal dural ring. Because of the low rupture risk, there are no treatment indications for small paraclinoid or ICA-superior hypophyseal artery unruptured aneurysms. In this case, because of the patient's age and the irregular shape of the aneurysm, treatment was considered necessary. Intraoperative findings using microscopic and endoscopic angiography showed that the aneurysm originated from the proximal branched PComA. The PComA originated from the same level as the ophthalmic artery, and the superior hypophyseal artery originated from the distal side of the PComA. CONCLUSIONS: According to numerous reports, small incidental paraclinoid aneurysms have a lower risk of rupture and growth than PComA aneurysms. Preoperative radiologic examination of unruptured small aneurysms was performed using magnetic resonance angiography and three-dimensional computed tomography angiography. Because the ability of magnetic resonance angiography and three-dimensional computed tomography angiography to detect small branches of the ICA is inferior to digital subtraction angiography, the location of an unruptured ICA aneurysm should be evaluated with other diagnostic modalities besides magnetic resonance angiography and three-dimensional computed tomography angiography.