[Unbranched-site aneurysm of intracranial internal carotid artery]

Most intracranial saccular aneurysms arise from the angle between a parent vessel and an arterial branch. Saccular aneurysms without relation to any arterial branch are rare. During the last 24 years, 210 patients with intracranial internal carotid artery (ICA) aneurysms were directly operated on at our institution. We found eight saccular aneurysms arising from intracranial ICA without relation to any arterial branch. So we call them unbranched-site aneurysms of intracranial ICA, and describe their unique characteristics. Of the eight cases with unbranched-site aneurysm of intracranial ICA, three were men and five were women. The age ranged from 31 to 61 years with an average age of 48. Seven cases suffered a subarachnoid hemorrhage due to ruptured unbranched-site aneurysm, and the other case had been admitted to our department because of accompanying ruptured left internal carotid-posterior communicating aneurysm. All cases of unbranched-site aneurysm presented intraoperative and/or angiographic findings of arteriosclerosis, and five of the patients had a past history of hypertension. Of the four unbranched-site aneurysms at the C2 portion of ICA, two arose from the lateral wall, one arose from the superior wall and the other arose from the infero-lateral wall of ICA. Of the four unbranched-site aneurysms at the C1 portion of ICA, three arose from the superior wall and the other arose from the supero-medial wall of ICA. Radical operation was performed in all cases and intraoperative rupture occurred in two cases. It was suggested that arteriosclerotic change in the arterial wall, and local hemodynamic stress played important roles in the development of unbranched-site aneurysms of intracranial ICA.