Clinical analysis of incidentally discovered unruptured aneurysms.

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Abstract
We analyzed the risk factors for rupture of an intracranial aneurysm based on a retrospective angiographic study of ruptured and unruptured aneurysms. The 44 cases of asymptomatic aneurysms were selected from 1612 patients whose lesions had been discovered fortuitously by angiography (2.7%) during the period from 1980 to 1989. All these patients were free from any sign of intracranial aneurysm. The variations in age, sex, and location of the aneurysms were analyzed compared with 638 ruptured aneurysms that had been treated in our institute during the same period. The size, shape, and arterial geometry of the unruptured aneurysms were examined angiographically. Unruptured aneurysm was discovered fortuitously in 44 (2.7%) of 1612 patients, with greater incidence in women aged older than 60 years. Unruptured aneurysms were less likely to occur in the anterior communicating artery (12.8%) and the middle cerebral artery (6.4%). However, they were frequently found in the internal carotid artery, with an incidence of 10.6% in cavernous portion of the internal carotid artery, 19.1% in the internal carotid-ophthalmic artery, 19.1% in the internal carotid-posterior communicating artery, and 12.8% in the internal carotid-anterior choroidal artery. Seven of the nine internal carotid-posterior communicating artery aneurysms showed a hypoplastic or aplastic posterior communicating artery. The mean diameter of the unruptured aneurysms was 4.8 mm, and 80% were smaller than 6 mm. Intracranial aneurysms are formed not only at the bifurcation of an artery but also at its branching and bending points. However, an aneurysm located at the bifurcation, such as the anterior communicating artery and the middle cerebral artery, bleeds easily in contrast with lateral aneurysms such as those found at the branching and bending points on the internal carotid artery.