CLINICAL ANALYSIS OF INCIDENTALLY DISCOVERED UNRUPTURED ANEURYSMS

Background and Purpose: We analyzed the risk factors for rupture of an intracranial aneurysm based on a retrospective angiographic study of ruptured and unruptured aneurysms. Methods: The 44 cases of asymptomat ic aneurysms were selected from 1612 patients whose lesions had been d iscovered fortuitously by angiography (2.7%) during the period from 19 80 to 1989. All these patients were free from any sign of intracranial aneurysm. The variations in age, sex, and location of the aneurysms w ere analyzed compared with 638 ruptured aneurysms that had been treate d in our institute during the same period. The size, shape, and arteri al geometry of the unruptured aneurysms were examined angiographically . Results: 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%). H owever, they were frequently found in the internal carotid artery, wit h an incidence of 10.6% in the cavernous portion of the internal carot id artery, 19.1% in the internal cartoid-ophthalmic artery, 19.1% in t he internal carotid-posterior communicating artery, and 12.8% in the i nternal 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 u nruptured aneurysms was 4.8 mm, and 80% were smaller than 6 mm. Conclu sions: Intracranial aneurysms are formed not only at the bifurcation o f an artery but also at its branching and bending points. However, an aneurysm located at the bifurcation, such as the anterior communicatin g artery and the middle cerebral artery, bleeds easily in contrast wit h lateral aneurysms such as those found at the branching and bending p oints on the internal carotid artery.