The annual incidence of aneurysmal rupture in patients with known aneurysms has previously been accepted as 1-2.3%. However, the recently published results of the International Study of Unruptured Intracranial Aneurysms (ISUIA) have shown a considerably lower annual incidence of rupture in certain situations . ISUIA followed 1,449 patients with 1,937 aneurysms. In the patients who had no history of SAH, the cumulative rate of rupture of aneurysms that were less than 10 mm in diameter at diagnosis was less than 0.05% per year, whilst in those who had suffered a SAH from a different aneurysm that had been repaired successfully, the rate was approximately 11 times higher (0.5% per year). The rupture rate of aneurysms that were 10 mm or more in diameter was less than 1% per year in both groups, except in patients with giant aneurysms (greater than 25 mm in diameter) who had not suffered a SAH, when the rupture rate was 6% in the first year. Other authors have confirmed that aneurysmal size is a major predictive factor for the risk of future rupture, although many aneurysms are documented as being considerably smaller than 10 mm when they rupture.
The ISUIA study also found that location was important. Among patients with no previous history of SAH, aneurysms situated at the basilar tip, vertebrobasilar, posterior cerebral or posterior communicating (PCOM) arteries were more likely to rupture, whilst in patients who had suffered an earlier SAH from a different aneurysm that had been repaired successfully, only the basilar tip location was predictive of rupture . SAH is rare in cavernous carotid aneurysms: if they rupture, they usually cause carotico-cavernous fistulae (CCF). Most authors would recommend treatment only of those projecting into the subarachnoid space and of those symptomatic with progressive ophthalmoplegia, facial pain or progressive visual loss. Hemorrhage risk is greater in symptomatic patients who represent one-third of all UIA patients but account for nearly three-quarters of those that bleed during observation. Multiple aneurysms increase long-term risk of rupture, with a 6.8% annual risk vs 1.9% for single aneurysms. This may be because the affected individuals are more susceptible to both aneurysmal formation and rupture.
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