Although UIAs are common, with an estimated prevalence in the general population of 2%,65 their average annual rupture rate is considered to be relatively low, ranging from 0.3% to 0.7%.66,67 However, the risk of rupture is heavily influenced by aneurysm size, location, and history of prior SAH from a separate but successfully treated aneurysm. In the retrospective arm of the International Study of Unruptured Intracranial Aneurysms (ISUIA), UIAs that were < 10 mm in diameter had a rupture rate of 0.05% per year, but this rate increased to 0.5% for patients who had a history of SAH from a previously treated aneurysm. For aneurysms > 10 mm, the rupture rate was near 1% per year irrespective of SAH history, but for giant aneurysms (> 25 mm), the rate was 6% in the first year.66
More recently, the prospective arm of the ISUIA, which followed 1692 patients, reported a cumulative rupture rate of 0%, 2.6%, 14.5%, and 40.0% over 5 years for anterior circulation aneurysms that were < 7 mm, 7 to 12 mm, 13 to 24 mm, and ≤ 25 mm in diameter, respectively. For posterior circulation aneurysms, the rates were 2.5%, 14.0%, 18.4%, and 50.0% over the same 5-year period (Table 21-2).67 In summary, larger aneurysms and those located in the vertebrobasilar circulation or the posterior communicating artery are at significantly higher risk of rupture than other aneurysms. Small aneurysms (< 7 mm in the ISUIA) have a very low risk of rupture 5 years from diagnosis unless the patient has a history of SAH.67 Other comorbidities, such as history of hypertension, abuse of alcohol or tobacco, and family history of aneurysms and SAH, should be taken into account as they may increase the risk of rupture.68