The incidence of intracranial aneurysms varies depending on the source from which the data are gleaned. Large autopsy series indicate that approximately 18% of adults harbor intracranial aneurysms. Large proportions of these aneurysms are very small and therefore the prevalence based on autopsy is higher than that detected radiographically. Radiological series derived from cerebral angiographic studies reveal that between 0.5 and 3% of the population is afflicted. Clinical series have confirmed an annual incidence (per 100,000 population) of 6-19. Collectively, these data demonstrate that between 1 and 12 million Americans have intracranial aneurysms, and that approximately 30,000 persons per year suffer SAH.

Aneurysmal SAH has enormous import to society. Aneurysmal rupture is responsible for 0.4-0.6% of all deaths, and despite an overall decrease in stroke rate, the incidence of aneurysmal SAH remains unaltered. Aneurysmal SAH is associated with a mortality rate of more than 25% and a significant morbidity rate exceeding 50%. Furthermore, it is estimated that approximately $1.75 billion and $522 million are spent annually on the care of patients with ruptured and unruptured aneurysms, respectively, in the United States.

SAH from aneurysm rupture most frequently occurs between the ages of 40 and 60. Aneurysms are rarely found in children, which lends credence to the assertion that most aneurysms arise from hemody-

Table V

Intracranial Aneurysms by Location Location Incidence (%)

Anterior circulation

Internal carotid artery 30

Petrous Cavernous

Paraclinoid (ophthalmic) Posterior communicating Anterior choroidal Carotid bifurcation Anterior cerebral artery 39

Precommunicating (A1 segment) Communicating artery segment Postcommunicating segment Middle cerebral artery 22

Sphenoidal (lenticulostriate) segment Bifurcation segment Distal segment Posterior circulation (vertebrobasilar) Vertebral artery 8

Main trunk

Posterior Inferior Cerebellar Artery Basilar artery Anterior Inferior Cerebellar Artery Trunk

Superior Cerebellar Artery Basilar bifurcation Posterior Cerebral Artery Precommunicating segment Distal (postcommunicating) segment namic vascular injury. Additionally, studies consistently reveal a female preponderance (54-62%).

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