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In medicine, intracranial aneurysms are "abnormal outpouching in the wall of intracranial blood vessels. Most common are the saccular (berry) aneurysms located at branch points in Circle of Willis at the base of the brain. Vessel rupture results in subarachnoid hemorrhage or intracranial hemorrhages. Giant aneurysms (>2.5 cm in diameter) may compress adjacent structures, including the oculomotor nerve. (From Adams et al., Principles of Neurology, 6th ed, p841)[1][2]
Almost 2% of adults may have intracranial aneurysms by magnetic resonance imaging.[3] Most are less than 7 mm in size and located in the anterior circulation.
The likelihood of rupture increases if a history of prior hemorrhage, larger size of the aneurysm, and location in posterior circulation.[4]