What is intracranial aneurysm? It is a localized bulge in the wall of a blood vessel in the brain, similar to a large bulge in an aging tire, which ruptures under the impact of blood flow, resulting in a subarachnoid hemorrhage and causing symptoms such as severe headache, vomiting and neck stiffness. The arrows in the picture below point to small bubbles on the aneurysm that have ruptured or are about to rupture. This condition requires intervention or surgery as soon as possible or it can cause fatal intracranial hemorrhage.