Cerebral aneurysms typically arise in the circle of Willis at vascular bifurcation points where hemodynamic stress is maximal. Ninety percent of aneurysms occur in the anterior circulation, and only 10% occur in the basilar system. Although cerebral aneurysms, if large enough, can manifest with symptoms of neural compression, the greatest concern is the occurrence of rupture and subarachnoid hemorrhage.

Intracranial aneurysms (ICAs) are responsible for 75% to 80% of episodes of subarachnoid hemorrhage (SAH), which has an incidence of 10-20/100,000, and is associated with high morbidity and mortality. One-third of patients will die from their initial bleed, with another third having severe disability or delayed death. Only the remaining third will have minimal morbidity and an acceptable outcome.

The prevalence of undiagnosed, asymptomatic aneurysms is estimated at 4%; however, surgical clipping only confers significant outcomes benefit when aneurysm size exceeds 10 mm. Timing of surgical intervention becomes more critical after a hemorrhage because the initial 72 hours presents a window for operative management, after which surgery is delayed 10 to 14 days until the risk of vasospasm has decreased. When surgery is indicated, the anesthesiologist must realize that anesthetic considerations for SAH and ICA are unique from those of routine craniotomy.

The primary concern of an anesthesiologist during surgery for aneurysm clipping is the prevention of rupture. ICA rupture at the time of induction has a mortality exceeding 75%. The likelihood of rupture is based on aneurysm size, wall strength, history of prior rupture, and transmural pressure. Transmural pressure is

CPP = MAP − ICP

where CPP is cerebral perfusion pressure and MAP is mean arterial pressure. Although mathematically equal to CPP, the concept of transmural pressure and its concern with aneurysmal wall stress is unique to aneurysms. The periods commonly associated with intraoperative rupture are at induction, dura and arachnoid exposure, hematoma evacuation, and during dissection to the aneurysm. Rupture during dissection, or at the time of aneurysm clipping,
carries with it much lower morbidity because it is usually rapidly controlled. Anesthetic priorities after a rupture are to maintain cerebral perfusion. Controlled hypotension in an attempt to reduce bleeding is detrimental but may be necessary under emergent conditions (e.g., aneurysmal rupture) to enable the neurosurgeon to clip the feeding vessel or aneurysm itself.