Intracranial Aneurysm

An intracranial aneurysm is a focal outpouching or ballooning of the wall of a brain artery, most commonly arising at branch points of the arteries at the base of the brain. Most are saccular ('berry') aneurysms and remain asymptomatic, but rupture causes subarachnoid hemorrhage, a serious form of stroke. Aneurysm management centers on estimating rupture risk and weighing it against the risk of treatment.

Definition

An intracranial aneurysm is a localized, abnormal dilation of an intracranial arterial wall, typically saccular and arising at arterial bifurcations of the circle of Willis, which may rupture and cause subarachnoid hemorrhage.

Scope

This entry defines intracranial aneurysms, describes their morphology and natural history, and introduces the concepts used to characterize rupture risk and the two main definitive treatments — microsurgical clipping and endovascular coiling. It is a reference and educational overview and does not provide diagnostic or treatment recommendations for any individual.

Core questions

• What distinguishes a saccular from a fusiform or mycotic aneurysm?
• How is the rupture risk of an unruptured aneurysm estimated?
• How do microsurgical clipping and endovascular coiling differ in approach and outcome?
• What is the natural history of an incidentally discovered unruptured aneurysm?

Key concepts

• Saccular (berry) aneurysm
• Fusiform and mycotic aneurysms
• Arterial bifurcation and hemodynamic stress
• Rupture risk and the PHASES score
• Microsurgical clipping
• Endovascular coiling and flow diversion
• Aneurysm size, location, and morphology

Mechanisms

Saccular aneurysms develop where hemodynamic stress concentrates at arterial branch points, contributing to degeneration of the internal elastic lamina and media of the vessel wall and progressive outpouching. As an aneurysm enlarges, wall tension and morphologic irregularity increase, and rupture releases arterial blood into the subarachnoid space. Rupture-risk estimation integrates factors such as aneurysm size, location, and patient characteristics; the PHASES score (Greving et al., 2014) operationalizes several of these predictors from pooled cohort data. Definitive treatment aims to exclude the aneurysm from the circulation, either by placing a clip across its neck microsurgically or by filling its sac endovascularly with coils, sometimes assisted by stents or flow-diverting devices.

Clinical relevance

Understanding aneurysm morphology, rupture-risk concepts, and treatment options supports critical reading of the neurovascular literature. This entry describes how rupture risk and treatment are framed; it is not a basis for deciding whether a particular aneurysm should be treated, which depends on individual factors and current guidelines and is determined by the treating team.

Epidemiology

Unruptured intracranial aneurysms are present in a few percent of the adult population, and the great majority never rupture. Rupture risk varies substantially with aneurysm size and location, as characterized by natural-history cohorts such as the International Study of Unruptured Intracranial Aneurysms (Wiebers et al., 2003). When rupture occurs it causes aneurysmal subarachnoid hemorrhage, which carries high early mortality and morbidity.

Evidence & guidelines

Key evidence includes the International Study of Unruptured Intracranial Aneurysms on natural history and treatment risk (Wiebers et al., 2003), the International Subarachnoid Aneurysm Trial comparing clipping and coiling for ruptured aneurysms (Molyneux et al., 2005), and the PHASES pooled rupture-risk model (Greving et al., 2014). Management of ruptured aneurysms is addressed in AHA/ASA guidance for aneurysmal subarachnoid hemorrhage (Hoh et al., 2023).

History

Microsurgical clipping of intracranial aneurysms became established in the mid-twentieth century and was refined with the operating microscope. The natural history of unruptured aneurysms was clarified by large prospective cohorts beginning with the International Study of Unruptured Intracranial Aneurysms (Wiebers et al., 2003). The International Subarachnoid Aneurysm Trial (Molyneux et al., 2005) then reshaped practice by demonstrating outcomes with endovascular coiling versus clipping for ruptured aneurysms, and rupture-risk modelling matured with the PHASES score (Greving et al., 2014).

Debates

Whether and when to treat small unruptured aneurysms

Because most small unruptured aneurysms never rupture while treatment carries its own risks, the threshold for intervening versus observing remains a central judgement informed by natural-history and rupture-risk evidence.

Clipping versus endovascular treatment

Randomized evidence in ruptured aneurysms informed a shift toward endovascular coiling in suitable cases, but durability, retreatment, and lesion-specific anatomy keep the choice between approaches an active consideration.

Key figures

• Andrew J. Molyneux
• David O. Wiebers
• Robert F. Spetzler
• Charles G. Drake

Seminal works

• wiebers-2003
• molyneux-2005
• greving-2014

Frequently asked questions

Do all intracranial aneurysms rupture?

No. Most unruptured intracranial aneurysms never rupture; rupture risk depends on factors such as size, location, and patient characteristics, and many small aneurysms are managed by observation.

What is the difference between clipping and coiling?

Clipping is an open microsurgical procedure that places a clip across the aneurysm neck, while coiling is an endovascular procedure that fills the aneurysm sac with coils through a catheter. The appropriate choice depends on the aneurysm and patient and is decided by the treating team.