Cerebral Aneurysm

Overview

Cerebral (Intracranial) aneurysm is a focal dilation of an intracranial artery due to weakness of the arterial wall. It occurs in roughly 1-5% of adults and can lead to permanent disability. The most common type is a saccular “berry” aneurysm. Most common location is anterior communicating artery. 

Cerebral aneurysms are typically asymptomatic until rupture, which leads to subarachnoid haemorrhage and presents with “thunderclap headache”.¹

Anatomy

Circle of Willis

Arterial ring located in the subarachnoid space and supplies majority of the brain

It connects the internal carotid arteries and the vertebral arteries

Vertebral arteries

• Arise from subclavian arteries
• Travel in vertebral foramina C1-C6
• Give the Posterior inferior cerebellar artery
• Combine to form basilar artery

Basilar artery

• Sits on the pons
• Branches:
  • Pontine arteries
  • Anterior inferior cerebellar artery
  • Superior cerebellar artery
  • Posterior cerebral artery

Internal carotids

• Arise from common carotid artery
• Travel in carotid sheath
• Branches:
  • Ophthalmic artery
  • Posterior communicating artery
  • Middle cerebral artery
  • Anterior cerebral artery

Communicating arteries

• Anterior communicating – connects the two Anterior Cerebral
  • Most common location of Cerebral aneurysm
• Posterior communicating – connects Internal carotid to Posterior Cerebral
  • Second most common location

Classification

Morphology

• Saccular (berry): round pouch attached to the artery via a neck – most common
• Fusiform: symmetrical stretching of the artery
• Dissecting (pseudo-aneurysm): blood accumulates between the layers of the artery
• Mycotic: caused by an infection

Diameter

• Small: <7 mm
• Medium: between 7 and 12 mm
• Large: between 13 and 24 mm
• Very large: 25 mm or more^{1 2}

Aetiology and Risk factors

Aetiology

Most intracranial aneurysms are acquired saccular (“berry”) aneurysms arising at arterial bifurcations in the Circle of Willis. They develop through:

• Haemodynamic shear stress at branch points
• Degeneration of the internal elastic lamina and tunica media
• Vascular inflammation, smooth-muscle-cell loss and extracellular-matrix breakdown
• Interaction between genetic susceptibility and acquired vascular injury

Risk factors for aneurysm formation

• Cigarette smoking—strongest modifiable association
• Uncontrolled hypertension
• Cocaine, amphetamine or other sympathomimetic use

• First-degree family history, especially ≥2 affected relatives
• Increasing age
• Previous intracranial aneurysm or aneurysmal subarachnoid haemorrhage
• Associated inherited/vascular disorders

• Autosomal dominant polycystic kidney disease 
• Genetic connective tissue diseases: Vascular Ehlers–Danlos syndrome, Loeys–Dietz syndrome, Marfan syndrome
• Fibromuscular dysplasia

Features increasing rupture risk (not formation)

• Aneurysm growth on serial imaging
• Large aneurysm size
• Posterior circulation, anterior communicating artery or posterior communicating artery location
• Irregular shape or daughter sac
• Previous SAH from another aneurysm
• Ongoing smoking and uncontrolled hypertension

Clinical Manifestation

Unruptured cerebral aneurysm

• Most are asymptomatic
• Potential symptoms – mainly due to compression:
  • Visual acuity loss
  • Oculomotor nerve palsy with pupil involvement
  • Facial pain³

Ruptured cerebral aneurysm

• Sudden thunderclap headache
  • “Worst headache of my life”
  • Maximal intensity within seconds to minutes
• Nausea and vomiting
• Photophobia
• Neck pain
• Brief loss of consciousness, collapse or syncope
• Confusion or reduced consciousness
• Seizure
• Focal neurological symptoms, such as weakness, numbness or speech disturbance
• Possible preceding sentinel headache from a minor leak
• May occur during exertion, coughing, straining or sexual activity

Clinical examination

• Reduced GCS
• Meningism: Kernig’s sign and Brudzinski’s sign
• Cranial nerve palsy
  • Especially a painful, pupil-involving third-nerve palsy
• Subhyaloid or retinal haemorrhage — Terson syndrome
• Hypertension and autonomic disturbance
• Raised ICP
  • Papilloedema on fundoscopy
  • Cushing’s triad: hypertension, bradycardia, irregular respiration

High-yield pattern: thunderclap headache with vomiting and meningism, with or without collapse, reduced consciousness, seizure or focal neurological deficits.

Diagnosis

Investigations

CT brain – first line

• Shows blood within the subarachnoid space
• Sensitivity decreases with time, meaning a delayed negative CT does not exclude haemorrhage

Lumbar puncture – required to exclude bleed for most patients with normal CT findings

• Elevated opening pressure
• Elevated RBC count that does not diminish between tubes
• Xanthochromia – represents haemoglobin degradation (pink or yellow tint)

Alternative imaging

• CT angiography
• MR angiography⁴

Differential diagnosis

• Cerebral infection (e.g. meningitis)
• Cerebral venous thrombosis
• Intracerebral haemorrhage
• Ischaemic stroke
• Acute hypertensive crisis
• Cervical artery dissection⁴

Treatment

Unruptured aneurysm – either observe or operate; decision is made on case-by-case basis

• Conservative management
  • Smoking cessation
  • BP control
  • Surveillance imaging

Ruptured aneurysm

• Surgery
  • Microsurgical clipping
• Endovascular
  • Coiling
  • Flow diversion (stenting)
• Prevention of vasospasm
  • Calcium channel blocker
• Blood pressure control
  • Treat severe hypertension while avoiding hypotension
  • Reduce rebleeding risk 
• Ventricular shunt
  • Used for acute hydrocephalus

Screening

• Considered in high risk groups
  • Two or more first-degree relatives with intravenous aneurysm or aneurysmal SAH
  • Autosomal dominant polycystic kidney disease with additional risk factors or family history
  • Previous aneurysmal SAH with risk of additional aneurysms
• Either done with CTA or MRA^{1 5}

Complications of ruptured aneurysms

Rupture of cerebral aneurysm leads to subarachnoid haemorrhage

• Vasospasm
• Hyponatremia
• Recurrence
• Rebleeding
• Seizures
• Hydrocephalus¹

References

1. Eby B. Cerebral aneurysm. In: BMJ Best Practice [Internet]. 2023 [cited 2026 Jun 5]. Available from: BMJ Best Practice
2. National Institute of Neurological Disorders and Stroke. Cerebral aneurysms [Internet]. Bethesda (MD): National Institute of Neurological Disorders and Stroke; [cited 2026 Jun 5]. Available from: https://www.ninds.nih.gov/health-information/disorders/cerebral-aneurysms
3. Singer RJ, Ogilvy CS, Rordorf G. Unruptured intracranial aneurysms. In: Goddeau RP, editor. UpToDate [Internet]. [updated 2026 Feb 13; cited 2026 Jun 5]. Available from: UpToDate
4. Singer RJ, Ogilvy CS, Rordorf G. Unruptured intracranial aneurysms. In: Goddeau RP, editor. UpToDate [Internet]. [updated 2026 Mar 6; cited 2026 Jun 5]. Available from: UpToDate
5. Singer RJ, Ogilvy CS, Rordorf G. Treatment of cerebral aneurysms. In: Goddeau RP, editor. UpToDate [Internet]. [updated 2025 Jan 3; cited 2026 Jun 5]. Available from: UpToDate