Intracranial Aneurysms

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Last updated: November 8, 2020
Revisions: 3

Last updated: November 8, 2020
Revisions: 3

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An aneurysm is defined as an abnormal dilatation of a blood vessel by more than 50% of its normal diameter. Aneurysms occur intracranially, with an approximate prevalence of intracranial aneurysms in adult populations around 3%.

Aneurysms often form at sites where blood vessels bifurcate or merge, and as such the majority form around the Circle of Willis (Fig. 1).

Whilst some caused by hereditary weakness in the arterial walls, other causes include hypertension or smoking (causing defects in the tunica media), trauma, or connective tissue diseases. The main concern regarding intracranial aneurysms is their risk of rupture.

Approximately 90% of all intracranial aneurysms are defined as saccular aneurysms, with the remainder being fusiform. They can be classified by size, with <10mm = small, 10-24mm = large, >24mm = giant.

Figure 1 – Circle of Willis

Risk Factors

Main risk factors include female gender, family history*, hypertension, and smoking

Autosomal dominant polycystic kidney disease (ADPKD) and connective tissue disorders (e.g. Marfans) are at increased risk of developing intracranial aneurysms.

*Patients are approximately 30% more likely to develop an intracranial aneurysm if a first-degree relative also has one

Figure 2 – Histology of an intracranial aneurysm, using an EVG stain

Risk Factors for Rupture

Risk factors for aneurysmal rupture include location and size (Table 1). The International Study of Unruptured Intracranial Aneurysms (ISUIA) revealed that the aneurysms with increasing size arising from the posterior circulation have a higher 5-year cumulative risk of rupture.


< 7mm

7 – 12mm 13 – 24mm

> 24mm

Cavernous carotid


0% 3.0% 6.4%
Anterior circulation


2.6% 14.5%


Posterior circulation

2.5% 14.5% 18.4%


Table 1 – Five year cumulative rupture rates of unruptured aneurysm, according to location and size

Clinical Features

Most small unruptured aneurysms produce no symptoms at all, typically being found incidentally found on MRI.

Unruptured symptomatic aneurysms can result in headaches or nausea, or in rarer cases resulting in focal neurological deficits, seizures, or isolated cranial nerve palsies*

Ruptured aneurysms will present with intracranial haemorrhage, most commonly as a subarachnoid haemorrhage (SAH)

*A well-described palsy in this scenario is a CN III palsy secondary to a posterior communicating artery aneurysm causing local compression


The mainstay of diagnosis of intracranial aneurysms is with a CT angiogram (CTA), allowing visualisation of the aneurysm and identifying its location. Additional imaging via a Magnetic Resonance Angiography (MRA) may be useful for operative planning.

Patients who present with clinical features of a SAH will typically a non-contrast CT head performed initially, prior to any further imaging performed if a ruptured aneurysm is suspected.


The management depends on patient factors (co-morbidities, functional status) and aneurysm factors (size, type, and position). All patients should have any modifiable risk factors optimised, most commonly via improved blood pressure control and smoking cessation.

The majority of aneurysms will be monitored only (at least initially), with regular interval imaging, assessing for any progression in size. Indeed, many incidental aneurysms will not grow and patients only ever require routine follow-up.

Surgical Management

For those that are requiring surgical intervention, the mainstay of treatment is either surgical clipping or endovascular coiling (Fig. 3).

  • Surgical clipping is seen more suitable in aneurysms that have branching arteries or are wide-necked, and often has lower reported rates or recurrence and rebleeding
  • Endovascular coiling is usually more favourable in posterior circulation aneurysms or co-morbid or older patients

Figure 3 – An anterior communicating artery aneurysm before and after coiling


Patients with unruptured aneurysms of favourable size and location carry less than 1% risk of rupture per year.

For patients with aneurysmal rupture, around half of all patients will even reach a hospital on time, and of those, around a half will die. Of those that survive, a half will develop significant neurological impairment.

Key Points

  • Intracranial aneurysms prevalence in adult populations is around 3%
  • Main risk factors include female gender, family history, hypertension, and smoking
  • Most are found incidentally and are best characterised on CT angiogram or MR angiogram
  • For those that are requiring surgical intervention, the mainstay of treatment is either surgical clipping or endovascular coiling
  • Ruptured intracranial aneurysms have poor outcomes