Neurosurgery for Cerebral Aneurysm

Updated: Oct 16, 2023
  • Author: Jeffrey E Florman, MD; Chief Editor: Brian H Kopell, MD  more...
  • Print


An aneurysm is an abnormal local dilatation in the wall of a blood vessel.

A false aneurysm (pseudoaneurysm) is a cavity lined by blood clot, such as is seen after trauma.

True aneurysms are composed of dilatations of all three layers (intima, media, and adventitia) of the walls of the arteries lining the cavity and include the following types:

  • Saccular aneurysms - Degenerative or developmental; traumatic; mycotic; oncotic; flow-related; vasculopathy-related; and drug-related
  • Fusiform aneurysms
  • Dissecting aneurysms

Common causes for aneurysms include the following:

  • Hemodynamically induced or degenerative vascular injury
  • Atherosclerosis (typically leading to fusiform aneurysms)
  • Underlying vasculopathy (eg, fibromuscular dysplasia [FMD])
  • High-flow states (as in  arteriovenous malformation [AVM] and fistula)

Uncommon causes for aneuryms include the following:

  • Trauma
  • Infection
  • Drugs
  • Neoplasms (primary or metastatic)

This article reviews the types, pathology, clinical picture, and management of intracranial aneurysms. For patient education resources, see the Headache Center, as well as Aneurysm, Brain.


Saccular Aneurysms: Degenerative or Developmental


Saccular aneurysms are rounded berrylike outpouchings that arise from arterial bifurcation points, most commonly in the circle of Willis (see the image below). These are true aneurysms—that is, they are dilatations of a vascular lumen caused by weakness of all vessel-wall layers.

White arrow on black card marks site of ruptured b White arrow on black card marks site of ruptured berry aneurysm in circle of Willis (major cause of subarachnoid hemorrhage).

A normal artery wall consists of the following three layers:

  • Intima, which is the innermost endothelial layer
  • Media, which consists of smooth muscle that can contract and expand
  • Adventitia, which is the outermost layer, consisting of connective tissue

The aneurysmal sac itself is usually composed of only intima and adventitia. The intima is typically normal, though subintimal cellular proliferation is common. The internal elastic membrane is reduced or absent, and the media ends at the junction of the aneurysm neck with the parent vessel. Lymphocytes and phagocytes may infiltrate the adventitia. The lumen of the aneurysmal sac often contains thrombotic debris. Atherosclerotic changes in the parent vessel are also common.


Studies have found scant evidence for congenital, developmental, or inherited weakness of the arterial wall. Although genetic conditions are associated with increased risk of aneurysm development (see below), most intracranial aneurysms probably result from hemodynamically induced degenerative vascular injury. The occurrence, growth, thrombosis, and even rupture of intracranial saccular aneurysms can be explained by abnormal hemodynamic shear stresses on the walls of large cerebral arteries, particularly at bifurcation points.


The incidence of intracranial aneurysms is not known with certainty but is estimated to be in the range of 1-6% of the population. [1] Published data vary according to the definition of what constitutes an aneurysm and whether the series is based on autopsy data or angiographic studies. In one series of patients undergoing coronary angiography, incidental intracranial aneurysms were found in 5.6% of cases; another series found aneurysms in 1% of patients undergoing four-vessel cerebral angiography for indications other than subarachnoid hemorrhage (SAH). Familial intracranial aneurysms have been reported. [2]  

Associated conditions

Conditions that have been associated with increased incidence of cerebral aneurysms include the following:

Autosomal dominant polycystic kidney disease (ADPKD) is by far the most common genetic abnormality associated with intracranial aneurysms, with an estimated 5-40% of ADPKD patients harboring such lesions. [3] These lesions are often multiple. All patients with ADPKD should undergo screening with magnetic resonance angiography (MRA). The proper age to begin screening patients with ADPKD and the optimal frequency of rescreening (if the initial MRA findings are negative) are issues that remain to be resolved. [4]

Intracranial aneurysms are uncommon in children and account for fewer than 2% of all cases. Aneurysms in the pediatric age group are often more posttraumatic or mycotic than degenerative and have a slight male predilection. Aneurysms found in children are also larger than those found in adults, averaging 17 mm in diameter.

Screening for intracranial aneurysms is recommended for people who have two immediate relatives with intracranial aneurysms. A study of 20 years of screening results of individuals with a positive family history of SAH found that the yield of long-term screening is substantial even after more than 10 years of follow-up and two initial negative screens. [5]  

In this study, MRA or computed tomography (CT) angiography (CTA) was done from age 16-18 years to age 65-70 years. [5] Aneurysms were identified in 51 (11%) of 458 individuals at first screening, in 21 (8%) of 261 at second screening, in seven (5%) of 128 at third screening, and in three (5%) of 63 at fourth screening. Five (3%) of 188 individuals without a history of aneurysms and with two negative screens had a de-novo aneurysm in a follow-up screen. These findings suggested that repeated screening should be considered in individuals with two or more first-degree relatives who had SAH or unruptured intracranial aneurysms.


Intracranial aneurysms are multiple in 10-30% of all cases (see the image below). [6] About 75% of patients with multiple intracranial aneurysms have two aneurysms, 15% have three, and 10% have four or more. A strong female predilection is observed with multiple aneurysms at an overall female-to-male ratio of 5:1. The ratio rises to 11:1 in patients with more than three aneurysms.

Circle of Willis has been dissected, and three ber Circle of Willis has been dissected, and three berry aneurysms are observed. Multiple aneurysms are observed in about 20-30% of cases of berry aneurysm. Such aneurysms are congenital in that defect in arterial wall may be present from birth, but actual aneurysm develops over years; thus, rupture is most likely to occur in middle-aged adults.

Multiple aneurysms are also associated with vasculopathies such as FMD and other connective-tissue disorders.

Multiple aneurysms can be bilaterally symmetric (ie, mirror aneurysms) or located asymmetrically. More than one aneurysm can be present on the same artery.


Aneurysms commonly arise at the bifurcations of major arteries (see the first image below). Most saccular aneurysms arise on the circle of Willis (see the second image below) or the middle cerebral artery (MCA) bifurcation.

Common locations of cerebral saccular aneurysms, w Common locations of cerebral saccular aneurysms, with relative incidences. Copyright 2006 Massachusetts Medical Society. All rights reserved.
White arrow on black card marks site of ruptured b White arrow on black card marks site of ruptured berry aneurysm in circle of Willis (major cause of subarachnoid hemorrhage).

Approximately 86.5% of all intracranial aneurysms arise on the anterior (carotid) circulation. Common locations include the anterior communicating artery (30%), the internal carotid artery (ICA) at the posterior communicating artery origin (25%), and the MCA bifurcation (20%). The ICA bifurcation (7.5%) and the pericallosal/callosomarginal artery bifurcation account for the remainder (4%).

About 10% of all intracranial aneurysms arise on the posterior (vertebrobasilar) circulation. About 7% arise from the basilar artery bifurcation, and the remaining 3% arise at the origin of the posterior inferior cerebellar artery (PICA) where it comes off of the vertebral artery..

Aneurysms that develop at distal sites in the intracranial circulation are often caused by trauma or infection (see Saccular Aneurysms: Traumatic). 

Clinical presentation

Aneurysms typically become symptomatic in people aged 40-60 years, with the peak incidence of SAH occurring in those aged 55-60 years. [7] Most aneurysms do not cause symptoms until they rupture; when they rupture, they are associated with significant morbidity and mortality.

Subarachnoid hemorrhage

The most common presentation of intracranial aneurysms is SAH (see the images below). In North America, 80-90% of nontraumatic SAHs are caused by the rupture of an intracranial aneurysm. Another 5% are associated with bleeding from an AVM or tumor, and the remaining 5-15% are idiopathic. 

White arrow on black card marks site of ruptured b White arrow on black card marks site of ruptured berry aneurysm in circle of Willis (major cause of subarachnoid hemorrhage).
Subarachnoid hemorrhage from ruptured aneurysm is Subarachnoid hemorrhage from ruptured aneurysm is more of an irritant-producing vasospasm than a mass lesion.
CT scan of aneurysmal subarachnoid hemorrhage in 5 CT scan of aneurysmal subarachnoid hemorrhage in 55-year-old woman shows subarachnoid blood within interpeduncular and ambient cisterns and right sylvian fissure caused by ruptured aneurysm at junction of right carotid artery and posterior communicating artery.
CT scan in 82-year-old woman shows extensive subar CT scan in 82-year-old woman shows extensive subarachnoid blood within cortical sulci, intraventricular hemorrhage, and intracerebral hematoma adjacent to large ruptured aneurysm of anterior communicating artery.

On presentation, patients typically report experiencing the worst headache of their lives; this presents at peak intensity, a so-called thunderclap headache. The association of meningeal signs should increase suspicion of SAH. For a full description of SAH, see Subarachnoid Hemorrhage.

The most widely used clinical methods for grading the clinical severity of aneurymal SAH are the Hunt and Hess scale and the World Federation of Neurological Surgeons (WFNS) grading scale, which measure the clinical severity of the hemorrhage on admission and have been shown to correlate well with outcome.

The Hunt and Hess scale specifies the following grades:

  • Grade 0 - Unruptured aneurysm
  • Grade 1 - Asymptomatic or minimal headache and slight nuchal rigidity
  • Grade 2 - Moderate-to-severe headache, nuchal rigidity, no neurologic deficit other than cranial nerve palsy
  • Grade 3 - Drowsiness, confusion, or mild focal deficit
  • Grade 4 - Stupor, moderate-to-severe hemiparesis, possible early decerebrate rigidity, and vegetative disturbances
  • Grade 5 - Deep coma, decerebrate rigidity, and moribund appearance

The WFNS scale specifies the following grades:

  • Grade I - Glasgow Coma Scale (GCS) score, 15; motor deficit absent
  • Grade II - GCS score, 13-14; motor deficit absent
  • Grade III - GCS score, 13-14; motor deficit present
  • Grade IV - GCS score, 7-12; motor deficit present or absent
  • Grade V - GCS score, 3-6; motor deficit present or absent

The Fisher grade, which describes the amount of blood seen on noncontrast CT of the head, is also useful in correlating the likelihood of developing vasospasm (see below), the most common cause of death and disability from SAH. Fisher grades are as follows:

  • Fisher 1 - No blood detected
  • Fisher 2 - Diffuse or vertical layers less than 1 mm thick
  • Fisher 3 - Localized clot or vertical layer 1 mm thick or thicker
  • Fisher 4 - Intracerebral or intraventricular clot with diffuse or no SAH

Vasospasm is overwhelmingly most common in Fisher grade 3 and is rarely found in patients with no blood on CT. [8]

Other symptoms

Signs and symptoms of aneurysm other than those associated with SAH are relatively uncommon. Some intracranial aneurysms produce cranial neuropathies. A common example is the third nerve palsy that is secondary to posterior communicating artery aneurysm. Other, less common symptoms include visual loss caused by an ophthalmic artery aneurysm that compresses the optic nerve, seizures, headaches, and transient ischemic attacks (TIAs) or cerebral infarction secondary to emboli (usually associated with large or giant partially thrombosed MCA aneurysms). So-called giant aneurysms (diameter >2.5 cm) are more often symptomatic because of their mass effect.

Clinical outcome

Of patients with SAH, 10% die before reaching medical attention, and another 50% die within 1 month; 50% of survivors have neurologic deficits. Ruptured aneurysms are most likely to rebleed within the first day (2-4%), and this risk remains very high (~25%) for the first 2 weeks if treatment is not provided. Vasospasm is a leading cause of disability and death from aneurysm rupture (see the images below).

White arrow on black card marks site of ruptured b White arrow on black card marks site of ruptured berry aneurysm in circle of Willis (major cause of subarachnoid hemorrhage).
Subarachnoid hemorrhage from ruptured aneurysm is Subarachnoid hemorrhage from ruptured aneurysm is more of an irritant-producing vasospasm than a mass lesion.
CT scan of aneurysmal subarachnoid hemorrhage in 5 CT scan of aneurysmal subarachnoid hemorrhage in 55-year-old woman shows subarachnoid blood within interpeduncular and ambient cisterns and right sylvian fissure caused by ruptured aneurysm at junction of right carotid artery and posterior communicating artery.
CT scan in 82-year-old woman shows extensive subar CT scan in 82-year-old woman shows extensive subarachnoid blood within cortical sulci, intraventricular hemorrhage, and intracerebral hematoma adjacent to large ruptured aneurysm of anterior communicating artery.

The following three factors have been correlated with improved outcomes:

  • Early referral to a hospital that has physicians experienced in treating intracranial aneurysms
  • Early treatment (open surgery and clipping or endovascular coiling)
  • Aggressive treatment of vasospasm

Outcomes associated with unruptured aneurysms are based primarily on whether they are treated and what the results of that treatment are.

Natural history

The true natural history risk of rupture of an incidentally discovered aneurysm is unknown. The ISUIA (International Study of Unruptured Intracranial Aneurysms), published in 1998 (retrospective component; N = 2621) and 2003 (prospective component; N =1692), assessed intracranial aneurysms without intervention to determine the true natural history risk. [9]

This study found rupture risk to be closely related to size. [9] For aneurysms smaller than 7 mm and those located in the anterior circulation in patients who had not had a hemorrhage from another aneurysm, the risk of subsequent rupture was extremely small (0.05%/y in the retrospective arm; 5-year cumulative risk of rupture of 0% in the prospective arm). Larger anterior-circulation aneuryms were demonstrated to have 5-year rupture risks of 2.6%, 14.5%, and 40% for aneurysms 3-7 mm, 7-12 mm, 13-24 mm, and 25 mm or larger ("giant aneurysms"), respectively.

Aneurysms at other locations (eg, the basilar tip and the posterior communicating artery), aneurysms larger than 10 mm, and aneurysms found in patients who had bled from a prior aneurysm were found to have higher risks (~0.5%/y). [9]  However, whereas ISUIA found aneurysms of the posterior circulation (including the posterior communicating artery) and larger aneurysms to have a higher rupture risk, a similar natural history study from Japan further identified aneurysms of the anterior communicating artery and aneurysms with daughter sacs as having a higher risk for rupture in the Japanese population. [10]

Critics of the ISUIA emphasized that the selection was biased because surgeons who entered patients into the study felt that these aneurysms were less likely to bleed. Nonetheless, the results of this study have significantly affected the way unruptured aneurysms are managed, with more and more aneurysms undergoing conservative management as opposed to invasive therapy, particularly if they are small and asymptomatic.

For untreated ruptured aneurysms, the risk of rebleeding after the initial hemorrhage is estimated at 20-50% in the first 2 weeks, and such rebleeding carries a mortality of nearly 85%. Aneurysms that have not ruptured but have manifested with other symptoms—for instance, new-onset third nerve palsy (considered a true emergency that necessitates urgent treatment of the aneurysm), brainstem compression due to a giant aneurysm, or visual loss (caused by an ophthalmic artery aneurysm)—should be treated because the natural history risk of rupture is believed to be significantly higher (6%/year) than that of incidentally discovered lesions.

Cigarette smoking, female gender, and younger age have been shown to correlate with aneurysm growth and rupture. [11, 12]  Although rare, growth of an unruptured aneurysm is associated with increased risk of hemorrhage. [13]

The apices of vessel bifurcations are the sites of maximum hemodynamic stress in a vascular network. Vascular and internal flow hemodynamics have a crucial effect on the origin, growth, and configuration of intracranial aneurysms. In the aneurysm, wall shear stress caused by the rapid changes of blood flow direction (the result of systole and diastole) continually damages the intima at an aneurysm cavity neck. These augmented hemodynamic stresses probably cause the initiation and subsequent progression of most saccular aneurysms. Thrombosis and rupture are also explained by intra-aneurysmal hemodynamic stresses.

Studies demonstrate that morphologic factors and geometric relationship between an aneurysm and its parent artery are relevant to natural history and determine intra-aneurysmal flow patterns. Patterns of intra-aneurysmal flow are important not only for the formation and progression of an aneurysm itself but also because they may influence the selection and placement of endovascular treatment devices. 


Saccular Aneurysms: Traumatic

Traumatic aneurysms account for fewer than 1% of all aneurysms. Aneurysmal lesions resulting from major trauma (eg, motor vehicle accident, blast injury from war, or gunshot) are often pseudoaneurysms (ie, false aneurysms). However, trauma to a vessel can sometimes lead to a true aneurysm involving all three layers of the vessel wall; this is usually more of a glacial phenomenon. Pathologically, traumatic false aneurysms lack any components of a vessel wall. They are cavities typically situated within adjacent blood clots that communicate with a vessel lumen. 

Two general types of traumatic aneurysms are identified:

  • Aneurysms secondary to penetrating trauma
  • Aneurysms secondary to nonpenetrating trauma

Penetrating trauma

Intracerebral aneurysms secondary to penetrating injuries are commonly due to high-velocity missile wounds of the head. Approximately 50% of civilian patients with penetrating missile injuries are found to have major vascular lesions. Nearly half of these patients have traumatic aneurysms. The diagnosis of posttraumatic aneurysm may be delayed; visualization on imaging can be obscured by the brain injury or retained foreign body, and these aneurysms can occur on a delayed basis.

Occasionally, the external carotid artery (ECA) is a site of traumatic injury. The superficial temporal artery (STA) is the most commonly affected vessel. STA traumatic pseudoaneurysm occurs as a complication of scalp trauma and may result from penetrating injury or blunt trauma. Meningeal vessels are uncommon sites of traumatic pseudoaneurysm development; most occur on branches of the middle meningeal artery. When a meningeal pseudoaneurysm hemorrhages, it is usually into the epidural space. Occasionally, cervical spine fracture-dislocations damage the vertebral artery (VA), producing vessel dissection or occlusion; pseudoaneurysms are rare.

Nonpenetrating trauma

Intracranial aneurysm secondary to nonpenetrating trauma is rare and usually occurs at the skull base (where it involves the petrous, cavernous, or supraclinoid ICA) or along the peripheral intracranial vessels. ICA aneurysms at the skull base can be caused by blunt trauma or skull fracture. Hyperextension and head rotation may stretch the ICA over the lateral mass of C1 or shear the artery at its intracranial entrance. Peripheral intracranial aneurysms, for example, can be caused by shearing forces between the inferior free margin of the falx cerebri and the distal ACA. 


Saccular Aneurysms: Mycotic

The term mycotic aneurysm refers to any aneurysm that results from an infectious process that involves the arterial wall; generally, it has a fusiform morphology and is friable. These aneurysms may be caused by a septic cerebral embolus that causes inflammatory destruction of the arterial wall, beginning with the endothelial surface. A more likely explanation is that infected embolic material reaches the adventitia through the vasa vasorum; inflammation then disrupts the adventitia and muscularis, resulting in aneurysmal dilatation. (See the images below.)

CT angiography reconstruction shows large, irregul CT angiography reconstruction shows large, irregularly shaped presumed mycotic middle cerebral artery aneurysm.
Coronal CT angiography shows large, irregularly sh Coronal CT angiography shows large, irregularly shaped presumed mycotic middle cerebral artery aneurysm (see previous image).
Digital subtraction angiogram, right internal caro Digital subtraction angiogram, right internal carotid injection, shows large, irregularly shaped presumed mycotic middle cerebral artery aneurysm.
Digital subtraction angiogram, right internal caro Digital subtraction angiogram, right internal carotid injection, three-dimensional reconstruction, shows large, irregularly shaped presumed mycotic middle cerebral artery aneurysm (see previous image).

With the increased incidence of drug abuse and immunocompromised states from various causes, mycotic aneurysms may have increased in frequency.

Most cases are treated on an emergency basis with antibiotics, which are continued for 4-6 weeks. Serial vascular imaging (at 1.5, 3, 6, and 12 mo) helps document the effectiveness of medical therapy. Even if aneurysms seem to be shrinking, they may subsequently grow, and new ones may form.

Direct surgical or endovascular intervention is often indicated in patients with SAH, those whose aneurysms enlarge during antibiotic therapy, and those whose aneurysm does not shrink after 4-6 weeks of antibiotics.


Saccular Aneurysms: Oncotic

Neoplastic aneurysms result from direct vascular invasion by a tumor or implantation of metastatic emboli that infiltrate and disrupt the vessel wall. Such oncotic aneurysms may be associated with either primary or metastatic tumors. Extracranial oncotic pseudoaneurysms with exsanguinating epistaxis are a common terminal event with malignant head and neck tumors. Intracranial oncotic aneurysms are less common. They are often bizarrely shaped and located on distal branches of the intracranial vessels, remote from the more typical saccular aneurysms located on the circle of Willis.


Saccular Aneurysms: Flow-Related

Flow-related aneurysms occur in 2.7-30% of patients with AVM. Proximal flow-related lesions arise in the circle of Willis or on vessels that feed the AVM and are probably related to increased hemodynamic stress. No increased frequency of hemorrhage is reported in patients with proximal feeding-artery aneurysms. Distal flow-related aneurysms are located in distal branches to the AVM.

Intranidal aneurysms have been reported in 8-12% of AVMs. Whether intranidal aneurysms arise from venous ectasias (dilatation) or from the flow-weakened walls of arterial vessels is unclear. Nevertheless, these thin-walled structures are exposed to arterial pressure and are considered a likely site for AVM hemorrhage.

Treatment of aneurysms associated with AVMs is similar to that of aneurysms not associated with AVMs, with the following differences:

  • Small flow-related aneurysms have been shown to disappear or shrink after successful treatment of the AVM, and this possibility must be considered, particularly if no hemorrhage has occurred
  • AVMs that bleed often have intranidal aneurysms; when these are found, they should be targeted for urgent therapy
  • In AVMs that manifest as SAH and circle-of-Willis aneurysms, one should presume that the aneurysm (not the AVM) is the source of the SAH and treat urgently to prevent rebleeding

Saccular Aneurysms: Vasculopathy-Related

Some vasculitides, such as systemic lupus erythematosus (SLE) and even Takayasu arteritis, have been associated with aneurysms. There is a 20-50% incidence of aneurysms in patients with cervical FMD. 


Saccular Aneurysms: Drug-Related

Substance abuse, especially with cocaine, can cause certain forms of vasculitis that contribute to aneurysm formation. In addition, these drugs can cause hemorrhage from preexisting vascular abnormalities such as AVMs or saccular aneurysms because of their ability to cause surges of increased systemic blood pressure to high values. Heroin, ephedrine, and methamphetamine use can also cause cerebral vasculitis and, potentially, aneurysms.


Fusiform Aneurysms

Fusiform aneurysms, often atherosclerotic in nature, are exaggerated arterial ectasias. Damage to the media results in arterial stretching and elongation that may extend over a considerable length of the vessel. These ectatic vessels may have more focal areas of fusiform or even saccular enlargement. Intraluminal clots are common, and perforating branches often arise along the involved vessel. They often have bizarre shapes with serpentine or giant configurations. 

Fusiform aneurysms usually occur in older patients, and the vertebrobasilar system is commonly affected. Fusiform aneurysms may thrombose, producing brainstem infarction as small ostia of perforating vessels that emanate from the aneurysm become occluded. They can also compress the adjacent brain or cause cranial nerve palsies via mass effect. Fusiform basilar artery aneurysms can be associated with abdominal aortic aneurysms.


Dissecting Aneurysms

In arterial dissections, blood accumulates within the vessel wall through a tear in the intima and the internal elastic lamina. The consequences of this intramural hemorrhage vary. If blood dissects subintimally, it causes luminal narrowing or even occlusion. If the intramural hematoma extends into the subadventitial plane, a saclike outpouching may be formed, termed a dissecting aneurysm. (See the image below.)

Image of dissecting aneurysm shows lumen of blood Image of dissecting aneurysm shows lumen of blood vessel in relation to aneurysm.

Dissecting aneurysms may arise spontaneously. More commonly, trauma or an underlying vasculopathy such as FMD is implicated. Most involve the extracranial segments of craniocerebral vessels. Intracranial dissections are rare and usually occur only with severe head trauma. Most dissections involve the midcervical ICA segment and terminate at the extracranial opening of the petrous carotid canal.

Although the common carotid artery (CCA) can be involved via cephalad extension of an aortic arch dissection, the CCA and carotid bulb are usually spared. The VA is also a common site of arterial dissection, most often between the VA exit from C2 and the skull base. Involvement of the first segment of the VA, which extends from the VA origin to its entry into the foramen transversarium (usually at the C6 level), is relatively rare. Dissecting intracranial vertebral aneurysms are extremely dangerous lesions with a higher risk of rerupture.


Imaging Options

The role of diagnostic imaging includes the following elements:

  • Identify the presence of aneurysms
  • Delineate the relationship between an aneurysm and the local vessels
  • Define the potential for collateral circulation to the brain
  • Assess for vasospasm
  • Help determine which treatment modality would best secure the lesion so as to prevent bleeding

The three major modalities used to study the size, location, and morphology of an intracranial aneurysm are as follows:

  • CTA 
  • MRA 
  • Catheter-based angiography 

The preferred initial method for evaluation of unruptured intracranial aneurysms is either MRA or CTA, whereas angiography is the preferred modality in patients who have had an SAH, though CTA alone has been used in this last setting. [14]  A patent intracranial aneurysm is visualized as a contrast-filled outpouching that commonly arises from an arterial wall or bifurcation. Thrombosed aneurysms can be occult on angiographic studies. Contrast extravasation from an aneurysm during angiography is extremely rare and can be fatal.

Aneurysms must be distinguished from infundibula. Infundibula are smooth funnel-shaped dilatations that are caused by incomplete regression of a vessel present in the developing fetus. Their most common location is at the origin of the posterior communicating artery from the ICA. Less commonly, an infundibulum arises from the anterior choroidal artery origin. Infundibula are 2 mm or less in diameter and regular in shape, and the distal vessel exits from their apices.

Similarly, aneurysms must be distinguished from vascular loops, and three-dimensional (3D) angiography is useful for distinguishing between a vascular loop and a true saccular aneurysm.

Artificial intelligence algorithms capable of identifying aneurysms on MRA or CTA are currently being introduced into the clinical world.

A focal parenchymal or cisternal hematoma surrounding an aneurysm on CT strongly suggests rupture. Larger, irregularly shaped aneurysms are also more likely to be the offending lesion. Lobulation or a smaller daughter dome (or teat) indicates possible rupture. Although focal vasospasm is a helpful finding, subarachnoid blood quickly spreads along the basal cisterns, making this a somewhat less reliable sign of aneurysm rupture. Positively determining the ruptured aneurysm is sometimes impossible. 

In approximately 15% of patients with nontraumatic SAH, no aneurysm is found despite a complete high-quality six-vessel cerebral angiogram. Two distinct subsets of these patients have been recognized. The first group consists of those with so-called benign perimesencephalic nonaneurysmal SAH (BPNSAH), in which bleeding on CT or magnetic resonance imaging (MRI) is localized immediately anterior to the brainstem and adjacent areas such as the interpeduncular fossa and ambient cisterns. The prognosis for BPNSAH patients is excellent, in that the SAH probably results from spontaneous rupture of small pontine or perimesencephalic veins with a low risk of rebleeding and delayed complications.

The second group consists of those in whom CT reveals an aneurysmal pattern of hemorrhage (ie, blood fills the suprasellar cistern, extends into the lateral sylvian or anterior interhemispheric fissures, or both) but angiography does not identify any aneurysm. In this group, repeat angiography (1-6 wk later) is indicated to look for an occult aneurysm that may have initially been compressed by hematoma, was thrombosed, or did not opacify because of local vasospasm. Repeat four-vessel cerebral angiography demonstrates a lesion in 10-20% of these cases. MRI of the brain and cervical spine with gadolinium contrast is also indicated to rule out vascular abnormality or tumor as the cause of the bleed.


Computed Tomography

Noncontrast CT, though of limited sensitivity for detecting intracranial aneuryms, is capable of demonstrating aneurysms that are large (usually ≥10 mm), are partially thrombosed, or contain calcium. Bone erosion can be observed in long-standing lesions that arise near the skull base. The typical nonthrombosed aneurysm appears as a well-delineated isodense–to–slightly hyperdense mass located somewhat eccentrically in the suprasellar subarachnoid space or the sylvian fissure. 

Patent aneurysms and the cerebral vasculature can be imaged by means of rapid contrast infusion and thin-section dynamic CTA. Such studies provide multiple projections of anatomically complex vascular lesions and delineate their relations to adjacent structures (see the image below). The accuracy of high-resolution CTA in the diagnosis of cerebral aneurysms 3 mm and larger has been reported to be about 97%.

CT angiography shows large basilar tip aneurysm. A CT angiography shows large basilar tip aneurysm. Adjacent ridges of skull-base anatomy relative to aneurysm are appreciated.

Subarachnoid hemorrhage

CT is the prefered imaging modality for detecting the presence of SAH. The reported ability of CT to reveal SAH caused by ruptured cerebral aneurysms in the acute phase is approximately 95%. This sensitivity decreases over time and is somewhat dependent on the CT scanner resolution and the interpreting radiologist. In one study, CT scans detected SAH 100% of the time within 12 hours of the ictus but in only 93% within 24 hours. [15]

Acute SAH appears as high attenuation within the subarachnoid cisterns. SAH may quickly spread diffusely throughout the cerebrospinal fluid (CSF) spaces, providing little clue to its site of origin. Some bleeding patterns have been associated with particular aneurysm locations. Hemorrhage located predominantly within the interhemispheric fissure is common in anterior communicating artery aneurysms, and sylvian fissure blood is often observed in MCA lesions. Fourth-ventricle hemorrhage is common in posterior fossa aneurysms, particularly those that arise at the PICA takeoff, and intraventricular blood also occurs with anterior communicating artery and basilar tip lesions.


Magnetic Resonance Imaging

The appearance of an aneurysm on MRI is highly variable and may be quite complex. The signal depends on the presence, direction, and rate of flow, as well as on the presence of clot, fibrosis, and calcification within the aneurysm itself.

Patent aneurysms can produce hyperintense or hypointense signals on routine MRI studies, depending on the specific flow characteristics and pulse sequences used. The typical patent aneurysm lumen with rapid flow is visible as a well-delineated mass that shows high-velocity signal loss (flow void) on T1- and T2-weighted images. Some signal heterogeneity may be observed if turbulent flow in the aneurysm is present. Gradient-refocused scans delineate the patent lumen of an aneurysm and are particularly helpful when acute thrombus makes the aneurysm difficult to identify.

Intravenous (IV) contrast agents typically do not enhance patent aneurysms with high flow rates, but wall enhancement may occur. Contrast in the intravascular space also often increases artifacts observed with rapid intraluminal flow.

Partially thrombosed aneurysms often have a complex signal on MRI. An area of high-velocity signal loss in the patent lumen with surrounding concentric layers of multilaminated clot and variable signal intensities can be observed. Larger aneurysms may have a thick signal void rim caused by hemosiderin-containing mural thrombus and a hemosiderin-laden fibrous capsule. If intraluminal flow is slow or turbulent, the residual lumen may be isointense with the remainder of the aneurysm and difficult to detect without contrast enhancement.

Multilayered clots can be observed in thrombosed aneurysms that have undergone repeated episodes of intramural hemorrhage. On occasion, recently thrombosed aneurysms may be isointense with brain parenchyma and difficult to distinguish from other intracranial masses.

MRA uses the macroscopic motion of flowing blood, together with background suppression of stationary tissue, to create images of the cerebral vasculature. Two standard techniques currently used for MRA are phase-contrast studies and time-of-flight acquisitions. The images can be viewed as individual thin sections (source images) or can be reprojected in the form of flow mapping or MRA (see the image below).

MR angiography demonstrates unruptured aneurysm at MR angiography demonstrates unruptured aneurysm at vertebrobasilar junction (upper image) and distal middle cerebral artery aneurysm (lower image).

Catheter-Based Angiography

Catheter-based angiography, or digital subtraction angiography (DSA), continues to be the criterion standard for revealing and delineating the features of an intracranial aneurysm. These techniques rely on placement of a catheter into the cerebral vasculature for direct injection of dye. Technologic advances, most notably 3D rotational angiography, have increased the utility of these studies for defining aneurysm anatomy. Images can be rotated in 3D space, giving a more accurate depiction of the aneurysm. (See the image below.)

Three-dimensional rotational digital subtraction a Three-dimensional rotational digital subtraction angiogram, carotid injection, reveals small anterior communicating artery aneurysm (arrow).

Principles of Treatment

This section highlights the basic principles of aneurysm treatment. Management of SAH is discussed more fully elsewhere (see Subarachnoid Hemorrhage). Guidelines for the management of SAH based on levels of evidence have been published by the American Heart Association (AHA) and the American Stroke Association (ASA). [16]  Future developments in the treatment of cerebral aneurysm and SAH are likely to rely increasingly on so-called personalized medicine. [17]

Treatment decisions for ruptured aneurysms differ significantly from those for unruptured aneurysms. Ruptured aneurysms should be treated urgently to prevent rebleeding and to permit aggressive management of vasospasm. Unruptured aneurysms are generally treated electively or followed. The following are three options for treating intracranial aneurysms:

  • Observation
  • Craniotomy and clipping (see the first image below)
  • Endovascular treatment (see the second image below)
Craniotomy and clipping of aneurysm. Skin incision Craniotomy and clipping of aneurysm. Skin incision and proposed craniotomy bone removal are indicated (A). Clip application to neck of aneurysm, permanently preventing blood flow into aneurysm, is also shown (B). Copyright 2006 Massachusetts Medical Society. All rights reserved.
Endovascular coiling of cerebral aneurysm. Transfe Endovascular coiling of cerebral aneurysm. Transfemoral approach to gain access to aneurysm via a small microcatheter (A) and final occlusion of aneurysm with coils (B). Copyright 2006 Massachusetts Medical Society. All rights reserved.

The decision regarding aneurysm treatment is individualized and should be made by a physician who is capable of offering clipping or coiling without bias. Whereas clipping is performed only by neurosurgeons, endovascular treatment is performed by interventional neuroradiologists or neurosurgeons or neurologists with endovascular training.

The risks of any treatment must outweigh the risks associated with no treatment. These risks vary, depending on many aneurysm-specific and patient-specific factors, including aneurysm size, location, and morphology, as well as patient age and medical comorbidities. Additionally, anxiety in patients who know they have an aneurysm can have a substantial influence on treatment decisions. One treatment strategy is outlined in an algorithm (see the image below).

Algorithm for treatment of intracranial aneurysms. Algorithm for treatment of intracranial aneurysms.

All ruptured aneurysms should be treated to avoid disastrous rebleeding (rare exceptions may include hemodynamic instability, extreme old age, or a clinical condition approaching brain death). Beyond attention to general medical supportive principles and successful securing of the aneurysm to prevent further bleeding, SAH patients have unique clinical needs. In particular, they need intensive monitoring and management for the possibility of developing hydrocephalus and vasospasm.

Hydrocephalus is usually the result of blood in the subarachnoid space obliterating the arachnoidal villi, but it can also be caused by obstruction from blood within the ventricles. When hydrocephalus leads to neurologic worsening because of the raised intracranial pressure (ICP), a ventriculostomy catheter should be placed for CSF diversion on an emergency basis. Not only can this be lifesaving, but a patient's neurologic examination can improve dramatically after the hydrocephalus has been treated.

In  one systematic review and meta-analysis (21 studies; N = 1511; patient age ≥ 65 y), long-term occlusion was achieved in 79% of patients. [18]  The rate of perioperative stroke (4%) was similar for patients with unruptured and ruptured aneurysms. Intraprocedural rupture occurred in 1% of patients with unruptured aneurysms and in 4% of patients with ruptured aneurysms. Perioperative mortality was 23% for patients with ruptured aneurysms and 1% for those with unruptured aneurysms. At 1-year follow-up, 93% of patients with unruptured aneurysms and 66% of patients with ruptured aneurysms had good outcomes.

Choice of surgical technique

Obliteration of an aneurysm (ruptured or unruptured) with endovascular techniques or surgical clipping is a matter of considerable controversy, and there are substantial regional variations. The endovascular techniques for treating aneruysms now extend beyond coiling to include stent/coiling, flow diversion, and intrasaccular devices.

With each passing year, endovascular techniques are becoming more commonly used in relation to surgical clipping (with or without bypass) for both ruptured and unruptured aneurysms. [19]  Currently, data suggest that endovascular techniques are safer than clipping for both ruptured and unruptured aneurysms in the acute perioperative period, whereas clipping is slightly more durable. Endovascular techniques may also lead to improved cognitive outcomes in patients treated for ruptured aneurysms. [20]


Exact risks for clipping or coiling an aneurysm are not known and depend on patient- and aneurysm-specific factors. Surgeon and institutional volume likely also plays a role, which has not yet been quantified. [21, 22, 23]

Historical data published by experienced vascular surgeons suggest that the morbidity and mortality figures associated with clipping a ruptured versus an unruptured aneurysm are 4-10.9% and 1-3%, respectively. [24, 25, 26] The ISUIA also examined the risks associated with clipping an unruptured aneurysm and found that those risks (15.7% morbidity at 1 y) were higher than previously believed; ISUIA studied and reported cognitive outcomes for the first time, and these represented one third of the reported morbidity. 

On the basis of several large studies, estimated risks associated with coiling are on the order of 3.7-5.3% morbidity and 1.1-1.5% mortality. [27, 28]  A major drawback associated with coiling is that over time, the coils can compact, leading to reopening or recanalization of the aneurysm. This is presumed to be the reason for the higher rerupture risk reported in the International Subarachnoid Aneurysm Trial (ISAT) follow-up studies for patients who underwent endovascular treatment. Fortunately, repeat coiling is a fairly safe technique, with morbidity and mortality rates of less than 2%.

Additionally, newer technologic advances, such as newer biologically active coils, stents, flow diverters, and intrasaccular devices, have been designed to prevent recanalization, and the results are encouraging. The challenge with many of the more advanced endovascular techniques is that they require dual antiplatelet therapy (DAPT), adding to complexity of care and morbidity in the patient with a ruptured intracranial aneurysm.

Surgical clipping vs endovascular treatment

Several studies have attempted to compare the two techniques and have found coiling to be safer but slightly less durable. Studying the California statewide database, Johnston et al found that outcomes after clipping unruptured aneurysms were significantly worse than those following coiling; coiling was also associated with lower inpatient mortality, shorter hospital stays, and lower costs. [29]

Increased safety of coiling over clipping was better demonstrated in the well-publicized ISAT, conducted mainly in Europe. [30]  In that study, 2143 patients who presented with SAH and were deemed to have an aneurysm that was felt to be treatable with either coiling or clipping were prospectively randomized to one of the two treatments. The study was stopped prematurely after a planned interim analysis found a 23.7% rate of dependency or death in the coiling cohort versus a 30.6% rate in the clipping cohort.

This controversial study significantly affected the treatment of ruptured aneurysms. The main criticism of the study was that the great majority of the aneurysms were felt to be better treated by one modality than by the other and therefore were not randomized. Only 22.4% of all aneurysms screened were randomized. Of those, the overwhelming majority were small and located in the anterior circulation. Therefore, although the ISAT is important, generalizing its results to all ruptured aneurysms is not possible.

Continued follow-up from the ISAT confirmed the original results, with a higher rate of seizures in the clipping group and a slightly higher rate of rebleeding in the coiling group. [31]  Subsequently, the ISAT investigators reported on a reanalysis of the data in which they stratified the benefit of coiling over clipping in patients with SAH. [32] Because of the small chance of aneurysm recurrence (recanalization) after coiling, which is higher than that after clipping, the investigators found that the results of the study may not necessarily apply to patients younger than 40 years.

In a review of hospital mortality associated with elective treatment of unruptured intracranial aneurysms, Alshekhlee et al found that endovascular treatment was associated with a lower death rate than surgical clipping (0.57% vs 1.6%), as well as lower rates of perioperative intracerebral hemorrhage and acute ischemic stroke. During the years 2000 through 2006, a trend toward greater use of endovascular coiling procedures was observed. [33]  This trend has continued in subsequent years.

A prospective multicenter study by Hammer et al (N = 661), designed to compare the results of endovascular treatment of ruptured intracranial aneurysms (n = 271) with those of microsurgical clipping (n = 390), found that the latter was associated with a lower rate of modality-associated complications and a higher occlusion rate of ruptured aneurysms. [34]

Ultimately, the decision for open surgery or for endovascular treatment should be made on an individual basis and may often involve difficult-to-quantify variables (eg, patient preference or the experience or availability of physician operators).


Surgical Clipping

The goal of surgical treatment is usually to place a clip across the neck of the aneurysm to exclude the aneurysm from the circulation (see the image below) without occluding normal vessels.

Craniotomy and clipping of aneurysm. Skin incision Craniotomy and clipping of aneurysm. Skin incision and proposed craniotomy bone removal are indicated (A). Clip application to neck of aneurysm, permanently preventing blood flow into aneurysm, is also shown (B). Copyright 2006 Massachusetts Medical Society. All rights reserved.

After performing a craniotomy, one should use microsurgical techniques with the operative microscope to dissect the aneurysm neck free from the feeding vessels without rupturing the aneurysm. Final treatment involves the placement of a surgical aneurysm clip around the neck of the aneurysm, thereby obliterating the flow into the aneurysm. The goal at surgery is to obliterate the aneurysm from the normal circulation without compromising any of the adjacent vessels or the small perforating branches of these vessels. The clips are available in various types, shapes, sizes, and lengths and are manufactured so as to be compatible with MRI.

Intraoperative angiography is now frequently used as an adjunct to clipping and permits confirmation of aneurysm occlusion and patency of nearby vessels. The frequency of its use varies widely; however, many vascular neurosurgeons choose to use this technique selectively for difficult aneurysms.

A technique called near-infrared indocyanine green videoangiography (ICGA) has gained some popularity as a less invasive way of assessing aneurysm and blood-vessel patency during aneurysm surgery. [35] After IV injection of the indocyanine green dye, an operating microscope equipped with appropriate software can within minutes detect blood flow within the vasculature using near-infrared technology (see the video below). The technique is less invasive than intraoperative angiography, but one disadvantage is that only vessels that can be seen by the operating microscope can be evaluated.

Near infrared indocyanine green videoangiography demonstrates blood flowing into ophthalmic artery aneurysm and internal carotid artery from which it arises.

When the aneurysm cannot be clipped because of the nature of the aneurysm or the poor medical condition of the patient, and the aneurysm is felt not to be a candidate for endovascular therapy (see below), the following alternatives may be considered:

  • Wrapping
  • Trapping
  • Proximal (hunterian) ligation

Although wrapping should never be the goal of surgery, situations may arise in which little else is possible (eg, fusiform basilar trunk aneurysms). Plastic resins may be slightly better than muscle or gauze for this purpose. Wrapping can be performed with cotton or muslin, with muscle, or with plastic or another polymer.

Trapping may be warranted in some cases. Effective treatment requires both distal and proximal arterial interruption with direct surgery (ligation or occlusion with a clip). Treatment may also incorporate extracranial-intracranial (EC-IC) bypass to maintain flow distal to the trapped segment. 

Proximal ligation has been used with some success for giant aneurysms, particularly those of the vertebrobasilar circulation. Advanced endovascular techniques, however, now often offer better alternatives for such lesions.

The details of the sophisticated surgical approaches and techniques used to explore and dissect in order to clip the various aneurysms are beyond the scope of this review.

Postoperative care

Most agree that angiography is necessary after surgery to confirm good clip placement with total obliteration of the aneurysm and patency of the surrounding vessels. In cases where these objectives are achieved, the rebleeding rate is negligibly low.

Whether a patient who undergoes intraoperative angiography should undergo postsurgery angiography with the better equipment available in a dedicated angiography suite is also controversial, and practices vary.

After successful obliteration of a ruptured aneurysm, the patient remains at significant risk for vasospasm, hydrocephalus, and medical complications (including hyponatremia, venous thromboembolism, infections, and cardiac stun) and should remain in an intensive care setting for at least 7-10 days. [36] Operative complications represent only a small portion of the morbidity and mortality associated with a ruptured intracranial aneurysm. The major causes of morbidity and mortality include misdiagnosis, rebleeding, hydrocephalus, and vasospasm.

Vasospasm is defined as angiographic narrowing that can lead to delayed ischemia. Clinically, it is diagnosed as deterioration in mental status or focal neurologic deficits, most commonly hemiparesis or dysphasia. Transcranial Doppler (TCD) ultrasonography (US) is frequently used as a noninvasive diagnostic tool and is sensitive to changes in the vessel caliber of the larger vessels of the circle of Willis. A trained technician should perform TCD US daily or every other day during the vasospasm period (days 3-12 after SAH, with some flexibility, depending on the extent of the hemorrhage).

If the patient's condition deteriorates, it is important to exclude all other causes of neurologic deterioration. If there is any doubt, cerebral angiography is indicated to confirm the diagnosis. Vasospasm must be aggressively managed once it is detected because it can lead to permanent disability and death.

So-called triple-H therapy (hypertension, hemodilution, and hypervolemia) remains the most important aspect of the medical management of vasospasm, but in refractory cases where medical management fails, endovascular methods are warranted. Transluminal balloon angioplasty and intra-arterial calcium-channel blockers are the most commonly used treatments. Papaverine and nitroglycerin have been used, but they can increase intracranial pressure suddenly and dramatically; consequently, they are now viewed as less safe than calcium-channel blockers and proximal angioplasty. [37]

Persistent hydrocephalus develops in approximately 20% of cases. In such instances, a shunting procedure, usually a ventriculoperitoneal shunt, is required.


Endovascular Treatment

Various endovascular methods have been developed and refined to treat intracranial aneurysms. Initially, endovascular balloon occlusion of a feeding artery was performed. However, this procedure was soon followed by direct obliteration of the aneurysmal lumen, first by detachable balloons and later by microcoils.

Guglielmi et al described a detachable platinum microcoil for use in treatment of intracranial aneurysms. These coils are soft and can be detached from the stainless steel guide by passing a very small direct current that causes electrolysis at the solder junction. Separation usually occurs within 2-10 minutes after satisfactory coil placement.

The endovascular toolbox has expanded greatly since the initial work of Gugliemi. Coils of complex shapes, stents, flow diverters and and a new generation of intrasaccular devices are now available and widely used. The great challenge for the more complex tools is that they require DAPT and can be unsafe in the early period for a ruptured aneurysm. Because of the need for DAPT, coiling remains the most common initial strategy for ruptured aneurysms, though intrasaccular devices have been used extensively by some practioners. [38]  There are  wide-necked or irregular ruptured aneurysms for which stents or flow diverters must be used with careful management of antiplatelet medication.

Since 1995, when it was approved by the US Food and Drug Administration (FDA), coiling has become the primary treatment modality for aneurysms in many centers. Whereas coiling was initially used for aneurysms not amenable to surgical clipping, the improved set of endovascular tools currently available allows wide-necked, irregular, and fusiform aneurysms not previously amenable to treatment to be treated effectively with endovascular techniques. Flow diversion in particular has been a disruptive technology, enabling endovascular treatment and cure of aneurysms that previously had no good treatment strategy or could only be bypassed and trapped.

Treatment strategies for treatment of wide-necked Treatment strategies for treatment of wide-necked aneurysms include balloon-assisted (above) and stent-assisted (below) techniques.
Endovascular coiling of large basilar tip aneurysm Endovascular coiling of large basilar tip aneurysm (precoiling, upper image) using two stents (not visible) and then coiling (lower image).

For each embolization procedure, access is gained in the femoral, radial, or brachial artery, and a guide catheter is placed in the cervical ICA or the VA. Microcatheters of varying sizes can then be navigated into the aneurysm or its parent vessel. Coils or intrasaccular devices are delivered into the aneurysm, and stents or flow diverters can be delivered into the parent vessel. Angiograms are sequentially performed to ensure preservation of the parent vessel and obliteration of the aneurysm. Follow-up studies are usually planned at 6 months, 1 year, and 2 years to check for stability and continued obliteration. If the aneurysm remains obliterated, then follow-up MRA is arranged every 5 years until the age of 80 years.