Cerebral Aneurysms

Cerebral Aneurysms

Deidre A. Buckley

Joanne V. Hickey


A cerebral aneurysm is a saccular outpouching of a cerebral artery. Intracranial saccular aneurysms or berry aneurysms account for approximately 80% to 90% of all intracranial aneurysms (IAs) and are the most common cause of nontraumatic subarachnoid hemorrhage (SAH). These small, berry-like projections occur at arterial bifurcations in the circle of Willis, with other shapes such as pedunculated, sessile, and multilobulated aneurysms occasionally seen.1 Rupture of a cerebral aneurysm usually results in an SAH, which is defined as bleeding into the subarachnoid space. The prevalence of unruptured IAs is variable based on current studies. Most recent prospective autopsy and angiographic studies indicate an overall frequency of IAs to be about 2% to 4%, suggesting that unruptured IAs will affect about 6 million individuals in the United States at some point in their lives.2, 3

The authors would like to acknowledge Michael Phillips, BA for his contributions of vignettes, and also his review and suggestions related to content for the chapter.

The mean age of the US population is increasing and IAs seem to become clinically apparent with increasing age. The incidence of SAH from aneurysms also increases with age. Most studies have found an increased occurrence of aneurysms in women compared with men, although in childhood and adolescence the ratio appears to be opposite, with a male-to-female ratio of 2:1.1 The annual incidence of SAH in North America is about 15 cases per 1,00,000 or about one hemorrhage every 18 minutes. About 80% of patients with nontraumatic SAH have ruptured saccular aneurysms, which occur in 30,000 patients annually in the United States.4

SAH is one of the most feared causes of acute headache upon presentation to the emergency department (ED). Headache accounts for 1% to 2% of ED visits and up to 4% of visits to primary care offices. Among the patients who present to the ED with headaches, approximately 1% has SAH. Two prospective studies found that if only patients with “the worst headache” of their lives and a normal neurological exam were considered, 12% of the patients would have SAH.5, 6, 7 Early and accurate diagnosis is critical. The initial hemorrhage may be fatal. For those who survive, patients may have minor to severe neurological deficits. Despite the widespread availability of neuroimaging, misdiagnosis or delays in diagnosis occur in up to 25% of patients with SAH8 when initially presenting for medical treatment. A list of common misdiagnoses is included in Table 24-1.9


No diagnosis made, or headache of unknown etiology

Transient ischemic attack/ischemic stroke

Headache (migraine, tension, or cluster headache)


Meningitis or encephalitis

Hypertensive crisis

Neck problems (arthritis or cervical disc disease)


Alcohol or drug intoxication

Psychiatric diagnoses


Cerebral aneurysms present in a variety of sizes, shapes, and etiologies. When classified by size, the following categories are used.

  • Small: up to 10 mm

  • Medium: 10 to 15 mm

  • Large: 15 to 25 mm

  • Giant: 25 to 50 mm

  • Super-giant: larger than 50 mm

Classification by shape and etiology includes the following categories.

  • Berry aneurysm: most common type; berry or saccular shaped with a neck or stem (Fig. 24-2)

  • Fusiform aneurysm: an outpouching of an arterial wall, without a stem (Fig. 24-2)

  • Traumatic aneurysm: any aneurysm resulting from a traumatic brain injury (accounts for a small number)

  • Mycotic (infectious) aneurysm: rare; caused by septic emboli from infections, such as bacterial endocarditis; may lead to aneurysmal formation

    Figure 24-1 ▪ Computed tomography angiogram demonstrating basilar apex aneurysm and right posterior communicating artery aneurysm. (Courtesy: Christopher S. Ogilvy, MD, Massachusetts General Hospital, Boston, MA.)

    Figure 24-2 ▪ Aneurysms. (A) Saccular (sac-like), with a well-defined neck. (B) Fusiform (spindle shaped) without a distinct neck. (Copyright 2003 American Stroke Association.)

  • Charcot-Bouchard aneurysm: microscopic aneurysmal formation associated with hypertension; involves the basal ganglia and brainstem

  • Dissecting aneurysm: related to atherosclerosis, inflammation, or trauma; an aneurysm in which the intimal layer is pulled away from the medial layer and blood is forced between the layers


Cerebral aneurysms usually occur at the bifurcations and branches of the large arteries at the base of the brain (circle of Willis). Eighty-five percent of aneurysms develop in the anterior part of the circle of Willis. The remaining 15% are found in the posterior circulation, known as the vertebrobasilar system (see Figure 24-3 for distribution of aneurysms).

Figure 24-3 ▪ Aneurysms located on arteries of the circle of Willis.

The most common sites of saccular aneurysms are the following.

  • 85% to 95% in the carotid system, with the following three most common locations.

    • Anterior communicating artery (Acomm) is the single most common: 30%

    • Anterior cerebral artery (ACA) are more common in males

    • Posterior communicating artery (Pcomm): 25% to 30%

    • Middle cerebral artery (MCA): 20%

  • 5% to 15% in the posterior circulation (vertebrobasilar arteries)

    • About 10% on the BA: basilar bifurcation, known as basilar tip, most common followed by basilar artery-superior cerebellar artery (SCA) (BA-SCA), basilar artery-vertebral artery (VA) (BA-VA) junction, and anterior inferior cerebellar artery (AICA)

    • About 5% on the VA and posterior inferior cerebellar artery (PICA) junction is the most common

    • Fusiform aneurysms are more common in the vertebrobasilar system

  • 20% to 30% of patients who suffer an aneurysm will have multiple aneurysms

Multiple aneurysms occur in 20% to 25% of patients with saccular aneurysms, with approximately 20% of patients with saccular aneurysms having a family history of IAs or SAH.10 A variety of other pathologic entities have been associated with IAs such as polycystic kidney disease, arteriovenous malformations, coarctation of the aorta, moyamoya disease, Marfan’s syndrome, Ehlers-Danlos syndrome, pituitary tumors, and pseudoxanthoma.3


Although the precise etiology of cerebral aneurysms remains unclear, many extrinsic, congenital, and genetic factors have been implicated in the formation and rupture of IAs. One theory suggests that a congenital/developmental defect exists in the medial and adventitial layers of the artery in the circle of Willis. There is little scientific basis for this theory, because these defects are commonly found post mortem in persons without aneurysms. The degenerative theory is strongly supported by current research and ascribes causation to hemodynamically induced degenerative vascular disease.11 According to this theory, the intima, covered only by the adventitia, bulges from a local weakness; by late midlife, stress causes vessel ballooning and rupture. There may be a predisposition to aneurysm formation in individuals with hypertension and in those in whom connective tissue disease promotes fragility of the arterial wall.11 Forbus12 suggested that aneurysms were acquired lesions resulting from degeneration of the elastic membrane due to a continued overstretching, combined with an underlying congenital defect in the muscularis portion of the arterial wall. Glynn proposed that the degeneration of the internal elastic lamina, possibly caused by atherosclerosis, was the leading cause in the formation of a saccular aneurysm. He stated that both congenital medial defects and acquired internal elastic defects had to be present before aneurysm formation.13

Phillips et al.14 noted that the distribution of the defects was inconsistent with the frequency of distribution of berry (saccular) aneurysms in humans. He further rationalized that the thinning of the arterial wall occurred early and was associated with the degeneration of the cells in the elastica and of the muscle cells of the intracellular matrix. He also noted that the increased rate of aneurysmal SAHs with increasing age suggests that they are not congenital lesions. This does not preclude congenital predisposition. More recently, research has demonstrated an association between the presence of specific human leukocyte antigen alleles and the genetic role they may play in aneurysm formation.15

Although the etiology of most IAs is as yet unknown, there are types of intracerebral aneurysms in which the etiology has been well demonstrated. Cerebral trauma can result in traumatic IA from localized arterial tear. Bacterial and fungal infections have also been known to cause infectious (mycotic) aneurysms. Infectious aneurysms form when bacteria, usually from septic emboli, break off and invade and destroy the vessel wall. Atherosclerotic aneurysms can form in vessel walls that have been damaged by deposits of atheromatous material, resulting in fusiform aneurysms. Fusiform aneurysms are rarely associated with SAH.

Familial Intracranial Aneurysms

There have been many families with documented IAs. Familial intracranial aneurysms (FIAs) are defined as the presence of two or more family members among first- and second-degree relatives with proven aneurysmal SAH or incidental IAs.16, 17, 18 Incidence of FIAs among SAH patients is 6% to 20%.19, 20 The estimated prevalence of FIAs among patients with SAH is 7% to 10%. Ogilvy21 states that in his experience with patients with ruptured and unruptured aneurysms, approximately 22% will have family members with aneurysms. Leblanc et al.22 reviewed 13 families with IAs and found the occurrence in the same decade in two affected siblings in 10 of 12 cases (83%). Although the usual incidence of morbidity and mortality from initial rupture of hemorrhage is about 40%, when familial patients are reviewed, the incidence of patients who suffered death or disability at the initial hemorrhage is 70%.

The familial occurrence does suggest the possibility of a genetically determined defect of the arterial wall. Several studies report that individuals with FIAs are more likely to have multiple aneurysms, and that these aneurysms are more likely to rupture at
a smaller size than those patients with an isolated aneurysm.23 Therefore, treatment considerations are different for patients with an unruptured isolated aneurysm.

The FIA study recruited a number of families with a goal to identify genes that underlie the development and rupture of IAs. This study was sponsored by the National Institutes for Health (NIH) and the National Institute of Neurological Disorders and Stroke (NINDS); it is the largest genetic linkage study to date. The study includes 26 clinical centers which have broad experience in clinical management and imaging of patients with IAs. Enrollment of 542 families with 2,874 subjects was completed in November, 2011.24 There is evidence suggesting that a genetic component plays an important role in the development of IAs, but specific loci affecting the risk of IAs have not been identified. The primary hypothesis of this study was that there are specific human chromosomal regions that are associated with an increased risk of IAs. The long-term objective is to identify susceptibility genes that are related to the formation of IAs.25

Population-based studies suggest that genetic factors play an important role in IA formation and aneurysm rupture. The risk of unruptured aneurysms, as determined by noninvasive screening such as magnetic resonance angiography (MRA) in unaffected relatives of families with two or more members who have an IA, is about four times greater than the risk among the general public.26 These disorders account for less than 1% of all IAs in the population and hence cannot explain the familial aggregation of IAs.25

Another important aspect of the systematic, genetic study of IA formation is the critical need to consider environmental factors in disease risk such as smoking. Nearly 80% of patients with an IA have a history of smoking at some time in their life.27 Not all individuals who smoke develop IA. Smoking may increase the risk of IA in individuals with specific genotypes at IA susceptibility loci.

The FIA study is powered to detect genes associated with the development of IA rupture and may hopefully determine the complex relationships between genes and the environment that can lead to death.25 The FIA study demonstrated that small unruptured IAs in patients who have a history of hypertension and smoking may have a higher risk of rupture than sporadic unruptured IAs of the same size. This needs to be taken into consideration when counseling a patient about treatment options. The FIA study identified sequence variants on chromosome 9p21.3, 8q11.12-12.1, and 2q33.1 which are associated with IAs.24

Various genetically determined conditions have been associated with IA; a definite link has only been associated with autosomal dominant polycystic kidney disease. The percentage of these patients that have IA varies from 10% to 40%. There is no identified definitive pattern of inheritance. Other diseases frequently mentioned such as Marfan’s syndrome, Ehlers-Danlos syndrome type 4, neurofibromatosis, and pseudoxanthoma elasticum are more often associated with carotid cavernous fistulas than with aneurysms.21 The current recommendation for screening is that each family member should undergo magnetic resonance imaging (MRI), MRA, or computed tomography angiography (CTA). It is recommended that family members be screened in their 20s and every 5 to 10 years thereafter. It is thought that patients who have a family history of IA may develop symptoms at a younger age than those with spurious aneurysms, although there is no conclusive study.21

After an IA is identified in a patient with a family history of aneurysms, treatment is often recommended. Endovascular, surgical, or combined options are considered, and the treatment with the lowest risk and highest efficacy of obliterating the IA is recommended to the individual.21

Intracranial Aneurysms in the Pediatric Population

IAs in children (younger than 18 years old) are rare, with a reported prevalence ranging from 0.5% to 4.6%;26 their epidemiology is poorly understood. Pediatric IAs occur more often in male patients and have a predilection for the terminal ICA bifurcation.

A total of 706 aneurysms have been reported in the pediatric population in English-language literature since 1939. Male-to-female occurrence is 1.8:1. SAH was the presentation in 80% of the children. The most common overall location was the ICA terminus which was the location in 26% of the cases. Only 17% of IAs occurred in the posterior circulation, and one fifth were giant lesions. Surgical treatment was performed in 79% of the cases. There was an overall good outcome in 60% of the cases, with mortality in 28%.28 The gender predominance (male-to-female ratio 1.8:1) may suggest the existence of differences in pathogenesis of IA formation in the pediatric patient. One interpretation is that congenital factors present in all IA patients may be expressed more in boys, but environmental factors may contribute to the increased incidence in girls.28

Pediatric aneurysms are likely to be pathologically distinct from adult saccular aneurysms.29 The characteristic abrupt termination of the internal elastic lamina and muscularis media at the entrance to adult saccular aneurysms at arterial bifurcations thought to be susceptible to sheer stress or to hypertensive and atherosclerotic deterioration was not found in the autopsy series of pediatric aneurysm specimens.28 Other theories for aneurysm formation in children include a connective tissue abnormality, an infectious process, a congenital anomaly causing an internal or medial elastic membrane defect, or head injury including birth trauma.30 More studies are necessary to better understand the differences in aneurysm formation in children versus adults. The male dominance and distinct anatomic location suggest the expression of a yet undetermined pathologic mechanism of aneurysm formation in children.28


Aneurysm Hemodynamics

The occurrence, growth, thrombosis, and rupture of intracranial saccular aneurysms can be directly related to the effect of hemodynamic forces. Strong evidence favors the idea that IAs of this nature occur because of hemodynamically induced degenerative vascular injury.11 Despite in vitro studies over the last decade, the exact mechanisms of action of these stresses remain to be completely understood, and guidelines for determining the likelihood that a particular IA will grow, rupture, regress, or thrombose do not exist.

Evolving and improved imaging techniques that depict accurate vascular morphology as well as quantification of particular aspects of cerebral hemodynamics provides an opportunity to add to the understanding of the natural history of saccular aneurysms.31 An assessment of the chance of rupture of an asymptomatic saccular aneurysm is almost entirely based on a statistical analysis of the natural history of “similar lesions.” Several factors combine to influence the magnitude of this risk (e.g., history of smoking, age, gender, number of aneurysms), and they all play a role in formulating techniques that would allow a more objective and individual estimation of risk. Research directed at the use of
several evolving techniques in angiography, magnetic resonance, and ultrasound aimed at the improved imaging of hemodynamic stresses that affect IAs may better define the relationship between the stresses and the vascular remodeling seen in aneurysm wall degeneration and healing.31

Aneurysm growth is a dynamic process that results from complex and incompletely understood interactions between hemodynamic forces and production of structural components that compose an aneurysm’s wall. Due to physical limitations in the collagen present at the site where IAs form, it would be expected that a growth much larger than 8 mm in diameter would result in rupture. However, in most instances, this does not occur. Changes in various hemodynamic factors (e.g., shear stress, pressure, and impingement force) as well as the presence of several humoral factors (e.g., inflammatory mediators and adhesion molecules) are potential signs of arterial injury. Sensing these signs, endothelium has the capacity to regulate the activity of substances that act to either promote or inhibit the repair of an aneurysm wall.31 Combinations of CT, CTA, MRI, MRA, and ultrasound can, in some instances, measure both geometric relationships and physiologic parameters such as shear stress, pulse pressure, and compliance before and after a given intervention. As these techniques evolve, improve, and become more widely used, they will provide data that will be valuable in both the laboratory and in a clinical setting.31

Natural History and Incidence of Subarachnoid Hemorrhage

Many questions regarding natural history of SAH can be answered from available data. SAH of any cause represents from 4.5% to 13% of all strokes. Age-adjusted incidence rates of SAH are available from numerous studies, with reported rates from 7.9 per 1,00,000 persons per year in Oxfordshire, England. In China, there are 2 cases per 1,00,000 compared to 22.5 cases per 1,00,000 in Finland. In Japan, there has been higher incident rates reported—21 to 25 per 1,00,000. Long-term trends are only available from Rochester, Minnesota, in population-based studies, which demonstrate that there was no change in the incidence rate of SAH through 1989. There are some gender-related differences, with female incidence higher than male in some population-based studies.32, 33

The incidence of aneurysmal SAH is higher than most other neurological diseases such as brain tumor, multiple sclerosis, and bacterial meningitis. The incidence of other types of stroke (intracerebral hemorrhage and cerebral infarction) has declined over the years, while the incidence of aneurysmal SAH remains the same.34 There is a possible reduced occurrence of SAH in some premenopausal women, especially those without a smoking history. In the same study, hormone replacement reduced the risk in postmenopausal women who had never smoked.35 Racial differences have been infrequently studied, although there are data that indicate that the incidence rate in the African American population is twice that of whites.36 Age-related differences are detected, with increasing incidence of SAH with age.

SAH is a type of intracranial hemorrhage in which bleeding occurs into the subarachnoid space. It accounts for 6% to 8% of all strokes and continues to be a significant cause of morbidity and mortality. Approximately 12% of patients die before receiving medical attention.23 SAH is associated with mortality rates between 25% and 50% from the consequences of the initial rupture. Approximately half of the untreated survivors will rebleed within the next 6 months, and among this group, morbidity and mortality are even higher.37 Approximately 30% of people die within the first 2 weeks following the acute event, and many survivors have persistent long-term deficits. Although advanced imaging techniques have allowed the noninvasive detection of an aneurysm or AVM as the potential source of a bleed, most aneurysms are detected following rupture. After SAH has occurred, medical complications are aggressively treated, but morbidity from factors other than rebleeding is still common.

Figure 24-5 ▪ Axial CT scan demonstrating large intraparenchymal hemorrhage within the right temporal lobe. (Courtesy: Christopher S. Ogilvy, MD, Massachusetts General Hospital, Boston, MA.)

Figure 24-6 ▪ Computed tomography angiography with 3D reconstruction of large 11-mm bi-lobed aneurysm arising from the distal right M1 segment. (Courtesy: Christopher S. Ogilvy, MD, Massachusetts General Hospital, Boston, MA.)

Figure 24-7 ▪ Axial CT scan demonstrating increase in size of large intraparenchymal hemorrhage within right temporal lobe with 15 mm of leftward midline shift, right uncal herniation, right subfalcine herniation, and compression and displacement of the midbrain. (Courtesy: Christopher S. Ogilvy, MD, Massachusetts General Hospital, Boston, MA.)

Jul 14, 2016 | Posted by in NURSING | Comments Off on Cerebral Aneurysms

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