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Intracranial Aneurysms and Subarachnoid Hemorrhage


An intracranial aneurysm is a cerebrovascular disorder in which weakness in the wall of a brain artery causes dilation or ballooning, which can grow and rupture over time. The term aneurysm originally comes from the Greek aneurysma—ana meaning “across” and eurys meaning “broad.” Rupture of an intracranial aneurysm causes aneurysmal subarachnoid hemorrhage (aSAH), which accounts for 5%–10% of all strokes. aSAH causes greater morbidity and mortality than other stroke types. At least one-quarter of patients who are admitted to the hospital die, and half of the survivors have permanent neurological deficit. Two-thirds of aSAH survivors regain functional independence but half have cognitive impairments, half are dissatisfied with life, and only one-third resume the same work as before the rupture ( ). Fortunately, there is a trend of improving outcomes in these patients thanks to early aneurysm repair, aggressive management of hydrocephalus and delayed cerebral ischemia (DCI), as well as better management of general medical complications. As there are an increasing number of neurointensivists and neurohospitalists as well as practicing vascular and general neurologists taking care of these patients, there is also a need to increase the exposure of neurology trainees to the management of aSAH.

Intracranial aneurysms are becoming more frequently detected, given the increased availability of advanced noninvasive diagnostic neurovascular imaging technologies such as computed tomographic angiography (CTA) and magnetic resonance angiography (MRA). Considering that intracranial aneurysm is a fairly common disease and is frequently discovered incidentally or presents symptomatically in young patients, at an age at which a disabling or fatal aSAH can jeopardize decades of otherwise healthy life, in-depth understanding of intracranial aneurysms and their complications is very important for modern comprehensive management of aSAH patients.

Epidemiology

The annual incidence of aSAH varies across the world, ranging from 2 to 16 cases per 100,000, with the highest rate in Tbilisi, Georgia and the lowest in Dijon, France ( ). In the United States, an analysis of the Nationwide Inpatient Sample provided an annual estimate of 14.5 discharges for aSAH per 100,000 adults ( ). However, up to 15% of patients with acute aSAH die before hospital admission, so the true incidence of aSAH might be even higher ( and ). aSAH is uncommon in children and increases with pediatric age. Resulting in an incidence from 0.18 to 2.0 per 100,000 ( and ). The average age of adult patients with aSAH is 55 years ( ). The incidence of aSAH is overall 1.2 times higher in women than in men. However, some evidence suggests an interaction of age and sex, with a higher incidence in younger men (25–45 years of age), women between 55 and 85 years of age, and men older than 85 ( ).

The prevalence of unruptured aneurysms in the general adult population is estimated at 3.2% in imaging studies and 5% adults in autopsy series ( ). Fortunately, most intracranial aneurysms do not rupture during the lifetime of an individual. While most persons harbor a single aneurysm, multiple aneurysms are common, occurring in 10%–30% of patients in large clinical series ( ). Multiple aneurysms can be mirror aneurysms or located asymmetrically in different locations in the circle of Willis.

Aneurysms can appear sporadically or in family clusters, with familial aneurysms, defined as ≥2 first-degree relatives affected, accounting for between 7% and 20% of patients ( ). Inheritance pattern is unclear. Leading possibilities are single-gene mutations with highly incomplete penetrance or multiple interacting genetic variations each with small effect size, and multiple intracranial aneurysms risk genes have been identified ( ). Familial aneurysms tend to be located in the middle cerebral artery and rupture at a younger patient age ( ).

Associated Conditions

Several hereditary and acquired conditions are associated with the formation of intracranial aneurysms and aSAH ( Box 67.1 ). More generally, any congenital or acquired condition that produces vascular wall weakening and/or increased hemodynamic stress could result in the formation of intracranial aneurysms.

BOX 67.1
Risk Factors and Associated Conditions for Intracranial Aneurysms

  • Hypertension

  • Tobacco smoking

  • Autosomal dominant inherited polycystic kidney disease

  • Ehlers-Danlos syndrome, vascular type (formally called type IV)

  • Fibromuscular dysplasia

  • Coarctation of the aorta

  • Moyamoya syndrome

  • Pseudoxanthoma elasticum

  • α 1 -Antitrypsin deficiency

  • Systemic lupus erythematosus

  • Sickle cell anemia

  • Bacterial endocarditis

  • Fungal infection

  • Neurofibromatosis type 1

  • Tuberous sclerosis

  • Pheochromocytoma

  • Arteriovenous malformations

  • Anomalous carotid-vertebrobasilar anastomoses

  • Cocaine use

Approximately 5% of aneurysms are associated with connective tissue disorders, the most important being Ehlers-Danlos syndrome, vascular type (type IV), neurofibromatosis type 1, and autosomal dominant polycystic kidney disease ( ). Among all patients with polycystic kidney disease, approximately 8% harbor saccular aneurysms ( ). The vascular type of Ehlers-Danlos syndrome, formally called type IV , has a defect of type-III collagen synthesis, which is a major component of distensible tissues such as vessels. A higher incidence of intracranial aneurysms in the cavernous segment of internal carotid artery (ICA) has been reported. The indication of arterial or venous puncture for catheter angiography in patients with Ehlers-Danlos syndrome should be carefully discussed because of increased vascular fragility and complications.

There are several known modifiable risk factors for aSAH, including—most prominently—hypertension, smoking, alcohol abuse, and the use of sympathomimetic drugs (e.g., cocaine). The role of elevated blood pressure in the formation of intracranial aneurysms has been supported by the association of aneurysms coarctation of the aorta, pheochromocytoma, and cocaine use—all conditions that can cause severe elevated blood pressure. Hypertension likely contributes to aneurysm formation and rupture via multiple mechanisms, including promoting wall injury and remodeling, and increasing radial force upon weakened wall segments. Active cigarette smoking is a risk factor for intracranial aneurysm formation ( ) and may increase the risk of aneurysm rupture. American Heart Association/American Stroke Association guidelines recommend treatment of these risk factors to avert first or recurrent subarachnoid hemorrhage ( ). Interestingly, despite the increased success in the treatment of these factors, the incidence of aSAH has not changed in 35 years, though this may, in part, reflect improved detection counterbalancing mild actual incidence decline ( ).

Pathophysiology

Intracranial aneurysms are classified according to their morphology, size, pathogenesis, and anatomical location ( Box 67.2 ). There are three morphological types of intracranial aneurysms: saccular, fusiform, and dissecting ( Fig. 67.1 ).

BOX 67.2
Classification of Intracranial Aneurysms
AICA, Anterior inferior cerebellar artery; PICA, Posterior inferior cerebellar artery; SCA, Superior cerebellar artery.

By Morphology

  • Saccular

  • Fusiform

  • Dissecting

By Pathogenesis

  • Genetic

  • Hemodynamic (arteriovenous malformation, contralateral carotid ligation, etc.)

  • Infection (bacterial, fungal)

  • Trauma

  • Atherosclerosis

  • Neoplasm (primary or metastatic)

  • Radiation

  • Drug abuse

  • Connective tissue disorders

  • Vasculopathy

  • Miscellaneous conditions (moyamoya disease, etc.)

By Size

  • <3 mm (small)

  • 3–6 mm (small)

  • 7–12 mm (small-medium)

  • 13–25 mm (large)

  • >25 mm (giant)

By Location

Internal Carotid Artery

  • 1.

    Cavernous

  • 2.

    Carotid cave

  • 3.

    Paraclinoid

  • 4.

    Ophthalmic

  • 5.

    Superior hypophyseal

  • 6.

    Posterior communicating

  • 7.

    Anterior choroidal

  • 8.

    Carotid bifurcation

Anterior Cerebral Artery

  • 1.

    A1

  • 2.

    Anterior communicating

  • 3.

    A1–2

  • 4.

    A2 and distal

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