Subarachnoid Hemorrhage and Aneurysms

Main Text

Preamble

Trauma is, by far, the most common cause of intracranial subarachnoid hemorrhage. Traumatic subarachnoid hemorrhage occurs when blood from contused brain or lacerated vessels extends into adjacent sulci; it was discussed in connection with craniocerebral trauma (Chapter 2). This chapter focuses on nontraumatic subarachnoid hemorrhage and aneurysms.

Subarachnoid Hemorrhage

Preamble

Spontaneous (i.e., nontraumatic) subarachnoid hemorrhage (SAH) accounts for 3-5% of all acute “strokes.” Approximately 80% of these are caused by a ruptured intracranial saccular aneurysm (SA). Other identifiable causes of nontraumatic SAH (ntSAH) include a variety of entities, such as dissections, venous hemorrhage or thrombosis, vasculitis, amyloid angiopathy, and reversible cerebral vasoconstriction syndrome (RCVS). No identifiable origin is found in 10-12% of patients presenting with ntSAH.

Neuroimaging plays a central role in the diagnosis and management of SAH. We begin this discussion with imaging aneurysmal SAH (aSAH) and its most devastating complications, vasospasm and secondary cerebral ischemia.

We then review two special types of nontraumatic, nonaneurysmal SAHs: Perimesencephalic SAH and an unusual pattern of SAH called convexity or convexal SAH (cSAH). Lastly, we discuss chronic repeated SAH and its rare but important manifestation, superficial siderosis (SS).

Aneurysmal Subarachnoid Hemorrhage

Terminology

aSAH is an extravasation of blood into the space between the arachnoid and pia. The typical location of aSAH (basal cisterns and sylvian and inferior interhemispheric fissures) usually helps distinguish it from other causes of ntSAH (see shaded box, p. 99).

Etiology

aSAH is most often caused by rupture of a saccular (“berry”) or (rarely) a blood blister-like aneurysm (BBA) (6-1). Other less common causes of aSAH include intracranial dissections and dissecting aneurysms.

Pathology

Location

Because most SAs arise from the circle of Willis or middle cerebral artery (MCA) bifurcation, the most common locations for aSAH are the suprasellar cistern and sylvian fissures (6-2)(6-3).

Occasionally, an aneurysm ruptures directly into the brain parenchyma rather than the subarachnoid space. This occurs most frequently when the apex of an anterior communicating artery (ACoA) aneurysm points upward and bursts into the frontal lobe.

Gross Pathology

The gross appearance of aSAH is typically characterized by blood-filled basal cisterns (6-17). SAH may extend into the superficial sulci and ventricles.

Clinical Issues

Demographics

The overall incidence of aSAH increases with age and peaks between 40-60 years old; M:F = 1:2.

aSAH is rare in children. Regardless of their relative rarity, however, cerebral aneurysms cause the majority of spontaneous (nontraumatic) SAHs in children and account for ~ 10% of all childhood hemorrhagic “strokes.”

Presentation

At least 75% of patients with aSAH present with sudden onset of the “worst headache of my life.” The most severe form is a “thunderclap” headache, an extremely intense headache that comes on “like a boom of thunder” and typically peaks within minutes or even seconds. There are many causes of “thunderclap” headache. The most serious and life threatening is aSAH, although it accounts for just 4-12% of these severe headaches.

1/3 of patients with aSAH complain of neck pain and 1/3 report vomiting. 10-25% experience a “sentinel headache” days or up to two weeks before the onset of overt SAH. These “sentinel headaches” are sudden, intense, persistent, and may represent minor bleeding prior to aneurysm rupture.

Natural History

Although aSAH causes just 3-5% of all “strokes,” nearly 1/3 of all stroke-related years of potential life lost before age 65 are attributable to aSAH. The mean age at death in patients with aSAH is significantly lower than in patients with other types of strokes.

Despite significant advances in diagnosis and management, aSAH is fatal or disabling in > 2/3 of patients. Massive SAH can cause coma and death within minutes. Approximately 1/3 of patients with aSAH die within 72 hours; another 1/3 survive but with disabling neurologic deficits.

Treatment Options

The goals of aSAH treatment in patients who survive their initial bleed are (1) to obliterate the aneurysm (preventing potentially catastrophic rebleeding) and (2) to prevent or treat vasospasm.

Imaging

General Features

NECT is an excellent screening examination for patients with “thunderclap” headache and suspected aSAH. In the first six hours after ictus, the sensitivity of modern CT scanners approaches 100%. Lumbar puncture is now considered unnecessary if the NECT is negative and the neurologic examination is normal.

The best imaging clue to aSAH is hyperdense cisterns and sulci on NECT.

CT Findings

The basal cisterns—especially the suprasellar cistern—are generally filled with hyperdense blood (6-4). Although SAH distribution generally depends on location of the “culprit” aneurysm, it is also somewhat variable and not absolutely predictive of aneurysm location.

ACoA aneurysms often rupture superiorly into the interhemispheric fissure. MCA bifurcation aneurysms usually rupture into the sylvian fissure. Internal carotid-posterior communicating artery (ICA-PCoA) aneurysms generally rupture into the suprasellar cistern (6-6A). Vertebrobasilar aneurysms often fill the fourth ventricle, prepontine cistern, and foramen magnum with blood.

Intraventricular hemorrhage (IVH) is present in nearly 1/2 of all patients with aSAH. Focal parenchymal hemorrhage is uncommon but, if present, is generally predictive of aneurysm rupture site.

MR Findings

Acute aSAH is isointense with brain on T1WI (6-7B)(6-8C). The CSF cisterns may appear smudged or “dirty”(6-8D). Hyperacute aSAH is isointense with CSF and may be difficult to identify. Acute aSAH is generally hypointense relative to CSF on T2WIs (6-7C)(6-8E).

FLAIR is the best sequence to depict acute aSAH. Hyperintensity in the sulci and cisterns is present (6-8F) but nonspecific. Other causes of “bright” CSF on FLAIR include hyperoxygenation, meningitis, neoplasm, and artifact.

Angiography

CTA is positive in 93% of aSAH cases if the “culprit” aneurysm is 3 mm or larger (6-6A). The addition of 3D postprocessing tools can improve detection of small saccular or BBAs on CTA.

Many patients with aSAH and positive CTA undergo surgical clipping without DSA. DSA identifies vascular pathology in 7-13% of patients with CTA-negative SAH, so such patients should be considered candidates for DSA.

So-called angiogram-negative SAH is found in ~ 15% of cases (6-8). With the addition of 3D rotational angiography and 3D shaded surface displays, the rate of “angiogram-negative” SAH decreases to 4-5% of cases. If the initial DSA is negative, many experts recommend repeat angiography in 2-4 weeks to exclude aneurysm as the cause of the SAH.

Differential Diagnosis

The major differential diagnosis of aSAH is traumatic SAH (tSAH). aSAH is generally much more widespread, often filling the basal cisterns. tSAH typically occurs adjacent to cortical contusions or lacerations and is therefore most common in the superficial sulci.

Perimesencephalic nonaneurysmal SAH (pnSAH) is much more limited than aSAH and is localized to the interpeduncular, ambient, and prepontine cisterns. Occasionally, pnSAH spreads into the posterior aspect of the suprasellar cistern. It rarely extends into the sylvian fissures.

Convexal SAH (cSAH) is, as the name implies, localized to superficial sulci over the cerebral convexities. Often, only a single sulcus is affected. Causes of cSAH are numerous and include cortical vein occlusion, amyloid angiopathy, vasculitis, and RCVS.

Pseudo-SAH is caused by severe cerebral edema. The hypodensity of the brain makes blood in the cerebral arteries and veins appear dense, mimicking the appearance of SAH.

Sulcal-cisternal FLAIR hyperintensity on MR is a nonspecific imaging finding. It occurs with hemorrhage, meningitis, carcinomatosis, hyperoxygenation, stroke, and gadolinium contrast (blood-brain barrier leakage or chronic renal failure). FLAIR “bright” CSF can also result from flow disturbances and technical artifacts (e.g., incomplete CSF nulling).

Pyogenic meningitis, meningeal carcinomatosis, and high inspired oxygen concentration may also cause CSF hyperintensity on FLAIR. Prior administration of gadolinium chelates (with or without decreased renal clearance) can result in diffuse delayed CSF enhancement.

Other etiologies of sulcal-cisternal FLAIR hyperintensity include hyperintense vessels with slow flow (e.g., acute arterial strokes, pial collaterals developing after cerebral ischemia-infarction, Sturge-Weber syndrome, moyamoya, and RCVS).

Post Aneurysmal Subarachnoid Hemorrhage Cerebral Ischemia and Vasospasm

Published data suggests delayed cerebral ischemia affects 20-40% of patients who survive their initial SAH and is the major cause of morbidity and death in this group. Cerebral vasospasm (CVS) is the most common cause of cerebral ischemia and typically occurs 4-10 days after aSAH (6-5). Recent studies have also shown that inflammation and microthromboembolism also contribute significantly to ischemic complications following aSAH.

Imaging Post Aneurysmal Subarachnoid Hemorrhage Complications

Noninvasive methods to detect early-stage post-aSAH complications include transcranial Doppler ultrasound, CTA, and MR/MRA. Multiple segments of vascular constriction and irregularly narrowed vessels are typical findings on CTA and DSA (6-6). MR with DWI and pMR is most sensitive for detecting early ischemic changes following aSAH.

DSA is typically performed if endovascular treatment or intraarterial administration of antispasmolytic agents, such as nicardipine, is anticipated.

Differential Diagnosis

The differential diagnosis of vasospasm within the context of existing SAH is limited. If the patient has a known aneurysm with recent SAH, the findings of multisegmental vascular narrowing indicate CVS. However, if the SAH is convexal, the differential diagnosis includes RCVS and vasculitis.

Other Complications of Aneurysmal Subarachnoid Hemorrhage

Obstructive hydrocephalus commonly develops in patients with aSAH, sometimes within hours of the ictus (6-6A), and may be exacerbated by the presence of IVH. Imaging studies show increased periventricular extracellular fluid with “blurred” lateral ventricle margins.

Perimesencephalic Nonaneurysmal Subarachnoid Hemorrhage

Terminology

pnSAH is also known as benign perimesencephalic SAH. pnSAH is a clinically benign SAH subtype that is anatomically confined to the perimesencephalic and prepontine cisterns (6-9).

Etiology

The precise etiology of pnSAH is unknown, although recent studies suggest it can be triggered by physical exertion in nearly 80% of cases. The bleeding source in pnSAH is usually undetermined; most investigators implicate venous—not aneurysmal—rupture as the most likely cause.

Clinical Issues

pnSAH is the most common cause of nontraumatic, nonaneurysmal SAH. The typical presentation is mild to moderate headache. Occasionally, patients experience severe “thunderclap” headache with meningismus.

The peak age of presentation in patients with pnSAH is between 40-60 years—identical to that of aSAH. There is no sex predilection.

Most cases of pnSAH follow a clinically benign and uneventful course, although MR demonstrates acute—and usually asymptomatic—ischemic lesions in nearly 1/2 of all cases. Rebleeding is uncommon (< 1%). In contrast to aSAH, vasospasm and delayed cerebral ischemia are rare.

Imaging

pnSAH has well-defined imaging features. NECT scans show focal accumulation of subarachnoid blood around the midbrain (in the interpeduncular and perimesencephalic cisterns) (6-10) and in front of the pons. High-resolution CTA is used to rule out underlying aneurysm or dissection (6-11). If the initial CTA is negative, there is no significant additional diagnostic yield from DSA.

Differential Diagnosis

The major differential diagnosis of pnSAH is aSAH. aSAH is significantly more extensive, spreading throughout the basal cisterns and often extending into the interhemispheric and proximal sylvian fissures.

tSAH would be suggested both by history and imaging appearance. tSAH occurs adjacent to contused brain. It is usually more peripheral, lying primarily within the sylvian fissure and over the cerebral convexities. During closed head injury, the midbrain may be suddenly and forcibly impacted against the tentorial incisura. In such cases, the presence of perimesencephalic blood can mimic pnSAH. In contrast to pnSAH, interpeduncular and prepontine hemorrhage is usually absent.

cSAH is found over the cerebral convexities, not in the perimesencephalic cisterns. Blood within a single sulcus or immediately adjacent sulci is common.

Convexal Subarachnoid Hemorrhage

Terminology

Isolated spontaneous ntSAH that involves the sulci over the brain vertex is called convexal or convexity SAH (cSAH). cSAH is a unique type of SAH with a very different imaging appearance from either aSAH or pnSAH. True cSAH is confined to the cortical surfaces, sparing the basal and perimesencephalic cisterns without extending into the sylvian or hemispheric fissures (6-12).

Etiology

A broad spectrum of vascular and even nonvascular pathologies can cause cSAH. These include dural sinus and cortical vein thrombosis (CoVT), arteriovenous malformations, dural arteriovenous fistulas, arterial dissection/stenosis/occlusion, mycotic aneurysm, vasculitides, amyloid angiopathy, coagulopathies, reversible cerebral vasoconstriction syndrome (RCVS), and posterior reversible encephalopathy syndrome (PRES).

Age is a significant predictor of cSAH etiology. In people over 60 years old, cerebral amyloid angiopathy (CAA) is a common cause and the presence of CT-visible cSAH indicates an increased risk for early recurrent ICH. In younger patients, RCVS and PRES are common. Venous occlusions can occur in all age groups. Recent studies show 25% of patients with cerebral venous sinus thrombosis have SAH, mostly in the cerebral convexities.

Clinical Issues

Although cSAH can occur at virtually any age, most patients are between 4th-8th decades. Peak age is 70 years.

The clinical presentation of cSAH varies with etiology but is quite different from that of aSAH. Most patients with cSAH have nonspecific headache without nuchal rigidity. Some present with focal or generalized seizures or neurologic deficits.

Patients with cSAH secondary to RCVS may present with a “thunderclap” headache. The vast majority are middle-aged women. cSAH caused by venous thrombosis or vasculitis may have milder symptoms with more insidious onset. Mean age of CoVT accompanied by cSAH is 33 years.

CAA is the major cause of cSAH in older adult patients. Worsening dementia and headache are the common presentations. cSAH in this age cohort is associated with cognitive impairment and CAA as well as APOE-ε4 overrepresentation compared with age-matched health controls. Between 40-45% experience recurrent cSAH and subsequent lobar hemorrhage (see Chapter 10).

The outcome of cSAH itself is generally benign and depends primarily on underlying etiology. Vasospasm and DCI are rare.

Imaging

CT Findings

Most cases of cSAH are unilateral, involving one (6-13A) or several dorsolateral convexity sulci. The basal cisterns are typically spared.

MR Findings

Focal sulcal hyperintensity on FLAIR is typical in cSAH (6-13B). T2* (GRE, SWI) shows “blooming” in the affected sulci (6-16C). If the etiology of the cSAH is dural sinus or cortical vein occlusion, a hypointense “cord” sign may be present. Patients with CAA have multifocal cortical and pial microbleeds (“blooming black dots”) on T2*. They may also show evidence of siderosis and prior lobar hemorrhages of differing ages.

Superficial Siderosis

Terminology

Linear hemosiderin deposition along brain surfaces, cranial nerves, &/or the spinal cord defines the condition known as superficial siderosis (SS) (6-14). There are two subtypes of siderosis that are distinguished by their anatomic distribution, causes and clinical features: Classic or infratentorial SS and cortical SS.

“Classic” SS of the CNS primarily affects the infratentorial regions and spinal cord. Classic infratentorial SS sometimes also affects the supratentorial regions.

The term “cortical” SS (cSS) describes a distinct pattern of iron-bearing blood-breakdown product deposition limited to cortical sulci over the convexities of the cerebral hemispheres. In cortical SS, the brainstem, cerebellum, and spinal cord are spared.

Etiology

SS is a consequence of chronic intermittent or continuous minor hemorrhage into the subarachnoid space. Trauma and surgery are the most common causes of classic SS. Other reported etiologies include hemorrhagic neoplasm, vascular malformations, venous obstruction(s) and intracranial hypotension with spinal CSF leaks, including CSF-venous fistulae. SS due to repeated aSAH is relatively uncommon.

Cortical SS has many potential causes, but, in older individuals, cortical SS is most often associated with CAA.

Pathology

Location

Although it can occur anywhere in the CNS, classic SS has a predilection for the posterior fossa (cerebellar folia and vermis, CNVIII) and brainstem.

Cortical SS is seen in 60% of patients with CAA but is rare in non-CAA forms of intracerebral hemorrhages.

Gross Pathology

Brownish yellow and blackish gray encrustations cover the affected structures, layering along the sulci and encasing cranial nerves.

Clinical Issues

Patients with classic SS often present with slowly progressive gait ataxia, dysarthria, and bilateral sensorineural hearing loss. Some patients present with progressive myelopathy. Often, decades pass between the putative event that causes SS and the development of overt symptoms.

As cortical SS most commonly occurs in the setting of CAA, progressive dementia and cognitive decline are common.

In contrast, a rare, relatively “acute” rapidly progressive SS syndrome can present as a consequence of ongoing and extensive intracranial hemorrhages.

Imaging

CT is usually normal in patients with SS.

SS on MR is best identified on T2* (GRE, SWI) imaging and is seen as a hypointense rim that follows along brain surfaces and coats the cranial nerves &/or spinal cord (6-15)(6-16). A characteristic bilinear track-like appearance is common with cortical SS (6-13B).

Despite extensive neuroimaging, the source of the SS often remains occult. In 50% of classic SS, a hemorrhage source is never identified despite extensive investigation of the entire neuraxis.

Differential Diagnosis

The major imaging mimic of classic SS is “bounce point” artifact, which makes the posterior fossa surfaces appear artifactually dark.

Most cortical SS mimics contain deoxygenated blood or blood products. Cerebral veins appear markedly hypointense on SWI but do not run parallel to the convexity sulci. Slow flow in sulcal arteries (pial collaterals in stroke or moyamoya) can appear hypointense on SWI as well.

In Sturge-Weber syndrome, hypointensity due to gyriform calcifications may be linear and cortical but can be easily identified on NECT and T1 C+ MR. Rare causes of cortical SS include hemorrhagic subarachnoid metastases, neurocutaneous melanosis (hyperintense on T1WI), and meningioangiomatosis (thickened enhancing, sometimes calcified and infiltrating proliferations of meningeal cells and blood vessels).

Aneurysms

Overview

Intracranial aneurysms are classified by their gross phenotypic appearance. The most common intracranial aneurysms are called saccular or “berry” aneurysms because of their striking sac- or berry-like configuration (6-17). Saccular aneurysms (SAs) are acquired lesions that arise from branch points of major cerebral arteries in which hemodynamic stresses are maximal. SAs lack some of the arterial layers (usually the internal elastic lamina and media) found in normal vessels. More than 90% of SAs occur on the “anterior” (carotid) circulation (6-18).

Pseudoaneurysms (PSAs) (also sometimes called “false” aneurysms) are focal arterial dilatations that are not contained by any layers of the normal arterial wall. They are often irregularly shaped and typically consist of a paravascular, noncontained blood clot that cavitates and communicates with the parent vessel lumen. Extracranial PSAs are more common than intracranial lesions. Intracranial PSAs usually arise from mid-sized arteries distal to the circle of Willis. Trauma, drug abuse, infection, and tumor are the usual etiologies.

Blood blister-like aneurysms (BBAs) are a special type of aneurysm recently recognized in the neurosurgical literature. BBAs are eccentric hemispherical arterial outpouchings that are covered by only a thin layer of adventitia. These dangerous lesions are both difficult to detect and difficult to treat. They have a tendency to rupture at a much smaller size and relatively younger age compared with SAs. Although BBAs can be found anywhere, they have a distinct propensity to occur along the supraclinoid ICA.

Fusiform aneurysms (FAs) are focal dilatations that involve the entire circumference of a vessel and extend for relatively short distances. FAs are more common on the vertebrobasilar (“posterior”) circulation. FAs can be atherosclerotic (more common) or nonatherosclerotic in origin. Nonatherosclerotic FAs (nASVD FAs) are often associated with collagen-vascular disorders, such as Marfan or Ehlers-Danlos type IV.

Saccular Aneurysm

Terminology

Saccular (“berry”) aneurysms (SAs) are sometimes called “true” aneurysms (to contrast them with PSAs).

An SA is a pathologic outward bulge that affects only part of the parent artery circumference. Most SAs lack two important structural components of normal intracranial arteries—the internal elastic lamina and the muscular layer (“media”)—and often have a focally thinned wall that is prone to rupture.

Etiology

General Concepts

Very few SAs are congenital (i.e., present at birth). SAs are acquired lesions that develop from abnormal extracellular matrix (ECM) maintenance and excessive hemodynamic stress. SA formation begins with endothelial dysfunction followed by inflammatory cascades, pathologic remodeling, and degenerative changes in vessel walls.

Genetics

Modeling studies have shown a distinct familial predisposition to forming SAs. Persons with first-degree relatives with known aSAH are at risk for developing unruptured intracranial aneurysms. Aneurysm risk at five years is 2-12%, 4-28% at 10 years and 7-40% at 15 years.

Autosomal dominant polycystic kidney disease (ADPCKD), inherited connective tissue disorders, anomalous blood vessels, familial predisposition, and “high-flow” states (i.e., vessels supplying an arteriovenous malformation) all increase the risk of SA development.

Systemic hypertension, smoking, and heavy alcohol consumption also contribute significantly to the risk of developing SAs and may augment any underlying genetic propensities.

Anomalous Blood Vessels

Bicuspid aortic valves, aortic coarctation, persistent trigeminal artery, and congenital anomalies of the anterior cerebral artery (ACA) (i.e., A1 asymmetries or infraoptic course of the A1 segment) all carry an increased risk of SA.

Inherited Vasculopathies and Syndromic Aneurysms

Some heritable connective tissue disorders (such as Marfan and Ehlers-Danlos II and IV syndromes or fibromuscular dysplasia) are associated with increased risk of intracranial aneurysms. Arteriopathy—not necessarily with aneurysm formation—is common in patients with neurofibromatosis type 1 (NF1). ADPCKD carries an increased lifetime risk (9-11.5%) of developing an SA, but aneurysm rupture is a rare event (0.04 per 100 patient years).

Familial Intracranial Aneurysms

A positive family history represents the strongest known risk factor for aSAH (4-10x general population). Up to 20% of patients with SAs have a family history of intracranial aneurysms. SAs in “clusters” of related individuals without any known heritable connective tissue disorder are termed familial intracranial aneurysms (FIAs).

Pathology

Location

Most intracranial SAs occur at points of maximal hemodynamic stress. The vast majority arise from major blood vessel bifurcations or branches (6-17). The circle of Willis and the MCA bifurcations are the most common sites (6-18) (6-1). Aneurysms beyond the circle of Willis are uncommon. Many peripheral aneurysms are actually PSAs secondary to trauma, infection, or tumor.

Anterior Circulation Aneurysms

90% of SAs occur on the “anterior” circulation (6-18). The anterior circulation consists of the ICA and its terminal branches, the ACA, and MCA. Approximately 1/3 of SAs occur on the ACoA with another 1/3 arising at the junction of the ICA and the PCoA. Approximately 20% of SAs occur at the MCA bi- or trifurcation.

Posterior Circulation Aneurysms

10% of SAs are located on the vertebrobasilar (“posterior”) circulation. The basilar artery bifurcation is the most common site, accounting for ~ 5% of all SAs (6-17). The posterior inferior cerebellar artery is the second most common location.

Size and Number

In autopsy studies, the reported prevalence of unruptured SAs across all age groups is 0.3-4%. Minute SAs (< 2 mm), diagnosed microscopically at autopsy, are present in 10-20% of the general population. Recent reviews suggest these minute unruptured SAs occur at a high rate. However, only a few (10%) of these small SAs enlarge to ≥ 2 mm, and 10% of these will eventually rupture within 10 years.

SAs vary in size from tiny (2-3 mm) (6-19) to large lesions > 1 cm (6-21). SAs that are ≥ 2.5 cm are called “giant” aneurysms. Between 15-20% of aneurysms are multiple and significantly more common in women.

Gross Pathology

The gross configuration of an SA changes with time as the arterial wall is remodeled in response to hemodynamic stresses. As it becomes progressively weakened, the wall begins to bulge outward, forming an SA. The opening (ostium) of an SA can be narrow or broad based. One or more lobules or an apical “tit” may develop. These outpouchings are the most vulnerable rupture site.

Microscopic Features

SAs demonstrate a disrupted or absent internal elastic lamina. The smooth muscle cell layer (media) is generally absent. The wall of an SA is usually quite fragile, consisting of intima and adventitia in a degraded ECM. Variable amounts of thrombus and atherosclerotic changes may also be present, especially in larger “giant” SAs. Inflammatory cell infiltration is a histologic hallmark of SAs.

Clinical Issues

Epidemiology

Unruptured intracranial aneurysms (UIAs) are found in 3% of the adult population and are increasingly detected due to more frequent cranial imaging. Asymptomatic unruptured SAs are at least 10x more prevalent than ruptured SAs.

Demographics

Peak presentation is between 40-60 years of age. There is a definite female predominance, especially with multiple SAs. SAs are rare in children, accounting for < 2% of all cases. Childhood aneurysms lack female predominance and are more often associated with trauma or infection.

Presentation

Between 80-90% of all ntSAHs are caused by ruptured SAs. The most common presentation is sudden onset of severe, excruciating headache (“thunderclap” or “worst headache of my life”).

Cranial neuropathy is a relatively uncommon presentation of SA. Of these, a pupil-involving CNIII palsy from a PCoA aneurysm is the most common. Occasionally, patients with partially or completely thrombosed aneurysms present with a transient ischemic attack (TIA) or stroke.

Natural History

The overall annual rupture rate of all SAs is 1-2%. However, the rupture risk varies according to size, location, and shape of the aneurysm.

Size and Rupture Risk

Aneurysms that are ≥ 5 mm are associated with a significantly increased risk of rupture compared with 2- to 4-mm aneurysms. Demonstrable growth on surveillance imaging is also associated with an increased rupture risk.

Shape/Configuration and Rupture Risk

In addition to size, shape and configuration also matter. Nonsaccular (nonspherical) shape increases rupture risk. The presence of a “bleb” (irregular wall protrusion) or elongated aspect ratio (length compared with width) are independent predictors of rupture risk.

Formation of a “bleb” is, in turn, related to the presence of strong and concentrated inflow jets, high speed, complex and unstable flow patterns and heterogeneous wall stress shear patterns. More distorted shapes are also associated with bleb formation.

Location and Rupture Risk

Vertebrobasilar artery aneurysms have a significantly higher rupture risk, as do ICA-PCoA aneurysms. MCA and ACA aneurysms are associated with modest risk. Bifurcation aneurysms are more prone to growth than sidewall aneurysms.

Treatment Options

aSAH is a catastrophic event with high mortality and significant morbidity. Approximately 1/3 of patients die and 1/3 survive with significant residual neurologic deficits. Only 1/4-1/3 of patients with aSAH recover with good functional outcome.

Ruptured SAs

Virtually all ruptured SAs are treated. Neuroendovascular options, such as coiling (with or without balloon/stent assistance) and flow diversion, are increasingly more common. The percentage of patients with cerebral aneurysms treated with craniotomy and clip ligation is decreasing.

Unruptured SAs

The management of unruptured SAs (UIAs) is controversial because of their unpredictable natural history. Initial size and multiplicity are significant factors related to aneurysm growth. Recent studies have shown that the growth rate for 7-mm UIAs is significantly faster than that for < 3-mm UIAs.

CLINICAL FEATURES OF SACCULAR ANEURYSMS

Epidemiology

• 3% of population; F > M

• Peak age of presentation: 40-60 years (rare in children)

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