Hemorrhage

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Hemorrhage

The term ‘intracranial hemorrhage’ describes extravasation of blood into brain parenchyma, regions defined and enclosed by the meninges and skull, and/or the ventricular cavities. Intracranial hematomas resulting from hemorrhage into various compartments can be classified on an anatomic basis as:

Extradural (epidural) hemorrhage/hematoma (EDH).

Subdural hemorrhage/hematoma (SDH), including acute and chronic types.

Subarachnoid hemorrhage (SAH).

Intraventricular hemorrhage (IVH), which is rare as a spontaneous event except in premature infants.

Encephalic/intraparenchymal brain hemorrhage (IPH/BH), which can be further subdivided into cerebral, cerebellar, or brain stem hemorrhage.

Strictly speaking, the term ‘cerebral hemorrhage’ should be restricted to describing bleeding into the parenchyma of the cerebral hemispheres, though it is often applied to describe extravasation of blood into any part of the brain parenchyma, including brain stem and cerebellum. Spontaneous spinal cord hemorrhages, especially intraparenchymal bleeds, are extraordinarily rare, whereas subdural spinal cord hematomas are slightly more common, e.g. in connection with anticoagulant use.

EXTRADURAL (EPIDURAL) HEMORRHAGE (EDH)

EDH is also considered in Chapter 11, in relation to craniocerebral trauma (CCT), with which it is almost invariably associated. Because both EDH and SDH have a very strong association with CCT, they are not usually considered a form of ‘stroke’, but a manifestation of such trauma.

MACROSCOPIC AND MICROSCOPIC APPEARANCES

EDH is easily recognized at necropsy as a biconvex hematoma that is readily seen upon removing the calvarium, but before breaching the dura (Fig. 10.1). Most cases result from rupture of the middle meningeal artery. As patients that have died from EDH are often the victims of a motor vehicle accident or foul play and therefore the subject of a forensic or medicolegal investigation, the size and site of any (causal) skull fracture and the volume of hematoma should be measured and recorded. Skull fractures are best identified after removal of dura from the inner table of the skull. Hematomas of 75–100 mL are usually fatal. The maximum volume that is likely to accumulate is approximately 300 mL. The neuropathologist should also document the effects of any EDH, such as distortion or herniation of brain substance, and the presence of any other traumatic lesions. Such documentation of EDH and its effects on the brain must be carried out at the time of necropsy and brain cutting; microscopic examination of the brain in such cases is often of limited value, except in documenting other evidence of CCT, such as diffuse axonal injury.

10.1 Large traumatic EDH over the right frontal lobe. (Courtesy of Professor Michael A. Farrell, Dublin, Ireland.)

Spinal EDH is rare and presents with acute severe pain in the region of bleeding and radiation of the pain to the extremities. In the lumbar spine, it probably results from internal rupture of Batson’s vertebral venous plexus, i.e. a non-arterial source of EDH.

EXTRADURAL HEMORRHAGE

Is much less common than SDH in large clinicopathologic series that incorporate autopsy data from head injury/trauma patients. EDH and SDH may occur together.

May be associated with a ‘lucid interval’ lasting minutes to hours (rarely days) after brain trauma. Injury to underlying brain may be minimal.

Occurs in 5–15% of fatal non-missile head injuries.

Usually results from skull fracture that produces tearing of a dural artery (especially the middle meningeal artery). This mechanism accounts for over 90% of cases. Much less often, EDH results from tearing of the sagittal or lateral sinus.

Is usually found in the temporal fossa in adults, but the posterior fossa in children, in whom it is less common.

SUBDURAL HEMORRHAGE/ HEMATOMA (SDH)

By comparison with EDH, SDH has a more diverse clinical presentation and etiology, and is not invariably associated with (documented) cranial trauma, i.e. it may occur spontaneously or after ‘minimally appreciated’ head trauma; e.g. in individuals that are being treated with anticoagulants or in the elderly with mild cognitive impairment.

Acute and chronic forms of SDH have distinctive clinicopathologic features. However, both types result from the rupture of ‘bridging veins’ between the brain and the dura; such veins become increasingly vulnerable to rupture in the elderly, as a consequence of ‘physiological’ brain atrophy or brain shrinkage associated with Alzheimer’s disease or other dementias. In this situation, the dura remains tightly adherent to the inner table of the calvarium, while the distance between it and the brain increases, putting tension on bridging veins. Because SDH results from venous (rather than arterial) rupture, clinical signs and symptoms develop more slowly, often in an insidious fashion.

MACROSCOPIC AND MICROSCOPIC APPEARANCES

As with EDH, the location, extent and volume of an acute SDH should be carefully documented at necropsy (preferably with the use of photographic evidence), along with its effects on the underlying brain (Figs 10.2, 10.3). Associated skull fractures (if any) and contusions (especially with acute SDH) should be noted when the brain is removed and subsequently sectioned. If the SDH is subacute or chronic, histologic sections of hematoma will show chronically inflamed vascular and variably fibrotic granulation tissue, with an admixture of blood breakdown products (Fig. 10.4). Portions of organizing or well organized SDH are often submitted as neurosurgical specimens, and described (inappropriately) as a ‘subdural membrane’. The chronicity of such pathology can be estimated from its degree of organization (Fig. 10.4). Eosinophils can be a surprisingly prominent feature of organizing SDH. As dural neoplasms (e.g. metastatic carcinoma from prostate or breast) and hemangiomas are, very rarely, associated with SDH, surgically removed SDH submitted for pathologic evaluation should be carefully examined with these diagnostic possibilities in mind. In childhood non-accidental injury, the volume of subdural blood may be small, often amounting to no more than a thin film over the cerebral hemispheres on either side of the falx cerebri. The posterior fossa may be involved more commonly in children than in adults.

10.2 Acute SDH.
This hematoma overlying the left cerebral hemisphere found at necropsy of a woman with acute leukemia and disseminated intravascular coagulation.

10.3 Bilateral (small) frontal SDHs with early organization (arrows).
The dura has been reflected from the convexities of the fixed brain from a 47-year-old woman with SDHs, attributed in part to antiphospholipid syndrome.

10.4 Organizing pseudomembranes in SDH.
(a) Dura is at top. A thin pseudomembrane (arrows) is composed of chronically inflamed granulation tissue, seen immediately beneath the dura and delineated by arrows. (b) Magnified view shows evidence (within the subdural membrane) of blood breakdown (hemosiderin-laden macrophages), capillaries, and many chronic inflammatory cells, among which eosinophils are prominent. (c) Well organized subdural membrane (arrows) composed of fibrous tissue and a few siderophages. (d) Thick subdural membrane (arrows) with acute hemorrhage, and prominent chronic inflammation. In both (c) and (d), dura is at upper right.

PATHOGENESIS OF SUBDURAL HEMATOMA (SDH)

Acute SDH usually results from trauma that results in the head being subjected to rapid acceleration or deceleration. Because of the brain’s inertia, its movement lags behind that of the skull. This causes traction on, and tearing of, bridging veins between brain and dura mater, since the latter is tightly adherent to interior of the skull.

Chronic SDH starts with rupture of bridging veins, followed by cycles of hematoma organization and rebleeding, due to formation of densely vascular granulation tissue (pseudomembrane) around the hematoma. Large caliber but thin-walled vessels in this tissue tend to bleed spontaneously (a propensity accentuated if an elderly patient is on anti-platelet agents or anticoagulated) and to perpetuate the pathologic process. Osmotic forces may also lead to enlargement of the hematoma and development of a subdural hygroma – a fluid-filled subdural space with relatively little blood in it.

SDH may rarely occur in association with dural neoplasms (meningiomas and metastases, e.g. from prostate or breast carcinoma) and dural hemangiomas.

Patients with abnormal hemostasis (e.g. with coagulopathy or thrombocytopenia from hematologic malignancy) are at increased risk of developing spontaneous acute or chronic SDH.

ACUTE SUBDURAL HEMATOMA

Should be clinically suspected in any patient of any age that has experienced significant head trauma. Its existence can be verified by appropriate imaging studies (CT/MRI), which show a layer of subdural blood that ‘accommodates’ to brain shape (versus the more elliptical shape of EDH).

Causes predominantly non-localizing signs and symptoms (headache/drowsiness) rather than a focal deficit.

May be bilateral.

Is recorded at necropsy in 25–50% of patients with fatal non-missile head injuries.

Appearing as a thin ‘smear’ of hematoma and without obvious cause is common at autopsy.

Carries a poorer prognosis than EDH because it is more often associated with underlying brain contusion, which can itself be a major cause of morbidity and mortality.

Has a mortality rate in patients under 10 years of age that is approximately half that in patients over 60 years of age.

CHRONIC SUBDURAL HEMATOMA

Chronic SDH mainly affects subjects over 50 years of age.

Precipitating closed head injury may be trivial, unrecognized, or not remembered, especially if the individual is in the early stages of mild cognitive impairment/dementia.

There is an equal frequency among men and women.

Progression of symptoms and signs may be stepwise and slow, including stroke-like episodes. In the elderly, chronic SDH is in the differential diagnosis of potentially treatable causes of age-associated cognitive decline.

Chronic SDH is especially common in elderly patients with cerebral atrophy due to aging or Alzheimer’s disease. It can be difficult to differentiate dementia secondary to parenchymal disease from that associated with chronic SDH.

SUBARACHNOID HEMORRHAGE/HEMATOMA (SAH)

SAH describes an acute extravasation of blood into the space between the arachnoid membrane and pia mater. Secondary SAH may occur whenever a primary intraparenchymal hematoma or contusional injury secondary to head trauma, extends into the subarachnoid space (SAS). Spontaneous SAH should be distinguished from SAH secondary to brain trauma, especially contusional brain injury (see Chapter 11). Spontaneous SAH occurs when a focally weakened artery in the subarachnoid space ruptures. Such weakening results from a localized abnormality due to either a pathologic process or a malformation (often erroneously considered congenital).

The relevant lesions are:

Berry (saccular) aneurysm, which is the commonest cause of spontaneous SAH (aneurysmal SAH or aSAH).

Fusiform aneurysm.

Infective (mycotic) aneurysm (IA).

Arteriovenous malformation (AVM) with a subarachnoid component.

Cerebral amyloid angiopathy (CAA).

Because AVM and CAA more often cause brain hemorrhage than spontaneous/primary SAH, they are considered together with other causes of brain hemorrhage. Systemic factors or diseases (thrombocytopenia, coagulopathy, hematologic malignancies) may also precipitate SAH, even in the presence of structurally normal arteries. In a small but significant proportion of cases, no etiology for SAH is discovered, even after careful imaging of the blood vessels (by angiography) or structural brain imaging.

BERRY (SACCULAR) ANEURYSM

This is an incidental finding in up to 1–3% of unselected autopsies (depending upon how carefully they are sought). Usually in this situation, there is no history and there are no pathologic features to suggest prior hemorrhage. Evidence that some patients report ‘warning/sentinel leaks’ prior to a major SAH has become controversial, because of issues such as ‘recall bias.’ Unlike the incidence of strokes due to brain parenchymal hemorrhage or infarction, which has declined over recent decades, the incidence of aSAH due to ruptured aneurysm has remained constant. The mortality rate from aSAH may have been decreasing in recent decades, but remains high:

25% mortality within 25 hours of aSAH

10–15% mortality prior to reaching hospital

10% mortality within 24 hours of hospitalization

1-month mortality of 50–60%.

PATHOGENESIS OF BERRY/SACCULAR ANEURYSMS

Though such aneurysms are said to be ‘congenital’, they are almost never discovered in infants or children. Certain segments of vascular tunica media, especially at arterial bifurcation points, where blood flow is most turbulent, are prone to develop aneurysms in later life.

The risk of developing aneurysms is increased by:

Hypertension.

Arteriovenous malformation; up to 10% are associated with coexisting berry aneurysm(s) involving arteries hemodynamically affected by increased blood flow though the arteriovenous malformation.

Some systemic vascular diseases (e.g. fibromuscular dysplasia).

Defective vascular collagen (especially type III), smooth muscle, or elastic tissue (e.g. Ehlers–Danlos syndrome, acid maltase deficiency).

Polycystic kidney disease (10–30% of patients have a berry aneurysm) and coarctation of the aorta – possibly through the mechanism of secondary hypertension.

Moyamoya disease.

Hypertension is not only a risk factor for development of berry aneurysms, but also increases the likelihood of rupture, and appears to increase the likelihood of multiple aneurysms, as does cigarette smoking. Rupture of a saccular aneurysm may be precipitated by cocaine abuse, probably due hypertensive surge.

ANEURYSMAL SUBARACHNOID HEMORRHAGE (aSAH)

aSAH has an annual incidence of approximately 10–12/100 000.

Incidence increases with age up to the 6th decade. Some studies suggest a decline in aSAH incidence beyond this age.

Age-adjusted mortality rates for aSAH are approximately 60% greater in females than males.

Median age at death from aSAH is 59 years, compared with 73 years for brain hemorrhage and 81 years for ischemic stroke.

aSAH accounts for 4.4% of stroke mortality, but over 25% of stroke-related years of potential life lost before the age of 65 years.

aSAH is fatal or disabling in over 2/3 of affected patients: one-third die from the initial hemorrhage within 72 hours, and a further third die or become significantly disabled due to complications of SAH (e.g. vasospasm with brain ischemia, rebleeding, acute or chronic hydrocephalus, complications of surgical and/or endovascular intervention).

In over 50% of patients, major aSAH may be preceded by warning symptoms, e.g. a ‘sentinel leak’ or small aSAH not associated with neurologic morbidity, hours to weeks before the large aSAH, though the significance and frequency of such ‘leaks’ has recently been questioned on the basis that they may be an artifact of ‘recall bias’.

Clinical manifestations of aSAH are determined by:

• location (e.g. aneurysm of the posterior communicating artery can compress cranial nerve III)

• size of hemorrhage

• regions into which hemorrhage extends (e.g. basal cisterns, brain parenchyma, ventricular cavities).

Massive aSAH can lead to coma and death within minutes.

Less extensive aSAH often produces prominent localizing signs and symptoms (e.g. hemiparesis, akinetic mutism, or paraparesis secondary to ruptured anterior communicating artery aneurysm).

If treated successfully (by surgical clipping, wrapping, endovascular coiling, embolization or a combination of techniques) and assuming no postoperative complications, an aneurysm will not usually cause further pathology or necessarily affect lifespan.

Saccular aneurysms that rupture are usually smaller than 1 cm in diameter; approximately 10–15% are smaller than 5 mm. Giant aneurysms (i.e. those larger than 2.5 cm diameter) bleed less frequently than small aneurysms and more commonly behave as a mass lesion.

For very small (<7 mm diameter) aneurysms that have not ruptured (i.e. discovered incidentally), the morbidity/mortality related to surgery probably means that surgical intervention is not warranted in such a situation, given the very low risk of rupture.

MACROSCOPIC APPEARANCES

Abundant SAH is obvious at necropsy. However, when the ruptured dome of an aneurysm is embedded within brain parenchyma, a purely intraparenchymal bleed (sometimes with negligible SAH) can result; this may occur particularly with anterior cerebral/anterior communicating artery junction or middle cerebral artery bi/trifurcation aneurysms. This should be borne in mind when someone, especially a young person, experiences or dies from a brain hemorrhage of occult origin (Figs 10.5, 10.6). Massive SAH in a young or middle-aged patient (Figs 10.7, 10.8) from a clinically unproven source of bleeding strongly suggests a berry aneurysm as the etiology. In an autopsy specimen, fresh blood at the base of the brain should be dissected away gently until the aneurysm is discovered (Fig. 10.8). This dissection should not be deferred until the brain has been fixed, because the fixed hematoma is then difficult to dissect, making the source of bleeding much more difficult to locate. The possibility that there is more than one aneurysm, or a combination of vascular abnormalities (berry aneurysm and AVM), should always be considered. Berry aneurysms may occur at branch points of arteries (with high flow) that supply an AVM. A ruptured aneurysm is usually identified by the proximity of abundant hematoma and a tear in the wall of the aneurysm, which can often be seen without the benefit of microscopy. Anterior circulation aneurysms occur at major branch points on the circle of Willis (Fig. 10.9), most commonly:

10.5 Right carotid–ophthalmic artery aneurysm treated by balloon embolization.
(a) Basal view showing severe right mesial temporal herniation (lower arrow) and balloon occlusion of the aneurysm adjacent to dusky discoloration on the undersurface of right frontal lobe in cortex overlying the hematoma (upper arrow). (b) Coronal section showing that the dome of the aneurysm had ruptured into the frontal lobe to produce a well-demarcated intracerebral hematoma.

10.6 All images are from the brain of a young man who presented with severe, rapid-onset headache and died within 72 hours, despite attempted evacuation of a cerebral hematoma.
(a) Large frontotemporal intracerebral hematoma adjacent to the left basal ganglia. Note left to right shift of midline structures and subfalcine herniation. (b) Large left middle cerebral artery bifurcation berry aneurysm within the Sylvian fissure (arrow) that has bled directly into the brain substance rather than the subarachnoid space. The left temporal tip has been cut away to show the aneurysm. (c) Left middle cerebral artery bifurcation berry aneurysm (arrow) clearly visible on the circle of Willis, dissected away from base of the brain.

10.7 Massive spontaneous SAH.
Coronal section of cerebrum shows extensive SAH, more prominent overlying the left cerebral hemisphere (arrows) than the right. Note how fresh blood extends into sulci. Small parenchymal hemorrhages are also seen (e.g. arrowhead).

10.8 (a) Basal view of freshly removed brain from a woman who complained of severe headache, then collapsed. A berry aneurysm on the circle of Willis should be suspected in all cases of spontaneous SAH. Abundant acute hemorrhage is seen within basal cisterns of an edematous brain, and obscuring the circle of Willis. (b) Dissection of blood from (a) at the time of necropsy revealed a ruptured left anterior cerebral artery/anterior communicating artery aneurysm (arrow).

10.9 Schematic illustration of the circle of Willis showing the most common locations for berry aneurysms at major arterial bifurcation points.
Approximately 30% of patients in whom an aneurysm is identified will have one or more aneurysms elsewhere on the circle of Willis.

the middle cerebral artery bi/trifurcation in the Sylvian fissure

the internal carotid artery/posterior communicating artery junction

the anterior cerebral artery/anterior communicating artery junction.

Posterior circulation aneurysms constitute 10–30% of cases; most arise at the basilar artery bifurcation (‘basilar tip’) (Fig. 10.9); less common sites for aneurysms in the posterior circulation are junctions between the basilar artery and one of its major branches. Massive rupture of a posterior circulation berry aneurysm is one of the few types of ‘stroke’ that can produce sudden unexpected death, sometimes within seconds.

For individuals that survive SAH by days or weeks, then come to necropsy, the pathologist should note:

The degree and extent of meningeal discoloration (e.g. with blood breakdown products) and fibrosis.

The existence and severity of hydrocephalus (often related to the above observation).

Ischemic lesions in the brain parenchyma that may have resulted from vasospasm.

The location and status of any aneurysm clip(s) (Fig. 10.10) or evidence of other modalities that may have been used to treat it, e.g. coiling, embolization (Fig. 10.11).

10.10 Two clipped aneurysms on the circle of Willis.
There is a large basilar tip aneurysm with a clip at its base and a much smaller anterior cerebral artery/anterior communicating artery aneurysm (arrow).

10.11 Many treatment options are available for patients with berry (saccular) aneurysm that has caused SAH.
(a) Note two clips on a large aneurysm in the vicinity of the right posterior cerebral/posterior communicating artery junction. Patient also had a large AVM in the right temporal lobe. (b) Note coils (arrow) placed in a large carotid-ophthalmic aneurysm. (Intraoperative photograph courtesy of Professor Neil A. Martin, UCLA.)

MICROSCOPIC APPEARANCES

Histologic features of a berry aneurysm, either ruptured or intact, are optimally demonstrated by an elastic stain (e.g. van Gieson or Movat pentachrome). The aneurysm may be unilobular or multilobular. Characteristic histopathologic features include attenuation and focal loss of both elastic tissue and smooth muscle cells of the muscularis, generally most marked close to the site of rupture. There is variable fibrosis of the aneurysm wall and foci of atherosclerosis may be evident (Figs 10.12–10.14). Blood breakdown products in the vicinity may reflect earlier, including asymptomatic or minimally symptomatic, bleeds. When an ‘incidental’ aneurysm is discovered at autopsy (Fig. 10.15), it may be quite large; in elderly patients, there may be superimposed intimal atherosclerotic change within its walls.

10.12 (a) Ruptured dome of a berry aneurysm.
Thinning and attenuation of the aneurysm wall and local absence of elastic tissue are seen near the site of rupture. (b) Control EVG stain (from another vessel) shows uniform (normal) staining of the elastica.

10.13 EVG-stained section of a berry aneurysm (unruptured, found incidentally at necropsy) shown at low magnification (a) and higher magnification (b) to highlight deficiency/discontinuity of internal elastic lamina (arrow in panel b).

10.14 Degenerative changes in wall of a berry aneurysm (ruptured dome, see Fig. 10.12).
(a) Fibrosis and focal atherosclerotic change with cholesterol clefts, and an absence of elastica. (b) A region of wall remote from the rupture site, with focal loss of the internal elastic lamina (arrow), and intimal fibromuscular hyperplasia.

10.15 Medium-sized berry aneurysm (arrow) at junction of anterior cerebral artery/anterior communicating artery. The patient had reported no symptoms to suggest prior rupture of the aneurysm.

Giant berry aneurysms (defined as having a diameter of ≥25 mm) often show mural calcification, and extensive thrombosis with or without recanalization; they are more likely to behave as a mass lesion than to produce massive SAH (Figs 10.16, 10.17).

10.16 Giant aneurysm in the left temporal lobe.
Note the mass effect caused by this large aneurysm, which has displaced and distorted adjacent temporal lobe and thalamic structures. The lumen of the aneurysm contains laminated thrombus and there are only small residual patent lumina. The cause of death was rupture of the aneurysm into the brain with intraventricular extension of hemorrhage.

10.17 (a) Giant middle cerebral artery aneurysm (arrow) – intraoperative photograph. (Courtesy of Professor Neil A. Martin, UCLA.) (b) Giant clipped basilar tip aneurysm. External view of the aneurysm. (c) Section demonstrating recent thrombus filling lumen of the aneurysm. Nearby regions of brain stem and cerebellum showed ischemic necrosis, probably secondary to thrombus propagation into the superior cerebellar artery. (Courtesy of Dr M.A. Verity, UCLA.)

INFECTIVE ANEURYSM (IA)

Diagnosis of an IA (also known as a ‘mycotic aneurysm,’ even though it is not always associated with fungal infection) depends upon a high index of suspicion when a patient with a predisposing medical condition (e.g. infective endocarditis) experiences a SAH or BH. IAs are usually situated on distal (e.g. meningeal) branches of cerebral arteries, are usually multiple, and may be inconspicuous or impossible to identify macroscopically. In such a situation, sections incorporating small arteries from the edge of a hematoma are likely to reveal an IA. Elastic stains demonstrate breaches in the elastic tissue of the affected arterial wall, while stains for fungi or bacteria are likely to show microorganisms within the vessel wall (Figs 10.18, 10.19). The natural history of IAs is not clearly defined, since they may be difficult to identify by neuroimaging. IA-related hemorrhage may be subarachnoid (~20% of patients), intraparenchymal (~25%) or even intraventricular (~5%). Treatment is largely medical (antibiotics) rather than surgical.

10.18 Infective aneurysm in a young intravenous drug abuser.
Intraoperative photograph showing the aneurysm involving a small leptomeningeal artery. (Courtesy of Professor Neil A. Martin, UCLA.)

10.19 Infective aneurysm in a young intravenous drug abuser.
Sections of the aneurysm shown in Fig. 10.18. (a) The elastic tissue is intact along part of the vessel wall (bottom of the illustration) but ends abruptly in a mass of fibrin and inflammatory cells. (b) Part of the aneurysm wall stained to show the presence of Gram-positive cocci, which had colonized the vessel wall and (together with inflammation) produced the aneurysmal dilatation.

INFECTIVE ANEURYSMS

Account for an estimated 5–6% of all intracranial aneurysms.

May present with SAH, brain hemorrhage, infarction, or headache.

Are often asymptomatic until the time of hemorrhage, but there may be prominent symptoms and signs of the predisposing condition (usually endocarditis).

Have a mortality of approximately 30% if bacterial and approximately 90% if fungal.

PATHOGENESIS OF IAs

IAs result from microbial colonization of the vessel wall.

As bacterial infection is the most common cause, the (older) term ‘mycotic’ aneurysm is best avoided, since this implies a fungal etiology for all IAs.

Infection-induced acute/chronic inflammation weakens the vessel wall, resulting in ectasia leading to formation of an IA.

Colonizing microorganisms may be of hematogenous origin (e.g. associated with subacute endocarditis) or from a local infection (e.g. meningitis, osteomyelitis).

As many as 3% of patients with infective endocarditis develop IAs.

IAs usually involve the distal branches of cerebral vessels, particularly where due to bacterial infection. Bacterial colonization may be facilitated by the absence of vasa vasorum in branches of these vessels. Inflammation and destruction of the artery appear to proceed from adventitia inwards.

FUSIFORM ANEURYSMS

These rare aneurysms are formed from ectatic, often tortuous, basal arteries. Most involve the vertebrobasilar system. They may be several centimeters in diameter. Most affected patients are elderly, though rare instances are described in children. They may present with:

brain stem and/or cranial nerve compression (basilar artery aneurysm)

ischemia secondary to thrombosis, especially brain stem/cerebellar infarcts with a basilar artery aneurysm

hemorrhage.

MACROSCOPIC AND MICROSCOPIC APPEARANCES

Fusiform aneurysms usually affect the basilar artery, especially its middle segment (Fig. 10.20), but may extend inferiorly to involve the upper part of one of the vertebral arteries. Except in children, there may be marked complicated atherosclerosis of the affected arterial wall, including foamy histiocytes, evidence of old intraplaque hemorrhage, calcification, thrombosis (either mural or occlusive), and variable degrees of inflammation.