18.1 Arteries of the Brain: Blood Supply and the Circle of Willis
A Overview of the arterial supply to the brain
Left lateral view. The parts of the brain in the anterior and middle cranial fossae receive their blood supply from branches of the internal carotid artery, while parts of the brain in the posterior cranial fossa are supplied by branches of the vertebral and basilar arteries; the latter is formed by the confluence of the two vertebral arteries. The carotid and basilar arteries are connected by a vascular ring called the circle of Willis (see C and D). In many cases the circle of Willis allows for compensation of decreased blood flow in one vessel with increased “collateral” blood flow through another vessel (important in patients with stenotic lesions of the afferent arteries, see E).
В The four anatomical divisions of the internal carotid artery
Anterior view of the left internal carotid artery. The internal carotid artery consists of four topographically distinct parts between the carotid bifurcation (see A) and the point where it divides into the anterior and middle cerebral arteries. The parts (separated in the figure by white disks) are:
(1) Cervical part (red): located in the lateral pharyngeal space.
(2) Petrous part (yellow): located in the carotid canal of the petrous bone.
(3) Cavernous part (green): follows an S-shaped curve in the cavernous sinus.
(4) Cerebral part (purple): located in the chiasmatic cistern of the subarachnoid space.
Except for the cervical part which generally does not give off branches, all the other parts of the internal carotid artery give off numerous branches (see p. 60). The intracranial parts of the internal carotid artery are subdivided into five segments (Cl -C5) based on clinical criteria:
• C1-C2: the supraclinoid segments, located within the cerebral part.
C1 andC2 lie above the anterior clinoid process of the lesser wing of the sphenoid bone.
• C3-C5: the infraclinoid segments, located within the cavernous sinus.
C Projection of the circle of Willis onto the base of the skull
Superior view. The two vertebral arteries enter the skull through the foramen magnum and unite behind the clivus to form the unpaired basilar artery. This vessel then divides into the two posterior cerebral arteries (additional vessels that normally contribute to the circle of Willis are shown in D).
Note: Each middle cerebral artery (MCA) is the direct continuation of the internal carotid artery on that side. Clots ejected by the left heart will frequently embolize to the MCA territory.
D Variants of the circle of Willis (after Lippert and Pabst)
The vascular connections within the circle of Willis are subject to considerable variation. As a rule, the segmental hypoplasias shown here do not significantly alter the normal functions of the arterial ring.
a In most cases, the circle of Willis is formed by the following arteries: the anterior, middle and posterior cerebral arteries; the anterior and posterior communicating arteries; the internal carotid arteries; and the basilar artery.
b Occasionally, the anterior communicating artery is absent.
c Both anterior cerebral arteries may arise from one internal carotid artery (10% of cases).
d The posterior communicating artery may be absent or hypoplastic on one side (10% of cases).
e Both posterior communicating arteries may be absent or hypoplastic (10% of cases).
f The posterior cerebral artery may be absent or hypoplastic on one side.
g Both posterior cerebral arteries may be absent or hypoplastic. In addition, the anterior cerebral arteries may arise from a common trunk (g).
E Stenoses and occlusions of arteries supplying the brain
Atherosclerotic lesions in older patients may cause the narrowing (stenosis) or complete obstruction (occlusion) of arteries that supply the brain. Stenoses most commonly occur at arterial bifurcations, and the sites of predilection are shown. Isolated stenoses that develop gradually may be compensated for by collateral vessels. When stenoses occur simultaneously at multiple sites, the circle of Willis cannot compensate for the diminished blood supply, and cerebral blood flow becomes impaired (varying degrees of cerebral ischemia, see p. 264).
Note: The damage is manifested clinically in the brain, but the cause is located in the vessels that supply the brain. Because stenoses are treatable, their diagnosis has major therapeutic implications.
F Anatomical basis of subclavian steal syndrome
“Subclavian steal” usually results from stenosis of the left subclavian artery (red circle) located proximal to the origin of the vertebral artery. This syndrome involves a stealing of blood from the vertebral artery by the subclavian artery. When the left arm is exercised, as during yard work, insufficient blood may be supplied to the arm to accommodate the increased muscular effort (the patient complains of muscle weakness). As a result, blood is “stolen” from the vertebral artery circulation and there is a reversal of blood flow in the vertebral artery on the affected side (arrows). This leads to deficient blood flow in the basilar artery and may deprive the brain of blood, producing a feeling of lightheadedness.
18.2 Arteries of the Cerebrum
A Arteries at the base of the brain
The cerebellum and temporal lobe have been removed on the left side to display the course of the posterior cerebral artery. This view was selected because most of the arteries that supply the brain enter the cerebrum from its basal aspect.
Note: the three principal arteries of the cerebrum, the anterior, middle and posterior cerebral arteries, arise from different sources. The anterior and middle cerebral arteries are branches of the internal carotid artery, while the posterior cerebral arteries are terminal branches of the basiler artery (see p.246f). The vertebral arteries, which fuse to form the basilar artery, distribute branches to the spinal cord, brainstem, and cerebellum (anterior spinal artery, posterior spinal arteries, superior cerebellar artery, and anterior and posterior inferior cerebellar arteries).
В Segments of the anterior, middle, and posterior cerebral arteries
Anterior cerebral artery
• Precommunicating part
• Postcommunicating part
• A1 = segment proximal to the anterior communicating artery
• A2 = segment distal to the anterior communicating artery
Middle cerebral artery (MCA)
• Insular part
• M1 = first horizontal segment of the artery (horizontal part)
• М2 = segment on the insula
Posterior cerebral artery
• Precommunicating part
• Postcommunicating part
• P1 = segment between the basilar artery bifurcation and posterior communicating artery
• P2 = segment between the posterior communicating artery and anterior temporal branches
• P3 = lateral occipital artery
• P4 = medial occipital artery
C Terminal branches of the middle cerebral artery on the lateral cerebral hemisphere
Left lateral view. Most of the blood vessels on the lateral surface of the brain are terminal branches of the middle cerebral artery (MCA). They can be subdivided into two main groups:
• Inferior terminal (cortical) branches: supply the temporal lobe cortex
• Superior terminal (cortical) branches: supply the frontal and parietal lobe cortex
Deeper structures supplied by these branches are not shown in the diagram (see p. 250f).
D Course of the middle cerebral artery in the interior of the lateral sulcus
Left lateral view. On its way to the lateral surface of the cerebral hemisphere, the middle cerebral artery first courses on the base of the brain; this is the sphenoidal part of the MCA. It then continues through the lateral sulcus along the insula, which is the sunken portion of the cerebral cortex. When the temporal and parietal lobes a re spread a part with a retractor, as shown here, we can see the arteries of the insula (which receive their blood from the insular part of the middle cerebral artery; see A). When viewed in an angiogram, the branches of the insular part of the MCA resemble the arms of a candelabrum, giving rise to the term “candelabrum artery” for that arterial segment.
E Branches of the anterior and posterior cerebral arteries on the medial surface of the cerebrum
Right cerebral hemisphere viewed from the medial side, with the left cerebral hemisphere and brainstem removed. The medial surface of the brain is supplied by branches of the anterior and posterior cerebral arteries. While the anterior cerebral artery arises from the internal carotid artery, the posterior cerebral artery arises from the basilar artery (which is formed by the junction of the left and right vertebral arteries).
18.3 Arteries of the Cerebrum, Distribution
A Distribution areas of the main cerebral arteries
a Lateral view of the left cerebral hemisphere, b medial view of the right cerebral hemisphere. Most of the lateral surface of the brain is supplied by the middle cerebral artery (green), whose branches ascend to the cortex from the depths of the insula. The branches of the anterior cerebral artery supply the frontal pole of the brain and the cortical areas near the cortical margin (red and pink). The posterior cerebral artery supplies the occipital pole and lower portions of the temporal lobe (blue). The central gray and white matter have a complex blood supply (yellow) that includes the anterior choroidal artery. The anterior and posterior cerebral arteries supply most of the medial surface of the brain.
В Distribution of the three main cerebral arteries in transverse and coronal sections
a, b Coronal sections at the level of the mammillary bodies, c Transverse section at the level of the internal capsule.
The internal capsule, basal ganglia, and thalamus derive most of their blood supply from perforating branches of the following vessels at the base of the brain:
• Anterior choroidal artery (from the internal carotid artery)
• Anterolateral central arteries (lenticulostriate arteries and striate branches) with their terminal branches (from the middle cerebral artery)
• Posteromedial central arteries (from the posterior cerebral artery)
• Perforating branches (from the posterior communicating artery)
The internal capsule, which is traversed by the pyramidal tract and other structures, receives most of its blood supply from the middle cerebral artery (anterior crus and genu) and from the anterior choroidal artery (posterior crus). If these vessels become occluded, the pyramidal tract and other structures will be interrupted, causing paralysis on the contralateral side of the body (stroke: central paralysis, see C on p. 265).
C Functional centers on the surface of the cerebrum
a Lateral view of the left cerebral hemisphere. Regions supplied by branches of the middle cerebral artery are shaded orange. Specific functions can be assigned to well-defined areas of the cerebrum. These areas are supplied by branches of the three main cerebral arteries. The sensorimotor cortex (pre- and postcentral gyrus) and the motor and sensory speech centers (Broca and Wernicke areas) are supplied by branches of the middle cerebral artery (see b). Therefore, a language deficit (aphasia) or the loss of motor or sensory function on one side of the body suggests an occlusion of the middle cerebral artery.
b Medial view of the right cerebral hemisphere. The “margin” of the sensorimotor cortex may be deprived of blood (clinically manifested by paralysis and sensory disturbances mainly affecting the lower limb) by an occlusion of the anterior cerebral artery. The visual cortex may lose its blood supply (causing blindness) through an occlusion of the posterior cerebral artery.
18.4 Arteries of the Brainstem and Cerebellum
A Arteries of the brainstem and cerebellum
a Basal view, b left lateral view.
The brainstem and cerebellum are supplied by the basilar and cerebellar arteries (see below). Because the basilar artery is formed by the union of the two vertebral arteries, blood supplied by the basilar artery is said to come from the vertebrobasilar complex. The vessels that supply the brainstem (mesencephalon, pons, and medulla oblongata) arise either directly from the basilar artery (e.g., the pontine arteries) and vertebral arteries or from their branches. The branches are classified by their sites of entry and distribution as medial, med iolateral, or lateral (paramedian branches; short and long circumferential branches). Decreased perfusion in or occlusion of these vessels leads to transient or permanent impairment of blood flow (brainstem syndrome) and may produce a great variety of clinical symptoms, given the many nuclei and tract systems that exist in the brainstem. The spinal cord, receives a portion of its blood supply from the anterior spinal artery (see b), which arises from the vertebral artery (see p. 286). The cerebellum is supplied by three large arteries:
• Posterior inferior cerebellar artery (PICA), the largest branch of the vertebral artery. This vessel is usually referred to by its acronym, PICA.
• Anterior inferior cerebellar artery (AICA), the first major branch of the basilar artery.
• Superior cerebellar artery (SCA), the last major branch of the basilar artery before it divides into the posterior cerebral arteries.
Note: the labyrinthine artery which supplies the inner ear (see also D, p. 155) usually arises from the anterior inferior cerebellar artery, as pictured here, although it may also spring directly from the basilar artery. Impaired blood flow in the labyrinthine artery leads to an acute loss of hearing (sudden sensorineural hearing loss), frequently accompanied by tinnitus (see D, p. 149).
D Distribution of the arteries of the pons in transverse section
The pons derives its blood supply from short and long branches of the basilar artery.
E Distribution of the arteries of the medulla oblongata in transverse section
The medulla oblongata is supplied by branches of the anterior spinal artery, and posterior inferior cerebellar artery (both arising from the vertebral artery), as well as the anterior inferior cerebellar artery (first large branch of the basilar artery).
18.5 Dural Sinuses, Overview
A Relationship of the principal dural sinuses to the skull
Oblique posterior view from the right side (brain removed and tentorium windowed on the right side). The dural sinuses are stiff-walled venous channels that receive blood from the internal and external cerebral veins, orbits, and calvaria, and convey it to the internal jugular veins on both sides. With few exceptions (inferior sagittal sinus, straight sinus), the walls of the dural sinuses are formed by both the periosteal and meningeal layers of the dura mater (see C, p.189). The valveless dural sinuses are lined internally by endothelium and are expanded at some sites (particularly in the superior sagittal sinus) to form “lateral lacunae" (see B). These expansions contain the arachnoid villi through which cerebrospinal fluid (CSF) is absorbed into the venous blood (see p. 194f). The system of dural sinuses is divided into an upper group and a lower group:
• Upper group: superior and inferior sagittal sinuses, straight sinus, occipital sinus, transverse sinus, sigmoid sinus, and the confluence of the sinuses.
• Lower group: cavernous sinus with anterior and posterior intercavernous sinuses, sphenoparietal sinus, superior and inferior petrosal sinuses.
The upper and lower groups of dural sinuses communicate with the venous plexuses of the vertebral canal through the marginal sinus at the inlet to the foramen magnum and through the basilar plexus on the clivus (see C).
В Structure of a dural sinus, shown here for the superior sagittal sinus
Transverse section, occipital view (detail from A). The sinus wall is composed of endothelium and tough, collagenous dural connective tissue with a periosteal and meningeal layer. Between the two layers is the sinus lumen.
Note the lateral lacunae, where the arachnoid villi open into the venous system. Superficial cerebral veins (superior cerebral veins, bridging veins, see pp. 186 and 262) open into the sinus itself along with diploic veins from the adjacent cranial bone. The sinus also receives emissary veins—valveless veins that establish connections among the sinuses, the diploic veins, and the extracranial veins of the scalp.
C Dural sinuses at the skull base
Transverse section at the level of the tentorium cerebelli, viewed from above (brain removed, orbital roof and tentorium windowed on the right side). The cavernous sinus forms a ring around the sella turcica, its left and right parts being interconnected at the front and behind by an anterior and a posterior intercavernous sinus. Behind the posterior intercavernous sinus, on the clivus, is the basilar plexus. This plexus also contributes to the drainage of the cavernous sinus.
18.6 Dural Sinuses:
Tributaries and Accessory Draining Vessels
A Dural sinus tributaries from the cerebral veins (after Rauber and Kopsch)
Right lateral view. Venous blood collected deep within the brain drains to the dural sinuses through superficial and deep cerebral veins (see p. 258). The red arrows in the diagram show the principal directions of venous blood flow in the major sinuses. Because of the numerous anastomoses, the isolated occlusion of even a complete sinus segment may produce no clinical symptoms.
В Accessory drainage pathways of the dural sinuses
Right lateral view. The dural sinuses have many accessory drainage pathways besides their principal drainage into the two internal jugular veins. The connections between the dural sinuses and extracranial veins mainly serve to equalize pressure and regulate temperature. These anastomoses are of clinical interest because their normal direction of blood flow may reverse (no venous valves), allowing blood from extracranial veins to reflux into the dural sinuses. This mechanism may give rise to sinus infections that lead, in turn, to vascular occlusion (venous sinus thrombosis). The most important accessory drainage vessels include the following:
• Emissary veins (diploic and superior scalp veins), see C.
• Superior ophthalmic vein (angular and facial veins).
• Venous plexus of foramen ovale (pterygoid plexus, retromandibular vein).
• Marginal sinus and basilar plexus (internal and external vertebral venous plexus), see C.
C Occipital emissary veins
Emissary veins establish a direct connection between the intracranial dural sinuses and extracranial veins. They run through small cranial openings such as the parietal and mastoid foramina. Emissary veins are of clinical interest because they create a potential route by which bacteria from the scalp may spread to the dura mater and incite a purulent meningitis.
18.7 Veins of the Brain:
Superficial and Deep Veins
Because the veins of the brain do not run parallel to the arteries, marked differences are noted between the regions of arterial supply and venous drainage. While all of the cerebral arteries enter the brain at its base, venous blood is drained from the entire surface of the brain, including the base, and also from the interior of the brain by two groups of veins: the superficial cerebral veins and the deep cerebral veins. The superficial veins
drain blood from the cerebral cortex (via cortical veins) and white matter (via medullary veins) directly into the dural sinuses. The deep veins drain blood from the deeper portions of the white matter, basal ganglia, corpus callosum, and diencephalon into the great cerebral vein, which enters the straight sinus. The two venous regions (those of the superficial and deep veins) are interconnected by numerous intracerebral anastomoses (see D).
A Superficial veins of the brain (superficial cerebral veins)
Left lateral view (a) and medial view (b).
a, b The superficial cerebral veins drain blood from the short cortical veins and long medullary veins in the white matter (see D) into the dural sinuses. (The deep cerebral veins are described in C, p. 261.) Their course is extremely variable, and veins in the subarachnoid space do not follow arteries, gyri, or sulci. Consequently, only the most important of these vessels are named here.
Just before terminating in the dural sinuses, the veins leave the subarachnoid space and run a short subdural course between the dura mater and arachnoid. These short subdural venous segments are called bridging veins. The bridging veins have great clinical importance because they may be ruptured by head trauma, resulting in a subdural hematoma (see p.262).
В Regions drained by the superficial cerebral veins
a Left lateral view, b view of the medial surface of the right hemisphere, c basal view.
The veins on the lateral surface of the brain are classified by their direction of drainage as ascending (draining into the superior sagittal sinus) or descending (draining into the transverse sinus). The superficial middle cerebral vein drains into both the cavernous and transverse sinuses (see A, p.254).
C Basal cerebral venous system
The basal cerebral venous system drains blood from both superficial and deep cerebral veins. A venous circle formed by the basilar veins (of Rosenthal, see below) exists at the base of the brain, analogous to the arterial circle of Willis. The basilar vein is formed in the anterior perforate substance by the union of the anterior cerebral and deep middle cerebral veins. Following the course of the optic tract, the basilar vein runs posteriorly around the cerebral peduncle and unites with the basilar vein from the opposite side on the dorsal aspect of the mesencephalon. The two internal cerebral veins also terminate at this venous junction, the posterior venous confluence. This junction gives rise to the midline great cerebral vein, which enters the straight sinus. The basilar vein receives tributaries from deep brain regions in its course (e.g., veins from the thalamus and hypothalamus, choroid plexus of the inferior horn, etc.). The two anterior cerebral veins are interconnected by the anterior communicating vein, creating a closed, ring-shaped drainage system.
D Anastomoses between the superficial and deep cerebral veins
Transverse section through the left hemisphere, anterior view. The superficial cerebral veins communicate with the deep cerebral veins through the anastomoses shown here (see p.260). Flow reversal (double arrows) may occur in the boundary zones between two territories.
18.8 Veins of the Brainstem and Cerebellum: Deep Veins
A Deep cerebral veins
Multiplanar transverse section (combining multiple transverse planes) with a superior view of the opened lateral ventricles. The temporal and occipital lobes and tentorium cer- ebelli have been removed on the left side to demonstrate the upper surface of the cerebellum and the superior cerebellar veins. On the lateral walls of the anterior horns of both lateral ventricles, the superior thalamostriate vein runs toward the interventricular foramen in the groove between the thalamus and caudate nucleus. After receiving the anterior vein of the septum pellucidum and the superior choroidal vein, it forms the internal cerebral vein and passes through the interventricular foramen along the roof of the diencephalon toward the quadrigeminal plate, which contains the superior and inferior colliculi. There it unites with the internal cerebral vein of the opposite side, and the basal veins to form the posterior venous confluence, which gives rise to the great cerebral vein.
В Cerebellar veins
Posterior view. Like the other veins of the brain, the cerebellar veins are distributed independently of the cerebellar arteries. Larger trunks cross over gyri and sulci, running mainly in the sagittal direction. A medial and a lateral group can be distinguished based on their gross topographical anatomy. The medial group of cerebellar veins drains the vermis and adjacent portions of the cerebellar hemispheres (precentral vein, superior and inferior veins of the vermis) and the medial portions of the superior and inferior cerebellar veins. The lateral group (petrosal vein and lateral portions of the superior and inferior cerebellar veins) drains most of the two cerebellar hemispheres. All of the cerebellar veins anastomose with one another; their outflow is exclusively infratentorial (i.e., below the tentorium cerebelli).
C Region drained by the deep cerebral veins
Coronal section. Three principal venous segments can be identified in each hemisphere:
• Thalamostriate vein
• Internal cerebral vein
• Basal vein
The region drained by the deep cerebral veins encompasses large portions of the base of the cerebrum, the basal ganglia, the internal capsule, the choroid plexuses of the lateral and third ventricles, the corpus callosum, and portions of the diencephalon and mesencephalon.
D Veins of the brainstem
a Anterior view of the brainstem in situ (the cerebellum and part of the occipital lobe have been removed on the left side), b Posterior view of the isolated brainstem with the cerebellum removed.
The veins of the brainstem are a continuation of the veins of the spinal cord and connect them with the basal veins of the brain. As on the spinal cord, the veins on the lower part of the brainstem form a venous plexus consisting of a powerfully developed longitudinal system and a more branched transverse system. The veins of the medulla oblongata, pons, and cerebellum make up the infratentorial venous system. Various anastomoses (e.g., anteromedial and lateral) exist at the boundary between the infra- and supratentorial systems.
18.9 Blood Vessels of the Brain: Intracranial Hemorrhage
Intracranial hemorrhages may be extracerebral (see A) or intracerebral (see C).
A Extracerebral hemorrhages
Extracerebral hemorrhages are defined as bleeding between the calvaria and brain. Because the bony calvaria is immobile, the developing hematoma exerts pressure on the soft brain. Depending on the source of the hemorrhage (arterial or venous), this may produce a rapidly or slowly developing incompressible mass with a rise of intracranial pressure that may damage not only the brain tissue at the bleeding site but also in more remote brain areas. Three types of intracranial hemorrhage can be distinguished based on their relationship to the dura mater:
a Epidural hematoma (epidural = above the dura). This type generally develops after a head injury involving a skull fracture. The bleeding most commonly occurs from a ruptured middle meningeal artery (due to the close proximity of the middle meningeal artery to the calvaria, a sharp bone fragment may lacerate the artery). The hematoma forms between the calvaria and the periosteal layer of the dura mater. Pressure from the hematoma separates the dura from the calvaria and displaces the brain. Typically there is an initial transient loss of consciousness caused by the impact, followed 1-5 hours later by a second decline in the level of consciousness, this time due to compression of the brain by the arterial hemorrhage. The interval between the first and second loss of consciousness is called the lucid interval (occurs in approximately 30-40% of all epidural hematomas). Detection of the hemorrhage (CT scanning of the head) and prompt evacuation of the hematoma are life-saving.
b Subdural hematoma (subdural = below the dura). Trauma to the head causes the rupture of a bridging vein (see p. 254) that bleeds between the dura mater and arachnoid. The bleeding occurs into a potential “subdural space,” which exists only when extravasated blood has dissected the arachnoid membrane from the dura (the spaces are described in C, p. 191). Because the bleeding source is venous, the increased intracranial pressure and mass effect develop more slowly than with an arterial epidural hemorrhage. Consequently, a subdural hematoma may develop chronically over a period of weeks, even after a relatively mild head injury.
c Subarachnoid hemorrhage is an arterial bleed caused by the rupture of an aneurysm (abnormal outpouching) of an artery at the base of the brain (see B). It is typically caused by a brief, sudden rise in blood pressure, like that produced by a sudden rise of intra-abdominal pressure (straining at stool or urine, lifting a heavy object, etc.). Because the hemorrhage is into the CSF-filled subarachnoid space, blood can be detected in the cerebrospinal fluid by means of lumbar puncture. The cardinal symptom of a subarachnoid hemorrhage is a sudden, excruciating headache accompanied by a stiff neck caused by meningeal irritation.
В Sites of berry aneurysms at the base of the brain (after Bahr and Frotscher)
The rupture of congenital or acquired arterial aneurysms at the base of the brain is the most frequent cause of subarachnoid hemorrhage and accounts for approximately 5 % of all strokes. These are abnormal saccular dilations of the circle of Willis and are especially common at the site of branching. When one of these thin-walled aneurysms ruptures, arterial blood escapes into the subarachnoid space. The most common site is the junction between the anterior cerebral and anterior communicating arteries (1 ); the second most likely site is the branching of the posteriorcommunicating artery from the internal carotid artery (2).
C Intracerebral hemorrhage
Coronal section at the level of the thalamus. Unlike the intracranial extracerebral hemorrhages described above, intracerebral hemorrhage occurs when damaged arteries bleed directly into the substance of the brain. This distinction is of very great clinical importance because extracerebral hemorrhages can be controlled by surgical hemostasis of the bleeding vessel, whereas intracerebral hemorrhages cannot. The most frequent cause of intracerebral hemorrhage (hemorrhagic stroke) is high blood pressure. Because the soft brain tissue offers very little resistance, a large hematoma may form within the brain. The most common sources of intracerebral bleeding are specific branches of the middle cerebral artery—the lenticulostriate arteries pictured here (known also as the “stroke arteries”). The hemorrhage causes a cerebral infarction in the region of the internal capsule, one effect of which is to disrupt the pyramidal tract, which passes through the capsule (see E, p. 377). The loss of pyramidal tract function below the lesion is manifested clinically by spastic paralysis of the limbs on the side of the body opposite to the injury (the pyramidal tracts cross below the level of the lesion). The hemorrhage is not always massive, and smaller bleeds may occur in the territories of the three main cerebral arteries, producing a typical clinical presentation.
18.10 Blood Vessels of the Brain: Cerebrovascular Disease
A Frequent causes of cerebrovascular disease (after Mumenthaler) Disturbances of cerebral blood flow that deprive the brain of oxygen (cerebral ischemia) are the most frequent cause of central neurological deficits. The most serious complication is stroke: the vast majority of all strokes are caused by cerebral ischemic disease. Stroke has become the third leading cause of death in western industrialized countries (approximately 700,000 strokes occur in the United States each year). Cerebral ischemia is caused by a prolonged diminution or interruption of blood flow and involves the distribution area of the internal carotid artery in up to 90% of cases. Much less commonly, cerebral ischemia is caused by an obstruction of venous outflow due to cerebral venous thrombosis (see B). A decrease of arterial blood flow in the carotid system most commonly results from an embolic or local thrombotic occlusion. Most emboli originate from atheromatous lesions at the carotid bifurcation (arterioarterial emboli) or from the expulsion of thrombotic material from the left ventricle (cardiac emboli). Blood clots (thrombi) may be dislodged from the heart as a resultof valvular disease or atrial fibrillation. This produces emboli that may be carried by the bloodstream to the brain, where they may cause the functional occlusion of an artery supplying the brain. The most common example of this involves all of the distribution region of the middle cerebral artery, which is a direct continuation of the internal carotid artery.
В Cerebral venous thrombosis
Coronal section, anterior view. The cerebral veins, like the cerebral arteries, serve specific territories (see pp. 258 and 260). Though much less common than decreased arterial flow, the obstruction of venous outflow is an important potential cause of ischemia and infarction. With a thrombotic occlusion, for example, the quantity of blood and thus the venous pressure are increased in the tributary region of the occluded vein. This causes a drop in the capillary pressure gradient, with an increased extravasation of fluid from the capillary bed into the brain tissue (edema). There is a concomitant reduction of arterial inflow into the affected region, depriving it of oxygen. The occlusion of specific cerebral veins (e.g., due to cerebral venous thrombosis) leads to brain infarctions at characteristic locations:
a Superior cerebral veins: Thrombosis and infarction in the areas drained by the:
• Medial superior cerebral veins (right, symptoms: contralateral lower limb weakness);
• Posterior superior cerebral veins (left, symptoms: contralateral hemiparesis).
Motor aphasia occurs if the infarction involves the motor speech center in the dominant hemisphere.
b Inferior cerebral veins: Thrombosis of the right inferior cerebral veins leads to infarction of the right temporal lobe (symptoms: sensory aphasia, contralateral hemianopia).
c Internal cerebral veins: Bilateral thrombosis leads to a symmetrical infarction affecting the thalamus and basal ganglia. This is characterized by a rapid deterioration of consciousness ranging to coma.
Because the dural sinuses have extensive anastomoses, a limited occlusion affecting part of a sinus often does not cause pronounced clinical symptoms, unlike the venous thromboses described here (see p. 256).
C Cardinal symptoms of occlusion of the three main cerebral arteries (after Masuhr and Neumann)
When the anterior, middle or posterior cerebral artery becomes occluded, characteristic functional deficits occur in the oxygen-deprived brain areas supplied by the occluded vessel (see p.250). In many cases the affected artery can be identified based on the associated neurological deficit:
• Bladder weakness (cortical bladder center) and paralysis of the lower limb (hemiplegia with or without hemisensory deficit, predominantly affecting the leg) on the side opposite the occlusion (see motor and sensory homunculi, pp. 329 and 339) indicate an infarction in the territory of the anterior cerebral artery.
• Contralateral hemiplegia affecting the arm and face more than the leg indicates an infarction in the territory of the middle cerebral artery. If the dominant hemisphere is affected, aphasia also occurs (the patient cannot name objects, for example).
• Visual disturbances affecting the contralateral visual field (hemianopia) may signify an infarction in the territory of the posterior cerebral artery, because the structures supplied by this artery include the visual cortex in the calcarine sulcus of the occipital lobe. If branches to the thalamus are also affected, the patient may also exhibit a contralateral hemisensory deficit because the afferent sensory fibers have already crossed below the thalamus.
The extent of the infarction depends partly on whether the occlusion is proximal or distal. Generally a proximal occlusion will cause a much more extensive infarction than a distal occlusion. MCA infarctions are the most common because the middle cerebral artery is essentially a direct continuation of the internal carotid artery.