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The brain and spinal cord are critically dependent on an uninterrupted supply of oxygenated blood, and thus are dependent on unimpeded flow through the cerebral vessels. About 18% of the total blood volume in the body circulates in the brain, which accounts for about 2% of the body weight. The blood transports oxygen, nutrients, and other substances necessary for proper functioning of the brain and carries away metabolites. Loss of consciousness occurs in less than 15 seconds after blood flow to the brain has stopped, and irreparable damage to the brain tissue occurs within 5 minutes.
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Cerebrovascular disease, or stroke, occurs as a result of vascular compromise or hemorrhage and is one of the most frequent sources of neurologic disability. Because the cerebral vessels each tend to irrigate specific territories in the brain, their occlusion results in highly stereotyped syndromes that, even prior to imaging studies, can suggest the site of the vascular lesion.
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Nearly half of the admissions to many busy neurologic services are because of strokes. Cerebrovascular disease is the third most common cause of death in industrialized societies. Because thrombolysis—if accomplished in the initial hours after a stroke occurs—can sometimes restore blood flow and improve clinical status, early recognition and treatment of stroke are essential.
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ARTERIAL SUPPLY OF THE BRAIN
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The circle of Willis (named after the English neuroanatomist Sir Thomas Willis) is a hexagon of vessels that gives rise to all of the major cerebral arteries. It is fed by the paired internal carotid arteries and the basilar artery. When the circle is complete, it contains a posterior communicating artery on each side and an anterior communicating artery. The circle of Willis shows many variations among individuals. The posterior communicating arteries may be large on one or both sides (embryonic type); the posterior cerebral artery may be thin in its first stretch (embryonic type); and the anterior communicating artery may be absent, double, or thin. Despite these variations, occlusion of each of the major cerebral arteries usually produces a characteristic clinical picture.
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Characteristics of the Cerebral Arteries
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The course of the large arteries (at least in their initial stretches) is largely ventral to the brain in a relatively small region. The arteries course in the subarachnoid space, often for a considerable distance, before entering the brain itself; rupture of a vessel (eg, from an aneurysm that has burst) tends to cause a subarachnoid hemorrhage.
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Each major artery supplies a certain territory, separated by border zones (watershed areas) from other territories; sudden occlusion in a vessel affects its territory immediately, sometimes irreversibly.
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The arterial blood for the brain enters the cranial cavity by way of two pairs of large vessels (Figs 12–1 and 12–2): the ...