Arteries of the Brain · 421 11
Vertebral artery. The vertebral arteries arise from the subclavian arteries on either side and are often of different caliber on the two sides. The left vertebral artery rarely arises directly from the aortic arch. The vertebral artery travels up the neck in the bony canal formed by the transverse foramina of the cervical vertebrae, which it enters at the C6 level (i.e., it does not pass through the transverse foramen of C7). At the level of the atlas (C1), it leaves this bony canal and curves around the lateral mass of the atlas dorsally and medially, sitting in the sulcus of the vertebral artery on the upper surface of the posterior arch of C1. It then runs ventrally between the occiput and the atlas and passes through the atlanto-occipital membrane. It usually penetrates the dura mater at the level of the foramen magnum.
In the subarachnoid space, the vertebral artery curves ventrally and cranially aroundthebrainstem,thenjoinsthecontralateralvertebralarteryinfrontofthe caudalportionoftheponstoformthebasilarartery.Thevertebralarterygivesoff manybranchestothemusclesandsofttissuesoftheneck;itsmajorintracranial branches are the posterior inferior cerebellar artery (PICA) and the anterior spinalartery(Fig. 11.2).TheoriginofthePICA(cf.alsop. 427)isjustdistaltothepoint wherethevertebralarteryentersthesubarachnoidspace;arupturedaneurysm attheoriginofthePICAmay,therefore,beextracranialandnonethelessproduce a subarachnoid hemorrhage. The branches of the vertebral artery to the spinal cordhaveavariableanatomy.Theysupplybloodtotheuppercervicalspinalcord andformanastomoseswithsegmentalspinalarteriesarisingfromtheproximal portion of the vertebral artery, and with the nuchal arteries.
Arteries of the Anterior and Middle Cranial Fossae
Internal Carotid Artery (ICA)
After it exits the carotid canal, the internal carotid artery courses rostrally, next to the clivus and beneath the dura mater, to the cavernous sinus. It curves upward and backward within the cavernous sinus, forming a loop that is open posteriorly (the carotid siphon, Fig. 11.1). Fine extradural branches of the internal carotid artery supply the floor of the tympanic cavity, the dura mater of the clivus, the semilunar ganglion, and the pituitary gland.
Injury or rupture of the internal carotid artery within the cavernous sinus produces a “short-circuit” connection between its arterial blood and the venous blood of the sinus (carotid-cavernous fistula). If an intracavernous aneurysm of the internal carotid artery ruptures, exophthalmos develops but there is no subarachnoid hemorrhage, because the aneurysm is extradural. The patient’s vision in the ipsilateral eye deteriorates thereafter because of outflow obstruction and congestion of the retinal veins.
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11 422 · 11 Blood Supply and Vascular Disorders of the Central Nervous System
anterior cerebral a. internal carotid a. middle cerebral a.
posterior communicating a.
anterior choroidal a.
posterior cerebral a.
superior cerebellar a. basilar a.
anterior inferior cerebellar a.
labyrinthine a.
posterior inferior cerebellar a.
vertebral a.
anterior spinal a.
Fig. 11.2 Arteries of the base of the skull
Ophthalmic artery. The internal carotid artery enters the subarachnoid space medial to the anterior clinoid process. The ophthalmic artery arises at this point from the internal carotid artery; it is thus already intradural at its site of origin (Fig. 11.1). It enters the orbit together with the optic nerve and supplies not only the contents of the orbit, but also the sphenoid sinus, the ethmoid air cells, the nasal mucosa, the dura mater of the anterior cranial fossa, and the skin of the forehead, root of the nose, and eyelids. The cutaneous branches of the ophthalmic artery form anastomoses with branches of the external carotid artery,
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Arteries of the Brain · 423 11
which can be an important path for collateral circulation around a stenosis or occlusion of the internal carotid artery (ophthalmic collaterals). Ruptured aneurysms or injuries of the ICA distal to the origin of the ophthalmic artery cause subarachnoid hemorrhage.
Posterior communicating artery. The next angiographically visible artery arising from the internal carotid artery along its intradural course is the posterior communicating artery (Figs. 11.1 and 11.2). In the early stages of embryonic development, this artery is the proximal segment of the posterior cerebral artery, which is at first a branch of the internal carotid artery and only later comes to be supplied by the basilar artery. In some 20% of cases, the posterior communicating artery remains the main source of blood for the posterior cerebral artery; this is equivalent to a direct origin of the posterior cerebral artery from the ICA, or fetal origin of the posterior cerebral artery, as it is traditionally called. The fetal pattern, if present, is usually seen only on one side, while the contralateral posterior cerebral artery arises from an asymmetric basilar tip. Sometimes, however, both posterior cerebral arteries arise directly from the ICA through unusually large posterior communicating arteries. In such cases, the basilar tip is smaller than usual, and the basilar artery appears to terminate where it gives off the two superior cerebellar arteries.
The posterior communicating artery ends where it joins the proximal segment of the posterior cerebral artery some 10 mm lateral to the basilar tip. It is a component of the circle of Willis and the most important anastomotic connection between the anterior and posterior circulations.
The posterior communicating artery gives off fine perforating branches to the tuber cinereum, mamillary body, rostral thalamic nuclei, subthalamus, and part of the internal capsule.
The origin of the posterior communicating artery from the ICA is a preferred site for the formation of aneurysms (so-called PComm aneurysms; see p. 481). Such aneurysms usually arise from the side wall of the internal carotid artery, and only rarely from the posterior communicating artery itself.
Anterior choroidal artery. This artery arises from the internal carotid artery immediately distal to the posterior communicating artery (Fig. 11.2), runs toward the occiput parallel to the optic tract, and then enters the choroidal fissure to supply the choroid plexus of the temporal horn of the lateral ventricle. Along its course, it gives off branches to the optic tract, uncus, hippocampus, amygdala, part of the basal ganglia, and part of the internal capsule. It is clinically significant that the anterior choroidal artery also supplies part of the pyramidal tract. It has anastomotic connections with the lateral posterior choroidal artery (see Fig. 11.10, p. 431).
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11 424 · 11 Blood Supply and Vascular Disorders of the Central Nervous System
Terminal branches. The internal carotid artery bifurcates above the clinoid process, giving rise to the anterior cerebral artery medially and the middle cerebral artery laterally.
Middle Cerebral Artery
The middle cerebral artery (MCA) is the largest branch of the internal carotid artery (Fig. 11.2). After its origin from the ICA above the anterior clinoid process, it travels laterally in the sylvian fissure (lateral sulcus). The main trunk of the middle cerebral artery gives off numerous perforating branches to the basal ganglia and to the anterior limb and genu of the internal capsule, as well as to the external capsule and claustrum (Fig. 11.3).
The middle cerebral artery divides into its major cortical branches within the insular cistern. These branches supply large areas of the frontal parietal, and temporal lobes.
The major branches of the middle cerebral artery (Fig. 11.4) are the orbitofrontal (I), prerolandic (II), rolandic (III), anterior parietal (IV), and posterior parietal (V) arteries, the artery of the angular gyrus (VI), and the temporooccipital, posterior temporal (VII), and anterior temporal (VIII) arteries. The cortical areas supplied by the middle cerebral artery include, among others, the primary sensory and motor cortices (except for their parasagittal and medial portions), the language areas of Broca and Wernicke, the primary auditory cortex, and the primary gustatory cortex.
The middle cerebral artery has cortical anastomotic connections with the anterior and posterior cerebral arteries.
Anterior Cerebral Artery
The anterior cerebral artery (ACA) originates from the bifurcation of the internal carotid artery and then courses medially and rostrally. The anterior cerebral arteries of the two sides come to lie adjacent to each other across the midline in front of the lamina terminalis; from this location, the two arteries course in parallel upward and posteriorly. This is also the site of the anastomotic connection between the two anterior cerebral arteries through the anterior communicating artery, a further important component of the circle of Willis (see Fig. 11.12, p. 434). The anterior communicating artery and the neighboring segments of the anterior cerebral arteries are preferred sites for the formation of aneurysms (so-called AComm aneurysms, p. 481).
Branches of the anterior cerebral artery. The proximal (basal) segment of the anterior cerebral artery gives off numerous small perforating branches that
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Anterior
choroidal a.