The Carotid Artery System

Internal Carotid Artery

There are four ICA segments: cervical, petrous, cavernous, and supraclinoid. The cervical segment ascends vertically in the neck, posterior and slightly medial to the ECA. Significant atherosclerotic disease is usually located at the ICA origin, with potential for artery-to-artery embolism, stenosis with eventual occlusion, or both. Unlike the ECA, this segment does not have branches, allowing differentiation between the two vessels on imaging scans.

The ICA enters the skull through the carotid canal within the petrous bone. This petrous segment has two small branches, the caroticotympanic and pterygoid branches, which are usually clinically irrelevant. The cavernous segment, usually called the carotid siphon because of its shape, is the portion of the ICA within the cavernous sinus and provides minor branches supplying the posterior pituitary (meningohypophyseal artery) and the abducens nerve. Of its many branches, the ophthalmic artery is the most significant. The ophthalmic artery arises from the ICA just as it pierces the dura and emerges from the cavernous sinus to pass through the optic canal into the orbit just below and lateral to the optic nerve. It supplies the globe and orbital contents through its 3 major branches: the ocular (central retinal and ciliary arteries), orbital, and extraorbital branches. The ophthalmic artery forms extensive anastomoses with branches of the ECA. The supraclinoid segment is the last portion of the ICA. It begins when this segment penetrates the dura. The posterior communicating artery (P-com) and the anterior choroidal artery are the two important branches originating at this level. The ICA then bifurcates into the anterior cerebral artery (ACA) and middle cerebral artery (MCA).

The P-com is often hypoplastic. When present, it travels posteriorly to communicate with the posterior circulation at the level of the posterior cerebral artery (PCA). The P-com also provides thalamoperforate branches that supply the anteromedial thalamus and parts of the cerebral peduncles. Its presence and size is variable but often serves as an important collateral pathway in extensive cerebrovascular disease allowing flow from the anterior to the posterior circulation or vice versa.

The anterior choroidal artery arises from the posterior surface of the ICA just above the P-com origin. This artery supplies an extensive cerebral area, including the visual system (optic tract, anterior portion of the lateral geniculate body and optic radiations), genu and posterior limb of the internal capsule, basal ganglia (medial globus pallidus and tail of the caudate), the diencephalon (portions of the lateral thalamus and the subthalamic nuclei), the midbrain (substantia nigra and portions of the cerebral peduncle), the medial temporal lobe (uncus, pyriform cortex, amygdala), and the choroidal plexus of the temporal horn and atrium.

The ACA travels medially and anteriorly toward the interhemispheric fissure. It supplies the anterior portions of the basal ganglia and internal capsule and most of the mesial portion of the frontal and parietal lobes. The first segment of the ACA, the A1 segment, begins at the carotid bifurcation and terminates at the level of the anterior communicating artery, which connects opposite A1 segments and constitutes an important collateral pathway in carotid artery occlusive disease. Occasionally, a single A1 exists supplying both medial frontal hemispheres from a single side and is termed an azygous ACA. The recurrent artery of Heubner is the most important branch of the A1 segment and supplies the anteroinferior portion of the head of the caudate, the putamen, and the anterior limb of the internal capsule.

The ACA continues as the A2 segment, where the orbitofrontal branch arises and travels around the genu of the corpus callosum to the orbital and medial surface of the frontal lobe whereas the frontopolar branch supplies the rest of the medial surface of the frontal lobe. The ACA then gives off its two major branches, the pericallosal artery that runs just above the corpus callosum and the callosomarginal artery paralleling the cingulate gyrus. These two arteries supply the mesial portions of the frontal and parietal lobes.

One of the major fail-safe systems within the cerebral circulation is the circle of Willis, formed by the connections between the ACAs, the anterior communicating arteries, the supraclinoid carotid, the P-coms, and PCAs. This vascular network often provides alternative conduits for perfusion avoiding the development of cerebral infarction when a major vessel becomes significantly diseased or occluded, as with cervical ICA atherosclerotic disease. The respective junctions of each of these vessels in the Circle of Willis is the primary site of berry aneurysm formation—the major cause of subarachnoid hemorrhages.

The MCA originates from the supraclinoid carotid stem and, subsequently, travels laterally to the sylvian fissure as the main-stem M1 segment, giving off lenticulostriate branches to the basal ganglia. As the MCA approaches the sylvian fissure, it usually divides into two large trunks, the superior and inferior divisions. Occasionally, the MCA trifurcates, and a middle trunk is also present. Different branches supply the frontal (orbitofrontal, ascending frontal, precentral, and central branches), parietal (anterior and posterior parietal and angular branches), and temporal (anterior and posterior temporal) lobes. The orbitofrontal, ascending frontal, precentral, and central branches usually arise from the superior division of the MCA, whereas the angular, anterior and posterior temporal branches arise from the inferior division. The anterior and posterior parietal branches can arise from either division (Fig. 54-2). The MCA stem or its distal bifurcation point are classic sites where large cerebral artery emboli lodge and are sometimes amenable to emergent intra-arterial thrombolytic therapy.

Figure 54-2 Arteries of Brain (Lateral and Medial Views).

Vertebrobasilar Arteries

The vertebral arteries (VAs) usually originate from the subclavian arteries on either side (see Fig. 54-1). They have 4 portions: 3 extracranial and one intracranial. From their origin, the VAs travel posteriorly (prevertebral segment) and enter the transverse foramen of the sixth cervical vertebrae. They then extend superiorly to exist at C2 (cervical segment), sharply turning posteriorly around the auricular process of the atlas (atlantic segment), then proceeding rostrally, piercing the posterior atlanto-occipital membrane and the dura mater to enter the intracranial cavity through the foramen magnum (intracranial or intradural segment). The vertebral arteries are prone to dissection at their entry and exit sites through the vertebra and are prone to temporal arteritis right at the dural junction.

The intracranial segments course anteriorly lateral to the medulla, then ascend medially to the pontomedullary junction, where they unite at the pontine midline to from the basilar artery (Fig. 54-3).

Figure 54-3 Arteries of Brain: Inferior Views.

The cervical branches of the VAs give muscular, vertebral body and radicular branches and may serve as collateral conduits in cases of cervical artery compromise or occlusion. The intracranial branches are neurologically more significant and, if diseased, often give definite neurologic syndromes. The first of these are the lateral medullary branches supplying the lateral portions of the medulla. Distally the posterior–inferior cerebellar arteries (PICAs) primarily supply the posterior and inferior regions of the cerebellum but also the dorsum of the medulla oblongata. Classic Wallenberg syndrome results from occlusion of medullary arteries of the PICA or penetrator branches of the vertebral arteries.

The anterior spinal artery arises from paired medial VA branches just before the basilar junction unites in the midline to form a single vessel running the full length of the spinal cord caudally in the anteromedial sulcus. It also supplies the medial portions of the medulla; however, adequate collateral circulation in this location makes the medial medullary syndrome rare. In contrast, the anterior spinal artery within the cord is crucial to spinal cord function, and its occlusion leads to an anterior spinal artery syndrome (Chapter 45). The posterior spinal arteries arise from the PICAs or intracranial VAs and run caudally, supplying the posterior and lateral aspect of the spinal cord.

The basilar artery courses rostrally on the anterior surface of the pons, and along the clivus to end at the pontomesencephalic junction providing a number of important branches on its way. The anterior–inferior cerebellar arteries (AICAs) usually arise from the midportion of the basilar artery and supply the brachium pontis, lateral pontine tegmentum, flocculus, and anteroinferior portions of the cerebellum. The internal auditory artery may arise from the AICAs or the basilar itself and supplies the vestibular and cochlear structures. The superior cerebellar arteries (SCAs) arise from the distal portion of the basilar artery before it bifurcates into the posterior cerebral arteries (PCAs). During their course around the midbrain, the SCAs provide branches to the superior lateral pontine tegmentum and the tectum of the mesencephalon. The SCAs then travel toward the cerebellum, supplying the superior vermis, lateral portion of the cerebellar hemispheres and most of the cerebellar nuclei and the cerebellar white matter. When the basilar artery reaches the level of the cerebral peduncles, it divides into opposite PCAs that loop laterally and posteriorly around the midbrain supplying the medial temporal lobe, portions of the parietal lobe and the occipital lobes. Perforator branches are given off to the thalamus. Distal to the posterior communicating artery, medial and lateral posterior choroidal branches off the PCA supply the posterior portion of the lateral geniculate body, optic tract, pulvinar, hippocampus, and parahippocampal gyrus, as well as the choroid plexus of the lateral and third ventricles.

The basilar artery is particularly prone to atherosclerotic deposition throughout its length, and at its extremes can cause either severe stenosis or occlusion, or formation of a fusiform aneurysm by weakening the vessel wall. The rostral end of the basilar, just before bifurcating into the PCAs, is the site most likely to be occluded by an embolus leading to the classic “top of the basilar” syndrome (Chapter 55). Similarly, this is one of the most common sites for berry aneurysms within the vertebrobasilar system.

Cerebral Sinuses and Veins

Surrounded by dura, cerebral sinuses and veins are the venous structures of the brain. They typically contain inpouchings of arachnoid cells, called arachnoid granulations, which allow CSF drainage. These granulations function as one-way valves and are pressure dependent. Malfunction of these valves can occur in subarachnoid hemorrhage or meningitis, leading to normal pressure hydrocephalus. The main venous sinuses include the superior and inferior sagittal sinuses, the straight sinus, the transverse sinuses, the sigmoid sinuses, the occipital sinus, the cavernous sinuses, the superior and inferior petrosal sinuses, and the sphenoparietal sinuses. Acute or subacute cerebral venous thrombosis can be the cause of a wide range of neurologic pathology from isolated chronic headache to venous infarction with seizures to obtundation and coma. Anatomic images and full discussion of this subject are provided in Chapter 56.

The superior sagittal sinus is located within the midline of the cerebral hemispheres surrounded by dura and tethered to the inner table of the skull via the pachymeninges. It runs posteriorly from the foramen cecum to the occipitocerebellar junction. The superior sagittal sinus drains the frontal and parietal lobes through the superior cerebral veins, the largest of which is the rolandic vein in the central sulcus. This sinus often drains into the right transverse sinus.

The inferior sagittal sinus parallels the corpus callosum, traveling in the inferior portion of the falx cerebri, and drains the region of the medial hemispheres and cingulate gyrus. The straight sinus is formed by the intersection of the inferior sagittal sinus and the great vein of Galen. The vein of Galen drains many smaller venous channels, including the choroidal, lateral ventricular, and thalamostriate veins and the basal vein of Rosenthal. These veins drain the choroid plexus, lateral ventricle, basal ganglia, thalamus, and medial temporal lobes. The straight sinus often drains into the left transverse sinus.

The transverse sinuses lie in the grooves of the occipital bone and run laterally and forward for a short distance before diving down to become the sigmoid sinuses. Each transverse sinus receives blood from the superior petrosal sinuses, mastoid and condyloid emissary veins, inferior cerebral and cerebellar veins, and diploic veins. The sigmoid sinuses are the continuation of the transverse sinuses and end at the jugular foramina, becoming the internal jugular veins.

The cavernous sinus is an intricate venous channel interconnecting with its contralateral partner via intercavernous channels around the infundibulum. The cavernous sinus is important for the structures that it drains and for the structures that run through it. Laterally in the cavernous sinus wall are CN-III, -IV, and -V (V1 and V2 segments), and through its center runs the intracavernous portion of the ICA, the sympathetic plexus, and CN-VI. The cavernous sinuses drain into paired superior and inferior petrosal sinuses that, in turn, drain into the transverse sinus and internal jugular veins, respectively.

The superior petrosal sinus connects the cavernous with the transverse sinus. It drains the tympanic cavity, cerebellum, and inferior portions of the cerebrum. The inferior petrosal sinus connects the cavernous sinus with the internal jugular vein and drains the inner ear, medulla, pons, and cerebellum. The sphenopalatine sinuses lie below the lesser wings of the sphenoid bone and drain the dura mater into the cavernous sinuses.