Anatomic Aspects of Cerebral Circulation

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54 Anatomic Aspects of Cerebral Circulation

Claudia J. Chaves

The brain and meninges are supplied by arteries derived from the common carotid artery (CCA) and vertebrobasilar system (Fig. 54-1). The right CCA usually originates from the brachiocephalic trunk, whereas the left CCA originates directly from the aortic arch. Both vertebral arteries (VAs) originate from the subclavian arteries. The morphologic variants of the CCA and VAs usually are not clinically significant.

Figure 54-1 Arteries to Brain and Meninges.

The CCA bifurcates at approximately the level of the sixth cervical vertebrae into the external and internal carotid arteries. The external carotid artery (ECA) supplies the neck, face, and scalp. The internal carotid artery (ICA) and its branches are mostly responsible for the arterial supply of the anterior two thirds of the cerebral hemispheres (anterior circulation).

The vertebrobasilar and posterior cerebral arteries (PCAs) supply blood to the brainstem, cerebellum, occipital lobes, and posterior portions of the temporal and parietal lobes (posterior circulation).

The Carotid Artery System

External Carotid Artery

At its origin, the ECA deviates anteriorly and medially in relation to the ICA in the neck and provides many branches to the neck (superior thyroid, ascending pharyngeal arteries) and face (lingual and facial arteries). As the artery ascends, occipital and posterior auricular branches supply the scalp in their named areas. The occipital artery, however, also has several meningeal branches that supply the posterior fossa and dura. Within the substance of the parotid gland, the ECA divides into its two terminal branches, the superficial temporal and maxillary arteries. The superficial temporal artery is the main supply to the scalp over the frontoparietal convexity and its underlying muscles. The more proximal branches also supply the masseter muscle. The superficial temporal artery is commonly involved in giant cell arteritis, an important consideration in the elderly with headaches, and can be palpated anterior to the tragus and in the temporal area (Chapter 11).

The maxillary artery supplies the face and, through its middle meningeal branch, provides most of the blood supply to the dura mater covering the brain. The middle meningeal artery is often implicated in the formation of epidural hematomas in patients with temporal or parietal bone skull fractures (Chapter 59).

The ECA occasionally has an important role in supplying collateral flow for ICA occlusive disease through anastomoses between its facial, maxillary, and superficial temporal branches and the ophthalmic artery.

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.

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