The vascular supply to the brain is derived from two sources: the anterior and posterior circulations, which supply blood to different regions of the brain. Hypoperfusion of specific areas of the brain leads to typical neurologic findings that can help clinically differentiate the location of the lesion and the vessels that may be involved.

The anterior circulation, which supplies blood to four-fifths of the brain, originates from the carotid system. The common carotid arteries divide into the right and left internal and external carotid arteries at the level of the angle of the mandible. The internal carotid arteries then course intracranially along the sella turcica within the cavernous sinus. The first branch off the internal carotid artery is the ophthalmic artery, which supplies the optic nerve and retina. Sudden onset of painless monocular blindness (amaurosis fugax) identifies the stroke as involving the anterior circulation (specifically, the carotid artery) at or below the level of the ophthalmic artery. The internal carotid arteries terminate by branching into the anterior and middle cerebral arteries at the circle of Willis. The anterior circulation supplies blood to the optic nerve, retina, and frontoparietal and anterotemporal lobes of the brain.

Although the posterior circulation is smaller (supplying blood to only one-fifth of the brain), it supplies the brainstem, which is critical for normal consciousness, movement, and sensation. The posterior circulation is derived from the two vertebral arteries that ascend through the transverse processes of the cervical vertebrae. The vertebral arteries enter the cranium through the foramen magnum, supplying the cerebellum via the posteroinferior cerebellar arteries. They join to form the basilar artery, which branches to form the posterior cerebral arteries. The posterior circulation supplies the brainstem, cerebellum, thalamus, auditory and vestibular functions of the ear, medial temporal lobe, and the visual occipital cortex.

The extent of stroke also depends on the presence of collateral blood flow distal to the vessel occlusion. A patient with excellent collateral blood flow from the contralateral hemisphere may have minimal clinical deficits despite a complete carotid occlusion. In contrast, a patient with poor collateral flow may be hemiplegic with the same lesion.

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