SCRN Exam

The chapters move from the heart outward: the aortic arch and the great vessels, the circle of Willis and the territory each artery feeds, the venous system, the structure and function of the brain itself, the fluid spaces and coverings that protect it, the physiology of cerebral blood flow and autoregulation, and finally the mechanisms of neuroplasticity that make recovery possible. Read with a map in mind: for every artery, ask what it feeds and what a patient looks like when it stops flowing.

This chapter traces blood from the left ventricle to the brain. You will learn the branches of the aortic arch, the course of the carotid and vertebral arteries, the division of the brain into anterior and posterior circulations, and why these distinctions matter for embolism, dissection and interventional access.

The brain receives about 15-20% of cardiac output even though it is only about 2% of body weight. All of that blood leaves the heart through the aortic arch. In the typical arrangement the arch gives off three great vessels from right to left: the brachiocephalic (innominate) trunk, the left common carotid artery and the left subclavian artery. The brachiocephalic trunk divides into the right subclavian artery and the right common carotid artery. Each subclavian artery gives off a vertebral artery.

Arch variants (such as a bovine arch, in which the left common carotid arises from the brachiocephalic trunk) and a tortuous arch in an older hypertensive patient can make femoral catheter access difficult and may lead the team to choose radial access. Complex arch atheroma is an embolic source, and aortic arch dissection is an absolute contraindication to thrombolysis.

Each common carotid artery ascends in the neck and bifurcates at about the level of the upper border of the thyroid cartilage (roughly C3-C4) into the external carotid artery and the internal carotid artery (ICA). The bifurcation and the proximal ICA bulb are the most common sites of atherosclerotic plaque in the extracranial circulation, because turbulent flow at the branch point injures the endothelium. This is the segment imaged by carotid duplex ultrasound and treated with carotid endarterectomy or stenting.

The external carotid supplies the face, scalp and dura. It does not normally feed the brain, but when the ICA is occluded it can send collateral blood through orbital branches backward into the ophthalmic artery and intracranial ICA.

The ICA has no branches in the neck. It enters the skull through the carotid canal, curves through the cavernous sinus (the carotid siphon), and gives off its first intracranial branch, the ophthalmic artery, which supplies the retina. Transient monocular vision loss like a shade coming down (amaurosis fugax) is a carotid-territory TIA from emboli to the retinal circulation. The ICA then gives off the PCom and anterior choroidal artery and terminates in the anterior cerebral artery (ACA) and middle cerebral artery (MCA). A clot at this "carotid T" blocks both ACA and MCA flow and typically produces a devastating stroke.

Each vertebral artery arises from the subclavian artery, ascends through the transverse foramina of the cervical vertebrae, loops behind the atlas and enters the skull through the foramen magnum. The two are often unequal in size, and the loop at C1-C2 is a classic site of dissection after neck manipulation or sudden rotation.

Inside the skull, each vertebral artery gives off the posterior inferior cerebellar artery (PICA), which supplies the lateral medulla and the inferior cerebellum, and contributes to the anterior spinal artery. At the pontomedullary junction the two vertebrals join to form the single basilar artery, which runs up the front of the pons. The basilar gives off the anterior inferior cerebellar artery (AICA), which also supplies the inner ear through the labyrinthine artery in most people, multiple small paramedian and circumferential pontine perforators, and the superior cerebellar artery (SCA). At the top of the midbrain the basilar divides into the two posterior cerebral arteries (PCAs), which supply the occipital lobes, the inferior temporal lobes and much of the thalamus.

The anterior circulation (both ICAs with their ACA and MCA branches) feeds most of the frontal and parietal lobes, the lateral temporal lobes, the basal ganglia and most of the internal capsule. The posterior circulation (vertebral, basilar and posterior cerebral arteries) feeds the brainstem, cerebellum, occipital lobes, medial temporal lobes and most of the thalamus.

• The aortic arch typically gives off the brachiocephalic trunk, left common carotid and left subclavian; vertebrals arise from the subclavians.
• The ICA has no neck branches; its first intracranial branch is the ophthalmic artery, so amaurosis fugax signals ipsilateral carotid disease.
• The ICA terminates in the ACA and MCA; a carotid T occlusion blocks both.
• The vertebrals give off PICA, join to form the basilar, which gives off AICA and SCA and ends as the two PCAs.
• Posterior circulation strokes cause vertigo, ataxia, diplopia and crossed findings and are underestimated by the NIHSS.