Cerebrovascular system

The cerebrovascular system comprises the vessels that transport blood to and from the brain. The brain's arterial supply is provided by a pair of internal carotid arteries and a pair of vertebral arteries, the latter of which unite to form the basilar artery. The anterior cerebral artery, a branch of the internal carotid artery, perfuses the anteromedial cerebral cortex; the middle cerebral artery, also a branch of the internal carotid artery, perfuses the lateral cerebral cortex; and the posterior cerebral artery, a branch of the basilar artery, perfuses the medial and lateral portions of the posterior cerebral cortex. The internal carotid arteries, the anterior cerebral arteries, and the posterior cerebral arteries anastomose through the anterior and posterior communicating arteries to form the circle of Willis, a vascular circuit surrounding the optic chiasm and pituitary stalk. The circle of Willis provides an alternative channel for blood flow in case of vascular occlusion and equalizes blood flow between the cerebral hemispheres. The cerebral hemispheres are drained by superficial cerebral veins (superior cerebral veins, middle cerebral veins, inferior cerebral veins) and deep cerebral veins (great cerebral vein, basal vein), which empty into the dural venous sinuses. Brain perfusion is regulated by the partial pressure of carbon dioxide (PaCO₂). The interruption of perfusion due to occlusion or hemorrhage of the cerebral vessels results in a stroke, which manifests with focal neurologic deficits in the body parts controlled by the affected brain territory.

The arterial supply of the brain is provided by the internal carotid arteries and the vertebral arteries, which are derivatives of the branches of the aortic arch.

Internal carotid arteries (ICA) ^[1][2]

• A terminal branch of the common carotid artery
• Course
  • Neck: lies within the carotid sheath and enters the cranium through the carotid canal
  • Cranium: lies on the roof of the cavernous sinus, in close proximity to CN VI
• Only has intracranial branches
  • Caroticotympanic artery (the first intracranial branch of the internal carotid artery that enters the middle ear cavity and anastomoses with the inferior tympanic artery)
  • Artery of the pterygoid canal (Vidian artery)
  • Meningohypophyseal trunk (posterior trunk)
  • Inferolateral trunk
  • Ophthalmic artery
  • Superior pituitary artery
  • Posterior communicating artery: anastomosis in Circle of Willis
  • Anterior choroidal artery
  • Terminal branches
    • Middle cerebral artery: supplies lateral cerebrum
    • Anterior cerebral artery: supplies anterior cerebrum

Vertebral arteries

• Arise from the subclavian arteries
• Course
  • Neck: The vertebral arteries pass through the foramina in the transverse processes of the cervical vertebrae of C1–C6 (C7 transverse foramina transmit only veins) and enter the cranium through the foramen magnum.
  • Cranium: The right and left vertebral arteries unite at the midline of the anterior surface of the pons to form the basilar artery.
• Branches
  • Anterior spinal artery and posterior spinal arteries: supply spinal cord
  • Posterior inferior cerebellar arteries (PICA): terminal branches that supply inferior cerebellum

• Definition: a vascular circuit formed by the anastomoses between branches of the internal carotid arteries (anterior circulation) and vertebral arteries (posterior circulation) around the optic chiasm and pituitary stalk ^[3]
• Consists of paired:
  • Internal carotid arteries (proximal to the origin of the middle cerebral arteries)
  • Anterior cerebral arteries (terminal branches of the internal carotids)
  • Posterior cerebral arteries (terminal branches of the vertebral arteries)
• Two anastomoses
  • Anterior communicating artery: connects the two anterior cerebral arteries
  • Posterior communicating artery (branch of internal carotid artery): connects the ICA to the posterior cerebral artery
• Functional significance
  • The circuit provides alternative channels to bypass a potential site of vascular occlusion.
  • Equalizes arterial flow to both cerebral hemispheres

Most saccular cerebral aneurysms, also known as berry aneurysms, occur in the anterior circulation of the brain, usually at the junction of the anterior cerebral artery and the anterior communicating artery in the circle of Willis. They are the most common cause of nontraumatic subarachnoid hemorrhage.

The cerebral hemispheres are drained by superficial and deep cerebral veins, which empty into the dural venous sinuses.

Superficial cerebral veins ^[8][9]

Deep cerebral veins ^[10]

Deep cerebral veins drain the cerebral medulla and drain into the straight sinus.

• Medullary veins: drain the gray matter
• Subependymal veins: receive blood from the medullary veins
• Basal vein (vein of Rosenthal): paired paramedian veins that receive blood from the temporal lobe and drain into the great cerebral vein
• Great cerebral vein (vein of Galen): receives blood from the deep veins

Dural venous sinuses ^[8][9]

• The dural venous sinuses drain blood from cerebral veins and CSF from the arachnoid granulations into the internal jugular vein.
• The sinuses are located intracranially between the two layers of dura mater (endosteal layer and meningeal layer).

Brain veins run in the subarachnoid space, have no valves to allow bidirectional blood flow, and have no muscular layer in the vessel wall!

• Cerebral perfusion is modulated by the partial pressure of carbon dioxide (pCO₂). ^[12]
  • Increased pCO₂ → vasodilation → increased cerebral blood flow
  • Decreased pCO₂ → vasoconstriction → decreased cerebral blood flow
• Therapeutic hyperventilation reduces pCO₂ → decreased cerebral blood flow → lower intracranial pressure (e.g., used when patients with acute cerebral edema are unresponsive to other treatments)
• Partial pressure of oxygen (pO₂) only modulates cerebral perfusion in severe hypoxic conditions
• Cerebral perfusion pressure (CPP) = mean arterial pressure (MAP) - intracranial pressure (ICP)
  • Decreased blood pressure or increased ICP → decreased cerebral perfusion
  • CPP of 0 indicates no brain perfusion (brain death).
  • Hypoxemia increases CPP if pO₂ < 50 mm Hg.
  • CPP linearly increases with pCO₂ until pCO₂ > 90 mm Hg.

• Arterial conditions
  • Stroke
  • Cerebral aneurysms
  • Subarachnoid hemorrhage
  • Epidural hematoma
  • Intraparenchymal hemorrhage
  • Trigeminal neuralgia (compression of superior cerebellar artery)
  • Hydrostatic cerebral edema (e.g., in severe hypertension)
• Venous conditions
  • Cerebral venous thrombosis
  • Cavernous sinus syndrome
  • Subdural hematoma (caused by ruptured bridging veins)