Cerebral Arterial Vasculature & Circle of Willis

Arterial Vasculature

• The arterial cerebrovascular system's main branches include vertebral and basilar arteries (supplying cerebellar, cerebral, spinal and pontine regions), internal carotid arteries (cerebral arteries), and the Circle of Willis (anastomosis of vessels).

Circle of Willis

• An anastomosis, which connects the vertebrobasilar and internal carotid systems to distribute blood
• Contains the vertebral and basilar artery: posterior cerebral, superior cerebellar, anterior inferior cerebellar, posterior inferior cerebellar, and anterior/posterior spinal arteries.
• Part of the internal carotid artery: posterior communicating, middle cerebral, anterior communicating, anterior cerebral, and ophthalmic arteries supply the brain.

Internal Carotid System & Vertebro-Basilar System

• Anterior/inferior view includes the internal carotid and vertebrobasilar systems.
• The internal carotid system consists of the internal carotid, ophthalmic, posterior communicating, anterior communicating, anterior cerebral (A1, A2), middle cerebral, M1 proximal branch, M2 segments, lenticulostriate and anterior choroidal arteries.
• The vertebro-basilar system is composed of vertebral, basilar, spinal, posterior inferior cerebellar, anterior inferior cerebellar, superior cerebellar, and posterior cerebral (P1, P2) arteries.

Vascular Supply

• The anterior choroidal artery supplies basal ganglia, hippocampus & posterior limb of internal capsule
• The M1 proximal branch of MCA, the lenticulostriate arteries supply basal ganglia & posterior limb of internal capsule
• The proximal ACA's penetrating branches, specifically the recurrent artery of Heubner, supply basal ganglia, limbic structures, and the anterior limb of internal capsule.
• Structures supplied by basilar artery/PCA include the thalamus and posterior limb of internal capsule, via thalamoperforator, thalamogeniculate, and posterior choroidal arteries.

Arterial Supply-Cortical

• ACA supplies the calloso-marginal & peri-callosal artery
• PCA supplies the medial temporal and occipital lobes.
• External cortical supply: ACA supplies antero-medial frontal lobe
• MCA supplies the lateral frontal, parietal, and temporal lobes

Examples of Cerebrovascular Accident (CVA)

• Aneurysm
• Atherosclerosis
• Dissection
• Arterio-Venous Malformation
• Infarction and Ischemia

Atherosclerosis

• Occurs at bifurcations of major vessels like the Circle of Willis due to plaque deposition on vessel walls.
• Within the circle, anastomoses offer collateral supply.
• Atherosclerosis leads to occlusive stroke/infarction and hemorrhage

###Aneurysms

• Ballooning occurs in Circle of Willis
• Micro-aneurysms occur intracerebrally
• Aneurysms lead to hemorrhage and infarction

Dissections

• Bleeding within a vessel
• Arteriovenous malformation, intracerebral.

Watershed Infarction

• Hypo-perfusion of border zones that can occur with occlusion of an end vessel, leading to focal ischemia.
• Hypo-perfusion results in border zone infarct
• Proximal limb weakness, aphasia can result.

Intra-Cerebral Hemorrhage

• Lacunar infarction, Charcot-Bouchard micro-aneurysm are examples
• Penetrating arteries to the thalamus, basal ganglia, and internal capsule, and no collateral perfusion.
• May result in pallidal or thalamic hemorrhage, small lacunes and present as pure hemi-paresis or pure hemi-sensory loss or progressive dementia

Case study

• Patient with history of diabetes and hypertension, presents with slurred speech, right-sided weakness, right facial weakness (spares forehead), hemiparesis
• Explanation: Occlusion of lenticulo-striate branches off left M1 segment into internal capsule

Ischemia vs. Hypoxia vs. Infarction

• Ischemia is lack of blood flow to a brain region
• Hypoxia is lack of oxygenated blood in a brain region
• Infarction is death of brain parenchyma due to circulatory failure
• Occlusive stroke caused by the thrombolytic or embolytic closure of a vessel
• Hemorrhagic stroke ruptures blood vessels

Types of Cerebral Ischemia

• Global cerebral ischemia: diffuse hypoxic or ischemic encephalopathy as a result of cardiac arrest/shock/hypo-perfusion
• Focal cerebral ischemia: occlusion from embolism/thrombus/vasculitis

Venous System

• Venous system eventually drains into the internal jugular vein.
• Superficial veins encompass the superior sagittal and cavernous sinuses.
• Deep veins include the Great Vein of Galen.

Additional Venous Participants

• Other veins include inferior sagittal sinus, superior anastomotic vein of Trolard/inferior anastomotic vein and torcula.
• "Empty delta sign" indicates superior sagittal thrombosis

Blood Brain Barrier (BBB)

• It is composed of astrocytes (inducing tight junctions), pericytes, and a metabolic barrier (MAO)
• It allows exchange from blood to the cerebral spinal fluid through lipid-soluble diffusion, receptor-mediated transport, and ion channel diffusion.

Functions of CSF

• Shock absorber
• Removal of metabolites
• pH buffer for blood

Blood CSF Barrier

• Accomplished through capillary filtration and active epithelial transport

Cerebrospinal Fluid (CSF) Characteristics

• Low protein and low ionic content
• Increased protein levels mean meningitis
• Red blood cells mean infarction or infection

Getting Dopamine Into the Brain

• Occurs in Parkinson's treatment
• Oral L-DOPA must be permeant
• Carbidopa must be combined with it to decrease L-DOPA metabolism
• Eventually L-DOPA is converted to Dopamine