Circle of willis

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Here is a comprehensive overview of the Circle of Willis drawn from multiple authoritative medical textbooks.

Circle of Willis

Overview

The circle of Willis (circulus arteriosus cerebri) is an anastomotic ring of arteries at the base of the brain that connects the anterior circulation (internal carotid arteries) and the posterior circulation (vertebrobasilar system). All major cerebral vessels arise from it, and it serves as the primary collateral pathway for cerebral blood flow. - Neuroanatomy through Clinical Cases, 3rd Ed.

Anatomy and Components

The circle is formed by 7 major vessels arranged in a heptagonal shape on the ventral surface of the brain:

Anterior part (from the internal carotid arteries):

Bilateral Internal Carotid Arteries (ICA) - each splits into ACA and MCA
Bilateral Anterior Cerebral Arteries (ACA) - A1 segments
Single Anterior Communicating Artery (AComm) - connects the two ACAs

Posterior part (from the vertebrobasilar system):

Bilateral Vertebral Arteries - arise from subclavian arteries, join to form the basilar artery
Single Basilar Artery - formed at the pontomedullary junction
Bilateral Posterior Cerebral Arteries (PCA) - terminal branches of the basilar artery

Connecting links:

Bilateral Posterior Communicating Arteries (PComm) - join the ICAs to the PCAs, linking anterior and posterior circulations

The main branches of the supraclinoid ICA can be remembered by the mnemonic OPAAM: Ophthalmic, Posterior communicating, Anterior choroidal, Anterior cerebral, and Middle cerebral arteries. - Neuroanatomy through Clinical Cases, 3rd Ed.

Circle of Willis and its main branches - inferior view of the brain

Circle of Willis schematic showing origin from aorta to cerebral vessels

Completeness and Anatomical Variants

A full-caliber, complete circle of Willis is present in only approximately 34% of individuals - Neuroanatomy through Clinical Cases, 3rd Ed. Barash's Clinical Anesthesia further notes that autopsy studies show the majority of specimens have anatomic anomalies: hypoplasia is the most frequently noted anomaly (24%), and complete absence of a vessel causing an incomplete circle occurs in an additional 6%.

Common variants include:

Hypoplastic or absent communicating arteries
Bilateral ACA supply from one carotid (one AComm supplying both)
Fetal origin of the PCA (PCA arising from the ICA instead of the basilar artery)
One dominant vertebral artery with contralateral hypoplasia

These variants are clinically significant - they can make occlusion of a particular vessel far more devastating than it would be in a patient with complete collateral circulation. - Schwartz's Principles of Surgery, 11th Ed.

Blood Supply Territories

Artery	Region Supplied
ACA	Medial frontal and parietal cortex (including medial sensorimotor cortex for the lower limb)
MCA	Lateral convexity: frontal, parietal, temporal lobes; internal capsule, basal ganglia (lenticulostriate branches)
PCA	Occipital lobe, inferior temporal lobe, thalamus, midbrain
Lenticulostriate aa. (from MCA)	Basal ganglia, internal capsule (deep penetrating)
Anterior choroidal a. (from ICA)	Posterior limb of internal capsule, optic tract, hippocampus

Deep structures (basal ganglia, thalamus, internal capsule) are supplied by small penetrating branches from the proximal segments of the main cerebral arteries near the circle of Willis. - Neuroanatomy through Clinical Cases, 3rd Ed.

Role in Collateral Circulation

The circle of Willis is the primary of three main collateral pathways for brain perfusion:

The circle of Willis itself (connecting carotid and vertebrobasilar systems)
Extracranial-intracranial anastomoses (e.g., ECA branches anastomosing with intracranial vessels)
Leptomeningeal anastomoses (between terminal branches of major cerebral arteries)

Localization in Clinical Neurology, 8th Ed.

Patients with complete proximal carotid occlusion may be entirely asymptomatic if a complete circle of Willis allows contralateral and posterior collateral flow to compensate. Conversely, an incomplete circle can make carotid occlusion catastrophic. - Schwartz's Principles of Surgery, 11th Ed.

Clinical Significance

Berry (Saccular) Aneurysms

The circle of Willis is the most common site for intracranial saccular aneurysms, forming at vessel bifurcations due to hemodynamic stress and medial defects. The frequency distribution of aneurysms is shown below:

Common sites of saccular (berry) aneurysms in the circle of Willis, with percentage distribution

Anterior communicating artery (AComm): ~40% - most common overall
Middle cerebral artery bifurcation: ~34%
Internal carotid / PComm junction: ~20%
Basilar tip / PCA: ~4%

Rupture causes subarachnoid hemorrhage (SAH). Blood in the basal subarachnoid space can trigger vasospasm of the major vessels of the circle of Willis, driven by endothelins, nitric oxide, and arachidonic acid metabolites, causing secondary ischemic injury days after the initial bleed. - Robbins, Cotran & Kumar Pathologic Basis of Disease

Stroke

Occlusion proximal to the circle of Willis may be tolerated due to collateral rerouting
Occlusion distal to the circle of Willis typically causes infarction in that artery's territory
85% of acute cerebrovascular events are ischemic strokes; collateral status is a key determinant of clinical outcome - Schwartz's Principles of Surgery, 11th Ed.

Carotid Endarterectomy (CEA)

During carotid cross-clamping, the ipsilateral hemisphere depends entirely on collateral flow through the circle of Willis. An incomplete circle (present in the majority of patients) may necessitate intraoperative shunting. - Barash's Clinical Anesthesia, 9th Ed.

CN III Compression

A posterior communicating artery aneurysm classically compresses CN III as it runs adjacent to it, causing a painful third nerve palsy with a dilated, "blown" pupil (pupil-involving CN III palsy) - a neurosurgical emergency.

Key Summary Table

Component	Origin	Connects
AComm	ACA-ACA junction	Left and right anterior circulation
PComm (bilateral)	ICA to PCA	Anterior to posterior circulation
Basilar artery	Convergence of 2 vertebrals	Posterior circulation to PCAs
ACA (bilateral)	ICA terminal branch	Medial hemisphere supply
MCA (bilateral)	ICA terminal branch	Lateral hemisphere supply
PCA (bilateral)	Basilar terminal branch	Posterior hemisphere/brainstem supply

Sources: Neuroanatomy through Clinical Cases 3rd Ed. | Schwartz's Principles of Surgery 11th Ed. | Barash's Clinical Anesthesia 9th Ed. | Robbins, Cotran & Kumar Pathologic Basis of Disease | Localization in Clinical Neurology 8th Ed. | Medical Physiology (Boron & Boulpaep)

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