Anatomy Atlas

Circle of Willis

Circulus arteriosus cerebri
Also: Cerebral arterial circle · Anterior communicating artery · Posterior communicating artery · Berry aneurysm · Saccular aneurysm
ArteryHigh-yielddraftNeuroCardio

Arterial ring at the base of the brain joining the carotid (anterior) and vertebrobasilar (posterior) circulations. It provides collateral flow, and its branch points are where berry aneurysms form.

Core anatomy

Origin

Formed by the terminal internal carotid arteries (common and internal carotid arteries) and the basilar artery (vertebrobasilar system).

Course

Lies in the interpeduncular cistern (subarachnoid space) around the optic chiasm, infundibulum and mammillary bodies. Components: the two anterior cerebral arteries (A1 segments) joined by the anterior communicating artery; the two internal carotid arteries; the two posterior communicating arteries connecting each ICA to the posterior cerebral artery (P1 segments) from the basilar bifurcation.

Branches
  • Anterior cerebral arteries and middle cerebral arteries (see cerebral arteries (ACA, MCA, PCA))
  • Anterior communicating artery (AComm)
  • Posterior communicating arteries (PComm) — beside oculomotor nerve (CN III)
  • Posterior cerebral arteries
  • Small central (perforating) branches to the hypothalamus, thalamus, internal capsule and basal ganglia; anterior choroidal artery (from the ICA)
Supplies

The whole brain via its branches; the perforators supply deep structures (hypothalamus, thalamus, internal capsule).

Anastomoses

Allows flow from one side or from the posterior circulation to compensate for a slowly narrowing carotid; a complete, symmetric circle is present in only a minority of people, so collateral capacity varies. verify

Relations

The AComm lies above the optic chiasm; the PComm runs alongside CN III.

Branches

Structures supplied

Found in

Mentioned in

Generated automatically from links in other entries.

Circle of Willis and vertebrobasilar system (inferior view)Seen from below with the front of the brain at the top. The two vertebral arteries give the posterior inferior cerebellar arteries and join to form the basilar artery, which gives the anterior inferior cerebellar, pontine and superior cerebellar arteries and divides into the two posterior cerebral arteries. Posterior communicating arteries join each posterior cerebral artery to the internal carotid artery. Each internal carotid gives the middle cerebral artery laterally and the anterior cerebral artery medially; the anterior cerebral arteries are joined by the anterior communicating artery. The oculomotor nerve passes between the posterior cerebral and superior cerebellar arteries. Common aneurysm sites: anterior communicating artery, posterior communicating–internal carotid junction, and middle cerebral artery bifurcation.Anterior cerebral a.Anterior communicating a.Internal carotid a. (cut)Posterior communicating a.Middle cerebral a.Posterior cerebral a.Superior cerebellar a.Pontine branchesAICABasilar a.PICAVertebral a.Lenticulostriate aa.CN IIIInfundibulumBerry aneurysm sites: AComm, PComm–ICA, MCA bifurcationInferior view · anterior ↑

Swipe sideways to see the whole diagram →

Circle of Willis and vertebrobasilar system. Inferior view, anterior at the top. The internal carotids (anterior circulation) and basilar artery (posterior circulation) are joined by the communicating arteries; CN III passes between the PCA and SCA. Red dots mark common berry aneurysm sites. A complete, symmetric circle is present in only a minority of people.
Original schematic, created for this atlas.
Diagram of the circle of Willis: internal carotid, anterior cerebral, anterior communicating, middle cerebral, posterior communicating and posterior cerebral arteries, with the basilar and vertebral arteries and their branches
Diagram of the arterial circle at the base of the brain.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 519. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))
Inferior surface of the brain with the arterial circle, the basilar and vertebral arteries and the middle cerebral artery in the lateral fissure
The arteries of the base of the brain.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 516. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))

Clinical correlations

Saccular (berry) aneurysms PANCE

Mechanism / lesion

Thin-walled outpouchings at arterial branch points — most at the AComm junction, then the PComm–ICA junction and the MCA bifurcation. Risk factors — hypertension, smoking, ADPKD, Ehlers–Danlos, coarctation of the aorta, family history.

Classic presentation

Rupture → subarachnoid hemorrhage (thunderclap headache; see cranial meninges and intracranial hemorrhage). Unruptured aneurysms can compress nearby structures: PComm aneurysm → pupil-involving CN III palsy; AComm aneurysm → visual field defects (chiasm). Treated by endovascular coiling or surgical clipping.

Collateral failure and watershed infarction

Mechanism / lesion

Incomplete circle or severe hypotension.

Classic presentation

Carotid occlusion may be silent with a good circle or cause a large MCA infarct without one; profound hypotension causes watershed (border-zone) infarcts between territories.

High-yield PANCE pearls

  • Most common berry aneurysm site → anterior communicating artery.
  • PComm aneurysm → CN III palsy with a dilated pupil.
  • Berry aneurysms + kidney cysts → ADPKD.
  • Circle of Willis lies in the subarachnoid space → aneurysm rupture = SAH.

Imaging anatomy

CT

CT angiography — aneurysms and vessel occlusion; noncontrast CT for SAH first.

MRI

MRA (time-of-flight, no contrast needed) for screening, e.g. in ADPKD with a family history of aneurysm.

Key views & landmarks
  • Digital subtraction (catheter) angiography — gold standard and route for coiling

Physical exam

Strength / function testing

Pupils and eye movements (CN III compression), visual fields (chiasm); in suspected SAH — level of consciousness and neck stiffness.

Self-quiz

Q1.

A ruptured berry aneurysm most commonly arises from which artery?

Show answer

B

About a third of saccular aneurysms arise at the anterior communicating artery, followed by the posterior communicating–ICA junction and the MCA bifurcation.

Q2.

A 38-year-old with autosomal dominant polycystic kidney disease has a mother who died of a brain hemorrhage. What screening test is most appropriate?

Show answer

B

ADPKD carries an increased risk of intracranial berry aneurysms; with a family history of aneurysm or SAH, screening with MRA (or CTA) is recommended.

Q3.

An unruptured posterior communicating artery aneurysm typically presents with which finding?

Show answer

E

The PComm runs alongside CN III, so an aneurysm compresses its superficial pupillomotor fibers, causing a pupil-involving CN III palsy that needs emergency vascular imaging.

Q4.

An anterior communicating artery aneurysm can compress which structure and cause visual field defects?

Show answer

C

The AComm lies above the optic chiasm, so an aneurysm there can cause visual field defects. AComm is the most common site of berry aneurysms.

Q5.

The circle of Willis lies in which space, so that aneurysm rupture causes which hemorrhage?

Show answer

A

The circle lies in the subarachnoid space (interpeduncular cistern), so a ruptured berry aneurysm causes subarachnoid hemorrhage with a thunderclap headache.

Flashcards

See also