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Structure

Circle of Willis

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The circle of Willis is an arterial anastomotic ring at the base of the brain that joins the carotid and vertebrobasilar systems. It allows collateral flow if one inflow vessel is narrowed.

Terminologia Anatomica (Latin): Circulus arteriosus cerebri · Open in the 3D atlas

Location

In the subarachnoid space of the interpeduncular and suprasellar cisterns, around the optic chiasm, infundibulum and mammillary bodies.

Structure

Anteriorly the two anterior cerebral arteries (A1 segments) are joined by the anterior communicating artery; laterally the posterior communicating arteries link the internal carotid arteries with the posterior cerebral arteries (P1 segments), which arise from the basilar bifurcation. The middle cerebral arteries leave the ring but are not part of it. The classic complete ring is present in only a minority of people; hypoplastic A1 or posterior communicating arteries and fetal-type posterior cerebral origin from the carotid are common variants.

Blood supply

Its inflow comes from the two internal carotid arteries (from the common carotids) and the basilar artery formed by the two vertebral arteries (from the subclavians). Intracranial arterial walls have few vasa vasorum and rely largely on luminal and cerebrospinal fluid diffusion.

Innervation

Perivascular plexuses carry sympathetic and parasympathetic autonomic fibres and sensory fibres, largely from the trigeminal nerve, that are implicated in headache.

Function

Distributes blood to both cerebral hemispheres and the brainstem and provides a potential collateral pathway between the anterior and posterior circulations and between the two hemispheres. Its effectiveness depends on the calibre of the communicating vessels.

Clinical correlations

Saccular (berry) aneurysms arise at branch points of the circle, most often at the anterior communicating artery complex and the posterior communicating artery origin, and rupture causes subarachnoid haemorrhage. A posterior communicating aneurysm may compress the oculomotor nerve (painful third-nerve palsy). Collateral flow through the circle can limit ischaemia in carotid stenosis when the ring is complete.

Common exam points

  • Components: anterior communicating (1), anterior cerebral A1 (2), internal carotid termination (2), posterior communicating (2), posterior cerebral P1 (2).
  • Anastomosis between the carotid (anterior) and vertebrobasilar (posterior) circulations; the middle cerebral artery is not part of the circle.
  • A complete classic circle is found in only a minority of individuals; fetal posterior cerebral origin from the ICA is a common variant.
  • Commonest berry aneurysm sites: anterior communicating artery complex and posterior communicating artery origin; a posterior communicating aneurysm can compress CN III.
  • Ischaemic stroke territory depends on the occluded branch, not on the circle itself.

References

  1. Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students. Elsevier — Tier 3 (textbook)
  2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. Wolters Kluwer, 8th edition (2018) — Tier 3 (textbook)
  3. Netter FH. Atlas of Human Anatomy. Elsevier — Tier 3 (textbook)
  4. Haines DE, Mihailoff GA. Fundamental Neuroscience for Basic and Clinical Applications. Elsevier, 5th edition (2018) — Tier 3 (textbook)

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