Circle of Willis: Anatomy, Aneurysms and Stroke Territories

The circle of Willis is an anastomotic ring at the base of the brain that connects the anterior (internal carotid) and posterior (vertebrobasilar) circulations. It earns its place in exams twice over — once as pure anatomy, and once as the place where subarachnoid haemorrhage begins.

At a glance

How the ring is built

Trace it as one continuous loop. Each internal carotid artery, having entered the skull, divides into an anterior cerebral artery and a middle cerebral artery. The two anterior cerebral arteries run forward and are joined across the midline by the short anterior communicating artery — that closes the front of the ring.

Behind, the two vertebral arteries fuse to form the basilar artery, which divides at the upper border of the pons into the two posterior cerebral arteries. Each posterior cerebral artery is joined to the internal carotid on the same side by a posterior communicating artery — that closes each side.

The middle cerebral artery is not part of the circle. It is the largest branch of the internal carotid and simply continues laterally; the circle passes it by. This is a favourite trick question.

Why the circle so often fails

A textbook-complete circle is present in a minority of people. Hypoplastic or absent communicating arteries, and a posterior cerebral artery arising from the internal carotid rather than the basilar (a “fetal” posterior cerebral artery), are all common. This matters clinically: the circle only provides collateral flow if the connecting segments are large enough to carry it, which is why one patient tolerates carotid occlusion and another infarcts.

Berry aneurysms

Saccular aneurysms form at branch points where the media is congenitally deficient and haemodynamic stress is highest. In rough order of frequency they occur at the anterior communicating artery, the posterior communicating artery’s origin from the internal carotid, and the middle cerebral artery bifurcation.

Rupture causes subarachnoid haemorrhage: sudden worst-ever thunderclap headache, neck stiffness, photophobia, vomiting, often with a brief loss of consciousness. Non-contrast CT is the first investigation; lumbar puncture for xanthochromia is used when CT is negative and suspicion remains high. Associations include polycystic kidney disease, Ehlers-Danlos and coarctation of the aorta.

Two localising signs are worth memorising. A posterior communicating artery aneurysm compresses the oculomotor nerve, producing a third nerve palsy with a dilated, unreactive pupil — the parasympathetic fibres run superficially on the nerve, so compression takes them out first, while ischaemic (diabetic) third nerve palsies typically spare the pupil. A pupil-involving third nerve palsy is a neurosurgical emergency. An anterior communicating artery aneurysm may compress the optic chiasm and produce a visual field defect.

Stroke territories

The leg-versus-arm pattern follows directly from the homunculus: the leg area sits on the medial surface in the anterior cerebral territory, the face and hand occupy the large lateral convexity supplied by the middle cerebral artery.

Last-minute checklist

• Two internal carotids, two vertebrals, three communicating arteries.
• The middle cerebral artery is not part of the circle.
• Anterior communicating artery is the commonest aneurysm site.
• Third nerve palsy with a blown pupil equals posterior communicating aneurysm until excluded.
• Anterior cerebral takes the leg; middle cerebral takes the face and arm; posterior cerebral takes the visual field.

Frequently asked questions

Is the middle cerebral artery part of the circle of Willis?

No. It is a terminal branch of the internal carotid artery that runs laterally and contributes nothing to the anastomotic ring, even though it arises immediately adjacent to it.

Why does a posterior communicating artery aneurysm dilate the pupil?

Parasympathetic fibres travel on the outer surface of the oculomotor nerve, so external compression damages them before the deeper motor fibres. Ischaemia, by contrast, affects the core first and usually spares the pupil.

Why is the macula spared in posterior cerebral artery stroke?

The occipital pole, which represents the macula, receives collateral supply from branches of the middle cerebral artery, so it often survives when the rest of the posterior cerebral territory infarcts.

References

• Gray’s Anatomy for Students — blood supply of the brain.
• Snell’s Clinical Neuroanatomy.
• Harrison’s Principles of Internal Medicine — cerebrovascular disease.