Nervous System Anatomy · Neurology · High Yield

Circle of Willis

The Circle of Willis is the arterial anastomosis at the base of the brain that allows collateral circulation between the anterior (carotid) and posterior (vertebrobasilar) circulations. Its completeness — and its vulnerabilities — determine the consequences of arterial occlusion.

✦ The Mnemonic

"Anterior Communicating Joins Anterior Cerebrals; Internal Carotids Post-Communicating Posterior"

Anterior communicating · ACAs · ICAs · PComms · PCAs — the six named components

AComm Anterior Communicating Artery Connects right and left ACAs; most common site of berry aneurysm (30–40%)

ACA Anterior Cerebral Arteries (×2) Supply medial frontal and parietal lobes; leg area of motor cortex

ICA Internal Carotid Arteries (×2) Give ACA + MCA; enter through carotid canal and cavernous sinus

MCA Middle Cerebral Arteries (×2) Largest branch of ICA; not part of circle but dominant territory — arm + face motor cortex

PComm Posterior Communicating Arteries (×2) Connect ICA to PCA; second most common aneurysm site; CN III palsy

PCA Posterior Cerebral Arteries (×2) From basilar artery; supply occipital lobe + thalamus; visual cortex

Basilar Basilar Artery From union of vertebral arteries; supplies brainstem + cerebellum; ends at PCAs

📚 Clinical Breakdown

The circle theoretically allows complete cerebral perfusion from a single vessel — in practice, the communicating arteries are often hypoplastic and cannot provide adequate collateral flow. Only ~20% of people have a 'complete' Circle of Willis with all segments fully patent.

Berry (saccular) aneurysms preferentially form at branching points due to haemodynamic stress: AComm (30–40%), PComm (25%), MCA bifurcation (20%). The classic presentation of rupture: 'thunderclap headache' — sudden worst-ever headache, maximal at onset, ± meningism. CT detects 98% of SAH in the first 6 hours; LP for xanthochromia if CT negative.

MCA stroke: most common stroke territory (largest cerebral artery). Contralateral face and arm weakness/sensory loss (the leg area is on the medial surface — ACA territory). Dysphasia if dominant hemisphere. Contralateral homonymous hemianopia (optic radiation). The MCA is the most common artery occluded in cardioembolic stroke.

Most common aneurysm site AComm (30–40%)

PComm aneurysm sign CN III palsy — blown pupil

MCA stroke Contralateral face + arm; dysphasia (dominant)

ACA stroke Contralateral leg weakness

⭐ Clinical Pearl

Posterior circulation stroke: PICA (posterior inferior cerebellar artery — from vertebral) occlusion causes lateral medullary (Wallenberg) syndrome: ipsilateral face pain/temperature loss, contralateral body pain/temperature loss, ipsilateral Horner's, dysphagia, ataxia. The pain/temperature dissociation across face and body is pathognomonic.

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