circle of Willis

**Components (anterior to posterior):** - Anterior communicating artery (AComm): connects the two ACAs. - Anterior cerebral arteries (ACAs): branch from the internal carotid arteries (ICAs). - Internal carotid arteries (supraclinoid portion). - Posterior communicating arteries (PComm): connect the ICA to the PCA. - Posterior cerebral arteries (PCAs): terminal branches of the basilar artery. - Basilar artery: formed by the union of the two vertebral arteries, and it gives off the PCAs. It is often included as part of the circle's posterior supply. - The middle cerebral arteries (MCAs) arise from the ICAs but are NOT part of the circle.

**Function:** The circle allows collateral circulation between the right and left hemispheres and between the anterior and posterior circulations. In theory, it keeps perfusion if a proximal vessel (e.g., ICA) is stenosed or occluded. Only about 20–25% of people have a complete, "textbook" circle. Hypoplastic or absent segments (e.g., a hypoplastic PComm or A1 segment) are common, so collateral flow is often unreliable.

**Clinical correlations (high-yield):** - **Berry (saccular) aneurysms:** These form at arterial bifurcations, where there is a congenital defect in the media and loss of the internal elastic lamina. The most common site is the AComm–ACA junction (about 30–40%). Next are the ICA–PComm junction and the MCA bifurcation. Rupture causes subarachnoid hemorrhage ("worst headache of my life," blood on noncontrast CT, xanthochromia on LP). Risk factors include hypertension, smoking, ADPKD, Ehlers-Danlos syndrome, coarctation of the aorta, and a family history. - **Complications of SAH:** Vasospasm (days 3–14, treated with nimodipine), rebleeding, hydrocephalus, and hyponatremia (SIADH or cerebral salt wasting). - **Aneurysm compression syndromes:** - PComm or ICA aneurysm compresses CN III, causing a "down and out" eye with ptosis and a dilated pupil (pupil involvement is a clue to compression rather than a diabetic microvascular cause). - AComm aneurysm can compress the optic chiasm, causing bitemporal hemianopia (usually the inferior fields first). - **Stroke territories:** ACA stroke causes contralateral leg weakness and sensory loss, with abulia and urinary incontinence. MCA stroke causes contralateral face and arm weakness, with aphasia (dominant hemisphere) or hemineglect (nondominant). PCA stroke causes contralateral homonymous hemianopia with macular sparing. The anterior and posterior circulations are linked by the circle but have distinct territories. - **Watershed (border zone) infarcts:** These occur at the borders between ACA/MCA/PCA territories during global hypoperfusion. They are not a circle of Willis phenomenon, but they are often tested alongside it. - **Lenticulostriate arteries** (MCA branches) supply the internal capsule and basal ganglia. They are the typical site of lacunar infarcts and Charcot-Bouchard microaneurysm hemorrhages in chronic hypertension.

**Embryology and anatomy pearls:** The fetal-type PCA, in which the PCA arises from the ICA via a large PComm, is a common variant. The circle sits in the subarachnoid space (suprasellar/interpeduncular cistern), which is why rupture causes SAH rather than intraparenchymal bleeding. Imaging uses CTA, MRA, or catheter angiography. Management of unruptured or ruptured aneurysms includes endovascular coiling or surgical clipping.