Overview

Components. Starting at the front, the circle is formed by the anterior communicating artery, the first parts (A1) of both anterior cerebral arteries, the ends of both internal carotid arteries, both posterior communicating arteries, and the first parts (P1) of both posterior cerebral arteries, which arise from the top of the basilar artery. It lies in the subarachnoid space, in the interpeduncular and chiasmatic cisterns.

Function. Normally little blood crosses the communicating arteries, because pressures on the two sides are balanced. When one feeding artery narrows slowly, the circle can act as a bypass. How well it does so depends on its anatomy, which varies greatly. The textbook complete, symmetrical circle is found in only a minority of people, about 15–40% in imaging studies. Hypoplastic communicating arteries are common, as is a "fetal" posterior cerebral artery that arises mainly from the internal carotid.

Perforating branches. Small central (perforating) branches leave the circle and its main vessels to supply the deep grey matter and the internal capsule. They include the lenticulostriate arteries, the thalamoperforating arteries and the recurrent artery of Heubner.

Location: Base of the brain, around the optic chiasm, the infundibulum (pituitary stalk) and the mammillary bodies.

Parts & subdivisions

Anterior communicating artery
Short vessel joining the two anterior cerebral arteries. It is the commonest site of berry aneurysms.
Anterior cerebral arteries (A1)
Run from the internal carotid termination to the anterior communicating artery.
Internal carotid arteries
Their terminal segments form the anterolateral corners of the circle.
Posterior communicating arteries
Join each internal carotid to the ipsilateral posterior cerebral artery. CN III runs close by.
Posterior cerebral arteries (P1)
Run from the basilar tip to the posterior communicating artery.
Middle cerebral artery
Not part of the ring itself, but the largest branch leaving it.

Function

Links the carotid and vertebrobasilar systems and the two sides of the brain.
Provides a possible bypass (collateral) route when a feeding artery narrows gradually.
Gives perforating branches to the hypothalamus, thalamus, basal ganglia and internal capsule.

Clinical significance

Berry (saccular) aneurysms

Berry aneurysms form at branch points around the circle. About 85% are in the anterior circulation, most often at the anterior communicating artery, then at the junction of the internal carotid and posterior communicating arteries, then at the middle cerebral bifurcation. Risk factors include adult polycystic kidney disease, connective tissue disorders, coarctation of the aorta, hypertension and smoking.

Rupture causes subarachnoid haemorrhage: a sudden "thunderclap", worst-ever headache, with neck stiffness. It is diagnosed by CT, followed if necessary by lumbar puncture to look for xanthochromia. Nimodipine is given to reduce later spasm of the cerebral arteries.

Posterior communicating aneurysm and CN III

An enlarging posterior communicating artery aneurysm compresses CN III. The result is a painful third nerve palsy in which the pupil is dilated, because the parasympathetic fibres lie on the outside of the nerve.

Mnemonics

Circle components

Moving around from the front: ACom, then A1 of each ACA, the ICA termination, PCom, and P1 of each PCA, closing at the basilar tip.

Compiled from standard anatomy and physiology references — see the Cardiovascular System sources. For education only; not medical advice.