Circle of Willis and intracranial haemorrhage: arteries, territories, berry aneurysms and the four bleeds

An arterial ring at the base of the brain

The brain receives its blood from the two internal carotid arteries in front and the two vertebral arteries behind, and the circle of Willis joins the two systems in a ring around the optic chiasm, the pituitary stalk and the mammillary bodies, in the suprasellar and interpeduncular cisterns. Each internal carotid artery enters the skull through the carotid canal, runs forward through the cavernous sinus and emerges through its dural roof medial to the anterior clinoid process. It then gives the ophthalmic artery, the posterior communicating artery and the anterior choroidal artery before dividing, below the anterior perforated substance, into the anterior and middle cerebral arteries. The two anterior cerebral arteries run forward and medially above the optic nerves and are joined in front of the chiasm by the short anterior communicating artery. Behind, the vertebral arteries unite at the lower border of the pons to form the basilar artery, which runs up the front of the pons and divides at its upper border into the two posterior cerebral arteries. Each posterior communicating artery runs back from the internal carotid to the posterior cerebral artery and closes the ring. The oculomotor nerve emerges between the posterior cerebral and superior cerebellar arteries and runs forward beside the posterior communicating artery towards the cavernous sinus, a relation that matters when that artery develops an aneurysm. Because the whole circle lies in the subarachnoid space, an aneurysm on it bleeds into the cerebrospinal fluid.

Variants, and why the circle does not prevent most strokes

The textbook ring is complete and symmetrical in only a minority of people, often quoted at about a fifth to a quarter. One or both posterior communicating arteries are commonly hypoplastic. In a fetal-type posterior cerebral artery the posterior cerebral fills mainly from the internal carotid through a large posterior communicating artery, with a small or absent P1 segment. An A1 segment may be hypoplastic, so that both anterior cerebral arteries fill from one carotid through the anterior communicating artery. The variants matter clinically. Whether a patient tolerates carotid clamping during endarterectomy depends on cross-flow through the circle, which is why surgeons monitor the awake patient under local anaesthesia, measure the stump pressure or insert a shunt. And because the communicating arteries are small, and the cerebral arteries beyond the circle behave largely as end arteries with only limited leptomeningeal collateral flow, an embolus that lodges beyond the circle produces infarction in that artery's territory however complete the ring may be.

Berry aneurysms and subarachnoid haemorrhage

About 85% of spontaneous subarachnoid haemorrhages are caused by a ruptured saccular (berry) aneurysm; most of the rest are non-aneurysmal perimesencephalic bleeds, and arteriovenous malformations and other causes are less common. Saccular aneurysms form at branch points on and near the circle, where haemodynamic stress falls on an arterial wall with a deficient media, and the large majority lie in the anterior circulation: the anterior communicating artery complex, the internal carotid at the origin of the posterior communicating artery, and the middle cerebral bifurcation. The basilar tip is the commonest posterior site. Risk factors are smoking, hypertension, heavy alcohol use, a family history in first-degree relatives and inherited conditions such as autosomal dominant polycystic kidney disease and vascular Ehlers–Danlos syndrome. The classic presentation is a thunderclap headache, sudden and severe and peaking within about 1 to 5 minutes, with vomiting, neck stiffness, photophobia, reduced consciousness or a seizure. An enlarging posterior communicating artery aneurysm may announce itself before it ruptures with a painful third nerve palsy.

1. Non-contrast CT of the head is the first-line test, and blood in the subarachnoid space confirms the diagnosis. CT is most accurate within 6 hours of symptom onset.
2. If a CT done within 6 hours of onset, and reported by a radiologist, shows no haemorrhage, a lumbar puncture is not routinely offered; look for another diagnosis.
3. If a CT done more than 6 hours after onset is normal, consider a lumbar puncture, waiting at least 12 hours from onset so that bilirubin has had time to form. Xanthochromia on spectrophotometry confirms the diagnosis.
4. Once the diagnosis is confirmed, CT angiography of the head is done without delay to find the cause and plan treatment, and the patient is discussed with the regional neurosciences centre.
5. Enteral nimodipine is considered for confirmed aneurysmal haemorrhage to reduce delayed cerebral ischaemia; the usual regimen is 60 mg every 4 hours for 21 days.
6. The aneurysm is secured at the earliest opportunity, because rebleeding is most likely in the first 24 hours. Where intervention is appropriate endovascular coiling is preferred, with clipping when coiling is unsuitable, a policy supported by the ISAT trial.

Extradural, subdural and the other bleeds

The pterion, where the frontal, parietal and squamous temporal bones meet the greater wing of the sphenoid, is the thinnest part of the side of the skull and overlies the anterior branch of the middle meningeal artery, which arises from the maxillary artery and enters the skull through the foramen spinosum. It lies about two finger-breadths above the zygomatic arch and a thumb's breadth behind the frontal process of the zygomatic bone. A blow there fractures the thin bone and tears the artery, and the blood strips the dura from the inner table to form an extradural haematoma. The dura is firmly attached at the sutures, so the clot is limited by them and bulges inwards as a biconvex lens. The classic history is a brief loss of consciousness, a lucid interval and then rapid deterioration with a fixed dilated pupil on the side of the clot, although a true lucid interval is seen in only a minority. An acute subdural haematoma comes from bridging veins torn as they cross the subdural space to the superior sagittal sinus, usually in a high-energy injury with underlying brain contusion, so the patient is often unconscious from the outset and the outlook is worse. The blood spreads over the hemisphere as a crescent, crossing suture lines but stopped by the falx and the tentorium. A chronic subdural haematoma follows trivial or forgotten trauma in older people, heavy drinkers and those taking anticoagulants, in whom brain atrophy stretches the bridging veins, and presents weeks later with headache, fluctuating confusion or a slowly progressive deficit.