Posterior communicating artery !

The posterior communicating artery links the front and back blood supply of the brain; an aneurysm here can press on the nerve that lifts the eyelid.

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What it is

A short artery joining the internal carotid to the posterior cerebral artery; a common aneurysm site that can press on nerve III.

What it does

Links front and back circulations; an aneurysm here can press on nerve III (droopy lid, wide pupil).

Key facts

Origin: Internal carotid
Branches: Small perforators to the thalamus and hypothalamus
Supplies: Links carotid and vertebrobasilar systems; aneurysm here compresses CN III

Relations

Course: Leaves the back of the internal carotid just above the cavernous sinus and runs back, above the oculomotor nerve, to join the posterior cerebral artery at the P1–P2 junction
Below: Oculomotor nerve, running forward below and lateral to it
Medial: Tuber cinereum, the pituitary stalk and the mammillary bodies
Above: Optic tract and the base of the hypothalamus

How to find it

It cannot be felt. It runs back from the internal carotid to the posterior cerebral artery, above and medial to the oculomotor nerve and below the optic tract; in a pterional craniotomy it is seen behind the carotid once the carotid is gently lifted.

Examination

Third nerve examination: lid position, eye position and movements, and the size and light reaction of both pupils

On imaging

CT or MR angiography shows its size and any aneurysm; catheter angiography confirms. A small funnel-shaped widening at its origin (infundibulum) can be mistaken for an aneurysm.

Common problems

Brain aneurysm: a weak bulge in the artery wall, most often at this junction.
Rupture causes a subarachnoid haemorrhage: a sudden 'worst headache of my life'.

How it is diagnosed

CT and CT angiography; angiography with a catheter to plan treatment; unruptured aneurysms are often found by chance on scans.

Treatment

Clipping: a small metal clip across the neck of the aneurysm through a craniotomy.
Coiling: soft platinum coils packed inside it through a catheter from the groin or wrist.
Watching some small unruptured aneurysms, with blood pressure control and no smoking.

Clinical notes

With a fetal-type posterior communicating artery, emboli from carotid disease can reach the occipital lobe, so an occipital stroke does not always mean a vertebrobasilar source
A small funnel-shaped widening at its origin (infundibulum) is common and harmless, but on angiography it can be mistaken for an aneurysm
Small perforating branches leave it for the thalamus, hypothalamus and internal capsule, so clipping or coiling of an aneurysm here must keep the artery open

Anatomical variants

Often small (hypoplastic) on one or both sides
Fetal pattern: a large posterior communicating artery is the main supply of the posterior cerebral artery

Development

It starts as the main caudal branch of the internal carotid, which first supplies the territory of the future posterior cerebral artery; the vertebrobasilar system later takes most of that flow, and when it does not, a fetal-type posterior communicating artery remains.

Memory aid

Rule of the pupil: pupil involved, think pressure (aneurysm); pupil spared, think small vessels (diabetes, hypertension).

Test yourself

Think of the answer, then tap the question to see it.

Which cranial nerve does a posterior communicating artery aneurysm compress, and what are the signs?

The oculomotor nerve (III): ptosis, eye down and out, and a dilated, unreactive pupil.

Why is a painful third nerve palsy with a dilated pupil treated as an emergency?

It suggests a posterior communicating artery aneurysm until imaging rules it out; microvascular palsies usually spare the pupil.

Which arteries does the posterior communicating artery join?

The internal carotid to the posterior cerebral artery (P1–P2 junction), linking the carotid and vertebrobasilar systems.

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Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2): arteria communicans posterior TA2 4521
Official Spanish (DeCS): Arteria comunicante posterior
English (TA2): Posterior communicating artery

Sources: Terminologia Anatomica (TA2); Neuroanatomy, Circle of Willis; Anatomy of the Human Body (20th ed.): Development of the Vascular System (Lea & Febiger (Gray's Anatomy, 1918), 1918) · adapted, Public domain.

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Representative model, for educational purposes only.

Posterior communicating artery highlighted on the Durgical 3D model