Anatomy Atlas

Vertebrobasilar system

Arteria basilaris
Also: Basilar artery · Posterior circulation · PICA · AICA · Superior cerebellar artery · Subclavian steal syndrome
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The posterior circulation — the vertebral arteries join to form the basilar artery, which supplies the brainstem, cerebellum and (through the PCAs) the occipital lobes. Its strokes cause vertigo, crossed signs and, with basilar occlusion, coma or locked-in syndrome.

Core anatomy

Origin

Vertebral arteries (vertebral artery), first branches of the subclavian arteries, ascending through the transverse foramina of C6–C1 and entering the skull through the foramen magnum.

Course

The two vertebral arteries run up the front of the medulla oblongata and join at the pontomedullary junction to form the basilar artery, which ascends in the groove on the front of the pons and divides into the two posterior cerebral arteries at the upper border of the pons/midbrain.

Branches
  • Vertebral: anterior spinal artery (joins its partner — medial medulla, anterior cord), posterior spinal arteries, PICA (lateral medulla, inferior cerebellum)
  • Basilar: AICA (lateral lower pons, anterior-inferior cerebellum; often gives the labyrinthine artery to the inner ear), pontine paramedian and circumferential branches, superior cerebellar artery (upper pons, superior cerebellum)
  • Posterior cerebral arteries (terminal branches; see cerebral arteries (ACA, MCA, PCA))
Supplies

Medulla oblongata, pons, midbrain, cerebellum, inner ear, thalamus, occipital and inferior temporal lobes, upper spinal cord.

Anastomoses

Joins the anterior circulation through the posterior communicating arteries (see circle of Willis).

Termination

Bifurcates into the posterior cerebral arteries at the midbrain.

Relations

CN III emerges between the PCA and SCA; CN VI runs up alongside the basilar artery.

Branches

Structures supplied

Generated automatically from links in other entries.

Circle of Willis and vertebrobasilar system (inferior view)Seen from below with the front of the brain at the top. The two vertebral arteries give the posterior inferior cerebellar arteries and join to form the basilar artery, which gives the anterior inferior cerebellar, pontine and superior cerebellar arteries and divides into the two posterior cerebral arteries. Posterior communicating arteries join each posterior cerebral artery to the internal carotid artery. Each internal carotid gives the middle cerebral artery laterally and the anterior cerebral artery medially; the anterior cerebral arteries are joined by the anterior communicating artery. The oculomotor nerve passes between the posterior cerebral and superior cerebellar arteries. Common aneurysm sites: anterior communicating artery, posterior communicating–internal carotid junction, and middle cerebral artery bifurcation.Anterior cerebral a.Anterior communicating a.Internal carotid a. (cut)Posterior communicating a.Middle cerebral a.Posterior cerebral a.Superior cerebellar a.Pontine branchesAICABasilar a.PICAVertebral a.Lenticulostriate aa.CN IIIInfundibulumBerry aneurysm sites: AComm, PComm–ICA, MCA bifurcationInferior view · anterior ↑

Swipe sideways to see the whole diagram →

Circle of Willis and vertebrobasilar system. Inferior view, anterior at the top. The internal carotids (anterior circulation) and basilar artery (posterior circulation) are joined by the communicating arteries; CN III passes between the PCA and SCA. Red dots mark common berry aneurysm sites. A complete, symmetric circle is present in only a minority of people.
Original schematic, created for this atlas.
Diagram of the circle of Willis: internal carotid, anterior cerebral, anterior communicating, middle cerebral, posterior communicating and posterior cerebral arteries, with the basilar and vertebral arteries and their branches
Vertebral and basilar arteries with the cerebellar and pontine branches.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 519. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))
Inferior surface of the brain with the arterial circle, the basilar and vertebral arteries and the middle cerebral artery in the lateral fissure
The basilar artery on the front of the pons.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 516. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))

Clinical correlations

Posterior circulation stroke PANCE

Mechanism / lesion

Embolism, atherosclerosis of the vertebral/basilar arteries, vertebral artery dissection (neck trauma or manipulation).

Classic presentation

"5 Ds": dizziness/vertigo, diplopia, dysarthria, dysphagia, dystaxia (ataxia), plus nausea, nystagmus and crossed signs (ipsilateral cranial nerve + contralateral limb deficits). PICA → lateral medullary (Wallenberg); AICA → lateral pontine syndrome (ipsilateral facial palsy, hearing loss, vertigo, ataxia, face sensory loss, contralateral body pain/temp loss); SCA → ipsilateral ataxia.

Basilar artery occlusion PANCE

Mechanism / lesion

Thrombosis or embolism — high mortality without reperfusion.

Classic presentation

Fluctuating or sudden coma, quadriparesis, pinpoint pupils, bilateral signs, or locked-in syndrome; "top of the basilar" syndrome — visual loss, eye movement abnormalities, altered consciousness. Emergency CTA and thrombectomy.

Subclavian steal syndrome PANCE

Mechanism / lesion

Stenosis of the proximal subclavian artery (before the vertebral origin, usually left) → blood flows retrograde down the ipsilateral vertebral artery to supply the arm during exercise.

Classic presentation

Arm claudication and vertebrobasilar symptoms (dizziness, syncope) with arm exercise; arm BP difference > 15–20 mm Hg and a weaker radial pulse on the affected side. Duplex shows reversed vertebral flow.

High-yield PANCE pearls

  • Vertigo + diplopia + dysarthria + ataxia + crossed signs → posterior circulation stroke → MRI (CT misses it).
  • Coma or locked-in state + pinpoint pupils → basilar occlusion → CTA + thrombectomy.
  • Dizziness when using one arm + unequal arm BPs → subclavian steal.
  • Vertigo + sudden unilateral hearing loss → AICA (labyrinthine artery) stroke.
  • Neck pain + posterior circulation stroke in a young patient → vertebral dissection.

Imaging anatomy

CT

Noncontrast CT has poor sensitivity in the posterior fossa; CTA for vertebral/basilar occlusion and dissection.

MRI

MRI with DWI is the test of choice for posterior circulation infarcts; MRA and fat-saturated T1 for dissection.

Ultrasound

Duplex of the vertebral arteries shows reversed flow in subclavian steal.

Physical exam

Strength / function testing

Eye movements and nystagmus, HINTS exam, facial sensation and strength, hearing, palate and tongue, limb ataxia and gait, crossed sensory findings; blood pressure in both arms and bilateral radial pulses.

Self-quiz

Q1.

A 66-year-old has dizziness and near-fainting whenever he chops wood with his left arm. BP is 150/85 in the right arm and 118/70 in the left. What is the most likely diagnosis?

Show answer

B

Proximal left subclavian stenosis makes the arm "steal" blood by reversing flow down the left vertebral artery during exercise, causing posterior circulation symptoms and a large inter-arm BP difference.

Q2.

A 58-year-old has sudden vertigo, vomiting, right-sided hearing loss, right facial weakness and right limb ataxia. Which artery is most likely occluded?

Show answer

B

AICA supplies the lateral lower pons (facial nucleus, vestibular/cochlear nuclei) and usually the labyrinthine artery — vertigo with hearing loss and facial palsy suggests the lateral pontine (AICA) syndrome.

Q3.

A 70-year-old is found comatose with quadriparesis and pinpoint pupils. CT shows no hemorrhage. What is the most important next step?

Show answer

A

Sudden coma, quadriparesis and pinpoint pupils suggest basilar artery occlusion, which has high mortality without reperfusion. It needs emergency CTA and thrombectomy.

Q4.

The posterior cerebral arteries are the terminal branches of which artery?

Show answer

E

The basilar artery divides into the two posterior cerebral arteries at the upper pons/midbrain. The PComm arteries connect them to the internal carotids.

Q5.

Which imaging test best detects an acute posterior circulation (brainstem or cerebellar) stroke?

Show answer

C

CT often misses posterior fossa infarcts because of bone artifact. MRI (diffusion-weighted) is the test of choice when a posterior circulation stroke is suspected.

Flashcards

See also