Vascular Territories & Watersheds

The vascular territories of the brain — which artery supplies which region — are the bedrock of stroke neurology. Recognizing the territory tells you the artery, and the artery tells you the likely mechanism, the prognosis, and often the treatment. Watershed zones between territories are particularly important because they identify the regions most vulnerable to hypoperfusion. This page consolidates the vascular territories covered in the anterior and posterior circulation pages, places them in clinical context, and reviews the watershed and end-zone vulnerability patterns.

Major Cortical Territories

Anterior Cerebral Artery (ACA)

Medial surface of frontal and parietal lobes, including:

• Medial frontal lobe (anterior cingulate, medial prefrontal).
• Supplementary motor area.
• Paracentral lobule (motor and sensory cortex for leg).
• Medial parietal lobe.
• Anterior corpus callosum.

Classical ACA stroke: contralateral leg weakness greater than arm or face, sometimes with abulia, akinetic mutism, urinary incontinence (medial frontal); transcortical motor aphasia if dominant; alien hand syndrome (callosal type) if callosum involved.

Middle Cerebral Artery (MCA)

Lateral surface of cerebral hemisphere, including:

• Lateral frontal lobe (Broca area, motor cortex face and arm).
• Lateral parietal lobe (Wernicke area on dominant side, postcentral gyrus face and arm).
• Lateral temporal lobe (most of superior and middle temporal gyri).
• Insula.

Classical MCA stroke: contralateral hemiparesis (face, arm) and hemisensory loss; aphasia (dominant) or neglect (non-dominant); homonymous hemianopia.

Posterior Cerebral Artery (PCA)

Occipital lobe and medial/inferior temporal lobe, including:

• Calcarine cortex (V1).
• Posterior medial temporal lobe (including hippocampus).
• Splenium of corpus callosum.
• Posterior thalamus (via perforator branches).

Classical PCA stroke: contralateral homonymous hemianopia with macular sparing; sometimes alexia without agraphia (dominant); memory deficits if hippocampus involved; visual agnosias if extrastriate cortex involved.

Deep Territories

Lenticulostriate Arteries (from MCA M1)

Supply:

• Putamen.
• Lateral globus pallidus.
• Caudate body and tail.
• Posterior limb of internal capsule (substantial portion).

Lacunar infarcts here produce small-vessel syndromes: pure motor, sensorimotor, dysarthria-clumsy hand. Hypertensive ICH in the putamen is the most common location of spontaneous intracerebral hemorrhage.

Recurrent Artery of Heubner (from ACA A2)

Supplies head of caudate and anterior limb of internal capsule.

Anterior Choroidal Artery (from ICA)

Supplies posterior limb of internal capsule, optic tract, parts of thalamus, choroid plexus.

Thalamoperforator Arteries (from PCA P1)

Paramedian thalamus and rostral midbrain. Artery of Percheron variant supplies bilateral paramedian thalami.

Thalamogeniculate Arteries (from PCA P2)

Lateral thalamus including VPL and VPM.

Brainstem Territories

Midbrain

Vascular supply primarily from PCA branches (paramedian and short circumferential) and SCA branches.

• Paramedian midbrain: from PCA P1 perforators. Affected in artery of Percheron occlusion (bilateral).
• Lateral midbrain: from PCA circumferential and SCA branches.

Pons

• Paramedian pons: from basilar paramedian perforators.
• Lateral lower pons: from AICA.
• Lateral upper pons: from SCA.

Medulla

• Medial medulla: from vertebral paramedian branches and anterior spinal artery. Medial medullary syndrome.
• Lateral medulla: from vertebral and PICA. Wallenberg syndrome.

Cerebellar Territories

• PICA: inferior cerebellum and lateral medulla.
• AICA: inferolateral cerebellum and lateral lower pons.
• SCA: superior cerebellum and parts of lateral upper pons/midbrain.

Watershed Zones

Watershed (border zone) regions lie between major arterial territories and have the most distal blood supply from each. They are vulnerable to hypoperfusion (global cerebral hypoperfusion from severe hypotension, cardiac arrest, severe carotid stenosis):

External (Cortical) Watershed

• Anterior watershed (ACA-MCA): superior frontal cortex, parasagittal region. Infarction here can produce “man in the barrel” syndrome — proximal shoulder and arm weakness with relative sparing of distal extremities.
• Posterior watershed (MCA-PCA): parieto-occipital region. Infarction here can produce visual symptoms, optic ataxia, simultanagnosia, sometimes the components of Balint syndrome.

Internal (Subcortical) Watershed

The deep watershed runs between the territories of the deep perforating arteries and the medullary branches penetrating from the cortical surface. These deep watersheds produce small infarcts often arranged in a “string of beads” pattern in the centrum semiovale.

Posterior Fossa Watersheds

Borders between PICA, AICA, and SCA cerebellar territories can be affected in hypoperfusion or in cerebellar stroke.

The Spinal Cord Watershed

The mid-thoracic cord (T4-T8 region) is a watershed between the cervical anterior spinal artery territory and the lower thoracic/lumbar territory supplied by the artery of Adamkiewicz. This is the region most vulnerable to anterior cord infarction from global hypoperfusion or aortic disease.

End Zones

“End zones” are the most distal parts of single arterial territories — areas dependent on the very terminal branches of an artery without significant collateral. The most distal hippocampus, the medial temporal lobe in PCA territory, and the central white matter are examples. Hypoperfusion may damage these regions selectively.

Clinical Applications

🔍 Did You Know?

The classical “man in the barrel” syndrome — bilateral proximal arm and shoulder weakness with relative sparing of distal extremities — reflects bilateral anterior watershed infarction at the ACA-MCA border, affecting the cortical representation of the proximal upper extremity. The cortical representation of the proximal shoulder and arm sits at the medial part of the lateral hemisphere, at the very edge of the MCA territory; the more distal hand and forearm representations sit more laterally, well within MCA territory. Severe hypotension or bilateral severe carotid stenosis preferentially infarcts the watershed border, producing this strikingly distinctive bilateral weakness pattern with hand function relatively preserved. The patient looks like they are stuck in a barrel — shoulders fixed, hands able to move. Recognition matters because the underlying cause is a global hypoperfusion event that may have other consequences and that often points to severe vascular disease.

Pitfalls and Pearls

• The vascular territory tells you the artery. Recognizing the territory at the bedside often allows the artery to be named before imaging confirms.
• Cortical MCA stroke spares the leg; ACA spares face and arm; capsular stroke affects all three equally.
• Watershed infarcts are at borders of major territories. ACA-MCA produces shoulder predominance (man in barrel). MCA-PCA produces visual and spatial deficits.
• Lenticulostriate infarcts produce lacunar syndromes; the small vessels are vulnerable to hypertensive damage.
• Bilateral paramedian thalamic infarction (artery of Percheron) presents with coma and vertical gaze palsy.
• Wallenberg syndrome is lateral medulla (PICA territory). The most pathognomonic stroke syndrome.
• Locked-in syndrome is ventral pons (basilar artery occlusion).
• Bilateral occipital infarction (PCA territory) can produce cortical blindness, sometimes with Anton denial.
• Spinal cord watershed at T4-T8 is vulnerable to anterior cord infarction from global hypoperfusion or aortic disease.
• End-zone hippocampal injury after cardiac arrest can produce dense amnesia with relative sparing of other functions.

References

1. Caplan LR. Caplan’s Stroke: A Clinical Approach. 5th ed. Cambridge University Press; 2016.
2. Mohr JP, Wolf PA, Grotta JC, et al, eds. Stroke: Pathophysiology, Diagnosis, and Management. 6th ed. Elsevier; 2016.
3. Mounier-Vehier F, Leys D, Rondepierre P, Godefroy O, Pruvo JP. Borderzone infarcts: anatomic basis and clinical patterns. Stroke. 1994;25(2):374-377.
4. Bogousslavsky J, Regli F. Borderzone infarctions distal to internal carotid artery occlusion. Ann Neurol. 1986;20(3):346-350.
5. Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 7th ed. Wolters Kluwer; 2017.