Cerebral Circulation: Distribution

What are the main components of the Circle of Willis?

Two anterior cerebral arteries joined by the anterior communicating artery, two internal carotid arteries, two posterior cerebral arteries from the basilar artery, and two posterior communicating arteries

What is the role of the communicating arteries in the Circle of Willis?

They can provide collateral blood flow when an artery is occluded

Is the middle cerebral artery (MCA) part of the Circle of Willis?

No. It is the largest branch leaving the Circle of Willis

Why can MCA occlusion be particularly damaging?

The MCA supplies a large territory of the cerebral hemisphere

Why is there normally little or no net flow through the communicating arteries?

Pressures on either side are normally equal

What happens to flow through the Circle of Willis after an arterial occlusion?

Pressure falls beyond the blockage, causing flow to reverse through communicating arteries so neighbouring vessels can provide collateral supply

Why can two people with similar arterial occlusions have different outcomes?

The effectiveness of collateral circulation through the Circle of Willis varies greatly between individuals

Why might a patient with a completely occluded left internal carotid artery have no symptoms?

The occlusion occurred slowly and the Circle of Willis supplies the affected side from elsewhere

What anatomical principle helps predict which cerebral artery supplies which functional region?

The position of the homunculus on the cerebral cortex

Where is the leg represented on the cortical homunculus?

On the medial surface of the hemisphere

Where are the face and hand represented on the cortical homunculus?

Low on the lateral convexity

Which artery mainly supplies the medial surface of the cerebral hemisphere?

Anterior cerebral artery (ACA)

Which artery mainly supplies the lateral surface of the cerebral hemisphere?

Middle cerebral artery (MCA)

Which artery mainly supplies the inferior and occipital surfaces?

Posterior cerebral artery (PCA)

What additional vascular system supplies the deep brain structures?

Deep perforating arteries from all three major arterial territories

What is the main cortical territory of the ACA?

Medial surface of the cerebral hemisphere

What motor deficit is characteristic of ACA occlusion?

Weakness affecting the leg more than the arm, with the face spared

What behavioural changes can occur with ACA occlusion?

Apathy and abulia

What bladder symptom can occur with ACA occlusion?

Urinary incontinence

What is the main cortical territory of the MCA?

The lateral convexity of the cerebral hemisphere

Approximately what proportion of anterior cerebral blood flow is associated with the MCA territory?

~75–80%

What motor deficit is characteristic of MCA occlusion?

Weakness affecting the face and arm more than the leg

What visual deficit can occur with MCA occlusion?

Homonymous hemianopia

What language deficit can occur with dominant-side MCA occlusion?

Aphasia

What perceptual/attention deficit can occur with non-dominant-side MCA occlusion?

Neglect

What is the main territory of the PCA?

Occipital cortex, medial temporal structures and thalamus

What visual deficit is characteristic of PCA occlusion?

Homonymous hemianopia with macular sparing

What memory problem can occur with PCA occlusion?

Memory impairment due to involvement of the hippocampus

What is a key feature distinguishing PCA occlusion from ACA/MCA cortical strokes?

Motor signs are usually absent

Where do perforating arteries supply?

The basal ganglia, internal capsule and thalamus

How do perforating arteries enter the deep brain?

They leave the parent artery at right angles and dive inward

Why are perforating arteries particularly vulnerable to occlusion or rupture?

They are end arteries with no anastomoses or collateral supply

What vascular risk factor particularly damages perforating arteries?

Chronic hypertension

What can happen to perforating arteries in chronic hypertension?

They can occlude or rupture

Why can a small internal capsule infarct cause severe disability?

The internal capsule contains tightly packed important pathways, so location matters more than lesion size

What is the key principle illustrated by deep infarcts?

Location, not size, determines disability

What are watershed zones?

Border-zone regions between major arterial territories that are vulnerable when cerebral perfusion is reduced

What deficit can occur with watershed injury after hypotension?

Bilateral proximal arm weakness

How does cerebral venous drainage occur?

Deep and superficial veins drain into dural venous sinuses, which drain into the internal jugular veins

Do cerebral veins have valves?

No

What is the consequence of cerebral veins having no valves?

Venous flow can reverse

Do cerebral veins follow the same territories as cerebral arteries?

No, they do not follow the arteries

How does a venous infarct differ from an arterial infarct?

It can cross arterial boundaries and is often haemorrhagic

What is the classic ACA deficit pattern?

Leg > arm weakness, apathy and urinary incontinence

What is the classic MCA deficit pattern?

Face and arm > leg weakness, aphasia or neglect, and hemianopia

What is the classic PCA deficit pattern?

Hemianopia with macular sparing and usually no motor signs

What is the classic deficit pattern of perforator infarction?

Dense neurological deficit without cortical signs

What is the classic watershed deficit pattern?

Bilateral proximal arm weakness following hypotension

If a patient has leg-predominant weakness with the face spared, which artery is most likely affected?

ACA

If a patient has face and arm-predominant weakness, which artery is most likely affected?

MCA

If a patient has aphasia following a cerebral infarct, which major artery is likely involved?

MCA territory on the dominant side

If a patient has neglect following a cerebral infarct, which major artery is likely involved?

MCA territory on the non-dominant side

If a patient has homonymous hemianopia with macular sparing and no motor signs, which artery is likely affected?

PCA

If a patient has a dense neurological deficit without cortical signs, what vascular territory should be considered?

Perforating arteries

If bilateral proximal arm weakness occurs after severe hypotension, what vascular region should be considered?

Watershed/border-zone regions

What is the key artery-to-deficit rule for ACA?

ACA → medial cortex → leg-predominant weakness

What is the key artery-to-deficit rule for MCA?

MCA → lateral cortex → face/arm-predominant weakness ± aphasia/neglect

What is the key artery-to-deficit rule for PCA?

PCA → occipital/inferior cortex → visual deficit with macular sparing

What is the key principle for solving cerebral circulation clinical questions?

Learn both directions: artery → deficit and deficit → artery