Anterior and Posterior Circulation Stroke Syndromes
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Direct answer
Anterior and posterior circulation stroke syndromes are the clinical patterns produced by infarction in the carotid (anterior) or vertebrobasilar (posterior) territories. Middle cerebral artery occlusion causes face and arm weakness greater than leg weakness, with aphasia when the dominant hemisphere is involved. Basilar artery occlusion is the most feared posterior event, causing coma, quadriparesis and cranial nerve signs.
What you must remember
- Anterior circulation: the internal carotid artery and its branches, including the middle and anterior cerebral arteries, supply the territories that produce hemispheric syndromes.
- MCA territory: contralateral face and arm weakness greater than leg weakness, with homonymous hemianopia; aphasia follows dominant-side lesions and neglect follows non-dominant ones.
- ACA territory: contralateral leg weakness greater than arm weakness, with abulia, urinary incontinence and a grasp reflex.
- PCA territory: contralateral homonymous hemianopia, often with macular sparing, and alexia without agraphia when the left occipital lobe and splenium are involved.
- Macular sparing: the occipital pole receives collateral supply from both MCA and PCA branches, so a PCA occlusion often spares the central field.
- Basilar occlusion: altered consciousness, quadriparesis, bilateral signs and cranial nerve palsies, with very high mortality if untreated.
- Locked-in syndrome: quadriplegia and mutism with preserved vertical eye movements and eyelid control, a classic pontine outcome of basilar thrombosis.
- Imaging: non-contrast CT excludes haemorrhage but misses early posterior fossa infarcts, so MRI with diffusion-weighted imaging is needed.
Localising a stroke by territory
A 68-year-old woman wakes with right-sided face and arm weakness, word-finding difficulty and a right visual field loss. The motor pattern and the visual field together point to a single vessel. Each territory leaves a recognisable signature, so the pattern identifies the artery before any imaging is reported.
| Territory | Motor pattern | Visual and cortical signs | Classic clue |
|---|---|---|---|
| MCA | Face and arm worse than leg | Homonymous hemianopia; aphasia or neglect | Gaze towards the lesion |
| ACA | Leg worse than arm | Abulia, incontinence | Grasp reflex |
| PCA | Usually no major motor deficit | Hemianopia with macular sparing | Alexia without agraphia |
| Basilar | Quadriparesis, often bilateral | Coma, cranial nerve palsies | Locked-in syndrome |
Read the table from the motor pattern and the visual field together, since these two findings separate most territories. In basilar occlusion, bilateral and brainstem signs matter more than any single territory pattern, so the examiner expects you to look for them first. A stem that mentions vertigo with a quadriparesis should trigger a basilar work-up immediately.
Where the exam tries to catch you
Most lost marks come from matching a pattern to the wrong vessel.
- Trap: Blaming a retinal or chiasmal lesion for a hemianopia with macular sparing. Correct: Macular sparing with homonymous hemianopia points to the occipital cortex, classically the PCA territory.
- Trap: Labelling every hemiparesis as MCA. Correct: Leg-dominant weakness with abulia points to ACA territory, and a grasp reflex supports that diagnosis.
- Trap: Dismissing vertigo or drowsiness as benign. Correct: Early basilar occlusion can present this way, so a falling level of consciousness needs immediate imaging.
- Trap: Ruling out posterior circulation stroke on a normal early CT. Correct: Non-contrast CT is insensitive in the posterior fossa, so MRI is the better test when posterior stroke is suspected.
Frequently asked questions
Which clinical features distinguish MCA from ACA stroke?
MCA stroke causes face and arm weakness greater than leg weakness, often with homonymous hemianopia and aphasia or neglect. ACA stroke causes leg-dominant weakness with abulia, urinary incontinence and a grasp reflex, which differ strongly from the MCA pattern.
Why does PCA infarction often spare the macula?
Macular sparing in PCA infarction occurs because the occipital pole receives collateral blood supply from both MCA and PCA branches. A homonymous hemianopia with macular sparing therefore points to occipital cortex rather than the retina or optic chiasm.
What are the clinical features of basilar artery occlusion?
Basilar artery occlusion causes altered consciousness or coma, quadriparesis, bilateral signs and cranial nerve palsies, and it may begin with vertigo or drowsiness. Locked-in syndrome, with preserved vertical eye movements, is a classic pontine outcome of basilar thrombosis.
Which imaging is needed for suspected posterior circulation stroke?
Non-contrast CT is performed first to exclude haemorrhage, followed by CT angiography to locate the occluded artery. MRI with diffusion-weighted imaging is needed because posterior fossa infarcts are often missed on early CT.
Which hemisphere causes aphasia in MCA stroke?
Aphasia follows infarction of the dominant hemisphere, which is the left side in right-handed people. Neglect and anosognosia follow non-dominant right hemisphere lesions, so the side of the lesion decides the cortical syndrome.