Anterior Cerebral Artery (ACA) Territory
This presentation of facial asymmetry with isolated right-sided lower extremity weakness localizes to the left anterior cerebral artery (ACA) territory, not the middle cerebral artery (MCA).
Anatomical Localization
The key to this localization is understanding the motor homunculus distribution:
- The ACA supplies the medial motor cortex, which contains the motor representation for the contralateral lower extremity 1, 2
- The MCA supplies the lateral motor cortex, which contains motor representation for the contralateral face and upper extremity 3
- When facial weakness occurs with predominantly lower extremity weakness, this indicates ACA territory involvement 4
Clinical Reasoning
The pattern of weakness is the critical distinguishing feature:
- MCA strokes typically present with contralateral hemiparesis affecting the face and arm more than the leg, with sensory deficits following a similar distribution 5, 3
- ACA strokes characteristically cause contralateral weakness that is most prominent in the lower extremity, often with relative sparing of the face and arm 1, 2
The presence of facial asymmetry in this case requires explanation: Research demonstrates that upper facial movements receive bilateral cortical innervation from both ACA and MCA territories 4. However, when facial weakness occurs in the context of isolated lower extremity weakness, the ACA territory remains the primary culprit, with the facial involvement representing either:
- Mild involvement of adjacent motor areas
- Extension of the infarct to border zones
- Pre-existing subclinical pathology
Additional ACA Territory Features to Assess
Beyond motor deficits, evaluate for other characteristic ACA stroke findings:
- Urinary incontinence (common with bilateral or dominant hemisphere ACA infarcts) 2
- Transcortical motor aphasia (if left hemisphere involved) 1, 2
- Apraxia and executive dysfunction 2
- Abulia or akinetic mutism (with bilateral ACA involvement)
Stroke Mechanism Considerations
Cardioembolism is the most common mechanism for ACA territory infarction (45% of cases), followed by atherothrombotic disease (29%) 1. This differs from the general stroke population and should guide your workup priorities.
ACA occlusions are relatively rare, accounting for only 1.8% of all cerebral infarctions 1, 6. The mechanisms include:
- Emboli in unusual hemodynamic circumstances (such as increased flow through the anterior communicating artery due to contralateral ICA occlusion) 6
- Propagation of thrombus from an occluded internal carotid artery 6
- Vasospasm or emboli associated with anterior communicating artery aneurysms 6
Imaging Confirmation
Obtain CT or MRI with vascular imaging (CTA/MRA) to confirm ACA territory infarction and identify the mechanism 7. Look specifically for:
- Infarction in the medial frontal lobe and paracentral lobule
- Involvement of the supplementary motor area
- Vascular occlusion or stenosis of the ACA