Left Anterior Cerebral Artery
Based on the clinical presentation of predominantly right lower extremity weakness (2/5) with less severe right upper extremity weakness (4/5), preserved speech, no facial droop, and intact cognition, the left anterior cerebral artery (ACA) territory is the most likely affected vascular distribution.
Anatomical Localization
The pattern of weakness strongly suggests an ACA territory infarction for the following reasons:
Leg-predominant weakness: The ACA supplies the medial aspect of the motor cortex, which controls the contralateral lower extremity 1. The motor homunculus representation shows that leg motor function is primarily mediated by the paracentral lobule supplied by the ACA 1.
Mild arm involvement: While the ACA primarily affects the leg, proximal arm weakness can occur due to involvement of the supplementary motor area or extension into watershed territories 1.
Lateralization: Since the weakness is on the right side of the body, the lesion must be in the left cerebral hemisphere (contralateral to symptoms) 1.
Preserved cortical functions: The absence of aphasia, facial droop, and clear speech indicates that the middle cerebral artery (MCA) territory—which supplies Broca's area, the facial motor cortex, and hand/arm motor areas—is likely spared 1.
Why Not Middle Cerebral Artery?
The MCA territory would typically present with:
- Arm-predominant or equal arm-leg weakness (not leg-predominant) 1
- Facial droop involving the lower face 1
- Aphasia if the dominant (left) hemisphere is affected 1
- Neglect or visuospatial deficits if the non-dominant hemisphere is affected 1
This patient lacks these characteristic MCA findings 1.
Clinical Context and Risk Factors
This patient's vascular risk profile (hypertension, hyperlipidemia, diabetes) places him at significant risk for intracranial atherosclerosis, which commonly affects the ACA and MCA distributions 1. Patients with diabetes, hyperlipidemia, and hypertension have increased susceptibility to both large vessel atherosclerosis and small vessel disease 1, 2.
Common Pitfall
A critical pitfall is assuming that all strokes with hemiparesis involve the MCA territory. The leg-predominant pattern is pathognomonic for ACA involvement and should immediately redirect localization away from the MCA 1. The preservation of speech and facial function further confirms this localization 1.