A 58-year-old man from Delhi presents to the emergency department with acute onset left hemiparesis and left facial droop. On examination, he has left-sided weakness (3/5) with hyperreflexia and an extensor plantar response. CT brain shows no hemorrhage. CT angiography reveals complete occlusion of the right middle cerebral artery (MCA) at its origin. On further imaging, a hypoplastic right anterior cerebral artery (ACA) is noted. Which vessel of the Circle of Willis is most likely compensating for the blood supply to the right ACA territory?

Explanation

## Clinical Context This patient has an acute right MCA occlusion with a hypoplastic right ACA. The left hemiparesis indicates right hemisphere involvement. The key anatomical finding is the hypoplastic right ACA — this means the right ACA is underdeveloped and cannot adequately supply the right frontal and medial parietal territories. ## Circle of Willis Anatomy & Collateral Flow **Key Point:** The anterior communicating artery (AComm) is the only direct anastomosis between the two anterior cerebral circulations. When the right ACA is hypoplastic or occluded, the left ACA can cross via the left anterior communicating artery to supply the right ACA territory. **High-Yield:** In cases of unilateral ACA hypoplasia or occlusion, the contralateral ACA (via AComm) is the primary collateral route. This is because: 1. The AComm directly connects the two A1 segments (proximal ACA) 2. It allows cross-flow from the patent left ACA to the right side 3. This is more effective than posterior communicating artery (PComm) routes, which supply more distal territories ## Why Left AComm? The **left anterior communicating artery** allows blood from the left ACA (which is patent and unaffected) to flow across the midline and supply the right ACA distribution. This is the most direct and physiologically relevant collateral pathway in this scenario. ## Anatomical Reference Table | Communicating Artery | Connects | Primary Role in Collateral Flow | | --- | --- | --- | | Anterior Communicating (AComm) | Left A1 ↔ Right A1 | ACA territory cross-flow; most effective for ACA occlusion | | Posterior Communicating (PComm) | ICA ↔ PCA | Supplies distal MCA/PCA territories; less useful for proximal ACA | | Fetal PComm variant | Large PComm | Rare; may provide some collateral but not primary route | **Clinical Pearl:** Hypoplastic ACAs are present in ~5–10% of the population and are often clinically silent until an acute vascular event occurs. The contralateral ACA then becomes critical for survival of the affected territory. ## Why Not the Other Options? - **Right AComm (Option 0):** Would connect right A1 to right A2, but the right A1 is already occluded — no blood to cross over. - **Right PComm (Option 2):** Connects the ICA to the PCA; supplies more distal territories and is not the primary route for ACA territory compensation. - **Right PCA (Option 3):** Supplies the occipital and inferior temporal lobes; not involved in ACA territory supply.