A 72-year-old woman from Mumbai is admitted with acute onset vertigo, nausea, and ataxia. On examination, she has nystagmus, loss of pain and temperature sensation over the right face and left body (crossed sensory loss), and right-sided Horner syndrome. MRI brain shows acute infarction in the right lateral medulla. The radiologist notes that the right vertebral artery is severely stenosed. Which artery within the circle of Willis would be the most critical collateral source to maintain brainstem perfusion in this patient?

Explanation

## Posterior Circulation and Collateral Supply in Vertebral Artery Stenosis **Key Point:** The posterior communicating artery (PComm) is the critical collateral pathway between the anterior and posterior circulation. In vertebral artery stenosis, the PComm allows blood from the internal carotid artery (ICA) to reach the posterior cerebral artery (PCA) and, via retrograde flow through the basilar artery, to perfuse the brainstem. ### Circle of Willis: Posterior Circulation Components | Component | Origin | Function | Collateral Role | | --- | --- | --- | --- | | Vertebral arteries (paired) | Subclavian artery | Merge to form basilar artery | Primary blood supply to brainstem | | Basilar artery | Vertebral artery confluence | Supplies pons, midbrain, cerebellum | Central brainstem perfusion | | Posterior cerebral arteries (PCAs) | Basilar artery bifurcation | Supply occipital, temporal lobes | Part of circle of Willis | | Posterior communicating arteries (PComm) | ICA to PCA | **Connects anterior and posterior circulation** | **Critical collateral in vertebral stenosis** | ### Wallenberg Syndrome (Lateral Medullary Infarction) This patient presents with classic **Wallenberg syndrome** (lateral medullary syndrome): **Clinical Features:** - **Ipsilateral (right):** Facial pain/temperature loss, Horner syndrome, ataxia - **Contralateral (left):** Body pain/temperature loss (crossed sensory loss) - **Vestibular:** Nystagmus, vertigo **Cause:** Occlusion of the **posterior inferior cerebellar artery (PICA)** or the vertebral artery itself, as in this case. **High-Yield:** Wallenberg syndrome is the most common brainstem stroke syndrome. Vertebral artery stenosis accounts for ~30% of cases; the remainder are due to PICA occlusion or vertebral artery dissection. ### Why PComm is the Critical Collateral ```mermaid flowchart TD A[Right ICA]:::outcome -->|normal flow| B[Right PCA] A -->|via PComm| C[Right PCA - Collateral source]:::action D[Right Vertebral Artery - Stenosed]:::urgent --> E[Basilar Artery - Reduced flow]:::urgent C -->|retrograde via PComm| F[Basilar Artery - Collateral perfusion]:::action F --> G[Brainstem supplied via basilar perforators]:::outcome H[Right PICA - Occluded]:::urgent --> I[Right lateral medulla - Infarction]:::urgent ``` **Mechanism of Collateral Flow:** 1. Right vertebral artery is stenosed → reduced flow into basilar artery 2. Right ICA blood flows normally into right PCA 3. The posterior communicating artery allows **retrograde flow** from the right PCA back into the basilar artery 4. This retrograde flow through the PComm helps maintain basilar artery perfusion pressure 5. Basilar perforating arteries can then supply the brainstem (pons, midbrain) and medulla **Clinical Pearl:** The posterior communicating artery is often hypoplastic (present in ~70% of people but <1 mm diameter in ~30%). Patients with hypoplastic PComm are at higher risk for posterior circulation stroke when vertebral artery stenosis develops, because this critical collateral is inadequate. **Mnemonic:** **PComm = Posterior Circulation Connector** — it is the only direct bridge between the anterior circulation (ICA) and the posterior circulation (PCA/basilar artery) within the circle of Willis. [cite:Snell's Anatomy Ch 7 - Cerebral Circulation; Harrison 21e Ch 435 - Cerebrovascular Diseases]