A 52-year-old woman presents to the emergency department with a sudden-onset, severe "thunderclap" headache described as the worst of her life, accompanied by neck stiffness and photophobia. Non-contrast CT head performed within 2 hours shows hyperdense material in the basal cisterns consistent with subarachnoid hemorrhage. CT angiogram reveals a saccular aneurysm at the structure marked **A** in the Circle of Willis diagram. Which of the following clinical findings would be MOST SPECIFIC to rupture of an aneurysm at this location?

Explanation

## Why "Bilateral leg weakness and abulia due to anterior cerebral artery territory ischemia" is right The anterior communicating artery (AComm) is the most common site of saccular aneurysm formation in the Circle of Willis (~30–35% of all intracranial aneurysms). Rupture causes subarachnoid hemorrhage with the classic "thunderclap" presentation. Critically, aneurysms at the AComm are supplied by and communicate with the anterior cerebral arteries (ACAs). When an AComm aneurysm ruptures or causes vasospasm, the ACA territories are at risk for ischemia, resulting in bilateral leg weakness (motor cortex involvement) and abulia or akinetic mutism (frontal lobe dysfunction). These neurologic findings are pathognomonic for AComm aneurysm rupture and distinguish it from other Circle of Willis aneurysms (Gray's Anatomy 42e Ch 28; Harrison 21e Ch 425). ## Why each distractor is wrong - **Homonymous hemianopia and contralateral hemiparesis due to posterior cerebral artery involvement**: This pattern is typical of posterior communicating artery (PComm) aneurysm rupture, which affects the PCA territory. PComm is the second most common site (~25%) but does not cause bilateral leg weakness or abulia. - **Ipsilateral facial droop and contralateral hemiplegia due to internal capsule infarction**: This presentation is characteristic of middle cerebral artery (MCA) aneurysm rupture (~20% of intracranial aneurysms), which affects the MCA territory and internal capsule. It does not produce the ACA-specific findings of bilateral leg weakness and abulia. - **Vertigo and ipsilateral ataxia due to vertebrobasilar insufficiency**: This pattern would suggest basilar artery or posterior circulation pathology, not an AComm aneurysm. The basilar artery is not a common site of saccular aneurysm formation in the Circle of Willis. **High-Yield:** AComm aneurysm rupture → bilateral leg weakness + abulia/akinetic mutism (ACA territory); PComm → homonymous hemianopia (PCA); MCA → contralateral hemiplegia (MCA territory). [cite: Gray's Anatomy 42e Ch 28; Harrison 21e Ch 425]