A 58-year-old hypertensive man presents to the emergency department with sudden-onset "thunderclap" headache, neck stiffness, and photophobia. Non-contrast CT head shows blood in the subarachnoid space. Angiography reveals a saccular aneurysm at the junction of the internal carotid artery and the structure marked **C** in the Circle of Willis diagram. Which clinical presentation is most characteristic of aneurysms at this location prior to rupture?

Explanation

## Why "Painful oculomotor nerve palsy with pupil-involving mydriasis (down-and-out eye and ptosis)" is right The structure marked **C** is the Posterior Communicating Artery (PCOM), which is the second most common site (30%) for berry aneurysms in the Circle of Willis after the anterior communicating artery. PCOM aneurysms have a classic and highly specific pre-rupture presentation: painful oculomotor nerve (CN III) palsy with pupil involvement (mydriasis), causing a down-and-out eye, ptosis, and pain. This occurs because the PCOM aneurysm compresses the ipsilateral CN III nerve as it passes nearby. The pupil-involving pattern distinguishes this compressive lesion from ischemic CN III palsy (which spares the pupil). This presentation is a neurosurgical emergency requiring urgent angiography and intervention. (Sutton Radiology; Harrison 21e Ch 426) ## Why each distractor is wrong - **Contralateral facial and arm weakness with expressive aphasia**: This is the classic presentation of Middle Cerebral Artery (MCA) territory stroke or MCA aneurysm, not PCOM. The MCA is marked as **B** in the diagram and supplies the motor cortex for face and arm. - **Contralateral homonymous hemianopia with visual hallucinations**: This is the presentation of Posterior Cerebral Artery (PCA) territory stroke or aneurysm, marked as **D** in the diagram. PCOM aneurysms do not classically cause visual field defects. - **Bilateral lower limb weakness with urinary incontinence**: This pattern suggests anterior cerebral artery (ACA) territory involvement (marked as **A**), which supplies the medial motor cortex for the legs. PCOM aneurysms do not cause this syndrome. **High-Yield:** PCOM aneurysm = painful CN III palsy with pupil involvement (pupil-involving = compressive, not ischemic); this is a pre-rupture warning sign requiring emergency angiography and intervention. [cite: Sutton Radiology; Harrison 21e Ch 426]