A CT angiogram of the brain in a 55-year-old with sudden severe headache ('thunderclap') shows subarachnoid blood in the basal cisterns. The CT angiogram identifies a saccular aneurysm at the junction of the internal carotid artery and posterior communicating artery (PCoA). Which cranial nerve palsy is classically associated with this location of aneurysm?
- A Third cranial nerve (oculomotor nerve) palsy with pupil involvement ✓
- B Sixth nerve palsy with lateral gaze paralysis
- C Fourth nerve palsy with vertical diplopia
- D Second nerve palsy with visual loss
Correct answer: A. Third cranial nerve (oculomotor nerve) palsy with pupil involvement
Explanation
An aneurysm at the ICA-posterior communicating artery junction classically compresses the third cranial nerve (CN III) from outside, producing a 'surgical' CN III palsy characterised by ptosis, ophthalmoplegia, and dilated unreactive pupil (pupil involvement). The pupillomotor fibres run on the periphery of CN III, making them vulnerable to extrinsic compression. Medical CN III palsy (from ischaemia) typically spares the pupil.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.