A 35-year-old woman develops sudden severe headache ('thunderclap'), third nerve palsy with a dilated, unreactive pupil (pupil-involving CN III palsy). The most urgent investigation and likely diagnosis are:
- A MRI brain with gadolinium; likely cavernous sinus thrombosis
- B Glucose and HbA1c; likely diabetic microvascular CN III palsy
- C Non-contrast CT head followed by lumbar puncture if CT negative; likely posterior communicating artery aneurysm with subarachnoid haemorrhage ✓
- D Orbital MRI; likely orbital apex syndrome
Correct answer: C. Non-contrast CT head followed by lumbar puncture if CT negative; likely posterior communicating artery aneurysm with subarachnoid haemorrhage
Explanation
A pupil-involving third nerve palsy with thunderclap headache is a posterior communicating artery (PCoA) aneurysm until proven otherwise. The pupillomotor fibres travel on the outer surface of CN III and are compressed first by external mass lesions such as an aneurysm. Urgent NCCT is the first investigation (detects SAH in 95% within 6 hours); if CT is negative, lumbar puncture is performed to detect xanthochromia. Diabetic CN III palsy classically spares the pupil (ischaemia affects the central core first, sparing the peripheral pupillomotor fibres). Delay is fatal.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.