A patient with a CN III palsy has a 'down and out' eye with a mid-dilated non-reactive pupil. The MOST urgent investigation to exclude a life-threatening cause is:

• A MRI brain with gadolinium enhancement
• B Lumbar puncture for CSF xanthochromia
• C CT angiography (CTA) of the head ✓
• D Carotid Doppler ultrasound

Explanation

A pupil-involving CN III palsy must be treated as 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 extrinsic compression (aneurysm, uncal herniation). CT angiography of the Circle of Willis provides rapid, highly sensitive (>95%) assessment for aneurysm and is preferred over MRI angiography in acute settings due to speed. If CTA is negative but clinical suspicion remains high, DSA (digital subtraction angiography) is the gold standard. A 'pupil-sparing' CN III palsy in a diabetic patient suggests ischaemic aetiology.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.