A 32-year-old woman presents with sudden onset diplopia and right ptosis. Examination reveals right eye with mydriasis, abducted and slightly depressed position, with failure of adduction, elevation, and depression. The light reflex is absent in the right eye. CT brain is unremarkable. MRI with gadolinium is urgently requested. The MOST likely diagnosis and the key MRI finding expected is:
- A Right CN III palsy from pituitary macroadenoma; sellar expansion with chiasmal compression
- B Diabetic CN III palsy; no significant MRI abnormality expected
- C Right posterior communicating artery aneurysm compressing CN III; enhancement of the aneurysm wall or flow void ✓
- D Cavernous sinus thrombosis; enhancement of the cavernous sinus with filling defect
Correct answer: C. Right posterior communicating artery aneurysm compressing CN III; enhancement of the aneurysm wall or flow void
Explanation
The PUPIL-INVOLVING CN III palsy (mydriasis + complete third nerve dysfunction) in a young patient without diabetes is a neurosurgical emergency until proven otherwise — it strongly suggests posterior communicating (PComm) artery aneurysm. The pupillomotor fibers travel on the outer periphery of CN III and are first compressed by an expanding aneurysm. CT brain can miss aneurysms <3 mm; MRI/MRA or conventional angiography is mandatory. Diabetic CN III palsy characteristically spares the pupil because microvascular ischemia affects the central fascicles first.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.