CBL: Painful Third Nerve Palsy — Suspected PCoA Aneurysm

Young patient with retro‑orbital headache progressing to ptosis, mydriasis, and impaired adduction — what’s the diagnosis and next step?

Topic: Vascular / Neuro-ophthalmology Level: Resident Tags: aneurysm, PCoA, CN III palsy, CTA/MRA

Vignette

A 28-year-old line chef presents with a new throbbing left retro‑orbital headache. Initial MRI brain is normal; trials of butalbital and amitriptyline provide no relief. One week later she develops left eyelid droop with persistent periorbital pain and comes to the ED.

Exam: BP 144/86, pulse 84. Uncomfortable but alert. Left ptosis; left pupil > right; impaired adduction of the left eye. Other cranial nerves intact. Motor, reflexes, sensation, coordination, and gait are normal.

Question 1

Which localization best explains this presentation?

  • A. Cavernous sinus affecting CN V1/V2
  • B. Subarachnoid segment of left CN III near the PCoA
  • C. Neuromuscular junction (myasthenia gravis)
  • D. Brainstem (oculomotor nucleus)

Question 2

What is the most appropriate next diagnostic step?

  • A. Observation and outpatient follow‑up in 1–2 weeks
  • B. CTA or MRA of the head/neck urgently
  • C. Start steroids and obtain orbital ultrasound
  • D. Routine MRI brain without vascular imaging

Question 3

Which management statement is most accurate?

  • A. All unruptured aneurysms ≥3 mm require immediate repair
  • B. Anterior‑circulation aneurysms <7 mm without high‑risk features are often observed with interval imaging
  • C. Nimodipine prevents rupture of unruptured aneurysms
  • D. Posterior circulation location does not affect treatment threshold

Question 4

A patient’s sister reports their father died from a cerebral aneurysm at 54. What screening advice is best?

  • A. Screen only if two or more first‑degree relatives are affected, or in high‑risk conditions (e.g., ADPKD); otherwise individualize
  • B. Everyone with one affected relative should have immediate DSA
  • C. Begin prophylactic nimodipine
  • D. No one requires screening regardless of family history

Teaching Points

  • Painful, pupil‑involving CN III palsy = compressive lesion until proven otherwise (classically PCoA aneurysm).
  • Obtain urgent CTA/MRA; DSA is gold standard and often precedes endovascular therapy.
  • Unruptured aneurysm management: weigh size, location, morphology, growth, age/comorbids, and patient values.
  • After SAH: treat aneurysm (coiling/clipping), give nimodipine, control BP, and monitor for delayed cerebral ischemia.

References

  1. Consensus reviews on unruptured intracranial aneurysms and subarachnoid hemorrhage management; neuroradiology texts on CTA/MRA/DSA.