On CT angiography, a patient with SAH has an aneurysm at the junction of the posterior communicating artery (PComA) and the internal carotid artery. The aneurysm is pointing postero-inferiorly. This direction of pointing indicates risk of:
- A Ipsilateral optic nerve compression and visual field defect
- B Contralateral hemiplegia due to mass effect on corticospinal tract
- C Ipsilateral oculomotor (CN III) nerve palsy with pupil involvement ✓
- D Trochlear nerve palsy and Parinaud syndrome
Explanation
PComA aneurysms are the classic cause of surgical CN III palsy with pupil involvement. The oculomotor nerve runs alongside the PComA from its origin at the ICA junction; when the aneurysm points postero-inferiorly and enlarges or ruptures, it compresses the superficially located pupillomotor fibers of CN III, causing complete III palsy with ptosis, mydriasis, and 'down and out' eye. This is the paradigmatic 'surgical pupil' — the pupil-sparing palsy indicates microvascular ischemia (e.g., diabetes). Visual symptoms would suggest ophthalmic artery or optic chiasm compression. Trochlear palsy is associated with dorsal midbrain lesions.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.