An aneurysm at the junction of the posterior communicating artery and the internal carotid artery most commonly compresses which cranial nerve?

An aneurysm at the junction of the posterior communicating artery and the internal carotid artery most commonly compresses which cranial nerve?

• A. CN II
• B. CN III
• C. CN IV
• D. CN VI

Answer: (B)

The main concept is the close relationship between the posterior communicating artery and the oculomotor nerve. An aneurysm at the PCom-ICA junction tends to press on CN III as it courses toward the orbit, producing an oculomotor palsy. This manifests as ptosis, a down-and-out eye position, and pupil dilation due to involvement of the parasympathetic fibers that control the pupil and ciliary muscle. The optic nerve, trochlear nerve, and abducens nerve aren’t in that immediate neighborhood, so they’re not the typical structures compressed by an aneurysm at this site.