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)

An aneurysm at the junction of the posterior communicating artery and the internal carotid artery sits right next to the oculomotor nerve as it travels from the brainstem toward the eye. Because the nerve lies so close to that arterial bend, a enlarging aneurysm tends to press on the nerve from the outside. The parasympathetic fibers that control pupil constriction run on the outer part of the oculomotor nerve, so they are affected early, producing a dilated pupil. Along with this, the motor fibers to most extraocular muscles are compromised, leading to ptosis and a “down and out” eye position with diplopia. That pupil-involving oculomotor palsy is characteristic of a PCom–internal carotid junction aneurysm, which is why the oculomotor nerve is the most commonly compressed structure. The other nerves listed are not in the typical vicinity of this aneurysm, so they’re less likely to be affected in this specific lesion.

An aneurysm at the junction of the posterior communicating artery and the internal carotid artery sits right next to the oculomotor nerve as it travels from the brainstem toward the eye. Because the nerve lies so close to that arterial bend, a enlarging aneurysm tends to press on the nerve from the outside. The parasympathetic fibers that control pupil constriction run on the outer part of the oculomotor nerve, so they are affected early, producing a dilated pupil. Along with this, the motor fibers to most extraocular muscles are compromised, leading to ptosis and a “down and out” eye position with diplopia. That pupil-involving oculomotor palsy is characteristic of a PCom–internal carotid junction aneurysm, which is why the oculomotor nerve is the most commonly compressed structure. The other nerves listed are not in the typical vicinity of this aneurysm, so they’re less likely to be affected in this specific lesion.