A CN III aneurysm is most likely to occur at the junction between which two arteries?

A CN III aneurysm is most likely to occur at the junction between which two arteries?

• A. Posterior communicating and Internal Carotid
• B. Posterior cerebral and Basilar
• C. Middle cerebral and Internal Carotid
• D. Anterior cerebral and Internal Carotid

Answer: (A)

The key idea is where the oculomotor nerve is closest to an aneurysm. A berry aneurysm most often forms at the junction where the posterior communicating artery branches from the internal carotid artery. The oculomotor nerve runs in that vicinity as it travels toward the cavernous sinus, so an enlarging aneurysm at this junction can press on the nerve and produce a third-nerve palsy. Because the parasympathetic fibers run on the surface of CN III, they are commonly affected first, leading to pupil dilation along with ptosis and the eye resting down and out. Other arterial junctions listed are not the typical profitably compressing sites for CN III, so they don’t fit the classic presentation as well as the posterior communicating–internal carotid junction.

The key idea is where the oculomotor nerve is closest to an aneurysm. A berry aneurysm most often forms at the junction where the posterior communicating artery branches from the internal carotid artery. The oculomotor nerve runs in that vicinity as it travels toward the cavernous sinus, so an enlarging aneurysm at this junction can press on the nerve and produce a third-nerve palsy. Because the parasympathetic fibers run on the surface of CN III, they are commonly affected first, leading to pupil dilation along with ptosis and the eye resting down and out.

Other arterial junctions listed are not the typical profitably compressing sites for CN III, so they don’t fit the classic presentation as well as the posterior communicating–internal carotid junction.