Teaching NeuroImages: Pupil-sparing oculomotor nerve palsy with posterior communicating artery aneurysm

A 66-year-old woman presented with ptosis and diplopia 2 weeks prior. She showed ptosis and limited adduction, elevation, and depression in the left eye (figure, A). Her pupils were isocoric and reactive (figure, B). Transfemoral left internal carotid angiography (figure, C) revealed a 5-mm elongated aneurysm with inferior projection (arrows) at the posterior communicating artery origin. Contrast-enhanced thin-section T1-weighted coronal image (figure, D) showed the aneurysm (short arrows) compressing the left oculomotor nerve at the cavernous sinus. The right oculomotor nerve traversing in the cavernous sinus was normally well identified as a round low signal (long arrow). This case violates the "rule of the pupil" proven with pupillography.(1,2)