Teaching NeuroImages: Pupil-sparing compression of oculomotor nerve by posterior cerebral artery vessel

A 54-year-old woman presented with a 30-year history of worsening left blepharoptosis. Neuro-ophthalmic examination was significant for blepharoptosis, limited levator function, and supraduction in the left eye with left hypotropia, and normal pupils without diplopia. MRI brain/orbits revealed subtle atrophy of the left superior rectus (figure 1, A and B), without any orbital pathology. Superior compression of the left oculomotor nerve by the posterior cerebral artery (PCA) was observed (figure 1, C-E). Given the microanatomy of oculomotor nerve in the cistern space,(1,2) this is a rare selective compression of the superolateral oculomotor nerve by the PCA vessel (figure 2), sparing the superomedial parasympathetic limb.