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.