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.

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Figure 1 Compression of the left CNIII nerve by left posterior cerebral artery

(A, B) Isolated asymmetric atrophy of the left superior rectus muscle (arrows). (C) Left posterior cerebral artery in cross-section (arrow) impinges upon the left CNIII (arrowhead). (D, E) Asymmetric flattening of the left CNIII (arrowheads) by the left posterior cerebral artery (arrows).

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Figure 2 Illustration of the CNIII microanatomy (not to scale)

Image is constructed based on similar images in Brazis.³

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• © 2020 American Academy of Neurology