Photo essay. Isolated fascicular third nerve palsy.

A 72-year-old hypertensive woman developed acute diplopia and right upper lid ptosis. Examination showed a partial right third nerve palsy with complete ptosis, moderately impaired adduction and supraduction, and sparing of infraduction and the pupil. There were no other neurologic deficits. Brain MRI with diffusion weighting demonstrated an infarct in the right paramedian midbrain tegmentum along the course of the third nerve fascicles. The clinical deficit and neuroimaging in this case provide further support for a previously suggested topographic organization of the fascicles of the third nerve within the brainstem.