Teaching video NeuroImage: isolated medial rectus palsy in midbrain infarction.

A 76-year-old man with hypertension suddenly developed horizontal diplopia. Examination disclosed adduction deficit of left eye without dissociated abducting nystagmus and ocular tilt reaction (OTR) (video). The impaired adduction was similar for saccades, smooth pursuit, the vestibulo-ocular reflex, and convergence. Vertical eye movements were normal. Diffusion-weighted MRI demonstrated a circumscribed acute infarction in the left paramedian midbrain (figure). Isolated medial rectus palsy due to midbrain lesion is extremely rare1 and should be differentiated from internuclear ophthalmoplegia (INO). Impaired convergence, absent abducting nystagmus, and OTR in our patient support medial rectus palsy rather than INO.