Vestibulo-ocular reflex pathways in internuclear ophthalmoplegia.

We measured the vestibulo-ocular reflex (VOR) during head impulses in a patient with right-sided internuclear ophthalmoplegia. Head impulses are rapid, passive, high-acceleration, low-amplitude head rotations in the direction of a particular semicircular canal (SCC). Adduction of the right eye was abnormally slow during right lateral SCC head impulses. The VOR during left posterior SCC impulses was severely deficient in both eyes, but the VOR during left anterior SCC impulses was only slightly deficient. We suggest that the vertical vestibulo-ocular pathways in humans are connected in SCC-plane coordinates, not the traditional roll and pitch coordinates, and that anterior SCC signals do not travel exclusively in the medial longitudinal fasciculus.