Monocular downbeat nystagmus.

Two patients with acute unilateral infarction of the medial thalamus and upper mesencephalon exhibited ipsilateral nuclear involvement of the third nerve, contralateral skew deviation with weakness of eye elevation, and monocular downbeat nystagmus. Monocular downbeat nystagmus is a rare manifestation of combined nuclear-supranuclear ophthalmoparesis that is seemingly secondary to dysfunction of cerebellar-modulated crossed oculovestibular fibers of the brachium conjunctivum, integrator neurons, or posterior commissure crossing fibers originating at the interstitial nucleus of Cajal and mediating vertical ocular reflexes.