Upbeat-torsional nystagmus and contralateral fourth-nerve palsy due to unilateral dorsal ponto mesencephalic lesion.

The central projections of the anterior semicircular canals are thought to be conveyed from the vestibular nuclei to the oculomotor nuclei in the midbrain by three distinct brainstem pathways: the medial longitudinal fasciculus, crossing ventral tegmental tract, and brachium conjunctivum. There is controversy as to whether upbeat nystagmus could result from lesions involving each of these pathways. We report a 52-year-old man who presented with a contralesional fourth-nerve palsy and primary-position upbeat-torsional nystagmus due to a small unilateral dorsal pontomesencephalic lymphomatous deposit. It is postulated that the upbeat-torsional nystagmus was caused by involvement of the brachium conjunctivum, which lies adjacent to the fourth-nerve fascicles at the dorsal pontomesencephalic junction, but involvement of the crossing ventral tegmental tract cannot be excluded. These observations suggest that, in humans, excitatory upward-torsional eye movement signals from the anterior semicircular canals could be partly conveyed to the midbrain by the brachium conjunctivum.