Torsional nystagmus during vertical pursuit.

We examined three patients with cavernous angioma within the middle cerebellar peduncle. Each patient had an unusual ocular motor finding: the appearance of a strong torsional nystagmus during vertical pursuit. The uncalled-for torsion changed direction when vertical pursuit changed direction. In one patient, we recorded eye movements with the magnetic field technique using a combined direction and torsion eye coil. The slow-phase velocity of the inappropriate torsional nystagmus was linearly related to the slow-phase velocity of vertical smooth pursuit, and changed direction when vertical pursuit changed direction. This torsional nystagmus also appeared during fixation suppression of the vertical vestibulo-ocular reflex (VOR), but was minimal during vertical head rotation when fixing a stationary target in the light. We suggest that inappropriately directed eye movements during pursuit might be another ocular motor sign of cerebellar dysfunction. Furthermore, we speculate that the signals used for vertical smooth pursuit are, at some stage, encoded in a semicircular canal VOR coordinate framework. To illustrate, for the vertical semicircular canals, vertical and torsional motion are combined on the same cells, with the anterior semicircular canals mediating upward movements and the posterior semicircular canals mediating downward movements. For the right labyrinth, however, both vertical semicircular canals produce clockwise slow phases (ipsilateral eye intorts, contralateral eye extorts). The opposite is true for the vertical semicircular canals in the left labyrinth; counterclockwise slow phases are produced. Hence, to generate a pure vertical VOR, the anterior or posterior semicircular canals on both sides of the head must be excited so that opposite-directed torsional components cancel. Thus, if pursuit were organized in a way similar to the VOR, pure vertical pursuit would require that oppositely-directed torsional components cancel in normals. If this did not happen, a residual torsional nystagmus could appear during attempted vertical pursuit.