TORSIONAL NYSTAGMUS DURING VERTICAL PURSUIT

We examined three patients with cavernous angioma within the middle ce rebellar peduncle. Each patient had an unusual ocular motor finding: t he appearance of a strong torsional nystagmus during vertical pursuit. The uncalled-for torsion changed direction when vertical pursuit chan ged direction. In one patient, we recorded eye movements with the magn etic field technique using a combined direction and torsion eye coil. The slow-phase velocity of the inappropriate torsional nystagmus was l inearly related to the slow-phase velocity of vertical smooth pursuit, and changed direction when vertical pursuit changed direction. This t orsional nystagmus also appeared during fixation suppression of the ve rtical vestibule-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 a nother ocular motor sign of cerebellar dysfunction. Furthermore, we sp eculate that the signals used for vertical smooth pur suit are, at som e stage, encoded in a semicircular canal VOR coordinate framework. To illustrate, for the vertical semicircular canals, vertical and torsion al motion are combined on the same cells, with the anterior semicircul ar canals mediating upward movements and the posterior semicircular ca nals mediating downward movements. For the right labyrinth, however, b oth vertical semicircular canals produce clockwise slow phases (ipsila teral eye intorts, contralateral eye extorts). The opposite is true fo r the vertical semicircular canals in the left labyrinth; counterclock wise slow phases are produced. Hence, to generate a pure vertical VOR, the anterior or posterior semicircular canals on both sides of the he ad must be excited so that opposite-directed torsional components canc el. Thus, if pursuit were organized in a way similar to the VOR, pure vertical pursuit would require that oppositely-directed torsional comp onents cancel in normals. If this did not happen, a residual torsional nystagmus could appear during attempted vertical pursuit.