Directional abnormalities of vestibular and optokinetic responses
in patients with cerebellar degeneration are reported. Three-axis
magnetic search-coil recordings of the eye and head were performed
in eight cerebellar patients. Among these patients, examples
of directional cross-coupling were found during (1) high-frequency,
high-acceleration head thrusts; (2) constant-velocity chair
rotations with the head fixed; (3) constant-velocity optokinetic
stimulation; and (4) following repetitive head shaking. Cross-coupling
during horizontal head thrusts consisted of an inappropriate
upward eye-velocity component. In some patients, sustained constant-velocity
yaw-axis chair rotations produced a mixed horizontal-torsional
nystagmus and/or an increase in the baseline vertical slow-phase
velocity. Following horizontal head shaking, some patients showed
an increase in the slow-phase velocity of their downbeat nystagmus.
These various forms of cross-coupling did not necessarily occur
to the same degree in a given patient; this suggests that different
mechanisms may be responsible. It is suggested that cross-coupling
during head thrusts may reflect a loss of calibration of brainstem
connections involved in the direct vestibular pathways, perhaps
due to dysfunction of the flocculus. Cross-coupling during constant-velocity
rotations and following head shaking may result from a misorientation
of the angular eye-velocity vector in the velocity-storage system.
Finally, responses to horizontal optokinetic stimulation included
an inappropriate torsional component in some patients. This
suggests that the underlying organization of horizontal optokinetic
tracking is in labyrinthine coordinates. The findings are also
consistent with prior animal-lesion studies that have shown
a role for the vestibulocerebellum in the control of the direction
of the VOR.
