Disorders of binocular control of eye movements in patients with cerebellar dysfunction

Recent research has implicated the cerebellum in conjugate ocular motor control, including steady gaze-holding and accuracy of pursuit and saccades. Whether the cerebellum also has a role in the control of the alignment of the eyes during fixation and of the yoking of the eyes during movement i. less certain. We have studied binocular (disconjugate) ocular motor control in nine patients with cerebellar dysfunction and compared the results with those of normal subjects. Eye alignment during fixation and the yoking of the eyes during and immediately after saccades were quantified by recording the movements of both eyes using scleral search coils. Patients had disturbances of ocular alignment. All had an esophoria during monocular viewing and many an esotropia during binocular viewing, implying an increase in convergence tone. Most had a vertical misalignment that varied with horizontal eye position ('alternating skew deviation'). Patients showed conjugate dysmetria (saccade under- or overshoot and postsaccade drift) and disconjugate dysmetria (the eyes were poorly yoked during and immediately after saccades). Both the conjugate and disconjugate abnormalities were incommitant, i.e. they varied with orbital eye position. Correlations amongst the various abnormalities suggested that one part of the cerebellum, perhaps the dorsal vermis and the underlying posterior fastigial nucleus, controls the conjugate size of saccades and that another part of the cerebellum, perhaps the flocculus/paraflocculus, controls the yoking of the eyes during saccades and both the disconjugate and conjugate components of postsaccade drift.