Caloric and optokinetic nystagmus in cerebellar patients

In certain cerebellar lesions, optokinetic tracking is deficient, especially on tracking or following to the ipsilateral side. However, in many cases optokinetic nystagmus can occur, and apparently in better degree than would be expected from a study of pendulum tracking. This may be due to the patient leading into the stimulus and picking it up as soon as it is presented in far lateral gaze (in the direction of the fast component) and allowing the drift caused by the tone deficit to assist the tracking. An added vestibular tone can eliminate such a pseudo reflex. Induced vestibular nystagmus is enhanced and assumes a more dominant role so that optokinetic nystagmus is overcome, not only on gaze to the side of the tracking, but also on straight ahead gaze and even on some deviation of the eyes in the opposite direction.