Unsteady gaze fixation enhances the severity of MS intention tremor

Abstract

Patients with arm intention tremor due to multiple sclerosis (MS) often manifest eye movement deficits, illustrating the role of infratentorial brain in both ocular and manual movement control. Our previous study showed that both the amplitude of intention tremor and eye fixational movements were greatly enhanced after coordinated eye-hand action toward stationary targets vs during hand movements with continuous target fixation. The present study tested, during coordinated step-tracking movements, the hypothesis that the amplitude of hand intention tremor was influenced by (unsteady) gaze fixation onto the target. Simultaneously recorded eye and hand tracking movements were compared between 13 MS patients with intention tremor and 14 healthy controls over conditions in which the magnitude of the primary eye and hand tracking movements, as well as their ratio, were altered. Patients always made larger fixational eye movements around the visual targets than the controls. In the patient group, the size of fixational eye movements decreased following a reduction in the magnitude of the preceding saccadic movement, and most interestingly, was accompanied with a decrease in tremor amplitude. An alteration in the magnitude of the primary hand movement did not affect the tremor severity. In patients with multiple sclerosis with intention tremor, the unsteady gaze fixation on the visual targets is proportional to the magnitude of the preceding saccades, and influences the severity of intention tremor during eye-hand coordinated visuomotor tasks.