Saccadic dynamic overshoot in normals and patients

Abstract

Dynamic overshoot (DO) of saccades was investigated in 17 normal subjects and 24 patients with central nervous lesions affecting the cerebellum and its connecting pathways, in order to detect and to define a pathological pattern of DO. Eye movements were recorded binocu-larly with the infrared reflection method. Normal subjects showed a high incidence of DO (average: 86% of all saccades) with an average amplitude of 0.16 deg (range of individual averages 0.08 to 0.41 deg). In 52% of all saccades DO occurred in both eyes and in 34% in one eye only. As in normals, the incidence of DO was too variable in patients to be used as a diagnostic tool. Six patients (out of 24) were considered to have pathological DO: their DO amplitudes (range of averages 0.57-0.99 deg) and/or left-right asymmetries exceeded the range of normal subjects. Three had multiple sclerosis, one a hereditary ataxia, one Friedreich's ataxia and one Wallenberg's syndrome.

DO is considered to result from an alteration in the saccade braking (deceleration) pulse of immediate premotor structures in the brainstem. Recent experimental evidence suggests that the deceleration of saccades is also under the influence of the cerebellum (more specifically the fastigial nucleus and vermis). A comparison of saccadic dysmetria, often a pathological feature in cerebellar patients, with DO shows that they are not specifically interrelated to saccadic dysmetria. We suggest that pathological DO is probably not related to cerebellar dysfunction.

Key Words:

• saccades
• dynamic overshoot
• cerebellum
• brainstem