Slow target-directed eye movements in ataxia-telangiectasia

PURPOSE. To analyze the slow eye movements that shift the direction of gaze in patients with ataxia-telangiectasia (A-T). METHODS. Eye and head movements were recorded with search coils in three patients with A-T during attempted gaze shifts, both with the head immobilized and free to move. RESULTS. Gaze shifts frequently included both saccadic and slow components. The slow movements were recorded after 42% of saccades and had an average peak velocity of 6.1 deg/sec and a mean amplitude of 2.0°. They occurred with the head stationary and moving, could be directed centripetally or centrifugally, had velocity waveforms that were relatively linear or exponential, and always moved the eyes toward the visual target. CONCLUSIONS. The slow movements appear to differ from pursuit and vestibular eye movements and are not fully explained by the various types of abnormal eye movements that can follow saccades, such as gaze-evoked nystagmus or postsaccadic drift. Their origin is uncertain, but they could represent very slow saccades, due to aberrant inhibition of burst cell activity during the saccade.