Myasthenia gravis: dynamic changes in saccadic waveform, gain, and velocity.

The effects of refixation fatigue, maintained gaze fatigue, and intravenous edrophonium on saccadic waveform, gain, and velocity were studied in 10 patients with ocular myasthenia gravis. Refixation fatigue was minimal. Gaze maintenance had four separate but differing effects on waveform and gain; normalized peak velocities did not decrease. Edrophonium caused hypermetria and increased gain but normalized peak velocities were either unchanged or decreased. These effects represent, in part, selective impairment of the tonic, as distinct from phasic, fibers in the extraocular muscles and compensatory increase in central saccadic gain induced by the muscle weakness.