Saccadic duration and intrasaccadic fatigue in myasthenic and nonmyasthenic ocular palsies

We measured the duration and amplitude of saccades in three normal subjects, eight patients with myasthenia gravis, and eight patients with nonmyasthenic ocular palsies. Saccades were examined at the start of a repetitive saccade task, after 3 minutes of activity, and 1 minute after administration of edrophonium. The duration of saccades was prolonged initially in both myasthenic and nonmyasthenic palsies. Activity did not produce significant differences among the three groups in either the slope of the duration-amplitude relationship or the predicted durations of saccades of 5 degrees, 10 degrees, or 15 degrees. However, durations decreased in myasthenia but increased in nonmyasthenic palsies after edrophonium. Much of this decrease in myasthenic saccadic duration was due to reduction in deceleration time, indicating resolution of intrasaccadic fatigue after edrophonium administration. However, the relation of deceleration fraction (deceleration time divided by total duration) to total duration remained constant in all subject groups. Analysis of saccadic duration is a useful means of interpreting responses to edrophonium because it incorporates data from saccades of a wide range of amplitudes into a linear relation between duration and amplitude.