A Comparison of Nystagmus and Saccadic Intrusions/Oscillations

Nystagmus can be classified into pendular and jerk waveforms, where both are generated by a slow, pathologic phase. Corrective phase (the position reset mechanism) differs. In pendular nystagmus, the eyes move back and forth with about the same velocity and amplitude, similar to that of a pendulum. Jerk nystagmus can have constant, increasing or decreasing slow phase velocity: vestibular nystagmus has a linear slow phase velocity due to vestibulo-ocular reflex imbalance; gaze-evoked nystagmus has a decreasing slow phase velocity due to a leaky neural integrator (the gaze-holding machinery); congenital nystagmus and certain posterior fossa disorders can cause an increasing slow phase velocity due to an unstable neural integrator. In jerk nystagmus, the slow (pathologic) phase is followed by the fast (named) phase. Nystagmus should be distinguished from oscillations and intrusions, particularly given disparate localizations and etiologies. These include saccadic intrusions (e.g., square wave jerks) and saccadic oscillations (e.g., ocular flutter, opsoclonus). Saccadic intrusions consist of saccades with an intersaccadic interval (i.e., a brief pause between movements), while saccadic oscillations lack this interval. Square wave jerks are small saccades that move the line of sight away from the target and then return to fixation 200 ms later. Macrosaccadic oscillations are saccades that take the eye off target and oscillate the line of sight about the object of regard - these movements straddle fixation, and are related to hypermetria. Ocular flutter is confined to the horizontal vector whereas opsoclonus has horizontal, vertical and torsional components, and occurs in bursts; however, the differential diagnosis, implications, and treatment of flutter and opsoclonus are essentially the same.