Nystagmus is a rhythmic, repetitive oscillation of the eyes, initiated by a slow eye movement that drives the eye off target, followed by a fast movement that is corrective (jerk nystagmus) or another slow eye movement in the opposite direction (pendular nystagmus). Saccadic intrusions (opsoclonus and flutter) are abnormal rapid eye movements (saccades) that have no slow phase. All such eye movements disrupt fixation and may interfere with vision.
Nystagmus may occur physiologically in response to an environmental stimulus or change in body position. It is also seen with diseases of the central nervous system or peripheral vestibular system and in some cases of visual loss.