Walsh & Hoyt: Types of Abnormal Eye Movements that Disrupt Stead Fixation: Nystagmus and Saccadic Intrusions

The essential difference between nystagmus and saccadic intrusions lies in the initial eye movement that takes the line of sight off the object of regard. For nystagmus, it is a slow drift (or ""slow phase""), as opposed to an inappropriate saccadic movement that intrudes on steady fixation. After the initial movement, corrective or other abnormal eye movements may follow. Thus, nystagmus may be defined as a repetitive, to-and-fro movement of the eyes that is initiated by a slow phase (drift). Saccadic intrusions, on the other hand, are rapid eye movements that take the eye off target. They include a spectrum of abnormal movements, ranging from single saccades to sustained saccadic oscillations.