Nystagmus is involuntary eye oscillations initiated by slow eye movements that drive the eye away from the target. In contrast, saccadic dyskinesia consists of involuntary, fast eye movements that take the fovea off target. Nystagmus usually arises from lesions in the 1. Vestibulo-ocular system (VOR) 2. Gaze-holding system 3. Smooth pursuit and optokinetic system. 1. Pendular versus jerk ■ Pendular (see A in the figure below): both phases are slow eye movements. ■ Jerk (see B, C, and D in the figure below): one phase consists of fast eye movements (quick phase), and the other consists of slow eye movements. By convention, the direction of nystagmus is named after the direction of quick phases that return the eye to the target. 2. Plane: horizontal, vertical, torsional, or combined form (e.g., rotary, elliptical) 3. Conjugacy ■ Conjugate: Both eyes move in the same direction with similar amplitude and frequency. ■ Disconjugate: Both eyes move in the same direction with different amplitude and frequency (e.g., internuclear ophthalmoplegia). ■ Disjunctive: The eyes move in opposite directions (e.g., oculomasticatory myorhythmia seen in Whipple’s disease).
Development of the optokinetic system in macaque monkeys