Mechanisms of nystagmus.

The slow phases of nystagmus can be divided into four types based on their wave forms: linear, exponentially decaying, exponentially increasing, and sinusoidal. The former three give risk to "jerk" nystagmus; the last to "pendular" nystagmus. The drift of the eyes that creates the slow phase of nystagmus can be analyzed by considering the mechanisms that normally hold the eyes still so that images are stable upon the retina for best visual acuity. Thus, we have a vestibular-optokinetic system to keep images of stationary objects stable on the retina during head rotation; a pursuit system to keep images of moving objects stable on the fovea during smooth trackings; and a gaze-holding network (neural integrator) to provide the appropriate tonic innervation to the ocular motor neurons to hold positions of gaze. It is disorders of these systems that create nystagmus.