The Mechanism of Downbeat Nystagmus

Eye movements were recorded from three patients with downbeat nystagmus. Unlike vestibular nystagmus, slow phase velocity did not change when fixation was eliminated. Patients could track upward smoothly without nystagmus, but tracking downward was saccadic. However, two patients who had Arnold-Chiari malformations produced brisk down slow phases of nystagmus during vertical head rotation. To better interpret the patients' performance, the pursuit, saccadic, and vestibular oculomotor control systems were modeled on a computer. Downbeat nystagmus was best simulated by assuming that down velocity information could not be transmitted to the neural integrator that generates smooth eye movements in response to visual system commands (a pursuit system defect). The pathology of downbeat nystagmus suggests that the archicerebellum or lower brain stem is important for smooth pursuit as well as other types of eye movements.