Positional nystagmus in the dark in normal subjects

Horizontal and vertical components of spontaneous nystagmus in the dark wer e assessed in 40 healthy subjects with monocular computerised video-oculogr aphy. Five different static head-in-space positions were used: sitting upri ght, face up, right ear down, left ear down, and face down; the head-on-tru nk position being kept the same in each position. In the sitting position, four subjects (10%) had down-beat nystagmus (slow-phase velocity [SPV] 0.7 degrees /s, SD 0.27) and 12 (30%) had upbeat nystagmus ( SPV 2.58 degrees / s, SD 1.38). In the supine position, three subjects (7.5%) had downbeat nys tagmus (SPV 5.60 degrees /s, SD 6.72) and 25 (62.5%) had upbeat nystagmus ( SPV 3.86 degrees /s, SD 4.54). The direction of nystagmus frequently change d with changes in head position. In 15 subjects, the behaviour of the verti cal nystagmus was investigated during a slow 1.2 degrees /s, 360 degrees re volution in pitch. All subjects had nystagmus at some point and the slow-ph ase velocity was modulated sinusoidally suggestive of gravitational modulat ion by otolithic signals with an offset towards upbeat nystagmus. Normal su bjects have a physiological spontaneous nystagmus of low velocity in the da rk, often upbeating, which is under the modulation of otolith input.