Positional nystagmus in the dark in normal subjects

Horizontal and vertical components of spontaneous nystagmus in the dark were assessed in 40 healthy subjects with monocular computerised video-oculography. Five different static head-in-space positions were used: sitting upright, face up, right ear down, left ear down, and face down; the head-on-trunk position being kept the same in each position. In the sitting position, four subjects (10%) had downbeat nystagmus (slow-phase velocity [SPV] 0.7 °/s, SD 0.27) and 12 (30%) had upbeat nystagmus (SPV 2.58 °/s, SD 1.38). In the supine position, three subjects (7.5%) had downbeat nystagmus (SPV 5.60 °/s, SD 6.72) and 25 (62.5%) had upbeat nystagmus (SPV 3.86 °/s, SD 4.54). The direction of nystagmus frequently changed with changes in head position. In 15 subjects, the behaviour of the vertical nystagmus was investigated during a slow 1.2 °/s, 360° revolution in pitch. All subjects had nystagmus at some point and the slow-phase velocity was modulated sinusoidally suggestive of gravitational modulation by otolithic signals with an offset towards upbeat nystagmus. Normal subjects have a physiological spontaneous nystagmus of low velocity in the dark, often upbeating, which is under the modulation of otolith input.