Impaired 'fixation suppression' of horizontal vestibular nystagmus during smooth pursuit: Pathophysiology and clinical implications.

BACKGROUND A peripheral spontaneous nystagmus (SN) is typically enhanced or revealed by removing fixation. Conversely, failure of fixation suppression of SN is usually a sign of a central disorder. Based on Luebke et al., (1988), who suggested the normal fixation mechanism is disengaged during pursuit, we hypothesized that vertical tracking in the light would bring out or enhance a horizontal SN. METHODS We studied 18 patients with acute vestibular neuritis (VN). Eye movements were recorded using video-oculography (VOG) at straight-ahead gaze with and without visual fixation, and during smooth pursuit. We compared the slow-phase velocity (SPV) and the fixation suppression indices of nystagmus (relative to SN in darkness) in each condition. RESULTS During vertical tracking the SPV of horizontal SN with eyes near straight-ahead gaze was significantly higher (median 2.7°/s) than under static visual fixation (median 1.2°/s). Likewise, the fixation index was significantly higher (worse suppression) during pursuit (median 48%) than during fixation (median 26%). A release of SN was also suggested during horizontal pursuit, if one assumes superposition of SN on a normal and symmetrical pursuit capability. CONCLUSIONS Visual suppression of horizontal SN during vertical pursuit was less effective than during static visual fixation, implying separate neural networks mediating visual fixation and pursuit. When horizontal pursuit appears relatively impaired; the degree may depend on how much an underlying horizontal SN is released during tracking. At the bedside, testing vertical pursuit may be a simple way to bring out or enhance a spontaneous horizontal vestibular nystagmus.