Study title

Importance: Visual evoked potential (VEP) testing is an important part of a diagnostic workup and prognostic indicator of visual ability in patients with early onset nystagmus. However, VEP testing requires stable fixation, which is impossible for those with nystagmus. Fixation instability is believed to reduce VEP amplitude, and therefore VEP reliability is low in this important patient group. Objective: To determine whether VEP amplitude can be increased by triggering acquisition only during slow periods of the nystagmus waveform. Design: Cross-sectional study. Setting: Data were collected at the Visual Electrodiagnostic Department at University Hospital Southampton, with clinical data from referring clinicians. Participants: Thirteen individuals with nystagmus (10 early onset nystagmus and three late onset nystagmus). Exposure: VEP obtained under continuous (standard) acquisition, or triggered during periods of low eye velocity, as detected by an eye tracker. Trigger velocity threshold was determined per-participant. Main outcome and measure: VEP amplitude. Results: VEP amplitude is significantly increased when triggered during low eye velocity in patients with early onset nystagmus (95% CI 1.42-6.83 µV, t(15) = 3.25, p = .0053), but not in patients with late onset nystagmus (95% CI -3.92-11.59 µV, t(5) = 1.27, p = .26). Conclusions and relevance: This study provides proof-of-concept that VEP amplitude (and therefore prognostic reliability) can be increased in patients with early onset nystagmus by triggering acquisition during periods of lower eye velocity.