Eyetracking-enhanced VEP for nystagmus

Abstract

Visual evoked potentials (VEPs) are an important prognostic indicator of visual ability in patients with nystagmus. However, VEP testing requires stable fixation, which is impossible with nystagmus. Fixation instability reduces VEP amplitude, and VEP reliability is therefore low in this important patient group. We investigated whether VEP amplitude can be increased using an eye tracker by triggering acquisition only during slow periods of the waveform. Data were collected from 10 individuals with early-onset nystagmus. VEP was obtained under continuous (standard) acquisition, or triggered during periods of low eye velocity, as detected by an eye tracker. VEP amplitude was compared using Bonferroni corrected paired samples t-tests. VEP amplitude is significantly increased when triggered during low eye velocity (95% CI 1.42-6.83 µV, t(15) = 3.25, p = 0.0053). This study provides proof-of-concept that VEP amplitude (and therefore prognostic reliability) can be increased in patients with early onset nystagmus by connecting an eye tracker and triggering acquisition during periods of lower eye velocity.

Figure 1

Amplitudes obtained for onset-offset VEPs using both standard and velocity-based triggering.

Figure 2

Amplitudes obtained for pattern reversal VEPs using both standard and velocity-based triggering. Note that P02 and P04 are coincident.

Figure 3

Example data (from P10) showing averaged onset-offset VEP waveform under (a) continuous triggering and (b) velocity triggering, and (c) eye speed trace with vertical dashed lines indicating trigger times. Note that VEP acquisition occurs ~ 24 ms after trigger.