Variability, repeatability and test-retest reliability of equine flash visual evoked potentials (FVEPs)

Abstract

Background: Visual evoked potentials (VEPs) are electrical potentials generated by neurons in the central nervous system in response to visual stimuli. A series of positive and negative wavelets in response to flash-stimuli (flash-VEP; FVEP) or reversing, iso-luminant patterns (pattern-VEP; PVEP) are recorded. Pathological conditions affecting the post-retinal pathways can alter overall waveform morphology, and also affect wavelet peak times and amplitudes. FVEPs have recently been described in horses, but more data on the variability within and between subjects is required, to adequately interpret results from clinical equine patients. Therefore, the purpose of this study was to describe the variability, repeatability and test-retest reliability of equine FVEPs in normal, adult horses.

Results: Equine FVEPs were recorded from one randomly selected eye in 17 horses, from both eyes in eight of these horses, and also at two separate recording sessions in six horses. N1, P2, N2 and P4 wavelets were present in 100% of the recordings in all horses, while P1, N2a, P3 and P5 were only present in some recordings. Coefficients of variation (CVs) were low for P2, N2 and P4 peak times, but higher for all amplitudes. There were no statistically significant differences comparing peak times and amplitudes between eyes or between sessions. Coefficients of repeatability (CRs) are reported for P2, N2 and P4 peak times between eyes (P2; 5 ms, N2; 18 ms, P4; 18 ms) and also between sessions (P2; 5 ms, N2; 16 ms, P4; 39 ms). Intraclass correlation coefficients (ICCs), as an estimate of test-retest reliability, was assessed to be fair to excellent for most parameters.

Conclusions: This study provides important data on variability, repeatability and test-retest reliability of FVEPs in normal, adult horses. We conclude that P2, N2 and P4 peak times should be included in the evaluation of equine FVEPs. The large inherent variability of FVEP amplitudes is likely to make them less suitable and useful for establishing a diagnosis on their own in most clinical patients, but they may occasionally provide support to a clinical diagnosis.

Keywords: FVEP; Horse; Repeatability; Test-retest; VEP; Variability; Vision; Visual evoked potentials.

Fig. 1

The equine FVEP. A series of positive and negative wavelets can be observed in the equine FVEP

Fig. 2

FVEP tracings from left and right eyes. Two tracings each from left and right eyes (OS and OD) recorded at the same session in four horses (horses 2–3, 12 and 14)

Fig. 3

FVEP tracings from separate sessions. Two representative tracings from each of two separate recording sessions in three horses (horses 2, 3 and 6)

Fig. 4

Bland-Altman plots to compare measurements between sessions. Bland-Altman plots with 95% limits of agreement for the mean difference (six horses). The mean measurement from the first and second recording session (x-axis) was plotted against the difference between the two measurements (y-axis). The mean difference was the estimated bias (dotted red lines). Limits of agreement are marked as blue dotted lines