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

Changes in the Parameters of Visual Evoked Potentials in Media Opacities

Purpose

This study compared flash visual evoked potentials (VEPs) parameter differences between eyes with unilateral opaque media and their normal contralateral counterparts.

Patients and Methods

We included 178 patients with unilateral media opacity and normal contralateral eye in each patient. The diseased eyes had normal ultrasonographic findings and were candidates for keratoplasty or anterior segment reconstruction. The patients underwent electrophysiological testing by the flash VEPs recording with the use of 1.4Hz (transient) and 8Hz (flicker) frequencies.

Results

The medians of P2 implicit times were significantly higher in eyes with media opacity than in normal eyes, with median differences between both eyes of -6.7 ms. Furthermore, the median N2P2 amplitude was 9.70 µv with media opacity compared with 11.00 µv in the normal eyes, with a significant difference of 1.3 (p = 0.002). The flash VEPs median parameters recorded at a frequency of 8-Hz showed significant differences in medians of spectral plots of 0.6 µv between normal and abnormal eyes. The same for the signal-to-noise ratio, where the difference in medians was 3 dB. Analysis of the difference between both eyes (normal-abnormal) in each patient showed in P2 implicit time a difference range of -49 to 54.7 ms with a median difference of -3 ms. The difference in amplitudes of recorded N2P2 showed a median of 2.13 µv. There was a statistically significant but weak negative correlation between N2P2 amplitude (r spearman = -0.173, p = 0.021) and the grades of corneal opacity.

Conclusion

Preoperative flash VEPs recording is useful for evaluating visual function in patients with unilateral media opacity to determine the future benefits of keratoplasty or anterior segment reconstruction. There are no correlations between the flash VEP parameters or the difference between the 2 eyes and the grades of corneal opacity.

Flash Visual Evoked Potentials in Conscious Horses: A Preliminary Study

The visual evoked potential (VEP) has many applications in veterinary neurology, but the test is not routinely used in a clinical setting. The aim of this study was to describe a reliable method for recording flash visual evoked potentials (F-VEPs) in nonsedated horses. F-VEPs were recorded from both eyes in 20 healthy and calm, adult horses. Recordings were accomplished without sedation, anaesthesia, or the use of mydriatic drugs. The mean and standard deviation of the latency of the most evident positive peak was 52.76±2.37 ms (P53). The mean latencies of the preceding and following negative peaks were 38.14±4.62 (N38) and 72.35±5.33 ms (N72), respectively. There were 2 mean peak-to-peak amplitudes (N38 - P53 and P53 - N72), and they were 11.85±6.21 and 22.81±11.50 µV, respectively. F-VEP was also recorded from 3 horses (6 eyes) before and during sedation with 2 doses each of xylazine (0.4 and 1.1 mg/kg) or detomidine (0.005 and 0.014 mg/kg). It was possible to obtain a reliable F-VEP with a P53 latency in horses without sedation that was similar to the P2 peak described in previous studies, and these data can be used in the future as a normal reference for comparisons in horses with different diseases using a similar methodology. Sedation affected the results by depressing peak amplitudes and increasing latencies or by completely obscuring any response. The exact impact of sedation on VEPs must be evaluated with much caution due to the small sample size.