Age‐associated changes in the equine flash visual evoked potential

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.

Grating Visual Acuity in phakic, aphakic, and pseudophakic Poodles

OBJECTIVE

To compare the grating visual acuity (VA) measured by visual evoked potentials (VEP) in phakic, aphakic, and pseudophakic Poodles.

ANIMALS STUDIED

Thirty-six Poodle dogs aged from 4 to 14 years.

PROCEDURES

Animals were allocated into three different groups according to their lens status: phakic group (n = 12), aphakic group (n = 12), and pseudophakic group (n = 12). Grating VA was measured in cycles/degree (cpd) in all animals using the electrodiagnosis system Roland RETIport^® in a dark room without using any mydriatic, sedative, or anesthetic drugs.

RESULTS

The mean grating VA in the phakic, aphakic, and pseudophakic groups was 5.9 ± 1.0 cpd (20/102-Snellen equivalent), 2.6 ± 0.7 cpd (20/231), and 5.2 ± 1.1 cpd (20/116), respectively. The VA from aphakic eyes was significantly lower when compared to the phakic and pseudophakic eyes (P < .05). There was no significant difference in VA between phakic and pseudophakic eyes.

CONCLUSIONS

The VEP is a useful tool for the evaluation of grating visual acuity in canines. The study showed that IOL implantation following phacoemulsification results in improved VA as measured by VEP compared to that of the aphakic eye and resulted in VA that was similar to that of the normal eye.

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

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.