Visual evoked potentials in the horse

Sammanfattning: Vision is an important sense for horses, both for survival in the wild and when horses are used for work, sport or recreation. However, it is often difficult to diagnose visual impairment in this species. Traditional techniques available in clinical equine practice, are based on subjective evaluations, and their results are many times difficult to interpret. Electrodiagnostic methods, flash electroretinography (FERG) and recording of flash visual evoked potentials (FVEP) are used to objectively evaluate the function of the retinal and post-retinal visual pathways. The electrical potentials generated in response to brief visual stimuli are measured non-invasively. Abnormal function in visual pathways can affect the FERG and FVEP waveforms, peak times and amplitudes. Lesions can thereby be detected, and their approximate localization evaluated. FVEPs are used in human medicine, and occasionally in animal species, but have not been described in the horse. The general aim of this thesis was to establish a technique for recording of FVEPs in horses in clinical practice. The results showed that FVEPs can be readily recorded in sedated horses in a clinical setting. The recorded waveform consisted of a series of positive (P1-P5) and negative (N1-N2) wavelets. The overall appearance of the waveform was shown to be similar in foals, young horses and adult horses. An age-related effect on peak times and amplitudes was observed, but most of the changes occurred early in life. Important data on FVEP variability and repeatability was reported, and it was concluded that P2, N2 and P4 peak times should be included in the evaluation of equine, clinical FVEPs. The large inherent variability of FVEP amplitudes made them less useful, but they occasionally provided support to a clinical diagnosis. In clinical patients, electrodiagnostic testing helped assessing functional impact of potentially visual-threatening diseases. By recording FERGs and FVEPs simultaneously, a subdivision into retinal vs post-retinal dysfunction could be made in many patients, such as horses with optic neuropathies and cortical visual impairment. FVEPs may also be of prognostic value in horses with traumatic optic neuropathy and possibly in cases with cortical visual impairment. The results from this thesis, opens up for the use of the FVEP as an adjunctive, objective method in the evaluation of equine patients with suspected visual impairment and neurological disease, but also for studies of development and function of the visual pathways in this species.