Abstract
Abstract: :
Purpose: Despite the frequent occurrence of retinal diseases in horses (e.g., equine recurrent uveitis, congenital stationary night blindness), no practical protocol has so far been suggested for the recording of flash electroretinograms (ERGs) in this species. The goal of our study was the evaluation of a practical method for the recording of ERGs in sedated, standing horses with the DTLTM microfiber electrode. Methods: One eye of each of ten healthy horses (median age: 9.5 years, range: 4 - 18 years) was used. Mydriasis was achieved with topical 1% tropicamide, and an auriculopalpebral nerve block was performed with 2% lidocaine HCl. The horses were sedated intravenously with 0.015 mg/kg detomidine HCl. The ERGs were recorded on the standing animal with the active electrode on the cornea (DTLTM), the reference electrode near the lateral canthus, and the ground electrode over the occipital bone. The bandwidth was set between 0.3 and 300 Hz (high-pass filter set at 75 Hz for oscillatory potentials). The light intensities of the white strobe light were 0.03 cds/m2 (scotopic) and 3 cds/m2 (scotopic and photopic). Adapting light level for photopic recording was 10 cd/m2 . The light source was modified to achieve Ganzfeld stimulation. Photopic and scotopic single flash and flicker responses were recorded. During the 20-minute dark adaptation period the retina was stimulated every five minutes with the 0.03 cds/m2 single flash. At the end of the recording session the cornea was stained with sodium fluorescein to check for abrasions. Results: The median b-wave amplitudes and implicit times were 38 µV and 33 ms (photopic), 43 µV and 63 ms (5-minute dark adaptation), 72 µV and 89 ms (10 minutes), 147 µV and 103 ms (15 minutes), 188 µV and 109 ms (20 minutes, 0.03 cds/m2), and 186 µV and 77 ms (20 minutes, 3 cds/m2). A steady increase in amplitude and implicit time was noted during dark adaptation. No oscillatory potentials could be isolated. The median amplitude of the photopic 30-Hz flicker was 25 µV, of the scotopic 10-Hz flicker (0.03 cds/m2) was 32 µV, and of the scotopic 30-Hz flicker (3 cds/m2) was 13 µV. No corneal abrasions were seen in any of the eyes tested. Detomidine sedation provided adequate sedation to perform the recordings on the standing horses without major artefacts. Conclusion: The use of detomidine and DTLTM microfiber electrode allowed the recording of good quality ERGs. This protocol should permit the detection of functional problems in the retina without the risk involved with general anesthesia. Our study could be the basis for the development of a standard ERG protocol for horses.
Keywords: 395 electroretinography: clinical • 316 animal model • 554 retina