Comparing Blind and Ultrasound-Guided Retrobulbar Nerve Blocks in Equine Cadavers: The Training Effect

SIMPLE SUMMARY: Standing ophthalmic surgeries have become more and more common in horses. For these standing surgeries, the blind retrobulbar block is often used for anesthesia and akinesia of the eye. However, placing a retrobulbar block using this blind technique can lead to complications, for example, penetration of the globe, nerve injury or chemosis due to spreading of the local anesthetic in the region of the globe. For this reason, it might be better to perform the retrobulbar block using guidance by ultrasound. Ultrasound-guided retrobulbar block has only been described twice in the literature using equine cadavers. Comparison of the ultrasound-guided peribulbar technique to the blind technique has only been carried out once. Furthermore, the learning curve of ultrasound-guided retrobulbar nerve block placement has not been evaluated. Our study aimed to compare the blind and ultrasound-guided approaches to retrobulbar block placement in horses and to evaluate the success and complication rates, analyzing the effect of training on ultrasound guidance. A trend towards a significant improvement in accuracy was seen for ultrasound guidance, and larger scale follow-up studies might show a significant training effect on the use of ultrasound in retrobulbar nerve block placement and that the use of ultrasound guidance could be promising. ABSTRACT: In standing ophthalmic surgery in horses, a retrobulbar nerve block (RNB) is often placed blindly for anesthesia and akinesia. The ultrasound (US)-guided RNB may have fewer complications, but the two techniques have only been compared once in equine cadavers. This study compares the techniques for success and complication rates and analyzes the effect of training on US guidance. Twenty-two equine cadavers were divided into three groups: blind RNBs were performed bilaterally in eight cadavers, US-guided RNBs were performed bilaterally in seven cadavers, and after US-guided training, blind RNBs were performed bilaterally in seven cadavers. All RNBs were performed by the same two inexperienced operators, and a combination of contrast medium (CM; 1.25 mL) and methylene blue dye (1.25 mL) were injected (2.5 mL total volume). Needle positioning in the periorbita and the distance of the CM to the optic foramen were assessed using computerized tomography (CT). Dye spreading was evaluated by dissection. In group 1, 37.5% of the injections were in the optimal central position in the periorbita; in group 2, 75% and in group 3, 71.4%. There was no significant difference between the groups regarding needle position (groups 1 and 2 p = 0.056; groups 1 and 3 p = 0.069, groups 2 and 3 p = 0.8). The mean CM distribution distance was not significantly different between all groups. Group 1 had 18.75% intraocular injections versus 0% in group 2 and 7.1% in group 3 (not significant). US guidance showed no significant increases in accuracy nor decreases in complications. However, the effects on accuracy showed a trend towards significant improvement, and larger scale follow-up studies might show significant training effects on US guidance.