# Preliminary Investigation of a Transcutaneous Ultrasound-Guided Technique for Pudendal Nerve Block in Six Horse Cadavers

## TL;DR

This study explores a new ultrasound-guided method to block pudendal nerves in horses for anogenital surgery, but the technique needs improvement.

## Contribution

A novel ultrasound-guided technique for pudendal nerve staining in equine cadavers is described and evaluated.

## Key findings

- The pudendal nerve was stained in 41.7% of injections using the ultrasound-guided method.
- Landmarks for dye injection were reliably imaged in all cadavers, but direct imaging of the pudendal nerve was not possible.
- The sciatic nerve was not stained, indicating some specificity of the technique.

## Abstract

Techniques for desensitising the area around the anus and genitals (anogenital) in horses are limited. This leaves veterinary surgeons with few proven options for local anaesthesia in horses undergoing anogenital surgery. Local anaesthetic has been used successfully in horses to block the two pudendal nerves that innervate the anogenital area, though current techniques carry risks such as damage to blood vessels and other organs. An ultrasound-guided (USG) technique for equine anogenital desensitisation may overcome these drawbacks. The aim of this study was to describe and assess the feasibility of using an USG technique, to identify the two pudendal nerves in horse cadavers and inject dye around them as a model of USG pudendal nerve blockade. After a pilot phase to determine the ultrasonographic and dissection approach, six fresh horse cadavers received USG injections of methylene blue dye (15 mL) to both pudendal nerves, guided by the landmarks identified in the pilot phase. Dissection revealed that the pudendal nerve was stained in 41.7% of injections. Based on these results, significant refinement of the technique in further cadaver studies is necessary to improve the staining success rate.

Regional anaesthesia of the equine anogenital tract is limited to local infiltration, extradural, blind palpation and nerve stimulator-guided techniques which risk iatrogenic damage, recumbency and ataxia. This study aimed to describe and assess the feasibility of transcutaneous ultrasound-guided (USG) pudendal nerve staining in equine cadavers. An initial pilot phase used two fresh equine cadavers and one standing unsedated horse to image the intrapelvic anatomy using ultrasound. One fixed equine cadaver specimen was also dissected to identify the pudendal nerve and refine the dissection approach. The main study of six fresh equine cadavers used ultrasound to identify the landmarks of pelvic brim, pelvic urethra, rectum and semimembranosus musculature. Bilateral USG injection of 15 mL of methylene blue dye was performed, followed by anatomical dissection. The pudendal nerve was stained in 41.7% of injections, with an average staining length of 8.26 cm. Landmarks guiding dye injection were reliably imaged in all cadavers. Imaging of the pudendal nerve was not possible. Staining of the sciatic nerve did not occur. Transcutaneous USG pudendal nerve staining with methylene blue dye (15 mL) is possible in equine cadavers. However, this technique cannot be currently recommended. Further refinement in cadavers is necessary to improve the staining success rate.