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

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.