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

Overview

• This study explores a new method using ultrasound to guide injections targeting the pudendal nerve in horses, aiming to improve regional anesthesia techniques for the equine anogenital area.
• The research tests the feasibility of this ultrasound-guided approach on horse cadavers and evaluates how effectively the nerve can be stained, which indicates successful targeting.

Introduction and Background

• Equine anogenital tract regional anesthesia currently relies on methods such as local infiltration, extradural injections, blind palpation, and nerve stimulator-guided techniques.
• These existing methods have drawbacks, including risks of causing unintended injury (iatrogenic damage), inducing the horse to lie down (recumbency), or causing loss of muscle coordination (ataxia).
• The pudendal nerve plays a crucial role in sensation and motor function of the anogenital region, making it an important target for anesthesia.
• The study aims to improve the safety and accuracy of pudendal nerve blocks by using ultrasound guidance, a technique widely used in human and small animal medicine but less explored in horses.

Study Design and Methodology

• The study had a pilot phase and a main study phase:
• Pilot Phase:
• Used two fresh horse cadavers and one standing, unsedated live horse to visualize pelvic intrapelvic anatomy through ultrasound.
• One fixed cadaver was dissected to accurately locate the pudendal nerve and refine the anatomical approach for the main study.
• Main Study Phase:
• Conducted on six fresh horse cadavers.
• Ultrasound was used to identify key anatomical landmarks: pelvic brim, pelvic urethra, rectum, and semimembranosus muscle.
• A bilateral transcutaneous (through the skin) ultrasound-guided injection of 15 mL methylene blue dye was administered to attempt staining the pudendal nerve.
• Following injection, anatomical dissections were performed to evaluate the staining pattern and verify nerve targeting.

Results

• Key anatomical landmarks guiding injection were consistently visualized in all cadaver ultrasound exams.
• Direct imaging of the pudendal nerve itself was not possible with the ultrasound settings or equipment used.
• After methylene blue injection:
• The pudendal nerve was successfully stained in 41.7% of the injections, indicating partial success in nerve targeting.
• The average length of stained nerve was 8.26 cm, which suggests reasonably extensive dye spread when successful.
• There was no unintended staining of the sciatic nerve, which suggests good specificity of the injection technique in avoiding off-target nerves.

Conclusions and Implications

• The study demonstrates that transcutaneous ultrasound-guided injections targeting the pudendal nerve in horses are feasible based on cadaver models.
• However, since the nerve itself could not be directly visualized and the staining success was only partial (<50%), the technique currently lacks sufficient reliability for clinical recommendation.
• Further refinement in technique, imaging technology, or injection approach is necessary to improve the success rate before this method can be adopted in live animals.
• The absence of sciatic nerve staining is encouraging, as it may reduce the risk of side effects like ataxia that result from inadvertent sciatic nerve block.
• This preliminary work lays the foundation for future studies aiming to enhance ultrasound-guided regional anesthesia methods for equine pelvic procedures.