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The procedure for bilateral internal pudendal (pudic) nerve block was first described by Larson [11] to facilitate relaxation of the bull's penis without causing locomotor impairment. The internal pudendal nerve block can be used in the standing bull for penile relaxation and analgesia distal to the sigmoid flexure and examination of the penis. In the standing female, the internal pudendal nerve block can be used to relieve straining caused by chronic vaginal prolapse. This technique may also be used for surgical procedures of the penis, such as repair of prolapses, removal of peripenile tumors, removal of penile papillomas or warts, and other minor surgeries of the penis and prepuce.

This procedure involves desensitizing the internal pudendal nerve and the anastomotic branch of the middle hemorrhoidal nerve using an ischiorectal approach. The internal pudendal nerve consists of fibers originating from the ventral branches of the third and fourth sacral nerves (S3 and S4) and the pelvic splanchnic nerves. The skin at the ischiorectal fossa on both sides is clipped, disinfected, and desensitized with approximately 2 ml of local anesthetic (Figure 17.7). A 14‐gauge, 1.25‐cm needle is inserted through the desensitized skin at the ischiorectal fossa to serve as a cannula. An 18‐gauge, 10‐ to 15‐cm spinal needle is then directed through the cannula to the pudendal nerve. The operator's left hand is placed into the rectum to the level of the wrist and the fingers directed laterally and ventrally to identify the lesser sacrosciatic foramen. The lesser sciatic foramen is first identified by rectal palpation as a soft depression in the sacrosciatic ligament. The internal pudendal nerve can be readily identified lying on the ligament immediately cranial and dorsal to the foramen and approximately one finger's width dorsal to the pudendal artery passing through the foramen. The internal pudendal artery can be readily palpated a finger's width ventral to the nerve. The spinal needle is held in the operator's right hand and introduced through the cannula in the ischiorectal fossa. The spinal needle is directed medial to the sacrosciatic ligament and directed cranioventrally (Figure 17.8). The needle is not felt until it has been introduced approximately 5–7 cm and can then be repositioned to the nerve. Once at the pudendal nerve, 20–75 ml of local anesthetic is deposited at the nerve (Figure 17.9). The needle is then removed and the site of deposition is massaged to aid in dispersal of the local anesthetic. This procedure is repeated on the opposite side of the pelvis. Relaxation of the penis varies and may take as long as 30–40 minutes for full effect. Effectiveness of the block can be assessed by firmly squeezing the tail of the epididymis of each testicle. The bull's inability to lift or retract the testicle signifies adequate analgesia. The duration of the internal pudendal nerve block lasts two to four hours [10].

Figure 17.7 Injection of 2 ml local anesthetic in the skin at the ischiorectal fossa.

Figure 17.8 Needle placement for pudendal nerve block.

Source: Image courtesy of Douglas Hostetler.

Figure 17.9 Pudendal nerve block.