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Sacral paravertebral anesthesia is used to relieve rectal tenesmus associated with rectal prolapse without affecting the sciatic nerve and function of the tail or the animal's ability to stand. The sacral paravertebral nerve block is used to provide analgesia to the pudendal nerve (pudic nerve), medial hemorrhoidal nerve (pelvic splanchnic nerve), and caudal hemorrhoidal nerve (caudal rectal nerve) by blocking S3, S4, and S5 as they branch off the spinal cord, thereby providing analgesia to the anus, vulva, and vagina [1, 10]. In bulls, S3 supplies motor function to the retractor penis muscles. Physical restraint in a squeeze chute and/or sedation may be beneficial in order to prevent lateral movement of the animal during the procedure. In addition, a caudal epidural may be helpful if the animal is fractious. The skin over the dorsal sacrum should be clipped of hair and surgically prepared for the procedure. The paired S5 foramina are 1–2 cm lateral to the sacral coccygeal joint. The S4 foramina are about 3–4 cm cranial and more lateral to the S5 foramina. The S3 foramina are an additional 3–4 cm cranial to the S4 foramina (Figure 17.5a and b). A stab incision can be made dorsal to each foramen to aid in the introduction of an 18‐gauge, 5‐ to 7‐cm needle. The foramina can be palpated rectally with a finger placed in or over the ring which allows for identification of the foramen and ensures correct needle placement (Figure 17.6). Once the needle has entered the osseous ring, inject 2–3 ml of lidocaine hydrochloride; this should be repeated for each foramen [10]. The use of a lidocaine/alcohol mixture has also been described to manage tenesmus following chronic cervicovaginal prolapse or rectal prolapse. A mixture of 1 ml of 2% lidocaine hydrochloride and 2 ml of 95% ethyl alcohol has been used effectively [10].

Figure 17.5 Dorsal view (a) and lateral view (b) of S3, S4, and S5 foramina with needle placement.

Source: Image courtesy of Douglas Hostetler.

Figure 17.6 Sacral paravertebral.

Source: Image courtesy of Douglas Hostetler.