Continuous Caudal Epidural Anesthesia
Continuous caudal epidural anesthesia is used in cattle with chronic rectal and vaginal prolapse that experience continuous straining after the initial epidural. This procedure is performed by placing a catheter into the epidural space for intermittent administration of local anesthetic. A 17‐gauge, 5‐cm spinal needle (Tuohy needle) with stylet in place is inserted into the epidural space at Co1 to Co2 with the bevel directed craniad. The stylet is removed, and 2 ml of local anesthetic is injected to determine if the needle is in the epidural space. A catheter is inserted into the needle and advanced cranially for 2–4 cm beyond the needle tip. The needle is then withdrawn while the catheter remains in place (Figure 17.4b). An adapter is placed on the end of the catheter and the catheter secured to the skin on the dorsum. Local anesthetic solution may then be administered as needed [2].
Additives or Alternatives to Local Anesthetic Epidural Anesthesia
More recently, α2‐adrenergic agonists and opioids either alone or in combination with local anesthetic solutions have been used for epidural anesthesia. Epidural administration of the α2‐agonist xylazine hydrochloride (0.05 mg/kg) diluted in 5–12 ml of sterile saline or xylazine hydrochloride (0.3 mg/kg) added to 5 ml of 2% lidocaine hydrochloride offer similar anesthesia to lidocaine. Although the duration of anesthesia is prolonged (four to five hours) using these combinations, systemic effects (sedation, salivation, ataxia) may also occur [1]. Epidural administration of opioids, such as morphine (0.1 mg/kg) diluted in 20 ml of sterile saline, is used to provide analgesia for a prolonged period (approximately 12 hours) without interfering with motor function. Disadvantages of using opioids for epidural anesthesia are that the analgesia is not as potent as lidocaine and the maximum effect of a morphine epidural may not occur for two to three hours. Caudal epidural administration of morphine may be used to help alleviate pain in the perineal area and straining [8].
Alcohol has been used previously to desensitize the perineum for a prolonged period and in some cases permanently. Alcohol administered by this route can lead to demyelination of nerves. This can result in damage more cranial than the perineum and could effectively paralyze the patient if it were to travel up the nerve root and into the sciatic nerve. Therefore its use in epidural anesthesia is not recommended [9].
Sacral Paravertebral Anesthesia
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.
Anesthesia of the Penis and Prepuce
Desensitization of the Internal Pudendal Nerve Block
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].