Surgery STAT: How to perform a caudal epidural in horses

In horses, administering a caudal epidural anesthetic into the sacrococcygeal or first intercoccygeal space provides regional analgesia to the tail and perineum, thus facilitating surgical procedures of the vagina, cervix, urethra, bladder or rectum, as well as select cases for correction of a prolapsed uterus; it is also helpful for managing dystocia.

A major benefit of performing caudal epidurals in horses is the ability to complete surgery and other invasive procedures with the patient standing; loss of sensory innervation without affecting motor control forgoes the need for and potential risks associated with general anesthesia. In addition, the technique accommodates placement of a catheter in the caudal epidural space for repeated dosing of medications and long-term pain management in horses with specific hindlimb musculoskeletal issues or other conditions.

Technique

The horse should be restrained in stocks; judicious use of sedation is paramount. Caution should be taken not to sedate the horse too heavily, which may cause ataxia and impede the animal’s ability to stand squarely, thereby making it more challenging to insert the needle on the midline.

The landmark for needle placement is the first intercoccygeal or sacrococcygeal space, which can be located by raising and lowering the tail. The exact distance from the tail base varies among horses; however, usually the space can be located 2.5 to 5 cm from the origin of the tail hairs.¹ This area should be clipped and aseptically prepared. Optionally, a small bleb of local anesthetic, either lidocaine or 2% mepivacaine, may be placed under the skin at the proposed site of injection.

An 18-ga, 1.5-inch needle should be inserted with the bevel facing cranially on the midline and perpendicular to the surface of the skin for a more accurate trajectory to the space; it should not be inserted perpendicular to the floor (Figure 1). Once the needle is through the skin, the hub can be filled with sterile saline or the anesthetic solution (Video 1).

As the needle is advanced in the correct location (Video 2), it will puncture the arcuate ligament, thus creating a popping sensation for the practitioner; fluid within the hub will then be aspirated into the epidural space via negative pressure. This procedure, known as the hanging drop technique, confirms accurate needle placement and accommodates using minimal pressure for injection of the local anesthetic. If the needle hits bone, it should be withdrawn approximately 0.5 cm, but not removed from the skin, and redirected. Loss of anal sphincter tone is a common finding when caudal epidural anesthesia is achieved.

Figure 1: Needle inserted on the midline, perpendicular to the surface of the skin. The hub of the needle is filled with anesthetic solution.

Video 1. With the hanging drop technique, the hub of the needle is filled with anesthetic solution and the desired needle position is confirmed.

Video 2. The needle is slowly advanced until the anesthetic solution is aspirated into the caudal epidural space, confirming accurate placement. Then the remaining anesthetic solution is injected.

Commonly used anesthetics are xylazine and lidocaine or 2% mepivacaine. Xylazine has the benefit of analgesia without ataxia, while detomidine has a profound analgesic effect but can cause undesirable ataxia. Using a combination of 2% mepivacaine (1-2 ml) and xylazine (50-100 mg) diluted to a total volume of 7 to 10 ml using sterile water or sterile saline has been successful in many surgical cases. Combining the two drugs can also produce longer analgesia with only mild ataxia, as compared with using either agent alone.² Morphine can also be combined with an alpha-2 agonist for prolonged analgesia in select hindlimb musculoskeletal disorders.

Risks and possible complications

Horses should be checked to ensure that sensory innervation has returned to the hindlimbs before they are moved out of the stocks. Turning horses out in a small paddock for a few hours after the procedure may be prudent, rather than possibly risking being cast if they are returned to their stall.

Contraindications to use of the caudal epidural technique include neurologic or musculoskeletal disease, or local inflammation surrounding the injection site. A caudal epidural presents low risk for puncture of the dura and subsequent leakage of cerebrospinal fluid, while keeping motor function intact with the use of appropriate analgesic doses.

In some cases, motor blockage may occur from cranial diffusion of alpha-2 agonists or anesthetic agents; however, this is associated with the use of doses above recommended values and with high volumes of solution. Studies have shown that cranial diffusion of the solution up to six to 10 vertebral spaces can occur when doses between 10 and 20 ml are used, with the consequential risk for motor nerve paralysis.³ Therefore, the total volume of solution should be less than 10 ml. In horses that have had previous caudal epidurals, a lower total volume (< 10 ml) should be used, as previous injections can make the space smaller, allowing for more cranial migration of the anesthetic solution.

Of note, although rare, some combinations of local anesthetics and alpha-2 agonists have reportedly caused severe ataxia or recumbency, even when used at recommended doses.⁴ Side effects can be reduced by good patient selection and the use of appropriate doses. Inadequate analgesia can result from incorrect injection technique or abnormal anatomy (e.g. presence of fibrous lesions from previous epidurals), which may result in unilateral effects. Infection can be minimized by thorough sterile preparation of the injection site and aseptic technique.

References

1. Robinson EP, Natalini CC. Epidural anesthesia and analgesia in horses. Vet Clin North Am Equine Pract 2002;18(1):61-82.

2. Grubb TL, Riebold TW, Huber MJ. Comparison of lidocaine, xylazine, and xylazine/lidocaine for caudal epidural analgesia in horses. J Am Vet Med Assoc 1992;201(8):1187-1190.

3. Hendrickson DA, Southwood LL, Johnson R, Kruse-Elliot K. Cranial migration of different volumes of new-methylene blue after caudal epidural injection in the horse. Equine Pract 1998;20(2):12-14.

4. Skarda RT, Muir WW. Analgesia, hemodynamic and respiratory effects of caudal epidurally administered ropivacaine hydrochloride in mares. Vet Anaesth Analg 2001;28(2):61-74.

Dr. Barton graduated from the Royal Veterinary College in London and is currently completing a surgery internship at Hagyard Equine Medical Institute in Lexington, Kentucky. In addition to equine surgery, she has an interest in neurology and lameness diagnosis. Dr. Sandow is a board-certified veterinary surgeon at Hagyard Equine Medical Institute. His interests include orthopedics, lameness and emergency celiotomy as well as dystocia.

Surgery STAT is a collaborative column between the American College of Veterinary Surgeons (ACVS) and dvm360 magazine. To locate a diplomate, visit ACVS's online directory, which includes practice setting, species emphasis and research interests, at acvs.org.