Epidural Analgesia

Designing an appropriate plan for analgesia in a horse, particularly for those with severe, chronic pain, can be challenging. Parenterally administered opioids and α₂-adrenergic agonists may be associated with side effects such as ataxia, excitement, and adverse effects on the gastrointestinal and cardiovascular systems. Nonsteroidal antiinflammatory drugs (NSAIDs) can cause gastrointestinal ulceration or renal disease, and they may be inadequate for horses with acute, intense pain. Epidural administration of drugs provides more localized analgesia and fewer systemic effects than parenteral administration.

Epidural drug administration places a drug in close proximity to its site of action within the spinal cord or the spinal nerves as they exit the spinal cord. A greater analgesic effect may be achieved with a smaller total dose of a drug. The duration of analgesia is usually longer than with parenteral administration. Pain that is related to a disease process of a hindlimb is particularly amenable to treatment with epidural analgesia. The site of injection for epidural drug administration in horses is usually the first coccygeal (caudal) interspace. When deposited into the epidural space, the drug diffuses across the meninges, into the cerebrospinal fluid (CSF), and then into the spinal cord. The degree of cephalad diffusion of the drug within the CSF depends on several factors, including the volume of the drug injected, concentration of the drug, and lipid solubility.² Although studies in dogs and people have reported analgesic effects with epidural morphine or α₂-agonists that extend to the midthoracic area or farther cranially,^3,4 this is generally not the case in horses.^5-8 This limits the use of epidural analgesia in horses to the treatment of hindlimb pain.

Technique for Epidural Administration

Caudal epidural drug administration can be done by single injection at the first coccygeal interspace or by placement of an epidural catheter at this same site for repeated drug administration. The lumbosacral space is also a potential site of injection, and its use may result in a more rapid onset of hindlimb analgesia. However, positioning the tip of the needle within the epidural space rather than the subarachnoid space can be difficult. Subarachnoid administration of drugs in the horse is certainly acceptable, but the dose should be reduced by 40% to 50%.

The first coccygeal interspace (between the first and second coccygeal vertebrae) is identified while raising and lowering the tail and palpating for the articulation between these two vertebrae. This space is just caudal to the most angular portion of the bend of the tail, about 5 cm cranial to the first long tail hairs (Figure 85-1). The site should be prepared aseptically. Administration of 2 to 3 mL of 2% lidocaine subcutaneously using a 25-gauge needle helps decrease the response of the horse to placement of the needle for epidural injection. An 18- or 20-gauge, 6.35-cm spinal needle with stylet is recommended for epidural injection in horses, although many clinicians use a standard 18-gauge needle. The bevel of a spinal needle is not as sharp as that of a standard needle and the bevel angle is less acute. This design makes it easier to identify penetration through the interarcuate ligament and the subsequent loss of resistance as the epidural space is entered. The needle is inserted at a 30- to 60-degree angle to horizontal, with the tip pointed cranioventrally, and is advanced until it contacts the floor of the vertebral canal (Figure 85-2). The depth of insertion is 3 to 6 cm, depending on the size of the horse and the angle of the needle. The needle placement can be tested by attempting to inject 2 to 3 mL of air or solution in a 3-mL syringe. Resistance to injection should be little or absent. Appropriate epidural injection of local anesthetic solution and xylazine often is confirmed when anal tone decreases and the tail relaxes, but because the drugs most commonly used for epidural analgesia of the hindlimb (morphine, detomidine) have little or no effect on motor nerves, these responses will be absent.

Fig. 85-1 The first coccygeal interspace between the first and second coccygeal vertebrae is identified while the clinician raises and lowers the tail and palpates for the articulation between these two vertebrae. This space is just caudal to the most angular portion of the bend of the tail.

Fig. 85-2 A, The needle is inserted at a 30- to 60-degree angle to horizontal and is advanced until it contacts the floor of the vertebral canal. B, Sagittal view of the sacral vertebrae and placement of the needle within the epidural space. Co-1, First coccygeal vertebra; L-6, sixth lumbar vertebra; S-1, first sacral vertebra.

Epidural Catheter Placement

The site for insertion of an epidural catheter is the same as for a single epidural injection. Placement of the catheter requires the use of a needle with a curved point (Tuohy, Becton, Dickinson, Franklin Lakes, New Jersey, United States) that will direct the catheter cranially, along the floor of the vertebral canal (Figure 85-3). The epidural catheter is made of polyamide (nylon) or Teflon, with a closed or open end, and it can be purchased with a wire stylet if desired. I use an 18-gauge, 8.89-cm Tuohy needle and a 20-gauge, 100-cm radiodense, polyamide catheter with a closed tip (bullet tip) without a stylet (Figure 85-4). Epidural catheters have marks every centimeter and multiple marks at 10, 15, and 20 cm from the end. Before needle and catheter placement, slide the catheter inside the needle and note the distance from the tip of the needle to the hub on the catheter. The clinician then should determine which mark on the catheter will be at the hub of the needle once the catheter has been advanced to the desired position.