Epidural Anaesthesia in the Horse

Epidural application of drugs is usually done using a spinal needle via the first coccygeal interspace (mostly palpable). After local application of lidocaine, a 18 gauge, 5-8 cm long needle with stylet is used. Two techniques exist: perpendicular punction to the skin or at a 45o angle. Epidural catheter placement is used f when prolonged medication is planned and a special “Tuohy” spinal needle will be needed. Correct position is indicated by a) the so called “pop” feeling when piercing the interarcuate ligament (not always present, more consistent with the larger tuohy needle) b) by observing the aspiration into the epidural space of drops of drug placed in the hub of the needle (often, not always) c) resistance free injection of air or drug (very subjective) d) advanced methods like epidural pressure measurement after injection. Despite experience there is still a considerable percentage of failures. In the first place the technique is used to obtain regional anaesthesia with a local anaesthetic (lidocaine) of the anus, rectum, vagina, vulva, urethra and bladder. However an exact prediction of the cranial spread, left- right repartition and duration of action not possible. Hind limb ataxia and occasionally lateral recumbency are possibly serious complications. More recently many other drugs have been used epidurally. Alpha-2 agonists and opioids are the most commonly used, alone or in combination with a local anesthetic (mostly lidocaine). Xylazine is often used but also detomidine can be used. Xylazine possesses alpha-2 agonistic properties but also some local anaesthetic action. In general alpha-2 agonists will produce regional analgesia and much less interference with motor function. Systemic resorption will also cause a degree of sedation but less than with iv administration. Epidural opioid analgesia is used successfully in horses without producing motor blockade and providing analgesia extending from coccyx to thoracic dermatomes. Morphine and methadone are often used. Methadone (0.1 mg/kg diluted with saline up to 20 ml saline) produces analgesia as early as 15 minutes and as cranial as the 13th rib, lasting up to 5 hours. Morphine can be used at the same dosage and has a longer duration of action than methadone (max 18 hours) but, opposed to methadone, a variable sometimes very slow onset time (1-8 hrs). Epidural opioid analgesia is a good tool with few side effects to be used in analgesic management of acute and more chronic pain originating in the hindquarters and posterior body (eg tarsocrural joint inflammation).