Presumed generalised seizure following caudal epidural administration of morphine and detomidine in a pony

Abstract

A 9-year-old Show Pony mare became acutely lame following removal of a bone sequestrum of the distal phalanx of the right thoracic limb. The mare also suffered from ongoing right dorsal colitis secondary to previous long-term non-steroidal antiinflammatory drug (NSAID) use. To avoid further NSAID use, a protocol for caudal epidural administration of morphine and detomidine in an increased volume was used to provide analgesia to the thoracic limbs. A total volume of 50 ml (0.2 ml/kg) was administered over approximately 90 seconds. Immediately following the injection, the pony collapsed into lateral recumbency, and experienced an apparent generalised seizure characterised by loss of consciousness and frantic paddling of all four limbs.The pony recovered rapidly without intervention and no residual neurological deficits were noted. The epidural analgesia resulted in a marked improvement in comfort levels. The speed of injection is thought to have caused a change in epidural and intracranial pressures resulting in a generalised seizure and highlights the importance of administering large volumes slowly.

Use of caudal epidural analgesia for management of orthopaedic limb pain in horses

“…Combining these personal experiences with the clinical findings of the case report by Tallon et al . (2021) and the study by Freitas et al . (2011), it seems reasonable to conclude that successful provision of analgesia for thoracic limb pain after caudal epidural injection probably requires a total volume of 0.15–0.2 mL/kg and that it should not be injected at a rate faster than 10 s/mL.…”

“…As hypothesised in the case report by Tallon et al . (2021), a rapid epidural injection (1.8 s/mL) of large volumes of solution (0.2 mL/kg) may also cause a change in the epidural and intracranial pressure resulting in generalised seizure. On an interesting note, we have had a very similar experience, in which a 500 kg Warmblood mare with severe thoracic trauma developed the exact same type of generalised seizure after caudal epidural injection of morphine (0.1 mg/kg bwt) and xylazine (0.17 mg/kg bwt) diluted with saline to a total volume of 0.1 mL/kg at a rate of 6 s/mL.…”

“…The case report by Tallon et al . (2021) in the current issue describes the development of a presumed generalised seizure in a pony following administration of morphine (0.2 mg/kg bwt) and detomidine (0.004 mg/kg bwt) diluted with saline to a relatively large total volume of 50 mL (0.2 mL/kg) into the caudal epidural space over approximately 90 s. The reason for performing the epidural injection was to treat severe pain of a thoracic limb, for which it proved to be successful. Despite the patient’s swift recovery after the generalised seizure, the reporting and discussion of the use of large volumes of solution for caudal epidural analgesia are of great interest to all clinicians dealing with pain management in horses.…”

“…2011; Hopster and van Eps 2019; Tallon et al . 2021). Furthermore, the perceived complexity of the epidural technique and risk of side effects may deter many practitioners from using caudal epidural analgesia for pain management in horses.…”

“…Slow injection of large volumes can, however, be particularly challenging if the horse is continuously weight shifting or in other ways difficult to keep standing still and if the syringe is attached directly to the epidurally placed hypodermic needle (Tallon et al . 2021). In these situations, the simplest solution for a single injection is to attach a flexible extension set between the needle and syringe since this will prevent movement of the needle when the horse moves ( Fig ).…”