Objective: To evaluate and compare effects of the caudal epidural administration of methadone (METH) and lidocaine (LIDO) on the tolerance to thermal stimulation over the dermatomes of the perineal, sacral, lumbar, and thoracic regions in the horse. ¢® Study design: A blinded, randomized, prospective, experimental cross-over study. ¢® Animals: Seven healthy horses, 15.7 ¾ 4.9 years (mean ¾ SD) of age, weighing 536 ¾ 37 kg. ¢® Methods: The horses received two treatments (group M: METH, 0,1 mg/kg or group L: LIDO, 0,35 mg/kg) at intervals of at least 28 days. An 18-gauge 80 mm Tuohy epidural needle was placed in the first inter-coccygeal space in awake standing horses restrained in stocks.
Analgesia was assessed by use of a probe maintained at constant temperature by circulating warm water. The maximum stimulation time was 30 seconds. Bilateral stimulation was performed at 5 defined points.
Before drug administration, base-line values of response time to thermal stimuli were obtained. Time to response was then measured 15 and 60 minutes after METH or LIDO administration and then hourly until the response returned to base-line. Development of any ataxia and/or sedation was recorded. Positive pain responses were defined as purposeful avoidance movements of the head, neck, trunk, limbs, and tail. Absence of attempts to kick, bite, and turning of the head toward the stimulation site were used to indicate analgesia. Results: Caudal epidural administration of METH and LIDO significantly increased reaction time to thermal stimulation (one-sample t-test; p Â 0.05). Analgesia in the perineal region was present 15 minutes after both METH and LIDO administration and progressed from caudal to cranial with time. The duration of a significant increase in reaction time was 5 hours after METH injection compared to 3 hours following LIDO. All horses defaecated and urinated normally and no excitement, sedation or ataxia were observed after METH administration. The horses were unable to defaecate normally and were moderately to severely ataxic with hindlimb weakness after LIDO. ¢® Conclusions: Caudal epidural administration of methadone has considerable potential in the management of perineal, lumbo-sacral, and thoracic pain in horses. Regional differences exist in the onset, duration and intensity of the pain relief.