Subcutaneous methadone is not different than transdermal fentanyl for postoperative analgesia in dogs with thoracolumbar disc disease, a prospective, randomised, blinded clinical study

Background Thoracolumbar disc disease is a common neurological condition in dogs, which incorporates different pain components. Multimodal analgesic treatments, especially postoperatively, are often based on opioids and require an intravenous catheter for drug application. This might impede early physiotherapy and mobilisation. Different composite pain scales and sensory testing devices exist to evaluate postoperative pain behaviour in dogs. At present, no data are available to clearly recommend one tool or technique after spinal surgery over others. Therefore, the primary aim of this study was to evaluate whether subcutaneously applied methadone or transdermal fentanyl can offer sufficient postoperative analgesia in dogs after thoracolumbar neurosurgery without the need to maintain intravenous access. A secondary aim was to evaluate which type of pain recognition tool would be suitable for dogs in this clinical setting. The hypothesis is that both subcutaneously applied methadone and transdermal fentanyl solution provide adequate pain relief in dogs after thoracolumbar spinal surgery. Methods In a prospective, randomised, blinded clinical study, fifty client-owned dogs were repeatedly evaluated for 96 h after undergoing spinal surgery. Treatment group M received 0.4 mg/kg methadone subcutaneously two hours before the start of surgery and every 6 h thereafter. Treatment group F received a topical application of 2.6 mg/kg transdermal fentanyl two hours before the start of surgery. Dogs were assessed via the Glasgow Composite Measure Pain Scale–Short Form (CMPS-SF), the Colorado State University Canine Acute Pain Scale (CPS), a Visual Analogue Scale (VAS) and von Frey Filaments. The treatment groups were compared via the Wilcoxon rank sum test. Correlations between the three pain scores were evaluated via Spearman’s rank correlation coefficient. Results Over the whole study course none of the pain evaluation methods could demonstrate a significant difference in analgesic requirements between groups M and F ( p < 0.05). In both treatment groups, the pain scores on all three scales decreased over time. The results of the different pain scales correlated moderately to strongly. Skin sensitivity assessed using von Frey filaments showed considerable individual variation among dogs, with most responding only to thicker filaments. Conclusion Subcutaneous methadone or transdermal fentanyl can provide adequate postoperative analgesia in dogs after spinal surgery without an intravenous catheter. The CMPS-SF and the CPS could reliably be used in this category of animals.