Facial expression of pain in horses after colic surgery or castration

To ensure that an animal in pain receives the correct analgesic treatment pain assessment is of great importance. Recent research has shown that horses change their facial expression during pain, and shows a so called pain face. Using pain face as a pain assessment tool in horses is only sparsely validated but is of great interest since studies in humans have shown that it is less possible to hide a pain face. The aim of this study was to investigate the clinical application of the equine pain face as a pain assessment tool during clinically relevant conditions. The hypothesis was that facial expression of pain in postoperative horses would disappear or diminish after analgesic treatment. The secondary aim of this study was to investigate the performance of the equine pain face as a pain assessment tool in postoperative horses. Eleven horses that underwent exploratory laparotomy due to colic and one horse that developed hemoperitoneum after castration were included in the study. The behaviour and face of the horses were filmed postoperatively at two occasions, before analgesic treatment and two hours after analgesic treatment. The randomised video films were shown to seven blinded observers, who pain scored each horse according to facial expression of pain (no pain face, pain face present or intense pain face) and a modified version of the Equine Pain Scale (EPS). Facial expression of pain was observed in all of the twelve horses included in the study. However, no significant difference was seen in grading of facial expression of pain or the score of the modified EPS before and after analgesic treatment. This indicates that either the analgesic treatment was not always optimized or that there could be several other conditions that contributed to the changes in the face mimic. Fleiss kappa for inter-observer reliability of facial expression of pain was determined to 0.11 (slight agreement). The low kappa value is most likely due to the subjective grading of facial expression of pain, concurrent or intermittent lidocaine and butorphanol treatment and differences in observers’ ability to evaluate facial expression. Investigations of the equine pain face regarding the number of present facial action units that are needed and a weighting of the facial action units is of great interest in order to create a higher inter-observer reliability. Intra-observer reliability was determined to a mean value of 0.51 (moderate agreement). Furthermore, five out of seven observers had a kappa value above 0.5, suggesting that facial expression of pain could be a good supplementary pain assessment tool when a single person observes a horse over a time. The score of the modified EPS was compared to the grading of facial expression of pain. The included horses were divided into three different groups on the basis of their total score of the modified EPS; group 1 (score of 0-3), group 2 (score of 4-7) and group 3 (score of 8-11). Sixty-three % of the horses in group 1 expressed a pain face and sixty % of the horses in group 2 expressed a pain face. Furthermore, hundred % of the horses in group 3 expressed a pain face. This study evaluated pain with a visceral component. Further studies regarding the specificity and sensitivity of facial expression of pain are required when complex pain types and medication are involved.