Influence of drug-induced nausea and opioid-associated behavioural alterations on pain scores in nonpainful cats: a prospective, randomised, experimental trial.

OBJECTIVE: To investigate whether pain scores (PS) obtained using the Feline Grimace Scale (FGS) or Glasgow Composite Measure Pain Scale - Feline (CMPS-F) are influenced by drug-induced nausea or opioid-associated behavioural alterations. STUDY DESIGN: Prospective, experimental, blinded, controlled, randomised, crossover trial. ANIMALS: A group of seven healthy adult laboratory cats. METHODS: Pain assessments (FGS followed by CMPS-F) were performed at baseline (BL) [three measurements (BL1-3), 20 minutes apart]. Ten minutes after BL3, cats were administered subcutaneous saline (0.5 mL), methadone (0.6 mg kg-1) or xylazine (0.5 mg kg-1). Assessments were repeated 10, 30 and 60 minutes after injection. At each time point, two blinded investigators determined PS before and after a standardised thermal nociceptive assay. Sedation, nausea and opioid-associated behaviours were recorded. Data were analysed using mixed linear models with time point and treatment as interacting predictors and Tukey-adjusted post hoc comparisons. RESULTS: Xylazine significantly increased CMPS-F and FGS scores at all post-treatment time points versus BL (all p < 0.001). CMPS-F increased from BL to T10-T60 (mean ± standard deviation, 2.0 ± 2.2, 6.6 ± 1.9, 8.3 ± 2.0 and 7.9 ± 2.2), respectively, whereas FGS increased from 1.4 ± 1.0 to 6.0 ± 1.4, 7.1 ± 2.1 and 6.3 ± 2.1. These increases coincided with nausea and/or emesis in six of seven cats and mild to moderate sedation. Methadone did not result in significant group-level changes over time (CMPS-F, 3.7 ± 3.2, 4.6 ± 0.8, 5.1 ± 2.3 and 4.7 ± 2.3; FGS, 2.6 ± 2.0, 3.9 ± 1.1, 3.7 ± 1.0 and 3.9 ± 2.3), although individual cats exceeded intervention thresholds. CONCLUSIONS: Drug-induced nausea was associated with clinically relevant increases in feline PS. Opioid-associated behavioural alterations may also influence individual assessments. CLINICAL RELEVANCE: These findings emphasise the importance of interpreting feline PS within the broader clinical context.