Consistent analgesic effect of intravenous dexamethasone on rebound pain after brachial plexus block: a causal machine learning approach.

BACKGROUND: Rebound pain, a sudden increase in pain intensity after resolution of regional anesthesia, remains a major challenge in postoperative patient care. Intravenous (IV) dexamethasone has been proposed as a preventive strategy, but whether its effectiveness varies across patient characteristics remains unclear. METHODS: This retrospective study included patients who underwent upper extremity orthopedic surgery under brachial plexus block (BPB). Rebound pain was defined as the occurrence of both severe pain (numeric rating scale ≥ 7) and rescue analgesic administration between 4 and 24 hours after BPB. The association between IV dexamethasone and rebound pain was evaluated using multivariable logistic and generalized random forest (GRF) models. RESULTS: Among the 1,171 patients, 651 (55.6%) received IV dexamethasone. The dexamethasone group had a significantly lower incidence of rebound pain (7.1% vs. 16.9%, P < 0.001) with delayed onset (median 14 vs. 11 hours). IV dexamethasone was an independent protective factor (odds ratio, 0.42; 95% confidence interval [CI], 0.27 to 0.64; P < 0.001). GRF analysis estimated an average treatment effect of -0.09 (95% CI -0.13 to -0.05), corresponding to a 9% absolute risk reduction. Calibration testing demonstrated accurate model (GRF) estimation (mean coefficient, 0.98; P < 0.001) and no statistically detectable heterogeneity in treatment effect (differential coefficient, -0.30; P = 0.713). CONCLUSIONS: IV dexamethasone was associated with reduced rebound pain without evidence of meaningful heterogeneity in treatment effect in patients undergoing upper extremity surgery under BPB. These findings suggest a potential preventive role for dexamethasone as an adjunct to perioperative pain management in this population.