Does dexamethasone have a perineural mechanism of action? A paired, blinded, randomized, controlled study in healthy volunteers.

BACKGROUND Dexamethasone prolongs block duration. Whether this is achieved via a peripheral or a central mechanism of action is unknown. We hypothesized that perineural dexamethasone added as an adjuvant to ropivacaine prolongs block duration compared with ropivacaine alone, by a locally mediated effect when controlled for a systemic action. METHODS We performed a paired, blinded, randomized trial, including healthy men. All subjects received bilateral blocks of the saphenous nerve with ropivacaine 0.5%, 20 ml mixed with dexamethasone 2 mg in one leg and saline in the other, according to randomization. The primary outcome was the duration of sensory block assessed by temperature discrimination in the saphenous nerve distribution. Secondary outcomes were sensory block assessed by mechanical discrimination, pain response to tonic heat stimulation, and warmth and heat pain detection thresholds. RESULTS We included 20 subjects; one had a failed block and was excluded from the paired analysis. Block duration was not statistically significantly longer in the leg receiving dexamethasone when assessed by temperature discrimination (primary outcome, estimated median difference 1.5 h, 95% confidence interval -3.5 to 0, P=0.050). For all other outcomes, the duration was statistically significantly longer in the leg receiving dexamethasone, but the median differences were <2.0 h. Individual subject analysis revealed that only eight subjects had a block prolongation of at least 2 h in the leg receiving dexamethasone perineurally. CONCLUSION Perineural administration of dexamethasone 2 mg showed a modest and inconsistent effect of questionable clinical relevance on block duration. CLINICAL TRIAL REGISTRATION NCT01981746.