Effect of preoperative oral dextromethorphan on immediate and late postoperative pain and hyperalgesia after total abdominal hysterectomy.

Dextromethorphan is a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist known to inhibit wind-up and NMDA-mediated nociceptive responses of dorsal horn neurons. Experimental and clinical studies indicate that NMDA-receptor antagonists may potentiate the effect of analgesics such as morphine, local anesthetics and NSAIDs. Results from previous clinical studies of dextromethorphan in postoperative pain are conflicting, possibly related to administration of insufficient doses of the drug. Fifty patients scheduled for non-malignant elective abdominal hysterectomy in general anesthesia were randomized to receive oral dextromethorphan 150 mg, or placebo 1 h before surgery. The patients received patient-controlled analgesia with morphine for 24 h postoperatively as the only analgesic. Patient-controlled analgesia (PCA) morphine consumption was reduced with 30% from 0-4 h after operation in patients receiving dextromethorphan compared with placebo (P=0.02); no differences were observed from 5-24 h postoperatively. There were no significant differences between groups for visual analogue scale scores at rest, during cough, or during mobilization, pressure pain detection thresholds, von Frey hair pain detection thresholds, or peak flow. At 24 h after operation, hyperalgesia to von Frey hair stimulation proximal to the surgical wound was easily detected in 23 of 25 patients receiving dextromethorphan, and in 22 of 25 patients receiving placebo, with no significant difference between groups. Pooled data from both groups showed a weak but significant correlation between the extent of hyperalgesia at 24 h after operation, and total 24 h postoperative PCA morphine consumption (Rs=0.28, P=0.05). Three months postoperatively, hyperalgesia was still detectable in 18 of 22 examined patients in the dextromethorphan group, and in 16 of 23 patients in the placebo group, without statistical differences between groups. There were no significant differences in side-effects (nausea, vomiting, sedation). In conclusion, oral dextromethorphan 150 mg reduced PCA morphine consumption immediately (0-4 h) after hysterectomy, without prolonged effects on pain or wound hyperalgesia. A positive correlation between the magnitude of wound hyperalgesia at 24 h after operation, and total 24 h postoperative PCA morphine consumption was demonstrated.