Clinical analgesia correlates with decline in temporal summation in response to remifentanil infusion in patients with chronic neuropathic (radicular) pain

Background: Animal studies have shown that in addition to their antinociceptive effects, opioids have attenuated the electrophysiological "wind-up" phenomenon. Although effects of opioids on clinical pain and on temporal summation (TS), the human correlatives of this phenomenon, have been tested repeatedly, correlations between these two parameters have not been reported so far. Objectives: To search for possible correlations between the effects of remifentanil on clinical pain intensity and on the magnitude of TS in patients with chronic pain. Design: A single-blinded prospective study. Setting: A tertiary care pain clinic. Patients: Thirty-one patients (24 men) with chronic lumbar (radicular) neuropathic pain. Intervention: Intervenous administration of saline followed by remifentanil infusions. Main Outcome Measures: Clinical pain intensity and thermal TS measured at baseline, during infusion of each drug and 20 minutes after termination of remifentnail infusion. Results: Friedman test revealed statistically significant differences in the magnitudes of both clinical pain intensity and TS (X2(3) = 73, p < 0.001 and X2(3)=11.38, p=0.01, respectively). Post hoc analysis (Wilcoxon signed-rank test) showed significant differences between clinical pain intensities measured at all time points but significant reductions in the magnitudes of TS were found only during remifentanil compared to baseline (p = 0.014) and to saline (p = 0.019). The difference in clinical pain between saline and remifentanil positively correlated with the difference in TS measured at the same time points (Spearman's test; r=0.444, p=0.012). Conclusions: These results point to a possible causative relationship between TS and opioid analgesia.