Evoked potentials after painful cutaneous electrical stimulation depict pain relief during a conditioned pain modulation

Background: Conditioned pain modulation (CPM) evaluates the pain modulating effect of a noxious conditioning stimulus (CS) on another noxious test stimulus (TS), mostly based solely on subjective pain ratings. We used painful cutaneous electrical stimulation (PCES) to induce TS in a novel CPM-model. Additionally, to evaluate a more objective parameter, we recorded the corresponding changes of cortical evoked potentials (PCES-EP). Methods: We examined the CPM-effect in 17 healthy subjects in a randomized controlled cross-over design during immersion of the non-dominant hand into 10 degrees C or 24 degrees C cold water (CS). Using three custom-built concentric surface electrodes, electrical stimuli were applied on the dominant hand, inducing pain of 40-60 on NRS 0-100 (TS). At baseline, during and after CS we assessed the electrically induced pain intensity and electrically evoked potentials recorded over the central electrode (Cz). Results: Only in the 10 degrees C-condition, both pain (52.6 +/- 4.4 (baseline) vs. 30.3 +/- 12.5 (during CS)) and amplitudes of PCES-EP (42.1 +/- 13.4 mu V (baseline) vs. 28.7 +/- 10.5 mu V (during CS)) attenuated during CS and recovered there after (all p < 0.001). In the 10 degrees C-condition changes of subjective pain ratings during electrical stimulation and amplitudes of PCES-EP correlated significantly with each other (r = 0.5) and with CS pain intensity (r = 0.5). Conclusions: PCES-EPs are a quantitative measure of pain relief, as changes in the electrophysiological response are paralleled by a consistent decrease in subjective pain ratings. This novel CPM paradigm is a feasible method, which could help to evaluate the function of the endogenous pain modulation processes.