Psychophysical or spinal reflex measures when assessing conditioned pain modulation?

Background Assessing conditioning pain modulation (CPM) with spinal reflex measures may produce more objective and stable CPM effects than using psychophysical measures. The aim of the study was to compare the CPM effect and test‐retest reliability between a psychophysical protocol with thermal test‐stimulus and a spinal reflex protocol with electrical test‐stimulus. Methods Twenty‐five healthy volunteers participated in two identical experiments separated by minimum 1 week. The thermal test‐stimulus was a constant heat stimulation of 120 seconds on the subjects' forearm with continuous ratings of pain intensity on a 10 cm visual analogue scale. The electrical test‐stimulus was repeated electrical stimulation on the arch of the foot for 120 seconds, which elicited a nociceptive withdrawal reflex recorded from the anterior tibial muscle. Conditioning stimulus was a 7°C water bath. Differences in the magnitude and test–retest reliability were investigated with repeated‐measures analysis of variance and by relative and absolute reliability indices. Results The CPM effect was 46% and 4.5% during the thermal and electrical test‐stimulus (p<0.001), respectively. Intraclass correlation coefficient of 0.5 and 0.4 was found with the electrical and thermal test‐stimulus, respectively. Wide limits of agreement were found for both the electrical (‐3.4 to 3.8 mA) and the thermal test‐stimulus (‐3.2...