Abstract
Different paradigms can assess the effect of conditioned pain modulation (CPM). The aim of the present study was to compare heat pain, as an often used test stimulus (TS), to painful cutaneous electrical stimulation (PCES), having the advantage of the additional recording of PCES-related evoked potentials. In 28 healthy subjects we applied heat and PCES at the dominant hand as test stimulus (TS) to compare the CPM-effect elicited by hand immersion into cold water (10 °C) as conditioning stimulus (CS). Subjects rated the pain intensity of TS at baseline, during and 5 min after CS application and additionally of CS, on a numerical rating scale (NRS) (0–100). The ‘early’ (during CS–before CS) and ‘late’ (after CS–before CS) CPM-effects were analyzed. Parallel to the PCES, the related evoked potentials were recorded via Cz to evaluate any changes in PCES-amplitudes. CS reduced significantly the pain intensity of both PCES and heat pain as TS. On a group level, the CPM-effect did not differ significantly between both paradigms. Both early and late CPM-effect based on PCES correlated significantly with the CS pain intensity (r = −0.630 and −0.503, respectively), whereas using heat pain the correlation was not significant. We found a significant reduction of PCES-amplitudes during CS, but this did not correlate with the PCES-induced pain intensity. Correlation with the CS painfulness (r = −0.464) did not achieve the significance level after Bonferroni correction. The extent of the CPM effects was similar in both testing paradigms at group level, despite intraindividual differences. Future studies should further elicit the exact mechanisms explaining the modality of these specific differences.
Highlights
Endogenous pain modulation seems to play an important role in the processing and transmission of nociceptive stimuli [1,2]
The aim of the present study was to compare extent of the induced Conditioned Pain Modulation (CPM)-effects by painful cold water as conditioning stimulus (CS) using pinprick-like painful cutaneous electrical stimulation (PCES) as test stimulus (TS) [20] with an established protocol using heat pain as TS [21], as it has been recently recommended to perform different protocols with the same subjects to increase the intraindividual reliability of different testing paradigms [9]
A further two subjects were excluded because the evoked potentials could not be utilized and a further two were excluded due to technical error during the PCES (Figure 2)
Summary
Endogenous pain modulation seems to play an important role in the processing and transmission of nociceptive stimuli [1,2]. Conditioned Pain Modulation (CPM) is the surrogate model for its assessment in humans, based on the analgesic effect of a noxious conditioning stimulus (CS) on a noxious test stimulus (TS) [3]. Brain Sci. 2020, 10, 684 in several pain syndromes like fibromyalgia, irritable bowel syndrome or osteoarthritis [4,5] and to play a role in the development of postoperative pain and in the transition from acute to chronic pain [5]. The recent recommendations on practice of CPM [9] highlighted the lack of uniform protocols for performing CPM and further data are needed to be able to compare different protocols, combine data from different studies and make conclusions on the clinical utilization of this test by meta-analyses and systematic reviews. In previous studies we proposed the term “early” CPM-effect for the difference between the pain intensity of the TS before and during the CS, and “late” CPM-effect for the difference between the pain intensity of the TS before and after the conditioning [10,11]
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