Cross-sectional experimental assessment of pain modulation as part of multidimensional profiling of people with cervicogenic headache: protocol for a feasibility study

BACKGROUND

An endogenous pain modulation profile, reflecting antinociceptive and pronociceptive mechanisms, may help to direct management by targeting the involved pain mechanism. For individuals with cervicogenic headache (CeH), the characteristics of such profiles were never investigated. However, the individual nature of experiencing pain demands profiling within a multidimensional framework including psychosocial lifestyle characteristics. The objective of the current protocol is to assess the pain modulation profile, which includes psychosocial lifestyle characteristics among people with CeH.

METHODS AND ANALYSIS

A protocol is described to map pain modulation profiles in people with CeH. A cross-sectional non-randomised experimental design will be used to assess feasibility of mapping these profiles. The pain modulation profile is composed based on results on the Depression, Anxiety, Stress Scale, Pittsburgh Sleep Quality Index, Headache Impact Test and on responses to temporal summation of pain (pinprick), conditioned pain modulation and widespread hyperalgesia (mechanical pressure pain threshold and cuff algometry). Primary analyses will report results relating to outcomes on feasibility. Secondary analyses will involve an analysis of proportions (%) of the different psychosocial lifestyle profiles and pain profiles.

ETHICS AND DISSEMINATION

Ethical approval was granted by the Ethics Committee Research UZ/KU Leuven (Registration number B3222024001434) on 30 May 2024. Results will be published in peer-reviewed journals, at scientific conferences and, through press releases. Protocol V.3. protocol date: 3 June 2024.

Effects of virtual reality on psychophysical measures of pain: superiority to imagination and nonimmersive conditions

ABSTRACT

Virtual reality (VR) has been shown to be effective in pain management. However, to date, little is known about the mechanisms by which immersive experiences influence pain processing. The aim of this study was to investigate the direct effects of an immersive VR environment on the perception of experimental pain in individuals with chronic pain and pain-free controls. The immersion in a VR landscape was compared with mental imagery and a nonimmersive control condition. Using a randomized within-crossover design, pressure pain detection and tolerance thresholds, spatial and temporal summation (SSP, TSP), and conditioned pain modulation (CPM) were measured in 28 individuals with chronic pain and 31 pain-free controls using phasic cuff pressure on the legs. Direct comparison between the groups showed that although individuals with pain had significantly lower pain thresholds, reduced CPM effects, and increased TSP, the VR condition had the same pain-inhibitory effect on pain thresholds as in pain-free controls. Conditioned pain modulation effects were reduced by all conditions compared with baseline. There were no significant differences between conditions and baseline for TSP and SSP. Overall, pain modulatory effects were largest for VR and smallest for imagery. These results demonstrate that immersion in a VR environment has an increasing effect on pain thresholds, reduces pain inhibition in a CPM paradigm, and has no effects on TSP. This applies for participants with chronic pain and pain-free controls. These VR effects exceeded the effects of mental imagery on the nonimmersive control condition. This indicates that VR effectively modulates pain perception in both patients and controls irrespective of differences in pain perception.

Pain Science in Practice (Part 7): How Is Descending Modulation of Pain Measured?

SYNOPSIS: Understanding the descending pain modulatory system allows for a neuroscientific explanation of naturally occurring pain relief. Evidence from basic science and clinical studies on the effectiveness of drugs in certain patient groups led to pharmacological manipulation of the descending pain modulatory system for analgesia. Understanding mechanisms and theories helps clinicians make sense of chronic musculoskeletal pain. This editorial explains how test paradigms, including conditioned pain modulation, offset analgesia, and stress-induced analgesia work, provide an overview of a placebo analgesia circuitry, and discusses how evoking activity in the descending pain modulatory system using specific paradigms can give new insights into how specific treatments work to reduce pain. J Orthop Sports Phys Ther 2024;54(2):1-6. doi:10.2519/jospt.2024.12113.

A back-translational study of descending interactions with the induction of hyperalgesia by high-frequency electrical stimulation in rat and human

ABSTRACT

In humans and animals, high-frequency electrocutaneous stimulation (HFS) induces an "early long-term potentiation-like" sensitisation, where synaptic plasticity is underpinned by an ill-defined interaction between peripheral input and central modulatory processes. The relative contributions of these processes to the initial pain or nociceptive response likely differ from those that underpin development of the heightened response. To investigate the impact of HFS-induced hyperalgesia on pain and nociception in perception and neural terms, respectively, and to explore the impact of descending inhibitory pathway activation on the development of HFS-induced hyperalgesia, we performed parallel studies utilising identical stimuli to apply HFS concurrent to (1) a conditioned pain modulation paradigm during psychophysical testing in healthy humans or (2) a diffuse noxious inhibitory controls paradigm during in vivo electrophysiological recording of spinal neurones in healthy anaesthetised rats. High-frequency electrocutaneous stimulation alone induced enhanced perceptual responses to pinprick stimuli in cutaneous areas secondary to the area of electrical stimulation in humans and increased the excitability of spinal neurones which exhibited stimulus intensity-dependent coded responses to pinprick stimulation in a manner that tracked with human psychophysics, supporting their translational validity. Application of a distant noxious conditioning stimulus during HFS did not alter perceived primary or secondary hyperalgesia in humans or the development of primary or secondary neuronal hyperexcitability in rats compared with HFS alone, suggesting that, upon HFS-response initiation in a healthy nervous system, excitatory signalling escapes inhibitory control. Therefore, in this model, dampening facilitatory mechanisms rather than augmenting top-down inhibitions could prevent pain development.

A novel computational approach to pain perception modelling within a Bayesian framework using quantitative sensory testing

Pain perception can be studied as an inferential process in which prior information influences the perception of nociceptive input. To date, there are no suitable psychophysical paradigms to measure this at an individual level. We developed a quantitative sensory testing paradigm allowing for quantification of the influence of prior expectations versus current nociceptive input during perception. Using a Pavlovian-learning task, we investigated the influence of prior expectations on the belief about the varying strength of association between a painful electrical cutaneous stimulus and a visual cue in healthy subjects (N = 70). The belief in cue-pain associations was examined with computational modelling using a Hierarchical Gaussian Filter (HGF). Prior weighting estimates in the HGF model were compared with the established measures of conditioned pain modulation (CPM) and temporal summation of pain (TSP) assessed by cuff algometry. Subsequent HGF-modelling and estimation of the influence of prior beliefs on perception showed that 70% of subjects had a higher reliance on nociceptive input during perception of acute pain stimuli, whereas 30% showed a stronger weighting of prior expectations over sensory evidence. There was no association between prior weighting estimates and CPM or TSP. The data demonstrates relevant individual differences in prior weighting and suggests an importance of top-down cognitive processes on pain perception. Our new psychophysical testing paradigm provides a method to identify individuals with traits suggesting greater reliance on prior expectations in pain perception, which may be a risk factor for developing chronic pain and may be differentially responsive to learning-based interventions.

No relevant differences in conditioned pain modulation effects between parallel and sequential test design. A cross-sectional observational study

Background

Conditioned pain modulation (CPM) is measured by comparing pain induced by a test stimulus with pain induced by the same test stimulus, either during (parallel design) or after (sequential design) the conditioning stimulus. Whether design, conditioning stimulus intensity and test stimulus selection affect CPM remains unclear.

Methods

CPM effects were evaluated in healthy participants (N = 89) at the neck, forearm and lower leg using the cold pressor test as the conditioning stimulus. In three separate experiments, we compared the impact of (1) design (sequential versus parallel), (2) conditioning stimulus intensity (VAS 40/100 versus VAS 60/100), and (3) test stimulus selection (single versus dual, i.e., mechanical and thermal). Statistical analyses of the main effect of design (adjusted for order) and experiment were conducted using linear mixed models with random intercepts.

Results

No significant differences were identified in absolute CPM data. In relative CPM data, a sequential design resulted in a slightly lower CPM effect compared to a parallel design, and only with a mechanical test stimulus at the neck (−6.1%; 95% CI [−10.1 to −2.1]) and lower leg (−5.9%; 95% CI [−11.7 to −0.1]) but not forearm (−4.5%; 95% CI [−9.0 to 0.1]). Conditioning stimulus intensity and test stimulus selection did not influence the CPM effect nor the difference in CPM effects derived from parallel versus sequential designs.

Conclusions

Differences in CPM effects between protocols were minimal or absent. A parallel design may lead to a minimally higher relative CPM effect when using a mechanical test stimulus. The conditioning stimulus intensities assessed in this study and performing two test stimuli did not substantially influence the differences between designs nor the magnitude of the CPM effect.

Medial Prefrontal High-Definition Transcranial Direct Current Stimulation to Improve Pain Modulation in Chronic Low Back Pain: A Pilot Randomized Double-blinded Placebo-Controlled Crossover Trial

Chronic low back pain (CLBP) is highly disabling, but often without identifiable source. Focus has been on impaired anti-nociceptive mechanisms contributing to pain maintenance, though methods of targeting this impairment remain limited. This randomised-controlled cross-over pilot trial used active versus sham medial prefrontal cortex (mPFC) high-definition transcranial direct current stimulation (HD-tDCS) for 3-consecutive days to improve descending pain inhibitory function. Twelve CLBP patients were included with an average visual analogue scale (VAS) pain intensity of 3.0 ± 1.5 and pain duration of 5.3 ± 2.6 years. Pressure pain thresholds (PPTs), conditioned pain modulation (CPM), and temporal summation of pain (TSP) assessed by cuff algometry, as well as pain symptomatology (intensity, unpleasantness, quality, disability) and related psychological features (pain catastrophizing, anxiety, affect), were assessed on Day1 before 3 consecutive days of HD-tDCS sessions (each 20 minutes), at 24-hours (Day 4) and 2-weeks (Day 21) following final HD-tDCS. Blinding was successful. No significant differences in psychophysical (PPT, CPM, TSP), symptomatology or psychological outcomes were observed between active and sham HD-tDCS on Day4 and Day21. CPM-effects at Day 1 negatively correlated with change in CPM-effect at Day4 following active HD-tDCS (P = .002). Lack of efficacy was attributed to several factors, not least that patients did not display impaired CPM at baseline. TRIAL REGISTRATION: : ClinicalTrials.gov (NCT03864822). PERSPECTIVE: Medial prefrontal HD-tDCS did not alter pain, psychological nor psychophysical outcomes, though correlational analysis suggested response may depend on baseline pain inhibitory efficacy, with best potential effects in patients with severe impairments in descending pain inhibitory mechanisms. Future work should focus on appropriate patient selection and optimising stimulation targeting.

Medial Prefrontal Transcranial Direct Current Stimulation Aimed to Improve Affective and Attentional Modulation of Pain in Chronic Low Back Pain Patients

Chronic low back pain (CLBP) is often without clear underlying pathology. Affective disturbance and dysfunctional pain mechanisms, commonly observed in populations with CLBP, have, therefore, been suggested as potential contributors to CLBP development and maintenance. However, little consensus exists on how these features interact and if they can be targeted using non-invasive brain stimulation. In this pilot trial, 12 participants completed two phases (Active or Sham) of high-definition transcranial direct current stimulation (HD-tDCS) to the medial prefrontal cortex, applied for 20 min on three consecutive days. Clinical pain ratings, questionnaires, and sensitivity to painful cuff pressure were completed at baseline, then 4 trials of conditioned pain modulation (CPM; alone, with distraction using a Flanker task, with positive affect induction, and with negative affect induction using an image slideshow) were performed prior to HD-tDCS on Day 1 and Day 4 (24 h post-HD-tDCS). At baseline, attentional and affective manipulations were effective in inducing the desired state (p < 0.001) but did not significantly change the magnitude of CPM-effect. Active HD-tDCS was unable to significantly alter the magnitude of the shift in valence and arousal due to affective manipulations, nor did it alter the magnitude of CPM under any basal, attentional, or affective manipulation trial significantly on Day 4 compared to sham. The CPM-effect was greater across all manipulations on Day 1 than Day 4 (p < 0.02) but also showed poor reliability across days. Future work is needed to expand upon these findings and better understand how and if HD-tDCS can be used to enhance attentional and affective effects on pain modulation.

The Effect of Stress on Repeated Painful Stimuli with and Without Painful Conditioning

OBJECTIVES

Stress and pain have been interrelated in clinical widespread pain conditions. Studies indicate that acute experimental stress in healthy volunteers has a negative effect on the descending inhibitory pain control system and thus the ability to inhibit one painful stimulus with another (conditioned pain modulation [CPM]) although without effect on general pain sensitivity. CPM effects can be assessed immediately after the stress induction, whereas some physiological stress responses (e.g., cortisol release) are delayed and longer lasting. It is unclear whether CPM may relate to stress-induced increases in cortisol.

DESIGN

Twenty-five healthy men had CPM effects measured over a period of 10 minutes. Pain detection thresholds (PDTs) were assessed by repeated test stimuli with cuff algometry on one leg, with and without painful cuff pressure conditioning on the contralateral leg. CPM effects, assessed as the increase in PDT during conditioning stimulation compared with without, were measured before and after experimental stress and a control condition (Montreal Imaging Stress Task [MIST]). Saliva cortisol levels and self-perceived stress were collected.

RESULTS

Participants reported the MIST to be more stressful compared with the MIST control, but cortisol levels did not change significantly from baseline. In all sessions, PDT increased during conditioning (P = 0.001), although the MIST compared with the MIST control had no significant effect on PDT or CPM effects. A negative correlation between changes in cortisol and conditioned PDT was found when applying the MIST (P < 0.03).

CONCLUSIONS

No significant effect of stress was found on CPM compared with a matched control condition. Individual changes in experimental stress and in conditioned pain sensitivity may be linked with cortisol.

The impact of paradigm and stringent analysis parameters on measuring a net conditioned pain modulation effect: A test, retest, control study

BACKGROUND

Reporting in conditioned pain modulation (CPM) studies is not standardised. Here, two CPM protocols were performed in populations of healthy human subjects in order to investigate the influence of the CPM paradigm and stringent analyses parameters on the identification of a net CPM effect.

METHODS

A standard thermal or mechanical CPM protocol was carried out on 25 and 17 subjects, respectively. The standard error of measurement (SEM) of the CPM effect was calculated in order to determine a change in pain thresholds greater than that due to measurement error or 'real' change in test scores. In addition, each individual underwent a minimum of two control CPM sessions, which were paired with the CPM test sessions. To quantify a net CPM effect, the intrasession difference between baseline and conditioning was subtracted from the difference calculated at the same time points during the control session.

RESULTS

For both protocols, excellent reliability for intrasession repeats of the test stimulus at baseline was demonstrated for thermal and mechanical stimulation (ICC > 0.9). Test-retest subject responses (in terms of experimental Session 1 versus. Session 2) showed excellent reliability for mechanical (ICC > 0.8), compared to thermal stimulation, which ranged from poor to moderate (ICC < 0.4->0.75). However, calculating the net CPM effect using control session data demonstrated poor-fair reliability for both protocols (ICC < 0.4-0.59).

CONCLUSION

Calculating the net CPM effect should be optimised and standardised for comparison of CPM data collected from global research groups. Recommendation is made for the performance of a multicentre, test-retest study.

Activation of the descending pain modulatory system using cuff pressure algometry: Back translation from man to rat

BACKGROUND

Diffuse noxious inhibitory controls (DNIC) as measured in rat and conditioned pain modulation (CPM), the supposed psychophysical paradigm of DNIC measured in humans, are unique manifestations of an endogenous descending modulatory pathway that is activated by the application of a noxious conditioning stimulus. The predictive value of the human CPM processing is crucial when deliberating the translational worth of the two phenomena.

METHODS

For CPM or DNIC measurement, test and conditioning stimuli were delivered using a computer-controlled cuff algometry system or manual inflation of neonate blood pressure cuffs, respectively. In humans (n = 20), cuff pain intensity (for pain detection and pain tolerance thresholds) was measured using an electronic visual analogue scale. In isoflurane-anaesthetized naïve rats, nociception was measured by recording deep dorsal horn wide dynamic range (WDR) neuronal firing rates (n = 7) using in vivo electrophysiology.

RESULTS

A painful cuff-pressure conditioning stimulus on the leg increased pain detection and pain tolerance thresholds recorded by cuff stimulation on the contralateral leg in humans by 32% ± 3% and 24% ± 2% (mean ± SEM) of baseline responses, respectively (p < .001). This finding was back-translated by revealing that a comparable cuff-pressure conditioning stimulus (40 kPa) on the hind paw inhibited the responses of WDR neurons to noxious contralateral cuff test stimulation to 42% ± 9% of the baseline neuronal response (p = .003).

CONCLUSIONS

These data substantiate that the noxious cuff pressure paradigm activates the descending pain modulatory system in rodent (DNIC) and man (CPM), respectively. Future back and forward translational studies using cuff pressure algometry may reveal novel mechanisms in varied chronic pain states.

SIGNIFICANCE

This study provides novel evidence that a comparable noxious cuff pressure paradigm activates a unique form of endogenous inhibitory control in healthy rat and man. This has important implications for the forward translation of bench and experimental pain research findings to the clinical domain. If translatable mechanisms underlying dysfunctional endogenous inhibitory descending pathway expression (previously evidenced in painful states in rat and man) were revealed using cuff pressure algometry, the identification of new analgesic targets could be expedited.

Delayed effects of attention on pain sensitivity and conditioned pain modulation

BACKGROUND

Efficacy of pain modulation is assessed as the difference in pain sensitivity during a painful conditioning, compared to before (conditioning pain modulation, CPM). Attention can be assessed with the Stroop task, in which participants report the number of words on a screen; either congruent or incongruent with the value of the words. Attention away from painful stimuli during CPM enhances the CPM effect. However, it is unknown if attention influences CPM effects when the two are done in sequence.

METHODS

Healthy men (n = 25) underwent cuff algometry CPM-assessment where the pressure-pain detection and tolerance thresholds (PTT) were recorded on one leg with and without contralateral conditioning. Two identical sessions of four test stimuli equal to PTT (5 s, 1-min interval, scored on a visual analogue scale, VAS) with a painful conditioning from the second to the last test-stimulus were performed. Stroop sessions were followed by test stimuli with or without painful conditioning.

RESULTS

The VAS scores in the first two sessions showed excellent reliability (ICC = 0.92). VAS scores were lower in sessions with Stroop compared to sessions without Stroop (p = .05) indicating an analgesic effect of Stroop. Participants were subgrouped into CPM responders and CPM non-responders according to CPM effects in the first two sessions. CPM non-responders (n = 13) showed facilitation to repeated noxious stimuli in all sessions with no effect of conditioning or Stroop (p = .02).

CONCLUSION

Attention and CPM both modulate pain in healthy men. Attention-induced analgesia works in CPM non-responders. Results indicate that attention and CPM are not the same and that they do not demonstrate additive effects when applied in sequence.

SIGNIFICANCE

Pain sensitivity is reduced after an attention task in healthy men. The delayed effects from attention only have minor effects on Conditioned Pain Modulation (CPM), and results support that attention-driven analgesia works independently of CPM. Results indicate that individual strategies for pain inhibition exist and that an overlap between the mechanisms of CPM and selective attention is limited. Moreover, painful phasic stimuli may increase the number of healthy volunteers with negative CPM effects.