Comparing test-retest reliability and magnitude of conditioned pain modulation using different combinations of test and conditioning stimuli

A novel clinical applicable bed-side tool for assessing conditioning pain modulation: proof-of-concept

Background and aims

In recent years, focus on assessing descending pain modulation or conditioning pain modulation (CPM) has emerged in patients with chronic pain. This requires reliable and simple to use bed-side tools to be applied in the clinic. The aim of the present pilot study was to develop and provide proof-of-concept of a simple clinically applicable bed-side tool for assessing CPM.

Methods

A group of 26 healthy volunteers participated in the experiment. Pressure pain thresholds (PPT) were assessed as test stimuli from the lower leg before, during and 5 min after delivering the conditioning tonic painful pressure stimulation. The tonic stimulus was delivered for 2 min by a custom-made spring-loaded finger pressure device applying a fixed pressure (2.2 kg) to the index finger nail. The pain intensity provoked by the tonic stimulus was continuously recorded on a 0–10 cm Visual Analog Scale (VAS).

Results

The median tonic pain stimulus intensity was 6.7 cm (interquartile range: 4.6–8.4 cm) on the 10 cm VAS. The mean PPT increased significantly (P = 0.034) by 55 ± 126 kPa from 518 ± 173 kPa before to 573 ± 228 kPa during conditioning stimulation. When analyzing the individual CPM responses (increases in PPT), a distribution of positive and negative CPM responders was observed with 69% of the individuals classified as positive CPM responders (increased PPTs = anti-nociceptive) and the rest as negative CPM responders (no or decreased PPTs = Pro-nociceptive). This particular responder distribution explains the large variation in the averaged CPM responses observed in many CPM studies. The strongest positive CPM response was an increase of 418 kPa and the strongest negative CPM response was a decrease of 140 kPa.

Conclusions

The present newly developed conditioning pain stimulator provides a simple, applicable tool for routine CPM assessment in clinical practice. Further, reporting averaged CPM effects should be replaced by categorizing volunteers/patients into anti-nociceptive and pro-nociceptive CPM groups.

Implications

The finger pressure device provided moderate-to-high pain intensities and was useful for inducing conditioning stimuli. Therefore, the finger pressure device could be a useful bed-side method for measuring CPM in clinical settings with limited time available. Future bed-side studies involving patient populations are warranted to determine the usefulness of the method.

Reliability of the conditioned pain modulation paradigm across three anatomical sites

Background and aims Conditioned Pain Modulation (CPM) is a measure of pain inhibition-facilitation in humans that may elucidate pain mechanisms and potentially serve as a diagnostic test. In laboratory settings, the difference between two pain measures [painful test stimulus (TS) without and with the conditioning stimulus (CS) application] reflects the CPM magnitude. Before the CPM test can be used as a diagnostic tool, its reliability on the same day (intra-session) and across multiple days (inter-session) needs to be known. Furthermore, it is important to determine the most reliable anatomical sites for both the TS and the CS. This study aimed to measure the intra-session and inter-session reliability of the CPM test paradigm in healthy subjects with the TS (pressure pain threshold-PPT) applied to three test sites: the face, hand, and dorsum of the foot, and the CS (cold pressor test-CPT) applied to the contralateral hand. Methods Sixty healthy participants aged 18-65 were tested by the same examiner on 3 separate days, with an interval of 2-7 days. On each day, testing was comprised of two identical experimental sessions in which the PPT test was performed on each of the three dominant anatomical sites in randomized order followed by the CPM test (repeating the PPT with CPT on the non-dominant hand). CPM magnitude was calculated as the percent change in PPT. The Intraclass Correlation Coefficient (ICC), Coefficient of Variation (CV), and Bland-Altman analyses were used to assess reliability. Results PPT relative reliability ranged from good to excellent at all three sites; the hand showed an intra-session ICC of 0.90 (0.84, 0.94) before CPT and ICC of 0.89 (0.83, 0.92) during CPT. The PPT absolute reliability was also high, showing a low bias and small variability when performed on all three sites; for example, CV of the hand intra-session was 8.0 before CPT and 8.1 during CPT. The relative reliability of the CPM test, although only fair, was most reliable when performed during the intra-session visits on the hand; ICC of 0.57 (0.37, 0.71) vs. 0.20 (0.03, 0.39) for the face, and 0.22 (0.01, 0.46) for the foot. The inter-session reliability was lower in all three anatomical sites, with the best reliability on the hand with an ICC of 0.40 (0.23, 0.55). The pattern of absolute reliability of CPM was similar to the relative reliability findings, with the reliability best on the hand, showing lower intra-session and inter-session variability (CV% = 43.5 and 51.5, vs. 70.1 and 73.1 for the face, and 75.9 and 78.9 for the foot). The CPM test was more reliable in women than in men, and in older vs. younger participants. Discussion The CPM test was most reliable when the TS was applied to the dominant hand and CS performed on the contralateral hand. These data indicate that using the CS and TS in the same but contralateral dermatome in CPM testing may create the most reliable results.

Central Sensitisation and functioning in patients with chronic low back pain: protocol for a cross-sectional and cohort study

INTRODUCTION

A relevant subsample of patients with chronic low back pain (CLBP) have manifested augmented central pain processing, central sensitisation (CS). Patients with CLBP have limited functioning and participation. Theoretically, physical functioning in patients with CLBP can plausibly be linked to CS; however, evidence to explain such association is scarce. Moreover, there is no gold standard for CS diagnosis. The objectives of the study are: (1) to analyse the association between instruments assessing reference symptoms and signs attributed to CS; (2) to analyse whether reference symptoms and signs attributed to CS are associated with functioning measurement outcomes; and (3) to analyse whether changes (between baseline and discharge) in reference symptoms and signs attributed to CS are related to changes in each of the functioning measurement outcomes.

METHODS AND ANALYSIS

A cross-sectional and longitudinal observational study is performed with measurements taken at baseline and discharge of an interdisciplinary rehabilitation programme. A sample size of 110 adult patients with CLBP has been calculated for the study. CS measurements are: Central Sensitisation Inventory, quantitative sensory testing and heart rate variability. Functioning measurements are: lifting capacity, maximal aerobic capacity, accelerometry and reported functioning. Statistical analyses to be performed are: (1) correlation between CS measurements, (2) multiple regression between functioning (dependent variable) and CS measurements (independent variable), and (3) multiple regression between changes in scores of functioning (dependent variable) and CS measurements (independent variable), and corrected for sex and age.

ETHICS AND DISSEMINATION

The study obtained the clearance to its implementation from the Medical Research Ethics Committee of the University Medical Center Groningen in July 2017. The results will be disseminated through scientific publications in peer-reviewed journals, presentations at relevant conferences, and reports to stakeholders.

TRIAL REGISTRATION NUMBER

NTR7167/NL6980.

Photobiomodulation therapy and transcutaneous electrical nerve stimulation on chronic neck pain patients: Study protocol clinical trial (SPIRIT Compliant)

INTRODUCTION

Chronic neck pain is a common musculoskeletal disorder that is associated with functional disability and decreased of quality of life. Electrophysical agents are commonly used to relieve pain, however the effects of combined use of these agents are little studied. The objective is to investigate the efficacy of photobiomodulation and electrical stimulation to relieve pain, both in isolation and combined.

MATERIALS AND METHODS

This a 4-arm randomized placebo-controlled trial with patient and evaluator blinded. This study will be performed in Department of Physical Therapy at Federal University of São Carlos, São Carlos/SP, Brazil. One hundred and forty-four patients with chronic neck pain will be randomized into 4 groups: active photobiomodulation therapy with active electrical stimulation, active photobiomodulation therapy, active electrical stimulation, or placebo treatment. They will receive 10 sessions of treatment.

PRIMARY OUTCOME

pain intensity (measured by pain numerical rating scale) posttreatment.

SECONDARY OUTCOMES

pain during movement, neck disability, range of motion, pressure pain threshold, temporal summation, conditioned pain modulation, depressive symptoms, pain catastrophizing, quality of life, analgesic intake, and global perceived effect at posttreatment (10 sessions). Pain intensity and global perceived effect will also be measured after 6 weeks randomization.

DISCUSSION

The findings of this study might clarify the importance of using the photobiomodulation therapy and transcutaneous electrical nerve stimulation for patients with chronic neck pain.

TRIAL REGISTRATION

NCT04020861. https://clinicaltrials.gov/ct2/show/NCT04020861?term=NCT04020861&draw=2&rank=1.

Determining Real Change in Conditioned Pain Modulation: A Repeated Measures Study in Healthy Volunteers

Conditioned pain modulation (CPM) is a potentially useful biomarker in pain populations; however, a statistically robust interpretation of change scores is required. Currently, reporting of CPM does not consider measurement error. Hence, the magnitude of change representing a "true" CPM effect is unknown. This study determined the standard error of measurement (SEM) and proportion of healthy participants showing a "true" CPM effect with a standard CPM paradigm. Fifty healthy volunteers participated in an intersession reliability study using pressure pain threshold (PPT) test stimulus and contact heat, cold water, and sham conditioning stimuli. Baseline PPTs were used to calculate SEM and >±2 × SEM to determine CPM effect. SEM for PPT was .21 kg/cm². An inhibitory CPM effect (>+2 SEM) was elicited in 59% of subjects in response to cold stimulus; in 44% to heat. Intrasession and intersession reliability of within-subject CPM response was poor (kappa coefficient <.36). Measurement error is important in determining CPM effect and change over time. Even when using reliable test stimuli, and incorporating measures to limit bias and error, CPM intersession reliability was fair and demonstrated a large degree of within-subject variation. Determining "true" change in CPM will underpin future interrogations of intraindividual differences in CPM. PERSPECTIVE: This study used a distribution-based statistical approach to identify real change in CPM, based on the SEM for the test stimulus. Healthy volunteers demonstrate substantial within-subject variation; CPM effect was paradigm dependent at intrasession testing and unstable to the same paradigm at intersession testing.

Association Between the 2011 Fibromyalgia Survey Criteria and Multisite Pain Sensitivity in Knee Osteoarthritis

OBJECTIVES

The present study evaluated the relationship between the 2011 American College of Rheumatology fibromyalgia (FM) survey criteria and quantitative sensory testing (QST).

MATERIALS AND METHODS

Patients with knee osteoarthritis scheduled to undergo knee arthroplasty completed the FM survey criteria and self-report measures assessing clinical symptoms. Patients also underwent a battery of QST procedures at the surgical knee and remote body sites, including pressure algometry, conditioned pain modulation, and temporal summation. All assessments were completed before surgery. FM survey criteria were used to calculate a continuous FM score indicating FM severity.

RESULTS

A total of 129 patients were analyzed. Of these, 52.7% were female, 93.8% were Caucasian, and 3.8% met the FM survey criteria for FM classification. Mean age for females (63.6 y) and males (64.7 y) was similar. Females and males differed significantly in nearly every outcome, including FM severity, clinical pain, anxiety, depression, and pressure pain sensitivity. In females, FM scores significantly correlated with pressure pain sensitivity, but not conditioned pain modulation or temporal summation, such that increased sensitivity was associated with greater FM severity at all body sites examined. In addition, as FM scores increased, the association between pain sensitivity at the surgical knee and pain sensitivity at remote body sites also increased. No relationship between FM score and QST was observed in males.

DISCUSSION

We demonstrated an association between diffuse hyperalgesia as measured by QST and FM severity in females with knee osteoarthritis. These results suggest that the FM survey criteria may represent a marker of pain centralization in females with potential utility in clinical decision making.

Systemic Exercise-Induced Hypoalgesia Following Isometric Exercise Reduces Conditioned Pain Modulation

Objective

Physically active individuals show greater conditioned pain modulation (CPM) compared with less active individuals. Understanding the effects of acute exercise on CPM may allow for a more targeted use of exercise in the management of pain. This study investigated the effects of acute isometric exercise on CPM. In addition, the between-session and within-session reliability of CPM was investigated.

Design

Experimental, randomized crossover study.

Setting

Laboratory at Marquette University.

Subjects

Thirty healthy adults (19.3±1.5 years, 15 males).

Methods

Subjects underwent CPM testing before and after isometric exercise (knee extension, 30% maximum voluntary contraction for three minutes) and quiet rest in two separate experimental sessions. Pressure pain thresholds (PPTs) at the quadriceps and upper trapezius muscles were assessed before, during, and after ice water immersions.

Results

PPTs increased during ice water immersion (i.e., CPM), and quadriceps PPT increased after exercise (P < 0.05). CPM decreased similarly following exercise and quiet rest (P > 0.05). CPM within-session reliability was fair to good (intraclass correlation coefficient [ICC] = 0.43-0.70), and the between-session reliability was poor (ICC = 0.20-0.35). Due to the variability in the systemic exercise-induced hypoalgesia (EIH) response, participants were divided into systemic EIH responders (N = 9) and nonresponders (N = 21). EIH responders experienced attenuated CPM following exercise (P = 0.03), whereas the nonresponders showed no significant change (P > 0.05).

Conclusions

Isometric exercise decreased CPM in individuals who reported systemic EIH, suggesting activation of shared mechanisms between CPM and systemic EIH responses. These results may improve the understanding of increased pain after exercise in patients with chronic pain and potentially attenuated CPM.

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.

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 s 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 s, 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 to 3.6 cm).

CONCLUSIONS

More pronounced CPM effect was demonstrated when using a psychophysical protocol with thermal test-stimulus compared to a spinal reflex protocol with electrical test-stimulus. Fair relative reliability and poor absolute reliability (due to high intraindividual variability) was found in both protocols.

SIGNIFICANCE

The large difference in CPM effect between the two protocols suggests that the CPM effect relates to pain perception rather than nociception on the spinal level. Due to poor absolute intrarater reliability, we recommend caution and further research before using any of the investigated CPM protocols in clinical decision making on an individual level.

Walking increases pain tolerance in humans: an experimental cross-over study

Background and aims

Exercise is commonly used as treatment for chronic pain with positive long-term effects on pain and pain-related disability. In pain-free subjects, hypoalgesia following an acute bout of exercise compared with a control condition has consistently been demonstrated also known as exercise-induced hypoalgesia (EIH). Walking exercise, a low intensity aerobic exercise, is frequently used in clinical practice as an easily applicable intervention for patients with chronic pain. Walking exercise is furthermore recommended as an effective treatment for patients with chronic musculoskeletal pain conditions to alleviate pain and reduce disability, however, the effect of walking on pain sensitivity is currently unknown. The aims of the present study were to investigate (1) the acute effect of walking on pain sensitivity, and (2) the relative (between-subjects) and absolute (within-subject) test-retest reliability of the hypoalgesic response across two sessions separated by 1 week.

Methods

In this randomised experimental cross-over study including two identical sessions, 35 pain-free subjects performed a standardized 6 min walking test and a duration-matched quiet rest condition in a randomized and counterbalanced order in each session. Before and after both conditions, handheld pressure pain thresholds (PPTs) were assessed at the thigh and shoulder, and pressure pain thresholds (cPPT) and pain tolerance (cPTT) were assessed with computer-controlled cuff algometry at the lower leg. Change in the pain sensitivity measures were analysed with repeated-measures ANOVAs, and test-retest reliability with intraclass correlation coefficients (ICC) and agreements in classification of EIH responders/non-responders between the two sessions.

Results

All subjects completed the walking conditions in both session 1 and session 2. The perceived intensity of walking assessed with rating of perceived exertion (RPE) and walking distance did not differ significantly between session 1 (distance: 632.5 ± 75.2 meters, RPE: 10.9 ± 1.9) and session 2 (distance: 642.1 ± 80.2 meters, RPE: 11.0 ± 2.4) (p > 0.11). Moreover, RPE showed excellent relative reliability with an ICC value of 0.95 [95%CI: 0.90–0.97]. Walking increased pain tolerance (mean difference: 2.6 kPa [95%CI: 0.5–4.9 kPa; p = 0.02]), but not pain thresholds compared with rest in both sessions. Hypoalgesia after walking demonstrated fair to good relative reliability (ICC = 0.61), however the agreement in classification of EIH responders/non-responders (absolute reliability) across sessions was low and not significant (κ = 0.19, p = 0.30).

Conclusions

Walking consistently increased pain tolerance but not pain thresholds compared with a duration-matched control condition with fair to good relative reliability between sessions. Based on classification of EIH responders/non-responders the absolute reliability between the two sessions was low indicating individual variance in the EIH response. Future studies should investigate the hypoalgesic effect of a walking exercise in a clinical pain population.

Conditioning pain modulation reduces pain only during the first stimulation of the temporal summation of pain paradigm in healthy participants

BACKGROUND

Pro-nociceptive and anti-nociceptive mechanisms are commonly assessed in clinical and experimental pain studies, but their potential interaction is not well understood.

OBJECTIVES

Investigate the effect of conditioning pain modulation (CPM) on temporal summation of pain (TSP).

METHODS

Twenty healthy participants underwent cuff algometry assessment on the lower legs to establish the pressure pain tolerance threshold (PTT). For the TSP assessment, ten stimuli at the level of the PTT were delivered by computerised cuff inflations (1-s stimulation, 1-s break) while participants rated pain intensity on a 10-cm electronic visual analogue scale (VAS). This TSP paradigm was then repeated with a simultaneous conditioning stimulus induced by a cuff on the contralateral leg, inflated to a constant pressure corresponding to 30% (mild), 70% (moderate) or 100% (severe) of the PTT. These were assessed in a randomised order, with a fifteen-minute break between tests. A final TSP test without conditioning was reassessed at the end (post-recording).

RESULTS

An interaction between stimuli (1-10) and repetition (p < 0.05) was found for VAS scores. VAS scores for the first stimulus were decreased during 30%, 70%, and 100% conditioning intensities, compared to baseline and post-recordings (p < 0.05). There was a significant increase in TSP during conditioning (p < 0.05). There were no significant differences between baseline and post-recordings for any stimuli (p > 0.05).

CONCLUSIONS

The current study indicates that mild to severe stimuli administered by cuff algometry does not modulate summation effect of temporal summation of pain, which could indicate that pain facilitatory mechanisms are more potent compared with pain inhibitory mechanisms.

SIGNIFICANCE

Current knowledge on the interaction effect of pro and anti-nociceptive paradigms are the lacking. The current study indicates that conditioning pain modulation does not modulate the summation effect of temporal summation of pain, when evaluated by computerized pressure algometry. This finding was independent of the mild, moderate and severe painful conditioning intensities.

Effects of Virtual Reality-Based Exercise Imagery on Pain in Healthy Individuals

Objective

Virtual reality (VR) is an advanced technology that can be used to attenuate pain. The present study aimed to investigate which method was more effective for pain management: VR combined with exercise imagery or VR distraction.

Methods

Fifty-two healthy students participated in this randomized cross-over controlled trial. One VR-based task aimed to passively use the imagery of driving a car as a distraction intervention (the driving group), whereas the other VR-based task aimed to use exercise imagery (running) to actively engage the participants in movement (the running group). The mechanical pressure pain thresholds of the quadriceps and forearm and the heat pain threshold of the hand of each subject were measured before, during, and after each VR task. The differences between the values at each time point and the differences between the groups were analyzed.

Results

The pressure and heat pain thresholds were significantly greater during VR task than those before VR task in both driving and running groups. The changes in the pressure pain thresholds that occurred during VR task were significantly higher in the running group than in the driving group. The difference between groups gradually declined after VR task. Conversely, there was no significant difference in the changes in the heat pain thresholds between the groups both during VR task and after VR task.

Conclusions

VR combined with exercise imagery has a greater effect on pressure pain thresholds, but not heat pain thresholds, than VR distraction.

Development of a new bed-side-test assessing conditioned pain modulation: a test-retest reliability study

Background and aims

Conditioned pain modulation (CPM) is of considerable interest within pain research. Often CPM testing is conducted in experimental settings using complicated instrumentation, thus challenging the implementation in clinical settings. Being able to assess CPM in a fast and reliable way in clinical settings could lead to a new diagnostic tool allowing improved profiling of pain patients.

Methods

A test-retest reliability study and a methodological development study were conducted based on different populations. The reliability study included 22 healthy subjects, mean age 23.6 years (SD: 2.4) and the methodological study included 29 healthy subjects, mean age 21.5 years (SD: 1.6). As painful phasic test stimulus, a 6–10 kg handheld, spring-based pressure algometer was applied perpendicularly to the muscle belly of the tibialis anterior muscle for 10 s and as painful tonic conditioning stimulus, 1–2 standard clamps, inducing a force of 1.3 kg, were applied extra-segmentally at the ipsilateral earlobe for 60–120 s. Four different test protocols were evaluated, of which one protocol was investigated for reliability. Test protocol 1 used a 6 kg pressure algometer as painful phasic test stimulus and a single clamp applied for 60 s as painful tonic conditioning stimulus. Test protocol 2 used a 10 kg pressure algometer as painful phasic test stimulus, and two clamps applied for 60 s as painful tonic conditioning stimulus. Test protocol 3 used a 10 kg pressure algometer as painful phasic test stimulus and a single clamp applied for 120 s as painful tonic conditioning stimulus. Test protocol 4 used a 6 kg pressure algometer as painful phasic test stimulus and a single clamp applied for 120 s as painful tonic conditioning stimulus.

Results

None of the stimuli caused any adverse events, e.g. bruises. In the reliability study (test protocol (1), non-significant CPM effects of 0.3 (SD: 1.6) and 0.2 (SD: 1.0) were observed in session 1 and 2, respectively. The intra-class correlations were 0.67 and 0.72 (p = < 0.01) and limits of agreement (LoA) ranged from −2.76 to 3.31. Non-significant CPM effects of 0.2 (SD: 1.0), −0.1 (SD: 1.1), and 0.0 (SD: 1.2) were observed for test protocol 2, 3, and 4, respectively).

Conclusions

The bedside test developed for investigating CPM was feasible and easy to use in healthy volunteers. No significant CPM effects were measured and a large variation in CPM effect ranging from −0.14 to 0.32 was observed. Intra-class correlation (ICC) values for the pressure algometer were interpreted as “good relative reliability” (test protocol 1), and LoA revealed a somewhat low absolute reliability.

Implications

The pressure algometer provided reproducible measurements and was useful for inducing phasic test stimuli. Since no significant CPM effects were detected, no recommendations for the bedside test can yet be made. Further examinations will have to establish if the “one size fits all” application of both test and conditioning stimuli is useful. Future bedside studies involving patient populations are warranted to determine the usefulness of the method.

Endogenous Pain Modulation Profiles Among Individuals With Chronic Pain: Relation to Opioid Use

It is generally assumed that individuals exhibiting high pain inhibition also tend to exhibit low pain facilitation, but little research has examined this association in individuals with pain. The aims of this cross-sectional study were 1) to examine the association between measures of conditioned pain modulation (CPM) and temporal summation (TS) in individuals with chronic pain, and 2) to examine whether this association was moderated by demographic (age, sex), psychological (depression, catastrophizing), or medication-related (opioid use) variables. Individuals (N= 190) with back or neck pain completed questionnaires and underwent a series of quantitative sensory testing procedures assessing CPM and TS. Results indicated that individuals with higher levels of CPM showed lower levels of TS, r = -.20, P < .01. Analyses, however, revealed that the magnitude of this association was substantially weaker among opioid users (r= -.08, NS) than nonusers (r= -.34, P < .01). None of the demographic or psychological variables included in our study influenced the association between CPM and TS. The magnitude of CPM was lower for opioid users than nonusers, suggesting that opioid use might dampen the functioning of endogenous pain-inhibitory systems and possibly contribute to a discordance between measures of pain inhibition and pain facilitation. PERSPECTIVE: Results of the present study indicated that greater endogenous pain-inhibitory capacity is associated with lower levels of pain facilitation. This association, however, was not significant among opioid users, suggesting that opioids might compromise the functioning and interrelationship between endogenous pain modulatory systems.

Alterations in Temporal Summation of Pain and Conditioned Pain Modulation Across an Episode of Experimental Exercise-Induced Low Back Pain

Persistent pain conditions, including low back pain (LBP), are often accompanied by alterations in pronociceptive and antinociceptive mechanisms, as quantified by temporal summation of pain (TSP) and conditioned pain modulation (CPM). It remains unclear whether altered pain sensitivity, CPM, and/or TSP are a consequence of pain presence or determine the degree of pain development. Pressure pain sensitivity, TSP, and CPM were assessed across an episode of exercise-induced LBP maintained for several days. Thirty healthy individuals participated in 3 experimental sessions: before (day 0), 2 days after fatiguing back muscle exercise with exercise-induced LBP present (day 2), and after pain resolution (day 7). Both handheld and cuff pressure-pain thresholds, along with TSP (10-cuff pain stimuli at .5 Hz) and CPM (cuff pain detection threshold prior versus during painful pressure conditioning) were assessed, alongside questionnaires pertaining to pain, disability, mood, sleep, menstruation, physical activity, and catastrophizing. The exercise-induced LBP model produced mild pain and disability, and reductions in pressure pain thresholds over both the lumbar and distant testing sites (p < .007). No pain-related changes were observed for TSP (p > .44) or CPM (p > .17). The baseline TSP was associated with the peak pain intensity of the exercise-induced LBP (p < .003). Perspective: Pressure-pain sensitivity was impacted by the presence of exercise-induced LBP, whereas TSP seemed to be more stable and was instead associated with the intensity of pain developed. No significant pain-related changes or associations were observed for CPM, suggesting this measure may have less usefulness in mild musculoskeletal pain conditions.

Pain Modulation: From Conditioned Pain Modulation to Placebo and Nocebo Effects in Experimental and Clinical Pain

Accumulating evidence reveal important applications of endogenous pain modulation assessment in healthy controls and in patients in clinical settings, as dysregulations in the balance of pain modulatory circuits may facilitate pain and promote chronification of pain. This article reviews data on pain modulation, focusing on the mechanisms and translational aspects of pain modulation from conditioned pain modulation (CPM) to placebo and nocebo effects in experimental and clinical pain. The specific roles of expectations, learning, neural and neurophysiological mechanisms of the central nervous system are briefly reviewed herein. The interaction between CPM and placebo systems in pain inhibitory pathways is highly relevant in the clinic and in randomized controlled trials yet remains to be clarified. Examples of clinical implications of CPM and its relationship to placebo and nocebo effects are provided. A greater understanding of the role of pain modulation in various pain states can help characterize the manifestation and development of chronic pain and assist in predicting the response to pain-relieving treatments. Placebo and nocebo effects, intrinsic to every treatment, can be used to develop personalized therapeutic approaches that improve clinical outcomes while limiting unwanted effects.

Assessment of CPM reliability: quantification of the within-subject reliability of 10 different protocols

Background and aims

Conditioned Pain Modulation (CPM) is a well-established phenomenon and several protocols have shown acceptable between-subject reliability [based on intraclass correlation coefficient (ICC) values] in pain-free controls. Recently, it was recommended that future CPM test-retest reliability studies should explicitly report CPM reliability based on CPM responders and non-responders (within-subject reliability) based on measurement error of the test stimulus. Identification of reliable CPM paradigms based on responders and non-responders may be a step towards using CPM as a mechanistic marker in diagnosis and individualized pain management regimes. The primary aim of this paper is to investigate the frequency of CPM responders/non-responders, and to quantify the agreements in the classification of responders/non-responders between 2 different days for 10 different CPM protocols.

Methods

Data from a previous study investigating reliability of CPM protocols in healthy subjects was used. In 26 healthy men, the test-stimuli used on both days were: Pain thresholds to electrical stimulation, heat stimulation, manual algometry, and computer-controlled cuff algometry as well as pain tolerance to cuff algometry. Two different conditioning stimuli (CS; cold water immersion and a computer-controlled tourniquet) were used in a randomized and counterbalanced order in both sessions. CPM responders were defined as a larger increase in the test stimulus response during the CS than the standard error of measurement (SEM) for the test-stimuli between repeated baseline tests without CS.

Results

Frequency of responders and non-responders showed large variations across protocols. Across the studied CPM protocols, a large proportion (from 11.5 to 73.1%) of subjects was classified as CPM non-responders when the test stimuli standard error of measurements (SEM) was considered as classifier. The combination of manual pressure algometry and cold water immersion induced a CPM effect in most participants on both days (n=16). However, agreement in the classification of CPM responders versus non-responders between days was only significant when assessed with computer-controlled pressure pain threshold as test-stimulus and tourniquet cuff as CS (κ=0.36 [95% CI, 0.04–0.68], p=0.037).

Conclusions and implications

Agreements in classification of CPM responders/non-responders using SEM as classifier between days were generally poor suggesting considerable intra-individual variation in CPM. The most reliable paradigm was computer-controlled pressure pain threshold as test-stimulus and tourniquet cuff as conditioning stimulus. However while this CPM protocol had the greatest degree of agreement of classification of CPM responders and non-responders across days, this protocol also failed to induce a CPM response in more than half of the sample. In contrast, the commonly used combination of manual pressure algometry and cold water immersion induced a CPM effect in most participants however it was inconsistent in doing so. Further exploration of the two paradigms and classification of responders and non-responders in a larger heterogeneous sample also including women would further inform the clinical usefulness of these CPM protocols. Future research in this area may be an important step towards using CPM as a mechanistic marker in diagnosis and in developing individualized pain management regimes.

Conditioned pain modulation identifies altered sensitivity in extremely preterm young adult males and females

BACKGROUND

Conditioned pain modulation is a potential biomarker for risk of persistent pain. As early-life experience can alter subsequent somatosensory processing and pain response, we evaluated conditioned pain modulation after extremely preterm birth.

METHODS

This observational study recruited extremely preterm (<26 weeks gestation; n=98) and term-born control (n=48) young adults (19-20 yr) from the longitudinal EPICure cohort. Pressure pain threshold (PPT; variable test stimulus lower leg) was measured before, during, and after a conditioning stimulus (contralateral hand immersion; 5°C water; 30 s). Questionnaires assessed current pain, medication use, anxiety, and pain catastrophising.

RESULTS

For participants tolerating conditioning, there were significant main effects of extremely preterm status, sex, and time on PPT during and after hand immersion. Inhibitory modulation was evoked in 64/98 extremely preterm (3, no change) and 38/48 term-born control (3, facilitation) subjects. The conditioned pain modulation effect (percentage change in PPT) did not differ between the extremely preterm and term-born control groups {53% [95% confidence interval (CI): 41-65] vs 57% [95% CI: 42-71]}. Reduced cold tolerance (<20 s) hampered conditioned pain modulation quantification in a higher proportion of extremely preterm participants [extremely preterm vs term-born control: 31/98 (32%) vs 7/48 (15%); P=0.03]. One-third of extremely preterm females withdrew the hand before parallel PPT (<15 s), and had lower baseline PPT than term-born control females [4.9 (95% CI: 4.8-5.1) vs 5.3 (95% CI: 5.1-5.5) ln kPa; P=0.02]. Higher anxiety, pain catastrophising, and medication use correlated with pain intensity, but not conditioned pain modulation effect.

CONCLUSIONS

Cold conditioning evoked inhibitory modulation in the majority of young adults and identified a subgroup of extremely preterm females with increased baseline sensitivity. Early-life experience and sex/gender should be considered when evaluating persistent pain risk with conditioned pain modulation.

Cuff Pressure Algometry in Patients with Chronic Pain as Guidance for Circumferential Tissue Compression for Wearable Soft Exoskeletons: A Systematic Review

In this article, we report on a systematic review of the literature on pressure-pain thresholds induced and assessed by computerized cuff pressure algometry (CPA). The motivation for this review is to provide design guidance on pressure levels for wearable soft exoskeletons and similar wearable robotics devices. In our review, we focus on CPA studies of patients who are candidates for wearable soft exoskeletons, as pain-related physiological mechanisms reportedly differ significantly between healthy subjects and patients with chronic pain. The results indicate that circumferential limb compression in patients most likely becomes painful at ∼10-18 kPa and can become unbearable even below 25 kPa. The corresponding ranges for healthy control subjects are 20-42 kPa (painful limits) and 34-84 kPa (unbearable levels). In addition, the increase of pain with time tends to be significantly higher, and the adaptation to pain significantly lower, than in healthy subjects. The results of this review provide guidance to designers of wearable robotics for populations with chronic pain regarding rates and magnitudes of tissue compression that may be unacceptable to users.

Age Interactions on Pain Sensitization in Patients With Severe Knee Osteoarthritis and Controls

OBJECTIVES

Widespread pressure hyperalgesia, facilitated temporal summation of pain (TSP), and impaired conditioned pain modulation (CPM) have been found in knee osteoarthritis (KOA) patients compared with controls and these parameters have further been suggested to be altered in the elderly. This study investigated the influence of age on pressure hyperalgesia, TSP, and CPM in patients with KOA and controls.

MATERIALS AND METHODS

One hundred thirty-three severe KOA patients and 50 age-matched and sex-matched asymptomatic controls were assessed by cuff algometry and handheld pressure algometry. Pain sensitivity was assessed around the head of the gastrocnemius muscle to identify mild pain detection threshold (MPDT) and pressure tolerance threshold (PTT). TSP was assessed by visual analogue scale scores of the pain evoked by 10 repetitive cuff stimulations. CPM was assessed as the difference in PTT before and during cuff-induced tonic arm pain. Pressure pain thresholds (PPTs) were assessed by handheld algometry at the tibialis anterior muscle. Two subgroups were analyzed in the age range below and above 65 years. Pearson correlations between age and pain parameters were applied.

RESULTS

Patients demonstrated reduced MPDT, PTT, and PPT (P<0.01), facilitated TSP (P<0.02), and a trend toward impaired CPM (P=0.06) compared with controls. A negative correlation was found between MPDT, PTT, and PPT and age (P<0.05) but no age-related association was found for TSP and CPM.

DISCUSSION

Pressure hyperalgesia was affected by age whereas dynamic pain mechanisms such as TSP and CPM were unaffected suggesting that these parameters are robust for a larger age range and reliable for long-term follow-up studies.

The effects of propranolol on heart rate variability and quantitative, mechanistic, pain profiling: a randomized placebo-controlled crossover study

Background and aims

The autonomic nervous system (ANS) is capable of modulating pain. Aberrations in heart rate variability (HRV), reflective of ANS activity, are associated with experimental pain sensitivity, chronic pain, and more recently, pain modulatory mechanisms but the underlying mechanisms are still unclear. HRV is lowered during experimental pain as well as in chronic pain conditions and HRV can be increased by propranolol, which is a non-selective β-blocker. Sensitization of central pain pathways have been observed in several chronic pain conditions and human mechanistic pain biomarkers for these central pain pathways include temporal summation of pain (TSP) and conditioned pain modulation (CPM). The current study aimed to investigate the effect of the β-blocker propranolol, and subsequently assessing the response to standardized, quantitative, mechanistic pain biomarkers.

Methods

In this placebo-controlled, double-blinded, randomized crossover study, 25 healthy male volunteers (mean age 25.6 years) were randomized to receive 40 mg propranolol and 40 mg placebo. Heart rate, blood pressure, and HRV were assessed before and during experimental pain tests. Cuff pressure pain stimulation was used for assessment of pain detection (cPDTs) and pain tolerance (cPTTs) thresholds, TSP, and CPM. Offset analgesia (OA) was assessed using heat stimulation.

Results

Propranolol significantly reduced heart rate (p<0.001), blood pressure (p<0.02) and increased HRV (p<0.01) compared with placebo. No significant differences were found comparing cPDT (p>0.70), cPTT (p>0.93), TSP (p>0.70), OA-effect (p>0.87) or CPM (p>0.65) between propranolol and placebo.

Conclusions

The current study demonstrated that propranolol increased HRV, but did not affect pressure pain sensitivity or any pain facilitatory or modulatory outcomes.

Implications

Analgesic effects of propranolol have been reported in clinical pain populations and the results from the current study could indicate that increased HRV from propranolol is not associated with peripheral and central pain pathways in healthy male subjects.

Assessment and manifestation of central sensitisation across different chronic pain conditions

Different neuroplastic processes can occur along the nociceptive pathways and may be important in the transition from acute to chronic pain and for diagnosis and development of optimal management strategies. The neuroplastic processes may result in gain (sensitisation) or loss (desensitisation) of function in relation to the incoming nociceptive signals. Such processes play important roles in chronic pain, and although the clinical manifestations differ across condition processes, they share some common mechanistic features. The fundamental understanding and quantitative assessment of particularly some of the central sensitisation mechanisms can be translated from preclinical studies into the clinic. The clinical perspectives are implementation of such novel information into diagnostics, mechanistic phenotyping, prevention, personalised treatment, and drug development. The aims of this paper are to introduce and discuss (1) some common fundamental central pain mechanisms, (2) how they may translate into the clinical signs and symptoms across different chronic pain conditions, (3) how to evaluate gain and loss of function using quantitative pain assessment tools, and (4) the implications for optimising prevention and management of pain. The chronic pain conditions selected for the paper are neuropathic pain in general, musculoskeletal pain (chronic low back pain and osteoarthritic pain in particular), and visceral pain (irritable bowel syndrome in particular). The translational mechanisms addressed are local and widespread sensitisation, central summation, and descending pain modulation.

Significance

Central sensitisation is an important manifestation involved in many different chronic pain conditions. Central sensitisation can be different to assess and evaluate as the manifestations vary from pain condition to pain condition. Understanding central sensitisation may promote better profiling and diagnosis of pain patients and development of new regimes for mechanism based therapy. Some of the mechanisms underlying central sensitisation can be translated from animals to humans providing new options in development of therapies and profiling drugs under development.

A tonic heat test stimulus yields a larger and more reliable conditioned pain modulation effect compared to a phasic heat test stimulus

Introduction

The interest in conditioned pain modulation (CPM) as a clinical tool for measuring endogenously induced analgesia is increasing. There is, however, large variation in the CPM methodology, hindering comparison of results across studies. Research comparing different CPM protocols is needed in order to obtain a standardized test paradigm.

Objectives

The aim of the study was to assess whether a protocol with phasic heat stimuli as test-stimulus is preferable to a protocol with tonic heat stimulus as test-stimulus.

Methods

In this experimental crossover study, we compared 2 CPM protocols with different test-stimulus; one with tonic test-stimulus (constant heat stimulus of 120-second duration) and one with phasic test-stimuli (3 heat stimulations of 5 seconds duration separated by 10 seconds). Conditioning stimulus was a 7°C water bath in parallel with the test-stimulus. Twenty-four healthy volunteers were assessed on 2 occasions with minimum 1 week apart. Differences in the magnitude and test-retest reliability of the CPM effect in the 2 protocols were investigated with repeated-measures analysis of variance and by relative and absolute reliability indices.

Results

The protocol with tonic test-stimulus induced a significantly larger CPM effect compared to the protocol with phasic test-stimuli (P < 0.001). Fair and good relative reliability was found with the phasic and tonic test-stimuli, respectively. Absolute reliability indices showed large intraindividual variability from session to session in both protocols.

Conclusion

The present study shows that a CPM protocol with a tonic test-stimulus is preferable to a protocol with phasic test-stimuli. However, we emphasize that one should be cautious to use the CPM effect as biomarker or in clinical decision making on an individual level due to large intraindividual variability.