Homotopic and heterotopic effects of endogenous analgesia in healthy volunteers

Increased functional connectivity between limbic brain areas in healthy individuals with high versus low sensitivity to cold pain: A resting state fMRI study

Background

The representation of variability in sensitivity to pain by differences in neural connectivity patterns and its association with psychological factors needs further investigation. This study assessed differences in resting-state functional connectivity (rsFC) and its association to cognitive-affective aspects of pain in two groups of healthy subjects with low versus high sensitivity to pain (LSP vs. HSP). We hypothesized that HSP will show stronger connectivity in brain regions involved in the affective-motivational processing of pain and that this higher connectivity would be related to negative affective and cognitive evaluations of pain.

Methods

Forty-eight healthy subjects were allocated to two groups according to their tolerability to cold stimulation (cold pressor test, CPT, 1°C). Group LSP (N = 24) reached the cut-off time of 180±0 sec and group HSP tolerated the CPT for an average of 13±4.8 sec. Heat, cold and mechanical evoked pain were measured, as well as pain-catastrophizing (PCS), depression, anxiety and stress (DASS-21). All subjects underwent resting state fMRI. ROI-to-ROI analysis was performed.

Results

In comparison to the LSP, the HSP had stronger interhemispheric connectivity of the amygdala (p = 0.01) and between the amygdala and nucleus accumbens (NAc) (p = 0.01). Amygdala connectivity was associated with higher pain catastrophizing in the HSP only (p<0.01).

Conclusions

These findings suggest that high sensitivity to pain may be reflected by neural circuits involved in affective and motivational aspects of pain. To what extent this connectivity within limbic brain structures relates to higher alertness and more profound withdrawal behavior to aversive events needs to be further investigated.

The development of a novel questionnaire assessing alterations in central pain processing in people with and without chronic pain

Background and aims The purpose of this study was to (a) develop and (b) conduct exploratory factor analysis on a novel self-report instrument for symptoms associated with altered central pain processing. Methods We first developed a 25-item questionnaire based on previous literature identifying symptoms and behaviours that may reflect altered spinal and supraspinal pain processing. We then administered this questionnaire to 183 people with chronic pain (n = 99) and healthy individuals (n = 84). Exploratory factor analysis was conducted to identify the factor structure of the questionnaire. Results Our results support a two-factor solution for the 25-item questionnaire that accounted for 57.2% of the total variance of responses in people with and without chronic pain. Factor one (11 items) included items related to alterations in sensation of pain, while factor two (seven items) included items associated with emotional and fatigue symptoms. Seven items showed weak factor loadings and were eliminated. Reliability was excellent, while both factors showed strong correlations with previously-validated self-report Instruments: (pain catastrophising, mood, vigilance, pain self-efficacy) and conditioned pain modulation, providing evidence for their validity. Conclusions We have developed a questionnaire containing two factors that appear to be related to two different symptom clusters, one of which is specifically related to pain and one of which contains other health-related symptoms related to mood and fatigue. These factors show excellent internal consistency and validity. This questionnaire may be a quick, easy and reliable instrument to assess central pain processing in clinical settings.

The tolerance to stretch is linked with endogenous modulation of pain

OBJECTIVES

The effect of stretching on joint range of motion is well documented, and although sensory perception has significance for changes in the tolerance to stretch following stretching the underlining mechanisms responsible for these changes is insufficiently understood. The aim of this study was to examine the influence of endogenous pain inhibitory mechanisms on stretch tolerance and to investigate the relationship between range of motion and changes in pain sensitivity.

METHODS

Nineteen healthy males participated in this randomized, repeated-measures crossover study, conducted on 2 separate days. Knee extension range of motion, passive resistive torque, and pressure pain thresholds were recorded before, after, and 10 min after each of four experimental conditions; (i) Exercise-induced hypoalgesia, (ii) two bouts of static stretching, (iii) resting, and (iv) a remote, painful stimulus induced by the cold pressor test.

RESULTS

Exercise-induced hypoalgesia and cold pressor test caused an increase in range of motion (p<0.034) and pressure pain thresholds (p<0.027). Moderate correlations in pressure pain thresholds were found between exercise-induced hypoalgesia and static stretch (Rho>0.507, p=0.01) and exercise-induced hypoalgesia and the cold pressor test (Rho=0.562, p=0.01). A weak correlation in pressure pain thresholds and changes in range of motion were found following the cold pressor test (Rho=0.460, p=0.047). However, a potential carryover hypoalgesic effect may have affected the results of the static stretch.

CONCLUSIONS

These results suggest that stretch tolerance may be linked with endogenous modulation of pain. Present results suggest, that stretch tolerance may merely be a marker for pain sensitivity which may have clinical significance given that stretching is often prescribed in the rehabilitation of different musculoskeletal pain conditions where reduced endogenous pain inhibition is frequently seen.

Conditioned pain modulation-A comprehensive review

Conditioned pain modulation (CPM) is a centrally processed measure of the net effect of the descending pain pathway. This comprises both the facilitatory as well as the inhibitory effect. In the past, CPM or similar effects have been previously described using different terminologies such as diffuse noxious inhibitory control (DNIC), heterotopic noxious conditioning stimulation (HNCS) or endogenous analgesia (EA). A variety of patient-related factors such as age, gender, hormones, race, genetic and psychological factors have been thought to influence the CPM paradigms. CPM paradigms have also been associated with a wide range of methodological variables including the mode of application of the 'test' as well as the 'conditioning' stimuli. Despite all these variabilities, CPM seems to reliably lend itself to the pain modulation profile concept and could in future become one of the phenotypic biomarkers for pain and also a guide for mechanism-based treatment in chronic pain. Future research should focus on establishing consistent methodologies for measuring CPM and thereby enhancing the robustness of this emerging biomarker for pain.

Distraction by a cognitive task has a higher impact on electrophysiological measures compared with conditioned pain modulation

Background

Conditioned pain modulation (CPM) evaluates the effect of a painful conditioning stimulus (CS) on a painful test stimulus (TS). Using painful cutaneous electrical stimulation (PCES) as TS and painful cold water as CS, the pain relief was paralleled by a decrease in evoked potentials (PCES-EPs). We now aimed to compare the effect of CPM with cognitive distraction on PCES-induced pain and PCES-EP amplitudes.

Methods

PCES was performed using surface electrodes inducing a painful sensation of 60 (NRS 0–100) on one hand. In a crossover design healthy subjects (included: n = 38, analyzed: n = 23) immersed the contralateral hand into 10 °C cold water (CS) for CPM evaluation and performed the 1-back task for cognitive distraction. Before and during the CS and 1-back task, respectively, subjects rated the pain intensity of PCES and simultaneously cortical evoked potentials were recorded.

Results

Both CPM and cognitive distraction significantly reduced PCES-EP amplitudes (CPM: 27.6 ± 12.0 μV to 20.2 ± 9.5 μV, cognitive distraction: 30.3 ± 14.2 µV to 13.6 ± 5.2 μV, p < 0.001) and PCES-induced pain (on a 0–100 numerical rating scale: CPM: 58 ± 4 to 41.1 ± 12.3, cognitive distraction: 58.3 ± 4.4 to 38.0 ± 13.0, p < 0.001), though the changes in pain intensity and PCES-amplitude did not correlate. The changes of the PCES-EP amplitudes during cognitive distraction were more pronounced than during CPM (p = 0.001).

Conclusions

CPM and cognitive distraction reduced the PCES-induced pain to a similar extent. The more pronounced decrease of PCES-EP amplitudes after distraction by a cognitive task implies that both conditions might not represent the general pain modulatory capacity of individuals, but may underlie different neuronal mechanisms with the final common pathway of perceived pain reduction.

Effects of Conditioned Pain Modulation on the Nociceptive Flexion Reflex in Healthy People: A Systematic Review

OBJECTIVES

The nociceptive flexion reflex (NFR) is a spinal reflex induced by painful stimuli resulting in a withdrawal response. Research has shown that the NFR is inhibited through endogenous pain inhibitory mechanisms, which can be assessed by conditioned pain modulation (CPM) paradigms. Although accumulating research suggests that the NFR can be affected by CPM, no clear overview of the current evidence exists. Therefore, the present review aimed at providing such a synthesis of the literature. In addition, the influence of personal factors on the CPM of the NFR was investigated.

MATERIALS AND METHODS

A systematic review was performed and reported following the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guidelines. Five electronic databases were searched to identify relevant articles. Retrieved articles were screened on eligibility using predefined inclusion criteria. Risk of bias was investigated according to the modified Newcastle-Ottawa Scale. Levels of evidence and strength of conclusion were assigned following the guidelines of the Dutch Institute for Healthcare Improvement.

RESULTS

Forty articles were included. There is some evidence that CPM produced by thermal or mechanical stimuli induces inhibitory effects on the NFR. However, inconclusive evidence exists with regard to the effect of electrical conditioning stimuli. While several personal factors do not affect CPM of the NFR, increased cognitive interference is associated with reduced NFR inhibition.

DISCUSSION

The present review demonstrates that certain types of nociceptive conditioning stimuli have the potential to depress, at the spinal level, nociceptive stimuli elicited from distant body regions. Although CPM of the NFR seems to be robust to the influence of several personal factors, it can be affected by cognitive influences.

The Association Between Conditioned Pain Modulation and Manipulation-induced Analgesia in People With Lateral Epicondylalgia

OBJECTIVES

Conditioned pain modulation (CPM) and manipulation-induced analgesia (MIA) may activate similar neurophysiological mechanisms to mediate their analgesic effects. This study assessed the association between CPM and MIA responses in people with lateral epicondylalgia.

MATERIALS AND METHODS

Seventy participants with lateral epicondylalgia were assessed for CPM followed by MIA. A single assessor measured pressure pain thresholds (PPT) before, during, and after cold water immersion (10°C) of the asymptomatic hand and contralateral lateral glide (CLG) mobilization of the neck. For analyses, linear mixed models evaluated differences in CPM and MIA responses. Pearson partial correlations and regression analyses evaluated the association between CPM and MIA PPT.

RESULTS

There was a significant increase (CPM and MIA, P<0.001) in PPT from baseline during the interventions (CPM mean: 195.84 kPa for elbow and 201.87 kPa for wrist, MIA mean: 123.01 kPa for elbow and 126.06 kPa for wrist) and after the interventions (CPM mean: 126.06 kPa for elbow, 114.24 kPa for wrist, MIA mean: 123.50 kPa for elbow and 122.16 kPa for wrist). There were also significant moderate and positive partial linear correlations (r: 0.40 to 0.54, P<0.001) between CPM and MIA measures, controlling for baseline measures. Regression analyses showed that CPM PPT was a significant predictor of MIA PPT (P<0.001) and the models explained between 73% and 85% of the variance in MIA PPT.

DISCUSSION

This study showed that CPM and MIA responses were significantly correlated and that the CPM response was a significant predictor of MIA response.

A Pleasant Sensation Evoked by Knee or Hand Icing Influences the Effect on Pain Intensity in Patients After Total knee Arthroplasty: A Prospective, Randomized, Cross-Over Study

Purpose

Cold therapy on the operated area after surgery is often used as an analgesic and to reduce pain, swelling, and increase range of motion. However, evidence to support the results of cold therapy is still scarce and the mechanism underlying its effectiveness remains unclear. The present study aimed to investigate whether a pleasant sensation evoked by icing the treated knee or a site distant from the treated site (the hand) influenced the acute effect on pain intensity in patients who have undergone total knee arthroplasty (TKA).

Patients and methods

A total of 37 patients with knee OA who underwent TKA were enrolled in this study. This prospective, randomized, cross-over study was performed for 2 days consecutively between days 8 and 15 postoperatively. Cold pack was placed on the anterior surface of the treated knee and palm for 10 mins, respectively. The main primary outcomes were the intensity of knee pain during maximal passive knee flexion.

Results

The two-way ANOVA showed significance only in the main effect of a pleasant sensation (F = 11.3, p = 0.001), but not in the icing site (F = 0.005, p = 0.94) and interaction (F = 0.65, p = 0.42).

Conclusion

This study shows that a pleasant sensation evoked by knee or hand icing influenced the effect on pain intensity during maximal knee flexion in patients after TKA. Even if knee icing has no effect on pain and evokes no pleasant sensation, it may be worthwhile to conduct hand icing to reduce pain.

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.

The relationship between sensitivity to pain and conditioned pain modulation in healthy people

BACKGROUND

The relationship between sensitivity to pain and conditioned pain modulation (CPM) - a paradigm reflecting the activity of the endogenous descending analgesic system - is still unclear. This study aimed at investigating CPM magnitude in two distinct subgroups of healthy subjects, presenting low vs. high sensitivity to pain (LSP vs. HSP, respectively), by employing two different thermal paradigms of CPM.

METHOD

Ninety-five healthy subjects (out of 293 tested) were identified as LSP (n = 48) or HSP (n = 47) according to their tolerance time to noxious cold stimulation (Cold Pressor Test, 1 °C). All subjects were exposed to two different paradigms of CPM: 1) Fixed temperature 'test-pain' (TP) where phasic, fixed painful heat stimuli of 47 °C were administered before and during a prolonged 'conditioning stimulus' (cold water at 12 °C for 30 s); and 2) Individually based 'pain-60' where TP was determined as the temperature that induced pain at a magnitude of 60 on a 0-100 rating scale (with the same conditioning stimulus).

RESULT

Using both thermal paradigms, LSP subjects showed decreased CPM magnitudes in comparison to HSP (p < 0.0001 in both paradigms). Within each group, no differences in the magnitudes of CPM were found between the two paradigms.

CONCLUSION

These findings show that regardless of the thermal CPM paradigm employed, healthy individuals exhibiting low sensitivity to pain have a low pain inhibition profile and vice-versa. It is suggested that in healthy subjects, pain sensitivity predisposes the magnitude of CPM and not the other way around.

Feasibility and reliability of electrical, mechanical and thermal nociceptive testing and assessment of diffuse noxious inhibitory control in dogs

PURPOSE

Quantitative sensory testing has been used to assess the somatosensory system. This study aimed to evaluate the feasibility and reliability of electrical (ENT), mechanical (MNT) and thermal (TNT) nociceptive testing and the effect of a conditioning stimulus on MNT.

PATIENTS AND METHODS

Sixteen healthy client-owned dogs were included in this study. Stimulation was applied bilaterally to the dorsal and plantar aspect of the metacarpus and metatarsus respectively, using transcutaneous electrical stimulator, algometry and a cold nociceptive device in a randomized order until a behavior response was observed or a cut-off reached. Tests were performed twice (60 seconds apart) by two observers. Retesting was performed 5 hours later. The diffuse noxious inhibitory control was tested by comparing MNT pre- and post-conditioning stimuli. Sham-testing was performed for ENT and TNT. Statistical analysis included linear model and intra-class correlation coefficient (P<0.05).

RESULTS

Feasibility was 99% (ENT), 93.5% (MNT) and 93.6% (TNT). Data for TNT were not analyzed due to inconsistent results. Mean ± SD were 48±22.6 mA (ENT) and 11.9±3.5 N (MNT). MNT was higher for thoracic than for pelvic limbs (P=0.002). Conditioning stimulus increased MNT (P=0.049). Inter-observer reliability was 91.4% (ENT) and 60.9% (MNT). False-positive responses were 15% (ENT) and 35.7% (TNT).

CONCLUSION

ENT was feasible, repeatable and superior to MNT and TNT. The assessment of the diffuse noxious inhibitory control with a conditioning stimulus showed promising results in dogs. These tools could be used in naturally-occurring disease to provide insight on their underlying mechanisms and therapeutics.

Blood Flow Restriction induces hypoalgesia in recreationally active adult male anterior knee pain patients allowing therapeutic exercise loading

OBJECTIVE

To evaluate if a single blood flow restriction (BFR)-exercise bout would induce hypoalgaesia in patients with anterior knee pain (AKP) and allow painless application of therapeutic exercise.

DESIGN

Cross-sectional repeated measures design.

SETTING

Institutional out-patients physiotherapy clinic.

PATIENTS

Convenience sample of 30 AKP patients.

INTERVENTION

BFR was applied at 80% of complete vascular occlusion. Four sets of low-load open kinetic chain knee extensions were implemented using a pain monitoring model.

MAIN OUTCOME MEASUREMENTS

Pain (0-10) was assessed immediately after BFR application and after a physiotherapy session (45 min) during shallow and deep single-leg squat (SSLS, DSLS), and step-down test (SDT). To estimate the patient rating of clinical effectiveness, previously described thresholds for pain change (≥40%) were used, with appropriate adjustments for baseline pain levels.

RESULTS

Significant effects were found with greater pain relief immediate after BFR in SSLS (d = 0.61, p < 0.001), DSLS (d = 0.61, p < 0.001), and SDT (d = 0.60, p < 0.001). Time analysis revealed that pain reduction was sustained after the physiotherapy session for all tests (d_(SSLS) = 0.60, d_(DSLS) = 0.60, d_(SDT) = 0.58, all p < 0.001). The reduction in pain effect size was found to be clinically significant in both post-BFR assessments.

CONCLUSION

A single BFR-exercise bout immediately reduced AKP with the effect sustained for at least 45 min.

Age and gender differences in mechanically induced intraoral temporal summation and conditioned pain modulation in healthy subjects

OBJECTIVE

The aim of this study was to investigate intraoral temporal summation (TS) and conditioned pain modulation (CPM) and compare the outcome with TS and CPM induced in the forearm. In addition, we aimed to study the effect of age and gender on intraoral and forearm TS and CPM.

STUDY DESIGN

Mechanical stimulation was induced with # 5.46 von Frey filament applying 26 grams of force. A single stimulus, followed by a train of 30 successive stimuli, was applied intraorally and to the dominant forearm. CPM was assessed with the TS test as the painful stimulus and with immersion of the nondominant hand in a hot water bath as the conditioning stimulus.

RESULTS

Gender was significantly associated with TS but not with CPM measures. Females had significantly lower mean TS measured in the face and in the dominant forearm compared with males. Age was significantly associated with CPM, but not with TS measures. In both sites examined, older patients had significantly lower mean CPM compared with younger patients.

CONCLUSIONS

Mechanical TM elicited in the oral cavity can be used as test stimulus for CPM testing. Intraoral modulation, both TS and CPM, has an extent similar to that of the standard cutaneous extremity. TS was lower in females, and CPM was reduced with age.

Exercise does not produce hypoalgesia when performed immediately after a painful stimulus

Background and aims:

Exercise-induced hypoalgesia (EIH) and conditioned pain modulation (CPM) are assumed to reflect descending pain inhibition. Potential interactions between EIH and CPM may be important in the therapy of chronic pain, as reduced CPM and increased pain after exercise are frequently observed. This study compared the EIH response after CPM was activated using a cold pressor task with the EIH response after a control condition.

Methods:

Thirty-one participants (age: 27.7±9.8; 15 female) completed two sessions: a cold pressor task (CPT) session, i.e. testing EIH with preceding CPM activation induced using a 2 min CPT at approximately 2°C, and a control session, i.e. testing EIH after a control condition (2 min of quiet rest). EIH was induced using a 15 min bicycling exercise at a target heart rate corresponding to 75% VO2 max. Repeated measures ANOVAs on pressure pain thresholds (PPTs) at the hand, back and leg were used to determine the effects of exercise after the cold pressor test and control condition. Furthermore, correlations between CPM and EIH, in the CPT session as well as control session, were calculated at each assessment site.

Results:

A significant time x condition interaction (F(1, 30)=43.61, p<0.001, partial η2=0.59), with Bonferroni-corrected post-hoc t-tests showed that PPTs increased after exercise in the control session (p<0.001), but not in the CPT session (p=0.125). Furthermore, there was a small positive correlation of EIH in the control session and CPM at the hand (r=0.37, p=0.043). There was a moderate negative correlation of EIH in the CPT session and CPM at the hand (r=−0.50, p=0.004), and smaller negative correlations at the back (r=−0.37, p=0.036) and at the leg (r=−0.35, p=0.054).

Conclusions:

Attenuated EIH after the CPM activation in comparison to a control condition suggests that EIH and CPM may share underlying pain inhibitory mechanisms on a systemic level. This assumption is further supported by the finding of small to moderate significant correlations between EIH and CPM at the hand. The attenuated EIH response furthermore suggests that these mechanisms are exhaustible, i.e. that its effects decline after a certain amount of inhibition.

Implications:

In patients with chronic pain, assessing the current capacity of the descending pain inhibitory system – as indicated by the CPM response – may aid to make better predictions about how patients will respond to exercise with respect to acute pain reduction.

Effects of two different intensities of transcutaneous electrical nerve stimulation on pain thresholds of contralateral muscles in healthy subjects

[Purpose] This study aimed to investigate the differential effects of high-intensity and low-intensity transcutaneous electrical nerve stimulation on the contralateral side on the pain threshold in healthy subjects. [Subjects and Methods] Twenty-five healthy adults, volunteers received two intensity levels (motor-level, 1.5 times the muscle motor threshold; sensory-level, sensory threshold of the common peroneal nerve), for 30 s on separate days. Pressure pain threshold was recorded on the contralateral tibialis anterior and deltoid muscle before, during, and after stimulation. [Results] Motor-level stimulation significantly increased the pressure pain threshold at both muscle sites, while effects of sensory-level stimulation on pressure pain thresholds were significant only at the deltoid site. The percent change in pressure pain thresholds at both sites was significantly higher during motor-level stimulation. [Conclusion] Motor-level stimulation, applied unilaterally to one leg, produced immediate contralateral diffuse and segmental analgesic effects. This may be of therapeutic benefit in patients for whom transcutaneous electrical nerve stimulation cannot be directly used at the painful site.

Central sensitization and changes in conditioned pain modulation in people with chronic nonspecific low back pain: a case-control study

Quantitative sensory testing is widely used in human research to investigate the state of the peripheral and central nervous system contributions in pain processing. It is a valuable tool to help identify central sensitization and may be important in the treatment of low back pain. The aim of this study was to evaluate changes in local and segmental hypersensitivity and endogenous pain inhibition in people with chronic nonspecific low back pain. Thirty patients with chronic low back pain and thirty healthy subjects were studied. Pressure pain thresholds (PPTs) were measured from the lumbar region and over the tibialis anterior muscle (TA). A cold pressor test was used to assess the activation of conditioned pain modulation (CPM), and PPTs in the lumbar region were recorded 30 s after immersion of participant's foot in a bucket with cold water. People with chronic low back pain have significantly lower PPT than controls at both the lumbar region [89.5 kPa (mean difference) 95 % CI 40.9-131.1 kPa] and TA [59.45 kPa (mean difference) 95 % CI 13.49-105.42 kPa]. During CPM, people with chronic low back pain have significantly lower PPT than controls in lumbar region [118.6 kPa (mean difference) 95 % CI 77.9-159.2 kPa]. Women had significantly lower PPTs than men in both lumbar region [101.7 kPa (mean difference) 95 % CI 37.9-165.7 kPa] and over the TA [189.7 kPa (mean difference) 95 % CI 14.2-145.2 kPa]. There was no significant difference in PPTs in men between healthy controls and those with low back pain, suggesting the significant differences are mediated primarily by difference between women.

Combined neuromodulatory interventions in acute experimental pain: assessment of melatonin and non-invasive brain stimulation

Transcranial direct current stimulation (tDCS) and melatonin can effectively treat pain. Given their potentially complementary mechanisms of action, their combination could have a synergistic effect. Thus, we tested the hypothesis that compared to the control condition and melatonin alone, tDCS combined with melatonin would have a greater effect on pain modulatory effect, as assessed by quantitative sensory testing (QST) and by the pain level during the Conditioned Pain Modulation (CPM)-task. Furthermore, the combined treatment would have a greater cortical excitability effect as indicated by the transcranial magnetic stimulation (TMS) and on the serum BDNF level. Healthy males (n = 20), (aged 18-40 years), in a blinded, placebo-controlled, crossover, clinical trial, were randomized into three groups: sublingual melatonin (0.25 mg/kg) + a-tDCS, melatonin (0.25 mg/kg) + sham-(s)-tDCS, or sublingual placebo+sham-(s)-tDCS. Anodal stimulation (2 mA, 20 min) was applied over the primary motor cortex. There was a significant difference in the heat pain threshold (°C) for melatonin+a-tDCS vs. placebo+s-tDCS (mean difference: 4.86, 95% confidence interval [CI]: 0.9 to 8.63) and melatonin+s-tDCS vs. placebo+s-tDCS (mean: 5.16, 95% CI: 0.84 to 8.36). There was no difference between melatonin+s-tDCS and melatonin+a-tDCS (mean difference: 0.29, 95% CI: -3.72 to 4.23). The mean change from the baseline on amplitude of motor evocate potential (MEP) was significantly higher in the melatonin+a-tDCS (-19.96% ± 5.2) compared with melatonin+s-tDCS group (-1.36% ± 5.35) and with placebo+s-tDCS group (3.61% ± 10.48), respectively (p < 0.05 for both comparisons). While melatonin alone or combined with a-tDCS did not significantly affect CPM task result, and serum BDNF level. The melatonin effectively reduced pain; however, its association with a-tDCS did not present an additional modulatory effect on acute induced pain.

Conditioned pain modulation is minimally influenced by cognitive evaluation or imagery of the conditioning stimulus

PURPOSE

Conditioned pain modulation (CPM) is an experimental approach for probing endogenous analgesia by which one painful stimulus (the conditioning stimulus) may inhibit the perceived pain of a subsequent stimulus (the test stimulus). Animal studies suggest that CPM is mediated by a spino-bulbo-spinal loop using objective measures such as neuronal firing. In humans, pain ratings are often used as the end point. Because pain self-reports are subject to cognitive influences, we tested whether cognitive factors would impact on CPM results in healthy humans.

METHODS

We conducted a within-subject, crossover study of healthy adults to determine the extent to which CPM is affected by 1) threatening and reassuring evaluation and 2) imagery alone of a cold conditioning stimulus. We used a heat stimulus individualized to 5/10 on a visual analog scale as the testing stimulus and computed the magnitude of CPM by subtracting the postconditioning rating from the baseline pain rating of the heat stimulus.

RESULTS

We found that although evaluation can increase the pain rating of the conditioning stimulus, it did not significantly alter the magnitude of CPM. We also found that imagery of cold pain alone did not result in statistically significant CPM effect.

CONCLUSION

Our results suggest that CPM is primarily dependent on sensory input, and that the cortical processes of evaluation and imagery have little impact on CPM. These findings lend support for CPM as a useful tool for probing endogenous analgesia through subcortical mechanisms.

Analysis of meaningful conditioned pain modulation effect in a pain-free adult population

Conditioned pain modulation (CPM) encompasses the effects of inhibitory and facilitatory pain modulatory systems and is inefficient in some chronic pain states. A proportion of healthy subjects also exhibit little or no CPM, perhaps suggesting that inherent factors such as gender or genetics may be influential. However, there is no consensus on how best to determine a meaningful CPM effect. This study aimed to determine the proportion of pain-free subjects exhibiting a meaningful CPM effect. Analyses of associations between 5HTTLPR (serotonin transporter-linked polymorphic region) polymorphisms on the serotonin transporter gene (SLC6A4), gender, and CPM effect were also carried out. A total of 125 healthy subjects (47 male; 78 female) underwent pressure pain threshold testing before, during, and after a cold pressor conditioning stimulus. A buccal cell sample was collected for analysis of 5HTTLPR genotype. Meaningful CPM effect was determined as an increase in pressure pain threshold values from baseline greater than the inherent error of measurement, calculated as 5.3%. During the conditioning stimulus, 116 subjects (92.8%) exhibited a CPM effect whereas 9 did not. CPM effect did not differ significantly between genders or between 5HTTLPR genotypes. This provides a clear basis on which to determine the proportion of patients with a chronic pain disorder that exhibit a meaningful CPM effect.

PERSPECTIVE

This study proposes a method for calculating meaningful CPM effect and reports the proportion and magnitude of effect elicited in a large sample. Associations between CPM, gender, and genotype were also analyzed. Clarification of normal CPM response may help to elucidate the mechanisms driving CPM inefficiency in chronic pain.

A large conditioned pain modulation response is not related to a large blood pressure response: a study in healthy men

BACKGROUND

Endogenous pain modulation has been studied with the conditioned pain modulation (CPM) paradigm with large differences in the magnitude of the CPM effect. We hypothesized that differences in CPM effects might be associated with differences in blood pressure responses to the conditioning stimulus when comparing the CPM effects using two different conditioning stimuli.

METHODS

A single-blind repeated-measures design with block-randomization was applied on 25 healthy male subjects. The test stimulus (TS; tonic heat pain for 120 s) was first presented alone, thereafter in parallel with a conditioning stimulus (CS). Conditioning stimuli were either a cold pressor test (CPT) or equally painful ischaemic muscle pain (ISC), both lasting 120 s. Finger blood pressure and heart rate were recorded continuously. Data were analysed in a linear mixed model framework with CS type (CPT or ISC) and conditioning (TS or TS + CS) as independent factors.

RESULTS

An inhibitory CPM effect was found for both types of conditioning (p < 0.001). The CPM effect was larger during CPT conditioning compared with ISC conditioning (p = 0.001). No association with the concomitant cardiovascular response (blood pressure and heart rate) was found (p > 0.34).

CONCLUSION

Cold pressor pain CS induces larger CPM effects than ischaemic pain CS. The larger CPM effect is, however, not associated with a larger blood pressure response. Other factors related to the CS should be investigated to understand why different CS modalities give different CPM effects.

Adaptability to pain is associated with potency of local pain inhibition, but not conditioned pain modulation: a healthy human study

This study investigated the relationship between pain sensitivity, adaptability, and potency of endogenous pain inhibition, including conditioned pain modulation (CPM) and local pain inhibition. Forty-one healthy volunteers (20 male, 21 female) received conditioning stimulation (CS) over 2 sessions in a random order: tonic heat pain (46 °C) on the right leg for 7 minutes and cold pressor pain (1 °C to 4 °C) on the left hand for 5 minutes. Participants rated the intensity of pain continuously using a 0 to 10 electronic visual analogue scale. The primary outcome measures were pressure pain thresholds (PPT) measured at the heterotopic and homotopic location to the CS sites before, during, and 20 minutes after CS. Two groups of participants, pain adaptive and pain nonadaptive, were identified based on their response to pain in the cold pressor test. Pain-adaptive participants showed a pain reduction between peak pain and pain at end of the test by at least 2 of 10 (n=16); whereas the pain-nonadaptive participants reported unchanged peak pain during 5-minute CS (n=25). Heterotopic PPTs during the CS did not differ between the 2 groups. However, increased homotopic PPTs measured 20 minutes after CS correlated with the amount of pain reduction during CS. These results suggest that individual sensitivity and adaptability to pain does not correlate with the potency of CPM. Adaptability to pain is associated with longer-lasting local pain inhibition.

The influence of intensity and duration of a painful conditioning stimulation on conditioned pain modulation in volunteers

BACKGROUND AND METHOD

The aim was to investigate influence from variations in intensity of a painful conditioning stimulation (CS) on early (0-6 min) and prolonged (6-12 min) conditioned pain modulation (CPM) in volunteers during concurrent exposure to test stimuli (TS). CS was applied to either forearm using painful heat with an intensity of 2/10 and 5/10, respectively, rated on a 0-10 numerical pain rating scale. At a second session, CS with an intensity of 7/10 was applied to the arm using a tourniquet. Threshold and suprathreshold painful heat and pressure as well as painful repeated monofilament pricking (RMP) were assessed as TS.

RESULTS

Regardless of TS, there was no significant difference in the magnitude of CPM within the same stimulus modality during the various intensities and phases of the CS. Significant modulation of heat pain thresholds (HPTs) was found during the early phase at 5/10 and 7/10, but not at 2/10. Only at 5/10 the prolonged CS resulted in a significant additional increase in HPT. During the early CS phase, CPM of suprathreshold heat pain was found at 2/10 and 5/10. The prolonged CS resulted in a significant additional temperature increase at 5/10. Only during the early phase significant CPM of pressure pain thresholds were found for all three pain intensities in conjunction with a significant CPM of suprathreshold pressure pain at 5/10. There was no CPM of RMP.

CONCLUSION

The CS intensity and the duration of CPM modulated pain sensitivity differentially across TS modalities.

Stability of conditioned pain modulation in two musculoskeletal pain models: investigating the influence of shoulder pain intensity and gender

Background

Several chronic pain populations have demonstrated decreased conditioned pain modulation (CPM). However there is still a need to investigate the stability of CPM paradigms before the measure can be recommended for implementation. The purpose of the present study was to assess whether shoulder pain intensity and gender influence CPM stability within and between sessions.

Methods

This study examined two different musculoskeletal pain models, clinical shoulder pain and an experimental model of shoulder pain induced with eccentric exercise in healthy participants. Patients in the clinical cohort (N = 134) were tested before surgery and reassessed 3 months post-surgery. The healthy cohort (N = 190) was examined before inducing pain at the shoulder, and 48 and 96 hours later.

Results

Our results provide evidence that 1) stability of inhibition is not related to changes in pain intensity, and 2) there are sex differences for CPM stability within and between days.

Conclusions

Fluctuation of pain intensity did not significantly influence CPM stability. Overall, the more stable situations for CPM were females from the clinical cohort and males from the healthy cohort.

Painful heat attenuates electrically induced muscle pain in men and women

Background and purpose

Women exhibit higher prevalence of most painful disorders. Several explanations have been proposed for this discrepancy, one being that endogenous pain modulatory pathways, which affect incoming nociceptive signals, act differently in men and women. A less efficient pain inhibitory system has been proposed as a contributing factor to explain why women exhibit higher prevalence of most painful disorders. The present study determined whether muscle pain, induced experimentally by electrical stimulation, is inhibited by a painful heat stimulus. This conditioned pain modulation (CPM) paradigm was used to determine whether women show signs of reduced inhibition compared to men.

Methods

Forty self-reported healthy individuals (20 female, 20 male) participated in a cross-over design with painful and non-painful heat as a conditioning stimulus. Test stimuli were painful intramuscular electrical stimulation of the tibialis anterior muscle at two intensities; low (1.1 × pain threshold) and high (1.6 × pain threshold). Painful conditioning was contact heat (45–49 ° C) to the contralateral forearm. Nonpainful conditioning was contact heat at 35 °C. Ten test stimuli were delivered in three blocks (before, during and after conditioning) in two sessions (painful and non-painful conditioning). The women were tested during days 12-14 of the menstrual cycle. This interval corresponds to the ovulatory phase of the menstrual cycle, the interval during which women are reported to show the largest inhibitory effects.

Results

Test stimuli were rated significantly lower during painful conditioning, compared with before conditioning. This was found for both low and high test stimulus intensities. Anonspecific attenuation was seen during non-painful conditioning for the low test stimulus intensity. Test stimuli were rated significantly lower also 3 min after conditioning, compared with before conditioning. The inhibitory effects were not different between men and women. Similar findings were obtained also if six non-CPM-responders (subjects rating test stimuli higher during conditioning than before conditioning) were excluded.

Conclusions and implications

The present findings indicate that painful contact heat inhibits electrically induced muscle pain and that inhibition was not different between men and women, when women were tested in the interval 12-14 days after their last menstruation. Some inhibition of muscle pain was seen during non-painful conditioning, indicating that nonspecific inhibitory effects were triggered. Also the nonspecific inhibitory effects were similar in men and women.

Different mechanisms for the short-term effects of real versus sham transcutaneous electrical nerve stimulation (TENS) in patients with chronic pain: a pilot study

Transcutaneous electrical nerve stimulation (TENS) has existed since the early 1970s. However, randomized placebo controlled studies show inconclusive results in the treatment of chronic pain. These results could be explained by assuming that TENS elicits a placebo response. However, in animal research TENS has been found to decrease hyperalgesia, which contradicts this assumption. The aim of this study is to use quantitative sensory testing to explore changes in pain processing during sham versus real TENS in patients with chronic pain. Patients with chronic pain (N = 20) were randomly allocated to real TENS or sham TENS application. Electrical pain thresholds (EPTs) were determined inside and outside the segment stimulated, before and after the first 20 minutes of the intervention, and after a period of 10 days of daily real/sham TENS application. Pain relief did not differ significantly for real versus sham TENS. However, by comparing time courses of EPTs, it was found that EPT values outside the segment of stimulation increased for sham TENS, whereas for real TENS these values decreased. There were, however, no differences for EPT measurements inside the segment stimulated. These results illustrate the importance of including mechanism-reflecting parameters in addition to symptoms when conducting pain research.

Dyspnea-pain counterirritation induced by inspiratory threshold loading: a laser-evoked potentials study

Background: experimentally induced dyspnea of the work/effort type inhibits, in a top-down manner, the spinal transmission of nociceptive inputs (dyspnea-pain counterirritation). Previous studies have demonstrated that this inhibition can be assessed by measuring the nociceptive flexion reflex (RIII). However, its clinical application is limited because of the strong discomfort associated with the electrical stimuli required to elicit the RIII reflex. Study objectives: we examined whether the dyspnea-pain counterirritation phenomenon can be evaluated by measuring the effect of work/effort type dyspnea on the magnitude of laser-evoked brain potentials (LEPs). Methods: 10 normal male volunteers were studied (age: 19–30 years). LEPs were elicited using a CO2 laser stimulator delivering 10- to 15-ms stimuli of 6 ± 0.7 W over a 12.5 mm2 area. The EEG was recorded using nine scalp channels. Non-nociceptive somatosensory-evoked potentials (SEPs) served as control. LEPs and SEPs were recorded before, during, and after 10 min of experimentally induced dyspnea [inspiratory threshold loading (ITL)]. Results: pain caused by the nociceptive laser stimulus was mild. ITL consistently induced dyspnea, mostly of the “excessive effort” type. Amplitude of the N2-P2 wave of LEPs decreased by 37.6 ± 13.8% during ITL and was significantly correlated with the intensity of dyspnea [ r = 0.66, CI 95% (0.08–0.92, P = 0.0319)]. In contrast, ITL had no effect on the magnitude of non-nociceptive SEPs. Discussion: experimentally induced dyspnea of the work/effort type reduces the magnitude of LEPs. This reduction correlates with the intensity of dyspnea. The recording of LEPs could constitute a clinically applicable approach to assess the dyspnea-pain counterirritation phenomenon in patients.

A systematic literature review of 10 years of research on sex/gender and experimental pain perception - part 1: are there really differences between women and men?

The purpose of this systematic review was to summarize and critically appraise the results of 10 years of human laboratory research on pain and sex/gender. An electronic search strategy was designed by a medical librarian and conducted in multiple databases. A total of 172 articles published between 1998 and 2008 were retrieved, analyzed, and synthesized. The first set of results (122 articles), which is presented in this paper, examined sex difference in the perception of laboratory-induced thermal, pressure, ischemic, muscle, electrical, chemical, and visceral pain in healthy subjects. This review suggests that females (F) and males (M) have comparable thresholds for cold and ischemic pain, while pressure pain thresholds are lower in F than M. There is strong evidence that F tolerate less thermal (heat, cold) and pressure pain than M but it is not the case for tolerance to ischemic pain, which is comparable in both sexes. The majority of the studies that measured pain intensity and unpleasantness showed no sex difference in many pain modalities. In summary, 10 years of laboratory research have not been successful in producing a clear and consistent pattern of sex differences in human pain sensitivity, even with the use of deep, tonic, long-lasting stimuli, which are known to better mimic clinical pain. Whether laboratory studies in healthy subjects are the best paradigm to investigate sex differences in pain perception is open to question and should be discussed with a view to enhancing the clinical relevance of these experiments and developing new research avenues.

A systematic literature review of 10 years of research on sex/gender and pain perception - part 2: do biopsychosocial factors alter pain sensitivity differently in women and men?

This systematic review summarizes the results of 10 years of laboratory research on pain and sex/gender. An electronic search strategy was designed by a medical librarian to access multiple databases. A total of 172 articles published between 1998 and 2008 were retrieved, analyzed, and synthesized. The second set of results presented in this review (129 articles) examined various biopsychosocial factors that may contribute to differences in pain sensitivity between healthy women and men. The results revealed that the involvement of hormonal and physiological factors is either inconsistent or absent. Some studies suggest that temporal summation, allodynia, and secondary hyperalgesia may be more pronounced in women than in men. The evidence to support less efficient endogenous pain inhibitory systems in women is mixed and does not necessarily apply to all pain modalities. With regard to psychological factors, depression may not mediate sex differences in pain perception, while the role of anxiety is ambiguous. Cognitive and social factors appear to partly explain some sex-related differences. Finally, past individual history may be influential in female pain responses. However, these conclusions must be treated with much circumspection for various methodological reasons. Furthermore, some factors/mechanisms remain understudied in the field. There is also a need to assess and improve the ecological validity of findings from laboratory studies on healthy subjects, and perhaps a change of paradigm needs to be considered at this point in time to better understand the factors that influence the experience of women and men who suffer from acute or chronic pain.

Cognitive manipulation targeted at decreasing the conditioning pain perception reduces the efficacy of conditioned pain modulation

Although painfulness of the conditioning stimulus (CS) is required for the activation of conditioned pain modulation (CPM), it is still unclear whether CPM expression depends on the objective physical intensity of the CS or the subjective perception of its pain. Accordingly, we cognitively manipulated the perceived CS pain, rendering the physical aspects of the CPM paradigm untouched. Baseline CPM was measured among 48 young healthy male subjects using the parallel paradigm with contact heat as test pain and hand immersion in hot water as CS. Subjects were then randomized into 4 groups, all of which were cognitively manipulated as to the CS-induced pain: group 1, placebo (CS less painful); group 2, nocebo (CS more painful); and groups 3 and 4, the informed control groups for groups 1 and 2, respectively. CPM was reassessed after the manipulation. Comparing the groups by MANCOVA (multivariate analysis of covariance) revealed that placebo exerted decreased CS pain and consequent attenuation of CPM magnitudes, while nocebo elicited increased CS pain, but without CPM elevation (P<.0001). Within the placebo group, the reduction in CS pain was associated with diminished CPM responses (r=0.767; P=.001); however, no such relationship characterized the nocebo group. Pain inhibition under CPM seems to depend on the perceived level of the CS pain rather than solely its physical intensity. Cognitively decreasing the perceived CS pain attenuates CPM magnitude, although a ceiling effect may limit CPM enhancement after cognitively increased CS pain. These findings emphasize the relevance of cognitive mechanisms in determining endogenous analgesia processes in humans.