Effect of Pain Induction or Pain Reduction on Conditioned Pain Modulation in Adults: A Systematic Review

Oral Delta-9-Tetrahydrocannabinol (THC) Increases Parasympathetic Activity and Supraspinal Conditioned Pain Modulation in Chronic Neuropathic Pain Male Patients: A Crossover, Double-Blind, Placebo-Controlled Trial

BACKGROUND

Disordered autonomic nervous system regulation and supraspinal pain inhibition have been repeatedly described in chronic pain. We aimed to explore the effects of δ-9-tetrahydrocannabinol (THC), an emerging treatment option, on autonomic nervous system and central pain modulation measures in patients with chronic pain.

METHODS

Twelve male patients with chronic radicular neuropathic pain participated in a randomized, double-blind, crossover, placebo-controlled, single-administration trial. Low/high frequency (LF/HF) heart rate variability (HRV) ratio and conditioned pain modulation (CPM) response were measured and resting-state functional magnetic resonance imaging (MRI) was performed at baseline and after sublingual administration of either 0.2 mg/kg oral THC or placebo.

RESULTS

THC significantly reduced the LF/HF ratio compared with placebo (interaction effect F(1,11) = 20.5; p < 0.005) and significantly improved CPM responses (interaction effect F(1,9) = 5.2; p = 0.048). The THC-induced reduction in LF/HF ratio correlated with increased functional connectivity between the rostral ventrolateral medulla and the dorsolateral prefrontal cortex [T(10) = 6.4, cluster p-FDR < 0.005].

CONCLUSIONS

THC shifts the autonomic balance towards increased parasympathetic tone and improves inhibitory pain mechanisms in chronic pain. The increase in vagal tone correlates with connectivity changes in higher-order regulatory brain regions, suggesting THC exerts top-down effects. These changes may reflect a normalizing effect of THC on multiple domains of supraspinal pain dysregulation.

CLINICAL TRIAL REGISTRY NUMBER

NCT02560545.

Assessment of Pain in Osteoarthritis of the Knee

Knee osteoarthritis (KOA) pain is a subjective and personal experience, making it challenging to characterise patients' experiences and assess their pain. In addition, there is no global standard for the assessment of pain in KOA. Therefore, this article examines the possible methods of assessing and characterising pain in patients with KOA using clinical symptoms, pain assessment tools, and imaging. We examine the current methods of assessment of pain in KOA and their application in clinical practice and clinical trials. Furthermore, we explore the possibility of creating individualised pain management plans to focus on different pain characteristics. With better evaluation and standardisation of pain assessment in these patients, it is hoped that patients would benefit from improved quality of life. At the same time, improvement in pain assessment would enable better data collection regarding symptom response in clinical trials for the treatment of osteoarthritis.

Modulation of central pain mechanisms using high-definition transcranial direct current stimulation: A double-blind, sham-controlled study

BACKGROUND

The use of high-definition transcranial direct current stimulation (HD-tDCS) has shown analgesic effects in some chronic pain patients, but limited anti-nociceptive effects in healthy asymptomatic subjects.

METHODS

This double-blinded sham-controlled study assessed the effects of HD-tDCS applied on three consecutive days on central pain mechanisms in healthy participants with (N = 40) and without (N = 40) prolonged experimental pain induced by intramuscular injection of nerve growth factor into the right hand on Day 1. Participants were randomly assigned to Sham-tDCS (N = 20 with pain, N = 20 without) or Active-tDCS (N = 20 with pain, N = 20 without) targeting simultaneously the primary motor cortex and dorsolateral prefrontal cortex for 20 min with 2 mA stimulation intensity. Central pain mechanisms were assessed by cuff algometry on the legs measuring pressure pain sensitivity, temporal summation of pain (TSP) and conditioned pain modulation (CPM), at baseline and after HD-tDCS on Day 2 and Day 3. Based on subject's assessment of received HD-tDCS (sham or active), they were effectively blinded.

RESULTS

Compared with Sham-tDCS, Active-tDCS did not significantly reduce the average NGF-induced pain intensity. Tonic pain-induced temporal summation at Day 2 and Day 3 was significantly lower in the NGF-pain group under Active-tDCS compared to the pain group with Sham-tDCS (p ≤ 0.05). No significant differences were found in the cuff pressure pain detection/tolerance thresholds or CPM effect across the 3 days of HD-tDCS in any of the four groups.

CONCLUSION

HD-tDCS reduced the facilitation of TSP caused by tonic pain suggesting that efficacy of HD-tDCS might depend on the presence of sensitized central pain mechanisms.

Immediate effects of dry needling on pain sensitivity and pain modulation in patients with chronic idiopathic neck pain: a single-blinded randomized clinical trial

BACKGROUND

Dry needling is frequently used for the treatment of neck pain but knowledge about its neurophysiological central effects is scarce.

OBJECTIVES

To compare the immediate effects of a single session of dry needling (DN) and sham needling (SN) on local and distant pressure pain thresholds and conditioned pain modulation in patients with chronic idiopathic neck pain.

METHOD

Participants with chronic idiopathic neck pain were randomly allocated to a DN or SN group. The primary outcome measure was the pressure pain threshold (PPT) at one peripheral location: quadriceps muscle (Q). Secondary outcome measures were local PPTs at the treated (most painful) (tUT) and non-treated upper trapezius muscle (ntUT), absolute and relative conditioned pain modulation (CPM) effects and pain during hot water immersion. Patients were assessed at baseline and immediately post intervention. Linear mixed models were used to examine interaction effects as well as between- and within-group differences.

RESULTS

Fifty-four participants were included for statistical analysis. Linear mixed model analyses showed no significant "group X time" interaction effects for any of the outcome measures. The relative CPM effect at the Q was significantly higher post-intervention, compared to baseline within the DN group (mean difference= 13.52%; 95% CI: 0.46, 26.59).

CONCLUSION

The present study shows no superior effect of DN, compared to SN, in the immediate effect on local and distant PPTs and CPM in patients with chronic idiopathic neck pain.

Questionnaire-based somatosensory profiling in breast cancer survivors: are we there yet? Associations between questionnaires and quantitative sensory testing

PURPOSE

Pain and sensory disturbances are common side effects of breast cancer treatment. Differential somatosensory functioning may reflect distinct pathophysiological backgrounds and therapeutic needs. Aim was to examine whether questionnaires evaluating signs and symptoms related to somatosensory functioning correlate sufficiently with quantitative sensory testing (QST) in breast cancer survivors to warrant consideration for somatosensory profiling in clinical practice.

METHODS

One year after breast cancer surgery, 147 women underwent QST and completed following questionnaires: Douleur Neuropathique en 4 questions (DN4), Central Sensitization Inventory, Margolis Pain Diagram and Visual Analog Scales (VAS). Associations between the questionnaires and QST were evaluated using Spearman correlation coefficients (rs).

RESULTS

Significant but weak (rs < 0.30) correlations were found between total DN4 score and QST results at the inner upper arm for detection of sharp stimuli (rs = 0.227), cold stimuli (rs = -0.186), and painful heat stimuli (rs = 0.179), as well as between QST evaluating conditioned pain modulation and the Margolis Pain Diagram on one hand (rs = 0.176) and minimum-maximum pain intensity differences (VAS) on the other (rs = -0.170).

CONCLUSION

Questionnaires evaluating signs and symptoms related to somatosensory functioning are insufficient for somatosensory profiling. Although somatosensory profiling may be valuable in a mechanism-based management, more research on the most appropriate clinical tools is needed.IMPLICATIONS FOR REHABILITATIONClinicians should be able to recognize that patients with persistent pain or sensory disturbances following breast cancer surgery may have a component of altered somatosensory processing as a significant contributor to their complaint in order to address it appropriately.Somatosensory profiling has yet to be implemented into clinical practice.No evidence-based recommendations can be made on the use of self-reported questionnaires to assess somatosensory processing in a breast cancer population based on the findings of this study.It is suggested to combine information on how individuals process and experience somatosensory stimulation with information from the patient interview or questionnaires to consider which biological, psychological and/or social factors may drive or sustain these neurophysiological processes.

Reimagining How We Treat Acute Pain: A Narrative Review

Acute pain may be influenced by biopsychosocial factors. Conditioned pain modulation, distraction, peripheral nerve stimulation, and cryoneurolysis may be helpful in its treatment. New developments in opioids, such as opioids with bifunctional targets and oliceridine, may be particularly suited for acute pain care. Allosteric modulators can enhance receptor subtype selectivity, offering analgesia with fewer and/or less severe side effects. Neuroinflammation in acute pain is caused by direct insult to the central nervous system and is distinct from neuroinflammation in degenerative disorders. Pharmacologic agents targeting the neuroinflammatory process are limited at this time. Postoperative pain is a prevalent form of acute pain and must be recognized as a global public health challenge. This type of pain may be severe, impede rehabilitation, and is often under-treated. A subset of surgical patients develops chronic postsurgical pain. Acute pain is not just temporally limited pain that often resolves on its own. It is an important subject for further research as acute pain may transition into more damaging and debilitating chronic pain. Reimagining how we treat acute pain will help us better address this urgent unmet medical need. 

Conditioned Pain Modulation Is Not Impaired in Individuals with Frozen Shoulder: A Case-Control Study

Frozen shoulder (FS) is a poorly understood condition resulting in substantial shoulder pain and mobility deficits. The mechanisms behind FS are not yet fully understood, but, similar to other persistent pain states, central pain mechanisms may contribute to ongoing symptoms in this population. The objective of this research was to investigate conditioned pain modulation (CPM) in people with FS compared with pain-free individuals. A total of 64 individuals with FS and 64 healthy volunteers participated in this cross-sectional study. CPM was assessed by using the pressure pain threshold (PPT) and an occlusion cuff (tourniquet test) as the test and conditioning stimulus, respectively. The absolute and percentage of change in PPT (CPM effect) as well as pain profiles (pro-nociceptive vs. anti-nociceptive) of individuals with FS and healthy controls were calculated. No significant differences in the absolute change in the PPT or CPM effect were found in people with FS compared to pain-free controls. Moreover, no between-group differences in the percentage of subjects with pro-nociceptive and anti-nociceptive pain profiles were observed. These results suggest that endogenous pain inhibition is normally functioning in people with FS. Altered central pain-processing mechanisms may thus not be a characteristic of this population.

Strength Exercise Has Different Effects on Pressure Pain Thresholds in Women with Endometriosis-Related Symptoms and Healthy Controls: A Quasi-experimental Study

OBJECTIVE

To evaluate the influence of strength exercises on remote pain sensitivity in women with endometriosis-related symptoms.

DESIGN

A quasi-experimental study.

SETTING

University Hospital, a tertiary health unit.

SUBJECTS

Twenty-one women with endometriosis-related symptoms and 21 healthy women provided written informed consent.

METHODS

The participants performed weekly exercise sessions on an extensor chair for four consecutive weeks. An electronic algometer was used to measure the pressure pain thresholds on the nondominant forearm. Heart rate and blood pressure were measured using a digital device. All measurements were taken before, immediately after, and 10 and 20 minutes after the exercise series.

RESULTS

Women with endometriosis-related symptoms had lower pain thresholds. Pressure pain thresholds increased immediately after exercise in healthy women, returning to baseline level 20 minutes after exercise. Women with endometriosis-related symptoms did not present significant pressure pain threshold alterations after exercise. However, they had a higher heart rate and systolic, diastolic, and average blood pressure than healthy women at all the timepoints. There were no consistent correlations between pressure pain thresholds and heart rate or blood pressure.

CONCLUSIONS

The strength exercise regimen used in this study increased pain thresholds in healthy women but not in women with endometriosis-related painful symptoms. The maintenance or even worsening of pain perception after exercise in women with persistent pain, such as those with endometriosis, may limit their adherence to a physical training program, which in turn could prevent them from experiencing the long-term beneficial effects of exercise.

Changes in pain catastrophization and neuropathic pain following operative stabilisation for patellofemoral instability: a prospective study with twelve month follow-up

PURPOSE

To determine the prevalence and change in neuropathic pain or pain catastrophizing before and 12 months following patellar stabilisation surgery for patellofemoral instability.

METHODS

We conducted a prospective clinical audit within a UK NHS orthopaedic surgical centre. Data from 84 patients with patellofemoral instability requiring stabilisation were analysed. Fifty percent (42/84) underwent MPFL reconstruction alone, and 16% (13/84) had both trochleoplasty and MPFL reconstruction. Neuropathic pain was assessed using painDETECT score. Pain catastrophizing was assessed using the Pain Catastrophizing Score. The Norwich Patellar Instability (NPI) Score and Kujala Patellofemoral Disorder Score were also routinely collected pre-operatively and one year post-operatively.

RESULTS

At 12 months post-operatively there was a statistically significant reduction in mean Pain Catastrophizing Scores (18.9-15.7; p < 0.02), but no change in mean painDETECT scores (7.3-7.8; p = 0.72). There was a statistically significant improvement in NPI scores (90.2-61.9; p < 0.01) and Kujala Patellofemoral Disorder Scores (48.7-58.1; p = 0.01). The prevalence of pain catastrophizing decreased from 31% pre-operatively to 24% post-operatively, whereas the prevalence of neuropathic pain remained consisted (10-11%).

CONCLUSIONS

Neuropathic pain and catastrophizing symptoms are not commonly reported and did not significantly change following patellofemoral stabilisation surgery. Whilst low, for those affected, there remains a need to intervene to improve outcomes following PFI surgery.

Perfil Nociceptivo e Uso de Analgésicos em Pacientes Submetidos à Cirurgia de Reparo do Manguito Rotador: Coorte Prospectiva

Objective  This study aimed to evaluate the nociceptive profile and the intake of analgesic drugs of patients submitted to rotator cuff repair surgery. Also, to evaluate the nociceptive thresholds and the integrity of the descending inhibitory system, pain catastrophism and prevalence of nociceptive or neuropathic pain.

Methods  Approved by the Ethics Committee of La Salle University (1.325.433/2015). 40 patients (>18 years old) who underwent rotator cuff repair surgery (divided in small and large injuries) were recruited. The used instruments were: Sociodemographic Questionnaire, Functional Pain Scale, Visual Analogue Scale (VAS), Quantitative Sensory Test (QST) and Conditioned Pain Modulation Task (CPM).

Results  Patients had a significant difference in pain thresholds QST heat (independent samples t test) and quality of sleep, mood and anxiety (paired t test) in groups preoperative. There was a significant correlation between preoperative CPM and postoperative VAS (Pearson Correlation). It was observed that, in preoperative, 38 patients used analgesics continuously. Besides that, in postoperative, use of opioid drugs was higher in patients with small injury (13 patients) than in those with large injury (9 patients).

Conclusion  Therefore, patients with rotator cuff injuries did not present alterations in the descending inhibitory system, but showed alterations in pain thresholds, which may interfere in the postoperative period and still be related to the consumption of analgesics.

A psychophysical study comparing massage to conditioned pain modulation: A single blind randomized controlled trial in healthy participants

INTRODUCTION

Pain-inducing massage results in greater pain inhibition than pain free massage, suggesting a mechanism dependent on conditioned pain modulation (CPM). The purpose of this study was to test the hypothesis that pain inducing massage produces similar magnitude of reduction in pain sensitivity as a cold pressor task and that baseline conditioned pain modulation efficiency predicts pain inducing massage related hypoalgesia.

METHODS

Sixty healthy participants were randomly assigned to receive either pain inducing massage to the neck, cold pressor task to the hand, or pain free massage to the neck. Participants also underwent pre and immediate post-intervention quantitative sensory testing. A repeated measures ANCOVA determined between group differences in pain sensitivity changes.

RESULTS

Pain inducing massage used as a conditioning stimulus resulted in comparable experimental pain sensitivity changes as a cold pressor task (p > 0.05). Pain intensity during the intervention demonstrated a weak correlation (r = 0.20, p = 0.12) with changes in pain sensitivity at a remote site. Individuals with an efficient CPM at baseline who received the pain inducing massage displayed greater increases in pressure pain threshold compared to individuals with a less efficient CPM indicating the potential benefit of treatment stratification by mechanism.

CONCLUSION

Although pain inducing massage resulted in less self-reported pain than a cold pressor task, both resulted in similar magnitude of the CPM response, suggesting shared underlying mechanisms. Understanding mechanisms of interventions can move us closer to mechanistic based treatments for pain which is consistent with a personalized medicine approach to care.

Central Sensitization Inventory is a useless instrument for detection of the impairment of the conditioned pain modulation in patients with chronic musculoskeletal pain

OBJECTIVE

The current study aimed to evaluate the concurrent validity and the diagnostic accuracy of the Central Sensitization Inventory (CSI) in detecting the impairment of the pain modulation in patients with chronic musculoskeletal pain.

METHODS

A cross-sectional study was conducted in 267 patients with chronic musculoskeletal pain enrolled consecutively in an outpatient department. The CSI (index method) were compared with the cold pressor test, which was the psychophysical test used to assess the conditioned pain modulation (CPM), (reference standard). Spearman's correlations assessed the concurrent validity, and measurements of the diagnostic accuracy were performed.

RESULTS

Ninety-three (34.8%) patients had CSI scores≥40. No significant correlation was found between CSI findings and the results of the CPT (dorsal forearm site or tibialis anterior site) was found. The cutoff point of 40 of the CSI showed values of sensitivity (35.1%, 95% CI: 22.6, 49.3) and specificity (65.2%, 95% CI: 58.4, 71.6) below 70%, and an accuracy of 59.1 (95% CI: 53.0, 65.1) when compared to the CPT to detect deficit. The ROC curve analysis yielded an area under the curve of 0.54 (95% CI: 0.45, 0.63, P>0.05).

CONCLUSIONS

The CSI is a useless instrument to detect the deficit in the CPM in patients with chronic musculoskeletal pain due to the absence of correlation with the psychophysical test result and the insufficient measurements of diagnostic accuracy.

Lack of correlation between central sensitization inventory and psychophysical measures of central sensitization in individuals with painful temporomandibular disorder

OBJECTIVE

To assess the correlation between the Central Sensitization Inventory (CSI) scores and the results of psychophysical tests and psychosocial questionnaires according to the presence of painful temporomandibular disorder (TMD).

DESIGN

It was a cross-sectional study involving 146 participants, aged 20-65 years. Painful TMD was classified using the Research Diagnostic Criteria for Temporomandibular Disorders. CSI was applied to assess "central sensitization-related symptoms", as has been suggested. Wind-up ratio, pressure pain threshold and conditioned pain modulation were used as psychophysical tests to evaluate signs and symptoms of central sensitization. Psychosocial factors were assessed by the presence of non-specific physical symptoms, depressive and anxiety symptoms. The sample was divided into two groups: Control (n = 31); Painful TMD (n = 115). Descriptive statistics characterized the sample. Correlation analysis were performed using Pearson's and Spearman's correlation coefficients (α = 5%).

RESULTS

Of the total sample, 78.8 % presented painful TMD, and the mean (standard deviation) age was 37.4 (±11.5) years. Anxiety symptoms (p = 0.028) and non-specific physical symptoms (p < 0.001) were more frequent in the painful TMD group than in controls. Painful TMD patients presented higher scores of the CSI (p < 0.001) and lower pressure pain thresholds (p ≤ 0.020) compared to controls. CSI scores were significantly correlated with psychosocial measures (p < 0.001) but not with psychophysical tests (p ≥ 0.089).

CONCLUSION

The CSI scores did not correlate with psychophysical measures of central sensitization but were positively correlated with the results of psychosocial questionnaires.

Diagnostic accuracy of the clinical indicators to identify central sensitization pain in patients with musculoskeletal pain

Background

The identification of central sensitization (CS) is an important aspect in the management of patients with chronic musculoskeletal pain. Several methods have been developed, including clinical indicators and psychophysical measures. However, whether clinical indicators coincide with the psychophysical test of CS-related sign and symptoms is still unknown. Therefore, the present study aimed to analyze the diagnostic accuracy of the clinical indicators in identifying CS-related sign and symptoms in patients with musculoskeletal pain.

Methods

One-hundred consecutive patients with musculoskeletal pain were included. Clinical indicators (index method) based on a combination of patient self-report pain characteristics and physical examination were used to identify the phenotype of patients with musculoskeletal pain and the predominance of the CS-related sign and symptoms. Conditioned pain modulation (CPM) was assessed by the Cold Pressor Test (reference standard), which is a psychophysical test used to detect impairment of CPM. Measurements of the diagnostic accuracy were performed.

Results

Twenty-seven patients presented predominance of CS-related sign and symptoms in the assessment of the clinical indicators, and 20 had impairment of CPM. Clinical indicators showed high accuracy (75.0%; 95% confidence interval = 65.3 to 83.1), high specificity (80.0%; 95% confidence interval = 69.6 to 88.1), high negative predictive value (87.7%; 95% confidence interval = 81.2 to 92.1), and a relevant positive likelihood ratio (2.8, 95% confidence interval = 1.5 to 5.0) when compared to the Cold Pressor Test.

Conclusion

Clinical indicators demonstrated a valuable tool for detecting the impaired CPM, which is a remarkable feature of the CS-related sign and symptoms. Clinicians are encouraged to use the clinical indicators in the management of patients with musculoskeletal pain.

Transcutaneous electrical nerve stimulation and heat to reduce pain in a chronic low back pain population: a randomized controlled clinical trial

BACKGROUND

Low back pain is the leading cause of disability worldwide. The therapeutic management of patients with chronic LBP is challenging.

OBJECTIVES

The aim of this study is to evaluate the effects of heat and transcutaneous electrical nerve stimulation combined on pain relief in participants with chronic low back pain.

METHODS

Fifty participants with chronic (≥3 months) low back pain were randomly assigned to two groups: HeatTens (n=25) and control group (n=25). Primary outcome was pain. Secondary outcomes were pressure pain thresholds, temporal summation, conditioned pain modulation, fear-avoidance and beliefs questionnaire, central sensitization inventory, quality of life, and medication use. The control group received no treatment and continued usual care. After four weeks of treatment, all measurements were repeated.

RESULTS

Fifty individuals participated in this study. Significant higher pressure pain threshold measures after both 30min and 4 weeks for the lower back region and the second plantar toe were found only in the experimental group.

CONCLUSION

The combination of heat and transcutaneous electrical nerve stimulation does not reduce pain scores in patients with chronic low back pain. Pressure pain threshold values significantly improved, showing beneficial effects of the experimental treatment. ClinicalTrials.gov: NCT03643731 (https://clinicaltrials.gov/ct2/show/NCT03643731).

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.

The Impact of Music on Nociceptive Processing

OBJECTIVE

Music has been shown to modulate pain, although the impact of music on specific aspects of nociceptive processing is less well understood. Using quantitative sensory testing (QST), we assessed the impact of a novel music app on specific aspects of nociceptive processing.

DESIGN

Within-subjects paired comparison of pain processing in control vs music condition.

SETTING

Human psychophysical laboratory.

SUBJECTS

Sixty healthy adult volunteers.

METHODS

Subjects were assessed for baseline anxiety, depression, and catastrophizing using validated questionnaires. QSTs measured included 1) pain threshold and tolerance to deep muscle pressure, 2) pain with mechanical pinprick, 3) temporal summation of pain (TSP) with a repeated pain stimulus, and 4) conditioned pain modulation (CPM) with a second painful stimulus. QSTs were performed in the absence and presence of music delivered through a music app.

RESULTS

We found an increase in pressure pain thresholds in both the forearm (P = 0.007) and trapezius (P = 0.002) with music, as well as a decrease in the amount of pinprick pain (P < 0.001) and TSP (P = 0.01) with music. Interestingly, CPM was also significantly diminished (P < 0.001) in the music condition. No significant difference in cold pain, anxiety, or situational catastrophizing was observed with music. Higher baseline pain catastrophizing scores were associated with less music-induced pressure pain reduction.

CONCLUSIONS

Several measures of mechanical pain sensitivity were reduced with music. TSP, a measure of central sensitization, also decreased with music, but CPM, a measure of descending modulation of pain, was not further augmented by music.

Conditioned Pain Modulation Effectiveness: An Experimental Study Comparing Test Paradigms and Analyzing Potential Predictors in a Healthy Population

Conditioned pain modulation (CPM) is an endogenous pain inhibition phenomenon that can be summarized simply as one type of pain being able to inhibit another, which must be in a remote area in relation to the first pain. We aimed to compare the effectiveness of four CPM test paradigms as well as the association of the CPM effect with potential predictors in 72 healthy volunteers. Pressure pain from an algometer was used as the test stimulus, and pain provoked by cold water or ischemic pressure was used as the conditioning stimulus, applied either sequentially or in parallel. No significant differences were found between the test paradigms, although the cold-parallel test showed the most significant effect size (ηP² = 0.614). No association was found between the CPM effect and sociodemographic variables (age or sex), nor anxiety, depression, catastrophizing, previous history of pain or self-perceived pain tolerance. Nevertheless, a strong association was found between the CPM effect and individual affinity for the stimulus in participants who underwent the cold water test paradigm; this explained around 45% of the total CPM effect when the paradigm (cold water) coincided with personal affinity for the stimulus (“I prefer cold to heat”, “cold is not unpleasant”).

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.

Human experimental pain models

Pain is an unpleasant sensory experience, associated with existing or potential tissue damage. It has also strong cognitive and emotional components. Stimuli that causes pain goes through process of nociception, which includes transduction, transmission, modulation and perception of said stimuli. Depending on the type of stimuli, we can classify human experimental pain models into mechanical, electrical, thermal and chemical. Information about pain mechanisms can be obtained from the following: 1) in vitro studies, 2) animal experiments, 3) human experimental pain studies and 4) clinical studies. Chosing the appropriate method for pain evaluation is a key step in the design of pain studies. Combining it with different electro-physiological and imaging methods, it can provide better objectivity and quantification of pain mechanisms. Focus in experimental pain studies is slowly shifting from static parameters of pain, such as pain threshold and maximum tolerance, to dynamic parameters, which can give us valuable insight in function of endogenous analgesic systems. This can be done using conditioned pain modulation. Using experimental pain on healthy voulenteers is key step in switching from animal models to clinical studies, foremost for validization of data from animals, making them important in translational research. Results from experimental pain studies can help us in understanding nociceptive mechanisms of acute and chronic pain, alongside development of new therapeutic modalities.

Biopsychosocial Influences on Shoulder Pain: Analyzing the Temporal Ordering of Postoperative Recovery

Shoulder surgery is a primary intervention for shoulder pain, yet many individuals experience persistent postoperative pain. Previously, we found individuals categorized as having a high-risk phenotype (comprised of COMT variation and pain catastrophizing) had approximately double the chance of not reaching a 12-month pain recovery criterion. As a means to better understand the development of persistent postoperative shoulder pain, this study advanced our previous work by examining temporal ordering of postoperative shoulder recovery based on potential mediating factors, and expansion of outcomes to include movement-evoked pain and shoulder active range of motion. Before surgery, individuals were categorized as either high-risk (high pain catastrophizing, COMT-genotype linked to low enzyme activity [n = 41]) or low-risk (low pain catastrophizing, COMT-genotype linked to normal enzyme activity [n = 107]). We then compared potential mediating variables at 3, 6, and 12 months postoperatively 1) endogenous pain modulation defined by a conditioned pain modulation paradigm; and 2) and emotion factors such as anxiety, fear of movement, and depressive symptoms. At 3 months, the high-risk subgroup had higher fear and movement-evoked pain, and causal mediation analysis confirmed the direct effect of risk subgroup on 12-month movement evoked pain. However, baseline to 12-month change in depressive symptoms were found to mediate 53% of the total effect of risk subgroup on 12-month movement-evoked pain. This study introduces potential temporal components and relationships to the development of persistent postoperative shoulder pain, which future studies will confirm and assess for potential therapeutic targets. PERSPECTIVE: This study expands upon postoperative shoulder recovery measures to include movement-evoked pain and depressive symptoms, and provides preliminary indication of temporal ordering to postoperative shoulder recovery for a preidentified high-risk subgroup. Future studies will distinguish temporal components of shoulder surgery that may optimize treatment targets of postoperative recovery.

Divergent effects of conditioned pain modulation on subjective pain and nociceptive-related brain activity

BACKGROUND AND OBJECTIVES

Pain is a complex experience involving both nociceptive and affective-cognitive mechanisms. The present study evaluated whether modulation of pain perception, employing a conditioned pain modulation (CPM) paradigm, is paralleled by changes in contact heat-evoked potentials (CHEPs), a brain response to nociceptive stimuli.

METHODS

Participants were 25 healthy, pain-free, college students (12 males, 13 females, mean age 19.24 ± 0.97 years). Twenty computer-controlled heat stimuli were delivered to the non-dominant forearm and CHEPs were recorded at Cz using a 32-channel EEG system. After each stimulus, participants rated the intensity of the heat pain using the 0-100 numerical rating scale. The latency and amplitude of N2, P2 components as well as single-sweep spectral analysis of individual CHEPs were measured offline. For CPM, participants had to submerge their dominant foot into a neutral (32 °C) or noxious (0 °C) water bath. CHEPs and heat pain ratings were recorded in 3 different conditions: without CPM, after neutral CPM (32 °C) and after noxious CPM (0 °C).

RESULTS

The noxious CPM induced a facilitatory pain response (p = 0.001) with an increase in heat pain following noxious CPM compared to neutral CPM (p = 0.001) and no CPM (p = 0.001). Changes in CHEPs did not differ between conditions when measured as N2-P2 peak-to-peak amplitude (p = 0.33) but the CPM significantly suppressed the CHEPs-related delta power (p = 0.03). Changes in heat pain in the noxious CPM were predicted by trait catastrophizing variables (p = 0.04).

CONCLUSION

The current study revealed that pain facilitatory CPM is related to suppression of CHEPs delta power which could be related to dissociation between brain responses to noxious heat and pain perception.

Clinical neurophysiology of pain

Clinical neurophysiologic investigation of pain pathways in humans is based on specific techniques and approaches, since conventional methods of nerve conduction studies and somatosensory evoked potentials do not explore these pathways. The proposed techniques use various types of painful stimuli (thermal, laser, mechanical, or electrical) and various types of assessments (measurement of sensory thresholds, study of nerve fiber excitability, or recording of electromyographic reflexes or cortical potentials). The two main tests used in clinical practice are quantitative sensory testing and pain-related evoked potentials (PREPs). In particular, PREPs offer the possibility of an objective assessment of nociceptive pathways. Three types of PREPs can be distinguished depending on the type of stimulation used to evoke pain: laser-evoked potentials, contact heat evoked potentials, and intraepidermal electrical stimulation evoked potentials (IEEPs). These three techniques investigate both small-diameter peripheral nociceptive afferents (mainly Aδ nerve fibers) and spinothalamic tracts without theoretically being able to differentiate the level of lesion in the case of abnormal results. In routine clinical practice, PREP recording is a reliable method of investigation for objectifying the existence of a peripheral or central lesion or loss of function concerning the nociceptive pathways, but not the existence of pain. Other methods, such as nerve fiber excitability studies using microneurography, more directly reflect the activities of nociceptive axons in response to provoked pain, but without detecting or quantifying the presence of spontaneous pain. These methods are more often used in research or experimental study design. Thus, it should be kept in mind that most of the results of neurophysiologic investigation performed in clinical practice assess small fiber or spinothalamic tract lesions rather than the neuronal mechanisms directly at the origin of pain and they do not provide objective quantification of pain.

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 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.

Chronic stress moderates the impact of social exclusion on pain tolerance: an experimental investigation

BACKGROUND

Experiences of social pain due to social exclusion may be processed in similar neural systems that process experiences of physical pain. The present study aimed to extend the findings on social exclusion and pain by examining the impact of social exclusion on an affective (ie, heat pain tolerance) and a sensory component of pain (ie, heat pain intensity). Whether a potential effect may be moderated by chronic life stress, social status, or social sup-port was further examined.

MATERIALS AND METHODS

A community-based sample of 59 women was studied. Social exclusion and inclusion were experimentally manipulated by using a virtual ball-tossing game called Cyberball in which participants were randomly assigned to either being excluded or being included by two other virtual players. Heat pain tolerance and intensity were assessed before and after the game. Potential psychosocial moderators were assessed via a questionnaire.

RESULTS

The main finding of this study is that chronic stress moderates the impact of social exclusion on pain tolerance (p<0.05). When chronic stress was high, socially excluded participants showed a lower heat pain tolerance than participants who were socially included. Contrary to the authors' hypothesis, pain sensitivity was increased in socially included participants compared with socially excluded participants after the game (p<0.05).

CONCLUSION

Higher levels of chronic stress may enhance the vulnerability of affective pain processing to acute social exclusion.