Determinants of endogenous analgesia magnitude in a diffuse noxious inhibitory control (DNIC) paradigm: Do conditioning stimulus painfulness, gender and personality variables matter?

Central sensitization inventory is associated with psychological functioning but not with psychophysical assessment of pain amplification

BACKGROUND

The central sensitization inventory (CSI) is a questionnaire that has been widely used as a tool for assessing symptoms associated with sensitization. However, its ability to identify individuals with this phenomenon has recently been questioned. The aim of this study was to assess the correlation of CSI with psychosocial and psychophysical factors in patients with painful TMD diagnosed according to diagnostic criteria for temporomandibular disorders (DC/TMD) and asymptomatic controls, as well as to determine the influence of these variables on the CSI scores variations.

METHODS

This cross-sectional study with 77 patients diagnosed with painful TMD according to DC/TMD and 101 asymptomatic controls realized correlations between CSI, WUR, PPT, CPM and psychosocial questionnaires (HADS, PSQI, PCS and PSS). In cases where significant correlations existed, the potential influence of these variables on CSI variation was explored through linear regression analysis.

RESULTS

It has been found that the CSI correlates with psychosocial variables (anxiety, depression, catastrophizing, sleep and stress) (p < 0.0006) regardless of the presence of TMD, and that 68.9% of the variation in CSI scores can be influenced by all these variables (except stress). On the contrary, the CSI does not correlate with psychophysical parameters indicative of pain amplification (wind-up ratio and conditioned pain modulation) (p > 0.320).

CONCLUSION

CSI is more associated with psychosocial factors than with more robust indicators of probable central sensitization (CS), thus limiting its utility in detecting this phenomenon both in TMD patients and healthy individuals.

SIGNIFICANCE STATEMENT

The research highlights a noteworthy relationship between the central sensitization inventory and psychological factors, emphasizing their substantial influence on inventory values. This correlation offers crucial insights into mental health markers within the questionnaire. Additionally, the lack of connection with pain amplification implies a necessary re-evaluation of the inventory's diagnostic suitability, especially in cases of painful temporomandibular disorders. Thus, caution is urged in its application for identifying CS in these individuals.

How side effects can improve treatment efficacy: a randomized trial

While treatment side effects may adversely impact patients, they could also potentially function as indicators for effective treatment. In this study, we investigated whether and how side effects can trigger positive treatment expectations and enhance treatment outcomes. In this pre-registered trial (DRKS00026648), 77 healthy participants were made to believe that they will receive fentanyl nasal sprays before receiving thermal pain in a controlled experimental setting. However, nasal sprays did not contain fentanyl, rather they either contained capsaicin to induce a side effect (mild burning sensation) or saline (inert). After the first session, participants were randomized to two groups and underwent functional MRI. One group continued to believe that the nasal sprays could contain fentanyl while the other group was explicitly informed that no fentanyl was included. This allowed for the independent manipulation of the side effects and the expectation of pain relief. Our results revealed that nasal sprays with a side effect lead to lower pain than inert nasal sprays without side effects. The influence of side effects on pain was dependent on individual beliefs about how side effects are related to treatment outcome, as well as on expectations about received treatment. Functional MRI data indicated an involvement of the descending pain modulatory system including the anterior cingulate cortex and the periaqueductal gray during pain after experiencing a nasal spray with side effects. In summary, our data show that mild side effects can serve as a signal for effective treatment thereby influencing treatment expectations and outcomes, which is mediated by the descending pain modulatory system. Using these mechanisms in clinical practice could provide an efficient way to optimize treatment outcome. In addition, our results indicate an important confound in clinical trials, where a treatment (with potential side effects) is compared to placebo.

Preliminary Characterization of Age and Chronic Low Back Pain Effects on Multimodal Pain Sensitivity: A Comparison Study in Older Adults with and Without Chronic Low Back Pain

Though pain sensitivity impairments contribute to chronic pain in younger adults, it is unclear if pain hypersensitivity manifests with aging and is heightened in the geriatric chronic low back pain population. The cross-sectional study preliminarily addressed this gap by measuring pain sensitivity in older adults with chronic low back pain (n = 25) as well as pain-free sex-matched older (n = 25) and younger adults (n = 25). Pain sensitivity was quantified by 8 distinct measures that were subdivided as static (ie, pressure pain thresholds, heat pain thresholds, fixed mechanical pain, and fixed cold pain) and dynamic pain sensitivity (ie, mechanical temporal summation, thermal ramp and hold, heat pain aftersensations, and conditioned pain modulation). Test-retest reliability values for pain sensitivity ranged from moderate to excellent (intraclass correlation coefficients ≥ .500; p's < .05). The main effect for the group was significant (partial η2 = .413, P < .001), revealing between-group differences in pain sensitivity on 5 out of 8 tests (p's ≤ .043). Predominantly, both older adult groups demonstrated increased pain facilitation and decreased pain inhibition during dynamic pain sensitivity testing compared to pain-free younger adults (p's ≤ .044). Despite qualitative differences, static and dynamic pain sensitivity responses were statistically similar between older adults with and without chronic LBP (p's > .05). Findings suggest pain sensitivity can be reliably measured in older adults and that pain hypersensitivity develops with chronological aging, providing partial support for the theory that pain hypersensitivity may impact geriatric chronic pain populations. Further study is needed to more definitively parse out whether pain hypersensitivity is comparatively heightened in older adults with chronic LBP beyond the influence of chronological aging. PERSPECTIVE: This article establishes that surrogate measures of centrally mediated pain sensitization are heightened with aging. Impaired endogenous pain modulation may influence chronic pain development, maintenance, treatment efficacy, and/or ensuing disability, necessitating research to comprehensively characterize how pain hypersensitivity contributes to geriatric chronic pain conditions.

Hypoalgesia and Conditioned Pain Modulation in Blood Flow Restriction Resistance Exercise

We compared the magnitude of exercise-induced hypoalgesia and conditioned pain modulation between blood-flow restriction (BFR) resistance exercise (RE) and moderate-intensity RE. Twenty-five asymptomatic participants performed unilateral leg press in two visits. For moderate-intensity RE, subjects exercised at 50% 1RM without BFR, whereas BFR RE exercised at 30% 1RM with a cuff inflated to 60% limb occlusion pressure. Exercise-induced hypoalgesia was quantified by pressure pain threshold changes before and after RE. Conditioned pain modulation was tested using cold water as the conditioning stimulus and mechanical pressure as the test stimulus and quantified as pressure pain threshold change. Difference in conditioned pain modulation pre- to post-RE was then calculated. The differences of RE on pain modulations were compared using paired t-tests. Pearson's r was used to examine the correlation between exercise-induced hypoalgesia and changes in conditioned pain modulation. We found greater hypoalgesia with BFR RE compared to moderate-intensity RE (p=0.008). Significant moderate correlations were found between exercise-induced hypoalgesia and changes in conditioned pain modulation (BFR: r=0.63, moderate-intensity: r=0.72). BFR RE has favorable effects on pain modulation in healthy adults and the magnitude of exercise-induced hypoalgesia is positively correlated with conditioned pain modulation activation.

Physical activity is predictive of conditioned pain modulation in healthy individuals: a cross-sectional study

Even in healthy populations, conditioned pain modulation (CPM) magnitude varies. This may be accounted for by (non-)modifiable factors, including physical activity. Yet, little research has thoroughly examined physical activity and its relation with CPM magnitude in a representative sample. Therefore, the present study investigated the predictive effect of physical activity on CPM magnitude in 105 healthy adults. Physical activity was assessed during seven consecutive days by self-report using the International Physical Activity Questionnaire and by monitored-based accelerometry. CPM was examined using a heterotopic noxious conditioning stimulation protocol during which the effect of a hot water conditioning stimulus on pressure pain thresholds was evaluated. Comparative, correlation, and hierarchical linear regression analyses were performed. Report-based walking predicts 4.8% of variance in pain modulatory capacity, moderate physical activity predicts 10.2% of variance in pain modulatory capacity, and report-based time spent on total physical activity predicts 7.0% of variance in pain modulatory capacity. More MET-minutes/week spent on total physical activity, including walking and moderate physical activity is associated with greater pain modulatory capacity. The findings of this study add to the limited evidence on the predictive effect of physical activity on CPM. It urges to consider physical activity a confounding factor when examining CPM. The current study provides evidence that a physically active lifestyle benefits endogenous pain modulation in healthy adults. Given its potential, walking and moderate-intensity physical activity might be achievable treatment strategies for pain patients known to have impaired CPM. PERSPECTIVE: The results of this article show that a physically active lifestyle including larger amounts of walking and moderate activity predict greater pain modulatory capacity. TRIAL REGISTRATION: This study has not been preregistered.

Regional and widespread pain sensitivity decreases following stretching in both men and women - Indications of stretch-induced hypoalgesia

INTRODUCTION

This study aimed to investigate the effect of sex on regional and widespread pain sensitivity following acute bouts of stretching and to investigate the acute effect of stretching on regional and widespread pain sensitivity following stretching.

METHODS

73 healthy adults (36 females; mean age 25.6 ± 6.7 years) with an age range from 19 to 62 years were recruited for this experimental study. Regional and distant pain pressure pain thresholds, passive knee extension range of motion and passive resistive torque were measured before and 30 s after four bouts of 30-s static muscle stretching of the knee flexors with 20-s rest between bouts.

RESULTS

No significant sex differences were found for pressure pain thresholds (p > 0.132), range of motion (p = 0.446) or passive resistive torque (p = 0.559) between pre-stretch and post-stretch measures. There were significant increases in pressure pain thresholds (p = 0.010), range of motion (p = 0.001) and passive resistive torque (p = 0.007) between pre-stretch and post-stretch measures.

CONCLUSION

Muscle stretching significantly decreased regional and widespread pain sensitivity, indicating that central pain-modulating mechanisms are engaged during muscle stretching, resulting in stretch-induced hypoalgesia. Moreover, the results showed that the effect of stretching on regional and widespread pain sensitivity is not sex-specific.

Adaptive and Compensatory Neural Signatures in Fibromyalgia: An Analysis of Resting-State and Stimulus-Evoked EEG Oscillations

This study aimed to investigate clinical and physiological predictors of brain oscillatory activity in patients with fibromyalgia (FM), assessing resting-state power, event-related desynchronization (ERD), and event-related synchronization (ERS) during tasks. We performed a cross-sectional analysis, including clinical and neurophysiological data from 78 subjects with FM. Multivariate regression models were built to explore predictors of electroencephalography bands. Our findings show a negative correlation between beta oscillations and pain intensity; fibromyalgia duration is positively associated with increased oscillatory power at low frequencies and in the beta band; ERS oscillations in the theta and alpha bands seem to be correlated with better symptoms of FM; fatigue has a signature in the alpha band-a positive relationship in resting-state and a negative relationship in ERS oscillations. Specific neural signatures lead to potential clusters of neural adaptation, in which beta oscillatory activity in the resting state represents a more adaptive activity when pain levels are low and stimulus-evoked oscillations at lower frequencies are likely brain compensatory mechanisms. These neurophysiological changes may help to understand the impact of long-term chronic pain in the central nervous system and the descending inhibitory system in fibromyalgia subjects.

CPM-Related Mechanisms Could Play a Key Role in the Effects on Pain Sensitivity Induced by Manual Therapy: Three Crossover Trials Investigating the Effects of Manual Pressure

Objective: The aim of this study is to assess whether pain-inducing manual pressure (PIMP) leads to effects on pressure pain threshold (PPT) mediated by conditioned pain modulation (CPM) and whether these effects are influenced by the intensity and repetition of the stimulus. Additionally, the influence of psychological factors and physical activity on the response to PIMP was explored. Methods: A total of 72 pain-free students were randomly assigned to three crossover trials. Trial 1 compared the effects of PIMP with the cold pressor task and pain-inducing electrostimulation. Trial 2 compared the effects of manual pressure that elicited moderate pain, mild pain, and no pain. Trial 3 compared a single PIMP stimulation with four stimuli applied at the same site or at different sites. Results: PIMP produced a lower increase in PPT than cold pressor task and no difference with electrostimulation. Manual pressure that caused moderate pain led to a greater increase in PPT compared to mild pain and pain-free application. Repetition of PIMP stimulus, whether at the same or different sites, did not significantly increase PPT compared to a single stimulation. No association with psychological factors or physical activity was found. Conclusions: PIMP produces an increase in PPT, suggesting the involvement of CPM-related mechanisms.

The proteus effect on human pain perception through avatar muscularity and gender factors

The Proteus effect, which occurs when using an avatar in virtual reality, influences user behavior, changes attitudes, and improves physical performance. Here, we show that human pain perception can be alleviated by the Proteus effect. To investigate the pain alleviation effect of using an avatar in a virtual environment, we conducted two experiments using a head-mounted display and a thermal pain stimulator to induce acute pain. The first experiment involved 20 adult participants, while the second experiment involved 44 adult participants. Experimental results show that participants reported significantly lower pain scores (15.982% reduction), as measured by the Pain Assessment Scale (PAS), when using a muscular avatar than when using a normal avatar. The experiments also revealed several significant gender factors. For example, participants reported significantly lower pain scores when using a gender-congruent avatar. In addition, the use of a muscular avatar was particularly effective for male participants. In contrast, female participants consistently reported lower pain scores when using the avatar regardless of its body type (muscular/normal). To further our understanding, we also measured participants' gender-related pain stereotypes using the Gender Role Expectations of Pain (GREP) questionnaire, as well as participants' sense of embodiment. The results of these questionnaires are consistent with the results of the PAS, suggesting possible relationships between stereotypes and the Proteus effect on pain perception, and between the degree of immersion in an avatar and the user's perception of pain.

Age- and Sex-Dependent Effects of Moderate Exercise on Endogenous Pain Inhibition in Rats

Diffuse noxious inhibitory controls (DNICs), or the pain inhibits pain phenomenon, refer to reduced pain-like behaviors that are displayed following a noxious conditioning stimulus located far from the test stimulus and have also been referred to as "descending control of nociception" when measured in awake-behaving animals. In this study, we sought to determine the impact of moderate long-term exercise on the DCN response and determine if this effect differed across age and sex. After a six-week exercise program consisting of 30 min of moderate treadmill running 5 days a week, the animals' forepaws were injected with capsaicin, and DCN responses were assessed using thermal withdrawal latencies of the hind paw. Young, exercised male and female rats displayed prolonged DCN responses relative to their sedentary counterparts, with the young exercised male group displaying longer-lasting DCN facilitation than the young exercised females. Exercise did not impact DCN responses in either male or female aged rats. Additionally, the serum testosterone levels did not change following exercise in any group. Importantly, the levels of corticosterone did not change following the exercise program, indicating that changes in the DCN response are not due to stress-induced analgesia. Our findings suggest that moderate exercise can facilitate the DCN response in young animals, even when this exercise does not change the levels of serum testosterone.

Influence of psychometric and sleep quality features on painful mechanical sensitivity and pain modulation in patients with temporomandibular disorders

Investigating the collective impact of psychometric properties and sleep quality on pain sensitivity in temporomandibular disorder (TMD) patients could improve clinical management strategies.

OBJECTIVE

Assessing whether combined psychometric properties and sleep quality impact painful mechanical sensitivity and pain modulation in TMD patients.

MATERIALS AND METHODS

A cross-sectional study using secondary data analysis of 77 TMD patients and 101 controls. All participants completed questionnaires characterizing their psychometric profile (anxiety, depression, stress and catastrophizing) and sleep quality, alongside psychophysical tests for painful mechanical sensory (mechanical pain threshold (MPT), pressure pain threshold (PPT), and wind-up ratio (WUR)) and conditioned pain modulation (CPM). Participants were grouped into "High distress" or "Low distress" categories based on psychometric properties and sleep quality using hierarchical cluster and k-means analyses. Multiple linear regression evaluated the influence of TMD, age, and the distress cluster on MPT, WUR, PPT, and CPM in masseter and thenar muscles. Differences were statistically significant when p < 0.05.

RESULTS

The presence of TMD was the strongest predictor of mechanical painful sensitivity in the trigeminal region (MPT[F(3,174) = 51.902;p < .001;R2 = .463]; TMD presence (β = -.682) / PPT[F(3,174) = 15.573;p < .001;R2 = .198] TMD presence (β = -.452), and extra-trigeminal (MPT[F(3,174) = 35.897;p < .001;R2 = .382] TMD (β = -.647) / CPM [F(3,174) = 4.106;p < .05;R2 = .050] TMD presence (β = .197). Furthermore, neither the high distress group nor the low distress group were able to significantly influence the variation of the values of any of the psychophysical variables evaluated (p > .05).

CONCLUSIONS

There is not a significant influence of impairment clusters based on psychological variables and sleep quality on painful mechanical sensitivity and pain modulation, regardless of the presence of TMD.

CLINICAL RELEVANCE

This outcome suggests that psychosocial factors and sleep quality may not play a decisive role in the sensory-discriminative aspect of pain, particularly concerning painful TMD.

Cross-sectional and longitudinal analysis of conditioned pain modulation and pain in fibromyalgia: CPM as an effect modifier of pain changes over time

Background and purpose

Fibromyalgia (FM) is associated with altered descending pain modulatory pathways, which can be assessed through Conditioned Pain Modulation (CPM). In this study, we aimed to explore the relationship between CPM and self-reported baseline characteristics in patients with fibromyalgia. We also performed a longitudinal analysis exploring CPM as a potential predictor of clinical improvement over time in individuals with FM.

Methods

We performed cross-sectional univariable and multivariable analyses of the relationship between CPM and other variables in 41 FM patients. We then performed longitudinal analyses, building linear mixed effects models with pain in the Visual Analogue Scale (VAS) as the dependent variable, and testing for the interaction between time and CPM. We also tested the interaction between CPM and time in models using other outcomes, such as the revised Fibromyalgia Impact Questionnaire (FIQR) and Quality of Life Scale (QOLs).

Results

We found no association between CPM and other demographic and clinical variables in the univariable analysis. We found a statistically significant association in the multivariable linear regression model between CPM and the QOLs at baseline, after controlling for age, sex, and duration of symptoms. In the longitudinal analyses, we found that CPM is an effect modifier for clinical improvement over time for the pain VAS, QOLs and FIQR: individuals with low-efficient CPM at baseline have a different (improved) pattern of response over time when compared to those with high-efficient CPM.

Conclusions

Our findings suggest that CPM is not a reliable biomarker of clinical manifestations in chronic pain patients during cross-sectional assessments. However, our results are consistent with previous findings that CPM can be used to predict the evolution of clinical pain over time. We expect that our findings will help in the selection of patients with the best profile to respond to specific interventions and assist clinicians in tailoring pain treatments.

Sex differences in visceral sensitivity and brain activity in a rat model of comorbid pain: a longitudinal study

ABSTRACT

Temporomandibular disorder (TMD) and irritable bowel syndrome (IBS) are 2 chronic overlapping pain conditions (COPCs) that present with significant comorbidity. Both conditions are more prevalent in women and are exacerbated by stress. While peripheral mechanisms might contribute to pain hypersensitivity for each individual condition, mechanisms underlying the comorbidity are poorly understood, complicating pain management when multiple conditions are involved. In this study, longitudinal behavioral and functional MRI-based brain changes have been identified in an animal model of TMD-like pain (masseter muscle inflammation followed by stress) that induces de novo IBS-like comorbid visceral pain hypersensitivity in rats. In particular, data indicate that increased activity in the insula and regions of the reward and limbic systems are associated with more pronounced and longer-lasting visceral pain behaviors in female rats, while the faster pain resolution in male rats may be due to increased activity in descending pain inhibitory pathways. These findings suggest the critical role of brain mechanisms in chronic pain conditions and that sex may be a risk factor of developing COPCs.

Is conditioned pain modulation (CPM) affected by negative emotional state?

BACKGROUND

Conditioned pain modulation (CPM) is an experimental paradigm, which describes the inhibition of responses to a noxious or strong-innocuous stimulus, the test stimulus (TS), by the additional application of a second noxious or strong-innocuous stimulus, the conditioning stimulus (CS). As inadequate CPM efficiency has been assumed to be predisposing for clinical pain, the search for moderating factors explaining inter-individual variations in CPM is ongoing. Psychological factors have received credits in this context. However, research concerning associations between CPM and trait factors relating to negative emotions has yielded disappointing results. Yet, the influence of anxious or fearful states on CPM has not attracted much interest despite ample evidence that negative affective states enhance pain. Our study aimed at investigating the effect of fear induction by symbolic threat on CPM.

METHODS

Thirty-seven healthy participants completed two experimental blocks: one presenting aversive pictures showing burn wounds (high-threat block) and one presenting neutral pictures (low-threat block). Both blocks contained a CPM paradigm with contact heat as TS and hot water as CS; subjective numerical ratings as well as contact-heat evoked potentials (CHEPs) were assessed.

RESULTS

We detected an overall inhibitory CPM effect for CHEPs amplitudes but not for pain ratings. However, we found no evidence for a modulation of CPM by threat despite threat ratings indicating that our manipulation was successful.

DISCUSSION

These results suggest that heat/thermal CPM is resistant to this specific type of symbolic threat induction and further research is necessary to examine whether it is resistant to fearful states in general.

SIGNIFICANCE

The attempt of modulating heat conditioned pain modulation (CPM) by emotional threat (fear/anxiety state) failed. Thus, heat CPM inhibition again appeared resistant to emotional influences. Pain-related brain potentials proved to be more sensitive for CPM effects than subjective ratings.

Sex differences in descending control of nociception (DCN) responses after chronic orofacial pain induction in rats and the contribution of kappa opioid receptors

Descending control of nociception (DCN), a measure of efficiency of descending pain inhibition, can be assessed in animals by the combined application of test and conditioning noxious stimuli. Evidence from pre-clinical and clinical studies indicates that this mechanism of pain control may differ between sexes and might be impaired in many chronic pain states. However, little is known about sex differences in DCN efficiency in models of acute and chronic orofacial pain. Herein, we first evaluated DCN responses in male and female rats by the applying formalin into the upper lip or capsaicin into the forepaw as the conditioning stimulus, followed by mechanical stimulation (Randall-Selitto) of the hind paw as the test stimulus. The same protocol (i.e., capsaicin in the forepaw followed by mechanical stimulation of the hind paw) was evaluated in male and female rats on day 3 after intraoral incision and on day 15 and 30 after chronic constriction injury of the infraorbital nerve (CCI-ION). Additionally, we assessed the effect of the kappa opioid receptor (KOR) antagonist Norbinaltorphimine (nor-BNI) on DCN responses of female nerve-injured rats. This study shows that naïve female rats exhibit less efficient DCN compared to males. Postoperative pain did not alter DCN responses in female and male rats, but CCI-ION induced loss of DCN responses in females but not in males. Systemic pretreatment with nor-BNI prevented the loss of DCN induced by CCI-ION in female rats. The results reveal sex differences in DCN responses and female-specific impairment of DCN following chronic orofacial pain. Moreover, the findings suggest that, at least for females, blocking KOR could be a promising therapeutic approach to prevent maladaptive changes in chronic orofacial pain.

Body-site effect on CPM efficiency in healthy subjects: Central vs. peripheral stimulation

Structural changes in the peripheral nerve system in neuropathic states alter sensory capacity of the affected area, thus biasing results of conditioned pain modulation (CPM) responses. The aim of this study was to evaluate CPM efficiency of central (i.e. trunk) vs. peripheral (i.e. limb) application of 'test' and 'conditioning' stimuli. Methods: Healthy volunteers (ages 18-73 yrs) underwent two CPM protocols: 'CPM Limb' and 'CPM Trunk'. Each included two types of test stimuli (Ts) (pressure pain threshold: PPT; and contact heat) conditioned either to hand immersion in cold noxious water (CPM limb), or to noxious contact heat applied on lower back (CPM trunk). Results: Both protocols generated efficient pain inhibition for each of the applied Ts; the PPT-based protocol induced more efficient CPM when the conditioned stimulus was applied on the trunk (p = 0.016). Moreover, the PPT-based CPM responses were significantly correlated (ρ = 0.349; p = 0.007). Conclusions: An efficient CPM induced by both central and peripheral stimulation, along with significant correlation between PPT-based responses, advances using the central 'CPM Trunk' protocol in patients with peripheral neuropathy.

Hyperalgesia, Increased Temporal Summation and Impaired Inhibitory Mechanisms in Episodic and Chronic Cluster Headache: An Observational Study

Cluster Headache (CH) is a primary headache that causes severe pain. Some evidence suggests that central mechanisms might be involved. The objective of this study was (1) to compare hyperalgesia signs, temporal summation and conditioned pain modulation among episodic (ECH) and chronic CH (CCH) patients and controls, (2) to compare these factors between sides in the patient groups and (3) to compare the psychophysical variables between the groups. This cross-sectional study included 71 subjects divided into three groups (ECH, CCH and controls). Pressure pain thresholds, temporal summation, conditioned pain modulation and other psychosocial variables were measured. The ANOVA showed differences for all physical outcome measures (p < 0.05). Bonferroni post hoc analyses showed differences when comparing the patient groups with the healthy subjects (p < 0.05), with large effect sizes (d > 0.8). No differences between the patient groups were found for almost all the variables (p > 0.05). Significant differences for all the variables were detected when comparing the symptomatic and non-symptomatic sides in both the ECH and CCH groups (p < 0.05). The ECH and CCH groups showed mechanical hyperalgesia, increased temporal summation and impaired inhibitory mechanisms compared to the controls. Side-to-side differences were also detected within the patient groups. Patients with CCH had poorer sleep quality and quality of life than the controls.

Opposing Effects on Descending Control of Nociception by µ and κ Opioid Receptors in the Anterior Cingulate Cortex

BACKGROUND

The efficiency of descending pain modulation, commonly assessed with the conditioned pain modulation procedure, is diminished in patients with chronic pain. The authors hypothesized that the efficiency of pain modulation is controlled by cortical opioid circuits.

METHODS

This study evaluated the effects of µ opioid receptor activation in the anterior cingulate cortex on descending control of nociception, a preclinical correlate of conditioned pain modulation, in male Sprague-Dawley rats with spinal nerve ligation-induced chronic pain or in sham-operated controls. Additionally, the study explored the consequences of respective activation or inhibition of κ opioid receptor in the anterior cingulate cortex of naive rats or animals with neuropathic pain. Descending control of nociception was measured as the hind paw withdrawal response to noxious pressure (test stimulus) in the absence or presence of capsaicin injection in the forepaw (conditioning stimulus).

RESULTS

Descending control of nociception was diminished in the ipsilateral, but not contralateral, hind paw of rats with spinal nerve ligation. Bilateral administration of morphine in the anterior cingulate cortex had no effect in shams but restored diminished descending control of nociception without altering hypersensitivity in rats with neuropathic pain. Bilateral anterior cingulate cortex microinjection of κ opioid receptor antagonists, including nor-binaltorphimine and navacaprant, also re-established descending control of nociception in rats with neuropathic pain without altering hypersensitivity and with no effect in shams. Conversely, bilateral injection of a κ opioid receptor agonist, U69,593, in the anterior cingulate cortex of naive rats inhibited descending control of nociception without altering withdrawal thresholds.

CONCLUSIONS

Anterior cingulate cortex κ opioid receptor activation therefore diminishes descending control of nociception both in naive animals and as an adaptive response to chronic pain, likely by enhancing net descending facilitation. Descending control of nociception can be restored by activation of μ opioid receptors in the anterior cingulate cortex, but also by κ opioid receptor antagonists, providing a nonaddictive alternative to opioid analgesics. Navacaprant is now in advanced clinical trials.

EDITOR’S PERSPECTIVE

The temporal effect of uncertain context on the perceptual processing of painful and non-painful stimulation

Uncertainty has been demonstrated to influence the perception of noxious stimuli, but little is known about the effects of prolonged uncertain contexts on the perception of painful and non-painful stimuli. To address this knowledge gap, the present study utilized a cue-based NPU-threat task, where uncertain and certain trials were separated into distinct blocks. The objective was to investigate the impact of uncertain contexts on the temporal dynamics of electroencephalogram (EEG) activity during the processing of painful and non-painful stimuli. The results revealed that the influence of uncertain contexts on neural responses extends beyond painful trials and is also evident in non-painful trials. In uncertain contexts, it has been observed that painful stimuli elicit larger P2 amplitudes and late beta band (13-30 Hz) event-related desynchronization (ERD) around 500-700 ms. However, in certain contexts, painful stimuli evoke stronger late gamma band (50-70 Hz) event-related synchronization (ERS) around 600-700 ms. For non-painful trials, in uncertain contexts, significantly higher amplitudes of the late positive potential (LPP) component and delta-theta band (2-7 Hz) ERS were observed compared to certain non-painful stimuli. These findings demonstrate that uncertain contexts exert a significant impact on the processing of both painful and non-painful stimuli, and this influence is mediated by distinct neural mechanisms.

Are clinical outcomes associated with baseline sensory profiles in people with musculoskeletal shoulder pain? Protocol for a prospective longitudinal observational study

BACKGROUND

Musculoskeletal shoulder pain is a common problem and its symptoms often become persistent. The experience of pain is multidimensional, and therefore, a range of patient characteristics may influence treatment response. An altered sensory processing has been associated with persistent musculoskeletal pain states and may contribute to outcomes in patients with musculoskeletal shoulder pain. The presence and potential impact of altered sensory processing in this patient cohort is not currently known. The aim of this prospective longitudinal cohort study is to investigate if baseline sensory characteristics are associated with clinical outcomes in patients presenting to a tertiary hospital with persistent musculoskeletal shoulder pain. If found, a relationship between sensory characteristics and outcome may lead to the creation of more effective treatment strategies and improvements in risk adjustment and prognosis.

METHODS

This is a single-centre prospective cohort study with 6-, 12- and 24-month follow-up. A total of 120 participants aged ≥18 years with persistent musculoskeletal shoulder pain (≥3 months) will be recruited from an Australian public tertiary hospital orthopaedic department. Baseline assessments, including quantitative sensory tests and a standardised physical examination, will be performed. In addition, information will be obtained from patient interviews, self-report questionnaires and medical records. Follow-up outcome measures will comprise information from the Shoulder Pain and Disability Index and a six-point Global Rating of Change scale.

ANALYSIS

Descriptive statistics will be used to report baseline characteristics and outcome measures over time. Change in outcome measures at the primary endpoint of six months from baseline will be calculated using paired t-tests. Associations between baseline characteristics and outcomes at a 6-month follow-up will be reported using multivariable linear and logistic regression models.

DISCUSSION

Understanding the relationship between sensory profile and the variable response to treatment in people with persistent musculoskeletal shoulder pain may enhance our understanding of the mechanisms contributing to the presentation. In addition, through better understanding of the contributing factors, the results of this study may contribute to the development of an individualised, patient-centred approach to treatment for people with this highly prevalent and debilitating condition.

Suffering from chronic tinnitus, chronic neck pain, or both: Does it impact the presence of signs and symptoms of central sensitization?

Chronic subjective tinnitus is a prevalent symptom, which has many similarities with chronic pain. Central sensitization is considered as a possible underlying mechanism of both symptoms. Central sensitization has already been investigated in chronic pain populations but not in patients with chronic subjective tinnitus. Therefore, the main objective of this cross-sectional study was to compare signs and symptoms, indicative for central sensitization, in tinnitus patients with and without chronic idiopathic neck pain, patients with chronic idiopathic neck pain only, and healthy controls. Also, differences in psychological and lifestyle factors, possibly influencing the association between central sensitization and tinnitus, were examined as well as correlations between signs and symptoms of central sensitization, and tinnitus, pain, psychological and lifestyle factors. Differences in signs and symptoms of central sensitization were examined using the self-report Central Sensitization Inventory and QST protocol (local and distant mechanical and heat hyperalgesia, conditioned pain modulation). Tinnitus, pain, psychological and lifestyle factors were evaluated using self-report questionnaires. Symptoms of central sensitization and local mechanical hyperalgesia were significantly more present in both tinnitus groups, compared to healthy controls, but were most extensive in the group with chronic tinnitus+chronic idiopathic neck pain. Distant mechanical hyperalgesia, indicative for central sensitization, was only observed in the group with both chronic tinnitus+chronic idiopathic neck pain. This group also displayed a significantly higher psychological burden and poorer sleep than patients with chronic tinnitus only and healthy controls. Signs and symptoms of central sensitization were also shown to be associated with tinnitus impact, pain-related disability, psychological burden and sleep disturbances. This study shows preliminary evidence for the presence of central sensitization in patients with chronic tinnitus+chronic idiopathic neck pain. This could be explained by the higher perceived tinnitus impact, psychological burden and sleep problems in this group. Trial registration: This study is registered as NCT05186259 (www.clinicaltrials.gov).

Relationship between Motor Corticospinal System, Endogenous Pain Modulation Mechanisms and Clinical Symptoms in Patients with Knee Osteoarthritis: New Perspectives on an Old Disease

Knee osteoarthritis (OA) is a painful condition characterized by joint and bone changes. A growing number of studies suggest that these changes only partially explain the pain experienced by individuals with OA. The purpose of the current study was to evaluate if corticospinal and bulbospinal projection measurements were interrelated in patients with knee OA, and to explore the relationship between these neurophysiological measures and temporal summation (excitatory mechanisms of pain) on one hand, and clinical symptoms on the other. Twenty-eight (28) patients with knee OA were recruited. Corticospinal projections were measured using transcranial magnetic stimulation, while bulbospinal projections were evaluated with a conditioned pain modulation (CPM) protocol using a counter-irritation paradigm. Validated questionnaires were used to document clinical and psychological manifestations. All participants suffered from moderate to severe pain. There was a positive association between corticospinal excitability and the effectiveness of the CPM (rs = 0.67, p = 0.01, n = 13). There was also a positive relationship between pain intensity and corticospinal excitability (rs = 0.45, p = 0.03, n = 23), and between pain intensity and temporal summation (rs = 0.58, p = 0.01, n = 18). The results of this study highlight some of the central nervous system changes that could be involved in knee OA and underline the importance of interindividual variability to better understand and explain the semiology and pathophysiology of knee OA.

Central sensitization in adolescents with hypermobility spectrum disorder or hypermobile Ehlers-Danlos syndrome-a feasibility study

BACKGROUND

Pain is a major symptom in adolescents with hypermobility spectrum disorder or hypermobile Ehlers-Danlos syndrome. Although the underlying mechanism causing generalized pain in children with hypermobility spectrum disorder or hypermobile Ehlers-Danlos syndrome is unclear, central sensitization has been suggested as a possible explanation. The aim of this study was to explore the feasibility of a study protocol for a future case-control study, investigating features of central sensitization in adolescents with hypermobility spectrum disorder or hypermobile Ehlers-Danlos syndrome.

METHODS

Central sensitization features were measured in ten patients and nine healthy controls aged 13-17 years via experimental pain measurement quantifying primary and secondary hyperalgesia, endogenous pain modulation, and exercise-induced hyperalgesia. Descriptive statistics were used. Frequency, median, and range values were calculated.

RESULTS

Eleven out of 57 patients chose to participate. No control could be recruited through public schools. Therefore, a convenience sampling strategy was used for the recruitment of the control group. The process of assessing primary and secondary hyperalgesia, endogenous pain modulation, and exercise-induced hyperalgesia was well tolerated by all participants (patients and controls). When assessing endogenous pain modulation via conditioned pain modulation, two participants in the patient group and three in the control group did not achieve a pain experience ≥ 3 on the numerical rating scale when immersing their hands in cold water.

CONCLUSION

This study investigated the feasibility, safety, and toleration of experimental pain measurements in adolescents with hypermobility spectrum disorder or hypermobile Ehlers-Danlos syndrome. Although the test protocol proved to be sufficiently feasible for use with the participant group, it will need to be adapted in the main study in order to obtain more reliable data. Recruitment, especially of participants for the control group, can be a major obstacle for future studies and requires careful planning.

TRIAL REGISTRATION

Researchweb.org, 270,501. Registered on 9 May 2019.

Efficiency of pain inhibition and facilitation of fibromyalgia patients is not different from healthy controls: Relevance of sensitivity-adjusted test stimuli

Background

Pain is a dynamic phenomenon dependent on the balance of endogenous excitatory and inhibitory systems, which can be characterized by quantitative sensory testing. Many previous studies of pain modulatory capacity of patients with fibromyalgia syndrome (FM) have reported decreased pain inhibition or increased pain facilitation. This is the first study to assess pain modulation, including conditioned pain modulation (CPM) and temporal pain summation, in the same healthy control (HC) and FM participants.

Methods

Only sensitivity-adjusted stimuli were utilized for testing of conditioned pain modulation (CPM) and temporal pain summation in 23 FM patients and 28 HC. All subjects received sensitivity-adjusted ramp-hold (sRH) during testing of pain facilitation (temporal summation) and pain inhibition (CPM). CPM efficacy was evaluated with test stimuli applied either concurrently or after application of the conditioning stimulus. Finally, the effects of CPM on pressure pain thresholds were tested.

Results

FM subjects required significantly less intense test and conditioning stimuli than HC participants to achieve standardized pain ratings of 50 ± 10 numerical rating scale (NRS) (p = 0.03). Using such stimuli, FM subjects' temporal pain summation and CPM efficacy was not significantly different from HC (all p > 0.05), suggesting similar pain facilitation and inhibition. Furthermore, the CPM efficacy of FM and HC participants was similar regardless of whether the test stimuli were applied during or after the conditioning stimulus (p > 0.05).

Conclusion

Similar to previous studies, FM participants demonstrated hyperalgesia to heat, cold, and mechanical stimuli. However, using only sensitivity-adjusted stimuli during CPM and temporal summation testing, FM patients demonstrated similarly effective pain inhibition and facilitation than HC, suggesting that their pain modulation is not abnormal.

Mechanisms of chronic pain in inflammatory rheumatism: the role of descending modulation

ABSTRACT

Persistent pain despite satisfactory disease treatment is frequent in rheumatoid arthritis (RA) and spondyloarthritis (Spa) and may result from specific changes in central pain processing. We assessed these mechanisms further by systematically comparing thermal pain thresholds and conditioned pain modulation (CPM) between patients with active RA or Spa and healthy controls. We included 50 patients with RA and 50 patients with Spa and 100 age-matched and sex-matched controls. Heat and cold pain thresholds (HPT-CPT) were measured on the dominant forearm, and CPM was assessed by applying conditioning stimuli (immersion in a cold-water bath) to one foot and the nondominant hand in 2 successive randomized sequences. Descending pain modulation was assessed as the difference in HPTs (in °C) before and after conditioning. Larger HPT differences (ie, a larger CPM effect) reflected more efficient descending inhibition. Potential associations between changes in CPM and clinical data, including disease activity, pain intensity, and psychological and functional variables, were systematically assessed. Heat pain threshold and cold pain threshold were similar in patients and controls. The mean CPM effect was significantly weaker in patients than that in controls for conditioning applied to either the foot (0.25°C ±2.57 vs 2.79°C ±2.31; P < 0.001) or the nondominant hand (0.57°C ±2.74 vs 2.68°C ±2.12; P < 0.001). The smaller CPM effect in patients was correlated with average pain intensity, but not with disease activity or other clinical characteristics, suggesting a significant pathophysiological role for changes in endogenous pain modulation in the mechanisms of chronic pain associated with inflammatory rheumatism.

Are central sensitization symptoms and psychosocial alterations interfering in the association between painful TMD, migraine, and headache attributed to TMD?

OBJECTIVE

To determine if somatosensory function and symptoms related to central sensitization (CS) differed in individuals with painful temporomandibular disorders (TMD) according to the presence of migraine (MIG) or MIG + headache attributed to TMD (HAT).

MATERIALS AND METHODS

This study evaluated 92 adults (20-65 years), presenting painful TMD. Standard diagnostic criteria were applied to classification of painful TMD, MIG, and HAT. CS was assessed through the central sensitization inventory (CSI), wind-up ratio (WUR), pressure pain thresholds (PPT), and the conditioned pain modulation test (CPM). Psychosocial factors were evaluated by validated instruments.

RESULTS

There was a significant difference regarding gender, with more women in the group TMD + MIG + HAT (p = 0.028). TMD + MIG and TMD + MIG + HAT had significantly lower PPTs than the TMD group. No group differences were found for the WUR, CPM, or CSI. TMD + MIG + HAT had higher chronic pain intensity (p = 0.001), disability points (p = 0.045), graded chronic pain scale (p = 0.007), and higher somatization (NSPS) scores (p = 0.012), compared to the other groups.

CONCLUSION

Mechanical hyperalgesia was more pronounced in the group with the highest pain and somatization scores, while CPM and WUR did not differ between groups. Altered somatosensory function and CS may partially underlie the pathophysiology of overlapping TMD pain conditions, pointing towards additive effects of comorbid head pains.

CLINICAL RELEVANCE

Our results demonstrate the importance of considering the association of primary and secondary headaches during TMD assessment and its implications for maintaining the signs and symptoms of CS. This can influence the conduct of treatment, which must be multidisciplinary, and must include management of mechanisms related to CS.

Inter-subject variability of pleasant pain relief using a data-driven approach in healthy volunteers

Background

The offset of a painful and unpleasant sensation can elicit pleasure. This phenomenon, namely pleasant pain relief (PPR), is attracting growing interest in research. While the cold pressor test (CPT) has been frequently used to study the inhibition of pain by the administration of another painful stimulation (inhibitory conditioned pain modulation; ICPM), a preliminary study from our research team has shown that CPT can also elicit a robust and long-lasting PPR. However, its effects on pain relief and inhibition vary greatly between subjects. Although substantial research has been carried out on inter-individual variability in the case of ICPM, the same cannot be said of PPR. Therefore, the current study sought to identify clusters of healthy volunteers with similar dynamic pain responses during the CPT, using a data-driven approach, and to investigate the inter-subject variability for PPR and ICPM.

Methods

One hundred and twenty-two healthy volunteers were recruited. A sequential ICPM paradigm was carried out with CPT (water at 10°C) and a Peltier Thermode to evaluate pain intensity and unpleasantness. Moreover, PPR was measured for four minutes at CPT offset. Statistical analyses were performed using group-based trajectory modelling.

Results

Four trajectories (groups) were identified for CPT pain intensity and unpleasantness ratings with varying levels of tonic pain and pain sensitization (e.g., temporal summation). PPR scores were correlated with both pain ratings trajectories (p &lt; 0.001). On the other hand, no differences were found between groups regarding ICPM efficacy (percentage pain inhibition).

Discussion

This study has provided a first step into the investigation of PPR and ICPM interindividual variability. Using a data-driven approach, it was shown that PPR at CPT offset differs between clusters of participants identified based on dynamic pain intensity and unpleasantness responses from CPT. Thus, it was brought to light that both the levels of tonic pain and pain sensitization underlie individual differences in PPR. The lack of correlation between CPT pain trajectories and ICPM efficacy may be explained by the hypotheses that eliciting ICPM requires only a certain threshold of stimulation which doesn’t need to be noxious. In the future, studies on the inter-subject variability of PPR in large samples of chronic pain patients are warranted.

Anti- and Pro-Nociceptive mechanisms in neuropathic pain after human spinal cord injury

BACKGROUND

Deficient endogenous pain modulation and increased nociceptive excitability are key features of central sensitization and can be assessed in humans by conditioned pain modulation (CPM, anti-nociceptive) and temporal summation of pain (TSP, pro-nociceptive), respectively. This study aimed to investigate these measures as proxies for central sensitization in subjects with chronic neuropathic pain (NP) after spinal cord injury (SCI).

METHODS

In paraplegic subjects with NP (SCI-NP; n = 17) and healthy controls (HC; n = 17), parallel and sequential sham-controlled CPM paradigms were performed using pressure pain threshold at the hand, that is, above lesion level, as test stimulus. The conditioning stimulus was a noxious cold (verum) or lukewarm water bath (sham) applied contralaterally. Regarding pro-nociceptive mechanisms, a TSP protocol with individually-adjusted pressure pain stimuli at the thenar eminence was used. CPM and TSP magnitudes were related to intensity and spatial extent of spontaneous NP.

RESULTS

Neither the parallel nor sequential sham-controlled CPM paradigm showed any significant inhibition of above-level pressure pain thresholds for SCI-NP or HC. Accordingly, no group difference in CPM capacity was found, however, subjects with more intense spontaneous NP showed lower inhibitory CPM capacity. TSP was observed for both groups but was not enhanced in SCI-NP.

CONCLUSIONS

Our results do not support altered above-level anti- or pro-nociceptive mechanisms in SCI-NP compared with HC; however, they also highlight the relevance of spontaneous NP intensity with regards to the capacity of endogenous pain modulation in SCI subjects.

SIGNIFICANCE

Central sensitization encompasses deficient endogenous pain modulation and increased nociceptive excitability. These two mechanisms can be assessed in humans by conditioned pain modulation and temporal summation of pain, respectively. Our data demonstrates a lack of descending pain inhibition only in subjects with severe neuropathic pain which may hint towards central sensitization at spinal and/or supra-spinal levels. Disentangling the mechanisms of endogenous pain modulation and neuronal hyperexcitability might improve mechanism-based treatment of neuropathic pain in subjects with spinal cord injury.

Sex-Related Effects of Adrenergic Drugs on Conditioned Pain Modulation: A Randomized Controlled Cross-Over Double-Blind Trial

Objective

Endogenous pain inhibition can be investigated using conditioned pain modulation (CPM). CPM efficacy has been reported to be influenced by various factors, such as gender and cardiovascular (autonomic) activity. The aim of this study is to describe the effect of pharmacological manipulations of autonomic activity on CPM efficacy.

Methods

Thirty healthy participants were enrolled to assess CPM efficacy in 4 experimental sessions. The first session consisted of the determination of baseline CPM effectiveness. The three following sessions were performed in a randomized order and consisted of the injection of (1) esmolol, (2) ephedrine, or (3) placebo, before the conditioning stimulus. Pain intensity induced by using a contact heat stimulation thermode was compared before and after a cold-pressure conditioning stimulus to evaluate CPM effectiveness.

Results

Our results show that inhibiting sympathetic nervous activity with esmolol did not have a significant effect on CPM. Conversely, enhancing sympathetic nervous activity with ephedrine increased CPM effectiveness in healthy women but decreased it in men.

Conclusions

Increasing sympathetic activity with adrenergic agonists, such as ephedrine, could improve CPM effectiveness in women. It will be interesting to verify if the same results are present in patients suffering from chronic pain and if adrenergic agonists could have better therapeutic effects in women showing reduced CPM effectiveness.

A wearable soft robot that can alleviate the pain and fear of the wearer

Social soft robotics may provide a new solution for alleviating human pain and fear. Here, we introduce a hand-held soft robot that can be clenched by the wearer. The robot comprises small airbags that can be inflated to provide the wearer with a feeling of being clenched. We then conducted an in-depth study of 66 adults who participated in a pain research protocol using thermal stimulation to investigate the effect of wearing the robot on pain perception and fear of injections. Pain assessment scale scores for perceived pain decreased significantly [Formula: see text] when participants wore the robot compared with the baseline condition in which the robot was not worn. In addition, the saliva test results showed a downward trend in oxytocin level when the robot provided the wearer with haptic feedback via the inflation of the internal airbags in response to the wearer's clench. Furthermore, the negative psychological state of participants, as measured using the positive and negative affect scale, improved significantly when wearing the robot. We also revealed that the salivary cortisol level, an indicator of stress, decreased significantly across all participants at the end of the experiment. In addition, participants' fear of injections was significantly improved after participation in the experiment. These results suggest that the wearable soft robot may alleviate the human perception of pain and fear in during medical treatments, such as vaccinations.

Body Composition does not influence Conditioned Pain Modulation and Exercise Induced Hyperalgesia in Healthy Males and Females

BACKGROUND

Obese individuals report a higher susceptibility to chronic pain. Females are more likely to have chronic pain and excess adipose tissue. Chronic pain is associated with dysfunctional pain modulatory mechanisms. Body composition differences may be associated with pain modulation differences in males and females. The purpose of this study was to investigate body composition (lean vs fat mass) differences and pain modulatory functioning in healthy males and females.

METHODS

Pressure pain thresholds (PPT) of 96 participants (47M; 49F) were assessed in both arms and legs before and after a double footed ice bath (2°C) for 1min and an isometric knee extension, time to failure task. The difference between post and pre measures was defined conditioned pain modulatory (CPM) response (ice bath) and exercise induced hypoalgesia (EIH) response. Whole body and site-specific fat and lean tissue were assessed via DXA scan.

RESULTS

Sex differences were found in whole body lean mass (61.5±6.7kg vs 41.2±5.4kg; P<0.001) but not fat mass amount (17.2±10.5kg vs 21.0±9.7kg; P=0.068). No effect of sex was found between limb CPM (P=0.237) and limb EIH (P=0.512). When controlling for lean mass there was no significant effect of sex on CPM (P=0.732) or EIH (P=0.474) response. Similar findings were found for fat mass.

CONCLUSION

The lack of difference suggests that males and females have similar modulatory functioning. It appears that in healthy adults free from chronic pain, neither fat mass nor lean mass has an influence on endogenous pain modulatory function.

Conditioned Pain Modulation in Children: The Effects of Painful and Nonpainful Conditioning Stimuli

Conditioned pain modulation (CPM), a psychophysical measure in which 1 pain stimulus (conditioning stimulus) is used to inhibit another pain stimulus (test stimulus), is an important indicator of endogenous pain inhibition in adults, but is understudied in children. Preliminary evidence suggests that CPM effects are present in healthy children and are more robust in adolescents. However, developmental differences in younger children are not well documented and few studies control for potential distraction effects of the conditioning stimulus (CS). Participants were 54 healthy children aged 6 to 12 years. After a baseline pressure pain threshold (PPT) test, participants underwent 2 conditioning trials in which PPT was assessed while they placed their left hand in a water bath maintained at either 12 °C (painful CS) or 22 °C (nonpainful sham CS) in counterbalanced order. Results revealed a significant CPM effect. PPT values were significantly higher relative to baseline during the painful CS trial; PPT during the nonpainful CS trial did not differ from baseline. There were no significant age differences in magnitude of CPM effect. The results indicate that children as young as 6 years of age demonstrate CPM, suggesting that descending inhibitory pathways may be better developed in young children than previously thought. PERSPECTIVE: This study was successful in producing inhibitory CPM effects in physically healthy children while controlling for sensory distraction. The findings provide strong evidence that the obtained CPM responses cannot be attributed to sensory distraction or other nonspecific effects. Future studies could utilize CPM paradigms to study various aspects of pediatric endogenous pain inhibition, in order to better predict pain responses and improve interventions.

Symptomatic but not Asymptomatic COVID-19 Impairs Conditioned Pain Modulation in Young Adults

Pain is a common symptom reported in COVID-19 patients. Impaired endogenous pain-modulatory mechanisms such as conditioned pain modulation (CPM), and exercise-induced hypoalgesia (EIH) have been found in chronic pain conditions but is often overlooked in acute conditions that evoke painful symptoms, such as COVID-19. The purpose was to compare pressure-pain sensitivity, CPM, and EIH function among individuals who previously had COVID-19, both symptomatically and asymptomatically, and a healthy control group. Pressure pain thresholds of 59 participants were assessed in the forearm and leg using a pressure algometer before and after 1) submersion of their dominant foot in cold water (2°C) for 1min; and 2) isometric knee extension performed to task-failure at 25% of their maximal contraction. The CPM response was attenuated in individuals who were infected with symptomatic COVID-19 (N = 26) compared to asymptomatic COVID-19 (N = 13) in arm (-1.0% ± 20.3 vs 33.3% ± 26.2; P < .001) and leg (12.8% ± 22.0 vs 33.8% ± 28.2; P = .014) and compared to controls (N = 20) in arm only (-1.0% ± 26.2 vs 23.4% ± 28.2; P = .004). The EIH response was not different between groups. CPM was impaired in individuals who had symptomatic COVID-19, which may have long-term implications on pain modulation.

Perspective

This study reveals that CPM was impaired in individuals who had symptomatic COVID-19 during the first wave of COVID-19, pre vaccine. These findings present a preliminary motive to study the long-term implications of COVID-19 and its effects on pain modulation.

Conditioned pain modulation is associated with heightened connectivity between the periaqueductal grey and cortical regions

Introduction

Conditioned pain modulation (CPM) is a psychophysical assessment used to estimate the efficiency of an individual's endogenous modulatory mechanisms. Conditioned pain modulation has been used as a predictive assessment for the development of chronic pain and responses to pain interventions. Although much is known about the spinal cord mechanisms associated with descending pain modulation, less is known about the contribution of supraspinal and especially cortical regions.

Objectives

We aimed to explore how whole-brain connectivity of a core modulatory region, the periaqueductal grey (PAG), is associated with conditioned pain modulation, and endogenous pain modulation more broadly.

Methods

We measured CPM and resting-state connectivity of 35 healthy volunteers, absent of chronic pain diagnoses. As a region of interest, we targeted the PAG, which is directly involved in endogenous modulation of input to the spinal cord and is a key node within the descending pain modulation network.

Results

We found that CPM was associated with heightened connectivity between the PAG and key regions associated with pain processing and inhibition, such as the primary and secondary somatosensory cortices, as well as the motor, premotor, and dorsolateral prefrontal cortices. These findings are consistent with connectivity findings in other resting-state and event-related fMRI studies.

Conclusion

These findings indicate that individuals who are efficient modulators have greater functional connectivity between the PAG and regions involved in processing pain. The heightened connectivity of these regions may contribute to the beneficial outcomes in clinical pain management, as quantified by CPM. These results may function as brain-based biomarkers for vulnerability or resilience to pain.

Sleep, Positive Affect, and Circulating Interleukin-6 in Women With Temporomandibular Joint Disorder

OBJECTIVE

Systemic inflammation is commonly observed in idiopathic chronic pain conditions, including temporomandibular joint disorder (TMD). Trait positive affect (PA) is associated with lower inflammation in healthy controls, but those effects may be threatened by poor sleep. The associations between PA with proinflammatory cytokine activity and potential moderation by sleep in chronic pain are not known. We thus investigated the association between PA and circulating interleukin-6 (IL-6) and moderation of that association by sleep in a sample of women with TMD and sleep difficulties.

METHODS

Participants (n = 110) completed the insomnia severity index and provided blood samples at five intervals throughout an evoked pain testing session. They then completed a 14-day diary assessing sleep and affect, along with wrist actigraphy.

RESULTS

There was not a significant main effect of PA on resting or pain-evoked IL-6 (b = 0.04, p = .33). Diary total sleep time (b = -0.002, p = .008), sleep efficiency (b = -0.01, p = .005), sleep onset latency (b = 0.006, p = .010), and wake after sleep onset (b = 0.003, p = .033) interacted with PA to predict IL-6, such that PA inversely predicted IL-6 at higher levels of total sleep time and sleep efficiency and at lower levels of sleep onset latency and wake after sleep onset. Surprisingly, when sleep was poor, PA predicted greater IL-6.

CONCLUSIONS

The potential salutary effects of PA on resting IL-6 erode when sleep is poor, underscoring the importance of considering sleep in conceptual and intervention models of TMD.

Absolute and relative reliability of a comprehensive quantitative sensory testing protocol in women treated for breast cancer

OBJECTIVE

Quantitative sensory testing (QST) are non-invasive psychophysical assessment techniques to evaluate functioning of the somatosensory nervous system. Despite the importance of reliability for correct use of QST results in research and clinical practice, the relative and absolute intra-and inter-rater reliability of a comprehensive QST protocol to evaluate the functioning of both peripheral and central somatosensory nervous system in a breast cancer population, has not yet been investigated.

SETTING

University Hospitals, Leuven, Belgium.

SUBJECTS

Thirty women at least six months after unilateral breast cancer surgery.

METHODS

The protocol included nine static and dynamic QST methods (mechanical detection-pain thresholds, pressure pain thresholds, thermal detection-pain thresholds for heat and cold, temporal summation and conditioned pain modulation (CPM)) performed in the surgical area and more distant regions. Absolute and relative intra (60-minutes interval) and inter-rater (one-week interval) reliability was evaluated using intraclass correlation coefficients, standard error of measurement and Bland-Altman plots.

RESULTS

A moderate to excellent relative intra- and inter-rater reliability was found for the evaluation of mechanical thresholds, pressure pain thresholds and temporal summation. Reliability of the CPM paradigm was considered weak. Systematic bias between raters was noticed for detection of mechanical and cold stimuli at the non-affected trunk and CPM.

CONCLUSIONS

Except for the evaluation of CPM, the QST protocol was found suitable for identifying differences between subjects (relative reliability) and individual follow-up after breast cancer surgery (limited systematic bias) during a one-week timeframe. Additional research is required to determine measurement properties that influence CPM test stability in order to establish a more reliable CPM test paradigm.

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

Background

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

Methods

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

Results

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

Conclusions

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

Descending pain modulatory efficiency in healthy subjects is related to structure and resting connectivity of brain regions

The descending pain modulatory system in humans is commonly investigated using conditioned pain modulation (CPM). Whilst variability in CPM efficiency, i.e., inhibition and facilitation, is normal in healthy subjects, exploring the inter-relationship between brain structure, resting-state functional connectivity (rsFC) and CPM readouts will provide greater insight into the underlying CPM efficiency seen in healthy individuals. Thus, this study combined CPM testing, voxel-based morphometry (VBM) and rsFC to identify the neural correlates of CPM in a cohort of healthy subjects (n =40), displaying pain inhibition (n = 29), facilitation (n = 10) and no CPM effect (n = 1). Clusters identified in the VBM analysis were implemented in the rsFC analysis alongside key constituents of the endogenous pain modulatory system. Greater pain inhibition was related to higher volume of left frontal cortices and stronger rsFC between the motor cortex and periaqueductal grey. Conversely, weaker pain inhibition was related to higher volume of the right frontal cortex - coupled with stronger rsFC to the primary somatosensory cortex, and rsFC between the amygdala and posterior insula. Overall, healthy subjects showed higher volume and stronger rsFC of brain regions involved with descending modulation, while the lateral and medial pain systems were related to greater pain inhibition and facilitation during CPM, respectively. These findings reveal structural alignments and functional interactions between supraspinal areas involved in CPM efficiency. Ultimately understanding these underlying variations and how they may become affected in chronic pain conditions, will advance a more targeted subgrouping in pain patients for future cross-sectional studies investigating endogenous pain modulation.

Can Central Sensitization after injury persist as an autonomous pain generator? - A comprehensive search for evidence

OBJECTIVE

To conduct a comprehensive search for evidence whether Central Sensitization following an injury can act as a persistent autonomous pain generator after the inducing injury has healed.

METHODS

We searched Medline on PubMed and the Cochrane Library, screening 3,572 abstracts, from which 937 full text articles were obtained, with 186 of these discarded as irrelevant to the question being posed. The remaining 751 articles were studied for evidence.

RESULTS

Fourteen publications were judged to provide weak evidence for the hypothesis of central sensitization as a persisting autonomous pain generator, but none addressed the question directly. No strong evidence for the affirmative answer was found.Sixty-two publications were judged to provide weak evidence for a negative answer, and nine judged to provide strong evidence.Unexpectedly, serious weaknesses were discovered in the literature underpinning the validity of the clinical diagnosis of Central Sensitization in man:(i)  Inappropriate extrapolation, in many publications, of laboratory animal data to humans.(ii) Failure to demonstrate the absence of peripheral pain generators which might be perpetuating Central Sensitization.(iii) Many factors now shown to confound what is being measured by quantitative sensory testing, conditioned pain modulation, and Central Sensitization Inventory.

CONCLUSIONS

We found no evidence proving that central sensitization can persist as an autonomous pain generator after the initiating injury has healed.Our review has also shown that the evidential basis for the diagnosis of CS in individual patients is seriously in question.

The association between pain-induced autonomic reactivity and descending pain control is mediated by the periaqueductal grey

There is a strict interaction between the autonomic nervous system (ANS) and pain, which might involve descending pain modulatory mechanisms. The periaqueductal grey (PAG) is involved both in descending pain modulation and ANS, but its role in mediating this relationship has not yet been explored. Here, we sought to determine brain regions mediating ANS and descending pain control associations. Thirty participants underwent conditioned pain modulation (CPM) assessments, in which they rated painful pressure stimuli applied to their thumbnail, either alone or with a painful cold contralateral stimulation. Differences in pain ratings between 'pressure-only' and 'pressure + cold' stimuli provided a measure of descending pain control. In 18 of the 30 participants, structural scans and two functional MRI assessments, one pain-free and one during cold-pain were acquired. Heart rate variability (HRV) was simultaneously recorded. Normalised low-frequency HRV (LF-HRVnu) and the CPM score were negatively correlated; individuals with higher LF-HRVnu during pain reported reductions in pain during CPM. PAG-ventro-medial prefrontal cortex (vmPFC) and PAG-rostral ventromedial medulla (RVM) functional connectivity correlated negatively with the CPM. Importantly, PAG-vmPFC functional connectivity mediated the strength of the LF-HRVnu-CPM association. CPM response magnitude was also negatively correlated with vmPFC GM volume. Our multi-modal approach, using behavioural, physiological and MRI measures, provides important new evidence of interactions between ANS and descending pain mechanisms. ANS dysregulation and dysfunctional descending pain modulation are characteristics of chronic pain. We suggest that further investigation of body-brain interactions in chronic pain patients may catalyse the development of new treatments. KEY POINTS: Heart rate variability (HRV) is associated with descending pain modulation as measured by the conditioned pain modulation protocol (CPM). There is an association between CPM scores and the functional connectivity between the periaqueductal grey (PAG) and ventro-medial prefrontal cortex (vmPFC). CPM scores are also associated with vmPFC grey matter volume. The strength of functional connectivity between the PAG and vmPFC mediates the association between HRV and CPM. Our data provide new evidence of interactions between the autonomic nervous system and descending pain mechanisms.

Inter-Individual Differences Explain More Variance in Conditioned Pain Modulation Than Age, Sex and Conditioning Stimulus Intensity Combined

Conditioned pain modulation (CPM) describes the reduction in pain evoked by a test stimulus (TS) when presented together with a heterotopic painful conditioning stimulus (CS). CPM has been proposed to reflect inter-individual differences in endogenous pain modulation, which may predict susceptibility for acute and chronic pain. Here, we aimed to estimate the relative variance in CPM explained by inter-individual differences compared to age, sex, and CS physical and pain intensity. We constructed linear and mixed effect models on pooled data from 171 participants of several studies, of which 97 had repeated measures. Cross-sectional analyses showed no significant effect of age, sex or CS intensity. Repeated measures analyses revealed a significant effect of CS physical intensity (p = 0.002) but not CS pain intensity (p = 0.159). Variance decomposition showed that inter-individual differences accounted for 24% to 34% of the variance in CPM while age, sex, and CS intensity together explained <3% to 12%. In conclusion, the variance in CPM explained by inter-individual differences largely exceeds that of commonly considered factors such as age, sex and CS intensity. This may explain why predictive capability of these factors has had conflicting results and suggests that future models investigating them should account for inter-individual differences.

Investigation of Correlations Between Pain Modulation Paradigms

OBJECTIVE

Endogenous pain modulation can be quantified through the use of various paradigms. Commonly used paradigms include conditioned pain modulation (CPM), offset analgesia (OA), spatial summation of pain (SSP), and temporal summation of pain (TSP), which reflect spatial and temporal aspects of pro- and antinociceptive processing. Although these paradigms are regularly used and are of high clinical relevance, the underlying physiological mechanisms are not fully understood.

DESIGN

The aim of this study is therefore to assess the association between these paradigms by using comparable protocols and methodological approaches.

SETTING

University campus.

SUBJECTS

Healthy and pain-free volunteers (n = 48) underwent psychophysical assessment of CPM, OA, SSP, and TSP (random order) at the same body area (volar nondominant forearm) with individualized noxious stimuli.

METHODS

CPM included heat stimuli before, during, and after a noxious cold-water bath, whereas for OA, three heat stimuli were applied: baseline trial, offset trial, and constant trial. For the SSP paradigm, two differently sized heat stimulation areas were evaluated, whereas for TSP, the first and last stimulus of 10 consecutive short heat stimuli were assessed. A computerized visual analog scale was used to continuously evaluate pain intensity. The magnitudes of all associations between all paradigm pairs were analyzed with Spearman's correlation, and individual influencing factors were assessed with a multivariate linear regression model.

RESULTS

Weak to moderate correlations among all four paradigms were found (P > 0.05), and no distinct influencing factors were identified.

CONCLUSIONS

A limited association between pain modulation paradigms suggests that CPM, OA, SSP, and TSP assess distinct aspects of endogenous analgesia with different underlying physiological mechanisms.

The Effects of Gender, Functional Condition, and ADL on Pressure Pain Threshold in Stroke Patients

Background

Somatosensory impairments and pain are common symptoms following stroke. However, the condition of perception and pain threshold for pressure stimuli and the factors that can influence this in individuals with stroke are still unclear. This study aimed to investigate the gender differences in pressure pain threshold (PPT) and positive somatosensory signs for pressure stimuli, and explore the effects of joint pain, motor function, and activities of daily living (ADL) on pain threshold in post-stroke patients.

Design

A cross-sectional study.

Methods

A total of 60 participants with stroke were recruited, and their pain condition, motor functions, and ADL were evaluated by the Fugl-Meyer assessment of joint pain scale, motor function scale, and Barthel index, respectively. PPTs in eight tested points at the affected and unaffected sides were assessed.

Results

Significant differences in PPTs were found between male and female patients in all measured muscles (p < 0.05). Positive somatosensory signs for pressure stimuli, including hypoalgesia and hyperalgesia, were frequently found at the affected side, particularly in the extremity muscles, but such signs were not significantly influenced by gender (p > 0.05). More equal PPTs between both sides and relatively lower PPTs at the affected side in the trunk and medial gastrocnemius muscles (p < 0.05) were observed in patients with less pain, better motor functions, and ADL.

Conclusion

Gender differences widely exist in post-stroke survivors either at the affected or unaffected side, which are multifactorial. Sensory loss and central and/or peripheral sensitization, such as hypoalgesia and hyperalgesia for pressure stimuli, caused by a brain lesion are common signs in male and female stroke patients. Moreover, patients who are in a better condition show a more symmetrical pain sensitivity between both sides in the trunk and in female lower extremities, indicating the bidirectional improvement of somatosensory abnormalities caused by a possible neural plasticity.

Test-retest reliability of pressure pain threshold and heat pain threshold as test stimuli for evaluation of conditioned pain modulation

OBJECTIVES

There is growing interest in conditioned pain modulation (CPM) protocols for evaluating the function of the descending pain-modulatory system in clinical settings. The aim of this study was to evaluate test-retest reliability of a CPM protocol with contact heat as a conditioning stimulus (CS) and two test stimuli (pressure pain threshold (PPT) and heat pain threshold (HPT)), all applied within one session. In addition, gender and age differences were evaluated.

METHODS

The study included 87 healthy subjects (average age 37.95 ± 12.02 years, 57.5% females). The magnitude of the results of the CPM test was calculated as the difference between subject-reported pain thresholds before and after application of the CS. To assess the reliability of the protocol, a subgroup of 66 subjects underwent re-test after 14±1 days. In order to investigate the influence of age on the CPM magnitude, subjects were divided into subgroups (20-29, 30-39, 40-49, 50-59 years).

RESULTS

The pain thresholds for both test stimuli (TS) were significantly higher following the application of the CS (p < 0.001). Values of the interclass correlation coefficient (ICC) for the CPM with PPT as the TS indicated good reliability (ICC=0.631, 95% CI:0.365-0.782), while those for the HPT indicated poor reliability (ICC=0.328, 95% CI:-0.067-0.582). Significant differences in CPM between genders were found for both TS. Significant CPM differences were not detected among the four age subgroups for either TS.

CONCLUSIONS

CPM effects can be successfully induced with both PPT and HPT as a TS, but PPT showed significantly higher reliability.

Brain-Derived Neurotrophic Factor Modulates the Effect of Sex on the Descending Pain Modulatory System in Healthy Volunteers

OBJECTIVES

We investigated sex differences and the influence of brain-derived neurotrophic factor (BDNF) in the descending pain modulatory system (DPMS), as measured by change on the numerical pain scale (NPS; 0-10) during conditioned pain modulation (CPM task; primary outcome) and by function of the corticospinal motor pathway and heat pain thresholds (HPTs; secondary outcomes).

METHODS

This cross-sectional study included healthy volunteers ranging in age from 18 to 45 years (32 male and 24 female). Assessment included serum BDNF, HPT, change on the NPS (0-10) during the CPM task, and motor-evoked potential (MEP) using transcranial magnetic stimulation (TMS).

RESULTS

The MEP (Mv) amplitude was larger in male participants compared with female participants (mean [SE] = 1.55 [0.34] vs mean [SE] = 1.27 [0.27], respectively, P = 0.001). The mean NPS (0-10) during CPM task changed more substantially for female compared with male participants (mean [SE] = -3.25 [2.01] vs mean [SE] = -2.29 [1.34], respectively, P = 0.040). In addition, a higher serum BDNF (adjusted index for age) was associated with a larger decrease of the NPS during CPM task (P = 0.003), although further regression analyses by sex showed that this was only significant for females (P = 0.010).

CONCLUSIONS

Significant sex differences were identified in DPMS function and corticospinal motor pathway integrity. Nevertheless, BDNF was associated with the function of the DPMS in female but not male participants, indicating that sex and neuroplasticity state are crucial factors for pain perception in healthy subjects.

Phenotyping peripheral neuropathic pain in male and female adolescents: pain descriptors, somatosensory profiles, conditioned pain modulation, and child-parent reported disability

ABSTRACT

Neuropathic pain (NeuP) can be difficult to diagnose and manage in children. Data regarding prevalence and sex-dependent differences are limited, and more detailed phenotyping is needed. This observational cohort study recruited adolescents (10-17 years) with NeuP or complex regional pain syndrome (CRPS). After pain history and NeuP questionnaires, quantitative sensory testing was performed. Individual z-score plots were calculated with body-region control measures and matched to mechanism-related sensory profiles (sensory loss, thermal hyperalgesia, and mechanical hyperalgesia). Conditioned pain modulation was assessed with pressure pain threshold and a contralateral cold conditioning stimulus, and meaningful conditioned pain modulation defined as twice the standard error of measurement. Patients and parents completed validated questionnaires for child quality of life (QoL), pain catastrophizing, and self-reported anxiety/depression. Males (n = 23) and females (n = 43) with NeuP (n = 52) or CRPS (n = 14) reported moderate-severe pain with neuropathic sensory descriptors. Mixed patterns of sensory gain/loss at pain sites were not sex-dependent. Thermal hyperalgesia was common in both NeuP and CRPS, whereas sensory loss occurred only with NeuP and in a smaller proportion than adult cohorts. Conditioned pain modulation was inhibitory in 54%, facilitatory in 14%, and nonresponders had variable cold conditioning sensitivity. Males and females reported marked impairment of QoL, increased emotional distress, and pain catastrophising. Child-parent QoL scores correlated, but catastrophizing scores were discordant when parents or adolescents reported higher anxiety/depression. NeuP in adolescents is associated with significant pain, physical impairment, and psychosocial impairment. Quantifying alterations in somatosensory profiles, descending modulation, child and parent psychological function will inform individualized therapy and stratification for future clinical trials.

The predictive value of quantitative sensory testing: a systematic review on chronic postoperative pain and the analgesic effect of pharmacological therapies in patients with chronic pain

Studies have suggested that quantitative sensory testing (QST) might hold a predictive value for the development of chronic postoperative pain and the response to pharmacological interventions. This review systematically summarizes the current evidence on the predictive value of QST for chronic postoperative pain and the effect of pharmacological interventions. The main outcome measures were posttreatment pain intensity, pain relief, presence of moderate-to-severe postoperative pain, responders of 30% and 50% pain relief, or validated questionnaires on pain and disability. A systematic search of MEDLINE and EMBASE yielded 25 studies on surgical interventions and 11 on pharmacological interventions. Seventeen surgical and 11 pharmacological studies reported an association between preoperative or pretreatment QST and chronic postoperative pain or analgesic effect. The most commonly assessed QST modalities were pressure stimuli (17 studies), temporal summation of pain (TSP, 14 studies), and conditioned pain modulation (CPM, 16 studies). Of those, the dynamic QST parameters TSP (50%) and CPM (44%) were most frequently associated with chronic postoperative pain and analgesic effects. A large heterogeneity in methods for assessing TSP (n = 4) and CPM (n = 7) was found. Overall, most studies demonstrated low-to-moderate levels of risk of bias in study design, attrition, prognostic factors, outcome, and statistical analyses. This systematic review demonstrates that TSP and CPM show the most consistent predictive values for chronic postoperative pain and analgesic effect, but the heterogeneous methodologies reduce the generalizability and hence call for methodological guidelines.

The Effect of Conditioned Pain Modulation on Tonic Heat Pain Assessed Using Participant-Controlled Temperature

OBJECTIVE

Descending pain modulation can be experimentally assessed by way of testing conditioned pain modulation. The application of tonic heat as a test stimulus in such paradigms offers the possibility of observing dynamic pain responses, such as adaptation and temporal summation of pain. Here we investigated conditioned pain modulation effects on tonic heat employing participant-controlled temperature, an alternative tonic heat pain assessment. Changes in pain perception are thereby represented by temperature adjustments performed by the participant, uncoupling this approach from direct pain ratings. Participant-controlled temperature has emerged as a reliable and sex-independent measure of tonic heat.

METHODS

Thirty healthy subjects underwent a sequential conditioned pain modulation paradigm, in which a cold water bath was applied as the conditioning stimulus and tonic heat as a test stimulus. Subjects were instructed to change the temperature of the thermode in response to variations in perception to tonic heat in order to maintain their initial rating over a two-minute period. Two additional test stimuli (i.e., lower limb noxious withdrawal reflex and pressure pain threshold) were included as positive controls for conditioned pain modulation effects.

RESULTS

Participant-controlled temperature revealed conditioned pain modulation effects on temporal summation of pain (P = 0.01). Increased noxious withdrawal reflex thresholds (P = 0.004) and pressure pain thresholds (P < 0.001) in response to conditioning also confirmed inhibitory conditioned pain modulation effects.

CONCLUSIONS

The measured interaction between conditioned pain modulation and temporal summation of pain supports the participant-controlled temperature approach as a promising method to explore dynamic inhibitory and facilitatory pain processes previously undetected by rating-based approaches.

Dissecting pain processing in adolescents with Non-Suicidal Self Injury: Could suicide risk lurk among the electrodes?

Background

Although non‐suicidal self‐injury (NSSI) disorder is highly prevalent in adolescents, its relationship with pain system function and suicidality is still controversial. The present study was designed to assess the function of the nociceptive afferent pathways and the endogenous pain modulation in adolescent patients with NSSI and to longitudinally register any suicide attempt, describe its frequency and find a possible association between suicide, neurophysiological measures and psychological measures.

Methods

We enrolled 30 adolescents suffering from NSSI and 20 age‐ and gender‐matched healthy controls. Patients underwent a comprehensive psychological evaluation. Each participant underwent thermal pain thresholds of the quantitative sensory testing, laser‐evoked potential recording to study the ascending nociceptive pathway and the conditioned pain modulation testing to test the endogenous pain modulation.

Results

We found that patients with NSSI had a reduced amplitude of the N2 component of laser‐evoked potentials and an abnormal conditioned pain modulation. The amplitude of the N2 was associated with suicidal risk.

Conclusions

The deficit of the endogenous pain modulation likely depends on a saturation due to continuous pain solicitation. The strong association of a reduced amplitude of the N2 component with suicide suggests that it may serve as a possible biomarker in self‐harming adolescents.

Significance

The present study identifies the N2 component of laser‐evoked potentials as a possible neurophysiological biomarker of suicidal risk in patients with non‐suicidal self‐injury, therefore, raising the possibility for a non‐invasive test to identify subjects at higher risk of suicide among self‐harming patients.