Role of endogenous pain modulation in chronic pain mechanisms and treatment

Age-related differences in functional connectivity associated with pain modulation

Growing evidence suggests that aging is associated with impaired endogenous pain modulation, and that this likely underlies the increased transition from acute to chronic pain in older individuals. Resting-state functional connectivity (rsFC) offers a valuable tool to examine the neural mechanisms behind these age-related changes in pain modulation. RsFC studies generally observe decreased within-network connectivity due to aging, but its relevance for pain modulation remains unknown. We compared rsFC within a set of brain regions involved in pain modulation between young and older adults and explored the relationship with the efficacy of distraction from pain. This revealed several age-related increases and decreases in connectivity strength. Importantly, we found a significant association between lower pain relief and decreased strength of three connections in older adults, namely between the periaqueductal gray and right insula, between the anterior cingulate cortex (ACC) and right insula, and between the ACC and left amygdala. These findings suggest that the functional integrity of the pain control system is critical for effective pain modulation, and that its function is compromised by aging.

Statistical learning shapes pain perception and prediction independently of external cues

The placebo and nocebo effects highlight the importance of expectations in modulating pain perception, but in everyday life we don't need an external source of information to form expectations about pain. The brain can learn to predict pain in a more fundamental way, simply by experiencing fluctuating, non-random streams of noxious inputs, and extracting their temporal regularities. This process is called statistical learning. Here, we address a key open question: does statistical learning modulate pain perception? We asked 27 participants to both rate and predict pain intensity levels in sequences of fluctuating heat pain. Using a computational approach, we show that probabilistic expectations and confidence were used to weigh pain perception and prediction. As such, this study goes beyond well-established conditioning paradigms associating non-pain cues with pain outcomes, and shows that statistical learning itself shapes pain experience. This finding opens a new path of research into the brain mechanisms of pain regulation, with relevance to chronic pain where it may be dysfunctional.

Could relationship-based learnt beliefs and expectations contribute to physiological vulnerability of chronic pain? Making a case to consider attachment in pain research

Pain is an interpersonal and inherently social experience. Pain perception and administration of medical treatment all occur in a particular environmental and social context. Early environmental influences, and early learning experiences and interactions, condition the body's response to different threats (like pain), ultimately shaping the underlying neurophysiology. These early interactions and experiences also determine what situations are perceived as threatening, as well as our belief in our own ability to self-manage, and our belief in others to offer support, during perceived threats. These beliefs intrinsically drive the combination of behaviours that emerge in response to perceived threats, including pain. Such behaviours can be categorised into attachment styles. In this interdisciplinary review, we synthesise and summarise evidence from the neurobiological, psychobiological, psychosocial and psychobehavioural fields, to describe how these beliefs are embedded in the brain's prediction models to generate a series of expectations/perceptions around the level of safety/threat in different contexts. As such, these beliefs may predict how one experiences, and responds to, pain; with potentially significant implications for the development and management of chronic pain. Little attention has been directed to the effect of adult attachment style on pain in research studies and in the clinical setting. Using interdisciplinary evidence, we argue why we think this interaction merits further consideration and research. PERSPECTIVE: This review explores the influence of attachment styles on pain perception, suggesting a link between social connections and chronic pain development. It aligns with recent calls to emphasise the social context in pain research and advocates for increased focus on adult attachment styles in research and clinical practice.

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

BACKGROUND

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

METHODS AND ANALYSIS

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

ETHICS AND DISSEMINATION

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

[Migraine and psychosomatic comorbidity]

As a common neurological disorder (10-15% of the population), migraine is associated with numerous comorbidities, particularly other pain syndromes, mental illnesses and functional disorders. These 'psychosomatic' comorbidities increase with migraine severity. Severely affected, comorbid patients also often have a poorer response to specific migraine therapy. Interestingly, migraine and the comorbidities mentioned have a number of common aetiological or facilitating factors, e.g. genetic factors, and show a higher incidence in women and in people with previous traumatic experiences, as well as (in the case of pain syndromes) signs of central sensitization. Another common feature is the association with current or chronic stress. We propose an extended diathesis-stress model that takes into account interrelated but individually different vulnerabilities and, depending on the stress experience, can depict both the occurrence of individual disorders (e.g. an isolated migraine) and the joint occurrence of migraine with other pain syndromes and other psychosomatic comorbidities. In summary, psychosomatic comorbidities should always be kept in mind in migraine therapy and, if necessary, treated early and multimodally.

Effect of dry needling on pain and central sensitization in women with chronic pelvic pain: A randomized parallel-group controlled clinical trial

Chronic pelvic pain (CPP) is a debilitating problem in women with clear evidence of myofascial dysfunction. It seems that Myofascial trigger points (MTrPs) contribute to the development of central sensitization (CS). This study aimed to investigate the effect of dry needling on pain and CS in women with CPP. Thirty-six women with CPP participated in this randomized controlled clinical trial and randomly assigned into three groups: dry needling group (DNG), placebo needling group (PNG) and control group (CG). The DNG received five sessions of DN using the "static needling", the PNG received non-penetrating method, and the CG did not receive any intervention. Assessment of outcomes including central sensitization inventory (CSI), short-form McGill pain questionnaire (SF-MPQ), electroencephalography (EEG), conditioned pain modulation (CPM), salivary cortisol concentration, 7-item general anxiety disorder scale (GAD-7), pain catastrophizing scale (PCS), and SF-36 questionnaire was performed pre-intervention, post-intervention, and three months post-intervention by a blind examiner. The result showed a significant group-by-time interaction for CSI, SF-MPQ, and PCS. There was a significant decrease in CSI score in post-intervention and three-months post-intervention compare to pre-intervention in the DNG and PNG. SF-MPQ-PPI score in DNG significantly decreased post-intervention. PCS-Total score decreased significantly post-intervention in DNG and PNG. No significant group-by-time interactions were observed for other variables. EEG results showed regional changes in the activity of frequency bands in both eye closed and eye open conditions. It seems that DN can affect central pain processing by removing the source of peripheral nociception. Trial registration: Iranian Registry of Clinical Trials (IRCT20211114053057N1, registered on: December 03, 2021. https://irct.behdasht.gov.ir/search/result?query=IRCT20211114053057N1).

Pupillometry as a Potential Objective Measurement of Pain Assessment in Healthy Volunteers

Background

Pain leads to activation of the autonomic nervous system and thus, among other things, to pupillary reflex dilation (PRD). Previous studies have already confirmed a correlation between the perception of pain and the pupillary reaction, measured using pupillometry. However, the previous study populations were under the influence of medication for analgesia in perioperative setting or suffered from pain. This study examines the relationship between pupillary reaction and pain perception in healthy controls and addresses the question of whether endogenous pain inhibition, clinically tested by conditioned pain modulation (CPM), can be quantified using pupillometry.

Methods

Forty-two healthy volunteers (21 females, 21 males, mean age 27.9 ± 5.8 years, range 20-39 years) were included in this study. The PRD, as a measure of the pupillary reaction (variance from the base diameter in percent), was investigated during baseline, heat application and during CPM testing and results compared to the reported pain intensity on the numerical rating scale (NRS).

Results

The volunteers showed higher variances under painful conditions compared to the measurement at rest corresponding to higher sympathetic activity during pain. Volunteers with a higher variance, ie a stronger pupillary reaction, gave higher pain ratings than subjects with a lower pupil variance. However, there was no correlation between the NRS and PRD. PRD and pain ratings during CPM were significantly lower compared to heat pain application alone. However, there was no correlation between the calculated CPM effect and the PRD.

Conclusion

Pupillometry is capable of objectively reflecting the pain response, eg pain relief through CPM testing. However, the CPM effect calculated from the subjective pain ratings and the objective PRD measurements is not associated suggesting that both measure different aspects of pain perception. It must be discussed whether the CPM effect can be the correct measure for the functionality of the pain system.

The impact of an immersive digital therapeutic tool on experimental pain: a pilot randomized within-subject experiment with an active control condition

Background

Pain is a complex and multifaced sensory and emotional experience. Virtual reality (VR) has shown promise in reducing experimental pain and chronic pain. This study examines an immersive VR environment initially designed for endometriosis patients, which demonstrated short-term analgesic effects. The research aims to determine the impact of the VR environment on experimental pain intensity and unpleasantness both during and after VR exposure (3D with binaural beats), while using an active control condition (2D with no binaural beats). Additionally, a secondary objective of the study was to identify the psychological and psychophysical factors that predict the analgesic effects of the immersive digital therapeutic tool.

Methods

The study involved twenty-one healthy individuals and used a within-subject design, comparing a VR treatment with an active control condition. Continuous heat stimulation was applied to the left forearm with a Peltier thermode. Pain ratings were collected for immediate and short-term effects.

Results

In both the VR and Control conditions, there were no significant differences in pain intensity before, during, and after exposure. However, during VR exposure, there was a significant decrease in pain unpleasantness as compared to before exposure (p < 0.001), with a 27.2% pain reduction. In the Control condition, there were no significant differences in pain unpleasantness during and after exposure. Furthermore, no psychological and psychophysical factors predicted the analgesic effects.

Discussion

The study investigated how a VR environment affected experimentally induced pain in healthy volunteers. It showed that VR reduced pain unpleasantness during exposure but had no lasting impact. The VR environment mainly influenced the emotional aspect of pain, possibly due to its inclusion of binaural beats and natural stimuli. The study suggests that the VR environment should be tested in chronic pain population with high distress levels.

Registration number clinicaltrialsgov

NCT06130267.

Does pain intensity after total knee arthroplasty depend on somatosensory functioning in knee osteoarthritis patients? A prospective cohort study

The objective of this study is to determine whether the change in pain intensity over time differs between somatosensory functioning evolution profiles in knee osteoarthritis (KOA) patients undergoing total knee arthroplasty (TKA). This longitudinal prospective cohort study, conducted between March 2018 and July 2023, included KOA patients undergoing TKA in four hospitals in Belgium and the Netherlands. The evolution of the Knee Injury and Osteoarthritis Outcome Score (KOOS) subscale pain over time (baseline, 3 months, and 1 year post-TKA scores) was the outcome variable. The evolution scores of quantitative sensory testing (QST) and Central Sensitization Inventory (CSI) over time (baseline and 1 year post-TKA scores) were used to make subgroups. Participants were divided into separate normal, recovered, and persistent disturbed somatosensory subgroups based on the CSI, local and widespread pressure pain threshold [PPT] and heat allodynia, temporal summation [TS], and conditioned pain modulation [CPM]. Linear mixed model analyses were performed. Two hundred twenty-three participants were included. The persistent disturbed somatosensory functioning group had less pronounced pain improvement (based on CSI and local heat allodynia) and worse pain scores 1 year post-TKA (based on CSI, local PPT and heat allodynia, and TS) compared to the normal somatosensory functioning group. This persistent group also had worse pain scores 1 year post-TKA compared to the recovered group (based on CSI). The study suggests the presence of a "centrally driven central sensitization" subgroup in KOA patients awaiting TKA in four of seven grouping variables, comprising their less pain improvement or worse pain score after TKA. Future research should validate these findings further. The protocol is registered at clinicaltrials.gov (NCT05380648).

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.

Examination of Conditioned Pain Modulation in Myofascial TMD With Consideration of Temporal Summation

Research on myofascial temporomandibular disorder (mTMD) has often focused on potential dysfunction in endogenous pain modulation. However, studies on the specific inhibitory and facilitatory components of endogenous pain modulation using conditioned pain modulation (CPM) and temporal summation of second pain (TSSP) have shown mixed results. This study aimed to 1) examine whether women with mTMD demonstrated efficient CPM compared to controls; 2) explore the association between independent measures of CPM and TSSP in women with mTMD relative to controls; and 3) determine whether resulting modulatory profiles differentially predicted pain intensity among cases. All participants were recruited from dental clinics. Cases were women who met the research diagnostic criteria for mTMD. Controls did not have facial pain on exam and were selected to be sociodemographically similar to cases. CPM and TSSP were assessed via independent psychophysical protocols. CPM was examined in linear mixed models predicting pain thresholds adjusted for age and stratified by TSSP. Mean CPM was estimated at a 2.2 (SD = 2.8) degree increase in pain thresholds (P ≤ .001), similar in cases and controls (P = .67). CPM was less efficient in cases with enhanced TSSP (P = .031), but not in controls. Although the double-pronociceptive profile of both low CPM and high TSSP trended higher among cases than controls, it did not predict higher levels of pain intensity among cases. This study does not support deficient inhibitory endogenous pain modulation in mTMD, but results suggest that inhibitory and facilitatory pain modulation should be examined concomitantly in the study of endogenous pain modulation. PERSPECTIVE: This manuscript presents a novel examination of inhibitory modulation by the level of facilitatory modulation in mTMD. The findings and approach may prove useful for mechanistic researchers studying endogenous pain modulation and clinical researchers seeking to jointly examine conditioned pain modulation and temporal summation in future research on chronic pain.

The rostral ventromedial medulla modulates pain and depression-related behaviors caused by social stress

ABSTRACT

The rostral ventromedial medulla (RVM) is a crucial structure in the descending pain modulatory system, playing a key role as a relay for both the facilitation and inhibition of pain. The chronic social defeat stress (CSDS) model has been widely used to study stress-induced behavioral impairments associated with depression in rodents. Several studies suggest that CSDS also causes changes related to chronic pain. In this study, we aimed to investigate the involvement of the RVM in CSDS-induced behavioral impairments, including those associated with chronic pain. We used chemogenetics to activate or inhibit the RVM during stress. The results indicated that the RVM is a vital hub influencing stress outcomes. Rostral ventromedial medulla activation during CSDS ameliorates all the stress outcomes, including social avoidance, allodynia, hyperalgesia, anhedonia, and behavioral despair. In addition, RVM inhibition in animals exposed to a subthreshold social defeat stress protocol induces a susceptible phenotype, facilitating all stress outcomes. Finally, chronic RVM inhibition-without any social stress stimulus-induces chronic pain but not depressive-like behaviors. Our findings provide insights into the comorbidity between chronic pain and depression by indicating the involvement of the RVM in establishing social stress-induced behavioral responses associated with both chronic pain and depression.

Therapeutic Patient Education as Part of the Physiotherapy Management of Adults with Headache: A Scoping Review

PURPOSE OF REVIEW

Physiotherapy interventions for headache mostly include exercise and manual therapy. Yet, the complex nature of headache, sometimes characterized by symptoms of facilitated central pain mechanisms, demands an individualized approach in which therapeutic patient education could be supportive. This scoping review aimed to summarize the position of therapeutic patient education within the physiotherapy management of adults with headache. PubMed, EMBASE, Web of Science, and Scopus were searched. The search-query comprised terminology relating to "headache", "education", and "physiotherapy". Eligibility criteria were: adults with headache, interventions including education within the domain of physiotherapy, reviews, clinical trials, cohort, case report, case-control studies.

RECENT FINDINGS

Eleven publications were included from the 281 retrieved publications. These publications were clinical trials (n = 4), reviews (n = 4), case-reports (n = 2), and a guideline (n = 1). Type of headaches studied were migraine (n = 3), post-traumatic headache (n = 2), tension-type headache (n = 2), cervicogenic headache (n = 1), primary headaches (n = 1), chronic daily headache (n = 1), and mixed migraine-cervicogenic headache (n = 1). Education seems an umbrella-term for postural education, lifestyle advice, and pain education. Three themes emerged across the publications: handling headache triggers (migraine, post-traumatic headache), promoting active lifestyle (post-traumatic headache, chronic daily headache, migraine), evaluating posture (post-traumatic headache, chronic daily headache, tension-type headache, cervicogenic headache). All publications recommended education in the management of headache. Only one (of the 11 included) publication described the educational program and determined its efficacy. Based on this scoping review, therapeutic patient education seems supported within physiotherapy management of headache. However, it is unclear how such education is tailored to the specific needs of the individual, the headache subtype, or when it should be added to physiotherapy management of headache.

Somatosensory and psychosocial profile of migraine patients: A cross-sectional study

BACKGROUND

Migraine is a chronic neurological disorder that involves the brain, characterized by a series of abnormal neuronal networks interacting at different levels of the central and peripheral nervous system. Furthermore, it is known that psychosocial features contribute to the exacerbation and chronicity of symptoms.

OBJECTIVE

To compare the somatosensory and psychosocial profiles of migraine patients with a control group.

METHODS

We conducted a cross-sectional study comparing the somatosensory and psychosocial profiles of patients with migraine and healthy volunteers. A total of 52 women were included. For the somatosensory profile, Mechanical Detection Threshold (MDT), Pressure Pain Threshold (PPT), Temporal Summation (TS), and Conditioned Pain Modulation (CPM) in the trigeminal and extra-trigeminal areas were evaluated. Psychosocial profiles were assessed using questionnaires, the Central Sensitization Inventory, the Generalized Anxiety Disorders, the Pain Catastrophizing Scale, and the Tampa Scale of Kinesiophobia. Mann-Whitney U test was used to compare differences in the profiles between groups. The significance level was set at 5%.

RESULTS

Migraine patients showed a loss of somatosensory function in the trigeminal area for MDT (p = 0.019, r = 0.34 and p = 0.011, r = 0.37 for the ophthalmic nerve and masseter muscle respectively), lower PPT in trigeminal and extra-trigeminal areas (p < 0.001, r=>0.60) and less efficient CPM (p < 0.001, r=>0.60). No statistically significant differences were found in the TS (p=>0.05). Statistically significant differences were found in all psychosocial variables (p = <0.001 r=>0.60).

CONCLUSION

Migraine patients showed loss of somatosensory function, lower pressure pain threshold, and an inhibitory pro-nociceptive profile with high scores on central sensitization and fear of movement compared to the control group.

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.

Prediction of duloxetine efficacy in addition to self-management in painful temporomandibular disorders: A randomised, placebo-controlled clinical trial

BACKGROUND

Conditioned pain modulation (CPM) is a potential predictor of treatment response that has not been studied in temporomandibular disorders (TMD).

OBJECTIVES

We conducted a randomised, double-blind, placebo-controlled trial (RCT) of duloxetine in addition to self-management (SM) strategies to investigate its efficacy to reduce pain intensity in painful TMD patients. Moreover, we investigated whether baseline CPM would predict the duloxetine efficacy to reduce TMD pain intensity.

METHODS

Eighty participants were randomised to duloxetine 60 mg or placebo for 12 weeks. The primary outcomes were the change in the pain intensity from baseline to week-12 and CPM-sequential paradigm at baseline. Safety, physical and emotional functioning outcomes were also evaluated.

RESULTS

Of 80 participants randomised, 78 were included in intention-to-treat analysis. Pain intensity decreased for SM-duloxetine and SM-placebo but did not differ between groups (p = .82). A more efficient CPM was associated with a greater pain intensity reduction regardless of the treatment group (p = .035). Physical and emotional functioning did not differ between groups, but adverse events (p = .014), sleep impairment (p = .003) and catastrophizing symptoms (p = .001) were more prevalent in SM-duloxetine group.

CONCLUSION

This study failed to provide evidence of a beneficial effect of adding duloxetine to SM strategies for treatment of painful TMD. Nonetheless, this RCT has shown the feasibility of applying pain modulation assessment to predict short-term treatment response in painful TMD patients, which confirms previous finds that CPM evaluation may serve a step forward in individualising pain treatment.

Low-frequency (5-Hz) stimulation of ventrolateral periaqueductal gray modulates the descending serotonergic system in the peripheral neuropathic pain

ABSTRACT

Neuropathic pain is a type of chronic pain that entails severe prolonged sensory dysfunctions caused by a lesion of the somatosensory system. Many of those suffering from the condition do not experience significant improvement with existing medications, resulting in various side effects. In this study, Sprague-Dawley male rats were used, and long-term deep brain stimulation of the ventrolateral periaqueductal gray was conducted in a rat model of spared nerve injury. We found that 5-Hz deep brain stimulation effectively modulated mechanical allodynia and induced neuronal activation in the rostral ventromedial medulla, restoring impaired descending serotonergic system. At the spinal level, glial cells were still activated but only the 5-HT1a receptor in the spinal cord was activated, implying its inhibitory role in mechanical allodynia. This study found that peripheral neuropathy caused dysfunction in the descending serotonergic system, and prolonged stimulation of ventrolateral periaqueductal gray can modulate the pathway in an efficient manner. This work would provide new opportunities for the development of targeted and effective treatments for this debilitating disease, possibly giving us lower chances of side effects from repeated high-frequency stimulation or long-term use of medication.

Quantitative sensory testing, psychological factors, and quality of life as predictors of current and future pain in patients with knee osteoarthritis

ABSTRACT

Substantial interindividual variability characterizes osteoarthritis (OA) pain. Previous findings identify quantitative sensory testing (QST), psychological factors, and health-related quality of life as contributors to OA pain and predictors of treatment outcomes. This exploratory study aimed to explain baseline OA pain intensity and predict OA pain after administration of a nonsteroidal anti-inflammatory drug in combination with paracetamol for 3 weeks. The Knee Injury and Osteoarthritis Outcome Score (KOOS) pain score was used to estimate OA pain presentation. One hundred one patients were assessed at baseline and follow-up using QST (pressure pain thresholds and temporal summation of pain [TSP]), symptoms of depression and anxiety, pain catastrophizing scales (PCSs), and health-related quality of life. Linear regression with backward selection identified that PCS significantly explained 34.2% of the variability in baseline KOOS pain, with nonsignificant contributions from TSP. Pain catastrophizing score and TSP predicted 29.3% of follow-up KOOS pain, with nonsignificant contributions from symptoms of anxiety. When assessed separately, PCS was the strongest predictor (32.2% of baseline and 24.1% of follow-up pain), but QST, symptoms of anxiety and depression, PCS, and quality of life also explained some variability in baseline and follow-up knee OA pain. Further analyses revealed that only TSP and PCS were not mediated by any other included variables, highlighting their role as unique contributors to OA pain presentation. This study emphasizes the importance of embracing a multimodal approach to OA pain and highlights PCS and TSP as major contributors to the baseline OA pain experience and the OA pain experience after OA treatment.

Exercise-Induced Hypoalgesia in Patients with Chronic Whiplash-Associated Disorders: Differences between Subgroups Based on the Central Sensitization Inventory

BACKGROUND

Physical exercise is an important element in the rehabilitation of chronic whiplash-associated disorders, with the physiological process underlying pain reduction called exercise-induced hypoalgesia. In chronic whiplash-associated disorders, exercise-induced hypoalgesia appears impaired, and the research suggests a relationship with symptoms of dysfunctional nociceptive processing, such as central sensitization. This study improves our understanding of exercise-induced hypoalgesia in chronic whiplash-associated disorders by examining the differences between the extent of exercise-induced hypoalgesia in subgroups based on scores on the central sensitization inventory (CSI).

METHODS

Data were collected from 135 participants with chronic whiplash-associated disorders who completed a set of questionnaires. Pain pressure thresholds and temporal summations were assessed before and after a submaximal aerobic bicycle exercise test.

RESULTS

We observed no interaction effect between exercise-induced hypoalgesia and the CSI scores for both pain pressure threshold and temporal summation. No overall statistical effect was measured in the analysis of the effect of time. The pain pressure threshold significantly related to the CSI. The temporal summation showed no correlation.

CONCLUSIONS

During this study, we did not find evidence for a difference in the presence of exercise-induced hypoalgesia when the subgroups were created based on the central sensitization cluster calculator. Limited evidence was found for the influence of CSI scores on the delta pain pressure threshold.

Does Active Inference Provide a Comprehensive Theory of Placebo Analgesia?

Placebo interventions generate mismatches between expected pain and sensory signals from which pain states are inferred. Because we lack direct access to bodily states, we can only infer whether nociceptive activity indicates tissue damage or results from noise in sensory channels. Predictive processing models propose to make optimal inferences using prior knowledge given noisy sensory data. However, these models do not provide a satisfactory explanation of how pain relief expectations are translated into physiological manifestations of placebo responses. Furthermore, they do not account for individual differences in the ability to endogenously regulate nociceptive activity in predicting placebo analgesia. The brain not only passively integrates prior pain expectations with nociceptive activity to infer pain states (perceptual inference) but also initiates various types of actions to ensure that sensory data are consistent with prior pain expectations (active inference). We argue that depending on whether the brain interprets conflicting sensory data (prediction errors) as a signal to learn from or noise to be attenuated, the brain initiates opposing types of action to facilitate learning from sensory data or, conversely, to enhance the biasing influence of prior pain expectations on pain perception. Furthermore, we discuss the role of stress, anxiety, and unpredictability of pain in influencing the weighting of prior pain expectations and sensory data and how they relate to the individual ability to regulate nociceptive activity (endogenous pain modulation). Finally, we provide suggestions for future studies to test the implications of the active inference model of placebo analgesia.

Physical activity, sitting time, and thermal quantitative sensory testing responses in African Americans

Introduction

Prior research suggests that African Americans (AAs) have more frequent, intense, and debilitating pain and functional disability compared with non-Hispanic Whites (NHWs). Potential contributing factors to this disparity are physical activity and sedentary behavior, given that AAs are less physically active, and physical activity is associated with antinociception (whereas sedentary behavior is linked to pronociception). However, impact of these factors on pain processing has largely been unexplored in AAs, especially before chronic pain onset.

Objective

This study examined relationships between physical activity, sedentary behavior (sitting time), and laboratory measures of pain and pain modulation in adult AAs. These included heat pain threshold and tolerance, temporal summation of pain (TSP, a marker of central sensitization), and conditioned pain modulation (CPM, a marker of descending pain inhibition).

Methods

Multiple regressions were conducted to examine the effects of physical activity and sitting time on heat threshold and tolerance. Multilevel models were conducted to assess the relationship between physical activity, sitting time, and temporal summation of pain. Additional multilevel models were conducted to assess the relationship between physical activity, sitting time, and conditioned pain modulation.

Results

Higher level of physical activity, but not sitting time, was associated with reduced TSP slopes. Neither physical activity nor sitting time was associated with CPM slopes. No significant relationships between physical activity or sitting time and heat pain threshold or tolerance were detected.

Conclusions

These findings suggest that physical activity is associated with reduced TSP, an effect which may be driven by reduced spinal hyperexcitability in more active individuals. Thus, structural and individual interventions designed to increase physical activity in healthy, young AAs may be able to promote antinociceptive processes (ie, reduced TSP/reduced pain facilitation) potentially protective against chronic pain.

Mechanisms and treatments of chronic pain after traumatic brain injury

While pain after trauma generally resolves, some trauma patients experience pain for months to years after injury. An example, relevant to both combat and civilian settings, is chronic pain after traumatic brain injury (TBI). Headache as well as pain in the back and extremities are common locations for TBI-related chronic pain to be experienced. TBI-related pain can exist alone or can exacerbate pain from other injuries long after healing has occurred. Consequences of chronic pain in these settings include increased suffering, higher levels of disability, serious emotional problems, and worsened cognitive deficits. The current review will examine recent evidence regarding dysfunction of endogenous pain modulatory mechanisms, neuroplastic changes in the trigeminal circuitry and alterations in spinal nociceptive processing as contributors to TBI-related chronic pain. Key pain modulatory centers including the locus coeruleus, periaqueductal grey matter, and rostroventromedial medulla are vulnerable to TBI. Both the rationales and existing evidence for the use of monoamine reuptake inhibitors, CGRP antagonists, CXCR2 chemokine receptor antagonists, and interventional therapies will be presented. While consensus guidelines for the management of chronic post-traumatic TBI-related pain are lacking, several approaches to this clinically challenging situation deserve focused evaluation and may prove to be viable therapeutic options.

Longitudinal changes in human supraspinal processing after RIII-feedback training to improve descending pain inhibition

The human body has the ability to influence its sensation of pain by modifying the transfer of nociceptive information at the spinal level. This modulation, known as descending pain inhibition, is known to originate supraspinally and can be activated by a variety of ways including positive mental imagery. However, its exact mechanisms remain unknown. We investigated, using a longitudinal fMRI design, the brain activity leading up and in response to painful electrical stimulation when applying positive mental imagery before and after undergoing a previously established RIII-feedback paradigm. Time course analysis of the time preceding painful stimulation shows increased haemodynamic activity during the application of the strategy in the PFC, ACC, insula, thalamus, and hypothalamus. Time course analysis of the reaction to painful stimulation shows decreased reaction post-training in brainstem and thalamus, as well as the insula and dorsolateral PFC. Our work suggests that feedback training increases activity in areas involved in pain inhibition, while simultaneously decreasing the reaction to painful stimuli in brain areas related to pain processing, which points to an activation of decreased spinal nociception. We further suggest that the insula and the thalamus may play a more important role in pain modulation than previously assumed.

Pain Informed Movement for people with knee osteoarthritis: Protocol for a pilot randomized controlled trial

Objective

Conservative pain management strategies for knee osteoarthritis (KOA) have limited effectiveness and do not employ a pain-mechanism informed approach. Pain Informed Movement is a novel intervention combining mind-body techniques with neuromuscular exercise and pain neuroscience education (PNE), aimed at improving endogenous pain modulation. While the feasibility and acceptability of this program has been previously established, it now requires further evaluation in comparison to standard KOA care.

Design

This protocol describes the design of a pilot two-arm randomized controlled trial (RCT) with an embedded qualitative component. The primary outcome is complete follow-up rate. With an allocation ratio of 1:1, 66 participants (33/arm) (age ≥40 years, KOA diagnosis or meeting KOA NICE criteria, and pain intensity ≥3/10), will be randomly allocated to two groups that will both receive 8 weeks of twice weekly in-person exercise sessions. Those randomized to Pain Informed Movement will receive PNE and mind-body technique instruction provided initially as videos and integrated into exercise sessions. The control arm will receive neuromuscular exercise and standard OA education. Assessment will include clinical questionnaires, physical and psychophysical tests, and blood draws at baseline and program completion. Secondary outcomes are program acceptability, burden, rate of recruitment, compliance and adherence, and adverse events. Participants will be invited to an online focus group at program completion.

Conclusion

The results of this pilot RCT will serve as the basis for a larger multi-site RCT aimed at determining the program's effectiveness with the primary outcome of assessing the mediating effects of descending modulation on changes in pain.

Trajectory of pain threshold and its association with acute pain after thoracic surgery: a prospective observational study

BACKGROUND

Postoperative analgesic management is an ongoing challenge. The pain threshold (PT) is an objective index that reflects the body's sensitivity to pain and can be used for quantitative pain assessment. We hypothesized that the PT is correlated with postoperative pain and can thus be used to guide postoperative pain management.

METHODS

This study involved 93 patients who underwent thoracoscopic surgery from December 2019 to February 2020. The PT was measured with transcutaneous electrical stimulation before surgery (T0) and at 1 h (T1), 6 h (T6), and 24 h (T24) after surgery. The visual analogue scale (VAS) score was used to evaluate the severity of postoperative pain at the same time. The PT variation (PTV) after surgery was calculated as the ratio of the postoperative PT to preoperative PT.

RESULTS

The postoperative PT was higher than the preoperative PT and showed a downward trend within 24 h after surgery; the PTV also showed a downward trend within 24 h after surgery. PT-T1 was negatively correlated with VAS-T1 at rest and during motion (rest: VAS-T1r = - 0.274, P = 0.008; motion: VAS-T1r = - 0.298, P = 0.004). PTV-T1 was negatively correlated with VAS-T1 during motion (r = - 0.213, P = 0.04). Lower VAS-T1 scores (< 4) at rest and during motion were associated with higher PT-T1 (rest: t = 2.452, P = 0.016; motion: t = 2.138, P = 0.035). The intraoperative sufentanil dose was associated with a postoperative increase in PTV-T1. Increased rescue analgesic administration was associated with PTV elevation. However, the incidence of dizziness in patients with moderate PTV-T24 was lower than that in patients with high or low PTV-T24 (χ2 = 8.297, P = 0.015).

CONCLUSIONS

The postoperative PT was higher than the preoperative PT and showed a downward trend within 24 h after surgery; PTV also showed a downward trend within 24 h after surgery. The PT and PTV were negatively correlated with the pain intensity at rest and during motion and were associated with perioperative analgesic consumption and the incidence of adverse events.

Effectiveness of different percutaneous electrolysis protocols in the endogenous modulation of pain: A Double-Blinded Randomized Clinical Trial

OBJECTIVE

This randomized clinical trial investigated if the application of percutaneous electrolysis (PE) enhances endogenous pain mechanisms (EPM) when compared with a simple needle application (acting as sham).

METHODS

Forty-six asymptomatic subjects, aged 18-40 years, were randomized into three groups receiving a single ultrasound-guided PE intervention consisting of a needle insertion on the lateral epicondyle: sham (without electrical current), low-intensity (0.3 mA, 90s), or high-intensity (three pulses of 3 mA, 3s) PE. Widespread pressure pain thresholds (PPT), conditioned pain modulation (CPM), and temporal summation (TS) were bilaterally assessed in the lateral epicondyle, bicipital groove, transverse process of C5 and tibialis anterior muscle. Outcomes were obtained by an assessor blinded to the treatment allocation of the subjects.

RESULTS

No significant changes in CPM were observed in either group (omnibus ANOVA all, P > .05). A significant bilateral increase in PPT in the lateral epicondyle in the high intensity group as compared with the sham group was observed (P < .01). A significant decrease of TS in both low (P = .002) and high (P = .049) intensity groups on the right, but not on the left, tibialis anterior was also observed when compared with the sham group.

CONCLUSIONS

One session of PE is able to slightly stimulate modulatory pathways related to nociceptive gain, particularly pressure pain sensitivity and temporal summation but not conditioning pain modulation, when compared with a sham needle intervention, with changes even contralaterally. No significant differences were found between low- and high-intensity doses of percutaneous electrolysis.

Beyond biopsychosocial: The keystone mechanism theory of pain

Pain is a deeply personal experience, with interindividual differences in its chronification and treatment presenting a formidable healthcare challenge. The biopsychosocial model (BPSm) has been hugely influential within nascent attempts at precision pain medicine, steering the field away from a reductionist biomechanical viewpoint and emphasising complex interactions of biological, psychological, and social factors which shape the individuality of pain. However, despite offering a strong theoretical foundation and holistic perspective, we contend that the BPSm remains limited in its capacity to deliver truly mechanistically informed treatment of pain. We therefore propose the keystone model of pain which offers a pragmatic balance between the dimensionality expansive BPSm and overly reductive approaches, providing both theoretical and practical advantages for the transition from treating populations to individual people.

Effects of conditioned pain modulation on Capsaicin-induced spreading muscle hyperalgesia in humans

OBJECTIVES

Muscle pain can be associated with hyperalgesia that may spread outside the area of primary injury due to both peripheral and central sensitization. However, the influence of endogenous pain inhibition is yet unknown. This study investigated how endogenous pain inhibition might influence spreading hyperalgesia in experimental muscle pain.

METHODS

Conditioned pain modulation (CPM) was assessed in 30 male volunteers by cold pressor test at the non-dominant hand as conditioning and pressure pain thresholds (PPT) at the dominant 2nd toe as test stimuli. Subjects were classified as having inhibitory or facilitating CPM based on published reference values. Subsequently, muscle pain and hyperalgesia were induced by capsaicin injection into the non-dominant supraspinatus muscle. Before and 5, 10, 15, 20, 30, 40, 50 and 60 min later, PPTs were recorded at the supraspinatus, infraspinatus and deltoid muscle, ring finger and toe.

RESULTS

Compared to baseline, PPTs decreased at the supraspinatus, infraspinatus and deltoid muscle (p≤0.03), and increased at the finger and toe (p<0.001). In facilitating CPM (n=10), hyperalgesia occurred at 5, 10, 15, 20 and 40 min (p≤0.026). In inhibitory CPM (n=20), hyperalgesia only occurred after 10 and 15 min (p≤0.03). At the infraspinatus muscle, groups differed after 5 and 40 min (p≤0.008).

CONCLUSIONS

The results suggest that facilitating CPM is associated with more spreading hyperalgesia than inhibitory CPM. This implies that poor endogenous pain modulation may predispose to muscle pain and spreading hyperalgesia after injury, and suggest that strategies to enhance endogenous pain modulation may provide clinical benefits.

Effect of Voluntary Exercise on Endogenous Pain Control Systems and Post-traumatic Headache in Mice

Traumatic brain injury (TBI) can cause acute and chronic pain along with motor, cognitive, and emotional problems. Although the mechanisms are poorly understood, previous studies suggest disruptions in endogenous pain modulation may be involved. Voluntary exercise after a TBI has been shown to reduce some consequences of injury including cognitive impairment. We hypothesized, therefore, that voluntary exercise could augment endogenous pain control systems in a rodent model of TBI. For these studies, we used a closed-head impact procedure in male mice modeling mild TBI. We investigated the effect of voluntary exercise on TBI-induced hindpaw nociceptive sensitization, diffuse noxious inhibitory control failure, and periorbital sensitization after bright light stress, a model of post-traumatic headache. Furthermore, we investigated the effects of exercise on memory, circulating markers of brain injury, neuroinflammation, and spinal cord gene expression. We observed that exercise significantly reduced TBI-induced hindpaw allodynia and periorbital allodynia in the first week following TBI. We also showed that exercise improved the deficits associated with diffuse noxious inhibitory control and reduced bright light stress-induced allodynia up to 2 months after TBI. In addition, exercise preserved memory and reduced TBI-induced increases in spinal BDNF, CXCL1, CXCL2, and prodynorphin expression, all genes previously linked to TBI-induced nociceptive sensitization. Taken together, our observations suggest that voluntary exercise may reduce pain after TBI by reducing TBI-induced changes in nociceptive signaling and preserving endogenous pain control systems. PERSPECTIVE: This article evaluates the effects of exercise on pain-related behaviors in a preclinical model of traumatic brain injury (TBI). The findings show that exercise reduces nociceptive sensitization, loss of diffuse noxious inhibitory control, memory deficits, and spinal nociception-related gene expression after TBI. Exercise may reduce or prevent pain after TBI.

Does feeling pain help prepare future clinicians to treat pain? A qualitative exploration of a novel experiential approach to teaching health professional students about pain

BACKGROUND AND INTRODUCTION

Pain is a subjective phenomenon, that is often misunderstood and invalidated. Despite recent advances in health professional training, it remains unclear how students should be taught about the subjectivity of pain. This study explored how a novel teaching activity that integrated physiotherapy students' first-hand experiences with laboratory-induced pain could address this gap.

OBJECTIVE

The study aimed to explore the experiences and perceptions of physiotherapy students in relation to a workshop where physiotherapy students experienced laboratory induced pain.

METHODS

We used a descriptive qualitative research design. Eighteen students participated in interviews. Transcripts were analyzed using an inductive conventional content analysis.

RESULTS

We identified four overarching themes: 1) First-hand pain experiences facilitated engagement in learning; 2) Reflecting on these personal and peer pain experiences helped students make sense of pain; 3) The learning activity helped students understand the inherent subjectivity and complexity of pain; and 4) Students saw benefits for clinical practice.

CONCLUSION

Integrating first-hand pain experiences within entry-level physiotherapy training appears to have novel value in helping students understand pain subjectivity. Future research should use robust and controlled designs to explore how this novel approach can be used to facilitate further understanding and empathy within clinical interactions with people living with pain.

Prolonged continuous theta burst stimulation increases motor corticospinal excitability and intracortical inhibition in patients with neuropathic pain: An exploratory, single-blinded, randomized controlled trial

OBJECTIVES

A new paradigm for Transcranial Magnetic Stimulation (TMS), referred to as prolonged continuous theta burst stimulation (pcTBS), has recently received attention in the literature because of its advantages over high frequency repetitive TMS (HF-rTMS). Clinical advantages include less time per intervention session and the effects appear to be more robust and reproducible than HF-rTMS to modulate cortical excitability. HF-rTMS targeted at the primary motor cortex (M1) has demonstrated analgesic effects in patients with neuropathic pain but their mechanisms of action are unclear and pcTBS has been studied in healthy subjects only. This study examined the neural mechanisms that have been proposed to play a role in explaining the effects of pcTBS targeted at the M1 and DLPFC brain regions in neuropathic pain (NP) patients with Type 2 diabetes.

METHODS

Forty-two patients with painful diabetic neuropathy were randomized to receive a single session of pcTBS targeted at the left M1 or left DLPFC. pcTBS stimulation consisted of 1,200 pulses delivered in 1 min and 44 s with a 35-45 min gap between sham and active pcTBS stimulation. Both the activity of the descending pain system which was examined using conditioned pain modulation and the activity of the ascending pain system which was assessed using temporal summation of pain were recorded using a handheld pressure algometer by measuring pressure pain thresholds. The amplitude of the motor evoked potential (MEP) was used to measure motor corticospinal excitability and GABA activity was assessed using short (SICI) and long intracortical inhibition (LICI). All these measurements were performed at baseline and post-pcTBS stimulation.

RESULTS

Following a single session of pcTBS targeted at M1 and DLPFC, there was no change in BPI-DN scores and on the activity of the descending (measured using conditioned pain modulation) and ascending pain systems (measured using temporal summation of pain) compared to baseline but there was a significant improvement of >13% in perception of acute pain intensity, increased motor corticospinal excitability (measured using MEP amplitude) and intracortical inhibition (measured using SICI and LICI).

CONCLUSION

In patients with NP, a single session of pcTBS targeted at the M1 and DLPFC modulated the neurophysiological mechanisms related to motor corticospinal excitability and neurochemical mechanisms linked to GABA activity, but it did not modulate the activity of the ascending and descending endogenous modulatory systems. In addition, although BPI-DN scores did not change, there was a 13% improvement in self-reported perception of acute pain intensity.

Patients with neuropathic pain from lumbosacral radiculopathy demonstrate similar pressure pain thresholds and conditioned pain modulation to those with fibromyalgia

OBJECTIVE

The purpose of the study was to evaluate pain thresholds, impairment of the endogenous pain modulatory system, and self-reported cognitive-emotional and central sensitization-related symptoms among three subject groups: a rarely studied patient cohort with neuropathic pain from lumbosacral radiculopathy (NPLSR), patients with fibromyalgia (FM) and healthy controls (HC).

METHODS

Patient-reported pain-related symptomology was evaluated with psychometricallyvalidated questionnaires. Pressure pain threshold (PPT), heat pain threshold (HPT), and cold pain threshold (CPT) were assessed in the low back and contralateral forearm. Conditioned pain modulation (CPM) was evaluated with a recently introduced methodology that accounts for a standard error of measurement.

RESULTS

Compared to the HC subjects, the FM and NPLSR subjects had significantly lower pain thresholds and more CPM impairment. No significant differences in PPT and CPM were observed between the FM and NPLSR groups. Significant group differences were found in self-reported symptoms of depression, anxiety, stress, and central sensitization. Self-reported symptom severity increased in a stair-step fashion, with the HC group scoring lowest and FM group scoring highest.

CONCLUSION

The NPLSR group manifested CPM dysfunction and pressure hyperalgesia at similar levels to the FM group, indicating that these two chronic pain syndromes, likely based on different pathophysiological mechanisms, in fact share some common pain processing features. However, though both patient groups demonstrated similarities in pain processing, self-reported cognitive-emotional and central sensitization-related symptom severity was significantly higher in the FM cohort, which distinguished them from the chronic NPLSR cohort.

Associations between psychological factors, pressure pain thresholds and conditioned pain modulation and disability in (sub)-acute low back pain: a three-month follow-up study

BACKGROUND

The clinical presentation and pain experience of patients with (sub)-acute low back pain ((S)ALBP) can strongly vary in clinical practice. However, despite growing evidence that psychological factors are associated with disability in chronic pain conditions including low back pain, studies examining the influence of psychological factors, quantitative sensory testing (QST) (i.e. pressure pain thresholds (PPTs)) and conditioned pain modulation (CPM) on future disability are still lacking in (S)ALBP.

OBJECTIVE

This prospective cohort study aims to determine associations between baseline psychological factors, PPTs and CPM in (S)ALBP and disability after 3 months.

METHODS

Fifty-two patients with (S)ALBP underwent a baseline PPT evaluation in rest and during a CPM protocol. Patients were asked to fill in self-report questionnaires: the Visual Analogue Scale (VAS), the Quebec Back Pain Disability Scale (QBPDS), the Pain Catastrophizing Scale (PCS), the Tampa Scale for Kinesiophobia (TSK) and the Illness Perception Questionnaire - Brief version (IPQ-B). At 3-month follow-up, participants were asked to fill in the QBPDS again. Multiple linear regression analysis was conducted to determine associations between baseline factors and disability at follow-up.

RESULTS

Thirty-eight patients participated at follow-up. Because of the multicollinearity issue, the TSK score was selected for analyses and the PCS and IPQ-B score were excluded from the model. No significant associations between baseline factors and disability at follow-up were found.

CONCLUSION

Neither baseline psychological factors nor PPTs or CPM in (S)ALBP were significantly associated with disability after 3 months. Our multiple linear regression analysis was likely underpowered to detect significant associations.

Pain Mechanisms Associated With Disease Activity in Patients With Rheumatoid Arthritis Treated With Disease-Modifying Antirheumatic Drugs: A Regression Tree Analysis

OBJECTIVE

Although pain affects the assessment of disease activity in patients with rheumatoid arthritis (RA), pain is not always directly related to peripheral joint inflammation. Peripheral and central nervous system regulatory mechanisms also affect pain perception. We used regression tree methodology to identify mechanisms most predictive of disease activity after disease-modifying antirheumatic drug (DMARD) treatment.

METHODS

Disease activity was evaluated using the Disease Activity Score in 28 joints (DAS28) in 176 patients with RA, before and after starting a DMARD. Quantitative sensory testing (QST), including pressure pain thresholds (PPTs), temporal summation, and conditioned pain modulation (CPM), were used to assess pain mechanisms. Regression tree methodology was used to determine the QST modalities most predictive of DAS28 after DMARD treatment.

RESULTS

This analysis identified 4 groups defined by baseline DAS28 category and either knee PPT (a combined measure of peripheral and central nervous system dysregulation) or CPM (a measure of descending pain inhibition). Among patients starting with low/moderate disease activity, lower knee PPT (PPT ≤ 4.65 kgf) most strongly predicted higher posttreatment disease activity (group 1 mean DAS28 2.8 [SD 1.0] vs group 2 mean DAS28 3.5 [SD 1.0]). Among patients starting with high baseline disease activity, less efficient descending pain modulation (CPM ≤ 1.55) most strongly predicted higher posttreatment disease activity (group 3 mean DAS28 3.4 [SD 1.4] vs group 4 mean DAS28 4.6 [SD 1.1]).

CONCLUSION

These results highlight the importance of identifying and treating aberrant peripheral and central pain regulation in patients with RA starting or switching DMARD therapy.

The Role of The Rostral Ventromedial Medulla in Stress Responses

The rostral ventromedial medulla (RVM) is a brainstem structure critical for the descending pain modulation system involved in both pain facilitation and inhibition through its projection to the spinal cord. Since the RVM is well connected with pain- and stress-engaged brain structures, such as the anterior cingulate cortex, nucleus accumbens, and amygdala, its involvement in stress responses has become a matter of great interest. While chronic stress has been proposed as a trigger of pain chronification and related psychiatric comorbidities due to maladaptive stress responses, acute stress triggers analgesia and other adaptative responses. Here we reviewed and highlighted the critical role of the RVM in stress responses, mainly in acute stress-induced analgesia (SIA) and chronic stress-induced hyperalgesia (SIH), providing insights into pain chronification processes and comorbidity between chronic pain and psychiatric disorders.

Pain Sensitization as a Potential Mediator of the Relationship Between Sleep Disturbance and Subsequent Pain in Rheumatoid Arthritis

OBJECTIVE

Many patients with rheumatoid arthritis (RA) experience sleep disturbances, commonly attributed to joint pain. Sleep disturbances could also influence pain. One mechanism may be through dysregulated pain processing, manifested by enhanced pain sensitivity. The present study was undertaken to examine the role of pain sensitization, measured by quantitative sensory testing (QST), as a mediator in the pathway of sleep disturbance leading to subsequent pain.

METHODS

We used longitudinal data from 221 patients with active RA who were followed for 12 weeks after initiating a disease-modifying antirheumatic drug. Baseline QST included pressure pain thresholds at articular (wrists, knees) and nonarticular (trapezius, thumbnails) sites, temporal summation (TS) at the wrist and forearm, and conditioned pain modulation (CPM). Baseline sleep disturbance and subsequent pain intensity were assessed using the Patient-Reported Outcomes Measurement Information System (PROMIS). We evaluated correlations between sleep disturbance, QSTs, and subsequent pain intensity. Mediation analyses separately assessed each QST as a mediator, adjusting for baseline confounding factors.

RESULTS

Sleep disturbance was correlated with all QST measures except wrist TS and CPM. Sleep disturbance significantly predicted subsequent pain (coefficient for a meaningful increase of 5 units in sleep disturbance = 0.32 (95% confidence interval 0.11, 0.50) in multiple regression. QST mediated 10-19% of this effect.

CONCLUSION

Pain sensitization may be one mechanism through which sleep disturbance contributes to pain. The small magnitude of association indicates that unmeasured pathways may contribute to this relationship. Intervention studies are needed to establish causality and determine whether improving sleep can improve pain in patients with RA.

Thermal grill illusion of pain in patients with chronic pain: a clinical marker of central sensitization?

ABSTRACT

The thermal grill illusion of pain (TGIP) is a paradoxical burning pain sensation elicited by the simultaneous application of innocuous cutaneous warm and cold stimuli with a thermode ("thermal grill") consisting of interlaced heated and cooled bars. Its neurophysiological mechanisms are unclear, but TGIP may have some mechanisms in common with pathological pain, including central sensitization in particular, through the involvement of N-methyl- d -aspartate receptors. However, few studies have investigated TGIP in patients with chronic pain and its clinical relevance is uncertain. We hypothesized that the TGIP would be increased in comparison with controls in patients with fibromyalgia or irritable bowel syndrome, which are regarded as typical "nociplastic" primary pain syndromes related to changes in central pain processing. We compared the sensations elicited by a large range of combinations of temperature differentials between the warm and cold bars of a thermal grill applied to the hand between patients with fibromyalgia (n = 30) or irritable bowel syndrome (n= 30) and controls (n = 30). The percentage of TGIP responses and the intensity and unpleasantness of TGIP were significantly greater in patients than controls. Furthermore, positive correlations were found between TGIP intensity and clinical pain intensity and between TGIP intensity and the cold pain threshold measured on the hand. These results are consistent with our working hypothesis of shared mechanisms between TGIP and clinical pain mechanisms in patients with nociplastic chronic pain syndromes and suggest that TGIP might represent a clinical marker of central sensitization in these patients.

Allodynia, Hyperalgesia, (Quantitative) Sensory Testing and Conditioned Pain Modulation in Patients With Complex Regional Pain Syndrome Before and After Spinal Cord Stimulation Therapy

OBJECTIVES

Complex regional pain syndrome (CRPS) is a chronic debilitating disease characterized by sensory abnormalities. Spinal cord stimulation (SCS) is an effective therapy for CRPS, but few studies have investigated the effects of SCS therapy on sensory characteristics. Therefore, this study investigated the effect of SCS on allodynia, hyperalgesia, electrical quantitative sensory testing (QST) parameters, and conditioned pain modulation (CPM) effect.

MATERIALS AND METHODS

This study is part of a multicenter randomized controlled trial (ISRCTN 36655259). Patients with CRPS in one extremity and eligible for SCS were included. The outcome parameters allodynia (symptom and sign), hyperalgesia (symptom), sensory thresholds with QST, CPM effect, and pain scores were tested before and after three months of SCS (40-Hz tonic SCS). Both the CRPS-affected extremity and the contralateral, clinically unaffected extremity were used to test three sensory thresholds with electrical QST: current perception threshold (CPT), pain perception threshold (PPT), and pain tolerance threshold (PTT). The PTT also was used as a test stimulus for the CPM paradigm both before and after the conditioning ice-water test. Nonparametric testing was used for all statistical analyses.

RESULTS

In total, 31 patients were included for analysis. Pain, allodynia (sign and symptom), and hyperalgesia (symptom) were all significantly reduced after SCS therapy. On the unaffected side, none of the QST thresholds (CPT, PPT, and PTT) was significantly altered after SCS therapy. However, the CPT on the CRPS-affected side was significantly increased after SCS therapy. A CPM effect was present both before and after SCS.

CONCLUSIONS

Standard 40-Hz tonic SCS significantly reduces pain, hyperalgesia, and allodynia in patients with CRPS. These findings suggest that SCS therapy should not be withheld from patients who suffer from allodynia and hyperalgesia, which contradicts previous findings derived from retrospective analysis and animal research. ISRCTN Registry: The ISRCTN registration number for the study is ISRCTN 36655259.

Response profile in a rat model of exercise-induced hypoalgesia is associated with duloxetine, pregabalin and diclofenac effect on constriction-induced neuropathy

BACKGROUND

Exercise is a known trigger of the inhibitory pain modulation system and its analgesic effect is termed exercise-induced hypoalgesia (EIH). Previous studies have demonstrated that rats with deficient analgesic response following exercise develop more significant hypersensitivity following nerve injury compared to rats with substantial analgesic response following exercise.

OBJECTIVES

A rat model of EIH as an indicator of the pain inhibitory system's efficiency was used to explore the association between EIH profiles and the effect of pharmacotherapy on rat's neuropathic pain.

METHODS

EIH profiles were assessed by evaluating paw responses to mechanical stimuli before and after exercise on a rotating rod. Rats with a reduction of ≤33% in responses were classified as low EIH and those with ≥67% as high EIH. Low and high EIH rats underwent sciatic nerve chronic constriction injury (CCI). Paw responses to mechanical stimuli were measured at baseline, following CCI, and after treatment with diclofenac, duloxetine or pregabalin. In a different group of low and high EIH rats, EIH was measured before and following treatment with the same medications.

RESULTS

Low EIH rats developed more significant hypersensitivity following CCI. Duloxetine and pregabalin successfully reduced hypersensitivity, although significantly more so in low EIH rats. Diclofenac had limited effects, and only on low EIH rats. Four days of duloxetine administration transformed low EIH rats' profiles to high EIH.

CONCLUSIONS

The findings of this study suggest that EIH profiles in rats can not only predict the development of hypersensitivity following injury but may also support targeted pharmacological treatment.

SIGNIFICANCE

Exercise is a known trigger of the inhibitory pain modulation. Rats with deficient analgesic response following exercise develop more significant hypersensitivity following nerve injury. Pain modulation profiles in rats can also support targeted pharmacological treatment; rats with deficient analgesic response following exercise benefit more from treatment with duloxetine and gabapentin. Treatment with duloxetine can improve pain modulation profile.

Sensitivity to movement-evoked pain, central sensitivity symptoms, and pro-nociceptive profiles in people with chronic shoulder pain: A parallel-group cross-sectional investigation

OBJECTIVE

To investigate whether sensitivity to movement-evoked pain (SMEP), central sensitivity symptom burden, and quantitative sensory testing (QST) outcomes differ between healthy controls and people with chronic shoulder pain.

METHODS

People with chronic shoulder pain (n = 39) and healthy controls (n = 26) completed validated questionnaires measuring demographic, pain characteristics, psychological factors, social support, sleep quality, central sensitivity inventory (CSI), and physical activity levels. A blinded assessor administered QST measuring pressure pain threshold, temporal summation, conditioned pain modulation, and cold hyperalgesia. All participants performed repeated lifting of weighted canisters and reported severity of pain over successive lifts of the weighted canisters. Between-group differences in the QST, SMEP and CSI scores were investigated. Demographic and psychosocial variables were adjusted in the analyses.

RESULTS

Dynamic mechanical allodynia, mechanical temporal summation, movement-evoked pain scores, SMEP index, and CSI scores were significantly (p ≤ 0.05) higher in the chronic shoulder pain group than in healthy controls. A significant proportion of people with chronic shoulder pain presented with pro-nociceptive profiles and experienced higher pain severity, interference, and disability.

CONCLUSIONS

People with chronic shoulder pain displayed symptoms and signs of central sensitization. Future research should investigate the predictive role of central sensitization on clinical outcomes in shoulder pain.

Neural mechanisms underlying the conditioned pain modulation response: a narrative review of neuroimaging studies

ABSTRACT

Processing spatially distributed nociceptive information is critical for survival. The conditioned pain modulation (CPM) response has become a common psychophysical test to examine pain modulation capabilities related to spatial filtering of nociceptive information. Neuroimaging studies have been conducted to elucidate the neural mechanisms underlying the CPM response in health and chronic pain states, yet their findings have not been critically reviewed and synthesized before. This narrative review presents a simplified overview of MRI methodology in relation to CPM assessments and summarizes the findings of neuroimaging studies on the CPM response. The summary includes functional MRI studies assessing CPM responses during scanning as well as functional and structural MRI studies correlating indices with CPM responses assessed outside of the scanner. The findings are discussed in relation to the suggested mechanisms for the CPM response. A better understanding of neural mechanisms underlying spatial processing of nociceptive information could advance both pain research and clinical use of the CPM response as a marker or a treatment target.

Precision Medicine in Neuropathic Pain

Neuropathic pain is a common chronic pain condition that is caused by a lesion or disease of the somatosensory nervous system. The multitude of sensory negative and positive sensations and associated comorbidities have a major impact on quality of life of affected patients. Current treatment options often only lead to a partial pain relief or are even completely ineffective. In addition, many clinical trials for the development of new drugs have not met the primary endpoint. Therefore, there is still an unmet clinical need in neuropathic pain syndromes. One reason for this therapeutic dilemma could be the heterogeneity of neuropathic pain with a variety of pathophysiological pain mechanisms that are expressed differently in each patient regardless of the underlying disease etiology. Reclassification of neuropathic pain syndromes therefore focuses on the underlying mechanisms of pain development rather than the disease etiology. A priori stratification of patients based on these individual mechanisms could allow the identification of potential treatment responders and thus realize the concept of a mechanism-based treatment. As no biomarkers for pain mechanisms have been discovered yet, one has to rely on surrogate markers that are thought to be closely related to these mechanisms. In this chapter, we present promising predictive biomarkers, focusing in particular on sensory symptoms and signs assessed by patient-reported outcome measures and sensory testing, and discuss how these tools might be used in clinical trials in the future.

Psychological mechanisms of offset analgesia: The effect of expectancy manipulation

A frequently used paradigm to quantify endogenous pain modulation is offset analgesia, which is defined as a disproportionate large reduction in pain following a small decrease in a heat stimulus. The aim of this study was to determine whether suggestion influences the magnitude of offset analgesia in healthy participants. A total of 97 participants were randomized into three groups (hypoalgesic group, hyperalgesic group, control group). All participants received four heat stimuli (two constant trials and two offset trials) to the ventral, non-dominant forearm while they were asked to rate their perceived pain using a computerized visual analogue scale. In addition, electrodermal activity was measured during each heat stimulus. Participants in both intervention groups were given a visual and verbal suggestion about the expected pain response in an hypoalgesic and hyperalgesic manner. The control group received no suggestion. In all groups, significant offset analgesia was provoked, indicated by reduced pain ratings (p < 0.001) and enhanced electrodermal activity level (p < 0.01). A significant group difference in the magnitude of offset analgesia was found between the three groups (F[2,94] = 4.81, p < 0.05). Participants in the hyperalgesic group perceived significantly more pain than the hypoalgesic group (p = 0.031) and the control group (p < 0.05). However, the electrodermal activity data did not replicate this trend (p > 0.05). The results of this study indicate that suggestion can be effective to reduce but not increase endogenous pain modulation quantified by offset analgesia in healthy participants.

Central Sensitization and Psychological State Distinguishing Complex Regional Pain Syndrome from Other Chronic Limb Pain Conditions: A Cluster Analysis Model

Complex regional pain syndrome (CRPS) taxonomy has been updated with reported subtypes and is defined as primary pain alongside other chronic limb pain (CLP) conditions. We aimed at identifying CRPS clinical phenotypes that distinguish CRPS from other CLP conditions. Cluster analysis was carried out to classify 61 chronic CRPS and 31 CLP patients based on evoked pain (intensity of hyperalgesia and dynamic allodynia, allodynia area, and after-sensation) and psychological (depression, kinesiophobia, mental distress, and depersonalization) measures. Pro-inflammatory cytokine IL-6 and TNF-α serum levels were measured. Three cluster groups were created: ‘CRPS’ (78.7% CRPS; 6.5% CLP); ‘CLP’ (64.5% CLP; 4.9% CRPS), and ‘Mixed’ (16.4% CRPS; 29% CLP). The groups differed in all measures, predominantly in allodynia and hyperalgesia (p < 0.001, η² > 0.58). ‘CRPS’ demonstrated higher psychological and evoked pain measures vs. ‘CLP’. ‘Mixed’ exhibited similarities to ‘CRPS’ in psychological profile and to ‘CLP’ in evoked pain measures. The serum level of TNF-αwas higher in the ‘CRPS’ vs. ‘CLP’ (p < 0.001) groups. In conclusion, pain hypersensitivity reflecting nociplastic pain mechanisms and psychological state measures created different clinical phenotypes of CRPS and possible CRPS subtypes, which distinguishes them from other CLP conditions, with the pro-inflammatory TNF-α cytokine as an additional potential biomarker.

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.

Clusters of facilitatory and inhibitory conditioned pain modulation responses in a large sample of children, adolescents, and young adults with chronic pain

Supplemental Digital Content is Available in the Text.

Findings from the current study add to the literature by describing different clinical phenotypes of central pain mechanisms of youth with chronic pain.

Introduction:

When investigating the role of facilitatory and inhibitory pain mechanisms such as conditioned pain modulation (CPM) and temporal summation of pain (TSP), it is important to take both into consideration in a single experimental model to provide the most information on subgroups of patients. Therefore, the objective of this study was to identify subgroups in a large population of pediatric patients with chronic pain based on their facilitatory and inhibitory pain mechanisms and compare them with control subjects.

Methods:

Five hundred twenty-one female subjects and 147 male subjects between 8 and 21 years old underwent a CPM assessment using a 2-minute tonic noxious heat stimulation as the test stimulus and a 2-minute cold-pressor task (CPT) (12°C) as the conditioning stimulus.

Results:

The best partition of clusters of patients was 3 clusters accounting for 27.15% of the total variation in the data. Cluster 1 (n = 271) was best characterized by high pain intensity during the CPT, lack of TSP during the test stimuli, and efficient inhibitory CPM. Cluster 2 (n = 186) was best characterized by low pain intensity during the CPT, lack of TSP during the test stimuli, and efficient inhibitory CPM. Cluster 3 (n = 151) was best characterized by high pain intensity during the CPT, presence of TSP during the test stimuli, and inefficient inhibitory CPM.

Discussion:

A single thermal CPM experimental design can identify combinations of facilitatory and inhibitory pain modulation responses. Findings from the current study add to the literature by describing different clinical phenotypes of central pain mechanisms of youth with chronic pain.

Mechanism-Based Pharmacological Treatment for Chronic Non-cancer Pain in Adolescents: Current Approaches and Future Directions

Chronic non-cancer pain in pediatrics is a widespread phenomenon that affects about 20% of adolescents (10-19 years old). Although interdisciplinary pain treatment programs, which often include pharmacological treatment, have emerged as the standard of care in management of this patient population, evidence regarding an optimal treatment is lacking. The efficacy and safety profiles of pharmacological treatments used to help adolescents suffering from chronic non-cancer pain remain understudied. This lack of evidence may increase polypharmacy and the risk of drug interactions and adverse events. This review examines evidence for the use of pharmacological treatments prescribed to treat chronic pain in adolescents (10-19 years old), with a focus on mechanism-based pharmacology. The objectives of this review are to: (a) review the evidence for mechanism-based pharmacological treatments for chronic non-cancer pain in adolescents and (b) describe the pharmacological agents that are commonly prescribed to manage chronic pain in adolescents, including dosage information, mechanism, and potential adverse effects. Pharmacological treatments should be used carefully with adolescents, ideally within an interdisciplinary treatment program that will incorporate physical rehabilitation, integrative medicine/active mind-body techniques, psychology, and global efforts to normalize daily activities.

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.

Monoamine control of descending pain modulation after mild traumatic brain injury

Traumatic brain injury (TBI) is a significant public health concern, with the majority of injuries being mild. Many TBI victims experience chronic pain. Unfortunately, the mechanisms underlying pain after TBI are poorly understood. Here we examined the contribution of spinal monoamine signaling to dysfunctional descending pain modulation after TBI. For these studies we used a well-characterized concussive model of mild TBI. Measurements included mechanical allodynia, the efficacy of diffuse noxious inhibitory control (DNIC) endogenous pain control pathways and lumber norepinephrine and serotonin levels. We observed that DNIC is strongly reduced in both male and female mice after mild TBI for at least 12 weeks. In naïve mice, DNIC was mediated through α2 adrenoceptors, but sensitivity to α2 adrenoceptor agonists was reduced after TBI, and reboxetine failed to restore DNIC in these mice. The intrathecal injection of ondansetron showed that loss of DNIC was not due to excess serotonergic signaling through 5-HT₃ receptors. On the other hand, the serotonin-norepinephrine reuptake inhibitor, duloxetine and the serotonin selective reuptake inhibitor escitalopram both effectively restored DNIC after TBI in both male and female mice. Therefore, enhancing serotonergic signaling as opposed to noradrenergic signaling alone may be an effective pain treatment strategy after TBI.