Diffuse noxious inhibitory controls (DNIC). II. Lack of effect on non-convergent neurones, supraspinal involvement and theoretical implications

Circadian functional changes of pain-processing brainstem nuclei and implications for cluster headache: A 7 Tesla imaging study

BACKGROUND

Pain thresholds and primary headaches, including cluster headache attacks, have circadian rhythmicity. Thus, they might share a common neuronal mechanism.

OBJECTIVE

This study aimed to elucidate how the modulation of nociceptive input in the brainstem changes from noon to midnight. Insights into the mechanism of these fluctuations could allow for new hypotheses about the pathophysiology of cluster headache.

METHODS

This repeated measure observational study was conducted at the University Hospital Zurich from December 2019 to November 2022. Healthy adults between 18 and 85 years of age were eligible. All participants were examined at noon and midnight. We tested the pain threshold on both sides of the foreheads with quantitative sensory testing, assessed tiredness levels, and obtained high-field (7 Tesla) and high-resolution functional magnetic resonance imaging (MRI) at each visit. Functional connectivity was assessed at the two visits by performing a region-of-interest analysis. We defined nuclei in the brainstem implicated in processing nociceptive input as well as the thalamus and suprachiasmatic nucleus as the region-of-interest.

RESULTS

Ten people were enrolled, and seven participants were included. First, we did not find statistically significant differences between noon and midnight of A-delta-mediated pain thresholds (median mechanical pain threshold at noon: left 9.2, right 9.2; at night: left 6.5, right 6.1). Second, after correction for a false discovery rate, we found changes in the mechanical pain sensitivity to have a statistically significant effect on changes in the functional connectivity between the left parabrachial nucleus and the suprachiasmatic nucleus (T = -40.79).

CONCLUSION

The MRI data analysis suggested that brain stem nuclei and the hypothalamus modulate A-delta-mediated pain perception; however, these changes in pain perception did not lead to statistically significantly differing pain thresholds between noon and midnight. Hence, our findings shed doubt on our hypothesis that the physiologic circadian rhythmicity of pain thresholds could drive the circadian rhythmicity of cluster headache attacks.

μ-Opioid Receptor Activation at the Dorsal Reticular Nucleus Shifts Diffuse Noxious Inhibitory Controls to Hyperalgesia in Chronic Joint Pain in Male Rats

BACKGROUND

The dorsal reticular nucleus is a pain facilitatory area involved in diffuse noxious inhibitory control (DNIC) through opioidergic mechanisms that are poorly understood. The hypothesis was that signaling of μ-opioid receptors is altered in this area with prolonged chronic inflammatory pain and that this accounts for the loss of DNICs occurring in this condition.

METHODS

Monoarthritis was induced in male Wistar rats (n = 5 to 9/group) by tibiotarsal injection of complete Freund's adjuvant. The immunolabeling of µ-opioid receptors and the phosphorylated forms of µ-opioid receptors and cAMP response element binding protein was quantified. Pharmacologic manipulation of μ-opioid receptors at the dorsal reticular nucleus was assessed in DNIC using the Randall-Selitto test.

RESULTS

At 42 days of monoarthritis, μ-opioid receptor labeling decreased at the dorsal reticular nucleus, while its phosphorylated form and the phosphorylated cAMP response element binding protein increased. [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin acetate (DAMGO) enhanced DNIC analgesia in normal animals (means ± SD: pre-DNIC: 126.9 ± 7.0 g; DNIC - DAMGO: 147.5 ± 8.0 g vs. DNIC + DAMGO: 198.1 ± 19.3 g; P < 0.001), whereas it produced hyperalgesia in monoarthritis (pre-DNIC: 67.8 ± 7.5 g; DNIC - DAMGO: 70.6 ± 7.7 g vs. DNIC + DAMGO: 32.2 ± 2.6 g; P < 0.001). An ultra-low dose of naloxone, which prevents the excitatory signaling of the μ-opioid receptor, restored DNIC analgesia in monoarthritis (DNIC - naloxone: 60.0 ± 6.1 g vs. DNIC + naloxone: 98.0 ± 13.5 g; P < 0.001), compared to saline (DNIC - saline: 62.5 ± 5.2 g vs. DNIC + saline: 64.2 ± 3.8 g). When injected before DAMGO, it restored DNIC analgesia and decreased the phosphorylated cAMP response element binding protein in monoarthritis.

CONCLUSIONS

The dorsal reticular nucleus is likely involved in a facilitatory pathway responsible for DNIC hyperalgesia. The shift of μ-opioid receptor signaling to excitatory in this pathway likely accounts for the loss of DNIC analgesia in monoarthritis.

EDITOR’S PERSPECTIVE

Simple Yet Effective-Sterile Water Injections to Treat Labor Pain in the Transport Environment

Sterile water injections (SWI) is a nonpharmacologic pain relief option to treat back pain in labor. This case report describes and discusses the use of SWI in the context of an obstetric retrieval of a 29-year-old woman who was transferred by the Royal Flying Doctor Service South Eastern Section. It provides an overview of SWI, discusses the relevance for medical transport, and offers suggestions for medical transport professionals.

Understanding of Spinal Wide Dynamic Range Neurons and Their Modulation on Pathological Pain

The spinal dorsal horn (SDH) transmits sensory information from the periphery to the brain. Wide dynamic range (WDR) neurons within this relay site play a critical role in modulating and integrating peripheral sensory inputs, as well as the process of central sensitization during pathological pain. This group of spinal multi-receptive neurons has attracted considerable attention in pain research due to their capabilities for encoding the location and intensity of nociception. Meanwhile, transmission, processing, and modulation of incoming afferent information in WDR neurons also establish the underlying basis for investigating the integration of acupuncture and pain signals. This review aims to provide a comprehensive examination of the distinctive features of WDR neurons and their involvement in pain. Specifically, we will examine the regulation of diverse supraspinal nuclei on these neurons and analyze their potential in elucidating the mechanisms of acupuncture analgesia.

Neurophysiological oscillatory markers of hypoalgesia in conditioned pain modulation

Introduction

Conditioned pain modulation (CPM) is an experimental procedure that consists of an ongoing noxious stimulus attenuating the pain perception caused by another noxious stimulus. A combination of the CPM paradigm with concurrent electrophysiological recordings can establish whether an association exists between experimentally modified pain perception and modulations of neural oscillations.

Objectives

We aimed to characterize how CPM modifies pain perception and underlying neural oscillations. We also interrogated whether these perceptual and/or neurophysiological effects are distinct in patients affected by chronic pain.

Methods

We presented noxious electrical stimuli to the right ankle before, during, and after CPM induced by an ice pack placed on the left forearm. Seventeen patients with chronic pain and 17 control participants rated the electrical pain in each experimental condition. We used magnetoencephalography to examine the anatomy-specific effects of CPM on the neural oscillatory responses to the electrical pain.

Results

Regardless of the participant groups, CPM induced a reduction in subjective pain ratings and neural responses (beta-band [15-35 Hz] oscillations in the sensorimotor cortex) to electrical pain.

Conclusion

Our findings of pain-induced beta-band activity may be associated with top-down modulations of pain, as reported in other perceptual modalities. Therefore, the reduced beta-band responses during CPM may indicate changes in top-down pain modulations.

Harnessing the conditioned pain modulation response in migraine diagnosis, outcome prediction, and treatment-A narrative review

OBJECTIVE

To present the potential use and relevance of the conditioned pain modulation (CPM) response to migraine diagnosis, outcome prediction, and treatment.

BACKGROUND

The CPM response is a widely used laboratory test to examine inhibitory pain modulation capabilities.

METHODS

This narrative review summarizes and synthesizes the findings on the CPM response in patients with migraine.

RESULTS

For diagnosis, we summarized the studies comparing CPM responses between patients with migraine and individuals without migraine or with other headache syndromes, as well as between patients with subtypes of migraine. For prediction, we summarized the studies utilizing the CPM response to predict migraine outcome, such as response to interventions. For treatment, we described a device that utilizes the CPM response for acute and preventative migraine treatment. In addition, we suggest the requirements needed for the CPM response to be used for migraine diagnosis, outcome prediction, and treatment.

CONCLUSIONS

Although more research is needed, the CPM response could be a useful tool for improving migraine management.

Temporal stability and responsiveness of a conditioned pain modulation test

OBJECTIVES

Conditioned pain modulation is a commonly used quantitative sensory test, measuring endogenous pain control. The temporal stability of the test is questioned, and there is a lack of agreement on the effect of different pain conditions on the conditioned pain modulation response. Thus, an investigation of the temporal stability of a conditioned pain modulation test among patients suffering from persistent or recurrent neck pain is warranted. Further, an investigation into the difference between patients experiencing a clinically important improvement in pain and those not experiencing such an improvement will aid the understanding between changes in pain and the stability of the conditioned pain modulation test.

METHODS

This study is based on a randomized controlled trial investigating the effect of home stretching exercises and spinal manipulative therapy vs. home stretching exercises alone. As no difference was found between the interventions, all participants were studied as a prospective cohort in this study, investigating the temporal stability of a conditioned pain modulation test. The cohort was also divided into responders with a minimally clinically important improvement in pain and those not experiencing such an improvement.

RESULTS

Stable measurements of conditioned pain modulation were observed for all independent variables, with a mean change in individual CPM responses of 0.22 from baseline to one week with a standard deviation of 1.34, and -0.15 from the first to the second week with a standard deviation of 1.23. An Intraclass Correlation Coefficient (ICC3 - single, fixed rater) for CPM across the three time points yielded a coefficient of 0.54 (p<0.001).

CONCLUSIONS

Patients with persistent or recurrent neck pain had stable CPM responses over a 2 week course of treatment irrespective of clinical response.

Intrinsic brain connectivity alterations despite intact pain inhibition in subjects with neuropathic pain after spinal cord injury: a pilot study

Endogenous pain modulation in humans is frequently investigated with conditioned pain modulation (CPM). Deficient pain inhibition is a proposed mechanism that contributes to neuropathic pain (NP) after spinal cord injury (SCI). Recent studies have combined CPM testing and neuroimaging to reveal neural correlates of CPM efficiency in chronic pain. This study investigated differences in CPM efficiency in relation to resting-state functional connectivity (rsFC) between 12 SCI-NP subjects and 13 age- and sex-matched healthy controls (HC). Twelve and 11 SCI-NP subjects were included in psychophysical and rsFC analyses, respectively. All HC were included in the final analyses. Psychophysical readouts were analysed to determine CPM efficiency within and between cohorts. Group differences of rsFC, in relation to CPM efficiency, were explored with seed-to-voxel rsFC analyses with pain modulatory regions, e.g. ventrolateral periaqueductal gray (vlPAG) and amygdala. Overall, pain inhibition was not deficient in SCI-NP subjects and was greater in those with more intense NP. Greater pain inhibition was associated with weaker rsFC between the vlPAG and amygdala with the visual and frontal cortex, respectively, in SCI-NP subjects but with stronger rsFC in HC. Taken together, SCI-NP subjects present with intact pain inhibition, but can be differentiated from HC by an inverse relationship between CPM efficiency and intrinsic connectivity of supraspinal regions. Future studies with larger cohorts are necessary to consolidate the findings in this study.

Quantitative sensory testing as an assessment tool to predict the response to standard pain treatment in knee osteoarthritis: a systematic review and meta-analysis

Emerging evidence suggest that quantitative sensory testing (QST) may predict the treatment response to pain-relieving therapies. This systematic review and meta-analysis focus on the predictive value of QST for pain management of knee osteoarthritis (OA). MEDLINE and EMBASE were systematically searched for all studies from year 2000 to 2023 on pretreatment QST and treatment of OA including surgical, pharmaceutical, and nonsurgical and nonpharmaceutical therapies. Preclinical studies and reviews were excluded. The systematic review followed the PRISMA guidelines and was pre-registered on the Open Science Framework website (link: https://osf.io/4FETK/, Identifier: DOI 10.17605/OSF.IO/4FETK). Meta-analysis were conducted to demonstrate the strength of the pre-treatment QST predictions on pain outcomes after OA treatments. Sixteen surgical (all on total knee arthroplasty [TKA], N = 1967), 5 pharmaceutical (4 on non-steroidal anti-inflammatory drugs [NSAIDs], N = 271), and 4 exercise-based therapy studies (N = 232) were identified. Pretreatment QST parameters predicted pain-relieving treatment outcomes in 81% of surgical, 100% of pharmaceutical, and 50% of exercise-based therapy studies. Meta-analyses found pretreatment QST profiles to predicted pain outcomes after TKA (random effects: 0.309, 95% confidence interval [CI]: 0.206-0.405, P < 0.001), NSAIDs (random effects: 0.323, 95% CI: 0.194-0.441, P < 0.001), and exercise-based therapies (random effects: 0.417, 95% CI: 0.138-0.635, P = 0.004). The overall risk of bias for the included studies was low to moderate. This systematic review and meta-analysis demonstrate weak-to-moderate associations between pretreatment QST and pain outcomes after standard OA pain treatments. Based on this work, it is hypothesized that a subset of specific pain sensitive patients with OA exist and that these patients do not respond adequately to standard OA pain treatments.

Neurons in the caudal ventrolateral medulla mediate descending pain control

Supraspinal brain regions modify nociceptive signals in response to various stressors including stimuli that elevate pain thresholds. The medulla oblongata has previously been implicated in this type of pain control, but the neurons and molecular circuits involved have remained elusive. Here we identify catecholaminergic neurons in the caudal ventrolateral medulla that are activated by noxious stimuli in mice. Upon activation, these neurons produce bilateral feed-forward inhibition that attenuates nociceptive responses through a pathway involving the locus coeruleus and norepinephrine in the spinal cord. This pathway is sufficient to attenuate injury-induced heat allodynia and is required for counter-stimulus induced analgesia to noxious heat. Our findings define a component of the pain modulatory system that regulates nociceptive responses.

Post-surgical contributors to persistent knee pain following knee replacement: The Multicenter Osteoarthritis Study (MOST)

Objective

Pain persistence following knee replacement (KR) occurs in ∼20-30% of patients. Although several studies have identified preoperative risk factors for persistent post-KR pain, few have focused on post-KR contributing factors. We sought to determine whether altered nociceptive signaling and other peripheral nociceptive drivers present post-operatively contribute to post-KR pain.

Design

We included participants from the Multicenter Osteoarthritis Study who were evaluated ∼12 months after KR. We evaluated the relation of measures of pain sensitivity [pressure pain threshold (PPT), temporal summation (TS), and conditioned pain modulation (CPM)] and the number of painful body sites to post-KR WOMAC knee pain, and of the number of painful sites to altered nociceptive signaling using linear or logistic regression models, as appropriate.

Results

171 participants (mean age 69 years, 62% female) were included. TS was associated with worse WOMAC pain post-KR (β ​= ​0.77 95% CI:0.19-1.35) and reduced odds of achieving patient acceptable symptom state (aOR ​= ​0.54 95%CI:0.34-0.88). Inefficient CPM was also associated with worse WOMAC pain post-KR (β ​= ​1.43 95% CI:0.15-2.71). In contrast, PPT was not associated with these outcomes. The number of painful body sites present post-KR was associated with TS (β ​= ​0.05, 95% CI:0.01, 0.05).

Conclusions

Post-KR presence of central sensitization and inefficient descending pain modulation was associated with post-KR pain. We also noted that presence of other painful body sites contributes to altered nociceptive signaling, and this may thus also contribute to the experience of knee pain post-KR. Our findings provide novel insights into central pain mechanisms and other peripheral pain sources contributing to post-KR persistent knee pain.

Optimizing and Accelerating the Development of Precision Pain Treatments for Chronic Pain: IMMPACT Review and Recommendations

Large variability in the individual response to even the most-efficacious pain treatments is observed clinically, which has led to calls for a more personalized, tailored approach to treating patients with pain (ie, "precision pain medicine"). Precision pain medicine, currently an aspirational goal, would consist of empirically based algorithms that determine the optimal treatments, or treatment combinations, for specific patients (ie, targeting the right treatment, in the right dose, to the right patient, at the right time). Answering this question of "what works for whom" will certainly improve the clinical care of patients with pain. It may also support the success of novel drug development in pain, making it easier to identify novel treatments that work for certain patients and more accurately identify the magnitude of the treatment effect for those subgroups. Significant preliminary work has been done in this area, and analgesic trials are beginning to utilize precision pain medicine approaches such as stratified allocation on the basis of prespecified patient phenotypes using assessment methodologies such as quantitative sensory testing. Current major challenges within the field include: 1) identifying optimal measurement approaches to assessing patient characteristics that are most robustly and consistently predictive of inter-patient variation in specific analgesic treatment outcomes, 2) designing clinical trials that can identify treatment-by-phenotype interactions, and 3) selecting the most promising therapeutics to be tested in this way. This review surveys the current state of precision pain medicine, with a focus on drug treatments (which have been most-studied in a precision pain medicine context). It further presents a set of evidence-based recommendations for accelerating the application of precision pain methods in chronic pain research. PERSPECTIVE: Given the considerable variability in treatment outcomes for chronic pain, progress in precision pain treatment is critical for the field. An array of phenotypes and mechanisms contribute to chronic pain; this review summarizes current knowledge regarding which treatments are most effective for patients with specific biopsychosocial characteristics.

A web app-based music intervention reduces experimental thermal pain: A randomized trial on preferred versus least-liked music style

Digital technologies are increasingly being used to strengthen national health systems. Music is used as a management technique for pain. The objective of this study is to demonstrate the effects of a web app-based music intervention on pain. The participants were healthy adults and underwent three conditions: Conditioned Pain Modulation (CPM), Most-Liked Music (MLM) and Least-Liked Music (LLM). The music used is MUSIC CARE©, a web app-based personalized musical intervention ("U" Sequence based on a musical composition algorithm). Thermal pain was measured before starting the 20-min music intervention and after three time points for each music condition: 2.20, 11.30, and 20 min. Mean pain perceptions were significantly reduced under both LLM and MLM conditions. Pain decrease was more important under MLM condition than LLM condition at 2.20 min with a mean difference between both conditions of 9.7 (±3.9) (p = 0.0195) and at 11.30 min [9.2 (±3.3), p = 0.0099]. LLM is correlated with CPM but not MLM, suggesting different mechanisms between LLM and MLM. Musical intervention, a simple method of application, fits perfectly into a multidisciplinary global approach and helps to treat the pain and anxiety disorders of participants. Clinical trial registration: [https://clinicaltrials.gov/ct2/show/NCT04862832], ClinicalTrials.gov [NCT04862832].

The Effect of Pre-Electroacupuncture on Nociceptive Discharges of Spinal Wide Dynamic Range Neurons in Rat

Purpose

Spinal wide dynamic range (WDR) neurons are well studied in pain models and they play critical roles in regulating nociception. Evidence has started to accumulate that acupuncture produces a good analgesic effect via activating different primary fibers with distinct intensities. The purpose of the present study was to compare the distinct intensities of pre-electroacupuncture (pre-EA) at local muscular receptive fields (RFs), adjacent or contralateral non-RFs regulating the nociceptive discharges of spinal WDR neurons evoked by hypertonic saline (HS).

Materials and Methods

Spinal segments of electrophysiological recording were identified by neural tracers applied at the left gastrocnemius muscle. The thresholds of Aβ (T_Aβ), Aδ (T_Aδ) and C (T_C) components of WDR neurons were measured to determine the intensity of pre-EA by extracellular recording. The discharges of WDR neurons induced by distinct intensities of pre-EA and 200 µL HS (6%) injection in left gastrocnemius muscle of rats were observed by extracellular recording.

Results

The spinal segments of WDR neurons were confirmed in lumbar (L)5-6 area according to the projective segments of dorsal root ganglion. T_Aβ, T_Aδ and T_C of WDR neurons was determined to be 0.5, 1, and 2 mA, respectively. The pre-EA with intensities of T_Aβ (P < 0.05), T_Aδ (P < 0.05), T_C (P < 0.05) or 2T_C (P < 0.01) at ipsilateral adjacent non-RFs significantly reduced the discharges of WDR neurons, while at local RFs only pre-EA of T_Aδ (P < 0.05), T_C (P < 0.05) and 2T_C (P < 0.01) could inhibit the nociceptive discharges. In addition, intensity of pre-EA at contralateral non-RFs should reach at least T_C to effectively inhibit the firing rates of WDR neurons (P < 0.01).

Conclusion

Pre-EA could suppress nociceptive discharges of WDR neurons and the inhibitory effects were dependent on the distinct intensities and locations of stimulation.

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.

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.

Monoaminergic mediation of hyperalgesic and analgesic descending control of nociception in mice

ABSTRACT

Descending control of nociception (DCN; also known as conditioned pain modulation [CPM], the behavioral correlate of diffuse noxious inhibitory controls) is the phenomenon whereby pain inhibits pain in another part of the body and is the subject of increasing study because it may represent a biomarker of chronic pain. We recently discovered that pain modulation upon application of a DCN paradigm involving low-intensity test stimuli occurs in the direction of hyperalgesia in healthy mice and rats, whereas the use of high-intensity stimuli produces analgesia. To elucidate the physiological mechanisms underlying hyperalgesic DCN, we administered agonists and antagonists of norepinephrine (NE) and serotonin (5-HT) receptors, key neurochemical players in the production of analgesic DCN. We find that three different monoamine reuptake inhibitors-the NE-selective reboxetine, the 5-HT-selective fluoxetine, and the dual NE/5-HT agonist duloxetine-all abolish hyperalgesic DCN when administered into the spinal cord (but not systemically), with no effect on heat or mechanical pain sensitivity. Reboxetine's attenuation of hyperalgesic DCN is mediated by α 2 -adrenergic receptors (i.e., blocked by atipamezole), and fluoxetine's effect is mediated by 5-HT 7 receptors (i.e., blocked by SB269970). In contrast, analgesic DCN was found to be reversed by atipamezole and SB269970 themselves, with no effect of reboxetine or fluoxetine. Thus, hyperalgesic DCN appears to be the neurochemical opposite to analgesic DCN. These data further validate and help elucidate a pre-clinical paradigm that mimics dysfunctional CPM, and thus may form the basis of translational experiments that aim to reveal preventative pharmacological strategies for individuals predisposed to persistent pain.

Blood flow restriction training promotes functional recovery of knee joint in patients after arthroscopic partial meniscectomy: A randomized clinical trial

Purpose: To explore the effect of blood flow restriction training (BFRT) on the recovery of knee function in patients after arthroscopic partial meniscectomy (APM).

Methods: Forty patients undergoing APM surgery were included in this parallel group, two-arm, single-assessor blinded, randomized clinical trial. The subjects were randomly divided into two groups: routine rehabilitation group (RR Group, n = 20) and routine rehabilitation + blood flow restriction training group (RR + BFRT Group, n = 20). One subject in each group dropped out during the experiment. All patients received 8 weeks of routine rehabilitation starting from the second day after APM. In addition, patients in the RR + BFRT group required additional BFRT twice a week. Visual analogue scale (VAS) score, range of motion (ROM), one-leg standing test (OLST) score, Lysholm knee score, quadriceps muscle strength, quadriceps thickness, and thigh circumference were evaluated at preoperative, postoperative, 4 and 8 weeks after surgery. SPSS 25.0 software was used for statistical analysis of the data. Repeated measures ANOVA was used if the data were normally distributed and had homogeneity of variance. Generalized estimating equations were chosen if the data were not normally distributed or had homogeneity of variance.

Results: There were no significant differences in VAS score, ROM, OLST score, Lysholm knee score, quadriceps muscle strength, quadriceps thickness, and thigh circumference between the two groups before surgery (p > 0.05). Compared with postoperative, VAS score, ROM, OLST score, Lysholm knee score, and thigh circumference were significantly improved in the RR group (p < 0.05), while quadriceps muscle strength and quadriceps thickness were not significantly enhanced at 8 weeks postoperatively (p > 0.05). However, VAS score, ROM, OLST score, Lysholm knee score, quadriceps muscle strength, quadriceps thickness, and thigh circumference were all significantly improved in the RR + BFRT group at 8 weeks postoperatively (p < 0.05). Furthermore, compared with the RR group, VAS score (50% vs. 86%), ROM (7.9% vs. 16.0%), OLST score (57.3% vs. 130.1%), Lysholm knee score (38.4% vs. 55.7%), relative peak torque (11.0% vs. 84.7%), mean power (20.6% vs. 88.1%), rectus femoris thickness (0.40% vs. 13.0%), vastus medialis (0.29% vs. 5.32%), vastus lateralis (0% vs. 6.2%), vastus internus (0% vs. 5.8%), and thigh circumference (2.7% vs. 5.8%) in the RR + BFRT group were significantly improved at 4 and 8 weeks postoperatively (p < 0.05).

Conclusion: BFRT combined with routine rehabilitation training can better promote the recovery of knee joint function in patients after APM, especially the improvement of quadriceps muscle strength and thickness.

Inhibition of current perception thresholds in A-delta and C fibers through somatosensory stimulation of the body surface

Somatosensory stimulation of the body surface, such as through tactile and noxious stimulation, is widely known to inhibit pain. However, no studies have measured the threshold changes due to somatosensory stimulation of each nerve fiber (Aβ, Aδ, and C) separately. We examined the changes in the current perception thresholds of Aδ, C, and Aβ fibers induced by non-noxious and noxious somatosensory stimulation of the body surface. The current stimuli were sinusoidal waves at frequencies of 2000 Hz, 250 Hz, and 5 Hz, which selectively stimulated the Aβ, Aδ, and C fibers, respectively. In the case of non-noxious stimulation, lightly rubbing the dorsal side of the forearm with a brush showed no significant physiological or clinical changes in the current perception thresholds of the Aδ, and C fibers; a significant increase was observed only in the Aβ fibers. However, applying noxious stimulation to the body surface through hand immersion in cold water increased pain thresholds in both the Aδ and C fibers, and sensory threshold of the Aβ fibers; changes in tactile thresholds were not significant. Inhibition of sensory information by nociceptive inputs may selectively suppress nociceptive stimuli.

The Current View on the Paradox of Pain in Autism Spectrum Disorders

Autism spectrum disorder (ASD) is a neurodevelopmental disorder, which affects 1 in 44 children and may cause severe disabilities. Besides socio-communicational difficulties and repetitive behaviors, ASD also presents as atypical sensorimotor function and pain reactivity. While chronic pain is a frequent co-morbidity in autism, pain management in this population is often insufficient because of difficulties in pain evaluation, worsening their prognosis and perhaps driving higher mortality rates. Previous observations have tended to oversimplify the experience of pain in autism as being insensitive to painful stimuli. Various findings in the past 15 years have challenged and complicated this dogma. However, a relatively small number of studies investigates the physiological correlates of pain reactivity in ASD. We explore the possibility that atypical pain perception in people with ASD is mediated by alterations in pain perception, transmission, expression and modulation, and through interactions between these processes. These complex interactions may account for the great variability and sometimes contradictory findings from the studies. A growing body of evidence is challenging the idea of alterations in pain processing in ASD due to a single factor, and calls for an integrative view. We propose a model of the pain cycle that includes the interplay between the molecular and neurophysiological pathways of pain processing and it conscious appraisal that may interfere with pain reactivity and coping in autism. The role of social factors in pain-induced response is also discussed. Pain assessment in clinical care is mostly based on subjective rather than objective measures. This review clarifies the strong need for a consistent methodology, and describes innovative tools to cope with the heterogeneity of pain expression in ASD, enabling individualized assessment. Multiple measures, including self-reporting, informant reporting, clinician-assessed, and purely physiological metrics may provide more consistent results. An integrative view on the regulation of the pain cycle offers a more robust framework to characterize the experience of pain in autism.

Conditioned pain modulation and offset analgesia: Influence of sex, sex hormone levels and menstrual cycle on the magnitude and retest reliability in healthy participants

BACKGROUND

Conditioned Pain Modulation (CPM) and offset analgesia quantify impairment of endogenous pain modulation, but magnitude and reliability vary broadly between studies, indicating influencing factors that are not currently controlled for. The aim of this study was to quantify magnitude and retest reliability of CPM and offset analgesia in healthy participants, whilst investigating the influence of sex and sex hormone levels.

METHODS

62 participants (30 female) completed the study. We tested CPM (heat-cold paradigm) and offset analgesia on six days within two menstrual cycles (tests were performed in each phase of two subsequent menstrual cycles, with similar time-points for men).

RESULTS

Median offset effect was -29.4% in female and -22.5% in male participants (as change from initial stimulus). Median early CPM effects were -16.7% for women versus -13.3% for men. Reliability (intra-class correlation coefficient (ICC)) was similar between the main measures, offset effect (female: 0.48, male: 0.47) and early CPM effect (female: 0.49, male: 0.43). There was significant variance between individual experimental parameters within protocols but not between sexes or menstrual phases. While oestradiol and progesterone did not correlate with the magnitude of effect within sexes, we found that testosterone levels explained an estimated 5-10% of variance within individual responses in all sexes.

CONCLUSIONS

Our results show that the reliability of both CPM effect and offset analgesia were independent of sex and menstrual cycle phase. The magnitude of CPM and offset effects were weakly influenced by sex and testosterone levels, indicating an area for future research, rather than clinical significance.

SIGNIFICANCE STATEMENT

This study investigated CPM and offset analgesia in parallel, across sexes and during two menstrual cycles while assessing the impact of sex hormones. Reliability seems to depend on experimental parameters rather than participant characteristics, while the magnitude of effect could be weakly linked to sex hormones and sex.

Virtual Reality and the Mediation of Acute and Chronic Pain in Adult and Pediatric Populations: Research Developments

The use of virtual reality (VR) in the mediation of acute pain in adults has shown real benefit to patients for the past 20 years. This review of the literature provides a descriptive synthesis of the types of VR technology, the mechanisms by which VR mediates pain, and a history of early research in the area. A review of the use of VR to mediate chronic pain in adults, and both acute and chronic pain in pediatric populations follows. The studies reviewed provide mixed results and it is noted that many studies have small sample sizes, are case studies, and do not control for extraneous variables such as the dosage and type of VR technology used. Although VR is an exciting area of inquiry that promises to yield multiple applications, there is a necessity to conduct larger random controlled trials to better understand the use cases for which VR is most effective.

Family History of Alcohol Use Disorder as a Predictor of Endogenous Pain Modulation Among Moderate to Heavy Drinkers

Family history of alcohol use disorder (AUD) is frequently endorsed by persons with chronic pain. Although individuals with a family history of AUD have demonstrated enhanced sensitivity to painful stimulation, previous research has not examined endogenous pain modulation in this population. The goal of this study was to test family history of AUD as a predictor of conditioned pain modulation, offset analgesia, and temporal summation among a sample of moderate and heavy drinkers. Adults with no current pain (N = 235; 58.3% male; M_age = 34.3; 91.9% non-Hispanic; 60% white) were evaluated for family history of AUD at baseline and pain modulatory outcomes were assessed via quantitative sensory testing. Participants with a family history of AUD (relative to those without) evinced a pro-nociceptive pain modulation profile in response to experimental pain. Specifically, family history of AUD was associated with deficits in pain-inhibitory processes. Approximately 4% of the variance in endogenous pain modulation was accounted for by family history, and exploratory analyses suggested these effects may be driven by paternal AUD. PERSPECTIVE: The current findings suggest individuals with a family history of AUD demonstrate pain modulatory function that may predispose them to the development of chronic pain. Clinically, these data may inform pain management approaches for individuals with a family history of AUD.

Impaired conditioned pain modulation was restored after a single exercise session in individuals with and without fibromyalgia

Supplemental Digital Content is Available in the Text.

Submaximal isometric exercise improved impaired conditioned pain modulation acutely in individuals with and without fibromyalgia, regardless of health status.

Introduction:

Exercise is an effective nonpharmacological intervention for individuals with fibromyalgia syndrome (FMS); however, considerable variability is observed in their pain response after a single exercise session that could be due to differences in baseline central pain inhibition (ie, conditioned pain modulation [CPM]).

Objectives:

This study examined the effect of isometric exercise on CPM in people with FMS and control participants. A subaim was to identify whether pain inhibition after exercise was due to differences in baseline CPM.

Methods:

Twenty-one individuals with FMS (50.5 ± 14.9 years) and 22 age-matched and sex-matched controls (49.2 ± 13.3 years) participated in a familiarization session and 2 randomized experimental sessions: (1) low-intensity isometric exercise and (2) quiet rest control. Conditioned pain modulation was measured before and after each experimental session. In addition, body composition and physical activity levels were collected to determine potential group differences.

Results:

Both groups had comparable body composition and physical activity levels and reported similar exercise-induced hypoalgesia (increase in pressure pain thresholds) at the exercising muscle (quadriceps muscle) and systemically (deltoid muscle). Both groups had a decrease in CPM after exercise and quiet rest; however, in both FMS and control participants with impaired baseline CPM, there was an increase in CPM at the deltoid muscle after exercise.

Conclusion:

In persons with low CPM, irrespective of health status, isometric exercise enhanced CPM at a site distal from the exercising muscle. Our results support the use of isometric exercise when initiating an exercise program especially for individuals with impaired CPM.

Sterile water injections for management of renal colic pain: a systematic review

BACKGROUND

Since the 1950s a small number of centres have used sterile water injections (SWI) to treat renal colic pain. We undertook this review to determine the efficacy of SWI to manage the pain of renal colic.

METHODS

We searched the electronic databases PubMed, Cochrane Central Register, CINAHL, and Scopus from database inception to 7 November 2021 for randomized controlled trials that met the inclusion criteria.

RESULTS

Six trials were included in the review (n = 894 patients). Two placebo controlled trials were included in the meta-analysis. Other trials compared SWI to Diclofenac, Morphine, or oral Paracetamol. The overall quality of the trial was low. Compared to a placebo SWI demonstrated a significant reduction in self-reported pain at 30 min (Mean difference [MD] = -4.68, 95% Confidence Interval [CI] = -5.21, -4.15. p < 0.001, I² = 0%) and at or beyond 60 min post-injection (MD = -5.34 95% CI = -5.85, -4.82, p ≤ 0.001, I² = 0%). Pain relief provided by SWI was significantly better than oral paracetamol and equivalent to Diclofenac and Morphine. No significant side-effects were attributed to SWI use in any trials.

DISCUSSION/CONCLUSION

SWI could be a suitable alternative for management of renal colic pain where alternatives such as non-steroidal anti-inflammatory and opioid drugs are either unavailable or contraindicated. However, further research is required to establish the role of SWI in renal colic pain management.

Sensory Thresholds and Peripheral Nerve Responses in Chronic Tension-Type Headache and Neuropsychological Correlation

Chronic tension-type headache (CTTH) is a common disease with no fully defined pathophysiological processes. We designed a study to value electrophysiological responses in these patients and their correlation with possible psychopathological manifestations in order to deepen understanding of central and peripheral mechanisms of CTTH. In 40 patients with CTTH and 40 healthy controls, we used electrical stimulation to determine sensory threshold (SPT) and pain perception threshold (PPT) and the characteristics of the electrophysiological sensory nerve action potential (SNAP): initial sensory response (ISR) and supramaximal response (SMR). We then calculated the intensity differences between thresholds (IDT), namely SPT-PPT, ISR-SMR and SMR-PPT, and correlated these IDTs with psychological characteristics: trait and state anxiety, depression, and emotional regulation. The SPT, together with the ISR and SMR thresholds, were higher (p < 0.01) in CTTH patients. The SMR-PPT IDT was smaller and correlated with significantly higher indicators of depression, state and trait anxiety, and poorer cognitive reappraisal. CTTH patients have less capacity to recognize non-nociceptive sensory stimuli, greater tendency toward pain facilitation, and a poor central pain control requiring higher stimulation intensity thresholds to reach the start and the peak amplitude of the SNAP. This is consistent with relative hypoexcitability of the Aβ nerve fibers in distant regions from the site of pain, and therefore, it could be considered a generalized dysfunction with a focal expression. Pain facilitation is directly associated with psychological comorbidity.

The Potential Clinical Utility of Pressure-Based vs. Heat-Based Paradigms to Measure Conditioned Pain Modulation in Healthy Individuals and Those With Chronic Pain

Conditioned pain modulation (CPM) is a physiological measure thought to reflect an individual's endogenous pain modulation system. CPM varies across individuals and provides insight into chronic pain pathophysiology. There is growing evidence that CPM may help predict individual pain treatment outcome. However, paradigm variabilities and practical issues have impeded widespread clinical adoption of CPM assessment. This study aimed to compare two CPM paradigms in people with chronic pain and healthy individuals. A total of 30 individuals (12 chronic pain, 18 healthy) underwent two CPM paradigms. The heat CPM paradigm acquired pain intensity ratings evoked by a test stimulus (TS) applied before and during the conditioning stimulus (CS). The pressure CPM paradigm acquired continuous pain intensity ratings of a gradually increasing TS, before and during CS. Pain intensity was rated from 0 (no pain) to 100 (worst pain imaginable); Pain50 is the stimulus level for a response rated 50. Heat and pressure CPM were calculated as a change in TS pain intensity ratings at Pain50, where negative CPM scores indicate pain inhibition. We also determined CPM in the pressure paradigm as change in pressure pain detection threshold (PDT). We found that in healthy individuals the CPM effect was significantly more inhibitory using the pressure paradigm than the heat paradigm. The pressure CPM effect was also significantly more inhibitory when based on changes at Pain50 than at PDT. However, in individuals with chronic pain there was no significant difference in pressure CPM compared to heat or PDT CPM. There was no significant correlation between clinical pain measures (painDETECT and Brief Pain Inventory) and paradigm type (heat vs. pressure), although heat-based CPM and painDETECT scores showed a trend. Importantly, the pressure paradigm could be administered in less time than the heat paradigm. Thus, our study indicates that in healthy individuals, interpretation of CPM findings should consider potential modality-dependent effects. However, in individuals with chronic pain, either heat or pressure paradigms can similarly be used to assess CPM. Given the practical advantages of the pressure paradigm (e.g., short test time, ease of use), we propose this approach to be well-suited for clinical adoption.

Sterile water injections for relief of labour pain (the SATURN trial): study protocol for a randomised controlled trial

Background

Up to 80% of women use some form of pharmacological analgesia during labour and birth. The side effects of pharmacological agents are often incompatible with the concurrent use of non-pharmacological pain relieving strategies, such as water immersion, ambulation and upright positioning, or may have negative effects on both the mother and fetus. Sterile water injections given into the skin of the lumbar region have been demonstrated to reduce back pain during labour. However, the injections given for back pain have no effect on abdominal contraction pain. The analgesic efficacy of sterile water injections for abdominal pain during childbirth is unknown. The injections cause an immediate, brief but significant pain that deters some women from using the procedure. This study aims to investigate the use of water injections given intradermally into the abdomen to relieve labour contraction pain. A vapocoolant spray will be applied to the skin immediately prior to the injections to reduce the injection pain.

Methods

In this pragmatic, placebo controlled trial 154 low-risk women in labour at term with a labour pain score ≥ 60 on a 100 millimitre visual analogue scale (VAS) will be randomly allocated to receive either six injections of sterile water or a sodium chloride 0.9% solution as a placebo (0.1–.0.3 ml per injection). Three injections are given along the midline from the fundus to the supra-pubis and three laterally across the supra-pubis. The primary outcome will be the difference in VAS score 30 min post injection between groups. Secondary outcomes include VAS score of the injection pain on administration, VAS score of labour pain at 60 and 90 min, maternal and neonatal birth outcomes.

Discussion

Access to effective pain relief during labour is fundamental to respectful and safe maternity care. Pharmacological analgesics should support rather than limit other non-pharmacological strategies. Sterile water injections have the potential to provide an alternative form of labour pain relief that is easy to administer in any labour and birth setting, and compatible with other non-pharmacological choices.

Trial registration

ANZCTR (ACTRN12621001036808) Date submitted: 22/06/2021. Date registered: 05/08/2021. https://www.anzctr.org.au/

Supplementary Information

The online version contains supplementary material available at 10.1186/s13063-022-06093-3.

Phenotyping Chronic Musculoskeletal Pain in Male and Female Adolescents: Psychosocial Profiles, Somatosensory Profiles and Pain Modulatory Profiles

Purpose

A major limitation in treatment outcomes for chronic pain is the heterogeneity of the population. Therefore, a personalized approach to the assessment and treatment of children and adolescents with chronic pain conditions is needed. The objective of the study was to subgroup pediatric patients with chronic MSK pain that will be phenotypically different from each other based on their psychosocial profile, somatosensory function, and pain modulation.

Patients and Methods

This observational cohort study recruited 302 adolescents (10–18 years) with chronic musculoskeletal pain and 80 age-matched controls. After validated self-report questionnaires on psychosocial factors were completed, quantitative sensory tests (QST) and conditioned pain modulation (CPM) were performed.

Results

Three psychosocial subgroups were identified: adaptive pain (n=125), high pain dysfunctional (n=115), high somatic symptoms (n=62). Based on QST, four somatosensory profiles were observed: normal QST (n=155), thermal hyperalgesia (n=98), mechanical hyperalgesia (n=34) and sensory loss (n=15). Based on CPM and temporal summation of pain (TSP), four distinct groups were formed, dysfunctional central processing group (n=27) had suboptimal CPM and present TSP, dysfunctional inhibition group (n=136) had suboptimal CPM and absent TSP, facilitation group (n=18) had optimal CPM and present TSP, and functional central processing (n=112) had optimal CPM and absent TSP. A significant association between the psychosocial and somatosensory profiles. However, no association was observed between the psychosocial or somatosensory profiles and pain modulatory profiles.

Conclusion

Our results provide evidence that adolescents with chronic musculoskeletal pain are a heterogenous population comprising subgroups that may reflect distinct mechanisms and may benefit from different treatment approaches. The combination of screening self-reported questionnaires, QST, and CPM facilitate subgrouping of adolescents with chronic MSK pain in the clinical context and may ultimately contribute to personalized therapy.

Bodily Illusions and Motor Imagery in Fibromyalgia

Fibromyalgia (FM) is characterised by chronic, continuous, widespread pain, often associated with a sense of fatigue, non-restorative sleep and physical exhaustion. Due to the nature of this condition and the absence of other neurological issues potentially able to induce disorders in body representations per se, it represents a perfect model since it provides an opportunity to study the relationship between pain and the bodily self. Corporeal illusions were investigated in 60 participants with or without a diagnosis of FM by means of an ad hoc devised interview. In addition, motor imagery was investigated and illusions relating to body part movements and changes in body size, feelings of alienness, and sensations of body parts not belonging to one’s own body (disownership and somatoparaphrenic-like sensations) were found. Crucially, these symptoms do not correlate with any of the clinical measures of pain or functional deficits. The results showed that motor imagery was also impaired, and the severity of the deficits found correlated with the functional impairment of the participant. This indicates that disorders in body representations and motor imagery are part of the clinical expression of FM. However, while motor imagery seems to be linked to reduced autonomy and functional deficits, bodily illusions are independent and potentially represent a concurrent symptom.

Ultrastructural Evidence for Oxytocin and Oxytocin Receptor at the Spinal Dorsal Horn: Mechanism of Nociception Modulation

Oxytocin is a hypothalamic neuropeptide involved in the inhibition of nociception transmission at spinal dorsal horn (SDH) level (the first station where the incoming peripheral signals is modulated). Electrophysiological, behavioral, and pharmacological data strongly support the role of this neuropeptide and its receptor (the oxytocin receptor, OTR) as a key endogenous molecule with analgesic properties. Briefly, current data showed that oxytocin release from the hypothalamus induces OTR activation at the SDH, inducing selective inhibition of the nociceptive Aδ- and C-fibers (probably peptidergic) activity, but not the activity of proprioceptive fibers (i.e. Aβ-fibers). The above inhibition could be a direct presynaptic mechanism, or a mechanism mediated by GABAergic interneurons. However, the exact anatomical localization of oxytocin and OTR remains unclear. In this context, the present study set out to analyze the role of OTRs, GABAergic cells and CGRP fibers in the SDH in rats by using electron microscopy. Ultrastructural analyses of the SDH tissue show that: (i) oxytocin and OTR are found in asymmetrical synapsis; (ii) OTR is found in GABAergic interneurons (near unmyelinated fibers), CGRPergic fibers and glial cells; (iii) whereas oxytocin is present in supraspinal descending projection fibers. These anatomical data strongly support the notion that oxytocin released at the SDH could presynaptically inhibit the nociceptive input from the peripheral primary afferent fibers. This inhibitory action could be direct or use a GABA interneuron. Furthermore, our findings that OTR is exhibited in glial tissue at the SDH requires further exploration in nociception assays.

Analysis and comparison of pain pressure threshold and active cervical range of motion after superficial and deep dry needling techniques of the upper trapezius muscle

OBJECTIVES

To evaluate the changes in pain pressure threshold (PPT) and active cervical range of motion (ACROM) after the application of superficial dry needling (DN) or deep DN in myofascial trigger point (MTrP) 1 of the upper trapezius versus a simulated DN technique in the gastrocnemius muscle (control group).

DESIGN

Double-blind, randomized controlled trial with 7-day follow-up.

PARTICIPANTS

Asymptomatic volunteers (n = 180; 76 men, 104 women) with a latent MTrP 1 in the upper trapezius were randomly divided into three groups: G1, receiving superficial DN in the upper trapezius; G2, receiving deep DN in the upper trapezius; and G3, control group, receiving simulated DN technique in the gastrocnemius muscle.

MAIN OUTCOME MEASURES

While sitting in a chair, each subject underwent measurements of PPT and ACROM (ipsilateral and contralateral side flexion and rotation, flexion and extension) preintervention, (immediately) postintervention, and at 24 h, 72 h and 7 days.

RESULTS

Superficial and deep DN produced an increase in PPT at 7 days with respect to preintervention levels. Furthermore, superficial and deep DN produced a decrease in cervical flexion at 24 h and an increase in ipsilateral rotation until 72 h, increasing to 7 days in the case of deep DN. On the contrary, superficial DN produced an increase in ipsilateral and contralateral side flexion after intervention, unlike deep DN that produced a decrease at 24 h. Furthermore, superficial DN produced an increase in contralateral rotation at 24 h and deep DN decreased extension at 72 h.

CONCLUSION

A single intervention of superficial or deep DN did not produce statistically significant changes in PPT or goniometry measurements.Trial registration number: NCT03719352 (ClinicalTrials.gov).

Endogenous Modulation of Pain: The Role of Exercise, Stress, and Cognitions in Humans

BACKGROUND

Pain is a complex and highly subjective phenomenon that can be modulated by several factors. On the basis of results from experimental and clinical studies, the existence of endogenous pain modulatory mechanisms that can increase or diminish the experience of pain is now accepted.

METHODS

In this narrative review, the pain modulatory effects of exercise, stress, and cognitions in humans are assessed.

RESULTS

Experimental studies on the effect of exercise have revealed that pain-free participants show a hypoalgesic response after exercise. However, in some patients with chronic pain, this response is reduced or even hyperalgesic in nature. These findings will be discussed from a mechanistic point of view. Stress is another modulator of the pain experience. Although acute stress may induce hypoalgesia, ongoing clinical stress has detrimental effects on pain in many patients with chronic pain conditions, which have implications for the understanding, assessment, and treatment of stress in patients with pain. Finally, cognitive strategies play differing roles in pain inhibition. Two intuitive strategies, thought suppression and focused distraction, will be reviewed as regards experimental, acute, and chronic pain.

CONCLUSION

On the basis of current knowledge on the role of exercise, stress, and cognitive pain control strategies on the modulation of pain, implications for treatment will be discussed.

Conditioned Pain Modulation: Comparison of the Effects on Nociceptive and Non-nociceptive Blink Reflex

Although conditioned pain modulation (CPM) is considered to represent descending pain inhibitory mechanisms triggered by noxious stimuli applied to a remote area, there have been no previous studies comparing CPM between pain and tactile systems. In this study, we compared CPM between the two systems objectively using blink reflexes. Intra-epidermal electrical stimulation (IES) and transcutaneous electrical stimulation (TS) were applied to the right skin area over the supraorbital foramen to evoke a nociceptive or a non-nociceptive blink reflex, respectively, in 15 healthy males. In the test session, IES or TS were applied six times and subjects reported the intensity of each stimulus on a numerical rating scale (NRS). Blink reflexes were measured using electromyography (R2). The first and second sessions were control sessions, while in the third session, the left hand was immersed in cold water at 10 °C as a conditioning stimulus. The magnitude of the R2 blink and NRS scores were compared among the sessions by 2-way ANOVA. Both the NRS score and nociceptive R2 were significantly decreased in the third session for IES, with a significant correlation between the two variables; whereas, TS-induced non-nociceptive R2 did not change among the sessions. Although the conditioning stimulus decreased the NRS score for TS, the CPM effect was significantly smaller than that for IES (p = 0.002). The present findings suggest the presence of a pain-specific CPM effect to a heterotopic noxious stimulus.

Nociception induces a differential presynaptic modulation of the synaptic efficacy of nociceptive and proprioceptive joint afferents

A previous study has indicated that during the state of central sensitization induced by the intradermic injection of capsaicin, there is a gradual facilitation of the dorsal horn neuronal responses produced by stimulation of the high-threshold articular afferents that is counteracted by a concurrent increase of descending inhibitory actions. Since these changes occurred without significantly affecting the responses produced by stimulation of the low-threshold articular afferents, it was suggested that the capsaicin-induced descending inhibition included a preferential presynaptic modulation of the synaptic efficacy of the slow conducting nociceptive joint afferents (Ramírez-Morales et al., Exp Brain Res 237:1629-1641, 2019). The present study was aimed to investigate more directly the contribution of presynaptic mechanisms in this descending control. We found that in the barbiturate anesthetized cat, stimulation of the high-threshold myelinated afferents in the posterior articular nerve (PAN) produces primary afferent hyperpolarization (PAH) in the slow conducting (25-35 m/s) and primary afferent depolarization (PAD) in the fast conducting (40-50 m/s) articular fibers. During the state of central sensitization induced by capsaicin, there is a supraspinally mediated shift of the autogenic PAH to PAD that takes place in the slow conducting fibers, basically without affecting the autogenic PAD generated in the fast conducting afferents. It is suggested that the change of presynaptic facilitation to presynaptic inhibition induced by capsaicin on the slow articular afferents is part of an homeostatic process aimed to keep the nociceptive-induced neuronal activity within manageable limits while preserving the proprioceptive information required for proper control of movement.

Immediate Effects of Acupuncture on Force and Delayed Onset of Muscle Soreness

Background: The ability to produce force is critical to several daily activities. Strategies to reduce delayed onsent muscle soreness (DOMS) and restore force-generating properties that occur in response to training and impair physical performance have been proposed. Acupuncture has been proposed to accelerate recovery and optimize muscle functioning. It was to determine the effects of acupuncture on DOMS and on the muscle force production capacity. Methods: Thirty physically active volunteers (26.3 ± 3.1 years old, 1.70 ± 0.08 m, 66.5 ± 12.6 kg) were allocated in a control (CG; n = 10), placebo (PG; n = 10), and acupuncture (AG; n = 10) groups. The groups were subjected to an exercise protocol to induce DOMS and were assessed for muscle force, threshold, and rate of perceived pain and activation of the biceps brachialis muscle before (PRE) and 20 minutes after (POST) a single acupuncture session using a mixed model analysis. Results: The threshold and rate of perceived pain were reduced in the AG (P < 0.05) but remained unchanged in the other groups (P > 0.05). Muscle activation also showed larger changes in the AG in comparison with the other groups (P < 0.05). No changes in the ability to produce force were found after acupuncture in any groups (P > 0.05). Conclusions: Acupuncture may reduce discomfort and improve muscle activation but was not effective to promote force improvement and/or restore the force-generating properties. It may be speculated that muscle disruption may have played a role and did not allow participants to restore their muscular performance. Rebec Number; RBR-8bh5k7 (www.ensaiosclinicos.gov.br/rg/RBR-8bh5k7); Clinical Trial Registry (UTN No.: U1111-1234-9553).

Hypnotic interventions in the management of chronic pain

This article describes the pain neuro-matrix and shows how hypnotic suggestions can be used directed at each part of this in order to be maximally effective. Although inducing the hypnotic state may be relatively simple, it is important to know how to utilise it effectively using suggestion and imagery. An understanding of the patient’s clinical condition and some counselling or psychological training are essential.

Responses of neurons in rostral ventromedial medulla to nociceptive stimulation of craniofacial region and tail in rats

The rostral ventromedial medulla (RVM) plays a key role in the endogenous modulation of nociceptive transmission in the central nervous system (CNS). The primary aim of this study was to examine whether the activities of RVM neurons were related to craniofacial nociceptive behaviour (jaw-motor response, JMR) as well as the tail-flick response (TF). The activities of RVM neurons and TF and JMR evoked by noxious heating of the tail or perioral skin were recorded simultaneously in lightly anaesthetized rats. Tail or perioral heating evoked the TF and JMR, and the latency of the JMR was significantly shorter (P < 0.001) than that of the TF. Of 89 neurons recorded in RVM, 40 were classified as ON-cells, 27 as OFF-cells, and 22 as NEUTRAL-cells based on their responsiveness to heating of the tail. Heating at either site caused an increase in ON-cell and decrease in OFF-cell activity before the occurrence of the TF and JMR, but did not alter the activity of NEUTRAL cells. Likewise, noxious stimulation of the temporomandibular joint had similar effects on RVM neurons. These findings reveal that the JMR is a measure of the excitability of trigeminal and spinal nociceptive circuits in the CNS, and that the JMR as well as TF can be used for studying processes related to descending modulation of pain. The findings also support the view that RVM ON- and OFF-cells play an important role in the elaboration of diverse nociceptive behaviours evoked by noxious stimulation of widely separated regions of the body.

Nonlinear increase of pain in distance-based and area-based spatial summation

ABSTRACT

When nociceptive stimulation affects a larger body area, pain increases. This effect is called spatial summation of pain (SSp). The aim of this study was to describe SSp as a function of the size or distance of a stimulated area(s) and to test how this function is shaped by the intensity and SSp test paradigm. Thirty-one healthy volunteers participated in a within-subject experiment. Participants were exposed to area-based and distanced-based SSp. For area-based SSp, electrocutaneous noxious stimuli were applied by up to 5 electrodes (5 areas) forming a line-like pattern; for distance-based SSp, the same position and lengths of stimuli were used but only 2 electrodes were stimulated. Each paradigm was repeated using pain of low, moderate, and high intensity. It was found that the pattern of pain intensity followed a logarithmic (power) rather than a linear function. The dynamics of the pain increase were significantly different across pain intensities, with more summation occurring when pain was perceived as low. Results indicated that area-based SSp is more painful than distance-based SSp when low and moderate but not when high pain intensity is induced. Presented findings have important implications for all studies in which the spatial dimension of pain is measured. When the area or separation between nociceptive stimulation increases, pain does not increase linearly and the pattern of the pain increase is a result of the interaction between intensity and the number of nociceptive sites. A power function should be considered when predicting the size of a nociceptive source.

Transcutaneous electrical nerve stimulation (TENS): towards the development of a clinic-friendly method for the evaluation of excitatory and inhibitory pain mechanisms

Background: Temporal summation and conditioned pain modulation (CPM) can be measured using a thermode and cold pressor test (CPTest). Unfortunately, these complex and expensive tools are ill-suited for routine clinical assessments.

Aims: We aimed to compare the temporal summation and CPM obtained with the thermode + CPTest paradigm to those obtained with a novel paradigm using transcutaneous electrical nerve stimulation (TENS).

Methods: We assessed temporal summation and CPM in 29 healthy participants, using two paradigms (random order): TENS, and thermode + CPTest. In the TENS paradigm, both the conditioning stimulus (CS) and the test stimulus (TS) were delivered using TENS; in the thermode + CPTest paradigm, the CS consisted of a CPTest and the TS was delivered using a thermode. We compared the average temporal summation and CPM evoked by the two paradigms.

Results: Average temporal summation was similar for both modalities (P = 0.90), and the number of participants showing temporal summation was similar in both paradigms (19 with thermode vs. 18 with TENS; P = 1.00). Average CPM response was larger following the thermode + CPTest than following the TENS (P = 0.005), and more participants showed CPM with the thermode + CPTest paradigm compared to the TENS paradigm (24 vs. 14; P = 0.01).

Conclusions: Both paradigms were roughly equivalent in the ability to evoke temporal summation (although response to one modality did not predict response to the other), but the TENS paradigm appeared to be less apt to induce a CPM response than the thermode + CPTest paradigm.

Enhanced Pronociceptive and Disrupted Antinociceptive Mechanisms in Nonspecific Chronic Neck Pain

OBJECTIVE

Evidence suggests altered pronociceptive and antinociceptive mechanisms in many chronic pain conditions. Knowledge about these mechanisms in nonspecific chronic neck pain (NSNP) would improve understanding of the causes and the design of more effective treatments. Pressure pain threshold (PPT) is often used to assess presence of altered nociceptive processing in NSNP; however, its usefulness to detect this is yet to be established. The purpose of this study was to determine the functional status of temporal summation of second pain (TSSP) and conditioned pain modulation (CPM) in NSNP and to characterize the association of both measures with PPT and clinical features of NSNP.

METHODS

Thirty-two participants with NSNP (mean [SD] age = 44 [11] years; 27 female) and 32 age- and sex-matched healthy controls were recruited. TSSP was assessed using an electrical stimulus at the dorsum of the hand, and CPM was evaluated with the Cold Pressor Test. PPT was assessed bilaterally at the neck and tibialis anterior muscles.

RESULTS

Participants with NSNP showed greater TSPP (mean difference = 0.23; 95% CI = 0.46-0.01; Cohen d = 0.51) and lower CPM (mean difference = 19.44; 95% CI = 10.42-28.46; Cohen d = 1.09). Pooled data from all participants showed lower PPTs at the neck than the tibialis anterior. However, PPT measures did not differ between groups at either location. PPT measures were not correlated with CPM and TSP.

CONCLUSION

NSNP is associated with enhanced pronociceptive and impaired antinociceptive mechanisms, which may explain long-lasting pain and failure of some treatments to resolve symptoms. However, due to the observational nature of this study, a clear cause-effect relationship cannot be established. Normal PPT values in the clinic should not be interpreted as absence of altered nociceptive processing.

IMPACT

This study fills in some gaps in knowledge. Changes in central nociceptive processing may explain persistent and recurrent symptoms in NSNP and failure of treatments to obtain long-lasting relief. Further research is required to ascertain if TSSP and CPM assessment in the clinic may help predict physical therapy treatment outcome. Whether symptomatic relief with physical therapy is mediated by an improvement in TSSP and CPM should also be explored. PPTs were unaltered in participants with NSNP despite evidence of impairment in the central pain modulatory systems. Normal PPTs should not be interpreted as evidence of unaltered central pain-related processing.

Challenges and opportunities in translational pain research - An opinion paper of the working group on translational pain research of the European pain federation (EFIC)

For decades, basic research on the underlying mechanisms of nociception has held promise to translate into efficacious treatments for patients with pain. Despite great improvement in the understanding of pain physiology and pathophysiology, translation to novel, effective treatments for acute and chronic pain has however been limited, and they remain an unmet medical need. In this opinion paper bringing together pain researchers from very different disciplines, the opportunities and challenges of translational pain research are discussed. The many factors that may prevent the successful translation of bench observations into useful and effective clinical applications are reviewed, including interspecies differences, limited validity of currently available preclinical disease models of pain, and limitations of currently used methods to assess nociception and pain in non-human and human models of pain. Many paths are explored to address these issues, including the backward translation of observations made in patients and human volunteers into new disease models that are more clinically relevant, improved generalization by taking into account age and sex differences, and the integration of psychobiology into translational pain research. Finally, it is argued that preclinical and clinical stages of developing new treatments for pain can be improved by better preclinical models of pathological pain conditions alongside revised methods to assess treatment-induced effects on nociception in human and non-human animals. Significance: For decades, basic research of the underlying mechanisms of nociception has held promise to translate into efficacious treatments for patients with pain. Despite great improvement in the understanding of pain physiology and pathophysiology, translation to novel, effective treatments for acute and chronic pain has however been limited, and they remain an unmet medical need.

Development and Validation of a Predictive Model of Pain Modulation Profile to Guide Chronic Pain Treatment: A Study Protocol

Introduction: Quantitative sensory testing is frequently used in research to assess endogenous pain modulation mechanisms, such as Temporal Summation (TS) and Conditioned Pain Modulation (CPM), reflecting excitatory and inhibitory mechanisms, respectively. Numerous studies found that a dysregulation of these mechanisms is associated with chronic pain conditions. In turn, such a patient's “profile” (increased TS and/or weakened CPM) could be used to recommend different pharmacological treatments. However, the procedure to evaluate these mechanisms is time-consuming and requires expensive equipment that is not available in the clinical setting. In this study, we aim to identify psychological, physiological and socio-demographic markers that could serve as proxies to allow healthcare professionals to identify these pain phenotypes in clinic, and consequently optimize pharmacological treatments.

Method: We aim to recruit a healthy participant cohort (n = 360) and a chronic pain patient cohort (n = 108). Independent variables will include psychological questionnaires, pain measurements, physiological measures and sociodemographic characteristics. Dependent variables will include TS and CPM, which will be measured using quantitative sensory testing in a single session. We will evaluate one prediction model and two validation models (for healthy and chronic pain participants) using multiple regression analysis between TS/CPM and our independent variables. The significance thresholds will be set at p = 0.05, respectively.

Perspectives: This study will allow us to develop a predictive model to compute the pain modulation profile of individual patients based on their biopsychosocial characteristics. The development of the predictive model is the first step toward the overarching goal of providing clinicians with a set of quick and cheap tests, easily applicable in clinical practice to orient pharmacological treatments.

Patient phenotyping in clinical trials of chronic pain treatments: IMMPACT recommendations

There is tremendous interpatient variability in the response to analgesic therapy (even for efficacious treatments), which can be the source of great frustration in clinical practice. This has led to calls for “precision medicine” or personalized pain therapeutics (ie, empirically based algorithms that determine the optimal treatments, or treatment combinations, for individual patients) that would presumably improve both the clinical care of patients with pain and the success rates for putative analgesic drugs in phase 2 and 3 clinical trials. However, before implementing this approach, the characteristics of individual patients or subgroups of patients that increase or decrease the response to a specific treatment need to be identified. The challenge is to identify the measurable phenotypic characteristics of patients that are most predictive of individual variation in analgesic treatment outcomes, and the measurement tools that are best suited to evaluate these characteristics. In this article, we present evidence on the most promising of these phenotypic characteristics for use in future research, including psychosocial factors, symptom characteristics, sleep patterns, responses to noxious stimulation, endogenous pain-modulatory processes, and response to pharmacologic challenge. We provide evidence-based recommendations for core phenotyping domains and recommend measures of each domain.

Conditioned pain modulation-A comprehensive review

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