Conditioned pain modulation in patients with nonspecific chronic back pain with chronic local pain, chronic widespread pain, and fibromyalgia

Widespread pain phenotypes impact treatment efficacy results in randomized clinical trials for interstitial cystitis/bladder pain syndrome: a Multidisciplinary Approach to the Study of Chronic Pelvic Pain network study

ABSTRACT

Pain clinical trials are notoriously complex and often inefficient in demonstrating efficacy, even for known efficacious treatments. A major issue is the difficulty in the a priori identification of specific phenotypes to include in the study population. Recent work has identified the extent of widespread pain as an important determinant of the likelihood of response to therapy, but it has not been tested in clinical trials for the treatment of interstitial cystitis/bladder pain syndrome (IC/BPS). We explored this hypothesis using data from 3 previously published trials testing treatments for IC/BPS, which suggested modest benefits but did not meet a priori primary outcome statistical significance criteria. Importantly, these studies also collected symptom questionnaire data that allowed us to retrospectively identify participants with and without widespread pain. Analyzing the treatment by the degree of widespread pain revealed a difference in outcome and statistical significance level for each trial. Participants with predominately local pain (ie, limited widespread pain symptoms) responded to therapy targeting local symptoms, whereas those with widespread pain did not. Alternatively, participants with widespread pain beyond their local pelvic pain responded to more centrally acting treatments. Our results suggest that differentiating patients based on widespread vs more localized pain is a key consideration for designing future clinical trials for conditions with variable pain profiles, such as IC/BPS and potentially other pain-based syndromic disorders.

Quantitative Sensory Testing in Fibromyalgia Syndrome: A Scoping Review

Background/Objectives: Quantitative sensory testing (QST) is one of the most reliable methods for assessing Fibromyalgia Syndrome (FMS). Despite its importance, there are still controversies regarding the correct interpretation of evoked responses, as they may vary depending on the protocol, individual characteristics, disease severity, and other factors. This study aims to examine how QST has been applied as an outcome measure in FMS. Methods: We considered three databases (Medline, Embase, and Web of Science) until June 2024. From a total of 2512 studies, 126 (39 RCTs and 87 non-RCTs) were selected for full reading after assessment for risk of bias and eligibility criteria. These criteria included at least one type of QST and a clear diagnosis of fibromyalgia (FMS). Results: The results highlighted a lack of standardization in QST, as no reported protocols were followed and there was no specific number of tender points tested for FMS. Additionally, there was inconsistency in the selection of sites and types of tests conducted. Conclusions: This heterogeneity in methodology may affect the comparability and interpretation of results, underscoring the urgent need for standardized guidelines for conducting QST in fibromyalgia studies. A clear understanding of how QST has been measured could prompt a reevaluation of current approaches to FMS assessment, leading to more accurate interpretations and, ultimately, improved management of this complex condition.

Exploring the value of a well-established conditioned pain modulation paradigm in women: a Translational Research in Pelvic Pain (TRiPP) study

Background

Conditioned pain modulation (CPM) is considered a human proxy for descending inhibitory pain pathways. However, there is wide variation in the CPM response described in the literature and ongoing debate about its utility.

Methods

Here we explored CPM in women with (n = 59) and without (n = 26) chronic pelvic pain (CPP), aiming to determine the magnitude of effect and factors influencing variability in the CPM response.

Results

Using a pressure pain threshold test stimulus and ischaemic pressure cuff conditioning stimulus (CS), we found no significant difference in the mean CPM effect between CPP and control participants. Using a robust statistical method (+/-2 standard error of measurement) to further investigate CPM, there was no significant difference in the proportion exhibiting inhibition between controls and CPP participants (X2  = 0.003, p = 0.96). Notably, only 23.1% of our healthy controls demonstrated a "true" CPM effect (n = 4 inhibitory, n = 2 facilitatory). Despite a rich data set, we were unable to identify any single questionnaire, clinical or psychophysical covariate correlating with the CPM effect.

Conclusions

Despite using one of the recommended CPM paradigms we were only able to demonstrate "true" CPM in 23.1% of control participants. Thus, the absence of differences between women with and without chronic pelvic pain must be interpreted with caution. Future studies using different CPM paradigms or larger sample sizes may find different results. Although CPM in chronic pain populations is of major theoretical mechanistic interest, the lack of an established assessment standard led us to question its added value in current clinical research.

Application of the grading system for "nociplastic pain" in chronic primary and chronic secondary pain conditions: a field study

ABSTRACT

The concept "nociplastic pain" has been developed for patients with features of nociceptive system sensitization that are not explained as nociceptive or neuropathic. Here, we tested how well the recently published grading system differentiates between chronic primary and secondary pain conditions. We recruited patients with fibromyalgia (FMS, n = 41), complex regional pain syndrome (CRPS, n = 11), osteoarthritis (OA, n = 21), or peripheral nerve injury (PNI, n = 8). We used clinical history, pain drawings, quantitative sensory testing (QST), and questionnaires to classify their pains as possibly or probably "nociplastic." All patients with chronic primary pain exhibited widespread/regional pain not explainable by either nociceptive or neuropathic mechanisms. Widespread pain occurred in 12 patients with OA but was identified as nociceptive in 11 of 12. Regional pain occurred in 4 patients with PNI but was identified as neuropathic in 3 of 4. At this step, the grading system had 100% sensitivity and 93% specificity. Clinical evidence for pain hypersensitivity by QST, and history of hypersensitivity and mental comorbidities did not differentiate between chronic primary pain (QST: 36/52 = 69%, history: 43/52 = 83%) and secondary pain conditions (QST: 20/29 = 69%, history: 24/29 83%). Based on these data, specificity remained excellent (93%), but sensitivity dropped substantially (60%) due to lacking evidence for pain hypersensitivity in many patients with FMS. This low sensitivity suggests that the published grading system is not suitable for screening purposes. We suggest structural and content modifications to improve sensitivity, including placement of patient history before clinical examination and addition of a high tender point count as evidence for widespread pain hypersensitivity.

Associations of preterm birth and neonatal stress exposure with chronic pain in adulthood - Results from the Gutenberg prematurity study

Although the effect of early childhood stress on central nervous pain processing is well known, studies on the association of prematurity and chronic pain are scarce. This study used data from a single-centre retrospective cohort study followed by a prospective clinical examination and pain assessment. The study was based on data from the local birth registry. Newborns born between 1969 and 2002 who had reached adulthood were eligible .. Using a selection algorithm, a study cohort stratified by gestational age (GA) was recruited. Chronic pain conditions were assessed using questionnaire and standardized pain drawings. Data on the pre-, peri- and postnatal clinical course was assessed from medical records. Multivariable logistic regression analyses were conducted to investigate associations between prematurity and chronic pain with adjustment for age, gender, socioeconomic status, and perinatal stress factors. 427 participants born preterm and full-term were included (age 28.5 ± 8.7 years). Chronic pain conditions were similarly common between groups with different levels of prematurity (GA ≥ 37 weeks: 34.5 %, GA33-36 weeks: 37.6 %, GA32-29 weeks: 25.2 %, GA < 29 weeks: 30.4 %, p = 0.20). In multivariable analyses, no association between low GA and the presence of chronic pain was found (OR = 0.99 (CI95 %: 0.94-1.04, p = 0.63); this was also true for a subanalysis of widespread pain. While neither fetal nutritional status nor perinatal stressors were associated with pain, exposure to maternal but not paternal smoking during pregnancy was associated with increased risk to develop pain (OR = 2.77 (CI95 %: 1.31-5.88, p = 0.008) in adults born preterm and full-term. This study suggests that prematurity by itself does not increase the risk of chronic pain later in life, but provides preliminary evidence for maternal smoking during pregnancy as risk factor.

What are the functional and clinical characteristics shared by fibromyalgia and low back pain? A scoping review

BACKGROUND

Fibromyalgia and chronic primary low back pain are two chronic pain conditions with a significant biopsychosocial burden. Recently, the International Association for the Study of Pain has grouped them under the term chronic primary pain. To further explore similarities and differences between these two conditions, the objective of this scoping review is to explore the pain-related, physiological and psychological outcomes in individuals with fibromyalgia and low back pain.

METHODS

The following databases were used to find relevant studies, using the PRISMA guidelines: Medline, Psycinfo, and CINAHL. Studies were included if they encompassed both participants with fibromyalgia or low back pain, with the objective to compare pain-related, physiological and/or psychological outcomes.

RESULTS

Nineteen studies were selected for extraction. Among the 2801 participants, 968 had fibromyalgia (mean age 48.56 ± 7.97 years, with 94% being female) and 896 had low back pain (mean age 47.48 ± 8.15 years, with 80% being female). Pain sensitivity, physical dysfunction, illness perception, psychological distress, alexithymia, depression, and anxiety were generally more severe in participants with fibromyalgia. Most studies found similar levels of pain intensity, kinesiophobia, quality of pain, quality of life, impact of pain, suicidal risk, anger, and social support comparing individuals with fibromyalgia and individuals with low back pain.

DISCUSSION

This scoping review highlights that although both conditions show similar pain intensity and impact on quality of life, fibromyalgia is associated with greater overall severity than low back pain, especially in sensitivity to pain and depression/anxiety.

The association between conditioned pain modulation and psychological factors in people with chronic spinal pain: A systematic review

Chronic spinal pain has negative effects on physical and mental well-being. Psychological factors can influence pain tolerance. However, whether these factors influence descending modulatory control mechanisms measured by conditioned pain modulation (CPM) in people with chronic spinal pain is unclear. This systematic review investigated the association between CPM response and psychological factors in people with chronic spinal pain. Published and unpublished literature databases were searched from inception to 23rd October 2023 included MEDLINE, EMBASE, CINAHL, and PubMed. Studies assessing the association between CPM response and psychological factors in people with chronic spinal pain were eligible. Data were pooled through meta-analysis. Methodological quality was assessed using the AXIS tool and the certainty of evidence measured through GRADE. From 2172 records, seven studies (n = 598) were eligible. Quality of included studies was moderate. There was very low certainty of evidence that depression (r = 0.01 [95% CI -0.10 to 0.12], I2 = 0%), and anxiety (r = -0.20 [95% CI -0.56 to 0.16], I2 = 84%), fear avoidance (r = -0.10 [95% CI -0.30 to 0.10], I2 = 70%) had no statistical associations with CPM responder status. Higher pain catastrophising was associated with CPM non-responder status (r = -0.19; 95% CI: -0.37 to -0.02; n = 545; I2: 76%) based on a very low certainty of evidence measured by GRADE. There is currently limited available evidence demonstrating an association between CPM response and psychological factors for people with chronic pain. Managing an individual's chronic pain symptoms irrespective of comorbid psychological distress, should continue until evidence offer insights that more targeted interventions are needed.

Does the distribution of musculoskeletal pain shape the fate of long-term sick leave? A prospective cohort study with register follow-up

ABSTRACT

Although multisite pain can markedly reduce work ability, the relevance of the bodily pain distribution as a predictor of long-term sick leave is still unknown. This study aimed to investigate the association between musculoskeletal pain distributions and long-term sick leave in the general working population of Denmark and included 66,177 currently employed wage earners without long-term sick leave during the prior 52 weeks. Participants reported whether they had pain in the lower extremity (hips/knees), upper extremity (neck/shoulders), or the low back. The analysis controlled for age, sex, year of survey reply, educational level, occupational group, psychosocial work factors, body max index, smoking, leisure-time physical activity, and mental health confounders. The results demonstrated that the risk of long-term sick leave increased with the number of pain sites. Compared with no pain, localized pain in any body region increased the risk/hazard by 25% to 29% (HR [95% CI]: 1.29 [1.07-1.54] for pain only in the low back), whereas pain in 2 regions increased the risk by 39% to 44% (HR [95% CI]: 1.41 [1.18-1.69] for pain in the low back + hips/knees). Workers reporting pain in all 3 regions experienced a 72% increased risk (HR [95% CI]: 1.72 [1.55-1.91]). Thus, the number of pain regions seems to matter more than the exact pain location. The spatial extension of musculoskeletal pain in workers functions as a gradient system, where pain spread throughout the body is an independent indicator of the high risk of long-term sick leave.

Low-intensity focused ultrasound to the posterior insula reduces temporal summation of pain

BACKGROUND

The insula and dorsal anterior cingulate cortex (dACC) are core brain regions involved in pain processing and central sensitization, a shared mechanism across various chronic pain conditions. Methods to modulate these regions may serve to reduce central sensitization, though it is unclear which target may be most efficacious for different measures of central sensitization.

OBJECTIVE/HYPOTHESIS

Investigate the effect of low-intensity focused ultrasound (LIFU) to the anterior insula (AI), posterior insula (PI), or dACC on conditioned pain modulation (CPM) and temporal summation of pain (TSP).

METHODS

N = 16 volunteers underwent TSP and CPM pain tasks pre/post a 10 min LIFU intervention to either the AI, PI, dACC or Sham stimulation. Pain ratings were collected pre/post LIFU.

RESULTS

Only LIFU to the PI significantly attenuated pain ratings during the TSP protocol. No effects were found for the CPM task for any of the LIFU targets. LIFU pressure modulated group means but did not affect overall group differences.

CONCLUSIONS

LIFU to the PI reduced temporal summation of pain. This may, in part, be due to dosing (pressure) of LIFU. Inhibition of the PI with LIFU may be a future potential therapy in chronic pain populations demonstrating central sensitization. The minimal effective dose of LIFU for efficacious neuromodulation will help to translate LIFU for therapeutic options.

Modulation of pain perception through transcranial alternating current stimulation and its nonlinear relationship with the simulated electric field magnitude

BACKGROUND

Oscillatory activities observed in multiple regions are closely associated with the experience of pain. Specifically, oscillatory activities within the theta- and beta-frequency bands, observed in the left dorsolateral prefrontal cortex (DLPFC), have been implicated in pain perception among healthy individuals and those with chronic pain. However, their physiological significance remains unclear.

METHODS

We explored the modulation of pain perception in healthy individuals by theta- and beta-band transcranial alternating current stimulation (tACS) over the left DLPFC and examined the relationship between the modulation effect and magnitude of the electric field elicited by tACS in the left DLPFC using computational simulation.

RESULTS

Our findings revealed that both theta- and beta-tACS increased the heat pain threshold during and after stimulation. Notably, the simulated electric field magnitude in the left DLPFC exhibited an inverted U-shaped relationship with the pain modulation effect for theta-tACS.

CONCLUSIONS

Our study findings suggested that there would be an optimal electric field strength to produce a high analgesic effect for theta-tACS.

SIGNIFICANCE

The application of theta- and beta-tACS interventions targeting the left DLPFC might facilitate the treatment of chronic pain. Furthermore, the attainment of effective pain modulation via theta-tACS over the DLPFC warrants the use of optimal stimulus intensity.

Is the Central Sensitization Inventory (CSI) associated with quantitative sensory testing (QST)? A systematic review and meta-analysis

Central sensitization (CS) involves an amplification of neural processing within the central nervous system that can result in widespread pain patterns and hypersensitivity to stimuli. The Central Sensitization Inventory (CSI) and various quantitative sensory testing (QST) methods purport to assess clinical markers of CS. The purpose of this systematic review and meta-analysis was to summarize and quantify the associations between total CSI scores and QST measures from previous studies. A systematic search identified 39 unique studies that were deemed eligible for the systematic review and 33 studies for meta-analyses (with 3314 subjects and 154 effect sizes), including five QST modalities: conditioned pain modulation, temporal summation, pressure pain threshold, heat pain threshold, and cold pain threshold. The meta-analysis yielded statistically significant CSI-QST correlations in total subject samples for all five QST modalities. The strongest associations were identified between CSI scores and pain threshold testing, especially pressure pain threshold, in which 51% of effects sizes, from 29 studies and 3071 subjects, were determined to be in a medium to large range.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

CLINICAL TRIAL REGISTRY NUMBER

NCT02560545.

Conditioned pain modulation (CPM) paradigm type affects its sensitivity as a biomarker of fibromyalgia

Fibromyalgia (FM) is a widespread chronic pain syndrome, possibly associated with the presence of central dysfunction in descending pain inhibition pathways. Conditioned Pain Modulation (CPM) has been proposed as a biomarker of FM. Nonetheless, the wide variety of methods used to measure CPM has hampered robust conclusions being reached. To clarify the validity of CPM as a biomarker of FM, we tested two CPM paradigms (parallel and sequential) in a sample of 23 female patients and 23 healthy women by applying test (mechanical) stimuli and conditioning (pressure cuff) stimuli. We evaluated whether CPM indices could correctly classify patients and controls, and we also determined the correlations between the indices and clinical variables such as symptomatology, disease impact, depression, quality of life, pain intensity, pain interference, fatigue and numbness. In addition, we compared the clinical status of CPM responders (efficient pain inhibitory mechanism) and non-responders. We observed that only parallel CPM testing correctly classified about 70% of patients with FM. In addition, more than 80% of healthy participants were found to be responders, while the rate was about 50% in the FM patients. The sequential CPM test was not as sensitive, with a decrease of up to 40% in the response rate for both groups. On the other hand, we did not observe any correlation between CPM measures and clinical symptoms. In summary, our findings demonstrate the influence of the CPM paradigm used and confirm that CPM may be a useful marker to complement FM diagnosis. However, the findings also cast doubts on the sensitivity of CPM as a marker of pain severity in FM.

Night time heart rate predicts next-day pain in fibromyalgia and primary back pain

Introduction

Primary chronic pain is pain that persists for over 3 months without associated measurable tissue damage. One of the most consistent findings in primary chronic pain is its association with autonomic hyperactivation. Yet whether the autonomic hyperactivation causes the pain or results from it is still unclear. It is also unclear to what extent autonomic hyperactivation is related to experienced pain intensity in different subtypes or primary chronic pain.

Objectives

Our first aim was to test lagged relationships between the markers of autonomic activation (heart rate) and pain intensity to determine its directionality. The main question here was whether autonomic biomarkers predict pain intensity or whether pain intensity predicts autonomic biomarkers. The second aim was to test whether this relationship is different between people with primary back pain and people with fibromyalgia.

Methods

Sixty-six patients with chronic pain were observed over an average of 81 days. Sleep heart rate and heart rate variability were measured with a wearable sensor, and pain intensity was assessed from daily subjective reports.

Results

The results showed a predictive relationship between sleep heart rate and next-day pain intensity (P < 0.05), but not between daily pain intensity and next night heart rate. There was no interaction with the type of chronic pain.

Conclusions

These findings suggest that autonomic hyperactivation, whether stress-driven or arising from other causes, precedes increases in primary chronic pain. Moreover, the present results suggest that autonomic hyperactivation is a common mechanism underlying the pain experience in fibromyalgia and chronic back pain.

Effects of virtual reality on psychophysical measures of pain: superiority to imagination and nonimmersive conditions

ABSTRACT

Virtual reality (VR) has been shown to be effective in pain management. However, to date, little is known about the mechanisms by which immersive experiences influence pain processing. The aim of this study was to investigate the direct effects of an immersive VR environment on the perception of experimental pain in individuals with chronic pain and pain-free controls. The immersion in a VR landscape was compared with mental imagery and a nonimmersive control condition. Using a randomized within-crossover design, pressure pain detection and tolerance thresholds, spatial and temporal summation (SSP, TSP), and conditioned pain modulation (CPM) were measured in 28 individuals with chronic pain and 31 pain-free controls using phasic cuff pressure on the legs. Direct comparison between the groups showed that although individuals with pain had significantly lower pain thresholds, reduced CPM effects, and increased TSP, the VR condition had the same pain-inhibitory effect on pain thresholds as in pain-free controls. Conditioned pain modulation effects were reduced by all conditions compared with baseline. There were no significant differences between conditions and baseline for TSP and SSP. Overall, pain modulatory effects were largest for VR and smallest for imagery. These results demonstrate that immersion in a VR environment has an increasing effect on pain thresholds, reduces pain inhibition in a CPM paradigm, and has no effects on TSP. This applies for participants with chronic pain and pain-free controls. These VR effects exceeded the effects of mental imagery on the nonimmersive control condition. This indicates that VR effectively modulates pain perception in both patients and controls irrespective of differences in pain perception.

Is low back pain a risk/prognostic factor for the development and/or progression of temporomandibular disorders? A systematic review with meta-analysis

OBJECTIVES

To evaluate the association between low back pain (LBP) and painful temporomandibular disorders (TMDs).

METHODS

Systematic review of observational studies. Searches were conducted using OVID MEDLINE, CINHAL, Web of Science and PUBMED databases up to 21 October 2022. Qualitative and quantitative analyses were performed. Risk of bias (RoB) was assessed using the Quality in Prognosis Studies tool (QUIPS).

RESULTS

Eight studies were included in the present review with meta-analysis. The first onset of TMDs was more likely in patients with previous chronic LBP (hazard ratio (HR) 1.53 [95% confidence interval (CI): 1.28; 1.83, p < .00001]). In addition, patients with chronic LBP had 3.25 times the odds (OR) [95% CI: 1.94; 5.43, p < .00001] of having chronic TMDs than those who did not have chronic LBP. In addition, the higher the exposure to chronic LBP, the higher the risk of developing a first onset of TMDs.

CONCLUSIONS

Chronic LBP can be considered a risk/contributing factor for painful TMDs. Although there is a high certainty in the evidence linking chronic LBP with the risk of a first onset of TMDs, there are insufficient studies to draw definitive conclusions. Furthermore, while an association between chronic LBP and chronic TMDs and a dose-effect was observed between these two conditions, a limited number of studies and evidence exist to support these findings. Future studies are needed to increase the body of evidence.

Fibromyalgia is associated with hypersensitivity but not with abnormal pain modulation: evidence from QST trials and spinal fMRI

Widespread pain and hyperalgesia are characteristics of chronic musculoskeletal pain conditions, including fibromyalgia syndrome (FM). Despite mixed evidence, there is increasing consensus that these characteristics depend on abnormal pain augmentation and dysfunctional pain inhibition. Our recent investigations of pain modulation with individually adjusted nociceptive stimuli have confirmed the mechanical and thermal hyperalgesia of FM patients but failed to detect abnormalities of pain summation or descending pain inhibition. Furthermore, our functional magnetic resonance imaging evaluations of spinal and brainstem pain processing during application of sensitivity-adjusted heat stimuli demonstrated similar temporal patterns of spinal cord activation in FM and HC participants. However, detailed modeling of brainstem activation showed that BOLD activity during "pain summation" was increased in FM subjects, suggesting differences in brain stem modulation of nociceptive stimuli compared to HC. Whereas these differences in brain stem activation are likely related to the hypersensitivity of FM patients, the overall central pain modulation of FM showed no significant abnormalities. These findings suggest that FM patients are hyperalgesic but modulate nociceptive input as effectively as HC.

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.

Exploratory analysis of 5 supervised machine learning models for predicting the efficacy of the endogenous pain inhibitory pathway in patients with musculoskeletal pain

OBJECTIVES

The identification of factors that influence the efficacy of endogenous pain inhibitory pathways remains challenging due to different protocols and populations. We explored five machine learning (ML) models to estimate the Conditioned Pain Modulation (CPM) efficacy.

DESIGN

Exploratory, cross-sectional design.

SETTING AND PARTICIPANTS

This study was conducted in an outpatient setting and included 311 patients with musculoskeletal pain.

METHODS

Data collection included sociodemographic, lifestyle, and clinical characteristics. CPM efficacy was calculated by comparing the pressure pain thresholds before and after patients submerged their non-dominant hand in a bucket of cold water (cold-pressure test) (1-4 °C). We developed five ML models: decision tree, random forest, gradient-boosted trees, logistic regression, and support vector machine.

MAIN OUTCOME MEASURES

Model performance were assessed using receiver operating characteristic curve (AUC), accuracy, sensitivity, specificity, precision, recall, F1-score, and the Matthews Correlation Coefficient (MCC). To interpret and explain the predictions, we used SHapley Additive explanation values and Local Interpretable Model-Agnostic Explanations.

RESULTS

The XGBoost model presented the highest performance with an accuracy of 0.81 (95% CI = 0.73 to 0.89), F1 score of 0.80 (95% CI = 0.74 to 0.87), AUC of 0.81 (95% CI: 0.74 to 0.88), MCC of 0.61, and Kappa of 0.61. The model was influenced by duration of pain, fatigue, physical activity, and the number of painful areas.

CONCLUSIONS

XGBoost showed potential in predicting the CPM efficacy in patients with musculoskeletal pain on our dataset. Further research is needed to ensure the external validity and clinical utility of this model.

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.

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.

[Nociplastic pain in research and practice : Overview of biopsychosocial principles, possibilities and difficulties]

Traditionally, two mechanistic pain categories were distinguished: nociceptive and neuropathic pain. After the definitions of these two mechanistic descriptors were refined more precisely in the International Association for the Study of Pain (IASP) taxonomy in 2011, a large group of patients remained whose pain could not be assigned to either of the two categories. Nociplastic pain was therefore proposed as a third mechanistic descriptor in 2016. This review article presents the current state of the integration of nociplastic pain into research and clinical practice. In particular, the possibilities and difficulties of applying this concept are addressed from a human and animal experimental research perspective.

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

Background

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

Methods

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

Results

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

Conclusion

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

Mechanisms of chiropractic spinal manipulative therapy for patients with chronic primary low back pain: protocol for a mechanistic randomised placebo-controlled trial

INTRODUCTION

Chronic low back pain (CLBP) is a highly prevalent and disabling condition. Identifying subgroups of patients afflicted with CLBP is a current research priority, for which a classification system based on pain mechanisms was proposed. Spinal manipulative therapy (SMT) is recommended for the management of CLBP. Yet, little data are available regarding its mechanisms of action, making it difficult to match this intervention to the patients who may benefit the most. It was suggested that SMT may influence mechanisms associated with central sensitisation. Therefore, classifying patients with CLBP according to central sensitisation mechanisms may help predict their response to SMT.

METHODS AND ANALYSIS

This protocol describes a randomised placebo-controlled trial aiming to examine which variables linked to central sensitisation may help predict the clinical response to SMT in a cohort of patients with CLBP. One hundred patients with chronic primary low back pain will be randomised to receive 12 sessions of SMT or placebo SMT over a 4-week period. Pain intensity and disability will be assessed as primary outcomes after completing the 4-week treatment (primary endpoint), and at 4-week and 12-week follow-ups. Baseline values of two pain questionnaires, lumbar pressure pain thresholds, concentrations of an inflammatory cytokine and expectations of pain relief will be entered as predictors of the response to SMT in a multiple regression model. Changes in these variables after treatment will be used in a second multiple regression model. The reference values of these predictors will be measured from 50 age and sex-matched healthy controls to allow interpretation of values in patients. Mixed analyses of variance will also be conducted to compare the primary outcomes and the predictors between groups (SMT vs placebo) over time (baseline vs post-treatment).

ETHICS AND DISSEMINATION

Ethical approval was granted by the Fundación Jiménez Díaz Clinical Research Ethics Committee.

TRIAL REGISTRATION NUMBER

NCT05162924.

A Comparison of Self-reported Pain Measures Between Sensory Phenotypes in HIV-associated Sensory Neuropathy

Painful HIV-associated neuropathy (HIV-SN) is a prevalent co-morbidity of HIV infection. Sensory phenotyping, using quantitative sensory testing (QST) could allow for improved stratification to guide personalized treatment. However, previous methods of QST interpretation have demonstrated limited association with self-reported pain measures. This study sought to identify differences in self-reported pain measures between composite QST-derived sensory phenotypes, and to examine any differences in participants reporting multi-site, multi-etiology chronic pain. In this cross-sectional observational study of participants with HIV (n = 133), individuals were allocated to neuropathy and neuropathic pain groups through clinical assessment and nerve conduction testing. They completed symptom-based questionnaires and underwent standardized QST. Participants were assigned, by pre-determined algorithm, to a QST-derived sensory phenotype. Symptoms were compared between sensory phenotypes. Symptom characteristics and Neuropathic Pain Symptom Inventory scores differed between QST-derived sensory phenotypes: 'sensory loss' was associated with more paroxysmal and paraesthetic symptoms compared to 'thermal hyperalgesia' and 'healthy' phenotypes (P = .023-0.001). Those with painful HIV-SN and additional chronic pain diagnoses were more frequently allocated to the 'mechanical hyperalgesia' phenotype compared to those with painful HIV-SN alone (P = .006). This study describes heterogeneous sensory phenotypes in people living with HIV. Differences in self-reported pain outcomes between sensory phenotypes has the potential to guide future stratified trials and eventually more targeted therapy. PERSPECTIVE: This article presents quantitative sensory testing derived phenotypes, thought to reflect differing pathophysiological pain mechanisms and relates them to self-reported pain measures in people with HIV infection. This could help clinicians stratify patients to individualize analgesic interventions more effectively.

Stress-induced hyperalgesia instead of analgesia in patients with chronic musculoskeletal pain

Many individuals with chronic musculoskeletal pain (CMP) show impairments in their pain-modulatory capacity. Although stress plays an important role in chronic pain, it is not known if stress-induced analgesia (SIA) is affected in patients with CMP. We investigated SIA in 22 patients with CMP and 18 pain-free participants. Pain thresholds, pain tolerance and suprathreshold pain ratings were examined before and after a cognitive stressor that typically induces pain reduction (SIA). Whereas the controls displayed a significant increase in pain threshold in response to the stressor, the patients with CMP showed no analgesia. In addition, increased pain intensity ratings after the stressor indicated hyperalgesia (SIH) in the patients with CMP compared to controls. An exploratory analysis showed no significant association of SIA or SIH with spatial pain extent. We did not observe significant changes in pain tolerance or pain unpleasantness ratings after the stressor in patients with CMP or controls. Our data suggest that altered stress-induced pain modulation is an important mechanism involved in CMP. Future studies need to clarify the psychobiological mechanisms of these stress-induced alterations in pain processing and determine the role of contributing factors such as early childhood trauma, catastrophizing, comorbidity with mental disorders and genetic predisposition.

Signature for Pain Recovery IN Teens (SPRINT): protocol for a multisite prospective signature study in chronic musculoskeletal pain

INTRODUCTION

Current treatments for chronic musculoskeletal (MSK) pain are suboptimal. Discovery of robust prognostic markers separating patients who recover from patients with persistent pain and disability is critical for developing patient-specific treatment strategies and conceiving novel approaches that benefit all patients. Given that chronic pain is a biopsychosocial process, this study aims to discover and validate a robust prognostic signature that measures across multiple dimensions in the same adolescent patient cohort with a computational analysis pipeline. This will facilitate risk stratification in adolescent patients with chronic MSK pain and more resourceful allocation of patients to costly and potentially burdensome multidisciplinary pain treatment approaches.

METHODS AND ANALYSIS

Here we describe a multi-institutional effort to collect, curate and analyse a high dimensional data set including epidemiological, psychometric, quantitative sensory, brain imaging and biological information collected over the course of 12 months. The aim of this effort is to derive a multivariate model with strong prognostic power regarding the clinical course of adolescent MSK pain and function.

ETHICS AND DISSEMINATION

The study complies with the National Institutes of Health policy on the use of a single internal review board (sIRB) for multisite research, with Cincinnati Children's Hospital Medical Center Review Board as the reviewing IRB. Stanford's IRB is a relying IRB within the sIRB. As foreign institutions, the University of Toronto and The Hospital for Sick Children (SickKids) are overseen by their respective ethics boards. All participants provide signed informed consent. We are committed to open-access publication, so that patients, clinicians and scientists have access to the study data and the signature(s) derived. After findings are published, we will upload a limited data set for sharing with other investigators on applicable repositories.

TRIAL REGISTRATION NUMBER

NCT04285112.

Hyper-connectivity between the left motor cortex and prefrontal cortex is associated with the severity of dysfunction of the descending pain modulatory system in fibromyalgia

INTRODUCTION

The association between descending pain modulatory system (DPMS) dysfunction and fibromyalgia has been previously described, but more studies are required on its relationship with aberrant functional connectivity (FC) between the motor and prefrontal cortices.

OBJECTIVES

The objective of this cross-sectional observational study was to compare the intra- and interhemispheric FC between the bilateral motor and prefrontal cortices in women with fibromyalgia, comparing responders and nonresponders to the conditioned pain modulation (CPM) test.

METHODS

A cross-sectional sample of 37 women (23 responders and 14 nonresponders to the CPM test) with fibromyalgia diagnosed according to the American College of Rheumatology criteria underwent a standardized clinical assessment and an FC analysis using functional near-infrared spectroscopy. DPMS function was inferred through responses to the CPM test, which were induced by hand immersion in cold water (0-1°C). A multivariate analysis of covariance for main effects between responders and nonresponders was conducted using the diagnosis of multiple psychiatric disorders and the use of opioid and nonopioid analgesics as covariates. In addition, we analyzed the interaction between the CPM test response and the presence of multiple psychiatric diagnoses.

RESULTS

Nonresponders showed increased FC between the left motor cortex (lMC) and the left prefrontal cortex (lPFC) (t = -2.476, p = 0.01) and right prefrontal cortex (rPFC) (t = -2.363, p = 0.02), even when both were considered as covariates in the regression analysis (lMC-lPFC: β = -0.127, t = -2.425, p = 0.021; lMC-rPFC: β = -0.122, t = -2.222, p = 0.033). Regarding main effects, a significant difference was only observed for lMC-lPFC (p = 0.035). A significant interaction was observed between the psychiatric disorders and nonresponse to the CPM test in lMC-lPFC (β = -0.222, t = -2.275, p = 0.03) and lMC-rPFC (β = -0.211, t = -2.2, p = 0.035). Additionally, a significant interaction was observed between the CPM test and FC in these two region-of-interest combinations, despite the psychiatric diagnoses (lMC-lPFC: β = -0.516, t = -2.447, p = 0.02; lMC-rPFC: β = -0.582, t = -2.805, p = 0.008).

CONCLUSIONS

Higher FC between the lMC and the bilateral PFC may be a neural marker of DPMS dysfunction in women with fibromyalgia, although its interplay with psychiatric diagnoses also seems to influence this association.

Use of the painDETECT to discriminate musculoskeletal pain phenotypes

Background

Musculoskeletal pain patients present similar pain characteristics regardless of the clinical diagnosis. PainDETECT questionnaire is useful for screening neuropathic-like symptoms in many musculoskeletal conditions. However, no previous studies compared pain phenotypes of patients with musculoskeletal pain using the painDETECT. Therefore, the current study aimed to compare the pain characteristics of patients with musculoskeletal pain classified according to the painDETECT as nociceptive pain, unclear, and neuropathic-like symptoms.

Methods

A cross-sectional study was conducted in 308 participants with musculoskeletal pain. Demographic and clinical characteristics of the participants were examined. Neuropathic-like symptoms, pain intensity, pain area, Central Sensitization-related sign and symptoms, functional limitation, and conditioned pain modulation were assessed in patients with musculoskeletal pain. Independent one-way analysis of variance (ANOVA) was used to test for between-group differences for the outcome measures with continuous variables and Pearson chi-square test verified between-group differences on the efficiency of the conditioned pain modulation.

Results

Participants had a mean age of 52.21 (±15.01) years old and 220 (71.42%) were females. One hundred seventy-three (56.16%) participants present nociceptive pain, 69 (22.40%) unclear, and 66 (21.42%) neuropathic-like symptoms. A one-way ANOVA showed differences for the pain intensity [F (2,305) = 20.097; p < .001], pain area [F (2,305) = 28.525; p < .001], Central Sensitization-related sign and symptoms [F (2,305) = 54.186; p < .001], and functional limitation [F (2,256) = 8.061; p < .001]. However, conditioned pain modulation was similarly impaired among the three groups (X² = 0.333, p = 0.847).

Conclusion

Patients with neuropathic-like symptoms revealed unfavorable pain characteristics compared to their counterparts, including pain intensity, generalized pain, Central Sensitization-related sign and symptoms, and functional limitation.

The Spatial Extent of Pain Is Associated with Pain Intensity, Catastrophizing and Some Measures of Central Sensitization in People with Frozen Shoulder

The aim of this cross-sectional study was to explore the spatial extent of pain and its association with clinical symptoms, psychological features, and pain sensitization in people with frozen shoulder (FS). Forty-eight individuals with FS completed pain drawings (PDs) and reported their clinical symptoms including pain intensity (Visual Analogue Scale) and shoulder disability (Shoulder Pain and Disability Index). Moreover, pain sensitization measurements (pressure pain thresholds, temporal summation, conditioned pain modulation, and Central Sensitization Inventory (CSI)) were assessed. Psychological features were assessed by Pain Catastrophizing Scale (PCS) and Pain Vigilance and Awareness Questionnaire. Pain frequency maps were generated, Margolis rating scale was used for pain location, and Spearman correlation coefficients were computed. The mean (SD) pain extent was 12.5% (6.7%) and the most common painful area was the anterolateral shoulder region (100%). Women presented a more widespread pain distribution compared with men. Significant positive associations were obtained between pain extent and current pain intensity (rs = 0.421, p < 0.01), PCS (rs = 0.307, p < 0.05) and CSI (rs = 0.358, p < 0.05). The anterolateral region of the shoulder was the most common painful area in people with FS. Women with FS presented more extended areas of pain; and a more widespread distribution of pain was correlated with higher levels of pain, pain catastrophizing and pain sensitization.

Aerobic Exercise Attenuates Pain Sensitivity: An Event-Related Potential Study

In this study, electroencephalography (EEG) was utilized to explore the neurophysiological mechanisms of aerobic exercise-induced hypoalgesia (EIH) and provide a theoretical basis for the application of aerobic exercise in pain assessment and treatment. Forty-five healthy subjects were randomly divided into moderate-intensity aerobic exercise [70% heart rate reserve (HRR)], low-intensity aerobic exercise (50% HRR), or control groups (sitting). Aerobic exercise was performed with cycling. Pressure pain threshold (PPT), heat pain threshold (HPT), event-related potential (ERP) induced by contact heat stimulus and pain scoring were measured before and after the intervention. We found that moderate-intensity aerobic exercise can increase the PPT (rectus femoris: t = -2.71, p = 0.017; tibialis anterior muscle: t = -2.36, p = 0.033) and HPT (tibialis anterior muscle: t = -2.219, p = 0.044) of proximal intervention sites rather than distal sites, and decreased pain scorings of contact heat stimulus. After moderate-intensity aerobic exercise, alpha oscillation power reflecting the central descending inhibitory function was enhanced (t = -2.31, p < 0.05). Low-intensity aerobic exercise mainly reduced the pain unpleasantness rating (Block 1: t = 2.415, p = 0.030; Block 2: t = 3.287, p = 0.005; Block 4: t = 2.646, p = 0.019; Block 5: t = 2.567, p = 0.022). Aerobic exercise had an overall EIH effect. Its hypoalgesic effect was related to exercise intensity and affected by the site and type of pain stimulus. Moderate-intensity aerobic exercise effectively reduced the sensitivity to various painful stimuli, and low-intensity aerobic exercise selectively inhibited the negative emotional pain response. The hypoalgesic mechanism of aerobic exercise involves the enhancement of the central descending inhibitory function.

Medial Prefrontal High-Definition Transcranial Direct Current Stimulation to Improve Pain Modulation in Chronic Low Back Pain: A Pilot Randomized Double-blinded Placebo-Controlled Crossover Trial

Chronic low back pain (CLBP) is highly disabling, but often without identifiable source. Focus has been on impaired anti-nociceptive mechanisms contributing to pain maintenance, though methods of targeting this impairment remain limited. This randomised-controlled cross-over pilot trial used active versus sham medial prefrontal cortex (mPFC) high-definition transcranial direct current stimulation (HD-tDCS) for 3-consecutive days to improve descending pain inhibitory function. Twelve CLBP patients were included with an average visual analogue scale (VAS) pain intensity of 3.0 ± 1.5 and pain duration of 5.3 ± 2.6 years. Pressure pain thresholds (PPTs), conditioned pain modulation (CPM), and temporal summation of pain (TSP) assessed by cuff algometry, as well as pain symptomatology (intensity, unpleasantness, quality, disability) and related psychological features (pain catastrophizing, anxiety, affect), were assessed on Day1 before 3 consecutive days of HD-tDCS sessions (each 20 minutes), at 24-hours (Day 4) and 2-weeks (Day 21) following final HD-tDCS. Blinding was successful. No significant differences in psychophysical (PPT, CPM, TSP), symptomatology or psychological outcomes were observed between active and sham HD-tDCS on Day4 and Day21. CPM-effects at Day 1 negatively correlated with change in CPM-effect at Day4 following active HD-tDCS (P = .002). Lack of efficacy was attributed to several factors, not least that patients did not display impaired CPM at baseline. TRIAL REGISTRATION: : ClinicalTrials.gov (NCT03864822). PERSPECTIVE: Medial prefrontal HD-tDCS did not alter pain, psychological nor psychophysical outcomes, though correlational analysis suggested response may depend on baseline pain inhibitory efficacy, with best potential effects in patients with severe impairments in descending pain inhibitory mechanisms. Future work should focus on appropriate patient selection and optimising stimulation targeting.

Do chronic low back pain subgroups derived from dynamic quantitative sensory testing exhibit differing multidimensional profiles?

OBJECTIVES

The relationship of pain sensitivity with pain and disability in low back pain (LBP) is complicated. It has been suggested increased understanding of dynamic quantitative sensory testing (QST) might be useful in increasing understanding of these relationships. This study aimed to create subgroups based on participant responses to dynamic QST, profile these subgroups based on multidimensional variables (including clinical measures of pain and disability, psychological and lifestyle variables and static QST), and investigate the association of subgroup membership with levels of pain intensity, LBP-related disability and disability risk at 12-month follow up.

METHODS

Participants (n=273) with dominant axial chronic non-specific LBP with duration of pain >3 months were included in this study. At baseline, eligible participants completed a self-report questionnaire to collect demographic, clinical, psychological and lifestyle data prior to dynamic and static QST. Dynamic QST measures were conditioned pain modulation (CPM) and temporal summation (TS). At 12-months follow up, clinical data were collected, including pain intensity and LBP-related disability. Sub-groups were formed by cross-tabulation. Analysis was undertaken to profile dynamic QST subgroup on demographic, clinical, psychological, lifestyle and static QST measures. Associations between dynamic QST subgroups and follow-up clinical variables were examined.

RESULTS

Based on dynamic QST, participants were allocated into four subgroups; normal CPM and normal TS (n=34, 12.5%); normal CPM and facilitated TS (n=6, 2.2%); impaired CPM and normal TS (n=186, 68.1%); impaired CPM and facilitated TS (n=47, 17.2%). At baseline no differences were demonstrated between subgroups across most clinical variables, or any psychological or lifestyle measures. The two subgroups with impaired CPM were more likely to have a higher number of painful body areas. Cold pain sensitivity was heightened in both the subgroups with facilitated TS. Subgroups did not differ across pain intensity, LBP-related disability and disability risk stratification at follow-up.

CONCLUSIONS

The profiles of people with axial LBP did not vary significantly across dynamic QST subgroups, save for those in groups with impaired CPM being more likely to have more widespread symptoms and those with facilitated TS having heightened cold pain sensitivity. Further, subgroup membership was not related to future pain and disability. The role of dynamic QST profiles in LBP remains unclear. Further work is required to understand the role of pain sensitivity in LBP. The utility of dynamic QST subgrouping might not be in determining of future disability. Future research might focus on treatment modifying effects of dynamic QST subgroups.

Conditioned pain modulation and pain sensitivity in functional somatic disorders: The DanFunD study

Background

Disrupted pain regulation has been proposed as a component in functional somatic disorders (FSD). The objective of this study was to examine a general population sample, encompassing three delimitations of FSD while assessing pain sensitivity and conditioning pain modulation (CPM).

Methods

Pressure pain thresholds (PPTs) at the tibialis and trapezius muscles were recorded at baseline. During cold pressor stimulation of the hand, the tibialis PPTs were re‐assessed and the difference from baseline measures defined the CPM effect. Participants (n = 2,198, 53% females) were randomly selected from the adult Danish population. FSD was established by self‐reported symptom questionnaires.

Results

With a few exceptions, only weak associations were seen between PPTs and CPM in cases with FSD (p > .1). A high PPT was associated with lower odds of having multi‐organ bodily distress syndrome (OR^{PPT trapezius}: 0.66, 95% CI: 0.49–0.88, p = .005), with the symptom profile characterized by all symptoms (OR^{PPT trapezius}: 0.72, 95% CI: 0.58–0.90, p = .003 and OR^{PPT tibialis}: 0.75, 95% CI: 0.62–0.91, p = .004), and with multiple chemical sensitivity (OR^{PPT trapezius}: 0.81, 95% CI: 0.67–0.97, p = .022). High CPM was associated with high odds of having irritable bowel (OR^{CPM relative}: 1.22, 95% CI: 1.04–1.43, p = .013 and OR^{CPM absolute} = 2.66, 95% CI: 1.07–6.45, p = .033).

Conclusion

However, only PPT measured over the trapezius muscle were still significant after correction for multiple testing for the symptom profile characterized by all symptoms. Findings from this study do not support altered pain regulation in questionnaire‐based FSD which is in contrast with the existing presumption. Further epidemiological studies in this field are needed.

Significance

Disrupted pain regulation as measured by abnormal pain thresholds has been hypothesized as a central mechanism in Functional Somatic Disorders (FSD). The hypothesis has been raised in clinical setting where patients presented subjective and objective features of hypersensitivity. The present population‐based study does not support this notion. This points to the importance of further studies into the underlying pathophysiology mechanisms of FSD.

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Pain referral area is reduced by remote pain

BACKGROUND

Endogenous pain inhibitory mechanisms are known to reduce pain intensity, but whether they influence the size and distribution of pain referral is unclear. This study aimed to determine if referred pain is reduced by applying a remote, conditioning painful stimulus.

METHODS

Twenty-four healthy men participated in this randomized, crossover study with a control and conditioning session. Referred pain was induced from the infraspinatus muscle (dominant side) by a painful pressure for 60 s. When applying pressure, the intensity was adjusted to a local pain intensity of 7/10 on a numerical rating scale. In the conditioning session, tonic painful pressure was simultaneously applied to the non-dominant leg during induction of referred pain. The area of referred pain was drawn onto a digital body chart and size extracted for data analysis.

RESULTS

For the total group and in a subgroup with distinct patterns of referred pain (n = 15/24), the pain area perceived in the back and front+back was smaller during the conditioning compared with the control (p < 0.05). No significant difference was found between sessions in a subgroup only demonstrating local pain (n = 9/24).

CONCLUSIONS

Engaging the descending noxious inhibitory control reduced the size of pain areas predominately when distinct pain referral was present. Assuming a conditioning effect of descending inhibitory control acting on dorsal horn neurons, these findings may indicate that mechanisms underlying pain referral can be modulated by endogenous control. The findings may indicate that referred pain may be a useful proxy to evaluate sensitivity of central pain mechanisms as previously suggested.

SIGNIFICANCE

The current results indicate a link between endogenous inhibition and pain referral. Descending inhibitory control effects on pain referral support a spinal mechanism involved in pain referral. Future studies should investigate whether the spatial characteristics of referred pain (e.g. size, frequency of affected body regions and distribution away from the primary nociceptive stimulus) can useful to evaluate the efficiency of endogenous pain modulation.

The inhibitory effect of conditioned pain modulation on temporal summation in low-back pain patients

OBJECTIVES

The literature on conditioned pain modulation (CPM) is inconclusive in relation to low-back pain and it is unclear how CPM affects temporal summation as a proxy of central pain integration. The aim of this study was to examine whether the CPM effect would be different on pain induced by temporal summation than single stimuli in a group of low back pain patients.

METHODS

A total of 149 low-back pain patients were included. CPM was examined using single, repeated and temporal summation (repeated-single difference) of mechanical pressure pain as test stimuli at an individualized, fixed supra-pain-threshold force, before and after 2 min of cold pressor test (0-2 degrees Celsius). Participants were categorized as CPM responders or non-responders according to three different criteria: simple (any pain inhibition), strict (pain inhibition of more than 10VAS) and reversed (pain inhibition or facilitation of less than 10VAS). Clinical data on back pain was collected for correlation and descriptive purposes.

RESULTS

Significant modulation was observed for all three test stimuli. Effects sizes were comparable in relative terms, but repeated pressure pain modulation was greater in absolute terms. No correlations to clinical data were observed, for any measure.

CONCLUSIONS

The current data suggests that repeated pressure pain may be better suited as the CPM test stimuli, than single pressure pain and temporal summation of pressure pain, as the CPM effect in absolute terms was greater. Employing temporal summation as the test stimulus in a CPM paradigm may be more sensitive than a single test stimulus.

Individuals with Persistent Greater Trochanteric Pain Syndrome Exhibit Impaired Pain Modulation, as well as Poorer Physical and Psychological Health, Compared with Pain-Free Individuals: A Cross-Sectional Study

OBJECTIVES

To compare physical, sensory, and psychosocial factors between individuals with greater trochanteric pain syndrome and controls and to explore factors associated with pain and disability.

DESIGN

Cross-sectional study.

SETTING

General community.

SUBJECTS

Patients with persistent, clinically diagnosed greater trochanteric pain syndrome and healthy controls.

METHODS

Participants completed tests of thermal and pressure pain threshold, conditioned pain modulation, temporal summation, muscle strength, physical function, physical activity, psychological factors, and health-related quality of life. Standardized mean differences between groups were calculated, and multiple linear regression identified factors associated with pain and disability.

RESULTS

Forty patients (95% female, average [SD] age = 51 [9] years) and 58 controls (95% female, average [SD] age = 53 [11] years) were included. Heat pain threshold, temporal summation, and pain catastrophizing were not different between groups. Compared with controls, patients displayed significantly poorer quality of life (standardized mean difference = -2.66), lower pressure pain threshold locally (-1.47, remotely = -0.57), poorer health status (-1.22), impaired physical function (range = 0.64-1.20), less conditioned pain modulation (-1.01), weaker hip abductor/extensor strength (-1.01 and -0.59), higher depression (0.72) and anxiety (0.61) levels, lower cold pain threshold locally (-0.47, remotely = -0.39), and less time spent in (vigorous) physical activity (range = -0.43 to -0.39). Twenty-six percent of pain and disability was explained by depression, hip abductor strength, and time to complete stairs.

CONCLUSIONS

Patients with greater trochanteric pain syndrome exhibited poorer health-related quality of life, physical impairments, widespread hyperalgesia, and greater psychological distress than healthy controls. Physical and psychological factors were associated with pain and disability.

Rat dorsal horn neurons primed by stress develop a long-lasting manifest sensitization after a short-lasting nociceptive low back input

Background

A single injection of nerve growth factor (NGF) into a low back muscle induces a latent sensitization of rat dorsal horn neurons (DHNs) that primes for a manifest sensitization by a subsequent second NGF injection. Repeated restraint stress also causes a latent DHN sensitization.

Objective

In this study, we investigated whether repeated restraint stress followed by a single NGF injection causes a manifest sensitization of DHNs.

Methods

Rats were stressed repeatedly in a narrow plastic restrainer (1 hour on 12 consecutive days). Control animals were handled but not restrained. Two days after stress paradigm, behavioral tests and electrophysiological in vivo recordings from single DHNs were performed. Mild nociceptive low back input was induced by a single NGF injection into the lumbar multifidus muscle just before the recording started.

Results

Restraint stress slightly lowered the low back pressure pain threshold (Cohen d = 0.83). Subsequent NGF injection increased the proportion of neurons responsive to deep low back input (control + NGF: 14%, stress + NGF: 39%; P = 0.041), mostly for neurons with input from outside the low back (7% vs 26%; P = 0.081). There was an increased proportion of neurons with resting activity (28% vs 55%; P = 0.039), especially in neurons having deep input (0% vs 26%; P = 0.004).

Conclusions

The results indicate that stress followed by a short-lasting nociceptive input causes manifest sensitization of DHNs to deep input, mainly from tissue outside the low back associated with an increased resting activity. These findings on neuronal mechanisms in our rodent model suggest how stress might predispose to radiating pain in patients.

Alterations in pronociceptive and antinociceptive mechanisms in patients with low back pain: a systematic review with meta-analysis

Altered pronociceptive and antinociceptive mechanisms are often implicated in painful conditions and have been increasingly studied over the past decade. For some painful conditions, alterations are well-established, but in populations with low back pain (LBP), there remains considerable debate whether these mechanisms are altered. The present systematic review aimed to address this issue by identifying studies assessing conditioned pain modulation (CPM) and/or temporal summation of pain (TSP) in patients with LBP, comparing with either a healthy control group or using a method with reference data available. Qualitative synthesis and quantitative meta-analysis of group differences were performed. For CPM and TSP, 20 and 29 original articles were eligible, with data for meta-analysis obtainable from 18 (1500 patients and 505 controls) and 27 (1507 patients and 1127 controls) studies, respectively. Most studies were of poor-to-fair quality with significant heterogeneity in study size, population, assessment methodology, and outcome. Nonetheless, CPM was impaired in patients with LBP compared with controls (standardized mean difference = -0.44 [-0.64 to -0.23], P < 0.001), and the magnitude of this impairment was related to pain chronicity (acute/recurrent vs chronic, P = 0.003), duration (RS = -0.62, P = 0.006), and severity (RS = -0.54, P = 0.02). Temporal summation of pain was facilitated in patients with LBP compared with controls (standardized mean difference = 0.50 [0.29-0.72], P < 0.001), and the magnitude of this facilitation was weakly related to pain severity (RS= 0.41, P = 0.04) and appeared to be influenced by test modality (P < 0.001). Impaired CPM and facilitated TSP were present in patients with LBP compared with controls, although the magnitude of differences was small which may direct future research on the clinical utility.

Efficacy of duloxetine for multisite pain in patients with knee pain due to osteoarthritis: An exploratory post hoc analysis of a Japanese phase 3 randomized study

BACKGROUND

Central sensitization, including dysfunction of descending inhibitory pain pathways, may contribute to multisite pain in patients with chronic musculoskeletal conditions. Duloxetine is a centrally acting analgesic that effectively reduces pain in patients with knee osteoarthritis. Here we assessed the efficacy of duloxetine (60 mg/day) in Japanese patients (N = 353) with pain due to knee osteoarthritis based on the number of painful body sites, determined using the Michigan Body Map.

METHODS

Post hoc analysis of a phase 3, randomized, placebo-controlled trial (ClinicalTrials.gov; NCT02248480).

RESULTS

At Week 14, the change from baseline in Brief Pain Inventory-Severity average pain score ("pain reduction") was significantly greater with duloxetine compared with placebo in patients with 3, 4, or ≥5 painful sites, but not in patients with 1 or 2 painful sites. In patients with ≥3 painful sites (57% of patients), pain reduction was significantly greater with duloxetine (n = 100) compared with placebo (n = 101) throughout the study (least squares mean change from baseline to Week 14: -2.68 vs -1.68). Greater pain reduction with duloxetine (n = 77) than placebo (n = 75) also occurred in patients with ≤2 painful sites, although the between-group difference was significant only at Week 4.

CONCLUSIONS

These results are consistent with duloxetine enhancing the activity of descending inhibitory pain pathways that are dysfunctional in patients with central sensitization and multisite pain. In addition, these results suggest that duloxetine may be an effective choice of analgesic for patients with knee osteoarthritis and multisite pain.

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

OBJECTIVES

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

MATERIALS AND METHODS

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

RESULTS

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

DISCUSSION

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

Emerging Clinical Technology: Application of Machine Learning to Chronic Pain Assessments Based on Emotional Body Maps

Depression and anxiety co-occur with chronic pain, and all three are thought to be caused by dysregulation of shared brain systems related to emotional processing associated with body sensations. Understanding the connection between emotional states, pain, and bodily sensations may help understand chronic pain conditions. We developed a mobile platform for measuring pain, emotions, and associated bodily feelings in chronic pain patients in their daily life conditions. Sixty-five chronic back pain patients reported the intensity of their pain, 11 emotional states, and the corresponding body locations. These variables were used to predict pain 2 weeks later. Applying machine learning, we developed two predictive models of future pain, emphasizing interpretability. One model excluded pain-related features as predictors of future pain, and the other included pain-related predictors. The best predictors of future pain were interactive effects of (a) body maps of fatigue with negative affect and (b) positive affect with past pain. Our findings emphasize the contribution of emotions, especially emotional experience felt in the body, to understanding chronic pain above and beyond the mere tracking of pain levels. The results may contribute to the generation of a novel artificial intelligence framework to help in the development of better diagnostic and therapeutic approaches to chronic pain.

Automated Nociceptive Withdrawal Reflex Measurements Reveal Normal Reflex Thresholds and Augmented Pain Ratings in Patients with Fibromyalgia

The nociceptive withdrawal reflex (NWR) is used to probe spinal cord excitability in chronic pain states. Here, we used an automated and unbiased procedure for determining the NWR threshold and compared the reflex thresholds and corresponding pain ratings in a well-characterized cohort of fibromyalgia (n = 29) and matched healthy controls (n = 21). Surface electrical stimuli were delivered to the foot in a stepwise incremental and decremental manner. The surface electromyographic activity was recorded from the ipsilateral tibialis anterior muscle. Fibromyalgia patients reported significantly higher scores for psychological distress and pain-related disability and a significantly lower score for perceived state of health compared to the matched controls. The subjective pain ratings were significantly higher in patients. The NWR thresholds were similar to the controls. In the patients, but not in controls, the NWR thresholds and subjective pain ratings were significantly correlated. Our results showed an increased subjective pain sensitivity in fibromyalgia, but we found no evidence for spinal sensitization based on the reflex measures.

Pain inhibition is not affected by exercise-induced pain

Introduction:

Offset analgesia (OA) and conditioned pain modulation (CPM) are frequently used paradigms to assess the descending pain modulation system. Recently, it was shown that both paradigms are reduced in chronic pain, but the influence of acute pain has not yet been adequately examined.

Objectives:

The aim of this study is to investigate OA and CPM after exercise-induced pain to evaluate whether these tests can be influenced by delayed-onset muscle soreness (DOMS) at a local or remote body site.

Methods:

Forty-two healthy adults were invited to 3 separate examination days: a baseline appointment, the consecutive day, and 7 days later. Participants were randomly divided into a rest (n = 21) and an exercise group (n = 21). The latter performed a single intensive exercise for the lower back. Before, immediately after, and on the following examination days, OA and CPM were measured at the forearm and the lower back by blinded assessor.

Results:

The exercise provoked a moderate pain perception and a mild delayed-onset muscle soreness on the following day. Repeated-measurements analysis of variance showed no statistically significant main effect for either OA or CPM at the forearm or lower back (P > 0.05).

Conclusion:

Delayed-onset muscle soreness was shown to have no effect on the inhibitory pain modulation system neither locally (at the painful body part), nor remotely. Thus, OA and CPM are robust test paradigms that probably require more intense, different, or prolonged pain to be modulated.

Reference values of conditioned pain modulation

Background and aims Endogenous pain modulation can be studied in humans by conditioned pain modulation (CPM): pain induced by a test stimulus is attenuated by a distantly applied noxious conditioning stimulus. The detection of impaired CPM in individual patients is of potential importance to understand the pathophysiology and predict outcomes. However, it requires the availability of reference values. Methods We determined reference values of CPM in 146 pain-free subjects. Pressure and electrical stimulation were the test stimuli. For electrical stimuli, we recorded both pain threshold and threshold for the nociceptive withdrawal reflex. Cold pressor test was the conditioning stimulus. The 5th, 10th and 25th percentiles for the three tests were computed by quantile regression analyses. Results The average thresholds increased after the conditioning stimulus for all three tests. However, a subset of subjects displayed a decrease in thresholds during the conditioning stimulus. This produced negative values for most of the computed percentiles. Conclusions This study determined percentile reference values of CPM that can be used to better phenotype patients for clinical and research purposes. The negative value of percentiles suggests that a slightly negative CPM effect can be observed in pain-free volunteers. Implications Pain facilitation rather than inhibition during the conditioning stimulus occurs in some pain-free volunteers and may not necessarily represent an abnormal finding.

Pain Expansion and Severity Reflect Central Sensitization in Primary Care Patients with Greater Trochanteric Pain Syndrome

OBJECTIVES

The aims of this study were twofold: 1) to evaluate the differences in pain sensitivity of patients with greater trochanteric pain syndrome (GTPS) and 2) to examine the association between pain expansion, pain severity, and pain-related central sensitization somatosensory variables in patients with GTPS.

STUDY DESIGN

A cross-sectional study.

SETTING

Primary health care centers.

SUBJECTS

The sample consisted of 49 participants with a mean age of 48.28 ± 8.13 years and included eight males (16.3%) and 41 females (83.7%).

METHODS

Conditioned pain modulation (CPM), pain location, temporal summation, pressure pain detection threshold (PPDT), and pain intensity were recorded. Pain severity was determined with the Graded Chronic Pain Scale (GCPS).

RESULTS

Several participants (34.7%) showed a negative conditioned pain modulation and a statistically significant negative moderate correlation (P < 0.05; r = -0.316) between conditioned pain modulation and right view percentage of pain location. Temporal summation at the major trochanter showed a statistically significant low negative correlation (P < 0.05; r = -0.298). The GCPS exhibited a statistically significant moderately positive correlation (P < 0.01; r = 0.467) with the PPDT at the trochanter and a significant correlation with the postero-superior iliac spine (P < 0.01; r = 0.515) and epicondyle (P < 0.01; r = 0.566).

CONCLUSIONS

Patients with GTPS presented altered CPM, a relationship with more pain areas associated with negative CPM, and a positive association between pain severity and mechanical hyperalgesia at remote sites. Thus, physicians could apply these outcome measurements to assess primary care patients with GTPS and determine the central sensitization presence to prescribe adequate multimodal treatment approaches.