Neural correlates of temporal summation of second pain in the human brainstem and spinal cord

Temporal Summation of the Thermal Grill Illusion is Comparable to That Observed Following Noxious Heat

The thermal grill illusion (TGI) describes a peculiar or even painful percept caused by non-noxious, interlaced warm and cold stimuli. It involves the glutamatergic system and is affected in putatively nociplastic syndromes such as fibromyalgia. The glutamatergic system is also involved in wind-up, that is, the increased activation of spinal neurons following repeated noxious stimulation leading to a temporal summation of perceived stimulus intensity. Here we combined both stimulation methods to further investigate whether non-noxious stimuli as employed in the TGI can lead to a similar summation of perceived stimulus intensity. In an experiment using a full crossover within-subjects design, 35 healthy volunteers received repeated stimuli, either in a thermal grill configuration or simply noxious heat. Both modalities were presented as sequences of 1 lead-in contact, followed by 11 consecutive contacts (each between 1.5 and 3 seconds), with either fast repetition ("wind-up" condition), or 2 slow-repeating control conditions. The main analyses concerned the relative pre-to-post sequence changes to quantify putatively wind-up-related effects. Pain ratings and skin conductance level (SCL) increased more strongly in "wind-up" than in control conditions. Interestingly, wind-up-related effects were of the same magnitude in TGI as compared to the pain control modality. Further, contact-by-contact SCL tracked how the effect emerged over time. These results indicate that although TGI does not involve noxious stimuli it is amenable to temporal summation and wind-up-like processes. Since both phenomena involve the glutamatergic system, the combination of wind-up with the TGI could yield a promising tool for the investigation of chronic pain conditions. PERSPECTIVE: Using thermal stimuli in an experimental protocol to combine 1) the TGI (painful or peculiar percept from simultaneous cold/warm stimulation) and 2) wind-up (increase in stimulus intensity after repeated exposure) holds promise to investigate pain and thermoceptive mechanisms, and chronic pain conditions.

Investigating Descending Pain Regulation in Fibromyalgia and the Link to Altered Autonomic Regulation by Means of Functional MRI Data

Fibromyalgia syndrome (FM) is a chronic pain condition that affects a significant portion of the population; yet, this condition is still poorly understood. Prior research has suggested that individuals with FM display a heightened sensitivity to pain and signs of autonomic dysfunction. Recent advances in functional MRI analysis methods to model blood-oxygenation-level-dependent (BOLD) responses across networks of regions, and structural and physiological modeling (SAPM) have shown the potential to provide more detailed information about altered neural activity than was previously possible. Therefore, this study aimed to apply novel analysis methods to investigate altered neural processes underlying pain sensitivity in FM in functional magnetic resonance imaging (fMRI) data from the brainstem and spinal cord. Prior fMRI studies have shown evidence of functional differences in fibromyalgia (FM) within brain regions associated with pain's motivational aspects, as well as differences in neural activity related to pain regulation, arousal, and autonomic homeostatic regulation within the brainstem and spinal cord regions. We, therefore, hypothesized that nociceptive processing is altered in FM compared to healthy controls (HCs) in the brainstem and spinal cord areas linked to autonomic function and descending pain regulation, including the parabrachial nuclei (PBN) and nucleus tractus solitarius (NTS). We expected that new details of this altered neural signaling would be revealed with SAPM. The results provide new evidence of altered neural signaling in FM related to arousal and autonomic homeostatic regulation. This further advances our understanding of the altered neural processing that occurs in women with FM.

Through the Lens of Movement-Evoked Pain: A Theoretical Framework of the "Pain-Movement Interface" to Guide Research and Clinical Care for Musculoskeletal Pain Conditions

Over 120 million Americans report experiencing pain in the past 3 months. Among these individuals, 50 million report chronic pain and 17 million report pain that limits daily life or work activities on most days (ie, high-impact chronic pain). Musculoskeletal pain conditions in particular are a major contributor to global disability, health care costs, and poor quality of life. Movement-evoked pain (MEP) is an important and distinct component of the musculoskeletal pain experience and represents an emerging area of study in pain and rehabilitation fields. This focus article proposes the "Pain-Movement Interface" as a theoretical framework of MEP that highlights the interface between MEP, pain interference, and activity engagement. The goal of the framework is to expand knowledge about MEP by guiding scientific inquiry into MEP-specific pathways to disability, high-risk clinical phenotypes, and underlying individual influences that may serve as treatment targets. This framework reinforces the dynamic nature of MEP within the context of activity engagement, participation in life and social roles, and the broader pain experience. Recommendations for MEP evaluation, encompassing the spectrum from high standardization to high patient specificity, and MEP-targeted treatments are provided. Overall, the proposed framework and recommendations reflect the current state of science in this emerging area of study and are intended to support future efforts to optimize musculoskeletal pain management and enhance patient outcomes. PERSPECTIVE: Movement-evoked pain (MEP) is a distinct component of the musculoskeletal pain experience and emerging research area. This article introduces the "Pain-Movement Interface" as a theoretical framework of MEP, highlighting the interface between MEP, pain interference, and activity engagement. Evaluating and treating MEP could improve rehabilitation approaches and enhance patient outcomes.

Recent developments and future avenues for human corticospinal neuroimaging

Non-invasive neuroimaging serves as a valuable tool for investigating the mechanisms within the central nervous system (CNS) related to somatosensory and motor processing, emotions, memory, cognition, and other functions. Despite the extensive use of brain imaging, spinal cord imaging has received relatively less attention, regardless of its potential to study peripheral communications with the brain and the descending corticospinal systems. To comprehensively understand the neural mechanisms underlying human sensory and motor functions, particularly in pathological conditions, simultaneous examination of neuronal activity in both the brain and spinal cord becomes imperative. Although technically demanding in terms of data acquisition and analysis, a growing but limited number of studies have successfully utilized specialized acquisition protocols for corticospinal imaging. These studies have effectively assessed sensorimotor, autonomic, and interneuronal signaling within the spinal cord, revealing interactions with cortical processes in the brain. In this mini-review, we aim to examine the expanding body of literature that employs cutting-edge corticospinal imaging to investigate the flow of sensorimotor information between the brain and spinal cord. Additionally, we will provide a concise overview of recent advancements in functional magnetic resonance imaging (fMRI) techniques. Furthermore, we will discuss potential future perspectives aimed at enhancing our comprehension of large-scale neuronal networks in the CNS and their disruptions in clinical disorders. This collective knowledge will aid in refining combined corticospinal fMRI methodologies, leading to the development of clinically relevant biomarkers for conditions affecting sensorimotor processing in the CNS.

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.

Structural and Physiological Modeling (SAPM) for the Analysis of Functional MRI Data Applied to a Study of Human Nociceptive Processing

A novel method has been developed for analyzing connectivity between regions based on functional magnetic resonance imaging (fMRI) data. This method, termed structural and physiological modeling (SAPM), combines information about blood oxygenation-level dependent (BOLD) responses, anatomy, and physiology to model coordinated signaling across networks of regions, including input and output signaling from each region and whether signaling is predominantly inhibitory or excitatory. The present study builds on a prior proof-of-concept demonstration of the SAPM method by providing evidence for the choice of network model and anatomical sub-regions, demonstrating the reproducibility of the results and identifying statistical thresholds needed to infer significance. The method is further validated by applying it to investigate human nociceptive processing in the brainstem and spinal cord and comparing the results to the known neuroanatomy, including anatomical regions and inhibitory and excitatory signaling. The results of this analysis demonstrate that it is possible to obtain reliable information about input and output signaling from anatomical regions and to identify whether this signaling has predominantly inhibitory or excitatory effects. SAPM provides much more detailed information about neuroanatomy than was previously possible based on fMRI data.

The current state of spinal cord functional magnetic resonance imaging and its application in clinical research

Since its development, spinal cord functional magnetic resonance imaging (fMRI) has utilized various methodologies and stimulation protocols to develop a deeper understanding of a healthy human spinal cord that lays a foundation for its use in clinical research and practice. In this review, we conducted a comprehensive literature search on spinal cord fMRI studies and summarized the recent advancements and resulting scientific achievements of spinal cord fMRI in the following three aspects: the current state of spinal cord fMRI methodologies and stimulation protocols, knowledge about the healthy spinal cord's functions obtained via spinal cord fMRI, and fMRI's exemplary usage in spinal cord diseases and injuries. We conclude with a discussion that, while technical challenges exist, novel fMRI technologies for and new knowledge about the healthy human spinal cord have been established. Empowered by these developments, investigations of pathological and injury states within the spinal cord have become the next important direction of spinal cord fMRI. Recent clinical investigations into spinal cord pathologies, for example, fibromyalgia, multiple sclerosis, spinal cord injury, and cervical spondylotic myelopathy, have already provided deep insights into spinal cord impairments and the time course of impairment-caused changes. We expect that future spinal cord fMRI advancement and research development will further enhance our understanding of various spinal cord diseases and provide the foundation for evaluating existing and developing new treatment plans.

Distinct neural signaling characteristics between fibromyalgia and provoked vestibulodynia revealed by means of functional magnetic resonance imaging in the brainstem and spinal cord

Introduction

Fibromyalgia and provoked vestibulodynia are two chronic pain conditions that disproportionately affect women. The mechanisms underlying the pain in these conditions are still poorly understood, but there is speculation that both may be linked to altered central sensitization and autonomic regulation. Neuroimaging studies of these conditions focusing on the brainstem and spinal cord to explore changes in pain regulation and autonomic regulation are emerging, but none to date have directly compared pain and autonomic regulation in these conditions. This study compares groups of women with fibromyalgia and provoked vestibulodynia to healthy controls using a threat/safety paradigm with a predictable noxious heat stimulus.

Methods

Functional magnetic resonance imaging data were acquired at 3 tesla in the cervical spinal cord and brainstem with previously established methods. Imaging data were analyzed with structural equation modeling and ANCOVA methods during: a period of noxious stimulation, and a period before the stimulation when participants were expecting the upcoming pain.

Results

The results demonstrate several similarities and differences between brainstem/spinal cord connectivity related to autonomic and pain regulatory networks across the three groups in both time periods.

Discussion

Based on the regions and connections involved in the differences, the altered pain processing in fibromyalgia appears to be related to changes in how autonomic and pain regulation networks are integrated, whereas altered pain processing in provoked vestibulodynia is linked in part to changes in arousal or salience networks as well as changes in affective components of pain regulation.

Proof-of-concept of a novel structural equation modelling approach for the analysis of functional magnetic resonance imaging data applied to investigate individual differences in human pain responses

A novel network analysis method is demonstrated for applications with functional magnetic resonance imaging (fMRI) data. The method is based on structural equation modeling (SEM) plus modeling of physiological responses in order to explain blood oxygenation-level dependent (BOLD) responses across interconnected regions. The method, termed structural and physiological modeling (SAPM) aims to overcome a weakness of previous analysis methods by estimating both input and output signaling of every region of a network. The results also provide weighting factors (B) which describe the influence of each input signal to a region on its output signaling to another region. The SAPM method is demonstrated by applying it to fMRI data from the brainstem and spinal cord in 55 healthy participants undergoing repeated applications of a heat pain stimulation paradigm. Data are also analyzed using our established SEM method for comparison. The results with both methods indicate that individual differences in nociceptive processing are mediated by differences in descending regulation of spinal cord neurons under the influence of both the nucleus tractus solitarius and periaqueductal gray region. The SAPM results show that BOLD responses in the entire network can be explained during all periods of the stimulation paradigm based on two latent (unobserved) input signaling sources, and a model of the predicted BOLD responses to the heat stimulus. The results demonstrate the concept of our novel SAPM method and provide evidence for its validity. Additional studies are needed to further develop the method and its applications to investigations of complex neural processes across networks.

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.

Effect of sleep loss on pain-New conceptual and mechanistic avenues

Introduction

Sleep disturbances increase pain sensitivity in clinical and preclinical settings, but the precise mechanisms are unknown. This represents a major public health issue because of the growing sleep deficiency epidemic fueled by modern lifestyle. To understand the neural pathways at the intersection between sleep and pain processes, it is critical to determine the precise nature of the sleep disruptions that increase pain and the specific component of the pain response that is targeted.

Methods

We performed a review of the literature about sleep disturbances and pain sensitivity in humans and rodents by taking into consideration the targeted sleep stage (REMS, non-NREMS, or both), the amount of sleep lost, and the different types of sleep disruptions (partial or total sleep loss, duration, sleep fragmentation or interruptions), and how these differences might affect distinct components of the pain response.

Results

We find that the effects of sleep disturbances on pain are highly conserved among species. The major driver for pain hypersensitivity appears to be the total amount of sleep lost, while REMS loss by itself does not seem to have a direct effect on pain sensitivity. Sleep loss caused by extended wakefulness preferentially increases pain perception, whereas interrupted and limited sleep strongly dysregulates descending controls such as DNIC, especially in women.

Discussion

We discuss the possible mechanisms involved, including an increase in inflammatory processes, a loss of nociceptive inhibitory pathways, and a defect in the cognitive processing of noxious input.

Beyond pain in the brain: A clinician's guide to interpreting the spinal cord's role in the pain experience

INTRODUCTION

Physical therapy practice has greatly improved in providing a biopsychosocial approach when considering persistent pain. However, the spinal cord is often overlooked as a structure with an important role in modulating nociceptive information.

PURPOSE

This article highlights the role of the dorsal horn (DH) in nociceptive processing and its impact on persistent pain conditions as they appear clinically. Key processes occurring in the spinal cord are described, including cellular changes and local spinal network responses to nociceptive stimuli. Additionally, associated clinical symptoms are discussed and some aspects of physical therapy evaluation are challenged based on the mechanisms of nociceptive processing presented in this commentary.

IMPLICATIONS

The spinal cord is an active participant in nociceptive processing, directly impacting the intensity, spread, and recurrence of pain, including within the context of central sensitization. Changes in the behavior of DH neurons are possible with sufficient stimulation and may occur after injury. Additionally, spinal cord activation patterns may lead to bilateral symptoms given adequate strength and duration despite a single peripheral driver. Viewing the spinal cord as a dynamic structure capable of up or down regulating its response to stimuli gives the clinician a better understanding of the nervous system's complex response to prolonged nociceptive input.

Predictive Value of Pain Sensitization Associated with Response to Exercise Therapy in Patients with Knee Osteoarthritis: A Prospective Cohort Study

PURPOSE

Knee osteoarthritis (KOA) is a degenerative disease with inflammation, becoming persistent as it progresses, resulting in reduced quality of life. Exercise is the recommended treatment for KOA; however, the extent of pain reduction with exercise is heterogeneous and the prognostic implications of baseline factors in patients undergoing exercise are still unknown. This study examined the association between the response to exercise therapy and clinical outcomes, radiologic severity, and pain sensitization, and investigated the optimal predictive value for the effectiveness of exercise.

PATIENTS AND METHODS

Demographics, radiologic severity, pressure pain threshold (PPT), and temporal summation of pain (TSP) at the knee, tibia, and forearm were assessed at baseline. The pain numeric rating scale (NRS) was assessed before and after 12 weeks of exercise. Patients were divided into responder/non-responder groups according to recommended criteria: responder, ≥30% reduction in pain; non-responder, <30% reduction in pain, and each variable was compared between the groups. The area under the curve (AUC) and cutoff points were determined by receiver operating characteristic curve analysis.

RESULTS

Sixty-five patients were categorized as responders and 26 as non-responders. In the non-responder group, baseline NRS (P<0.01), pain duration (P<0.01), and TSP at the knee (P<0.001) and tibia (P<0.05) were significantly higher, and PPT at the knee (P<0.001), tibia (P<0.001), and forearm (P<0.001) were significantly lower, than those in the responder group; however, no significant differences between groups were found in other demographics and radiologic severity. The variables that showed moderate or better predictive ability (AUC≥0.7) were PPT at the knee (cutoff points: 241.5 kPa), tibia (307.5 kPa), forearm (318.5 kPa), and TSP at the knee (15.5 mm).

CONCLUSION

Our findings suggest that pain sensitization is associated with the response to exercise therapy. Furthermore, we provide clinically predictive values for PPT and TSP in predicting the outcome to exercise in KOA.

Effects of variability in manually contoured spinal cord masks on fMRI co-registration and interpretation

Functional magnetic resonance imaging (fMRI) of the human spinal cord (SC) is a unique non-invasive method for characterizing neurovascular responses to stimuli. Group-analysis of SC fMRI data involves co-registration of subject-level data to standard space, which requires manual masking of the cord and may result in bias of group-level SC fMRI results. To test this, we examined variability in SC masks drawn in fMRI data from 21 healthy participants from a completed study mapping responses to sensory stimuli of the C7 dermatome. Masks were drawn on temporal mean functional image by eight raters with varying levels of neuroimaging experience, and the rater from the original study acted as a reference. Spatial agreement between rater and reference masks was measured using the Dice Similarity Coefficient, and the influence of rater and dataset was examined using ANOVA. Each rater's masks were used to register functional data to the PAM50 template. Gray matter-white matter signal contrast of registered functional data was used to evaluate the spatial normalization accuracy across raters. Subject- and group-level analyses of activation during left- and right-sided sensory stimuli were performed for each rater's co-registered data. Agreement with the reference SC mask was associated with both rater (F(7, 140) = 32.12, P < 2 × 10-16, η2 = 0.29) and dataset (F(20, 140) = 20.58, P < 2 × 10-16, η2 = 0.53). Dataset variations may reflect image quality metrics: the ratio between the signal intensity of spinal cord voxels and surrounding cerebrospinal fluid was correlated with DSC results (p < 0.001). As predicted, variability in the manually-drawn masks influenced spatial normalization, and GM:WM contrast in the registered data showed significant effects of rater and dataset (rater: F(8, 160) = 23.57, P < 2 × 10-16, η2 = 0.24; dataset: F(20, 160) = 22.00, P < 2 × 10-16, η2 = 0.56). Registration differences propagated into subject-level activation maps which showed rater-dependent agreement with the reference. Although group-level activation maps differed between raters, no systematic bias was identified. Increasing consistency in manual contouring of spinal cord fMRI data improved co-registration and inter-rater agreement in activation mapping, however our results suggest that improvements in image acquisition and post-processing are also critical to address.

Evidence for Integration of Cognitive, Affective, and Autonomic Influences During the Experience of Acute Pain in Healthy Human Volunteers

Our psychological state greatly influences our perception of sensations and pain, both external and visceral, and is expected to contribute to individual pain sensitivity as well as chronic pain conditions. This investigation sought to examine the integration of cognitive and emotional communication across brainstem regions involved in pain modulation by comparing data from previous functional MRI studies of affective modulation of pain. Data were included from previous studies of music analgesia (Music), mood modulation of pain (Mood), and individual differences in pain (ID), totaling 43 healthy women and 8 healthy men. The Music and Mood studies were combined into an affective modulation group consisting of runs with music and positive-valenced emotional images plus concurrent presentation of pain, and a control group of runs with no-music, and neutral-valenced images with concurrent presentation of pain. The ID group was used as an independent control. Ratings of pain intensity were collected for each run and were analyzed in relation to the functional data. Differences in functional connectivity were identified across conditions in relation to emotional, autonomic, and pain processing in periods before, during and after periods of noxious stimulation. These differences may help to explain healthy pain processes and the cognitive and emotional appraisal of predictable noxious stimuli, in support of the Fields’ Decision Hypothesis. This study provides a baseline for current and future investigation of expanded neural networks, particularly within higher limbic and cortical structures. The results obtained by combining data across studies with different methods of pain modulation provide further evidence of the neural signaling underlying the complex nature of pain.

Music to My Senses: Functional Magnetic Resonance Imaging Evidence of Music Analgesia Across Connectivity Networks Spanning the Brain and Brainstem

Pain is often viewed and studied as an isolated perception. However, cognition, emotion, salience effects, and autonomic and sensory input are all integrated to create a comprehensive experience. Music-induced analgesia has been used for thousands of years, with moderate behavioural effects on pain perception, yet the neural mechanisms remain ambiguous. The purpose of this study was to investigate the effects of music analgesia through individual ratings of pain, and changes in connectivity across a network of regions spanning the brain and brainstem that are involved in limbic, paralimbic, autonomic, cognitive, and sensory domains. This is the first study of its kind to assess the effects of music analgesia using complex network analyses in the human brain and brainstem. Functional MRI data were collected from 20 healthy men and women with concurrent presentation of noxious stimulation and music, in addition to control runs without music. Ratings of peak pain intensity and unpleasantness were collected for each run and were analysed in relation to the functional data. We found that music alters connectivity across these neural networks between regions such as the insula, thalamus, hypothalamus, amygdala and hippocampus (among others), and is impacted by individual pain sensitivity. While these differences are important for how we understand pain and analgesia, it is essential to note that these effects are variable across participants and provide moderate pain relief at best. Therefore, a therapeutic strategy involving music should use it as an adjunct to pain management in combination with healthy lifestyle changes and/or pharmaceutical intervention.

Altered Pain in the Brainstem and Spinal Cord of Fibromyalgia Patients During the Anticipation and Experience of Experimental Pain

Chronic pain associated with fibromyalgia (FM) affects a large portion of the population but the underlying mechanisms leading to this altered pain are still poorly understood. Evidence suggests that FM involves altered neural processes in the central nervous system and neuroimaging methods such as functional magnetic resonance imaging (fMRI) are used to reveal these underlying alterations. While many fMRI studies of FM have been conducted in the brain, recent evidence shows that the changes in pain processing in FM may be linked to autonomic and homeostatic dysregulation, thus requiring further investigation in the brainstem and spinal cord. Functional magnetic resonance imaging data from 15 women with FM and 15 healthy controls were obtained in the cervical spinal cord and brainstem at 3 tesla using previously established methods. In order to investigate differences in pain processing in these groups, participants underwent trials in which they anticipated and received a predictable painful stimulus, randomly interleaved with trials with no stimulus. Differences in functional connectivity between the groups were investigated by means of structural equation modeling. The results demonstrate significant differences in brainstem/spinal cord network connectivity between the FM and control groups which also correlated with individual differences in pain responses. The regions involved in these differences in connectivity included the LC, hypothalamus, PAG, and PBN, which are known to be associated with autonomic homeostatic regulation, including fight or flight responses. This study extends our understanding of altered neural processes associated with FM and the important link between sensory and autonomic regulation systems in this disorder.

An Investigation of Descending Pain Modulation in Women With Provoked Vestibulodynia (PVD): Alterations of Spinal Cord and Brainstem Connectivity

The most common subtype of vulvodynia (idiopathic chronic vulvar pain) is provoked vestibulodynia (PVD). Previous imaging studies have shown that women with vulvodynia exhibit increased neural activity in pain-related brain regions (e.g., the secondary somatosensory cortex, insula, dorsal midcingulate, posterior cingulate, and thalamus). However, despite the recognized role of the spinal cord/brainstem in pain modulation, no previous neuroimaging studies of vulvodynia have examined the spinal cord/brainstem. Sixteen women with PVD and sixteen matched Control women underwent a spinal cord/brainstem functional magnetic resonance imaging (fMRI) session consisting of five runs with no painful thermal stimuli (No Pain), interleaved randomly with five runs with calibrated, moderately painful heat stimulation (Pain). Functional connectivity was also assessed in periods before, during, and after, pain stimulation to investigate dynamic variations in pain processing throughout the stimulation paradigm. Functional connectivity in the brainstem and spinal cord for each group was examined using structural equation modeling (SEM) for both Pain and No Pain conditions. Significant connectivity differences during stimulation were identified between PVD and Control groups within pain modulatory regions. Comparisons of Pain and No Pain conditions identified a larger number of connections in the Control group than in the PVD group, both before and during stimulation. The results suggest that women with PVD exhibit altered pain processing and indicate an insufficient response of the pain modulation system. This study is the first to examine the spinal cord/brainstem functional connectivity in women with PVD, and it demonstrates altered connectivity related to pain modulation in the spinal cord/brainstem.

An Investigation of Descending Pain Modulation in Women With Provoked Vestibulodynia: Alterations of Brain Connectivity

Provoked Vestibulodynia (PVD) is the most common vulvodynia subtype (idiopathic chronic vulvar pain). Functional magnetic resonance imaging (fMRI) studies indicate that women with PVD exhibit altered function in a number of pain modulatory regions in response to noxious stimulation, such as in the secondary somatosensory cortex, insula, dorsal midcingulate, posterior cingulate, and thalamus. However, previous neuroimaging studies of PVD have not examined periods of time before and after noxious stimulation or investigated functional connectivity among pain modulatory regions. Fourteen women with PVD and 14 matched Control participants underwent five fMRI runs with no painful stimuli interleaved randomly with five runs with calibrated, moderately painful heat stimuli applied to the thenar eminence. As recent findings indicate that pain processing begins before and continues after painful stimulation, 2-min periods were included in each run before and after the stimulus. Functional brain connectivity was assessed during both trials of Pain and No Pain stimulation for each group using structural equation modeling (SEM). Analyses of variance (ANOVAs) on connectivity values demonstrated significant main effects of study condition, and group, for connectivity among pain modulatory regions. Most of the differences between the Pain and No Pain conditions found only in the PVD group take place before (i.e., thalamus to INS, ACC to S1, thalamus to S1, and thalamus to S2) and after pain stimulation (i.e., INS to amygdala, PPC to S1, and thalamus to S2). Such differences were not observed in the Control group. These findings further support previous results indicating that women with PVD have altered pain processing compared to pain-free women.

Investigation of Correlations Between Pain Modulation Paradigms

OBJECTIVE

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

DESIGN

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

SETTING

University campus.

SUBJECTS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Spinal cord neural activity of patients with fibromyalgia and healthy controls during temporal summation of pain: an fMRI study

The cause for the increased sensitivity of patients with fibromyalgia (FM) to painful stimuli is unclear but sensitization of dorsal horn spinal cord neurons has been suggested. There, critical changes of sensory information occur which depend on the plasticity of second-order neurons and descending pain modulation, including facilitation and inhibition. This study used repetitive stimuli that produce temporal-summation-of-second-pain (TSSP) and central sensitization, relevant mechanisms for patients with chronic pain. We examined spinal cord neural activation during TSSP in patients with FM and healthy controls (HC) and used its functional connectivity with several brainstem nuclei to model the observed blood-oxygen-level-dependent (BOLD) time-course with pain ratings. Sixteen HC and 14 FM participants received repetitive heat stimuli to the hand at 0.4 Hz to achieve TSSP during functional imaging with a 3 T-Philips Achieva MRI scanner. Stimuli were adjusted to each individual's pain sensitivity to achieve maximal pain ratings of 50 ± 10 on a numerical pain scale (0-100). Using a 16-channel neurovascular coil, multiple image series were obtained from the cervical spinal cord to the brainstem using single-shot turbo-spin echo sequences. During repetitive, sensitivity-adjusted heat stimuli, pain ratings of all subjects increased as predicted, consistent with TSSP. HC and FM participants had similar temporal patterns of spinal activation: initial BOLD increase followed by deactivation. Structural equation modeling showed that the observed spinal activity during TSSP was associated with more BOLD activity across/within the brainstem in FM subjects than HC, suggesting differences in pain modulation.NEW & NOTEWORTHY "Windup" and its behavioral correlate "temporal-summation-of-second pain" (TSSP) represent spinal cord mechanisms of pain augmentation associated with central sensitization and chronic pain. Fibromyalgia (FM) is a chronic pain disorder, where abnormal TSSP has been demonstrated. We used fMRI to study spinal cord and brainstem activation during TSSP. We characterized the time course of spinal cord and brainstem BOLD activity during TSSP which showed abnormal brainstem activity in patients with FM, possibly due to deficient pain modulation.

Investigation of the neural basis of expectation-based analgesia in the human brainstem and spinal cord by means of functional magnetic resonance imaging

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Expectation of lower pain results in lower perceived pain in healthy humans.

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This expectation analgesia is mediated by descending regulation of the spinal cord.

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Connectivity analyses showed effects of expecting lower pain prior to stimulation.

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Expectation analgesia involves regions linked to arousal and autonomic regulation.

Purpose

The expected intensity of pain resulting from a noxious stimulus has been observed to have a strong influence on the pain that is perceived. The neural basis of pain reduction, as a result of expecting lower pain, was investigated using functional magnetic resonance imaging (fMRI) in the brainstem and spinal cord.

Methods

Functional MRI studies were carried out in a region spanning the brainstem and cervical spinal cord in healthy participants. Participants were familiarized with a noxious heat stimulus and study procedures in advance, and were informed during each trial that either a heat calibrated to produce moderate pain (Base state), or a temperature 1 °C lower (Low state), would be applied to their hand. However, the Base temperature was applied in every trial.

Results

Pain ratings were significantly reduced as a result of expecting lower temperatures. FMRI results demonstrate blood oxygenation-level dependent (BOLD) signal variations in response to participants being informed of the stimulus to expect, in advance of stimulation, and in response to stimulation. Significant coordination of BOLD signals was also detected across specific brainstem and spinal cord regions, with connectivity strengths that varied significantly with the study condition, and with individual pain ratings. The results identify regions that are known to be involved with arousal and autonomic regulation.

Conclusions

Expectation-based analgesia is mediated by descending regulation of spinal cord nociceptive responses. This regulation appears to be related to arousal and autonomic regulation, consistent with the cognitive/affective dimension of pain.

Brainstem Pain-Modulation Circuitry and Its Plasticity in Neuropathic Pain: Insights From Human Brain Imaging Investigations

Acute pain serves as a protective mechanism that alerts us to potential tissue damage and drives a behavioural response that removes us from danger. The neural circuitry critical for mounting this behavioural response is situated within the brainstem and is also crucial for producing analgesic and hyperalgesic responses. In particular, the periaqueductal grey, rostral ventromedial medulla, locus coeruleus and subnucleus reticularis dorsalis are important structures that directly or indirectly modulate nociceptive transmission at the primary nociceptive synapse. Substantial evidence from experimental animal studies suggests that plasticity within this system contributes to the initiation and/or maintenance of chronic neuropathic pain, and may even predispose individuals to developing chronic pain. Indeed, overwhelming evidence indicates that plasticity within this circuitry favours pro-nociception at the primary synapse in neuropathic pain conditions, a process that ultimately contributes to a hyperalgesic state. Although experimental animal investigations have been crucial in our understanding of the anatomy and function of the brainstem pain-modulation circuitry, it is vital to understand this system in acute and chronic pain states in humans so that more effective treatments can be developed. Recent functional MRI studies have identified a key role of this system during various analgesic and hyperalgesic responses including placebo analgesia, offset analgesia, attentional analgesia, conditioned pain modulation, central sensitisation and temporal summation. Moreover, recent MRI investigations have begun to explore brainstem pain-modulation circuitry plasticity in chronic neuropathic pain conditions and have identified altered grey matter volumes and functioning throughout the circuitry. Considering the findings from animal investigations, it is likely that these changes reflect a shift towards pro-nociception that ultimately contributes to the maintenance of neuropathic pain. The purpose of this review is to provide an overview of the human brain imaging investigations that have improved our understanding of the pain-modulation system in acute pain states and in neuropathic conditions. Our interpretation of the findings from these studies is often guided by the existing body of experimental animal literature, in addition to evidence from psychophysical investigations. Overall, understanding the plasticity of this system in human neuropathic pain conditions alongside the existing experimental animal literature will ultimately improve treatment options.

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.

Association of Chronic Pain with Radiologic Severity and Central Sensitization in Hip Osteoarthritis Patients

Purpose

Pain and joint deformity are the most common symptoms of hip osteoarthritis (OA). However, no significant association between pain and severity of radiographic lesions has been reported. Recently, central sensitization has been suggested as an underlying mechanism of pain in OA. We investigated the involvement of radiologic severity or central sensitization in the clinical manifestation of hip OA with various degrees of joint deformity.

Patients and Methods

We included 39 patients with hip OA and divided them into two groups according to the severity of the hip pain: strong/severe (numerical rating scale, NRS≥6) and mild/moderate (NRS<6). We assessed the radiologic severity of OA using the Kellgren-Lawrence (K-L) scale and minimum joint space width (mJSW). We conducted quantitative sensory testing (QST) that included pressure pain threshold (PPT) and temporal summation of pain (TSP) at hip, tibialis anterior (leg), and extensor carpi radialis longus (arm) on the affected side. We examined the difference of radiologic assessment and QST results between each group and the correlation of the NRS with the radiologic assessment and QST results.

Results

There was no significant difference in the K-L scale and mJSW between patients with strong/severe and mild/moderate joint pain. Strong/severe pain patients demonstrated a lower PPT at all measurement sites and higher TSP at the hip and leg than the mild/moderate pain patients. In addition, NRS was significantly negatively correlated with PPT and positively correlated with TSP at all measurement sites, but not with the K-L scale and mJSW.

Conclusion

We reported no significant difference in radiologic severity between patients with strong/severe and mild/moderate joint pain. By contrast, we found a significant difference in central sensitization represented by QST between strong/severe and mild/moderate joint pain groups. These results suggest that central sensitization may be involved in the joint pain of patients with hip OA who complain of severe pain despite less severe joint deformity.

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.

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.

A comparison of the effectiveness of functional MRI analysis methods for pain research: The new normal

Studies of the neural basis of human pain processing present many challenges because of the subjective and variable nature of pain, and the inaccessibility of the central nervous system. Neuroimaging methods, such as functional magnetic resonance imaging (fMRI), have provided the ability to investigate these neural processes, and yet commonly used analysis methods may not be optimally adapted for studies of pain. Here we present a comparison of model-driven and data-driven analysis methods, specifically for the study of human pain processing. Methods are tested using data from healthy control participants in two previous studies, with separate data sets spanning the brain, and the brainstem and spinal cord. Data are analyzed by fitting time-series responses to predicted BOLD responses in order to identify significantly responding regions (model-driven), as well as with connectivity analyses (data-driven) based on temporal correlations between responses in spatially separated regions, and with connectivity analyses based on structural equation modeling, allowing for multiple source regions to explain the signal variations in each target region. The results are assessed in terms of the amount of signal variance that can be explained in each region, and in terms of the regions and connections that are identified as having BOLD responses of interest. The characteristics of BOLD responses in identified regions are also investigated. The results demonstrate that data-driven approaches are more effective than model-driven approaches for fMRI studies of pain.

Cortico-spinal imaging to study pain

Functional magnetic resonance imaging of the brain has helped to reveal mechanisms of pain perception in health and disease. Recently, imaging approaches have been developed that allow recording neural activity simultaneously in the brain and in the spinal cord. These approaches offer the possibility to examine pain perception in the entire central pain system and in addition, to investigate cortico-spinal interactions during pain processing. Although cortico-spinal imaging is a promising technique, it bears challenges concerning data acquisition and data analysis strategies. In this review, we discuss studies that applied simultaneous imaging of the brain and spinal cord to explore central pain processing. Furthermore, we describe different MR-related acquisition techniques, summarize advantages and disadvantages of approaches that have been implemented so far and present software that has been specifically developed for the analysis of spinal fMRI data to address challenges of spinal data analysis.

Coordinated Human Brainstem and Spinal Cord Networks during the Expectation of Pain Have Elements Unique from Resting-State Effects

Functional magnetic resonance imaging (fMRI) research on the human brainstem (BS) and spinal cord (SC) has identified extensive BS/SC resting-state networks (RSNs) by showing spontaneous coordinated blood oxygenation-level dependent (BOLD) signal fluctuations in the absence of a stimulus. Studies have shown that these networks can be influenced by participants’ level of arousal or attention (e.g., watching a video), and linked network function to autonomic homeostatic regulation. Here we explore how the cognitive state of expecting pain can influence connectivity in these networks. Data from two studies (a predictable pain stimulus study, and a resting-state study) were compared to show the effects of expecting pain on BS/SC networks, and how networks differed from networks associated with the resting-state. In each study, BOLD fMRI data were obtained from the cervical SC and brainstem in healthy participants at 3 tesla using a T₂-weighted single-shot fast spin-echo imaging method. Functional connectivity was investigated within the entire 3D volume by means of structural equation modeling (SEM) and analyses of covariance (ANCOVA). Results showed extensive connectivity within/across BS and SC regions during the expectation of pain, and ANCOVA analyses showed that connectivity in specific components of these networks varied with individual pain sensitivity. Comparing these results to RSN fluctuations revealed commonalities in coordination between BS and SC regions, and specific BS–BS connectivity fluctuations unique to the expectation of pain. Based on the regions involved, these results provide evidence of brainstem regulation specific to the expectation of pain.

Novel Characterization Of Thermal Temporal Summation Response By Analysis Of Continuous Pain Vs Time Curves And Exploratory Modeling

Background

Temporal summation (TS) refers to the increased perception of pain with repetitive noxious stimuli. While thermal TS is generally considered a behavioral correlate of spinal windup, noxious heat pulses also trigger additional sensory processes which were modeled in this study.

Methods

Nineteen healthy volunteers (9 females, mean age 29.2, SD 10.5) underwent two identical TS experiments, spaced a week apart. The TS paradigm consisted of 10 identical heat pulses with individualized temperatures at the thenar eminence (0.5Hz). We extracted 3 features from continuous TS response curves: Lag, time to first feel pain; Slope, the rate of pain increase between the first and most painful heat pulse; and Delta, the maximum drop in pain after peak pain is reached. We then examined the within-individual stability of these features, followed by the Pearson’s correlations among these features and between the features and negative affect.

Results

All 3 features were stable over 1 week. Lag and Delta were negatively correlated (r = −0.5, p = 0.042). Slope did not correlate with Lag or Delta, but strongly correlated with a traditional TS measure, first pulse pain and peak pain difference (r = 0.91, p < 0.0001). Negative affects such as trait and state anxiety were negatively correlated with baseline (r = −0.49, p = 0.031) and peak stimulating temperature (r = −0.48, p = 0.039), respectively, suggesting an association between anxiety and greater pain sensitivity.

Conclusion

We were able to decouple spinal windup from other perceptual processes generated by phasic thermal TS paradigms and demonstrate temporal stability of these curve features. These curve features may help better characterize the complex sensory response to noxious heat pulses and serve as biomarkers to profile patients with chronic pain.

Childhood Adversity and Pain Facilitation

OBJECTIVE

This study investigated whether childhood adversity would be associated with hypersensitivity on two measures of central pain facilitation: area of secondary allodynia and temporal summation of second pain (TSSP), and whether pain facilitation would be explained by adult posttraumatic stress disorder (PTSD) symptoms.

METHOD

Participants endorsing high (n = 31) and low (n = 31) childhood adversity underwent capsaicin-induced secondary allodynia and TSSP testing. The tests were conducted a week apart with test order counterbalanced.

RESULTS

Larger areas of secondary allodynia were observed in the high adversity group compared with the low adversity group (F(1,60) = 4.81, p = .032). This group difference was largely (62%) explained by greater PTSD symptoms in the high adversity group. Although no overall difference was found in TSSP slopes (p = .886), this was attributed to an order by group interaction (F(1,58) = 5.07, p = .028) and low power. Subsequent analyses revealed positive TSSP slopes in the high adversity group when TSSP testing was performed first, and this order effect was associated with blunted sympathetic responses to TSSP on the first visit. The two facilitation measures were unrelated (p = .631).

CONCLUSIONS

Larger areas of secondary allodynia were observed in the high adversity group, which was explained largely by PTSD symptoms. This suggests that adversity-related changes in pain facilitation may underlie the association between childhood adversity and generalized widespread pain. Although TSSP was affected by previous testing, adversity-related pain facilitation was observed when TSSP testing occurred first. Finally, adversity was not associated with a consistent pattern of hypersensitivity across the two measures of central pain facilitation.

Usefulness of Ramp & Hold Procedures for Testing of Pain Facilitation in Human Participants: Comparisons With Temporal Summation of Second Pain

Quantitative sensory testing (QST) is used to systematically interrogate normal responding and alterations of nervous system function, including pain-related central sensitization (CS). However, up to now, QST of CS in human subjects has been mostly focused on temporal summation of second pain (TSSP), has been difficult to perform, and has been associated with low reliability. In contrast, slow ramp & hold (RH) procedures are simpler tests of temporal summation and easier to perform. We examined the usefulness of RH procedures as reliable generators of CS using 2 validated QST procedures: decay of pain aftersensations and wind-down. Twenty-seven pain-free subjects (74% female) were enrolled into the study. Trains of sensitivity-adjusted TSSP or RH heat stimuli were applied to the hands of participants to achieve moderate temporal pain summation (50 Numerical Rating Scale [NRS] [0-100]). Fifteen-second aftersensations and 30-second wind-down related to TSSP or RH were used for CS comparisons. Reliability of all test procedures was tested over 24 hours. Use of sensitivity-adjusted TSSP and RH heat stimuli resulted in average pain ratings of 48.2 and 49.6 NRS, respectively. Aftersensations or wind-down decay were not significantly different after either TSSP or RH, (all P > .05), indicating that each procedure achieved similar levels of short-term CS. Sensitivity-adjusted RH stimuli were well tolerated and resulted in reliable pain increases of ∼50 NRS. The magnitude of short-term CS, determined by aftersensations and wind-down was similar after sensitivity-adjusted TSSP and RH stimuli (P > .05), suggesting that pain facilitation of healthy participants and likely chronic pain patients can not only be tested with TSSP but also with RH procedures. PERSPECTIVE: This article examines the ability of RH procedures to generate similar central sensitivity augmentation than TSSP. The results suggest that RH is similarly well suited as TSSP to explore central pain mechanisms in healthy subjects and most likely also in chronic pain patients.

Brainstem neuroimaging of nociception and pain circuitries

The brainstem is known to be an important brain area for nociception and pain processing, and both relaying and coordinating signaling between the cerebrum, cerebellum, and spinal cord. Although preclinical models of pain have characterized the many roles that brainstem nuclei play in nociceptive processing, the degree to which these circuitries extend to humans is not as well known. Unfortunately, the brainstem is also a very challenging region to evaluate in humans with neuroimaging. The challenges for human brainstem imaging arise from the location of this elongated brain structure, proximity to cardiorespiratory noise sources, and the size of its constituent nuclei. These challenges can require dedicated approaches to brainstem imaging, which should be adopted when study hypotheses are focused on brainstem processing of nociception or modulation of pain perception. In fact, our review will highlight many pain neuroimaging studies that have reported some brainstem involvement in nociceptive processing and chronic pain pathology. However, we note that with recent advances in neuroimaging leading to improved spatial and temporal resolution, more studies are needed that take advantage of data collection and analysis methods focused on the challenges of brainstem neuroimaging.

Inhibition of electroacupuncture on nociceptive responses of dorsal horn neurons evoked by noxious colorectal distention in an intensity-dependent manner

Background

The transmission of visceral nociception can be inhibited by electroacupuncture (EA) at the spinal level. However, relationships between current intensity and EA-induced analgesia are still lacking. This study compares the effects of different intensities of EA at local acupoints and heterotopic acupoints on nociceptive responses of spinal wide dynamic range (WDR) neurons induced by noxious colorectal distension (CRD).

Materials and methods

Experiments were conducted on 40 Sprague Dawley rats anesthetized with 10% urethane. Discharges of WDR neurons in the L1–L3 segments of the dorsal horn of the spinal cord were recorded extracellularly by glass micropipettes. Different intensities of EA (0.5, 1, 2, 4, 6, and 8 mA, 0.5 ms, 2 Hz) were applied to contralateral “Zusanli” (ST 36) or “Neiguan” (PC 6), with either the same or different segmental innervation of the colon.

Results

In local acupoints, the increased discharges of WDR neurons evoked by CRD were significantly inhibited by EA at 0.5–8 mA. A positive relationship between current intensity and the inhibiting rate was observed within 0.5–4 mA, but the inhibiting rate reached a plateau when EA exceeded 4 mA. In heterotopic acupoints, the increased discharges of WDR neurons evoked by CRD were significantly inhibited by EA at 2–8 mA. A positive relationship between current intensity and the inhibiting rate was observed within 2–6 mA. Further increase in the current beyond 6 mA also resulted in a plateau effect.

Conclusion

Within a certain range, the nociceptive responses of dorsal horn neurons induced by CRD could be inhibited by EA in an intensity-dependent manner.

Developing an optimized strategy with transcranial direct current stimulation to enhance the endogenous pain control system in fibromyalgia

INTRODUCTION

Fibromyalgia affects more than 5 million people in the United States and has a detrimental impact on individuals' quality of life. Current pharmacological treatments provide limited benefits to relieve the pain of fibromyalgia, along with a risk of adverse effects; a scenario that explains the increasing interest for multimodal approaches. A tailored strategy to focus on this dysfunctional endogenous pain inhibitory system is transcranial direct current stimulation (tDCS) of the primary motor cortex. By combining tDCS with aerobic exercise, the effects can be optimized. Areas covered: The relevant literature was reviewed and discussed the methodological issues for designing a mechanistic clinical trial to test this combined intervention. Also, we reviewed the neural control of different pathways that integrate the endogenous pain inhibitory system, as well as the effects of tDCS and aerobic exercise both alone and combined. In addition, potential neurophysiological assessments are addressed: conditioned pain modulation, temporal slow pain summation, transcranial magnetic stimulation, and electroencephalography in the context of fibromyalgia. Expert commentary: By understanding the neural mechanisms underlying pain processing and potential optimized interventions in fibromyalgia with higher accuracy, the field has an evident potential of advancement in the direction of new neuromarkers and tailored therapies.

Ten Key Insights into the Use of Spinal Cord fMRI

A comprehensive review of the literature-to-date on functional magnetic resonance imaging (fMRI) of the spinal cord is presented. Spinal fMRI has been shown, over more than two decades of work, to be a reliable tool for detecting neural activity. We discuss 10 key points regarding the history, development, methods, and applications of spinal fMRI. Animal models have served a key purpose for the development of spinal fMRI protocols and for experimental spinal cord injury studies. Applications of spinal fMRI span from animal models across healthy and patient populations in humans using both task-based and resting-state paradigms. The literature also demonstrates clear trends in study design and acquisition methods, as the majority of studies follow a task-based, block design paradigm, and utilize variations of single-shot fast spin-echo imaging methods. We, therefore, discuss the similarities and differences of these to resting-state fMRI and gradient-echo EPI protocols. Although it is newly emerging, complex connectivity and network analysis is not only possible, but has also been shown to be reliable and reproducible in the spinal cord for both task-based and resting-state studies. Despite the technical challenges associated with spinal fMRI, this review identifies reliable solutions that have been developed to overcome these challenges.

Evidence for a spinal involvement in temporal pain contrast enhancement

Spatiotemporal filtering and amplification of sensory information at multiple levels during the generation of perceptual representations is a fundamental processing principle of the nervous system. While for the visual and auditory system temporal filtering of sensory signals has been noticed for a long time, respective contrast mechanisms within the nociceptive system became only recently subject of investigations, mainly in the context of offset analgesia (OA) subsequent to noxious stimulus decreases. In the present study we corroborate in a first experiment the assumption that offset analgesia involves a central component by showing that an OA-like effect accounting for 74% of a corresponding OA reference can be evoked by decomposing the stimulus offset into two separate box-car stimuli applied within the same dermatome but to separate populations of primary afferent neurons. In order to draw conclusions about the levels of the CNS at which temporal filtering of nociceptive information takes place during OA we investigate in a second experiment neuronal activity in the spinal cord during a painful thermal stimulus offset employing high-resolution fMRI in healthy volunteers. Pain-related BOLD responses in the spinal cord were significantly reduced during OA and their time course followed widely behavioral hypoalgesia, but not the thermal stimulation profile. In summary, the results suggest that temporal pain contrast enhancement during OA comprises a central mechanism and this mechanism becomes already effective at the level of the spinal cord.

Functional Neuroimaging of Nociceptive and Pain-Related Activity in the Spinal Cord and Brain: Insights From Neurovascular Coupling Studies

Spinal cord and brain processes underlie pain perception, which produces systemic cardiovascular changes. In turn, the autonomic nervous system regulates vascular function in the spinal cord and brain in order to adapt to these systemic changes, while neuronal activity induces local vascular changes. Thus, autonomic regulation and pain processes in the brain and spinal cord are tightly linked and interrelated. The objective of this topical review is to discuss work on neurovascular coupling during nociceptive processing in order to highlight supporting evidence and limitations for the use of cerebral and spinal fMRI to investigate pain mechanisms and spinal nociceptive processes. Work on functional neuroimaging of pain is presented and discussed in relation to available neurovascular coupling studies and related issues. Perspectives on future work are also discussed with an emphasis on differences between the brain and the spinal cord and on different approaches that may be useful to improve current methods, data analyses and interpretation. In summary, this review highlights the lack of data on neurovascular coupling during nociceptive stimulation and indicates that hemodynamic and BOLD responses measured with fMRI may be biased by nonspecific vascular changes. Future neuroimaging studies on nociceptive and pain-related processes would gain further understanding of neurovascular coupling in the brain and spinal cord and should take into account the effects of systemic vascular changes that may affect hemodynamic responses. Anat Rec, 301:1585-1595, 2018. © 2018 Wiley Periodicals, Inc.

Frequency-dependent top-down modulation of temporal summation by anodal transcranial direct-current stimulation of the primary motor cortex in healthy adults

BACKGROUND

Transcranial direct-current stimulation (tDCS) applied over the primary motor cortex has been shown to be effective in the treatment of a number of chronic pain conditions. However, there is a lack of understanding of the top-down analgesic mechanisms involved.

METHOD

In this study, we investigated the effects of tDCS on the facilitation of subjective sensory and pain scores using a transcutaneous electrically evoked measure of temporal summation. In this randomized, blinded, cross-over study healthy subjects received a single stimulus given at 0.9× pain threshold (pTh) over the L5 dermatome on the lateral aspect of the right leg, followed by a train of 5 stimuli given at 0.5, 1, 5 and 20 Hz before and after 20 min of sham or anodal tDCS (2 mA) applied over the primary motor cortex. Ratings of sensation and pain intensity were scored on a visual analogue scale (VAS).

RESULTS

Temporal summation leading to pain only occurred at higher frequencies (5 and 20 Hz). Sham or real tDCS had no effect over temporal summation evoked at 5 Hz; however, there was a significant analgesic effect at 20 Hz. Sham or real tDCS had no effect over acute, single stimuli-evoked responses.

CONCLUSION

These results indicate that anodal tDCS applied to the primary motor cortex preferentially modulates temporal summation induced by high-frequency electrical stimulation-induced pain. The inhibitory effects of tDCS appear to be dynamic and dependent on the degree of spinal cord excitability and may explain the higher analgesic efficacy in patients with moderate to severe chronic pain symptoms.

SIGNIFICANCE

The analgesic effects of tDCS are dependent on spinal cord excitability. This work provides insight into top-down modulation during acute pain and temporal summation. This knowledge may explain why tDCS has a higher analgesic efficacy in chronic pain patients.

The role of fMRI in drug development

Functional magnetic resonance imaging (fMRI) has been known for over a decade to have the potential to greatly enhance the process of developing novel therapeutic drugs for prevalent health conditions. However, the use of fMRI in drug development continues to be relatively limited because of a variety of technical, biological, and strategic barriers that continue to limit progress. Here, we briefly review the roles that fMRI can have in the drug development process and the requirements it must meet to be useful in this setting. We then provide an update on our current understanding of the strengths and limitations of fMRI as a tool for drug developers and recommend activities to enhance its utility.

Continuous Descending Modulation of the Spinal Cord Revealed by Functional MRI

Spontaneous variations in spinal cord activity may arise from regulation of any of a number of functions including sensory, motor, and autonomic control. Here, we use functional MRI (fMRI) of healthy participants to identify properties of blood oxygenation-level dependent (BOLD) variations in the spinal cord in response to knowledge that either a noxious stimulus is impending, or that no stimulus is to be expected. Expectation of a noxious stimulus, or no stimulus, is shown to have a significant effect on wide-spread BOLD signal variations in the spinal cord over the entire time period of the fMRI acquisition. Coordination of BOLD responses between/within spinal cord and brainstem regions are also influenced by this knowledge. We provide evidence that such signal variations are the result of continuous descending modulation of spinal cord function. BOLD signal variations in response to noxious stimulation of the hand are also shown, as in previous studies. The observation of both continuous and reactive BOLD responses to emotional/cognitive factors and noxious peripheral stimulation may have important implications, not only for our understanding of endogenous pain modulation, but also in showing that spinal cord activity is under continuous regulatory control.