Unravelling the mystery of pain, suffering, and relief with brain imaging

Network targets for therapeutic brain stimulation: towards personalized therapy for pain

Precision neuromodulation of central brain circuits is a promising emerging therapeutic modality for a variety of neuropsychiatric disorders. Reliably identifying in whom, where, and in what context to provide brain stimulation for optimal pain relief are fundamental challenges limiting the widespread implementation of central neuromodulation treatments for chronic pain. Current approaches to brain stimulation target empirically derived regions of interest to the disorder or targets with strong connections to these regions. However, complex, multidimensional experiences like chronic pain are more closely linked to patterns of coordinated activity across distributed large-scale functional networks. Recent advances in precision network neuroscience indicate that these networks are highly variable in their neuroanatomical organization across individuals. Here we review accumulating evidence that variable central representations of pain will likely pose a major barrier to implementation of population-derived analgesic brain stimulation targets. We propose network-level estimates as a more valid, robust, and reliable way to stratify personalized candidate regions. Finally, we review key background, methods, and implications for developing network topology-informed brain stimulation targets for chronic pain.

Endogenous opioid systems alterations in pain and opioid use disorder

Decades of research advances have established a central role for endogenous opioid systems in regulating reward processing, mood, motivation, learning and memory, gastrointestinal function, and pain relief. Endogenous opioid systems are present ubiquitously throughout the central and peripheral nervous system. They are composed of four families, namely the μ (MOPR), κ (KOPR), δ (DOPR), and nociceptin/orphanin FQ (NOPR) opioid receptors systems. These receptors signal through the action of their endogenous opioid peptides β-endorphins, dynorphins, enkephalins, and nociceptins, respectfully, to maintain homeostasis under normal physiological states. Due to their prominent role in pain regulation, exogenous opioids-primarily targeting the MOPR, have been historically used in medicine as analgesics, but their ability to produce euphoric effects also present high risks for abuse. The ability of pain and opioid use to perturb endogenous opioid system function, particularly within the central nervous system, may increase the likelihood of developing opioid use disorder (OUD). Today, the opioid crisis represents a major social, economic, and public health concern. In this review, we summarize the current state of the literature on the function, expression, pharmacology, and regulation of endogenous opioid systems in pain. Additionally, we discuss the adaptations in the endogenous opioid systems upon use of exogenous opioids which contribute to the development of OUD. Finally, we describe the intricate relationship between pain, endogenous opioid systems, and the proclivity for opioid misuse, as well as potential advances in generating safer and more efficient pain therapies.

Pain in Persons with Disorders of Consciousness

Pain and suffering in persons with disorders of consciousness (DoC) remain poorly understood, frequently unaddressed or inadequately addressed, and controversial on numerous levels. This narrative literature review will address a number of critical issues germane to pain and suffering in this challenging group of patients, providing an introductory overview of the topic, perspectives on current knowledge regarding pain pathoanatomy and pathophysiology, and a review of common pain generators and factors that can lead to the chronifcation of pain. Caveats on bedside pain assessment challenges, as well as electrophysiologic and neuroimaging findings in these patients, will also be explored. Pain management techniques, including non-pharmacological and pharmacological, will be reviewed. Ethical considerations in the context of pain and suffering in persons with disorders of consciousness will round out the review prior to our concluding comments.

The prevalence of painful physical symptoms in major depressive disorder: A systematic review and meta-analysis of observational studies

BACKGROUND

Painful physical symptoms (PPS) are common in patients with major depressive disorder (MDD), but their prevalence has been mixed. This is a systematic review and meta-analysis of the pooled prevalence of PPS in MDD patients.

METHODS

Systematic literature searches were independently conducted in major databases (PubMed, EMBASE, PsycINFO and Web of Science). Data analyses were conducted using a random-effects model.

RESULTS

A total of 20 studies with 53,852 patients were included. The overall prevalence of PPS in MDD patients was 55.2% (95%CI: 47.9-62.3%), with a point prevalence of 64.2% (95%CI: 53.2-73.8%) and a 12-month prevalence of 57.0% (95%CI: 23.9-84.8%). No significant publication bias was found in this meta-analysis.

CONCLUSION

PPS are common in MDD patients. Considering the negative impact of PPS on daily functioning, effective preventive measures and routine screening should be conducted for MDD patients, and timely treatments should be offered to those in need. Registration number: CRD42020179471.

Brain Responses to Noxious Stimuli in Patients With Chronic Pain: A Systematic Review and Meta-analysis

IMPORTANCE

Functional neuroimaging is a valuable tool for understanding how patients with chronic pain respond to painful stimuli. However, past studies have reported heterogenous results, highlighting opportunities for a quantitative meta-analysis to integrate existing data and delineate consistent associations across studies.

OBJECTIVE

To identify differential brain responses to noxious stimuli in patients with chronic pain using functional magnetic resonance imaging (fMRI) while adhering to current best practices for neuroimaging meta-analyses.

DATA SOURCES

All fMRI experiments published from January 1, 1990, to May 28, 2019, were identified in a literature search of PubMed/MEDLINE, EMBASE, Web of Science, Cochrane Library, PsycINFO, and SCOPUS.

STUDY SELECTION

Experiments comparing brain responses to noxious stimuli in fMRI between patients and controls were selected if they reported whole-brain results, included at least 10 patients and 10 healthy control participants, and used adequate statistical thresholding (voxel-height P < .001 or cluster-corrected P < .05). Two independent reviewers evaluated titles and abstracts returned by the search. In total, 3682 abstracts were screened, and 1129 full-text articles were evaluated.

DATA EXTRACTION AND SYNTHESIS

Thirty-seven experiments from 29 articles met inclusion criteria for meta-analysis. Coordinates reporting significant activation differences between patients with chronic pain and healthy controls were extracted. These data were meta-analyzed using activation likelihood estimation. Data were analyzed from December 2019 to February 2020.

MAIN OUTCOMES AND MEASURES

A whole-brain meta-analysis evaluated whether reported differences in brain activation in response to noxious stimuli between patients and healthy controls were spatially convergent. Follow-up analyses examined the directionality of any differences. Finally, an exploratory (nonpreregistered) region-of-interest analysis examined differences within the pain network.

RESULTS

The 37 experiments from 29 unique articles included a total of 511 patients and 433 controls (944 participants). Whole-brain meta-analyses did not reveal significant differences between patients and controls in brain responses to noxious stimuli at the preregistered statistical threshold. However, exploratory analyses restricted to the pain network revealed aberrant activity in patients.

CONCLUSIONS AND RELEVANCE

In this systematic review and meta-analysis, preregistered, whole-brain analyses did not reveal aberrant fMRI activity in patients with chronic pain. Exploratory analyses suggested that subtle, spatially diffuse differences may exist within the pain network. Future work on chronic pain biomarkers may benefit from focus on this core set of pain-responsive areas.

Subjective suffering in patients with low back pain

AIM

Low back pain (LPB) is a prevalent rheumatic and musculoskeletal disease and patients often experience physical discomfort and functional limitations. This study aimed to evaluate how subjective suffering relates to illness perceptions, pain intensity and psychological morbidity as well as to understand the mediator (direct and indirect) effects on functional disability.

METHODS

A cross-sectional study with 125 patients who received physiotherapy and answered the following instruments: Illness Perception Questionnaire-Revised; Illness Subjective Suffering Inventory; Hospital Anxiety and Depression Scale and the Oswestry LBP Incapacity Questionnaire.

RESULTS

In the structural equation modeling analysis, subjective suffering totally mediated the relationship between both emotional response (estimate = 0.068, 95% highest posterior density interval [HPDI] = [0.017, 0.133]), consequences (estimate = 0.118, 95% HPDI = [0.035, 0.211]) and functional disability. The relationship between coherence and disability was partially mediated by subjective suffering (estimate = -0.067, 95% HPDI = [-0.134, -0.016]). Only identity and pain intensity showed direct effects on functional disability.

CONCLUSION

Intervention in patients with LBP should focus on subjective suffering to reduce functional disability, providing information and promoting the acceptance of disease and pain.

Sensitized brain response to acute pain in patients using prescription opiates for chronic pain: A pilot study

BACKGROUND

Chronic opiate use leads to a sensitized behavioral response to acute pain, which in turn, leads to escalating doses of opiates. This study was designed to test the hypothesis that chronic opiate usage is also associated with a sensitized neurobiological response to acute pain in individuals that have used prescription opiates for 6 or more months.

METHODS

Fourteen patients with non-alcoholic chronic pancreatitis that have been taking prescription opiates for 6 or more months and 14 gender matched, non-opiate using controls were enrolled. Functional neuroimaging data was acquired while participants received blocks of thermal stimulation to their wrist (individually-tailored to their pain threshold).

RESULTS

Self-reported pain was significantly greater in opiate using patients (3.4 ± 3.4) than controls (0.2 ± 0.8: Brief Pain Inventory p < 0.005), however no significant difference between groups was observed in the individually-tailored pain thresholds. Opiate using patients evidenced a significantly greater response to pain than controls in two established nodes of the "Pain Matrix": somatosensory cortex (p_FWE≤0.001) and anterior cingulate cortex (p ≤ 0.01). This response was positively correlated with prescribed morphine equivalent dosages (average: 133.5 ± 94.8 mg/day).

CONCLUSION

The findings suggest that in chronic pancreatitis patients, a dose of opiates that normalizes their behavioral response to acute pain is associated with an amplified neural response to acute pain. Further longitudinal studies are needed to determine if this neural sensitization hastens a behavioral tolerance to opiates or the development of an opioid use disorder.

Are Multidimensional Pain Inventory Coping Strategy Profiles Associated with Long-Term Spinal Cord Stimulation Effectiveness?

Introduction

It is acknowledged that the way patients cope with pain may influence treatment outcome. In particular, psychological factors are deemed important when considering patients for suitability for spinal cord stimulation (SCS).

Objective

The aim of the study is to observe how pre-implantation psychological characteristics impact the effectiveness of SCS for chronic pain.

Methods

The analysis comprised data from 137 patients who underwent an SCS implant. Screening evaluation included a coping strategies profile (Multidimensional Pain Inventory) and psychiatric disorders (Mini-International Neuropsychiatric Interview). Based on SCS implant outcome collected during follow-up visits, patients were divided into three groups: subjects with long-term pain relief (long-term group), subjects who failed the SCS treatment and decided to explant trial device (trial explanter group [TE]), and those who chose a permanent device (permanent explanter group [PE]).

Results

Results showed that most of the patients who failed with the SCS (TE and PE groups) demonstrated a dysfunctional coping profile and showed a higher presence of psychiatric disorders, which significantly influenced the experience and perception of pain.

Conclusions

The findings of this study support the value of a multidisciplinary screening. Addressing psychological issues before SCS implantation can reduce the failure rate of SCS.

Real-Time Accumbal Dopamine Response to Negative Stimuli: Effects of Ethanol

Activity in the mesolimbic dopamine (DA) pathway is known to have a role in reward processing and related behaviors. The mesolimbic DA response to reward has been well-examined, while the response to aversive or negative stimuli has been studied to a lesser extent and produced inconclusive results. However, a brief increase in the DA concentration in terminals during nociceptive activation has become an established but not well-characterized phenomenon. Consequently, the interpretation of the significance of this neurochemical response is still elusive. The present study was designed to further explore these increases in subsecond DA dynamics triggered by negative stimuli using voltammetry in anesthetized rats. Our experiments revealed that repeated exposure to a tail pinch resulted in more efficacious DA release in rat nucleus accumbens. This fact may suggest a protective nature of immediate DA efflux. Furthermore, a sensitized DA response to a neutral stimulus, such as a touch, was discovered following several noxious pinches, while a touch applied before these pinches did not trigger DA release. Finally, it was found that the pinch-evoked DA efflux was significantly decreased by ethanol acutely administrated at an analgesic dose. Taken together, these results support the hypothesis that subsecond DA release in the nucleus accumbens may serve as an endogenous antinociceptive signal.

Does monetary reward operantly enhance pain sensitivity over time? An experiment in healthy individuals

Aim

Operant conditioning has long been believed to influence the pain experience through a psychological reward pathway. This study was formulated to test the hypothesis that pain sensitivity may be enhanced >3 months if a monetary reward works as a reinforcement.

Methods

Forty healthy subjects volunteered to participate in this study. The subjects repeatedly underwent pain testing via mechanical stimuli, and they rolled dice three (or six) times to gain money at the following five time points: baseline, three reinforcement sessions, and last session. The payoff was determined by roll of the dice. The subjects were instructed to roll the dice into a masked stand three times per session and informed that no one monitored the number of dice actually appeared. The subjects were also informed that they could roll the dice another three times when they reported strong pain during reinforcement sessions.

Results

The amount of individual payoff had significantly increased at last session compared with the values obtained at baseline; however, no changes were identified in terms of the pain ratings for mechanical stimuli during all sessions.

Conclusion

The results suggest that the psychological reward pathway does not always involve pain perception, and it is difficult to conclude whether pain sensitivity is operantly changed through the monetary reward in healthy individuals. Further investigation is required.

An Integrative Neuroscience Framework for the Treatment of Chronic Pain: From Cellular Alterations to Behavior

Chronic pain can result from many pain syndromes including complex regional pain syndrome (CRPS), phantom limb pain and chronic low back pain, among others. On a molecular level, chronic pain syndromes arise from hypersensitization within the dorsal horn of the spinal cord, a process known as central sensitization. Central sensitization involves an upregulation of ionotropic and metabotropic glutamate receptors (mGluRs) similar to that of long-term potentiation (LTP). Regions of the brain in which LTP occurs, such as the amygdala and hippocampus, are implicated in fear- and memory-related brain circuity. Chronic pain dramatically influences patient quality of life. Individuals with chronic pain may develop pain-related anxiety and pain-related fear. The syndrome also alters functional connectivity in the default-mode network (DMN) and salience network. On a cellular/molecular level, central sensitization may be reversed through degradative glutamate receptor pathways. This, however, rarely happens. Instead, cortical brain regions may serve in a top-down regulatory capacity for the maintenance or alleviation of pain. Specifically, the medial prefrontal cortex (mPFC), which plays a critical role in fear-related brain circuits, the DMN, and salience network may be the driving forces in this process. On a cellular level, the mPFC may form new neural circuits through LTP that may cause extinction of pre-existing pain pathways found within fear-related brain circuits, the DMN, and salience network. In order to promote new LTP connections between the mPFC and other key brain structures, such as the amygdala and insula, we propose a holistic rehabilitation program including cognitive behavioral therapy (CBT) and revolving around: (1) cognitive reappraisals; (2) mindfulness meditation; and (3) functional rehabilitation. Unlike current medical interventions focusing upon pain-relieving medications, we do not believe that chronic pain treatment should focus on reversing the effects of central sensitization. Instead, we propose here that it is critical to focus on non-invasive efforts to promote new neural circuits originating from the mPFC.

The causal role of the somatosensory cortex in prosocial behaviour

Witnessing another person’s suffering elicits vicarious brain activity in areas that are active when we ourselves are in pain. Whether this activity influences prosocial behavior remains the subject of debate. Here participants witnessed a confederate express pain through a reaction of the swatted hand or through a facial expression, and could decide to reduce that pain by donating money. Participants donate more money on trials in which the confederate expressed more pain. Electroencephalography shows that activity of the somatosensory cortex I (SI) hand region explains variance in donation. Transcranial magnetic stimulation (TMS) shows that altering this activity interferes with the pain–donation coupling only when pain is expressed by the hand. High-definition transcranial direct current stimulation (HD-tDCS) shows that altering SI activity also interferes with pain perception. These experiments show that vicarious somatosensory activations contribute to prosocial decision-making and suggest that they do so by helping to transform observed reactions of affected body-parts into accurate perceptions of pain that are necessary for decision-making.

When we experience physical pain, certain areas in our brain that process bodily sensation and emotions switch on. If we see someone else in pain, many of the same regions also get activated. In contrast, convicted criminals with psychopathic traits have less activation in these areas of the brain when witnessing someone’s pain; they also show less empathy and disregard the needs of others. This suggests that a lack of this ‘shared activations’ may lead to problems in empathy. In fact, many scientists believe that shared activations are why we feel empathy for people in pain, and why we are driven to help them. Yet, there is little direct evidence about how the activity in the pain processing parts of the brain actually influences helpful behavior. As a result, some scientists now argue that empathy-related processes may actually contribute very little to helping behavior.

Gallo et al. designed an experiment where participants watched videos of someone having their hand swatted with a belt, and showing different levels of pain as a result. The volunteers could decide to reduce the amount of pain the person received by donating money they could have taken home. The more pain the participants thought the victim was in, the more money they gave up to lessen it. During the study, the activity in the brain region that processes pain in the hand was also measured in the participants. The more active this region was, the more money people donated to help.

Then, Gallo et al. used techniques that interfered with the activity of the brain area involved in perceiving sensations from the hand. This interference changed how accurately participants assessed the victim's pain. It also disrupted the link between donations and the victim's perceived pain: the amount of money people gave no longer matched the level of pain they had witnessed. This suggests that the brain areas that perceive sensations of pain in the self, which evolved primarily to experience our own sensations, also have a social function. They transform the sight of bodily harm into an accurate feeling for how much pain the victim experiences. The findings also show that we need this feeling so we can adapt our help to the needs of others. In the current debate about the role of empathy in helping behaviors, this study demonstrates that empathy-related brain activity indeed promotes helping by allowing us to detect those that need our assistance.

Understanding the relationship between helping behavior and the activity of the brain may further lead to treatments for individuals with antisocial behavior and for children with callous and unemotional traits, a disorder that is associated with a lack of empathy and a general disregard for others.

HIV Distal Neuropathic Pain Is Associated with Smaller Ventral Posterior Cingulate Cortex

Objective

. Despite modern antiretroviral therapy, HIV-associated neuropathy is one of the most prevalent, disabling and treatment-resistant complications of HIV disease. The presence and intensity of distal neuropathic pain is not fully explained by the degree of peripheral nerve damage. A better understanding of brain structure in HIV distal neuropathic pain may help explain why some patients with HIV neuropathy report pain while the majority does not. Previously, we reported that more intense distal neuropathic pain was associated with smaller total cerebral cortical gray matter volumes. The objective of this study was to determine which parts of the cortex are smaller.

Methods

. HIV positive individuals with and without distal neuropathic pain enrolled in the multisite (N = 233) CNS HIV Antiretroviral Treatment Effects (CHARTER) study underwent structural brain magnetic resonance imaging. Voxel-based morphometry was used to investigate regional brain volumes in these structural brain images.

Results

. Left ventral posterior cingulate cortex was smaller for HIV positive individuals with versus without distal neuropathic pain (peak P  = 0.017; peak t = 5.15; MNI coordinates x = -6, y = -54, z = 20). Regional brain volumes within cortical gray matter structures typically associated with pain processing were also smaller for HIV positive individuals having higher intensity ratings of distal neuropathic pain.

Conclusions

. The posterior cingulate is thought to be involved in inhibiting the perception of painful stimuli. Mechanistically a smaller posterior cingulate cortex structure may be related to reduced anti-nociception contributing to increased distal neuropathic pain.

Disease-related differences in resting-state networks: a comparison between localized provoked vulvodynia, irritable bowel syndrome, and healthy control subjects

Localized provoked vulvodynia (LPVD) affects approximately 16% of the female population, but biological mechanisms underlying symptoms remain unknown. Like in other often comorbid chronic pain disorders, altered sensory processing and modulation of pain, including central sensitization, dysregulation of endogenous pain modulatory systems, and attentional enhancement of pain perception, have been implicated. The aim of this study was to test whether regions of interest showing differences in LPVD compared to healthy control subjects (HCs) in structural and evoked-pain neuroimaging studies, also show alterations during rest when compared with HCs and a chronic pain control group (irritable bowel syndrome [IBS]). Functional magnetic resonance imaging was performed during resting state in 87 age-matched premenopausal females (29 LPVD, 29 HCs, and 29 IBS). Group-independent component analysis and general linear models were applied to investigate group differences in the intrinsic connectivity of regions comprising sensorimotor, salience, and default mode resting-state networks. Subjects with LPVD showed substantial alterations in the intrinsic connectivity of these networks compared with HCs and IBS. The intrinsic connectivity of many of the regions showing group differences during rest were moderately associated with clinical symptom reports in LPVD. Findings were robust to controlling for affect and medication usage. The current findings indicate that subjects with LPVD have alterations in the intrinsic connectivity of regions comprising the sensorimotor, salience, and default mode networks. Although shared brain mechanisms between different chronic pain disorders have been postulated, the current findings suggest that some alterations in functional connectivity may show disease specificity.

Unexplained Painful Physical Symptoms in Patients with Major Depressive Disorder: Prevalence, Pathophysiology and Management

Patients with major depression often report pain. In this article, we review the current literature regarding the prevalence and consequences, as well as the pathophysiology, of unexplained painful physical symptoms (UPPS) in patients with major depressive disorder (MDD). UPPS are experienced by approximately two-thirds of depressed patients. The presence of UPPS makes a correct diagnosis of depression more difficult. Moreover, UPPS are a predictor of a poor response to treatment and a more chronic course of depression. Pain, in the course of depression, also has a negative impact on functioning and quality of life. Frequent comorbidity of depression and UPPS has inspired the formulation of an hypothesis regarding a shared neurobiological mechanism of both conditions. Evidence from neuroimaging studies has shown that frontal-limbic dysfunction in depression may explain abnormal pain processing, leading to the presence of UPPS. Increased levels of proinflamatory cytokines and substance P in patients with MDD may also clarify the pathophysiology of UPPS. Finally, dysfunction of the descending serotonergic and noradrenergic pathways that normally suppress ascending sensations has been proposed as a core mechanism of UPPS. Psychological factors such as catastrophizing also play a role in both depression and chronic pain. Therefore, pharmacological treatment and/or cognitive therapy are recommended in the treatment of depression with UPPS. Some data suggest that serotonin and noradrenaline reuptake inhibitors (SNRIs) are more effective than selective serotonin reuptake inhibitors (SSRIs) in the alleviation of depression and UPPS. However, the pooled analysis of eight randomised clinical trials showed similar efficacy of duloxetine (an SNRI) and paroxetine (an SSRI) in reducing UPPS in depression. Further integrative studies examining genetic factors (e.g. polymorphisms of genes for interleukins, serotonin transporter and receptors), molecular factors (e.g. cytokines, substance P) and neuroimaging findings (e.g. functional studies during painful stimulation) might provide further explanation of the pathophysiology of UPPS in MDD and therefore facilitate the development of more effective methods of treatment.

Lost but making progress--Where will new analgesic drugs come from?

There is a critical need for effective new pharmacotherapies for pain. The paucity of new drugs successfully reaching the clinic calls for a reassessment of current analgesic drug discovery approaches. Many points early in the discovery process present significant hurdles, making it critical to exploit advances in pain neurobiology to increase the probability of success. In this review, we highlight approaches that are being pursued vigorously by the pain community for drug discovery, including innovative preclinical pain models, insights from genetics, mechanistic phenotyping of pain patients, development of biomarkers, and emerging insights into chronic pain as a disorder of both the periphery and the brain. Collaborative efforts between pharmaceutical, academic, and public entities to advance research in these areas promise to de-risk potential targets, stimulate investment, and speed evaluation and development of better pain therapies.

Assessing pain objectively: the use of physiological markers

Pain diagnosis and management would benefit from the development of objective markers of nociception and pain. Current research addressing this issue has focused on five main strategies, each with its own advantages and disadvantages. These encompass: (i) monitoring changes in the autonomic nervous system; (ii) biopotentials; (iii) neuroimaging; (iv) biological (bio-) markers; and (v) composite algorithms. Although each strategy has shown areas of promise, there are currently no validated objective markers of nociception or pain that can be recommended for clinical use. This article introduces the most important developments in the field and highlights shortcomings, with the aim of allowing the reader to make informed decisions about what trends to watch in the future.

Reward and motivation in pain and pain relief

Pain is fundamentally unpleasant, a feature that protects the organism by promoting motivation and learning. Relief of aversive states, including pain, is rewarding. The aversiveness of pain, as well as the reward from relief of pain, is encoded by brain reward/motivational mesocorticolimbic circuitry. In this Review, we describe current knowledge of the impact of acute and chronic pain on reward/motivation circuits gained from preclinical models and from human neuroimaging. We highlight emerging clinical evidence suggesting that anatomical and functional changes in these circuits contribute to the transition from acute to chronic pain. We propose that assessing activity in these conserved circuits can offer new outcome measures for preclinical evaluation of analgesic efficacy to improve translation and speed drug discovery. We further suggest that targeting reward/motivation circuits may provide a path for normalizing the consequences of chronic pain to the brain, surpassing symptomatic management to promote recovery from chronic pain.

Cerebral responses to innocuous somatic pressure stimulation following aerobic exercise rehabilitation in chronic pain patients: a functional magnetic resonance imaging study

OBJECTIVE

The purpose of this research was to assess the functional brain activity and perceptual rating of innocuous somatic pressure stimulation before and after exercise rehabilitation in patients with chronic pain.

MATERIALS AND METHODS

Eleven chronic pain patients and eight healthy pain-free controls completed 12 weeks of supervised aerobic exercise intervention. Perceptual rating of standardized somatic pressure stimulation (2 kg) on the right anterior mid-thigh and brain responses during functional magnetic resonance imaging (fMRI) were assessed at pre- and postexercise rehabilitation.

RESULTS

There was a significant difference in the perceptual rating of innocuous somatic pressure stimulation between the chronic pain and control groups (P=0.02) but no difference following exercise rehabilitation. Whole brain voxel-wise analysis with correction for multiple comparisons revealed trends for differences in fMRI responses between the chronic pain and control groups in the superior temporal gyrus (chronic pain > control, corrected P=0.30), thalamus, and caudate (control > chronic, corrected P=0.23). Repeated measures of the regions of interest (5 mm radius) for blood oxygen level-dependent signal response revealed trend differences for superior temporal gyrus (P=0.06), thalamus (P=0.04), and caudate (P=0.21). Group-by-time interactions revealed trend differences in the caudate (P=0.10) and superior temporal gyrus (P=0.29).

CONCLUSION

Augmented perceptual and brain responses to innocuous somatic pressure stimulation were shown in the chronic pain group compared to the control group; however, 12-weeks of exercise rehabilitation did not significantly attenuate these responses.

Chronic musculoskeletal pain: review of mechanisms and biochemical biomarkers as assessed by the microdialysis technique

Chronic musculoskeletal pain conditions are multifaceted, and approximately 20% of the adult population lives with severe chronic pain, with a higher prevalence in women and in lower income groups. Chronic pain is influenced by and interacts with physical, emotional, psychological, and social factors, and a biopsychosocial framework is increasingly applied in clinical practice. However, there is still a lack of assessment procedures based on the activated neurobiological pain mechanisms (ie, the biological part of the biopsychosocial model of pain), which may be a necessary step for further optimizing outcomes after treatments for patients with chronic pain. It has been suggested that chronic pain conditions are mainly driven by alterations in the central nervous system with little or no peripheral stimuli or nociception. In contrast, other authors argue that such central alterations are driven by peripheral alterations and nociceptive input. Microdialysis is an in vivo method for studying local tissue alterations and allows for sampling of substances in the interstitium of the muscle, where nociceptor free nerve endings are found close to the muscle fibers. The extracellular matrix plays a key role in physiologic functions of cells, including the primary afferent nociceptor. The present review mainly concerns the results of microdialysis studies and how they can contribute to the understanding of activated peripheral nociceptive and pain mechanisms in humans with chronic pain. The primary aim was to review molecular studies using microdialysis for the investigation of human chronic muscle pain, ie, chronic masticatory muscle pain, chronic trapezius myalgia, chronic whiplash-associated disorders, and chronic widespread pain/fibromyalgia syndrome. Several studies clearly showed elevated levels of serotonin, glutamate, lactate, and pyruvate in localized chronic myalgias and may be potential biomarkers. These results indicate that peripheral muscle alterations are parts of the activated pain mechanisms in common chronic pain conditions. Muscle alterations have been reported in fibromyalgia syndrome and chronic widespread pain, but more studies are needed before definite conclusions can be drawn. For other substances, results are inconclusive across studies and patient groups.

HIV-associated distal neuropathic pain is associated with smaller total cerebral cortical gray matter

Despite modern antiretroviral therapy, HIV-associated sensory neuropathy affects over 50 % of HIV patients. The clinical expression of HIV neuropathy is highly variable: many individuals report few symptoms, but about half report distal neuropathic pain (DNP), making it one of the most prevalent, disabling, and treatment-resistant complications of HIV disease. The presence and intensity of pain is not fully explained by the degree of peripheral nerve damage, making it unclear why some patients do, and others do not, report pain. To better understand central nervous system contributions to HIV DNP, we performed a cross-sectional analysis of structural magnetic resonance imaging volumes in 241 HIV-infected participants from an observational multi-site cohort study at five US sites (CNS HIV Anti-Retroviral Treatment Effects Research Study, CHARTER). The association between DNP and the structural imaging outcomes was investigated using both linear and nonlinear (Gaussian Kernel support vector) multivariable regression, controlling for key demographic and clinical variables. Severity of DNP symptoms was correlated with smaller total cerebral cortical gray matter volume (r = -0.24; p = 0.004). Understanding the mechanisms for this association between smaller total cortical volumes and DNP may provide insight into HIV DNP chronicity and treatment-resistance.

The costs and consequences of adequately managed chronic non-cancer pain and chronic neuropathic pain

BACKGROUND

Chronic pain is distressing for patients and a burden on healthcare systems and society. Recent research demonstrates different aspects of the negative impact of chronic pain and the positive impact of successful treatment, making an overview of the costs and consequences of chronic pain appropriate.

OBJECTIVE

To examine recent literature on chronic noncancer and neuropathic pain prevalence, impact on quality and quantity of life, societal and healthcare costs, and impact of successful therapy.

METHODS

Systematic reviews (1999 to February 2012) following PRISMA guidelines were conducted to identify studies reporting appropriate outcomes.

RESULTS

Chronic pain has a weighted average prevalence in adults of 20%; 7% have neuropathic pain, and 7% have severe pain. Chronic pain impeded activities of daily living, work and work efficiency, and reduced quality and quantity of life. Effective pain therapy (pain intensity reduction of at least 50%) resulted in consistent improvements in fatigue, sleep, depression, quality of life, and work.

CONCLUSION

Strenuous efforts should be put into obtaining good levels of pain relief for people in chronic pain, including the opportunity for multiple drug switching, using reliable, validated, and relatively easily applied patient-centered outcomes. Detailed, thoughtful and informed decision analytic policy modeling would help understand the key elements in organizational change or service reengineering to plan the optimum pain management strategy to maximize pain relief and its stream of benefits against budgetary and other constraints. This paper contains the information on which such models can be based.

Diminished neurokinin-1 receptor availability in patients with two forms of chronic visceral pain

Central sensitization and dysregulation of peripheral substance P and neurokinin-1 receptor (NK-1R) signaling are associated with chronic abdominal pain in inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS). Although positron emission tomography (PET) has demonstrated that patients with injury-related chronic pain have diminished NK-1R availability in the brain, it is unknown whether these deficits are present in IBD and IBS patients, who have etiologically distinct forms of non-injury-related chronic pain. This study's aim was to determine if patients with IBD or IBS exhibit deficits in brain expression of NK-1Rs relative to healthy controls (HCs), the extent to which expression patterns differ across patient populations, and if these patterns differentially relate to clinical parameters. PET with [(18)F]SPA-RQ was used to measure NK-1R availability by quantifying binding potential (BP) in the 3 groups. Exploratory correlation analyses were performed to detect associations between NK-1R BP and physical symptoms. Compared to HCs, IBD patients had NK-1R BP deficits across a widespread network of cortical and subcortical regions. IBS patients had similar, but less pronounced deficits. BP in a subset of these regions was robustly related to discrete clinical parameters in each patient population. Widespread deficits in NK-1R BP occur in IBD and, to a lesser extent, IBS; however, discrete clinical parameters relate to NK-1R BP in each patient population. This suggests that potential pharmacological interventions that target NK-1R signaling may be most effective for treating distinct symptoms in IBD and IBS.

Pain ratings, psychological functioning and quantitative EEG in a controlled study of chronic back pain patients

Objectives

Several recent studies report the presence of a specific EEG pattern named Thalamocortical Dysrhythmia (TCD) in patients with severe chronic neurogenic pain. This is of major interest since so far no neuroscientific indicator of chronic pain could be identified. We investigated whether a TCD-like pattern could be found in patients with moderate chronic back pain, and we compared patients with neuropathic and non-neuropathic pain components. We furthermore assessed the presence of psychopathology and the degree of psychological functioning and examined whether the strength of the TCD-related EEG markers is correlated with psychological symptoms and pain ratings.

Design

Controlled clinical trial with age and sex matched healthy controls.

Methods

Spontaneous EEG was recorded in 37 back pain patients and 37 healthy controls.

Results

We were not able to observe a statistically significant TCD effect in the EEG data of the whole patient group, but a subsample of patients with evidence for root damage showed a trend in this direction. Pain patients showed markedly increased psychopathology. In addition, patients' ratings of pain intensity within the last 1 to 12 months showed strong correlations with EEG power, while psychopathology was correlated to the peak frequency.

Conclusion

Out of several possible interpretations the most likely conclusion is that only patients with severe pain as well as root lesions with consecutive thalamic deafferentation develop the typical TCD pattern. Our primary method of defining ‘neuropathic pain’ could not reliably determine if such a deafferentation was present. Nevertheless the analysis of a specific subsample as well as correlations between pain ratings, psychopathology and EEG power and peak frequency give some support to the TCD concept.

Trial Registration

ClinicalTrials.gov NCT00744575

Correlations between EEG and clinical outcome in chronic neuropathic pain: surgical effects and treatment resistance

Chronic neuropathic pain may require a neurosurgical treatment, but for reasons that have not been fully explored yet, a significant number of patients do not benefit from the intervention. We compared the resting EEG of 15 healthy controls to the EEG of 23 chronic neuropathic pain patients before and 12 months after treatment by the central lateral thalamotomy (CLT). A patient subgroup had a high (n = 14, pain relief (PR)  ≥ 50%) and another subgroup a low (n = 9, PR < 50%) postoperative PR. EEG spectral power and source localization of the high PR patients were normalized postoperatively. In contrast, low PR patients showed postoperative maintenance of insular, cingulate and prefrontal overactivities, and their frustration values were positively correlated with cingulate and prefrontal activity. These findings demonstrate a normalizing effect of CLT on cortical activity and suggest that treatment resistance is associated with a frustration-based dynamics.