Neurobiological basis for pain vulnerability: why me?

The relationship between traumatic exposure and pain perception in children: the moderating role of posttraumatic symptoms

ABSTRACT

Adverse childhood experiences (ACEs) affect approximately half of all children worldwide. These experiences have been linked to increased pain sensitivity in adulthood and a higher likelihood of developing severe chronic pain. However, most studies have assessed the effects of ACEs retrospectively, long after they occurred, leaving room for other factors to influence the observed outcomes. We investigated, for the first time, the association between ACEs and concurrent pain perception among young children who live in a conflict zone and are consistently exposed to potentially traumatic experiences. Participants were 60 elementary school children (ages 8-11 years) living in conflict regions (n = 39) or nonconflict regions (n = 21). Posttraumatic stress symptom (PTSS) severity, traumatic exposure, pressure pain threshold (PPT), and mechanical detection threshold (MDT) were measured. Trauma-exposed children had significantly lower PPT than did controls, but MDT was similar across groups. Pressure pain threshold correlated positively with proximity to the conflict zone and inversely with traumatic exposure magnitude and PTSS severity. In addition, PTSSs moderated the relationship between repeated traumatic exposure and PPT. Children with higher PTSS severity displayed pain hypersensitivity regardless of their traumatic exposure level, whereas in children with lower PTSS severity, greater traumatic exposure correlated with pain hypersensitivity. The results suggest that ACEs among children lead to concurrent pain hypersensitivity and distress and may put them at elevated risk of chronic pain early in life. In addition, our findings emphasize the need for identifying children with various PTSS levels to provide tailored interventions and mitigate the long-term negative effects of ACEs.

Patients' Perspectives on Axial Pain in Relation to Inflammation and Structural Damage in a Large Cohort of Axial Spondyloarthritis Patients

OBJECTIVE

The objective of this study was to explore to what extent patients with axial spondyloarthritis (axSpA) link experienced pain in the neck, back, and hips to inflammation and/or structural damage.

METHODS

Patients from the Groningen Leeuwarden Axial Spondyloarthritis (GLAS) cohort visiting the outpatient clinic between 2016 and 2019 filled out two additional questions in relation to the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) question 2: (1) "To what extent do you think the pain you experience in your neck, back, and hips is related to inflammation caused by axSpA?" and (2) "To what extent do you think the pain you experience in your neck, back, and hips is related to damage of the spine and joints caused by axSpA?" Answers had to be depicted on a numeric rating scale from 0 (none) to 10 (very much); a difference of ≥2 points between the scores of these questions was considered clinically relevant in favor of the highest scoring question.

RESULTS

A total of 688 patients with axSpA (24% with nonradiographic axSpA [nr-axSpA]) were included (62% male, mean ± SD age 48 ± 14 years, and mean ± SD Ankylosing Spondylitis Disease Activity Score [ASDAS] 2.3 ± 1.0). Seventy-five percent of patients could not link the origin of their pain, 15% linked axial pain predominantly to inflammation, and 10% linked axial pain predominantly to damage. Patients in the inflammation group were younger, had shorter symptom duration, were more frequently diagnosed with nr-axSpA, had higher ASDASCRP , had more often elevated CRP levels, had fewer comorbidities, had better spinal mobility, and had less spinal radiographic damage.

CONCLUSION

In our large observational cohort, the majority of patients with axSpA could not differentiate the origin of experienced axial pain. If patients were able to link axial pain to clinical inflammation or damage, it was in concordance with clinical assessments and radiographic outcome, which may be helpful in establishing the origin of pain and supporting better patient-centered treatment decisions.

Rasch analysis of the Depression, Anxiety, and Stress Scale in patients with chronic pain

OBJECTIVE

Questionnaires are commonly used to assess and screen pain-related, psychological symptoms in patients with chronic pain. In Germany, the "German pain questionnaire" provided by the German Pain Society used for this purpose includes the Depression, Anxiety, and Stress Scale (DASS). This study aims to analyze the DASS by fitting its data to the Rasch model to test the psychometric quality.

STUDY DESIGN AND SETTING

In this cross-sectional study, 932 patients with chronic pain answered the DASS. The 21-item short version was tested via Rasch analysis using the parameters threshold order, fit to the model, Differential Item Functioning, unidimensionality and reliability.

RESULTS

The Rasch analysis results showed a low reliability, misfitting items, Differential Item Functioning or multidimensionality. It was necessary to remove items from the subscales to improve fit to the Rasch model. A revised depression subscale of the DASS-21 was the only scale that achieved all the required psychometric parameters. The summation of all items to a total scale was not supported.

CONCLUSION

More research is required on somatic free measurement of psychological symptoms in patients with chronic pain. The results demonstrate that the development of a new instrument or a revision of existing instruments for screening of psychological symptoms in chronic pain are needed.

The impact of the social context on the development of secondary hyperalgesia: an experimental study

ABSTRACT

Social support has been shown to reduce pain ratings and physiological responses to acute pain stimuli. Furthermore, this relationship is moderated by adult attachment styles. However, these effects have not been characterized in experimentally induced symptoms of chronic pain, such as secondary hyperalgesia (SH) which is characterized by an increased sensitivity of the skin surrounding an injury. We aimed to examine whether social support by handholding from a romantic partner can attenuate the development of experimentally induced SH. Thirty-seven women, along with their partners, participated in 2 experimental sessions 1 week apart. In both sessions, SH was induced using an electrical stimulation protocol. In the support condition, the partner was seated across from the participant holding the participant's hand during the electrical stimulation, whereas in the alone condition, the participant went through the stimulation alone. Heart rate variability was measured for both the participant as well as the partner before, during, and after the stimulation. We found that the width of the area of hyperalgesia was significantly smaller in the support condition. Attachment styles did not moderate this effect of social support on the area width. Increasing attachment avoidance was associated with both a smaller width of hyperalgesia and a smaller increase in the sensitivity on the stimulated arm. For the first time, we show that social support can attenuate the development of secondary hyperalgesia and that attachment avoidance may be associated with an attenuated development of secondary hyperalgesia.

Neurotransmitter receptor densities are associated with changes in regional Cerebral blood flow during clinical ongoing pain

Arterial spin labelling (ASL) plays an increasingly important role in neuroimaging pain research but does not provide molecular insights regarding how regional cerebral blood flow (rCBF) relates to underlying neurotransmission. Here, we integrate ASL with positron emission tomography (PET) and brain transcriptome data to investigate the molecular substrates of rCBF underlying clinically relevant pain states. Two data sets, representing acute and chronic ongoing pain respectively, were utilised to quantify changes in rCBF; one examining pre-surgical versus post-surgical pain, and the second comparing patients with painful hand Osteoarthritis to a group of matched controls. We implemented a whole-brain spatial correlation analysis to explore associations between change in rCBF (ΔCBF) and neurotransmitter receptor distributions derived from normative PET templates. Additionally, we utilised transcriptomic data from the Allen Brain Atlas to inform distributions of receptor expression. Both datasets presented significant correlations of ΔCBF with the μ-opioid and dopamine-D2 receptor expressions, which play fundamental roles in brain activity associated with pain experiences. ΔCBF also correlated with the gene expression distributions of several receptors involved in pain processing. Overall, this is the first study illustrating the molecular basis of ongoing pain ASL indices and emphasises the potential of rCBF as a biomarker in pain research.

Heritability of musculoskeletal pain and pain sensitivity phenotypes: two generations of the Raine Study

ABSTRACT

There is a need to better understand biological factors that increase the risk of persistent musculoskeletal pain and heightened pain sensitivity. Knowing the heritability (how genes account for differences in people's traits) can enhance the understanding of genetic versus environmental influences of pain and pain sensitivity. However, there are gaps in current knowledge, including the need for intergenerational studies to broaden our understanding of the genetic basis of pain. Data from Gen1 and Gen2 of the Raine Study were used to investigate the heritability of musculoskeletal pain, and pressure and cold pain sensitivity. Participants included parents (Gen 1, n=1092) and their offspring (Gen 2, n=688) who underwent a battery of testing and questionnaires including pressure and cold pain threshold testing and assessments of physical activity, sleep, musculoskeletal pain, mental health and adiposity. Heritability estimates were derived using the Sequential Oliogenic Linkage Analysis Routines (SOLAR) software. Heritability estimates for musculoskeletal pain and pressure pain sensitivity were significant, accounting for between 0.190 and 0.289 of the variation in the phenotype. In contrast, heritability of cold pain sensitivity was not significant. This is the largest intergenerational study to date to comprehensively investigate the heritability of both musculoskeletal pain and pain sensitivity, using robust statistical analysis. This study provides support for the heritability of musculoskeletal pain and pain sensitivity to pressure, suggesting the need for further convergence of genetic and environmental factors in models for the development and/or maintenance of these pain disorders.

Epigenetics and postsurgical pain: A scoping review

OBJECTIVE

Multiple factors are involved in the physiology and variability of postsurgical pain, a great part of which can be explained by genetic and environmental factors and their interaction. Epigenetics refers to the mechanism by which the environment alters the stability and expression of genes. We conducted a scoping review to examine the available evidence in both animal models and clinical studies on epigenetic mechanisms involved in regulation of postsurgical and chronic postsurgical pain.

METHODS

The Arksey & ÓMalley framework and the PRISMA-ScR (Preferred Reporting Items for Systematic Review and Meta-Analysis, scoping reviews extension) guidelines were used. The PubMed, Web of Science and Google Scholar databases were searched, and the original articles cited in reviews located through the search were also reviewed. English-language articles without time limits were retrieved. Articles were selected if the abstract addressed information on the epigenetic or epigenomic mechanisms, histone, or DNA methylation and microribonucleic acids involved in postsurgical and chronic postsurgical pain in animal models and clinical studies.

RESULTS

The initial search provided 174 articles, and 81 were used. The available studies to date, mostly in animal models, have shown that epigenetics contributes to regulation of gene expression in the pathways involved in postsurgical pain and in maintaining long-term pain.

CONCLUSION

Research on possible epigenetic mechanisms involved in postsurgical pain and chronic postsurgical pain in humans is scarce. In view of the evidence available in animal models, there is a need to evaluate epigenetic pain mechanisms in the context of human and clinical studies.

Pelvic Pain Alters Functional Connectivity Between Anterior Cingulate Cortex and Hippocampus in Both Humans and a Rat Model

The anterior cingulate cortex (ACC) and hippocampus (HIPP) are two key brain regions associated with pain and pain-related affective processing. However, whether and how pelvic pain alters the neural activity and connectivity of the ACC and HIPP under baseline and during social pain, and the underlying cellular and molecular mechanisms, remain unclear. Using functional magnetic resonance imaging (fMRI) combined with electrophysiology and biochemistry, we show that pelvic pain, particularly, primary dysmenorrhea (PDM), causes an increase in the functional connectivity between ACC and HIPP in resting-state fMRI, and a smaller reduction in connectivity during social exclusion in PDM females with periovulatory phase. Similarly, model rats demonstrate significantly increased ACC-HIPP synchronization in the gamma band, associating with reduced modulation by ACC-theta on HIPP-gamma and increased levels of receptor proteins and excitation. This study brings together human fMRI and animal research and enables improved therapeutic strategies for ameliorating pain and pain-related affective processing.

The Importance of Perceived Relevance: A Qualitative Evaluation of Patient's Perceptions of Value and Impact Following a Low-Intensity Group-Based Pain Management Program

Background and objectives: Limited evidence exists exploring perceptions of which aspects of a pain management program are perceived as valuable and impactful. The aim of this study was to explore patient beliefs about which aspects of a pain management program were valued and/or had perceived impact. Materials and Methods: One-on-one structured interviews were conducted with 11 adults three months after their completion of the Spark Pain Program at Westmead Hospital, Sydney, Australia. Concepts in the transcripts were inductively identified and explored, utilizing thematic analysis to better understand their relevance to the study aim. Results: Four themes emerged: (1) "The program overall was positive, but…"; (2) "I valued my improved knowledge and understanding of pain, but…"; (3) "I valued the stretching/relaxation/pacing/activity monitoring"; and (4) "I valued being part of a supportive and understanding group". Participants reported that they liked being treated as an individual within the group. A lack of perceived personal relevance of key messages was identified in some participants; it appears that patients in pain programs must determine that changes in knowledge, beliefs, and attitudes are personally relevant in order for the changes to have a significant impact on them. Conclusions: This study provides new insights into aspects of a pain management program that were perceived as valuable and impactful, areas that "missed the mark", and hypotheses to guide the implementation of service delivery and program redesign.

Central Nervous System Changes in Pelvic Inflammation/Pain Patients

Purpose of Review

Centralized pain syndromes (CPS), including chronic pelvic pain (CPP) syndrome, are significant public health problems with prevalence more than diabetes, cancer, or cardiovascular disease. A variety of pathologies are linked with CPP syndrome; however, pain often continues without the presence of pathology, or when an underlying pelvic disease is found, the extent and severity of pain are disproportionate. Although this is not a systematic review, we performed a detailed literature search to identify relevant papers and to provide the available evidence for central changes in association with CPP syndrome.

Recent Findings

Recent advances in brain imaging techniques have provided more accurate data on gray matter volume, functional connectivity, and metabolite levels in the pain-relevant areas of the brain. The present evidence shows that like other chronic pain conditions, the CPP syndrome is associated with central nervous system (CNS) alterations. In particular, these include changes in brain structure, in the activity of both the hypothalamic–pituitary–adrenal (HPA) axis and the autonomic nervous system, and in the behavioral and central response to noxious stimulation.

Summary

A growing body of evidence, mostly from neuroimaging, suggests that for many patients with CPP, the pain may be associated to changes in both structure and function of the CNS. The treatment of pain symptoms, even without the presence of identifiable pathology, may prevent the development or at least minimize the progression of long-term central changes. These findings support the use of new therapeutic strategies targeting the CNS for controlling of pain in CPP conditions.

Exploring the concept of pain of Australian children with and without pain: qualitative study

OBJECTIVE

A person's concept of pain can be defined as how they understand what pain actually is, what function it serves and what biological processes are thought to underpin it. This study aimed to explore the concept of pain in children with and without persistent pain.

DESIGN

In-depth, face-to-face interviews with drawing tasks were conducted with 16 children (aged 8-12 years) in New South Wales, Australia. Thematic analysis was used to analyse and synthesise the data.

SETTING

Children with persistent pain were identified from a pain clinic waiting list in Australia, and children without pain were identified through advertising flyers and email bulletins at a university and hospital.

PARTICIPANTS

Eight children had persistent pain and eight children were pain free.

RESULTS

Four themes emerged from the data: 'my pain-related knowledge', 'pain in the world around me', 'pain in me' and 'communicating my concept of pain'. A conceptual framework of the potential interactions between the themes resulting from the analysis is proposed. The concept of pain of Australian children aged 8-12 years varied depending on their knowledge, experiences and literacy levels. For example, when undertaking a drawing task, children with persistent pain tended to draw emotional elements to describe pain, whereas children who were pain free did not.

CONCLUSIONS

Gaining an in-depth understanding of a child's previous pain-related experiences and knowledge is important to facilitate clear and meaningful pain science education. The use of age-appropriate language, in combination with appropriate assessment and education tasks such as drawing and discussing vignettes, allowed children to communicate their individual concept of pain.

Linking Lifestyle Factors to Complex Pain States: 3 Reasons Why Understanding Epigenetics May Improve the Delivery of Patient-Centered Care

Persistent pain is determined by a diverse and ever-changing combination of biology, psychology, and society. Research suggests a need to embrace a patient-centered, biopsychosocial approach to improve outcomes. Only through in-depth understanding of complex mechanisms and by using mechanism-based reasoning can the clinician tailor interventions-the basic tenet of patient-centered care. Epigenetics is helping to unravel complex underlying mechanisms and might have at least 3 major clinical implications for orthopaedic and sports physical therapists. First, it promotes mechanism-based clinical reasoning by improved understanding of the pathophysiology of many health conditions and the underlying mechanisms of action of commonly used interventions. Second, it might help patient subgrouping, allowing more targeted interventions. Finally, it might be used as a biomarker to monitor the effects of environmental factors and lifestyle interventions on health. For these reasons, the authors urge clinicians and clinical researchers to follow this rapidly growing area of research, as it might be soon contributing to patient assessment. J Orthop Sports Phys Ther 2019;49(10):683-687. doi:10.2519/jospt.2019.0612.

Transcultural adaptation and French validation of the Pain Sensitivity Questionnaire

PURPOSE

To validate a French translation of the Pain Sensitivity Questionnaire (PSQ), which is a valuable tool to predict an individual's natural disposition to feel pain that could be used after surgery.

METHODS

We studied content validity, internal consistency, convergent validity (anxiety, depression and catastrophism) and test-retest reliability of the French version of the PSQ (PSQ-F) in 146 patients either before scheduled surgery or during pregnancy; then, convergent and concurrent validity in 85 healthy volunteers submitted to nociceptive tests.

RESULTS

Internal consistency of the PSQ-F was found to be excellent, with Cronbach's α at 0.866, 0.886, and 0.927, respectively for its "minor", "moderate" and "total" scores. Test-retest reliability was significant, with intraclass correlation coefficients at 0.629, 0.629, and 0.635, respectively for the above- mentioned scores. These three scores correlated with anxiety, depression and catastrophizing scores in patients, but not in healthy volunteers, possibly because of low and few variant psychometric scores in this group. They were inversely correlated to the temperature needed to evoke heat pain rated 6 out of 10, but not to the mechanical pain threshold (electronic von Frey), nor to the heat pain threshold. Finally, they directly correlated to the pain induced by the cold pressor test (minor and total scores only).

DISCUSSION

This validated version can now be used by French-speaking researchers and physicians.

TRIAL REGISTRATION

www.ClinicalTrials.gov (NCT03113903); 14 April, 2017.

Genetic components of human pain sensitivity: a protocol for a genome-wide association study of experimental pain in healthy volunteers

INTRODUCTION

Pain constitutes a major component of the global burden of diseases. Recent studies suggest a strong genetic contribution to pain susceptibility and severity. Whereas most of the available evidence relies on candidate gene association or linkage studies, research on the genetic basis of pain sensitivity using genome-wide association studies (GWAS) is still in its infancy. This protocol describes a proposed GWAS on genetic contributions to baseline pain sensitivity and nociceptive sensitisation in a sample of unrelated healthy individuals of mixed Latin American ancestry.

METHODS AND ANALYSIS

A GWAS on genetic contributions to pain sensitivity in the naïve state and following nociceptive sensitisation will be conducted in unrelated healthy individuals of mixed ancestry. Mechanical and thermal pain sensitivity will be evaluated with a battery of quantitative sensory tests evaluating pain thresholds. In addition, variation in mechanical and thermal sensitisation following topical application of mustard oil to the skin will be evaluated.

ETHICS AND DISSEMINATION

This study received ethical approval from the University College London research ethics committee (3352/001) and from the bioethics committee of the Odontology Faculty at the University of Antioquia (CONCEPTO 01-2013). Findings will be disseminated to commissioners, clinicians and service users via papers and presentations at international conferences.

DOLORisk: study protocol for a multi-centre observational study to understand the risk factors and determinants of neuropathic pain

Background: Neuropathic pain is an increasingly prevalent condition and has a major impact on health and quality of life. However, the risk factors for the development and maintenance of neuropathic pain are poorly understood. Clinical, genetic and psychosocial factors all contribute to chronic pain, but their interactions have not been studied in large cohorts. The DOLORisk study aims to study these factors. Protocol: Multicentre cross-sectional and longitudinal cohorts covering the main causes leading to neuropathic pain (e.g. diabetes, surgery, chemotherapy, traumatic injury), as well as rare conditions, follow a common protocol for phenotyping of the participants. This core protocol correlates answers given by the participants on a set of questionnaires with the results of their genetic analyses. A smaller number of participants undergo deeper phenotyping procedures, including neurological examination, nerve conduction studies, threshold tracking, quantitative sensory testing, conditioned pain modulation and electroencephalography. Ethics and dissemination: All studies have been approved by their regional ethics committees as required by national law. Results are disseminated through the DOLORisk website, scientific meetings, open-access publications, and in partnership with patient organisations. Strengths and limitations: Large cohorts covering many possible triggers for neuropathic painMulti-disciplinary approach to study the interaction of clinical, psychosocial and genetic risk factorsHigh comparability of the data across centres thanks to harmonised protocolsOne limitation is that the length of the questionnaires might reduce the response rate and quality of responses of participants.

DOLORisk: study protocol for a multi-centre observational study to understand the risk factors and determinants of neuropathic pain

Background: Neuropathic pain is an increasingly prevalent condition and has a major impact on health and quality of life. However, the risk factors for the development and maintenance of neuropathic pain are poorly understood. Clinical, genetic and psychosocial factors all contribute to chronic pain, but their interactions have not been studied in large cohorts. The DOLORisk study aims to study these factors.

Protocol: Multicentre cross-sectional and longitudinal cohorts covering the main causes leading to neuropathic pain (e.g. diabetes, surgery, chemotherapy, traumatic injury), as well as rare conditions, follow a common protocol for phenotyping of the participants. This core protocol correlates answers given by the participants on a set of questionnaires with the results of their genetic analyses. A smaller number of participants undergo deeper phenotyping procedures, including neurological examination, nerve conduction studies, threshold tracking, quantitative sensory testing, conditioned pain modulation and electroencephalography.

Ethics and dissemination: All studies have been approved by their regional ethics committees as required by national law. Results are disseminated through the DOLORisk website, scientific meetings, open-access publications, and in partnership with patient organisations.

Strengths and limitations:

Large cohorts covering many possible triggers for neuropathic pain

Multi-disciplinary approach to study the interaction of clinical, psychosocial and genetic risk factors

High comparability of the data across centres thanks to harmonised protocols

One limitation is that the length of the questionnaires might reduce the response rate and quality of responses of participants

Prevalence and Determinants of Painful and Painless Neuropathy in Type 1 Diabetes Mellitus

Aims: To evaluate (1) the prevalence of diabetic distal symmetrical sensory-motor polyneuropathy (DSPN) and painful DSPN among patients with type 1 diabetes mellitus (DM1) aged over 18 years and (2) the determinant factors of neuropathy and pain in those patients. Materials and Methods: An epidemiological, cross-sectional, observational study was performed; 330,386 people were included, and a total of 444 people were diagnosed with DM1. After exclusion of possible confounders, 360 patients were assessed for painless and painful DSPNs using neurological examination and questionnaires for neuropathy and pain. Odds ratio (OR) and confidence intervals (95% CI) were estimated using multinomial logistic regression models. The analysis was based on a framework with four conceptual levels that consider feasible pathways between several risk factors: (1) socio-demographic factors and diabetes duration, (2) patient habits, (3) co-morbidities, and (4) metabolic factors and disease complications. Results: The prevalence of DSPN and painful DSPN were 42.8 and 18.9%, respectively. Diabetes duration was positively associated with painful (OR = 1.107, 95% CI: 1.107-1.139) and painless DSPN (OR = 1.069, 95% CI: 1.043-1.096). Education level was negatively associated with painful DSPN (OR = 0.889, 95% CI: 0.826-0.957). Sex (female) was positively associated only with painless DSPN (OR = 1.769, 95% CI: 1.007-3.107). Being a current or former smoker was positively associated only with painless DSPN (OR = 1.940, 95% CI: 1.069-3.518). Hypertension was positively associated with painful DSPN (OR = 2.474, 95% CI: 1.110-5.512) and painless DSPN (OR = 2.565, 95% CI: 1.252-5.256). Glycated hemoglobin (HbA1c) was positively associated only with painless DSPN (OR = 1.193, 95% CI: 1.018-1.399). Conclusions: Diabetes duration and hypertension have a direct impact on the development of painful and painless DSPN. However, female sex and HbA1c have a direct effect only on the development of painless DSPN, and education level has an indirect effect on the development of painful DSPN. Therefore, it can be concluded that different etiological factors have different contributions to the development of neuropathy and pain.

Negative Evidence for a Functional Role of Neuronal DNMT3a in Persistent Pain

Traditionally, neuroscience has had to rely on mixed tissue analysis to examine transcriptional and epigenetic changes in the context of nervous system function or pathology. However, particularly when studying chronic pain conditions, this approach can be flawed, since it neglects to take into account the shifting contribution of different cell types across experimental conditions. Here, we demonstrate this using the example of DNA methyltransferases (DNMTs) – a group of epigenetic modifiers consisting of Dnmt1, Dnmt3a, and Dnmt3b in mammalian cells. We used sensory neuron-specific knockout mice for Dnmt3a/3b as well as pharmacological blockade of Dnmt1 to study their role in nociception. In contrast to previous analyses on whole tissue, we find that Dnmt3a and 3b protein is not expressed in adult DRG neurons, that none of the DNA methyltransferases are regulated with injury and that interfering with their function has no effect on nociception. Our results therefore currently do not support a role for neuronal DNA methyltransferases in pain processing in adult animals.

Pain chronification: what should a non-pain medicine specialist know?

OBJECTIVE

Pain is one of the most common reasons for an individual to consult their primary care physician, with most chronic pain being treated in the primary care setting. However, many primary care physicians/non-pain medicine specialists lack enough awareness, education and skills to manage pain patients appropriately, and there is currently no clear, common consensus/formal definition of "pain chronification".

METHODS

This article, based on an international Change Pain Chronic Advisory Board meeting which was held in Wiesbaden, Germany, in October 2016, provides primary care physicians/non-pain medicine specialists with a narrative overview of pain chronification, including underlying physiological and psychosocial processes, predictive factors for pain chronification, a brief summary of preventive strategies, and the role of primary care physicians and non-pain medicine specialists in the holistic management of pain chronification.

RESULTS

Based on currently available evidence, we propose the following consensus-based definition of pain chronification which provides a common framework to raise awareness among non-pain medicine specialists: "Pain chronification describes the process of transient pain progressing into persistent pain; pain processing changes as a result of an imbalance between pain amplification and pain inhibition; genetic, environmental and biopsychosocial factors determine the risk, the degree, and time-course of chronification."

CONCLUSIONS

Early intervention plays an important role in preventing pain chronification and, as key influencers in the management of patients with acute pain, it is critical that primary care physicians are equipped with the necessary awareness, education and skills to manage pain patients appropriately.

Psychosocial Stress Delays Recovery of Postoperative Pain Following Incisional Surgery in the Rat

Psychosocial factors such as anxiety, depression and catastrophizing, commonly associated with established chronic pain, also may be associated with an increased risk of chronic postsurgical pain (CPSP) when present preoperatively. We used a repeat social defeat (RSD) paradigm to induce psychosocial stress in rodents prior to incisional surgery of the paw. Mixed effects growth curve models were utilized to examine resolution of mechanical hypersensitivity in rats for four weeks following surgery. Eight days following surgery, immunohistochemistry was conducted to examine glial activation as well as evoked neuronal activation in the spinal cord. Here we document that RSD resulted in reduced weight gain and increased depressive symptoms prior to surgery. Rats exposed to RSD displayed delayed resolution of mechanical hypersensitivity in the ipsilateral paw following surgery compared to non-defeated rats. Prior exposure to RSD significantly increased microglial activation and neuronal sensitization (pERK-IR) within the ipsilateral spinal cord. In conclusion, we found that chronic social stress alters the neurobiological response to surgical injury, resulting in slowed recovery. This model maybe useful for future interventional studies examining the mechanistic interactions between depression and risk of CPSP.

A neurobiologist's attempt to understand persistent pain

This topical review starts with a warning that despite an impressive wealth of neuroscientific data, a reductionist approach can never fully explain persistent pain. One reason is the complexity of clinical pain (in contrast to experimentally induced pain). Another reason is that the "pain system" shows degeneracy, which means that an outcome can have several causes. Problems also arise from lack of conceptual clarity regarding words like nociceptors, pain, and perception. It is, for example, argued that "homeoceptor" would be a more meaningful term than nociceptor. Pain experience most likely depends on synchronized, oscillatory activity in a distributed neural network regardless of whether the pain is caused by tissue injury, deafferentation, or hypnosis. In experimental pain, the insula, the second somatosensory area, and the anterior cingulate gyrus are consistently activated. These regions are not pain-specific, however, and are now regarded by most authors as parts of the so-called salience network, which detects all kinds of salient events (pain being highly salient). The networks related to persistent pain seem to differ from the those identified experimentally, and show a more individually varied pattern of activations. One crucial difference seems to be activation of regions implicated in emotional and body-information processing in persistent pain. Basic properties of the "pain system" may help to explain why it so often goes awry, leading to persistent pain. Thus, the system must be highly sensitive not to miss important homeostatic threats, it cannot be very specific, and it must be highly plastic to quickly learn important associations. Indeed, learning and memory processes play an important role in persistent pain. Thus, behaviour with the goal of avoiding pain provocation is quickly learned and may persist despite healing of the original insult. Experimental and clinical evidence suggest that the hippocampal formation and neurogenesis (formation of new neurons) in the dentate gyrus are involved in the development and maintenance of persistent pain. There is evidence that persistent pain in many instances may be understood as the result of an interpretation of the organism's state of health. Any abnormal pattern of sensory information as well as lack of expected correspondence between motor commands and sensory feedback may be interpreted as bodily threats and evoke pain. This may, for example, be an important mechanism in many cases of neuropathic pain. Accordingly, many patients with persistent pain show evidence of a distorted body image. Another approach to understanding why the "pain system" so often goes awry comes from knowledge of the dynamic and nonlinear behaviour of neuronal networks. In real life the emergence of persistent pain probably depends on the simultaneous occurrence of numerous challenges, and just one extra (however small) might put the network into a an inflexible state with heightened sensitivity to normally innocuous inputs. Finally, the importance of seeking the meaning the patient attributes to his/her pain is emphasized. Only then can we understand why a particular person suffers so much more than another with very similar pathology, and subsequently be able to help the person to alter the meaning of the situation.