Central sensitization: a biopsychosocial explanation for chronic widespread pain in patients with fibromyalgia and chronic fatigue syndrome

In addition to the debilitating fatigue, the majority of patients with chronic fatigue syndrome (CFS) experience chronic widespread pain. These pain complaints show the greatest overlap between CFS and fibromyalgia (FM). Although the literature provides evidence for central sensitization as cause for the musculoskeletal pain in FM, in CFS this evidence is currently lacking, despite the observed similarities in both diseases. The knowledge concerning the physiological mechanism of central sensitization, the pathophysiology and the pain processing in FM, and the knowledge on the pathophysiology of CFS lead to the hypothesis that central sensitization is also responsible for the sustaining pain complaints in CFS. This hypothesis is based on the hyperalgesia and allodynia reported in CFS, on the elevated concentrations of nitric oxide presented in the blood of CFS patients, on the typical personality styles seen in CFS and on the brain abnormalities shown on brain images. To examine the present hypothesis more research is required. Further investigations could use similar protocols to those already used in studies on pain in FM like, for example, studies on temporal summation, spatial summation, the role of psychosocial aspects in chronic pain, etc.

Exercise-Induced Hypoalgesia in Pain-Free and Chronic Pain Populations: State of the Art and Future Directions

Exercise is considered an important component of effective chronic pain management and it is well-established that long-term exercise training provides pain relief. In healthy, pain-free populations, a single bout of aerobic or resistance exercise typically leads to exercise-induced hypoalgesia (EIH), a generalized reduction in pain and pain sensitivity that occurs during exercise and for some time afterward. In contrast, EIH is more variable in chronic pain populations and is more frequently impaired; with pain and pain sensitivity decreasing, remaining unchanged or, in some cases, even increasing in response to exercise. Pain exacerbation with exercise may be a major barrier to adherence, precipitating a cycle of physical inactivity that can lead to long-term worsening of both pain and disability. To optimize the therapeutic benefits of exercise, it is important to understand how EIH works, why it may be impaired in some people with chronic pain, and how this should be addressed in clinical practice. In this article, we provide an overview of EIH across different chronic pain conditions. We discuss possible biological mechanisms of EIH and the potential influence of sex and psychosocial factors, both in pain-free adults and, where possible, in individuals with chronic pain. The clinical implications of impaired EIH are discussed and recommendations are made for future research, including further exploration of individual differences in EIH, the relationship between exercise dose and EIH, the efficacy of combined treatments and the use of alternative measures to quantify EIH. PERSPECTIVE: This article provides a contemporary review of the acute effects of exercise on pain and pain sensitivity, including in people with chronic pain conditions. Existing findings are critically reviewed, clinical implications are discussed, and recommendations are offered for future research.

Central sensitisation in chronic pain conditions: latest discoveries and their potential for precision medicine

Chronic pain is a leading cause of disability globally and associated with enormous health-care costs. The discrepancy between the extent of tissue damage and the magnitude of pain, disability, and associated symptoms represents a diagnostic challenge for rheumatology specialists. Central sensitisation, defined as an amplification of neural signalling within the CNS that elicits pain hypersensitivity, has been investigated as a reason for this discrepancy. Features of central sensitisation have been documented in various pain conditions common in rheumatology practice, including fibromyalgia, osteoarthritis, rheumatoid arthritis, Ehlers-Danlos syndrome, upper extremity tendinopathies, headache, and spinal pain. Within individual pain conditions, there is substantial variation among patients in terms of presence and magnitude of central sensitisation, stressing the importance of individual assessment. Central sensitisation predicts poor treatment outcomes in multiple patient populations. The available evidence supports various pharmacological and non-pharmacological strategies to reduce central sensitisation and to improve patient outcomes in several conditions commonly seen in rheumatology practice. These data open up new treatment perspectives, with the possibility for precision pain medicine treatment according to pain phenotyping as a logical next step. With this view, studies suggest the possibility of matching non-pharmacological approaches, or medications, or both to the central sensitisation pain phenotypes.

Genetic determinants of bone mineral content at the spine and radius: a twin study

The possible role of genetic and/or environmental factors in determining bone mass has been investigated in 30 pairs of twins (16 monozygotic and 14 dizygotic) divided in two age groups (below and above 25 years of age). Bone mineral content was evaluated by single- and dual photon absorptiometry at the distol third of the radius for peripheral cortical bone and in the lumbar spine for the axial bone. The "within pair" variance has been used as an index of genetic influence. A significant (p less than 0.01) genetic determinant was found for the bone mass of the radius in adults and for the spinal bone mass in the age group younger than 25 years. The heritability index h2 was 0.75 for cortical BMC and 0.88 for axial BMC. Such a genetic determinant could not conclusively be demonstrated in adult twins for the spine and in youngsters for the cortical bone, suggesting that environmental factors may play a more dominant role in growth of cortical bone during adolescence and diminution of axial bone during adult life.

Assessment of the strength of proximal femur in vitro: relationship to femoral bone mineral density and femoral geometry

Femoral neck axis length, neck width, and neck-shaft angle were measured on radiographs of right proximal femora from 64 cadavers (28 female, 36 male). Bone mineral density (BMD) was measured using dual energy X-ray absorptiometry (DXA) for various regions of interest, and quantitative computed tomography (QCT) was used to determine BMD and bone areas for cortical and trabecular bone at the trochanter and femoral neck. The strength of the femur was determined by a mechanical test simulating a fall on the greater trochanter, and the fracture type (cervical or trochanteric) was subsequently determined from radiographs. Twenty-six cervical fractures and 38 trochanteric fractures were observed, with no significant sex difference in the distribution of fracture types. Femoral strength was significantly elevated in males compared to females. DXA trochanteric BMD was more strongly (p < 0.05) correlated with femoral strength (r2 = 0.88) than were any of the other DXA BMD measurements (r2 = 0.59-0.76). In multiple regression models, a combination of different DXA BMD measurements produced only a small increase (1%) in the explained variability of femoral strength. Of the QCT measurements, trochanteric cortical area yielded the optimal correlation with femoral strength (r2 = 0.83). Weak, but significant, correlations were observed between femoral strength and cortical BMD at trochanteric (r2 = 0.28) and neck regions (r2 = 0.07). In multiple regression models, combining QCT parameters yielded, at best, an r2 of 0.87. Of the geometrical parameters, both neck axis length and neck width were significantly correlated with femoral strength (r2 = 0.24, 0.22, respectively), but no significant correlation was found between strength and the neck-shaft angle. Combining DXA trochanteric BMD with femoral neck width resulted in only a small increase in the explained variability (1%) compared to trochanteric BMD alone. The results demonstrated that DXA and QCT had a similar ability to predict femoral strength in vitro. Trochanteric BMD was the best DXA parameter, and cortical area (not cortical BMD) was the optimal QCT parameter. Geometric measurements of the proximal femur were only weakly correlated with the mechanical strength, and combinations of DXA, QCT, and geometric parameters resulted in only small increases in predictive power compared to the use of a single explanatory variable alone.

Effect of Pain Neuroscience Education Combined With Cognition-Targeted Motor Control Training on Chronic Spinal Pain: A Randomized Clinical Trial

Importance

Effective treatments for chronic spinal pain are essential to reduce the related high personal and socioeconomic costs.

Objective

To compare pain neuroscience education combined with cognition-targeted motor control training with current best-evidence physiotherapy for reducing pain and improving functionality, gray matter morphologic features, and pain cognitions in individuals with chronic spinal pain.

Design, Setting, and Participants

Multicenter randomized clinical trial conducted from January 1, 2014, to January 30, 2017, among 120 patients with chronic nonspecific spinal pain in 2 outpatient hospitals with follow-up at 3, 6, and 12 months.

Interventions

Participants were randomized into an experimental group (combined pain neuroscience education and cognition-targeted motor control training) and a control group (combining education on back and neck pain and general exercise therapy).

Main Outcomes and Measures

Primary outcomes were pain (pressure pain thresholds, numeric rating scale, and central sensitization inventory) and function (pain disability index and mental health and physical health).

Results

There were 22 men and 38 women in the experimental group (mean [SD] age, 39.9 [12.0] years) and 25 men and 35 women in the control group (mean [SD] age, 40.5 [12.9] years). Participants in the experimental group experienced reduced pain (small to medium effect sizes): higher pressure pain thresholds at primary test site at 3 months (estimated marginal [EM] mean, 0.971; 95% CI, -0.028 to 1.970) and reduced central sensitization inventory scores at 6 months (EM mean, -5.684; 95% CI, -10.589 to -0.780) and 12 months (EM mean, -6.053; 95% CI, -10.781 to -1.324). They also experienced improved function (small to medium effect sizes): significant and clinically relevant reduction of disability at 3 months (EM mean, -5.113; 95% CI, -9.994 to -0.232), 6 months (EM mean, -6.351; 95% CI, -11.153 to -1.550), and 12 months (EM mean, -5.779; 95% CI, -10.340 to -1.217); better mental health at 6 months (EM mean, 36.496; 95% CI, 7.998-64.995); and better physical health at 3 months (EM mean, 39.263; 95% CI, 9.644-66.882), 6 months (EM mean, 53.007; 95% CI, 23.805-82.209), and 12 months (EM mean, 32.208; 95% CI, 2.402-62.014).

Conclusions and Relevance

Pain neuroscience education combined with cognition-targeted motor control training appears to be more effective than current best-evidence physiotherapy for improving pain, symptoms of central sensitization, disability, mental and physical functioning, and pain cognitions in individuals with chronic spinal pain. Significant clinical improvements without detectable changes in brain gray matter morphologic features calls into question the relevance of brain gray matter alterations in this population.

Trial Registration

clinicaltrials.gov Identifier: NCT02098005.

Applying Modern Pain Neuroscience in Clinical Practice: Criteria for the Classification of Central Sensitization Pain

Background: The awareness is growing that central sensitization is of prime importance for the assessment and management of chronic pain, but its classification is challenging clinically since no gold standard method of assessment exists. Objectives: Designing the first set of classification criteria for the classification of central sensitization pain. Methods: A body of evidence from original research papers was used by 18 pain experts from 7 different countries to design the first classification criteria for central sensitization pain. Results: It is proposed that the classification of central sensitization pain entails 2 major steps: the exclusion of neuropathic pain and the differential classification of nociceptive versus central sensitization pain. For the former, the International Association for the Study of Pain diagnostic criteria are available for diagnosing or excluding neuropathic pain. For the latter, clinicians are advised to screen their patients for 3 major classification criteria, and use them to complete the classification algorithm for each individual patient with chronic pain. The first and obligatory criterion entails disproportionate pain, implying that the severity of pain and related reported or perceived disability are disproportionate to the nature and extent of injury or pathology (i.e., tissue damage or structural impairments). The 2 remaining criteria are 1) the presence of diffuse pain distribution, allodynia, and hyperalgesia; and 2) hypersensitivity of senses unrelated to the musculoskeletal system (defined as a score of at least 40 on the Central Sensitization Inventory). Limitations: Although based on direct and indirect research findings, the classification algorithm requires experimental testing in future studies. Conclusion: Clinicians can use the proposed classification algorithm for differentiating neuropathic, nociceptive, and central sensitization pain. Key words: Chronic pain, diagnosis, hypersensitivity, classification, neuropathic pain

Low Back Pain: Guidelines for the Clinical Classification of Predominant Neuropathic, Nociceptive, or Central Sensitization Pain

Background: Low back pain (LBP) is a heterogeneous disorder including patients with dominant nociceptive (e.g., myofascial low back pain), neuropathic (e.g., lumbar radiculopathy), and central sensitization pain. In order to select an effective and preferably also efficient treatment in daily clinical practice, LBP patients should be classified clinically as either predominantly nociceptive, neuropathic, or central sensitization pain. Objective: To explain how clinicians can differentiate between nociceptive, neuropathic, and central sensitization pain in patients with LBP. Study Design: Narrative review and expert opinion. Setting: Universities, university hospitals and private practices. Methods: Recently, a clinical method for the classification of central sensitization pain versus neuropathic and nociceptive pain was developed. It is based on a body of evidence of original research papers and expert opinion of 18 pain experts from 7 different countries. Here we apply this classification algorithm to the LBP population. Results: The first step implies examining the presence of neuropathic low back pain. Next, the differential diagnosis between predominant nociceptive and central sensitization pain is done using a clinical algorithm. Limitations: The classification criteria are substantiated by several original research findings including a Delphi survey, a study of a large group of LBP patients, and validation studies of the Central Sensitization Inventory. Nevertheless, these criteria require validation in clinical settings. Conclusion: The pain classification system for LBP should be an addition to available classification systems and diagnostic procedures for LBP, as it is focussed on pain mechanisms solely. Key words: Chronic pain, neuroscience, diagnosis, clinical reasoning, examination, assessment

Measurement Properties of the Central Sensitization Inventory: A Systematic Review

BACKGROUND AND OBJECTIVE

Central sensitization (CS) is a phenomenon associated with several medical diagnoses, including postcancer pain, low back pain, osteoarthritis, whiplash, and fibromyalgia. CS involves an amplification of neural signaling within the central nervous system that results in pain hypersensitivity. The purpose of this systematic review was to gather published studies of a widely used outcome measure (the Central Sensitization Inventory [CSI]), determine the quality of evidence these publications reported, and examine the measurement properties of the CSI.

DATABASES AND DATA TREATMENT

Four databases were searched for publications from 2011 (when the CSI was developed) to July 2017. The Consensus-Based Standards for the Selection of Health Measurement Instruments (COSMIN) checklist was applied to evaluate methodological quality and risk of bias. In instances when COSMIN did not offer a scoring system for measurement properties, qualitative analyses were performed.

RESULTS

Fourteen studies met inclusion criteria. Quality of evidence examined with the COSMIN checklist was determined to be good to excellent for all studies for their respective measurement property reports. Interpretability measures were consistent when publications were analyzed qualitatively, and construct validity was strong when examined alongside other validated measures relating to CS.

CONCLUSIONS

An assessment of the published measurement studies of the CSI suggest the tool generates reliable and valid data that quantify the severity of several symptoms of CS.

Nociplastic Pain Criteria or Recognition of Central Sensitization? Pain Phenotyping in the Past, Present and Future

Recently, the International Association for the Study of Pain (IASP) released clinical criteria and a grading system for nociplastic pain affecting the musculoskeletal system. These criteria replaced the 2014 clinical criteria for predominant central sensitization (CS) pain and accounted for clinicians' need to identify (early) and correctly classify patients having chronic pain according to the pain phenotype. Still, clinicians and researchers can become confused by the multitude of terms and the variety of clinical criteria available. Therefore, this paper aims at (1) providing an overview of what preceded the IASP criteria for nociplastic pain ('the past'); (2) explaining the new IASP criteria for nociplastic pain in comparison with the 2014 clinical criteria for predominant CS pain ('the present'); and (3) highlighting key areas for future implementation and research work in this area ('the future'). It is explained that the 2021 IASP clinical criteria for nociplastic pain are in line with the 2014 clinical criteria for predominant CS pain but are more robust, comprehensive, better developed and hold more potential. Therefore, the 2021 IASP clinical criteria for nociplastic pain are important steps towards precision pain medicine, yet studies examining the clinimetric and psychometric properties of the criteria are urgently needed.

Dysfunctional Endogenous Analgesia During Exercise in Patients with Chronic Pain: To Exercise or Not to Exercise?

Background: Exercise is an effective treatment for various chronic pain disorders, including fibromyalgia, chronic neck pain, osteoarthritis, rheumatoid arthritis, and chronic low back pain. Although the clinical benefits of exercise therapy in these populations are well established (i.e. evidence based), it is currently unclear whether exercise has positive effects on the processes involved in chronic pain (e.g. central pain modulation). Objectives: Reviewing the available evidence addressing the effects of exercise on central pain modulation in patients with chronic pain. Methods: Narrative review. Results: Exercise activates endogenous analgesia in healthy individuals. The increased pain threshold following exercise is due to the release of endogenous opioids and activation of (supra)spinal nociceptive inhibitory mechanisms orchestrated by the brain. Exercise triggers the release of β-endorphins from the pituitary (peripherally) and the hypothalamus (centrally), which in turn enables analgesic effects by activating µ-opioid receptors peripherally and centrally, respectively. The hypothalamus, through its projections on the periaqueductal grey, has the capacity to activate descending nociceptive inhibitory mechanisms. However, several groups have shown dysfunctioning of endogenous analgesia in response to exercise in patients with chronic pain. Muscle contractions activate generalized endogenous analgesia in healthy, pain-free humans and patients with either osteoarthritis or rheumatoid arthritis, but result in increased generalised pain sensitivity in fibromyalgia patients. In patients having local muscular pain (e.g. shoulder myalgia), exercising non-painful muscles activates generalized endogenous analgesia. However, exercising painful muscles does not change pain sensitivity either in the exercising muscle or at distant locations. Limitations: The reviewed studies examined acute effects of exercise rather than long-term effects of exercise therapy. Conclusions: A dysfunctional response of patients with chronic pain and aberrations in central pain modulation to exercise has been shown, indicating that exercise therapy should be individually tailored with emphasis on prevention of symptom flares. The paper discusses the translation of these findings to rehabilitation practice together with future research avenues. Key words: Whiplash, fibromyalgia, chronic pain, low back pain, exercise, rehabilitation, chronic fatigue syndrome, osteoarthritis, rheumatoid arthritis, sensitization, shoulder

Long-term effect of omega-3 fatty acid supplementation in active rheumatoid arthritis. A 12-month, double-blind, controlled study

OBJECTIVE

To study the long-term effects of supplementation with omega-3 fatty acids (omega 3) in patients with active rheumatoid arthritis.

METHODS

Ninety patients were enrolled in a 12-month, double-blind, randomized study comparing daily supplementations with either 2.6 gm of omega 3, or 1.3 gm of omega 3 + 3 gm of olive oil, or 6 gm of olive oil.

RESULTS

Significant improvement in the patient's global evaluation and in the physician's assessment of pain was observed only in those taking 2.6 gm/day of omega 3. The proportions of patients who improved and of those who were able to reduce their concomitant antirheumatic medications were significantly greater with 2.6 gm/day of omega 3.

CONCLUSION

Daily supplementation with 2.6 gm of omega 3 results in significant clinical benefit and may reduce the need for concomitant antirheumatic medication.

Reduced pressure pain thresholds in response to exercise in chronic fatigue syndrome but not in chronic low back pain: an experimental study

OBJECTIVE

The aims of this study were to examine: (i) baseline pressure pain thresholds in patients with chronic fatigue syndrome and those with chronic low back pain compared with healthy subjects; (ii) the change in mean pain threshold in response to exercise; and (iii) associations with exercise-induced increase in nitric oxide.

PARTICIPANTS

Twenty-six patients with chronic fatigue syndrome suffering of chronic pain, 21 patients with chronic low back pain and 31 healthy subjects.

METHODS

Participants underwent a submaximal aerobic exercise protocol on a bicycle ergometer, preceded and followed by venous blood sampling (nitric oxide) and algometry (hand, arm, calf, low back).

RESULTS

Patients with chronic fatigue syndrome presented overall lower pain thresholds compared with healthy subjects and patients with chronic low back pain (p < 0.05). No significant differences were found between healthy subjects and patients with chronic low back pain. After submaximal aerobic exercise, mean pain thresholds decreased in patients with chronic fatigue syndrome, and increased in the others (p < 0.01). At baseline, nitric oxide levels were significantly higher in the chronic low back pain group. After controlling for body mass index, no significant differences were seen between the groups at baseline or in response to exercise. Nitric oxide was not related to pain thresholds in either group.

CONCLUSION

The results suggest hyperalgesia and abnormal central pain processing during submaximal aerobic exercise in chronic fatigue syndrome, but not in chronic low back pain. Nitric oxide appeared to be unrelated to pain processing.

Brain-derived neurotrophic factor as a driving force behind neuroplasticity in neuropathic and central sensitization pain: a new therapeutic target?

INTRODUCTION

Central sensitization is a form of maladaptive neuroplasticity underlying many chronic pain disorders, including neuropathic pain, fibromyalgia, whiplash, headache, chronic pelvic pain syndrome and some forms of osteoarthritis, low back pain, epicondylitis, shoulder pain and cancer pain. Brain-derived neurotrophic factor (BDNF) is a driving force behind neuroplasticity, and it is therefore crucial for neural maintenance and repair. However, BDNF also contributes to sensitization of pain pathways, making it an interesting novel therapeutic target.

AREAS COVERED

An overview of BDNF's sensitizing capacity at every level of the pain pathways is presented, including the peripheral nociceptors, dorsal root ganglia, spinal dorsal horn neurons, and brain descending inhibitory and facilitatory pathways. This is followed by the presentation of several potential therapeutic options, ranging from indirect influencing of BDNF levels (using exercise therapy, anti-inflammatory drugs, melatonin, repetitive transcranial magnetic stimulation) to more specific targeting of BDNF's receptors and signaling pathways (blocking the proteinase-activated receptors 2-NK-κβ signaling pathway, administration of phencyclidine for antagonizing NMDA receptors, or blockade of the adenosine A2A receptor).

EXPERT OPINION

This section focuses on combining pharmacotherapy with multimodal rehabilitation for balancing the deleterious and therapeutic effects of BNDF treatment in chronic pain patients, as well as accounting for the complex and biopsychosocial nature of chronic pain.

Scapular positioning and movement in unimpaired shoulders, shoulder impingement syndrome, and glenohumeral instability

The purpose of this manuscript is to review the knowledge of scapular positioning at rest and scapular movement in different anatomic planes in asymptomatic subjects and patients with shoulder impingement syndrome (SIS) and glenohumeral shoulder instability. We reviewed the literature for all biomechanical and kinematic studies using keywords for impingement syndrome, shoulder instability, and scapular movement published in peer reviewed journal. Based on the predefined inclusion and exclusion criteria, 30 articles were selected for inclusion in the review. The literature is inconsistent regarding the scapular resting position. At rest, the scapula is positioned approximately horizontal, 35° of internal rotation and 10° anterior tilt. During shoulder elevation, most researchers agree that the scapula tilts posteriorly and rotates both upward and externally. It appears that during shoulder elevation, patients with SIS demonstrate a decreased upward scapular rotation, a decreased posterior tilt, and a decrease in external rotation. In patients with glenohumeral shoulder instability, a decreased scapular upward rotation and increased internal rotation is seen. This literature overview provides clinicians with insight into scapular kinematics in unimpaired shoulders and shoulders with impingement syndrome and instability.

In the mind or in the brain? Scientific evidence for central sensitisation in chronic fatigue syndrome

BACKGROUND

Central sensitisation entails several top-down and bottom-up mechanisms, all contributing to the hyperresponsiveness of the central nervous system to a variety of inputs. In the late nineties, it was first hypothesised that chronic fatigue syndrome (CFS) is characterised by hypersensitivity of the central nervous system (i.e. central sensitisation). Since then, several studies have examined central sensitisation in patients with CFS. This study provides an overview of such studies.

MATERIALS AND METHODS

Narrative review.

RESULTS

Various studies showed generalised hyperalgesia in CFS for a variety of sensory stimuli, including electrical stimulation, mechanical pressure, heat and histamine. Various tissues are affected by generalised hyperalgesia: the skin, muscle tissue and the lungs. Generalised hyperalgesia in CFS is augmented, rather than decreased, following various types of stressors like exercise and noxious heat pain. Endogenous inhibition is not activated in response to exercise and activation of diffuse noxious inhibitory controls following noxious heat application to the skin is delayed.

CONCLUSIONS

The observation of central sensitisation in CFS is in line with our current understanding of CFS. The presence of central sensitisation in CFS corroborates with the presence of several psychological influences on the illness, the presence of infectious agents and immune dysfunctions and the dysfunctional hypothalamus-pituitary-adrenal axis as seen in these severely debilitated patients.