Advances in targeting central sensitization and brain plasticity in chronic pain

Rethinking plasticity: Analysing the concept of "destructive plasticity" in the light of neuroscience definitions

As a multilevel and multidisciplinary field, neuroscience is designed to interact with various branches of natural and applied sciences as well as with humanities and philosophy. The continental tradition in philosophy, particularly over the past 20 years, tended to establish strong connections with biology and neuroscience findings. This cross fertilization can however be impeded by conceptual intricacies, such as those surrounding the concept of plasticity. The use of this concept has broadened as scientists applied it to explore an ever-growing range of biological phenomena. Here, we examine the consequences of this ambiguity in an interdisciplinary context through the analysis of the concept of "destructive plasticity" in the philosophical writings of Catherine Malabou. The term "destructive plasticity" was coined by Malabou in 2009 to refer to all processes leading to psycho-cognitive and emotional alterations following traumatic or nontraumatic brain injuries or resulting from neurodevelopmental disorders. By comparing it with the neuroscientific definitions of plasticity, we discuss the epistemological obstacles and possibilities related to the integration of this concept into neuroscience. Improving interdisciplinary exchanges requires an advanced and sophisticated manipulation of neurobiological concepts. These concepts are not only intended to guide research programmes within neuroscience but also to organize and frame the dialogue between different theoretical backgrounds.

Combining Transcranial Direct Current Stimulation With Non-Invasive Interventions for Chronic Primary Pain: A Systematic Review and Meta-Analysis

BACKGROUND

A growing number of studies has combined transcranial direct current stimulation (tDCS) with other non-invasive non-pharmacological therapies (NINPT) to enhance effects in pain reduction. However, the efficacy of these combined approaches in treating chronic primary pain (CPP) warrants thorough investigation.

OBJECTIVE

This study aims to evaluate the efficacy of tDCS in conjunction with other NINPT in alleviating pain severity among CPP patients.

METHODS

We conducted a systematic search for randomized controlled trials (RCTs) comparing the efficacy of tDCS combined with NINPT against control treatments in adult CPP patients. The search spanned multiple databases, including PubMed, EMBASE, LILACS, Scopus, Web of Science, and CENTRAL.

RESULTS

Our systematic review included 11 RCTs with a total of 449 participants. In our meta-analysis, which comprised 228 participants receiving active-tDCS and 221 receiving sham-tDCS, we found a significant reduction in pain intensity (Standard Mean Difference = -0.73; 95% Confidence Interval (CI) = -1.18 to -0.27; P = .002) with the use of active-tDCS combined with NINPT.

CONCLUSION

These findings substantiate the therapeutic potential of combining tDCS with other NINPT, highlighting it as an effective treatment modality for reducing pain intensity in CPP patients.

An Innovative EEG-Based Pain Identification and Quantification: A Pilot Study

OBJECTIVE

The present pilot study aimed to propose an innovative scale-independent measure based on electroencephalographic (EEG) signals for the identification and quantification of the magnitude of chronic pain.

METHODS

EEG data were collected from three groups of participants at rest: seven healthy participants with pain, 15 healthy participants submitted to thermal pain, and 66 participants living with chronic pain. Every 30 s, the pain intensity score felt by the participant was also recorded. Electrodes positioned in the contralateral motor region were of interest. After EEG preprocessing, a complex analytical signal was obtained using Hilbert transform, and the upper envelope of the EEG signal was extracted. The average coefficient of variation of the upper envelope of the signal was then calculated for the beta (13-30 Hz) band and proposed as a new EEG-based indicator, namely Piqβ, to identify and quantify pain.

MAIN RESULTS

The main results are as follows: (1) A Piqβ threshold at 10%, that is, Piqβ ≥ 10%, indicates the presence of pain, and (2) the higher the Piqβ (%), the higher the extent of pain.

CONCLUSIONS

This finding indicates that Piqβ can objectively identify and quantify pain in a population living with chronic pain. This new EEG-based indicator can be used for objective pain assessment based on the neurophysiological body response to pain.

SIGNIFICANCE

Objective pain assessment is a valuable decision-making aid and an important contribution to pain management and monitoring.

Healthy women show more experimentally induced central sensitization compared with men

ABSTRACT

Women more often experience chronic pain conditions than men. Central sensitization (CS) is one key mechanism in chronic pain that can differ between the sexes. It is unknown whether CS processes are already more pronounced in healthy women than in men. In 66 subjects (33 women), a thermal CS induction protocol was applied to the dorsum of one foot and a sham protocol to the other. Spatial extent [cm 2 ] of secondary mechanical hyperalgesia (SMH) and dynamic mechanical allodynia were assessed as subjective CS proxy measures, relying on verbal feedback. Changes in nociceptive withdrawal reflex magnitude (NWR-M) and response rate (NWR-RR) recorded through surface electromyography at the biceps and rectus femoris muscles were used as objective CS proxies. The effect of the CS induction protocol on SMH was higher in women than in men (effect size 2.11 vs 1.68). Nociceptive withdrawal reflex magnitude results were statistically meaningful for women (effect size 0.31-0.36) but not for men (effect size 0.12-0.29). Differences between men and women were not meaningful. Nociceptive withdrawal reflex response rate at the rectus femoris increased in women after CS induction and was statistically different from NWR-RR in men (median differences of 13.7 and 8.4% for 120 and 140% reflex threshold current). The objective CS proxy differences indicate that dorsal horn CS processes are more pronounced in healthy women. The even larger sex differences in subjective CS proxies potentially reflect greater supraspinal influence in women. This study shows that sex differences are present in experimentally induced CS in healthy subjects, which might contribute to women's vulnerability for chronic pain.

Psycho-Neuroendocrinology in the Rehabilitation Field: Focus on the Complex Interplay between Stress and Pain

Chronic stress and chronic pain share neuro-anatomical, endocrinological, and biological features. However, stress prepares the body for challenging situations or mitigates tissue damage, while pain is an unpleasant sensation due to nociceptive receptor stimulation. When pain is chronic, it might lead to an allostatic overload in the body and brain due to the chronic dysregulation of the physiological systems that are normally involved in adapting to environmental challenges. Managing stress and chronic pain (CP) in neurorehabilitation presents a significant challenge for healthcare professionals and researchers, as there is no definitive and effective solution for these issues. Patients suffering from neurological disorders often complain of CP, which significantly reduces their quality of life. The aim of this narrative review is to examine the correlation between stress and pain and their potential negative impact on the rehabilitation process. Moreover, we described the most relevant interventions used to manage stress and pain in the neurological population. In conclusion, this review sheds light on the connection between chronic stress and chronic pain and their impact on the neurorehabilitation pathway. Our results emphasize the need for tailored rehabilitation protocols to effectively manage pain, improve treatment adherence, and ensure comprehensive patient care.

Development, refinement, and validation of an equine musculoskeletal pain scale

Musculoskeletal disease is a common cause of chronic pain that is often overlooked and inadequately treated, impacting the quality of life of humans and horses alike. Lameness due to musculoskeletal pain is prevalent in horses, but the perception of pain by owners is low compared with veterinary diagnosis. Therefore, this study aims to establish and validate a pain scale for chronic equine orthopaedic pain that is user-friendly for horse owners and veterinarians to facilitate the identification and monitoring of pain in horses. The newly developed musculoskeletal pain scale (MPS) was applied to 154 horses (mean age 20 ± 6.4 years SD) housed at an equine sanctuary, of which 128 (83%) suffered from chronic orthopaedic disease. To complete the MPS, the horses were observed and videotaped from a distance while at rest in their box or enclosure. In addition, they received a complete clinical and orthopaedic exam. The need for veterinary intervention to address pain (assessed and executed by the sanctuary independent from this study) was used as a longitudinal health outcome to determine the MPS's predictive validity. To determine the interrater agreement, the MPS was scored for a randomly selected subset of 30 horses by six additional blinded raters, three equine veterinary practitioners, and three experienced equestrians. An iterative process was used to refine the tool based on improvements in the MPS's correlation with lameness evaluated at the walk and trot, predictive validity for longitudinal health outcomes, and interrater agreement. The intraclass correlation improved from 0.77 of the original MPS to 0.88 of the refined version (95% confidence interval: 0.8-0.94). The refined MPS correlated significantly with lameness at the walk (r = 0.44, p = 0.001) and trot (r = 0.5, p < 0.0001). The refined MPS significantly differed between horses that needed veterinary intervention (mean MPS = 8.6) and those that did not (mean MPS = 5.0, p = 0.0007). In summary, the MPS showed good interrater repeatability between expert and lay scorers, significant correlation with lameness at the walk and trot, and good predictive validity for longitudinal health outcomes, confirming its ability to identify horses with orthopaedic health problems.

Itch: from the skin to the brain - peripheral and central neural sensitization in chronic itch

Similar to chronic pain, chronic itch is frequently linked to neural sensitization, a phenomenon wherein the nervous system becomes hypersensitive to stimuli. This process of neural sensitization of chronic itch is orchestrated by various signaling pathways and mediators in both the peripheral and central nervous systems. At the level of the peripheral nervous system, inflammation and neuroimmune interactions induce plastic changes to peripheral nerve fibers, thereby amplifying the transmission of itch signaling. Neural sensitization in the central nervous system occurs at both the spinal cord and brain levels. At the level of the spinal cord, it involves hyperactivity of itch-activating spinal pathways, dysfunction of spinal inhibitory circuits, and attenuation of descending supraspinal inhibitory pathways. In the brain, neural sensitization manifests as structural and functional changes to itch-associated brain areas and networks. Currently, we have a diverse array of neuroimmune-modulating therapies targeting itch neural sensitization mechanisms to help with providing relief to patients with chronic itch. Itch research is a dynamic and continually evolving field, and as we grow in our understanding of chronic itch mechanisms, so will our therapeutic toolbox. Further studies exploring the peripheral and central neural sensitization mechanisms in the context of chronic itch are needed.

Suprascapular Nerve Pulsed Radiofrequency as an Effective Pain Relief Strategy in Supraspinatus Muscle Tendon Tears

INTRODUCTION

The supraspinatus muscle tendon is the most frequently rotator cuff muscle torn. Reliable shoulder pain relief strategies are needed for patients with severe pain, refractory to conservative management, and without surgical indication.

MATERIALS AND METHODS

We conducted a retrospective analysis in a Portuguese reference Rehabilitation Centre during the 1st of January 2020 and the 30th of June 2021, including all patients with partial or complete supraspinatus tendon tear, older than 50 years, who presented with severe pain and who were submitted to suprascapular nerve pulsed radiofrequency.

RESULTS

We included 32 patients in our retrospective analysis, mainly female (53%) with a mean age of 66.50 years old. Most of the patients reported right shoulder pain (21 patients, 66%). The mean baseline pain, reported on the numeric rating scale, was 8.00 ± 0.88. Compared to baseline, mean pain reduced 4.00 ± 3.19 at three months (p<0.001), 3.59 ± 3.13 at six months (p<0.001) and 2.94 ± 2.78 at 12 months (p<0.001). From the 3rd to the 12th month there was an increase of 1.06 ± 2.77 in mean pain (p=0.038). There was no difference (p>0.05) in average pain at 0, 3, 6, or 12 months between patients who were simultaneously submitted to an intra-articular shoulder injection.

DISCUSSION

Our analysis revealed 36% average shoulder pain reduction for, at least, 12 months following suprascapular nerve pulsed radiofrequency, with a peak pain reduction of 50% at three months. Slow fading of pain reduction in the following nine months was seen, however, compared to baseline, pain reduction was always statistically significant. Cortico-anaesthetic intra-articular shoulder injections seem not to add benefit in shoulder pain reduction when performed simultaneously with suprascapular nerve pulsed radiofrequency.

CONCLUSION

Suprascapular nerve pulsed radiofrequency seems to be an effective strategy for shoulder pain reduction, in patients with severe pain, refractory to other management modalities. Nonetheless, larger prospective studies, analyzing shoulder functionality and quality of life lost scores, besides pain reduction, should be pursued.

Current Evidence for Biological Biomarkers and Mechanisms Underlying Acute to Chronic Pain Transition across the Pediatric Age Spectrum

Chronic pain is highly prevalent in the pediatric population. Many factors are involved in the transition from acute to chronic pain. Currently, there are conceptual models proposed, but they lack a mechanistically sound integrated theory considering the stages of child development. Objective biomarkers are critically needed for the diagnosis, risk stratification, and prognosis of the pathological stages of pain chronification. In this article, we summarize the current evidence on mechanisms and biomarkers of acute to chronic pain transitions in infants and children through the developmental lens. The goal is to identify gaps and outline future directions for basic and clinical research toward a developmentally informed theory of pain chronification in the pediatric population. At the outset, the importance of objective biomarkers for chronification of pain in children is outlined, followed by a summary of the current evidence on the mechanisms of acute to chronic pain transition in adults, in order to contrast with the developmental mechanisms of pain chronification in the pediatric population. Evidence is presented to show that chronic pain may have its origin from insults early in life, which prime the child for the development of chronic pain in later life. Furthermore, available genetic, epigenetic, psychophysical, electrophysiological, neuroimaging, neuroimmune, and sex mechanisms are described in infants and older children. In conclusion, future directions are discussed with a focus on research gaps, translational and clinical implications. Utilization of developmental mechanisms framework to inform clinical decision-making and strategies for prevention and management of acute to chronic pain transitions in children, is highlighted.

Pulsed Radiofrequency Decreases pERK and Affects Intracellular Ca2+ Influx, Cytosolic ATP Level, and Mitochondrial Membrane Potential in the Sensitized Dorsal Root Ganglion Neuron Induced by N-Methyl D-Aspartate

Background

The molecular mechanism of pulsed radiofrequency (PRF) in chronic pain management is not fully understood. Chronic pain involves the activation of specific N-Methyl D-Aspartate receptors (NMDAR) to induce central sensitization. This study aims to determine the effect of PRF on central sensitization biomarker phosphorylated extracellular signal-regulated kinase (pERK), Ca²⁺ influx, cytosolic ATP level, and mitochondrial membrane potential (Δψm) of the sensitized dorsal root ganglion (DRG) neuron following NMDAR activation.

Methods

This study is a true experimental in-vitro study on a sensitized DRG neuron induced with 80 µM NMDA. There are six treatment groups including control, NMDA 80 µM, Ketamine 100 µM, PRF 2Hz, NMDA 80 µM + PRF 2 Hz, and NMDA 80 µM + PRF 2 Hz + ketamine 100 µM. We use PRF 2 Hz 20 ms for 360 seconds. Statistical analysis was performed using the One-Way ANOVA and the Pearson correlation test with α=5%.

Results

In the sensitized DRG neuron, there is a significant elevation of pERK. There is a strong correlation between Ca²⁺, cytosolic ATP level, and Δψm with pERK intensity (p<0.05). PRF treatment decreases pERK intensity from 108.48 ± 16.95 AU to 38.57 ± 5.20 AU (p<0.05). PRF exposure to sensitized neurons also exhibits a Ca²⁺ influx, but still lower than in the unexposed neuron. PRF exposure in sensitized neurons has a higher cytosolic ATP level (0.0458 ± 0.0010 mM) than in the unexposed sensitized neuron (0.0198 ± 0.0004 mM) (p<0.05). PRF also decreases Δψm in the sensitized neuron from 109.24 ± 6.43 AU to 33.21 ± 1.769 AU (p<0.05).

Conclusion

PRF mechanisms related to DRG neuron sensitization are by decreasing pERK, altering Ca²⁺ influx, increasing cytosolic ATP level, and decreasing Δψm which is associated with neuron sensitization following NMDAR activation.

Effects of manual therapy combined with therapeutic exercise versus routine physical therapy on brain biomarkers in patients with chronic non-specific neck pain in Thailand: a study protocol for a single-blinded randomised controlled trial

INTRODUCTION

Structural brain alterations in pain-related areas have been demonstrated in patients with non-specific neck pain. While manual therapy combined with therapeutic exercise is an effective management for neck pain, its underlying mechanisms are poorly understood. The primary objective of this trial is to investigate the effects of manual therapy combined with therapeutic exercise on grey matter volume and thickness in patients with chronic non-specific neck pain. The secondary objectives are to assess changes in white matter integrity, neurochemical biomarkers, clinical features of neck pain, cervical range of motion and cervical muscle strength.

METHODS AND ANALYSIS

This study is a single-blinded, randomised controlled trial. Fifty-two participants with chronic non-specific neck pain will be recruited into the study. Participants will be randomly allocated to either an intervention or control group (1:1 ratio). Participants in the intervention group will receive manual therapy combined with therapeutic exercise for 10 weeks (two visits per week). The control group will receive routine physical therapy. Primary outcomes are whole-brain and regional grey matter volume and thickness. Secondary outcomes are white matter integrity (fractional anisotropy and mean diffusivity), neurochemical biomarkers (N-acetylaspartate, creatine, glutamate/glutamine, myoinositol and choline), clinical features (neck pain intensity, duration, neck disability and psychological symptoms), cervical range of motion and cervical muscle strength. All outcome measures will be taken at baseline and postintervention.

ETHICS AND DISSEMINATION

Ethical approval of this study has been granted by Faculty of Associated Medical Science, Chiang Mai University. The results of this trial will be disseminated through a peer-reviewed publication.

TRIAL REGISTRATION NUMBER

NCT05568394.

Diagnosis and Management of Neuropathic Pain in Spine Diseases

Neuropathic pain is generally defined as a non-physiological pain experience caused by damage to the nervous system. It can occur spontaneously, as a reaction to a given stimulus, or independently of its action, leading to unusual pain sensations usually referred to as firing, burning or throbbing. In the course of spine disorders, pain symptoms commonly occur. According to available epidemiological studies, a neuropathic component of pain is often present in patients with spinal diseases, with a frequency ranging from 36% to 55% of patients. Distinguishing between chronic nociceptive pain and neuropathic pain very often remains a challenge. Consequently, neuropathic pain is often underdiagnosed in patients with spinal diseases. In reference to current guidelines for the treatment of neuropathic pain, gabapentin, serotonin and norepinephrine reuptake inhibitors and tricyclic antidepressants constitute first-line therapeutic agents. However, long-term pharmacologic treatment often leads to developing tolerance and resistance to used medications. Therefore, in recent years, a plethora of therapeutic methods for neuropathic pain have been developed and investigated to improve clinical outcomes. In this review, we briefly summarized current knowledge about the pathophysiology and diagnosis of neuropathic pain. Moreover, we described the most effective treatment approaches for neuropathic pain and discussed their relevance in the treatment of spinal pain.

BDNF Therapeutic Mechanisms in Neuropsychiatric Disorders

Brain-derived neurotrophic factor (BDNF) is the most abundant neurotrophin in the adult brain and functions as both a primary neurotrophic signal and a neuromodulator. It serves essential roles in neuronal development, maintenance, transmission, and plasticity, thereby influencing aging, cognition, and behavior. Accumulating evidence associates reduced central and peripheral BDNF levels with various neuropsychiatric disorders, supporting its potential utilization as a biomarker of central pathologies. Subsequently, extensive research has been conducted to evaluate restoring, or otherwise augmenting, BDNF transmission as a potential therapeutic approach. Promising results were indeed observed for genetic BDNF upregulation or exogenous administration using a multitude of murine models of neurological and psychiatric diseases. However, varying mechanisms have been proposed to underlie the observed therapeutic effects, and many findings indicate the engagement of disease-specific and other non-specific mechanisms. This is because BDNF essentially affects all aspects of neuronal cellular function through tropomyosin receptor kinase B (TrkB) receptor signaling, the disruptions of which vary between brain regions across different pathologies leading to diversified consequences on cognition and behavior. Herein, we review the neurophysiology of BDNF transmission and signaling and classify the converging and diverging molecular mechanisms underlying its therapeutic potentials in neuropsychiatric disorders. These include neuroprotection, synaptic maintenance, immunomodulation, plasticity facilitation, secondary neuromodulation, and preservation of neurovascular unit integrity and cellular viability. Lastly, we discuss several findings suggesting BDNF as a common mediator of the therapeutic actions of centrally acting pharmacological agents used in the treatment of neurological and psychiatric illness.

Sciatic nerve stimulation alleviates acute neuropathic pain via modulation of neuroinflammation and descending pain inhibition in a rodent model

Background

Neuropathic pain (NP) is characterized by abnormal activation of pain conducting pathways and manifests as mechanical allodynia and thermal hypersensitivity. Peripheral nerve stimulation is used for treatment of medically refractory chronic NP and has been shown to reduce neuroinflammation. However, whether sciatic nerve stimulation (SNS) is of therapeutic benefit to NP remains unclear. Moreover, the optimal frequency for SNS is unknown. To address this research gap, we investigated the effect of SNS in an acute NP rodent model.

Methods

Rats with right L5 nerve root ligation (NRL) or Sham surgery were used. Ipsilateral SNS was performed at 2 Hz, 20 Hz, and 60 Hz frequencies. Behavioral tests were performed to assess pain and thermal hypersensitivity before and after NRL and SNS. Expression of inflammatory proteins in the L5 spinal cord and the immunohistochemical alterations of spinal cord astrocytes and microglia were examined on post-injury day 7 (PID7) following NRL and SNS. The involvement of the descending pain modulatory pathway was also investigated.

Results

Following NRL, the rats showed a decreased pain threshold and latency on the von Frey and Hargreaves tests. The immunofluorescence results indicated hyperactivation of superficial spinal cord dorsal horn (SCDH) neurons. Both 2-Hz and 20-Hz SNS alleviated pain behavior and hyperactivation of SCDH neurons. On PID7, NRL resulted in elevated expression of spinal cord inflammatory proteins including NF-κB, TNF-α, IL-1β, and IL-6, which was mitigated by 2-Hz and 20-Hz SNS. Furthermore, 2-Hz and 20-Hz SNS suppressed the activation of spinal cord astrocytes and microglia following NRL on PID7. Activity of the descending serotoninergic pain modulation pathway showed an increase early on PID1 following 2-Hz and 20-Hz SNS.

Conclusions

Our results support that both 2-Hz and 20-Hz SNS can alleviate NP behaviors and hyperactivation of pain conducting pathways. We showed that SNS regulates neuroinflammation and reduces inflammatory protein expression, astrocytic gliosis, and microglia activation. During the early post-injury period, SNS also facilitates the descending pain modulatory pathway. Taken together, these findings support the therapeutic potential of SNS for acute NP.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02513-y.