Disinhibition of the primary somatosensory cortex in patients with fibromyalgia

An exploration of knowledge-organizing technologies to advance transdisciplinary back pain research

Chronic low back pain (LBP) is influenced by a broad spectrum of patient-specific factors as codified in domains of the biopsychosocial model (BSM). Operationalizing the BSM into research and clinical care is challenging because most investigators work in silos that concentrate on only one or two BSM domains. Furthermore, the expanding, multidisciplinary nature of BSM research creates practical limitations as to how individual investigators integrate current data into their processes of generating impactful hypotheses. The rapidly advancing field of artificial intelligence (AI) is providing new tools for organizing knowledge, but the practical aspects for how AI may advance LBP research and clinical are beginning to be explored. The goals of the work presented here are to: (1) explore the current capabilities of knowledge integration technologies (large language models (LLM), similarity graphs (SGs), and knowledge graphs (KGs)) to synthesize biomedical literature and depict multimodal relationships reflected in the BSM, and; (2) highlight limitations, implementation details, and future areas of research to improve performance. We demonstrate preliminary evidence that LLMs, like GPT-3, may be useful in helping scientists analyze and distinguish cLBP publications across multiple BSM domains and determine the degree to which the literature supports or contradicts emergent hypotheses. We show that SG representations and KGs enable exploring LBP's literature in novel ways, possibly providing, trans-disciplinary perspectives or insights that are currently difficult, if not infeasible to achieve. The SG approach is automated, simple, and inexpensive to execute, and thereby may be useful for early-phase literature and narrative explorations beyond one's areas of expertise. Likewise, we show that KGs can be constructed using automated pipelines, queried to provide semantic information, and analyzed to explore trans-domain linkages. The examples presented support the feasibility for LBP-tailored AI protocols to organize knowledge and support developing and refining trans-domain hypotheses.

Reduced somatosensory evoked potentials and paired-pulse inhibition in the primary somatosensory cortex of athletes with chronic pain

PURPOSE

Chronic pain impedes athletic training and performance. However, it is challenging to identify the precise causes of chronic pain for effective treatment. To examine possible neuroplastic changes in sensory transmission and cortical processing, we compared somatosensory evoked potentials (SEPs) and paired-pulse inhibition (PPI) in primary sensory cortex (S1) between athletes with chronic pain and control athletes.

METHODS

Sixty-six intercollegiate athletes (39 males and 27 females) were recruited for this study, 45 control athletes and 21 reporting persistent pain for > 3 months. Sensory-evoked potentials were induced in S1 by constant-current square-wave pulses (0.2-ms duration) delivered to the right median nerve, while PPI was induced by paired stimulation at interstimulus intervals of 30 and 100 ms (PPI-30 and PPI-100 ms, respectively). All participants were randomly presented with total 1,500 (each 500 stimuli) single stimuli and stimulus pairs at 2 Hz.

RESULTS

Both N20 amplitude and PPI-30 ms were significantly lower in athletes with chronic pain compared to control athletes, while P25 amplitude and PPI-100 ms did not differ significantly between groups.

CONCLUSION

Chronic pain in athletes is associated with substantially altered excitatory-inhibitory balance within the primary somatosensory cortex, possibly due to reduced thalamocortical excitatory transmission and suppressed cortical inhibitory transmission.

Augmented pain-evoked primary sensorimotor cortex activation in adolescent girls with juvenile fibromyalgia

ABSTRACT

Juvenile fibromyalgia (JFM) is a chronic widespread pain condition that primarily affects adolescent girls. Previous studies have found increased sensitivity to noxious pressure in adolescents with JFM. However, the underlying changes in brain systems remain unclear. The aim of this study was to characterize pain-evoked brain responses and identify brain mediators of pain hypersensitivity in adolescent girls with JFM. Thirty-three adolescent girls with JFM and 33 healthy adolescent girls underwent functional magnetic resonance imaging scans involving noxious pressure applied to the left thumbnail at an intensity of 2.5 or 4 kg/cm 2 and rated pain intensity and unpleasantness on a computerized Visual Analogue Scale. We conducted standard general linear model analyses and exploratory whole-brain mediation analyses. The JFM group reported significantly greater pain intensity and unpleasantness than the control group in response to noxious pressure stimuli at both intensities ( P < 0.05). The JFM group showed augmented right primary somatosensory cortex (S1) activation to 4 kg/cm 2 (Z > 3.1, cluster-corrected P < 0.05), and the peak S1 activation magnitudes significantly correlated with the scores on the Widespread Pain Index ( r = 0.35, P = 0.048) with higher activation associated with more widespread pain. We also found that greater primary sensorimotor cortex activation in response to 4 kg/cm 2 mediated the between-group differences in pain intensity ratings ( P < 0.001). In conclusion, we found heightened sensitivity to noxious pressure stimuli and augmented pain-evoked sensorimotor cortex responses in adolescent girls with JFM, which could reflect central sensitization or amplified nociceptive input.

Neural indices of multimodal sensory and autonomic hyperexcitability in fibromyalgia

Fibromyalgia (FM) is characterized by chronic widespread musculoskeletal pain and psychological distress. Research suggests people with FM experience increased somatosensory sensitization which generalizes to other sensory modalities and may indicate neural hyperexcitability. However, the available evidence is limited, and studies including measures of neural responsivity across sensory domains and both central and peripheral aspects of the neuraxis are lacking. Thirty-nine participants (51.5 ± 13.6 years of age) with no history of neurological disorders, psychosis, visual, auditory, or learning deficits, were recruited for this study. People with FM (N = 19) and control participants (CNT, N = 20) did not differ on demographic variables and cognitive capacity. Participants completed a task that combined innocuous auditory stimuli with electrocutaneous stimulation (ECS), delivered at individually-selected levels that were uncomfortable but not painful. Event-related potentials (ERPs) and electrodermal activity were analyzed to examine the central and sympathetic indices of neural responsivity. FM participants reported greater sensitivity to ECS and auditory stimulation, as well as higher levels of depression, anxiety, ADHD, and an array of pain-related experiences than CNT. In response to ECS, the P50 deflection was greater in FM than CNT participants, reflecting early somatosensory hyperexcitability. The P50 amplitude was positively correlated with the FM profile factor obtained with a principal component analysis. The N100 to innocuous tones and sympathetic reactivity to ECS were greater in FM participants, except in the subgroup treated with gabapentinoids, which aligns with previous evidence of symptomatic improvement with GABA-mimetic medications. These results support the principal tenet of generalized neural hyperexcitability in FM and provide preliminary mechanistic insight into the impact of GABA-mimetic pharmacological therapy on ameliorating the neural excitation dominance.

Functional Magnetic Resonance Imaging Signal Variability Is Associated With Neuromodulation in Fibromyalgia

OBJECTIVES

Although primary motor cortex (M1) transcranial direct current stimulation (tDCS) has an analgesic effect in fibromyalgia (FM), its neural mechanism remains elusive. We investigated whether M1-tDCS modulates a regional temporal variability of blood-oxygenation-level-dependent (BOLD) signals, an indicator of the brain's flexibility and efficiency and if this change is associated with pain improvement.

MATERIALS AND METHODS

In a within-subjects cross-over design, 12 female FM patients underwent sham and active tDCS on five consecutive days, respectively. Each session was performed with an anode placed on the left M1 and a cathode on the contralateral supraorbital region. The subjects also participated in resting-state functional magnetic resonance imaging (fMRI) at baseline and after sham and active tDCS. We compared the BOLD signal variability (SDBOLD), defined as the standard deviation of the BOLD time-series, between the tDCS conditions. Baseline SDBOLD was compared to 15 healthy female controls.

RESULTS

At baseline, FM patients showed reduced SDBOLD in the ventromedial prefrontal cortex (vmPFC), lateral PFC, and anterior insula and increased SDBOLD in the posterior insula compared to healthy controls. After active tDCS, compared to sham, we found an increased SDBOLD in the left rostral anterior cingulate cortex (rACC), lateral PFC, and thalamus. After sham tDCS, compared to baseline, we found a decreased SDBOLD in the dorsomedial PFC and posterior cingulate cortex/precuneus. Interestingly, after active tDCS compared to sham, pain reduction was correlated with an increased SDBOLD in the rACC/vmPFC but with a decreased SDBOLD in the posterior insula.

CONCLUSION

Our findings suggest that M1-tDCS might revert temporal variability of fMRI signals in the rACC/vmPFC and posterior insula linked to FM pain. Changes in neural variability would be part of the mechanisms underlying repetitive M1-tDCS analgesia in FM.

Neurophysiological and Psychosocial Mechanisms of Fibromyalgia: A Comprehensive Review and Call for An Integrative Model

Research into the neurobiological and psychosocial mechanisms involved in fibromyalgia has progressed remarkably in recent years. Despite this, current accounts of fibromyalgia fail to capture the complex, dynamic, and mutual crosstalk between neurophysiological and psychosocial domains. We conducted a comprehensive review of the existing literature in order to: a) synthesize current knowledge on fibromyalgia; b) explore and highlight multi-level links and pathways between different systems; and c) build bridges connecting disparate perspectives. An extensive panel of international experts in neurophysiological and psychosocial aspects of fibromyalgia discussed the collected evidence and progressively refined and conceptualized its interpretation. This work constitutes an essential step towards the development of a model capable of integrating the main factors implicated in fibromyalgia into a single, unified construct which appears indispensable to foster the understanding, assessment, and intervention for fibromyalgia.

Effect of whole-hand water flow stimulation on the neural balance between excitation and inhibition in the primary somatosensory cortex

Sustained peripheral somatosensory stimulations, such as high-frequency repetitive somatosensory stimulation (HF-RSS) and vibrated stimulation, are effective in altering the balance between excitation and inhibition in the somatosensory cortex (S1) and motor cortex (M1). A recent study reported that whole-hand water flow (WF) stimulation induced neural disinhibition in the M1. Based on previous results, we hypothesized that whole-hand WF stimulation would lead to neural disinhibition in the S1 because there is a strong neural connection between M1 and S1 and aimed to examine whether whole-hand WF stimulation would change the neural balance between excitation and inhibition in the S1. Nineteen healthy volunteers were studied by measuring excitation and inhibition in the S1 before and after each of the four 15-min interventions. The excitation and inhibition in the S1 were assessed using somatosensory evoked potentials (SEPs) and paired-pulse inhibition (PPI) induced by single- and paired-pulse stimulations, respectively. The four interventions were as follows: control, whole-hand water immersion, whole-hand WF, and HF-RSS. The results showed no significant changes in SEPs and PPI following any intervention. However, changes in PPI with an interstimulus interval (ISI) of 30 ms were significantly correlated with the baseline value before whole-hand WF. Thus, the present findings indicated that the whole-hand WF stimulation had a greater decreased neural inhibition in participants with higher neural inhibition in the S1 at baseline. Considering previous results on M1, the present results possibly show that S1 has lower plasticity than M1 and that the duration (15 min) of each intervention may not have been enough to alter the balance of excitation and inhibition in the S1.

Low test–retest reliability of a protocol for assessing somatosensory cortex excitability generated from sensory nerves of the lower back

In people with chronic low back pain (CLBP), maladaptive structural and functional changes on a cortical level have been identified. On a functional level, somatosensory cortical excitability has been shown to be reduced in chronic pain conditions, resulting in cortical disinhibition. The occurrence of structural and/or functional maladaptive cortical changes in people with CLBP could play a role in maintaining the pain. There is currently no measurement protocol for cortical excitability that employs stimulation directly to the lower back. We developed a protocol for the measurement of single pulse somatosensory evoked potential (SEP) waveforms and paired-pulse behavior (PPB) generated from sensory nerves of the lower back and quantified its test–retest reliability in a sample of 30 healthy individuals to gain insights into the normal variability of cortical responses, which could then be compared to results from people with CLBP. We investigated cortical excitability by measuring SEPs and PPB. PPB was defined as the ratio of the amplitude of the second cortical response (A2s) divided by the first cortical response (A1). A2s was determined by subtracting the response to single-pulse stimuli from the paired pulse stimuli response to account for linear superposition effects. The test–retest reliability of the protocol was very poor with no evidence of systematic bias but a high amount of random variability between sessions. There was no significant difference in the right side PPB for session 1 (Mean ratio A2s/A1 = 0.66, SD = 0.54) and session 2 (Mean ratio A2s/A1 = 0.94, SD = 1.56); mean session difference [(95% CI) = −0.44 (−1.23 to 0.34); t (22) = −1.17, p = 0.26]. The ICC₃.₁ (absolute agreement) for the outlier-removed right side PPB were 0.19 (95% CI: −0.84 to 0.66) and 0.43 for left side PPB (95% CI: −0.37 to 0.76). This finding potentially has wider implications for PPB protocols. If these findings were replicated in other groups and other nerves, it would question the validity of this measure more generally. However, these findings are restricted to healthy people and sensory nerves of the lower back and may not be generalizable.

Transcriptional and cellular signatures of cortical morphometric remodelling in chronic pain

ABSTRACT

Chronic pain is a highly debilitating and difficult to treat condition, which affects the structure of the brain. Although the development of chronic pain is moderately heritable, how disease-related alterations at the microscopic genetic architecture drive macroscopic brain abnormalities is currently largely unknown. Here, we examined alterations in morphometric similarity (MS) and applied an integrative imaging transcriptomics approach to identify transcriptional and cellular correlates of these MS changes, in 3 independent small cohorts of patients with distinct chronic pain syndromes (knee osteoarthritis, low back pain, and fibromyalgia) and age-matched and sex-matched pain-free controls. We uncover a novel pattern of cortical MS remodelling involving mostly small-to-medium MS increases in the insula and limbic cortex (none of these changes survived stringent false discovery rate correction for the number of regions tested). This pattern of changes is different from that observed in patients with major depression and cuts across the boundaries of specific pain syndromes. By leveraging transcriptomic data from Allen Human Brain Atlas, we show that cortical MS remodelling in chronic pain spatially correlates with the brain-wide expression of genes related to pain and broadly involved in the glial immune response and neuronal plasticity. Our findings bridge levels to connect genes, cell classes, and biological pathways to in vivo imaging correlates of chronic pain. Although correlational, our data suggest that cortical remodelling in chronic pain might be shaped by multiple elements of the cellular architecture of the brain and identifies several pathways that could be prioritized in future genetic association or drug development studies.

Involvement of Parvalbumin-Positive Neurons in the Development of Hyperalgesia in a Mouse Model of Fibromyalgia

Fibromyalgia (FM) presents as chronic systemic pain, which might be ascribed to central sensitization, in which pain information processing is amplified in the central nervous system. Since patients with FM display elevated gamma oscillations in the pain matrix and parvalbumin (PV)-positive neurons play a critical role in induction of gamma oscillations, we hypothesized that changes in PV-positive neurons are involved in hyperalgesia in fibromyalgia. In the present study, to investigate a role of PV-positive neurons in neuropathic pain, mice received reserpine administration for 3 consecutive days as an animal model of FM (RES group), while control mice received vehicle injections in the same way (VEH group). The mice were subjected to hot-plate and forced swim tests, and immuno-stained PV-positive neurons were counted in the pain matrix. We investigated relationships between PV-positive neuron density in the pain matrix and pain avoidance behaviors. The results indicated that the mice in the RES group showed transient bodyweight loss and longer immobility time in the forced swim test than the mice in the VEH group. In the hot-plate test, the RES group showed shorter response latencies and a larger number of jumps in response to nociceptive thermal stimulus than the VEH group. Histological examination indicated an increase in the density of PV-positive neurons in the primary somatosensory cortex (S1) in the RES group. Furthermore, response latencies to the hot-plate were significantly and negatively correlated with the density of PV-positive neurons in the S1. These results suggest a critical role for PV-positive neurons in the S1 to develop hyperalgesia in FM.

Identifying Motor Control Strategies and Their Role in Low Back Pain: A Cross-Disciplinary Approach Bridging Neurosciences With Movement Biomechanics

Persistent low back pain (LBP) is a major health issue, and its treatment remains challenging due to a lack of pathophysiological understanding. A better understanding of LBP pathophysiology has been recognized as a research priority, however research on contributing mechanisms to LBP is often limited by siloed research within different disciplines. Novel cross-disciplinary approaches are necessary to fill important knowledge gaps in LBP research. This becomes particularly apparent when considering new theories about a potential role of changes in movement behavior (motor control) in the development and persistence of LBP. First evidence points toward the existence of different motor control strategy phenotypes, which are suggested to have pain-provoking effects in some individuals driven by interactions between neuroplastic, psychological and biomechanical factors. Yet, these phenotypes and their role in LBP need further validation, which can be systematically tested using an appropriate cross-disciplinary approach. Therefore, we propose a novel approach, connecting methods from neuroscience and biomechanics research including state-of-the-art optical motion capture, musculoskeletal modeling, functional magnetic resonance imaging and assessments of psychological factors. Ultimately, this cross-disciplinary approach might lead to the identification of different motor control strategy phenotypes with the potential to translate into clinical research for better treatment options.

Structural and functional thalamocortical connectivity study in female fibromyalgia

Dysfunctional thalamocortical interactions have been suggested as putative mechanisms of ineffective pain modulation and also suggested as possible pathophysiology of fibromyalgia (FM). However, it remains unclear which specific thalamocortical networks are altered and whether it is related to abnormal pain perception in people with FM. Here, we conducted combined vertex-wise subcortical shape, cortical thickness, structural covariance, and resting-state functional connectivity analyses to address these questions. FM group exhibited a regional shape deflation of the left posterior thalamus encompassing the ventral posterior lateral and pulvinar nuclei. The structural covariance analysis showed that the extent of regional deflation of the left posterior thalamus was negatively covaried with the left inferior parietal cortical thickness in the FM group, whereas those two regions were positively covaried in the healthy controls. In functional connectivity analysis with the left posterior thalamus as a seed, FM group had less connectivity with the periaqueductal gray compared with healthy controls, but enhanced connectivity between the posterior thalamus and bilateral inferior parietal regions, associated with a lower electrical pain threshold at the hand dorsum (pain-free point). Overall, our findings showed the structural thalamic alteration interacts with the cortical regions in a functionally maladaptive direction, leading the FM brain more responsive to external stimuli and potentially contributing to pain amplification.

Cross-modal relationships of neural gating with the subjective perception of respiratory and somatosensory sensations

Neural gating is a phenomenon whereby the response to a stimulus in the electroencephalogram (EEG) is attenuated when preceded by an identical stimulus. Attenuation of paired auditory clicks has repeatedly been shown to be affected in mental disorders, for example, schizophrenia. Neural gating has also been measured for respiratory and somatosensory sensations, however the attenuation of bodily relevant stimuli has not yet been systematically related to the subjective perception of bodily sensations. This research direction is potentially relevant to explaining disease trajectories in psychosomatic conditions characterized by chronic breathlessness and/or pain. In the present study, we recorded high-density EEG from 85 healthy young adults while they experienced brief paired respiratory occlusions and brief paired electrocutaneous stimulation of the wrist. The event-related potential N1 was measured centro-laterally in response to the second relative to the first stimulus to quantify neural gating in both sensory domains. Participants experienced resistive loaded breaths and electrocutaneous stimuli of various intensities, rated their perceived intensity and unpleasantness, and performed magnitude estimation. Relationships of respiratory and somatosensory neural gating to the subjective intensity and unpleasantness of sensations, as well as the ability to discriminate sensations of varying intensities, were investigated intra-modally and cross-modally. We report significant relationships of the somatosensory neural gating to perceived intensity and unpleasantness of respiratory and somatosensory sensations, with the stronger neural gating relating to a stronger subjective intensity and unpleasantness. We discuss these unexpected findings through the lens of individual differences and different theoretical accounts on the origins of cortical attenuation of repetitive stimuli.

Phantom Perception of Sound and the Abnormal Cortical Inhibition System: An Electroencephalography (EEG) Study

Objectives:

Despite no observable external sound present, a perceived feeling of a recurrent unpleasant sound is a main complaint in the patients with chronic tinnitus. This phantom perception of sound is considered as the auditory equivalent of phantom limb pain, and altered excitability may be involved in its underlying pathology. Tinnitus-related hyper-excitation is suppressed by inhibitory repetitive transcranial magnetic stimulation (rTMS). However, the neural mechanism underlying the treatment is not fully understood, and quantifying the suppression induced by rTMS has yet to be considered.

Methods:

We evaluated the effect of rTMS on the cortical inhibition status following single-site stimulation over the auditory temporal cortex (T group) or dual-site stimulation over the auditory temporal and the frontal regions (TF group). These effects were also compared with outcomes following sham stimulation (S group). Subjective response was recorded using tinnitus-related handicap index (THI), and changes in the cortical inhibition status were assessed using an auditory paired-pulse suppression index (PPSI).

Results:

TF group showed the greatest benefit from the treatment evidenced in the reduced PPSI and THI scores. T and S groups did not benefit much. TF group overlapped mostly with the responder group, indicating improvement in both subjective THI and objective PPSI measurements.

Conclusion:

Our results suggest that rTMS is a beneficial therapeutic treatment for chronic tinnitus patients and the dual-site treatment was the most effective in terms of both tinnitus complaint and quantitative indices. Thus, subjective reports and electrophysiological signatures may be complementary for the diagnosis/prognosis of tinnitus.

Central sensitization in chronic pain and medically unexplained symptom research: A systematic review of definitions, operationalizations and measurement instruments

OBJECTIVE

Central sensitization (CS), a mechanism explaining the persistence of symptoms, has been the focus of many research projects. Explanations given to patients with chronic pain are often based on this mechanism. It is hypothesized that CS also plays an important role in the persistence of medically unexplained symptoms (MUS). However, definitions and operationalizations of CS vary. We conducted a systematic review of definitions, operationalizations and measurement instruments of CS.

METHODS

We searched in PubMed, EMBASE, PsycINFO, Cinahl and The Cochrane Library till September 2017 and included papers that addressed CS in relation to chronic pain and/or MUS. Two reviewers independently selected, analysed and classified information from the selected publications. We performed a thematic analysis of definitions and operationalizations. We listed the measurement instruments.

RESULTS

We included 126 publications, 79 publications concerned chronic pain, 47 publications concerned MUS. Definitions of CS consistently encompass the theme hyperexcitability of the central nervous system (CNS). Additional themes are variably present: CNS locations, nature of sensory input, reduced inhibition and activation and modulation of the NDMA receptor. Hyperalgesia and allodynia are widely mentioned as operationalizations of CS. Quantitative sensory testing (QST) and (f)MRI are the most reported measurement instruments.

CONCLUSIONS

There is consensus that hyperexcitability is the central mechanism of CS. Operationalizations are based on this mechanism and additional components. There are many measurement instruments available, whose clinical value has still to be determined. There were no systematic differences in definitions and operationalizations between the publications addressing MUS and those addressing chronic pain.

Somatosensory temporal discrimination is impaired in fibromyalgia

OBJECTIVE

Fibromyalgia is the prototypical central sensitivity syndrome which is associated with increased sensitivity to pain and other stimuli. In this study, we aimed to evaluate whether somatosensory temporal discrimination ability, which provides information about central processing of sensory stimuli, was impaired in patients with fibromyalgia.

METHODS

Fifteen patients with fibromyalgia and 15 healthy subjects participated in the study. Demographic characteristics of participants and severity for fatigue, sleep quality, cognitive symptoms, somatic symptoms and health-related quality of life in fibromyalgia patients were recorded. Somatosensory temporal discrimination thresholds were measured from the dorsum of the dominant hands of the participants by using a constant current stimulator (Medtronic, Keypoint).

RESULTS

Patients with fibromyalgia had higher somatosensory temporal discrimination thresholds than healthy subjects (p < 0.001). There were significant correlations between STDTs and pain intensity, FIQ scores and symptom severity scale scores in fibromyalgia group (p = 0.006, r = 0.68; p = 0.037, r = 0.54; p = 0.017, r = 0.61 respectively).

CONCLUSION

Somatosensory temporal discrimination ability is impaired in fibromyalgia patients compared to healthy subjects. Disrupted somatosensory temporal discrimination ability correlates with increased widespread pain and severity of other symptoms including fatigue, sleep quality, cognitive symptoms, somatic symptoms and decreased functional status. The impaired somatosensory temporal discrimination ability indicates an alteration in higher cognitive sensory processing in fibromyalgia patients.

The effect of occipital nerve field stimulation on the descending pain pathway in patients with fibromyalgia: a water PET and EEG imaging study

Background

Fibromyalgia is a chronic disorder characterized by widespread musculoskeletal pain accompanied by fatigue, sleep, memory, and mood problems. Recently, occipital nerve field stimulation (ONS) has been proposed as an effective potential treatment for fibromyalgia-related pain. The aim of this study is to unravel the neural mechanism behind occipital nerve stimulation’s ability to suppress pain in fibromyalgia patients.

Materials and methods

Seven patients implanted with subcutaneous electrodes in the C2 dermatoma were enrolled for a Positron Emission Tomography (PET) H₂¹⁵O activation study. These seven patients were selected from a cohort of 40 patients who were part of a double blind, placebo-controlled study followed by an open label follow up at six months. The H₂¹⁵O PET scans were taken during both the “ON” (active stimulation) and “OFF” (stimulating device turned off) conditions. Electroencephalogram (EEG) data were also recorded for the implanted fibromyalgia patients during both the “ON” and “OFF” conditions.

Results

Relative to the “OFF” condition, ONS stimulation resulted in activation in the dorsal lateral prefrontal cortex, comprising the medial pain pathway, the ventral medial prefrontal cortex, and the bilateral anterior cingulate cortex as well as parahippocampal area, the latter two of which comprise the descending pain pathway. Relative deactivation was observed in the left somatosensory cortex, constituting the lateral pain pathway as well as other sensory areas such as the visual and auditory cortex. The EEG results also showed increased activity in the descending pain pathway. The pregenual anterior cingulate cortex extending into the ventral medial prefrontal cortex displayed this increase in the theta, alpha1, alpha2, beta1, and beta2 frequency bands.

Conclusion

PET shows that ONS exerts its effect via activation of the descending pain inhibitory pathway and the lateral pain pathway in fibromyalgia, while EEG shows activation of those cortical areas that could be responsible for descending inhibition system recruitment.

Trial Registration

This study is registered with ClinicalTrials.gov, number NCT00917176 (June 10, 2009).

[Analgesic effects of transcutaneous electrical nerve stimulation (TENS) in patients with fibromyalgia: A systematic review]

OBJECTIVE

To determine whether transcutaneous electrical nerve stimulation (TENS) has an analgesic effect greater than placebo or other treatments in patients with fibromyalgia. Furthermore, it was intended to analyze the optimal application parameters to achieve a greater reduction of pain.

DESIGN

A systematic review.

DATA SOURCE

Randomized clinical trials on the effect of TENS on fibromyalgia in the databases Pubmed, Cochrane and PEDro until November 2016.

SELECTION OF STUDIES

8 studies out of a total of 62 were selected. Controlled clinical trials in which TENS was applied in patients with fibromyalgia were included.

DATA EXTRACTION

Pain was analyzed as the main variable, although other variables such as fatigue, quality of life and impact, range of motion and depression were also included.

RESULTS

6 out of 8 studies obtained a significant decrease of pain. In 2 studies, TENS was applied as complementary treatment to therapeutic exercise with results evidencing a decrease in pain. The rest of the variables studied presented a great variability and conclusive results could not be established.

CONCLUSIONS

Treatment with TENS is effective for reducing pain in people with fibromyalgia. In addition, the inclusion of TENS in therapeutic exercise programs seems to have a greater effect than practicing therapeutic exercise in isolation. However, no efficacy has been demonstrated in other variables different to pain. Further studies are needed to investigate the optimization of the parameters of the TENS and a greater consensus among the variables used.

Disrupted Resting State Network of Fibromyalgia in Theta frequency

Fibromyalgia (FM), chronic widespread pain, exhibits spontaneous pain without external stimuli and is associated with altered brain activities during resting state. To understand the topological features of brain network in FM, we employed persistent homology which is a multiple scale network modeling framework not requiring thresholding. Spontaneous magnetoencephalography (MEG) activity was recorded in 19 healthy controls (HCs) and 18 FM patients. Barcode, single linkage dendrogram and single linkage matrix were generated based on the proposed modeling framework. In theta band, the slope of decrease in the number of connected components in barcodes showed steeper in HC, suggesting FM patients had decreased global connectivity. FM patients had reduced connectivity within default mode network, between middle/inferior temporal gyrus and visual cortex. The longer pain duration was correlated with reduced connectivity between inferior temporal gyrus and visual cortex. Our findings demonstrated that the aberrant resting state network could be associated with dysfunction of sensory processing in chronic pain. The spontaneous nature of FM pain may accrue to disruption of resting state network.

Corticospinal excitability as a biomarker of myofascial pain syndrome

INTRODUCTION

Myofascial pain syndrome (MPS) is a common chronic pain disorder that lacks effective diagnostic criteria. To better understand neurophysiological changes in chronic pain, several trials exploring corticospinal excitability in different populations of patients with chronic pain have been performed.

OBJECTIVES

In this systematic review, we aimed to investigate the current literature on MPS and intracortical disinhibition, by means of increased intracortical facilitation and decreased intracortical inhibition (ICI).

METHODS

We performed a search on PubMed to identify clinical trials on MPS and transcranial magnetic stimulation measurements. We then applied the Harford Hill criteria to the identified studies to assess the possible causal relationship between intracortical disinhibition measurements and MPS. Finally, we compared our findings on MPS with other chronic pain conditions.

RESULTS

Four studies assessing corticospinal excitability in patients with MPS were found. Although the amount of trials available is limited, all the reported studies indicated an increased intracortical disinhibition in patients with MPS. Importantly, these measurements were also correlated with psychological factors, such as pain catastrophism, or anxiety. However, based on the Harford Hill criteria, we could not assert a strong causal relationship between these markers and MPS. Although intracortical disinhibition has been consistently found in patients having MPS, this lack of cortical inhibition was not only observed in this specific chronic pain syndrome but also in fibromyalgia and neuropathic pain conditions.

CONCLUSION

Intracortical disinhibition seems to be a marker that has been consistently observed in MPS. Future prospective cohort studies could provide new insights in the development of neoplastic and maladaptive changes occurring in chronic pain syndromes and help the development of new therapeutic options.

Chronic stress-induced mechanical hyperalgesia is controlled by capsaicin-sensitive neurones in the mouse

BACKGROUND

Clinical studies demonstrated peripheral nociceptor deficit in stress-related chronic pain states, such as fibromyalgia. The interactions of stress and nociceptive systems have special relevance in chronic pain, but the underlying mechanisms including the role of specific nociceptor populations remain unknown. We investigated the role of capsaicin-sensitive neurones in chronic stress-related nociceptive changes.

METHOD

Capsaicin-sensitive neurones were desensitized by the capsaicin analogue resiniferatoxin (RTX) in CD1 mice. The effects of desensitization on chronic restraint stress (CRS)-induced responses were analysed using behavioural tests, chronic neuronal activity assessment in the central nervous system with FosB immunohistochemistry and peripheral cytokine concentration measurements.

RESULTS

Chronic restraint stress induced mechanical and cold hypersensitivity and increased light preference in the light-dark box test. Open-field and tail suspension test activities were not altered. Adrenal weight increased, whereas thymus and body weights decreased in response to CRS. FosB immunopositivity increased in the insular cortex, dorsomedial hypothalamic and dorsal raphe nuclei, but not in the spinal cord dorsal horn after the CRS. CRS did not affect the cytokine concentrations of hindpaw tissues. Surprisingly, RTX pretreatment augmented stress-induced mechanical hyperalgesia, abolished light preference and selectively decreased the CRS-induced neuronal activation in the insular cortex. RTX pretreatment alone increased the basal noxious heat threshold without influencing the CRS-evoked cold hyperalgesia and augmented neuronal activation in the somatosensory cortex and interleukin-1α and RANTES production.

CONCLUSIONS

Chronic restraint stress induces hyperalgesia without major anxiety, depression-like behaviour or peripheral inflammatory changes. Increased stress-induced mechanical hypersensitivity in RTX-pretreated mice is presumably mediated by central mechanisms including cortical plastic changes.

SIGNIFICANCE

These are the first data demonstrating the complex interactions between capsaicin-sensitive neurones and chronic stress and their impact on nociception. Capsaicin-sensitive neurones are protective against stress-induced mechanical hyperalgesia by influencing neuronal plasticity in the brain.

Augmented Pain Processing in Primary and Secondary Somatosensory Cortex in Fibromyalgia: A Magnetoencephalography Study Using Intra-Epidermal Electrical Stimulation

The aim of this study was to investigate augmented pain processing in the cortical somatosensory system in patients with fibromyalgia (FM). Cortical evoked responses were recorded in FM (n = 19) and healthy subjects (n = 21) using magnetoencephalography after noxious intra-epidermal electrical stimulation (IES) of the hand dorsum (pain rating 6 on a numeric rating scale, perceptually-equivalent). In addition, healthy subjects were stimulated using the amplitude corresponding to the average stimulus intensity rated 6 in patients with FM (intensity-equivalent). Quantitative sensory testing was performed on the hand dorsum or thenar muscle (neutral site) and over the trapezius muscle (tender point), using IES (thresholds, ratings, temporal summation of pain, stimulus-response curve) and mechanical stimuli (threshold, ratings). Increased amplitude of cortical responses was found in patients with FM as compared to healthy subjects. These included the contralateral primary (S1) and bilateral secondary somatosensory cortices (S2) in response to intensity-equivalent stimuli and the contralateral S1 and S2 in response to perceptually-equivalent stimuli. The amplitude of the contralateral S2 response in patients with FM was positively correlated with average pain intensity over the last week. Quantitative sensory testing results showed that patients with FM were more sensitive to painful IES as well as to mechanical stimulation, regardless of whether the stimulation site was the hand or the trapezius muscle. Interestingly, the slope of the stimulus-response relationship as well as temporal summation of pain in response to IES was not different between groups. Together, these results suggest that the observed pain augmentation in response to IES in patients with FM could be due to sensitization or disinhibition of the cortical somatosensory system. Since the S2 has been shown to play a role in higher-order functions, further studies are needed to clarify the role of augmented S2 response in clinical characteristics of FM.

Increased Low- and High-Frequency Oscillatory Activity in the Prefrontal Cortex of Fibromyalgia Patients

Recent human neuroimaging studies have suggested that fibromyalgia (FM), a chronic widespread pain disorder, exhibits altered thalamic structure and function. Since the thalamus has extensive reciprocal connection with the cortex, structural and functional thalamic alterations in FM might be linked to aberrant thalamocortical oscillation. This study investigated the presence of abnormal brain rhythmicity in low- and high-frequency bands during resting state in patients with FM and their relationship to clinical pain symptom. Spontaneous magnetoencephalography (MEG) activity was recorded in 18 females with FM and 18 age- and sex-matched healthy control (HC) subjects. The most remarkable finding was that FM patients had general increases in theta, beta and gamma power along with a slowing of the dominant alpha peak. Increased spectral powers in the theta-band were primarily localized to the left dorsolateral prefrontal (DLPFC) and orbitofrontal cortex (OFC). Beta and gamma over-activation were localized to insular, primary motor and primary and secondary somatosensory (S2) cortices, as well as the DLPFC and OFC. Furthermore, enhanced high-frequency oscillatory activities in the DLPFC and OFC were associated with higher affective pain scores in patients with FM. Our results demonstrate that FM patients feature enhanced low- and high-frequency oscillatory activity in the brain areas related to cognitive and emotional modulation of pain. Increased low- and high-frequency activity of the prefrontal cortex may contribute to persistent perception of pain in FM. Therapeutic intervention based on manipulating neural oscillation to restore normal thalamocortical rhythmicity may be beneficial to pain relief in FM.