Pain matrices and neuropathic pain matrices: A review

Does intra-epidermal electrical stimulation activate mechano- and thermo-nociceptors? A discrimination approach

BACKGROUND

Objective laboratory tests are needed to diagnose lesions within the nociceptive system accurately. One approach is assessing pain-related evoked potentials (PREPs) in response to intra-epidermal electrical stimulation (IES). In this context, peripheral characterization of the specificity of nociceptor activation with IES is needed.

NEW METHOD

As IES directly depolarizes free nerve endings, it might allow a more comprehensive nociceptor activation than classical contact heat stimulation. Hence, this study aimed to investigate whether mechano-nociceptors are activated by IES. To test this hypothesis, a heat pain model was used to assess whether IES would show comparable pain hypersensitivity in the experimentally-induced area of secondary mechanical hyperalgesia (SMH), as known for pinprick but not for contact heat stimuli. Pain ratings and PREPs were recorded in response to 15 contact heat and pinprick stimuli as well as IES applied to the volar forearm before (PRE) and after (POST) a heat pain model inducing an area of SMH (EXP) or a control model (CTRL).

RESULTS AND COMPARISON WITH EXISTING METHODS

All 24 participants (25.5 ± 4.7 y, 10 f/14 m) presented with SMH in POST-EXP condition. Pain ratings were significantly increased in EXP versus CTRL for IES (p = 0.016) and pinprick (p = 0.006) but not for contact heat (p = 0.683). PREP NP-amplitude between EXP and CTRL was only increased in response to pinprick (p = 0.027), but not to IES (p = 0.547) and contact heat stimuli (p = 0.070).

CONCLUSIONS

Psychophysical assessments suggest mechano-nociceptor activation by IES, while PREPs do not support this assumption, indicating the predominant activation of thermo-nociceptors by IES.

Modulation of thermal perception by VR-based visual stimulation to the embodied virtual body

Visual stimulation to the embodied virtual body could modulate human perception, however the associated neurophysiological mechanisms have not elucidated yet. The present study aimed to reveal the underlying neurophysiological mechanisms from a neurophysiological viewpoint. Fifteen healthy participants were subjected to three visual conditions (i.e., fire, water, and non-visual effect conditions) and psychological pain stimulation (thermal grill stimulation). Oscillatory neural activities during stimulation were measured with electroencephalogram. The association between accessory visual stimulation applied to the embodied virtual body, induced by virtual reality, and perception was examined through neuronal oscillatory analysis using electroencephalogram data. Regression analysis was performed to obtain data on brain regions contributing to sensory modulation with body illusion. The results of subjective measures under the fire and water conditions showed that thermal perception were modulated by a visual stimulus to the virtual hand. Furthermore, we found that the insula was commonly associated with thermal perception under the fire and water conditions. This result indicate that the insula may control sensory information as a gatekeeper as well as facilitate the access to human attention and cognition as a hub, suggesting the influence on perception and cognition.

Identifying neural circuitry abnormalities in neuropathic pain with transcranial magnetic stimulation and electroencephalogram co-registration

Non-invasive brain stimulation (NIBS) technology such as transcranial magnetic stimulation (TMS) represents a promising treatment for neuropathic pain. However, neural circuitries underlying analgesia remain to be established, which is largely limiting treatment responses. Using TMS and electroencephalogram co-registration (TMS-EEG), this study quantified the circuitry abnormalities in neuropathic pain and their associations with pain symptoms. A group of 21 neuropathic pain individuals and 21 healthy controls were assessed with TMS-EEG delivering to the primary motor cortex (M1). With source modelling, local current density and current propagation were analysed with significant current density (SCD) and scattering (SCS) respectively. The SCS and SCD data converged on higher activities in neuropathic pain individuals than healthy controls, within the emotional affective (perigenual anterior cingulate cortex, pgACC), sensory nociceptive (primary somatosensory cortex, S1), and the attentional cognitive (anterior insula, aINS; supracallosal anterior cingulate cortex, scACC) structures of pain. Moreover, current propagation to the pgACC was associated with lower pain-related negative emotions, while current propagation to the aINS with higher pain-related negative emotions. Using concurrent TMS-EEG, our data identified abnormal pain circuitries that could be utilised to improve treatment efficacy with brain stimulation technologies.

The effect of the EXOPULSE Mollii Suit on pain and fibromyalgia-related symptoms-A randomized sham-controlled crossover trial

BACKGROUND

Fibromyalgia pain and related symptoms are poorly managed by approved pharmacological and alternative interventions. This trial aimed to evaluate the effects of the EXOPULSE Mollii Suit-a multisite transcutaneous electrical nerve stimulation device-on fibromyalgia pain, fatigue, affective symptoms, disease impact, and quality of life.

METHODS

Adult patients with fibromyalgia were enrolled. Phase 1 implied a randomized, sham-controlled, cross-over, double-blind trial, applying daily 1 h sessions of active or sham intervention, over 2 weeks (2-week washout). In the open-label phase 2, all patients received daily active intervention for 4 weeks. Comparisons on pain, fatigue, disease impact, affective symptoms, quality of life, clinical impression, and comfort ratings were performed using Friedman, Wilcoxon signed rank, and Chi2 tests.

RESULTS

Thirty-three patients completed the study (93.9% female, mean age: 51.3 years). Pain (primary endpoint assessed via a visual analog scale) was significantly reduced after the active (pre-active: 6.9 ± 1.4, post-active: 5.9 ± 1.8, pre-sham: 6.8 ± 1.4, post-sham: 6.6 ± 1.5) versus the sham intervention (X2 = 10.60, p = 0.014). This was also the case of other secondary endpoints (i.e., fatigue, anxiety, and disease impact), except depression and quality of life. The Clinical Global Impression of Change (CGI-C) was significantly different between the active and sham intervention periods (X2 p = 0.035), and the different proportions of categories were as follows: 'worsening' (sham: 18.2% vs. active: 0.0%), 'improvement' (sham: 48.5% vs. active 63.6%) or 'no change (sham: 33.3% vs. active 36.4%) respectively. After phase 2, significant positive effects were observed for most of the outcomes, and 78.8% of patients reported improvement according to CGI-C.

CONCLUSIONS

This study suggests the clinical benefits of the EXOPULSE Mollii Suit in alleviating pain and fibromyalgia-related fatigue, emotional symptoms, and disease impact. It is worth noting that the study has several limitations related to the low number of participants, the short-term analysis of effects in the first blinded and controlled phase, and the open-label nature of phase 2. Future studies with a larger cohort and longer protocol treatment are needed, to further confirm the current results, and evaluate the long-term effects of this technique.

SIGNIFICANCE

Patients with fibromyalgia suffer from pain as well as fatigue, sleep impairment, emotional disturbances, and altered quality of life. Transcutaneous electrical nerve stimulation might help manage those symptoms, but the available systems are limited by the fact that they could be applied at best over two sites. This randomized controlled study is the first to apply a multi-site transcutaneous electrical nerve stimulation device, the EXOPULSE Mollii Suit, with significant effects on fibromyalgia pain and related symptoms.

Fibromyalgia and the painful self: A meta-analysis of resting-state fMRI data

Fibromyalgia (FM) is a complex medical condition. The nested hierarchical model of self and its extension to the pain matrix could represent an integrated theoretical framework that might comprehensively captures FM clinical feautres. A multi-level meta-analysis was conducted. Resting-state functional connectivity (RS-FC) studies that compared patients with FM and healthy controls (HCs) were included. The association between RS-FC among self-related brain regions and pain intensity was also explored in the FM group. Eleven studies were eligible for meta-analytic procedures. Patients with FM, compared to HCs, were characterized by an increased RS-FC between the default mode network (DMN) and areas ascribed to interoceptive (e.g., insula) and exteroceptive (e.g., premotor, visual/auditory cortices) self layers. The clinical group also showed a reduced RS-FC among regions of the pain matrix (i.e., periaqueductal gray matter, somatosensory areas) involved in pain modulation. An increased RS-FC within DMN together with a heightened RS-FC between DMN and interoceptive self areas were positively associated to pain intensity reported by patients with FM. The nested hierarchical model of self and its extension to the pain matrix might represent comprehensive neurobiological backgrounds for clarifying core mind-body clinical features of FM.

Adolescent alcohol exposure promotes mechanical allodynia and alters synaptic function at inputs from the basolateral amygdala to the prelimbic cortex

Binge drinking is common among adolescents despite mounting evidence linking it to various adverse health outcomes that include heightened pain perception. The prelimbic (PrL) cortex is vulnerable to insult from adolescent alcohol exposure and receives input from the basolateral amygdala (BLA) while sending projections to the ventrolateral periaqueductal gray (vlPAG) - two brain regions implicated in nociception. In this study, adolescent intermittent ethanol (AIE) exposure was carried out in male and female rats using a vapor inhalation procedure. Assessments of mechanical and thermal sensitivity revealed that AIE exposure induced protracted mechanical allodynia. To investigate synaptic function at BLA inputs onto defined populations of PrL neurons, retrobeads and viral labelling were combined with optogenetics and slice electrophysiology. Recordings from retrobead labelled cells in the PrL revealed AIE reduced BLA driven feedforward inhibition of neurons projecting from the PrL to the vlPAG, resulting in augmented excitation/inhibition (E/I) balance and increased intrinsic excitability. Consistent with this finding, recordings from virally tagged PrL parvalbumin interneurons (PVINs) demonstrated that AIE exposure reduced both E/I balance at BLA inputs onto PVINs and PVIN intrinsic excitability. These findings provide compelling evidence that AIE alters synaptic function and intrinsic excitability within a prefrontal nociceptive circuit.

Friction-induced pain: from skin surface to brain activation

The interface friction between prosthetic socket and residual limb tends to cause skin injury and pain. The frictional pain was systematically studied based on skin tribological behaviors and brain activation. The results showed that the skin frictional pain was affected by the combination of friction and mechanical properties of anatomic regions and liner materials. The low elastic modulus and good viscoelasticity of anatomic regions or high adhesion and good compliance of liner materials both can increase the frictional pain threshold and decease the injury. The main brain activation related to frictional pain located in the primary somatosensory cortex (SI), secondary somatosensory cortex (SII), and prefrontal cortex (PFC). The brain negative activation increased and the activation area decreased with the increased pain intensity. The features of α activity, β activity, αpeak and extracted from EEG signals were effective in the recognition of pain state, but cannot recognize the pain intensities.

Abnormal alterations in structure-function coupling at the modular level in patients with postherpetic neuralgia

To investigate the presence of modular loss of coupling and abnormal alterations in functional and structural networks in the brain networks of patients with postherpetic neuralgia (PHN). We collected resting-state functional magnetic resonance imaging data and diffusion tensor imaging data from 82 healthy controls (HCs) and 71 PHN patients, generated structural connectivity (SC) and functional connectivity (FC) networks, and assessed the corresponding clinical information assessment. Based on AAL(90) mapping, the brain network was divided into 9 modules, and the structural-functional connectivity (SC-FC) coupling was compared at the whole-brain level and within the modules, as well as alterations in the topological properties of the brain network in the patient group. Finally, correlation analyses were performed using the following clinical scales: Visual Analogue Scale (VAS), Hamilton Anxiety Scale (HAMA), and Hamilton Depression Scale (HAMD). Compared with HCs, patients with PHN had reduced global efficiency (Eg) and local efficiency (Eloc) of structural and functional networks. The FC in the PHN group presented abnormal node clustering coefficients (NCp), local node efficiencies (NLe), and node efficiencies (Ne), and the SC presented abnormal node degrees (Dc), NCp, NLe, characteristic path lengths (NLp), and Ne. In addition, SC-FC coupling was reduced in the patient default network (DMN), salient network (SN), and visual network (VIS). Moreover, the degree of impairment of graph theory indicators was significantly positively correlated with scales such as VAS scores, and the coupling of modules was significantly negatively correlated with the early course of the patient's disease. Large-scale impaired topological properties of the FC and SC networks were observed in patients with PHN, and SC-FC decoupling was detected in these modules of the DMN, SN, and VIS. These aberrant alterations may have led to over-transmission of pain information or central sensitization of pain.

Chronic pain-induced functional and structural alterations in the brain: A multi-modal meta-analysis

Chronic pain is a debilitating condition associated with brain alterations. However, the variability in neuroimaging results across modalities necessitates a comprehensive multi-modal meta-analysis for a cohesive understanding. This study aims to elucidate brain alterations in chronic pain patients using a multi-modal meta-analysis approach encompassing structural, resting-state functional connectivity, and pain processing paradigms in functional magnetic resonance imaging. A systematic literature search was conducted across PubMed, OVID Embase, OVID Medline, and Web of Science, encompassing studies published up to May 2022, to identify relevant research articles on chronic pain and MRI techniques in three modalities. Inclusion criteria encompassed experiments reporting three modality brain alterations in chronic pain patients, with sufficient statistical thresholds and enough sample size. We conducted voxel-wise meta-analyses using seed-based d mapping to identify significant alterations in each modality. Additionally, conjunction analyses were executed to identify common alterations across these modalities. Ultimately, 47 structure studies, 37 resting state functional connectivity studies, and 41 pain-processing studies were selected for formal analysis. Chronic pain patients displayed notable structural and functional alterations in the insular cortex, characterized by reduced gray matter, disruptions in functional connectivity with the frontoparietal network, and enhanced activation during painful stimuli processing. Distinct activation patterns were observed in the left and right insular cortex for pain stimulus processing versus anticipation. Furthermore, the superior temporal gyrus and superior frontal gyrus exhibited joint alterations across modalities. This multi-modal meta-analysis reveals consistent brain alterations in chronic pain patients, shedding light on the complex interplay between structural and functional changes. PERSPECTIVE: This multi-modal meta-analysis integrates findings from structural, resting-state functional connectivity, and pain processing paradigms in fMRI, revealing consistent brain alterations in chronic pain patients. Notable brain changes highlight the intricate interplay between structural and functional brain changes, advancing our understanding of chronic pain's neural underpinnings.

18 kDa Translocator protein (TSPO) is upregulated in rat brain after peripheral nerve injury and downregulated by diroximel fumarate

Neuroimmune signaling is a key process underlying neuropathic pain. Clinical studies have demonstrated that 18 kDa translocator protein (TSPO), a putative marker of neuroinflammation, is upregulated in discrete brain regions of patients with chronic pain. However, no preclinical studies have investigated TSPO dynamics in the brain in the context of neuropathic pain and in response to analgesic treatments. We used positron emission tomography-computed tomography (PET-CT) and [18F]-PBR06 radioligand to measure TSPO levels in the brain across time after chronic constriction injury (CCI) of the sciatic nerve in both male and female rats. Up to 10 weeks post-CCI, TSPO expression was increased in discrete brain regions, including medial prefrontal cortex, somatosensory cortex, insular cortex, anterior cingulate cortex, motor cortex, ventral tegmental area, amygdala, midbrain, pons, medulla, and nucleus accumbens. TSPO was broadly upregulated across these regions at 4 weeks post CCI in males, and 10 weeks in females, though there were regional differences between the sexes. Using immunohistochemistry, we confirmed TSPO expression in these regions. We further demonstrated that TSPO was upregulated principally in microglia in the nucleus accumbens core, and astrocytes and endothelial cells in the nucleus accumbens shell. Finally, we tested whether TSPO upregulation was sensitive to diroximel fumarate, a drug that induces endogenous antioxidants via nuclear factor E2-related factor 2 (Nrf2). Diroximel fumarate alleviated neuropathic pain and reduced TSPO upregulation. Our findings indicate that TSPO is upregulated over the course of neuropathic pain development and is resolved by an antinociceptive intervention in animals with peripheral nerve injury.

Neuroscience of the human thalamus related to acute pain and chronic "thalamic" pain

The association of posterior thalamic strokes with the presence of chronic "thalamic" pain was described in the early 1900s and revisited in a recent review of these patients. Acute pain in corporal structures is associated with the spinothalamic tract (STT), which originates in the dorsal horn of the spinal cord, whereas that associated with cranial structures is associated with the spinal division of the trigeminal nucleus. These pathways terminate in the ventral posterior nucleus (VP), including its posterior and inferior subnuclei and its core, which is classically associated with tactile and haptic functions. In medial nuclei (medial dorsal and intralaminar) receptive fields are large and stimulation evokes diffuse unpleasant sensations and pain while neurons in these nuclei subserve cognitive processes of attention, alerting, and conditioning. In the lateral nuclei neurons have small receptive and projected fields and high resolution of responses to somatic stimuli. Neurons in the lateral nuclei respond to stimuli producing pain, temperature, and visceral sensations while stimulation evokes similar sensations. Small strokes in VP core versus structures located inferior and posterior are associated with thalamic pain and decreased tactile, painful, and cold sensations and with decreased evoked potentials for painful (laser) heat and median nerve stimulation (electrical). Lesions of VP, but not ventral medial posterior nucleus (VMpo), are associated with thalamic pain, contrary to the recent "disinhibition" model. We review the evidence that the lateral nuclei are associated with multiple processes including tactile, nociceptive, visceral, and thermal content of stimuli, whereas the medial nuclei are related to cognitions about those stimuli.

A distributed brain response predicting the facial expression of acute nociceptive pain

Pain is a private experience observable through various verbal and non-verbal behavioural manifestations, each of which may relate to different pain-related functions. Despite the importance of understanding the cerebral mechanisms underlying those manifestations, there is currently limited knowledge of the neural correlates of the facial expression of pain. In this functional magnetic resonance imaging (fMRI) study, noxious heat stimulation was applied in healthy volunteers and we tested if previously published brain signatures of pain were sensitive to pain expression. We then applied a multivariate pattern analysis to the fMRI data to predict the facial expression of pain. Results revealed the inability of previously developed pain neurosignatures to predict the facial expression of pain. We thus propose a facial expression of pain signature (FEPS) conveying distinctive information about the brain response to nociceptive stimulations with minimal or no overlap with other pain-relevant brain signatures associated with nociception, pain ratings, thermal pain aversiveness, or pain valuation. The FEPS may provide a distinctive functional characterization of the distributed cerebral response to nociceptive pain associated with the socio-communicative role of non-verbal pain expression. This underscores the complexity of pain phenomenology by reinforcing the view that neurosignatures conceived as biomarkers must be interpreted in relation to the specific pain manifestation(s) predicted and their underlying function(s). Future studies should explore other pain-relevant manifestations and assess the specificity of the FEPS against simulated pain expressions and other types of aversive or emotional states.

Neuroanatomy of the nociceptive system: From nociceptors to brain networks

This chapter reviews the neuroanatomy of the nociceptive system and its functional organization. We describe three main compartments of the nervous system that underlie normal nociception and the resulting pain percept: Peripheral, Spinal Cord, and Brain. We focus on how ascending nociceptive processing streams traverse these anatomical compartments, culminating in the multidimensional experience of pain. We also describe neuropathic pain conditions, in which nociceptive processing is abnormal, not only because of the primary effects of a lesion or disease affecting peripheral nerves or the central nervous system (CNS), but also due to secondary effects on ascending pathways and brain networks. We discuss how the anatomical components (circuits/networks) reorganize under various etiologies of neuropathic pain and how these changes can give rise to pathological pain states.

Clinical neurophysiology of neuropathic pain

Timely and accurate diagnosis of neuropathic pain is critical for optimizing therapeutic outcomes and minimizing treatment delays. According to current standards, the diagnosis of definite neuropathic pain requires objective confirmation of a lesion or disease affecting the somatosensory nervous system. This can be provided by specialized neurophysiological techniques as conventional methods like nerve conduction studies and somatosensory evoked potentials may not be sufficient as they do not assess pain pathways. These specialized techniques apply various stimuli, such as thermal, electrical, or mechanical, alongside assessments of spinal/cortical potential or electromyographic reflex recordings. The selection of techniques is guided by the patient's clinical history and examination. The most common neurophysiological tests used in clinical practice are pain-related evoked potentials (PREPs) providing an objective evaluation of nociceptive pathways. Four types of PREPs are employed: laser evoked potentials, contact-heat evoked potentials, intra-epidermal electrical stimulation evoked potentials, and pinprick evoked potentials, with the two former ones being the most robust and reliable ones. These techniques investigate small-diameter fibers, primarily Aδ-fibers, and spinothalamic tracts allowing the identification of peripheral or central nervous system lesions. Yet, they are limited in capturing neuronal mechanisms underlying neuropathic pain or in providing objective quantification of pain sensation. Two neurophysiological measures which investigate the pain system beyond its integrity are the nociceptive withdrawal reflex and the N13 component of somatosensory evoked potentials. Both of these methods are more commonly used in research than clinical practice, but they pose interesting approaches to quantify central sensitization, a key underlying mechanism of neuropathic pain. Future investigations in neuropathic pain are therefore warranted.

Pain-Discriminating Information Decoded From Spatiotemporal Patterns of Hemodynamic Responses Measured by fMRI in the Human Brain

Functional magnetic resonance imaging (fMRI) has been widely used in studying the neural mechanisms of pain in the human brain, primarily focusing on where in the brain pain-elicited neural activities occur (i.e., the spatial distribution of pain-related brain activities). However, the temporal dynamics of pain-elicited hemodynamic responses (HDRs) measured by fMRI may also contain information specific to pain processing but have been largely neglected. Using high temporal resolution fMRI (TR = 0.8 s) data acquired from 62 healthy participants, in the present study we aimed to test whether pain-distinguishing information could be decoded from the spatial pattern of the temporal dynamics (i.e., the spatiotemporal pattern) of HDRs elicited by painful stimuli. Specifically, the peak latency and the response duration were used to characterize the temporal dynamics of HDRs to painful laser stimuli and non-painful electric stimuli, and then were compared between the two conditions (i.e., pain and no-pain) using a voxel-wise univariate analysis and a multivariate pattern analysis. Furthermore, we also tested whether the two temporal characteristics of pain-elicited HDRs and their spatial patterns were associated with pain-related behaviors. We found that the spatial patterns of HDR peak latency and response duration could successfully discriminate pain from no-pain. Interestingly, we also observed that the Pain Vigilance and Awareness Questionnaire (PVAQ) scores were correlated with the average response duration in bilateral insula and secondary somatosensory cortex (S2) and could also be predicted from the across-voxel spatial patterns of response durations in the middle cingulate cortex and middle frontal gyrus only during painful condition but not during non-painful condition. These findings indicate that the spatiotemporal pattern of pain-elicited HDRs may contain pain-specific information and highlight the importance of studying the neural mechanisms of pain by taking advantage of the high sensitivity of fMRI to both spatial and temporal information of brain responses.

The Role of the Insular Cortex and Serotonergic System in the Modulation of Long-Lasting Nociception

The insular cortex (IC) is a brain region that both receives relevant sensory information and is responsible for emotional and cognitive processes, allowing the perception of sensory information. The IC has connections with multiple sites of the pain matrix, including cortico-cortical interactions with the anterior cingulate cortex (ACC) and top-down connections with sites of descending pain inhibition. We explored the changes in the extracellular release of serotonin (5HT) and its major metabolite, 5-hydroxyindoleacetic acid (5HIAA), after inflammation was induced by carrageenan injection. Additionally, we explored the role of 5HT receptors (the 5HT1A, 5HT2A, and 5HT3 receptors) in the IC after inflammatory insult. The results showed an increase in the extracellular levels of 5HT and 5-HIAA during the inflammatory process compared to physiological levels. Additionally, the 5HT1A receptor was overexpressed. Finally, the 5HT1A, 5HT2A, and 5HT3 receptor blockade in the IC had antinociceptive effects. Our results highlight the role of serotonergic neurotransmission in long-lasting inflammatory nociception within the IC.

Precuneal hyperperfusion in patients with attention-deficit/hyperactivity disorder-comorbid nociplastic pain

Introduction

Nociplastic pain (NP), a third category of chronic pain, offers a framework for elucidating its pathophysiology and treatment strategies. One of the characteristics of NP is comorbidity of symptoms other than pain, such as psychological and cognitive problems; therefore, these can be clues to understanding NP. Recently, we reported several cases with comorbid symptoms of attention-deficit/hyperactivity disorder (ADHD). Notably, ADHD medications, including methylphenidate (MP) and atomoxetine, improved chronic pain as well as the symptoms of ADHD. However, in clinical settings, identifying comorbid ADHD in patients with chronic pain is challenging, and underlying mechanisms have not been elucidated. To explore the common characteristics of brain function in patients with ADHD-comorbid NP, we identified brain regions where cerebral blood flow (CBF) distributions changed between pre- and post-treatment using single-photon emission computed tomography (SPECT). Additionally, we examined brain regions where CBF values correlated with clinical scores.

Methods

We retrospectively studied 65 patients (mean age 53 ± 14 years; 30 males and 35 females) with ADHD-comorbid NP who underwent CBF-SPECT before and after ADHD medication initiation. Clinical scores included the clinical global impression severity (CGI-S), pain numerical rating scale, hospital anxiety and depression scale, pain catastrophizing scale, and Conners' adult ADHD rating scale-self report scores. Voxel-based statistical methods were used to compare pre- and post-treatment CBF-SPECT images to identify significant differences and investigate brain regions correlated with clinical scores.

Results

The CBF was higher in the precuneus, insular gyrus, and thalamus before treatment than after treatment (paired t-test, cluster-definition p < 0.001, cluster-extent threshold p < 0.05, with family-wise error [FWE] correction). The hyperperfusion in the precuneus was positively correlated with the CGI-S score and significantly reduced after treatment with MP (paired t-test, cluster-definition p < 0.005, cluster-extent threshold p < 0.05, with FWE correction).

Conclusion

The finding of precuneal hyperperfusion may provide insight into the mechanisms of NP and help identify patients who would benefit most from ADHD medications.

Bedside Neuromodulation of Persistent Pain and Allodynia with Caloric Vestibular Stimulation

BACKGROUND

Caloric vestibular stimulation (CVS) is a well-established neurological diagnostic technique that also induces many phenomenological modulations, including reductions in phantom limb pain (PLP), spinal cord injury pain (SCIP), and central post-stroke pain.

OBJECTIVE

We aimed to assess in a variety of persistent pain (PP) conditions (i) short-term pain modulation by CVS relative to a forehead ice pack cold-arousal control procedure and (ii) the duration and repeatability of CVS modulations. The tolerability of CVS was also assessed and has been reported separately.

METHODS

We conducted a convenience-based non-randomised single-blinded placebo-controlled study. Thirty-eight PP patients were assessed (PLP, n = 8; SCIP, n = 12; complex regional pain syndrome, CRPS, n = 14; non-specific PP, n = 4). Patients underwent 1-3 separate-day sessions of iced-water right-ear CVS. All but four also underwent the ice pack procedure. Analyses used patient-reported numerical rating scale pain intensity (NRS-PI) scores for pain and allodynia.

RESULTS

Across all groups, NRS-PI for pain was significantly lower within 30 min post-CVS than post-ice pack (p < 0.01). Average reductions were 24.8% (CVS) and 6.4% (ice pack). CRPS appeared most responsive to CVS, while PLP and SCIP responses were less than expected from previous reports. The strongest CVS pain reductions lasted hours to over three weeks. CVS also induced substantial reductions in allodynia in three of nine allodynic CRPS patients, lasting 24 h to 1 month. As reported elsewhere, only one patient experienced emesis and CVS was widely rated by patients as a tolerable PP management intervention.

CONCLUSIONS

Although these results require interpretative caution, CVS was found to modulate pain relative to an ice pack control. CVS also modulated allodynia in some cases. CVS should be examined for pain management efficacy using randomised controlled trials.

An approach to the detection of pain from autonomic and cortical correlates

OBJECTIVE

To assess the value of combining brain and autonomic measures to discriminate the subjective perception of pain from other sensory-cognitive activations.

METHODS

20 healthy individuals received 2 types of tonic painful stimulation delivered to the hand: electrical stimuli and immersion in 10 Celsius degree (°C) water, which were contrasted with non-painful immersion in 15 °C water, and stressful cognitive testing. High-density electroencephalography (EEG) and autonomic measures (pupillary, electrodermal and cardiovascular) were continuously recorded, and the accuracy of pain detection based on combinations of electrophysiological features was assessed using machine learning procedures.

RESULTS

Painful stimuli induced a significant decrease in contralateral EEG alpha power. Cardiac, electrodermal and pupillary reactivities occurred in both painful and stressful conditions. Classification models, trained on leave-one-out cross-validation folds, showed low accuracy (61-73%) of cortical and autonomic features taken independently, while their combination significantly improved accuracy to 93% in individual reports.

CONCLUSIONS

Changes in cortical oscillations reflecting somatosensory salience and autonomic changes reflecting arousal can be triggered by many activating signals other than pain; conversely, the simultaneous occurrence of somatosensory activation plus strong autonomic arousal has great probability of reflecting pain uniquely.

SIGNIFICANCE

Combining changes in cortical and autonomic reactivities appears critical to derive accurate indexes of acute pain perception.

Non-linear blood-brain barrier transport and dosing strategies influence receptor occupancy ratios of morphine and its metabolites in pain matrix

BACKGROUND AND PURPOSE

Morphine is important for treatment of acute and chronic pain. However, there is high interpatient variability and often inadequate pain relief and adverse effects. To better understand variability in the dose-effect relationships of morphine, we investigated the effects of its non-linear blood-brain barrier (BBB) transport on μ-receptor occupancy in different CNS locations, in conjunction with its main metabolites that bind to the same receptor.

EXPERIMENTAL APPROACH

CNS exposure profiles for morphine, M3G and M6G for clinically relevant dosing regimens based on intravenous, oral immediate- and extended-release formulations were generated using a physiology-based pharmacokinetic model of the CNS, with non-linear BBB transport of morphine. The simulated CNS exposure profiles were then used to derive corresponding μ-receptor occupancies at multiple CNS pain matrix locations.

KEY RESULTS

Simulated CNS exposure profiles for morphine, M3G and M6G, associated with non-linear BBB transport of morphine resulted in varying μ-receptor occupancies between different dose regimens, formulations and CNS locations. At lower doses, the μ-receptor occupancy of morphine was relatively higher than at higher doses of morphine, due to the relative contribution of M3G and M6G. At such higher doses, M6G showed higher occupancy than morphine, whereas M3G occupancy was low throughout the dose ranges.

CONCLUSION AND IMPLICATIONS

Non-linear BBB transport of morphine affects the μ-receptor occupancy ratios of morphine with its metabolites, depending on dose and route of administration, and CNS location. These predictions need validation in animal or clinical experiments, to understand the clinical implications.

Contributions of neuroimaging in central poststroke pain: a review

BACKGROUND

 Central neuropathic poststroke pain (CNPSP) affects up to 12% of patients with stroke in general and up to 18% of patients with sensory deficits. This pain syndrome is often incapacitating and refractory to treatment. Brain computed tomography and magnetic resonance imaging (MRI) are widely used methods in the evaluation of CNPSP.

OBJECTIVE

 The present study aims to review the role of neuroimaging methods in CNPSP.

METHODS

 We performed a literature review of the main clinical aspects of CNPSP and the contribution of neuroimaging methods to study its pathophysiology, commonly damaged brain sites, and possible differential diagnoses. Lastly, we briefly mention how neuroimaging can contribute to the non-pharmacological CNPSP treatment. Additionally, we used a series of MRI from our institution to illustrate this review.

RESULTS

 Imaging has been used to explain CNPSP pathogenesis based on spinothalamic pathway damage and connectome dysfunction. Imaging locations associated with CNPSP include the brainstem (mainly the dorsolateral medulla), thalamus (especially the ventral posterolateral/ventral posteromedial nuclei), cortical areas such as the posterior insula and the parietal operculum, and, more recently, the thalamocortical white matter in the posterior limb of the internal capsule. Imaging also brings the prospect of helping search for new targets for non-pharmacological treatments for CNPSP. Other neuropathic pain causes identified by imaging include syringomyelia, multiple sclerosis, and herniated intervertebral disc.

CONCLUSION

 Imaging is a valuable tool in the complimentary evaluation of CNPSP patients in clinical and research scenarios.

[Insular lobe epilepsy. Part 1: semiology]

The insula is often referred to as "the fifth lobe" of the brain, and its accessibility used to be very limited due to the deep location under the opercula as well as the sylvian vasculature. It was not until the availability of modern stereo-electroencephalography (SEEG) technique that the intracranial electrodes could be safely and chronically implanted within the insula, thereby enabling anatomo-electro-clinical correlations in seizures of this deep origin. Since the first report of SEEG-recorded insular seizures in late 1990s, the knowledge of insular lobe epilepsy (ILE) has rapidly expanded. Being on the frontline for the diagnosis and management of epilepsy, neurologists should have a precise understanding of ILE to differentiate it from epilepsies of other lobes or non-epileptic conditions. Owing to the multimodal nature and rich anatomo-functional connections of the insula, ILE has a wide range of clinical presentations. The following symptoms should heighten the suspicion of ILE: somatosensory symptoms involving a large/bilateral cutaneous territory or taking on thermal/painful character, and cervico-laryngeal discomfort. The latter ranges from slight dyspnea to a strong sensation of strangulation (laryngeal constriction). Other symptoms include epigastric discomfort/nausea, hypersalivation, auditory, vestibular, gustatory, and aphasic symptoms. However, most of these insulo-opercular symptoms can easily be masked by those of extra-insular seizure propagation. Indeed, sleep-related hyperkinetic (hypermotor) epilepsy (SHE) is a common clinical presentation of ILE, which shows predominant hyperkinetic and/or tonic-dystonic features that are often indistinguishable from those of fronto-mesial seizures. Subtle objective signs, such as constrictive throat noise (i.e., laryngeal constriction) or aversive behavior (e.g., facial grimacing suggesting pain), are often the sole clue in diagnosing insular SHE. Insular-origin seizures should also be considered in temporal-like seizures without frank anatomo-electro-clinical correlations. All in all, ILE is not the epilepsy of an isolated island but rather of a crucial hub involved in the multifaceted roles of the brain.

Disrupted Balance of Short- and Long-Range Functional Connectivity Density in Patients with Herpes Zoster or Postherpetic Neuralgia: A Resting-State fMRI Study

Purpose

This study aimed to explore the abnormal changes in short- and long-range functional connectivity density (FCD) in patients with herpes zoster (HZ) and postherpetic neuralgia (PHN).

Patients and Methods

Twenty HZ patients, 22 PHN patients, and 19 well-matched healthy controls (HCs) underwent resting-state functional magnetic resonance imaging scans. We used FCD mapping, a data-driven graph theory method, to investigate local and global functional connectivity patterns. Both short- and long-range FCD were calculated and compared among the PHN, HZ, and HC groups. Then, the abnormal regions were used to calculate seed-based functional connectivity. Finally, correlation analyses were performed between the altered FCD values and clinical datas.

Results

Compared with HCs, HZ patients showed significantly increased long-range FCD of the bilateral cerebellum, thalamus, parahippocampal gyrus, superior temporal gyrus and lingual gyrus. HZ patients also displayed significantly decreased short-range FCD of the bilateral posterior cingulate gyrus, median cingulate/paracingulate gyri, and left precuneus. Compared with HCs, PHN patients displayed significantly decreased long-range FCD of the bilateral superior frontal gyrus and decreased short-range FCD in the bilateral posterior cingulate gyrus, median cingulate/paracingulate gyri, and precuneus. However, there was no significant difference in either long-range or short-range FCD between the PHN and HZ patients. Long-range FCD deficit areas and the right insula showed altered functional connectivity in PHN patients. Furthermore, pain duration in patients with PHN was correlated with abnormal long-range FCD.

Conclusion

Herpes zoster pain widely affects intra- and inter-regional functional connectivity, leading to disrupted short-range FCD and increased long-range FCD during different stages of the disease. Long-term chronic pain in PHN patients may impair the pain emotion regulation pathway. These findings could improve our understanding of the pathophysiological mechanisms of HZ and PHN and offer neuroimaging markers for HZ and PHN.

Data-driven analysis of whole-brain intrinsic connectivity in patients with chronic low back pain undergoing osteopathic manipulative treatment

BACKGROUND

Chronic Low Back Pain (cLBP) poses a significant health challenge, leading to functional disability and reduced quality of life. Osteopathic Manipulative Treatment (OMT) is emerging as a therapeutic option for cLBP, but the brain mechanisms underlying its analgesic effect remain unclear.

MATERIALS AND METHODS

Thirty cLBP patients were randomly exposed to either four weekly sessions of OMT (N=16) or Sham treatment (N=14). Resting-state Magnetic Resonance Imaging (rs-MRI) scans and pain perception questionnaires were collected before and after treatment. A voxel-wise, rs-fMRI data-driven analysis was conducted to identify changes in the intrinsic functional connectivity across the whole brain that were associated with the OMT. Spearman's correlations were used to test for the association between changes in intrinsic connectivity and individual reports of pain perception.

RESULTS

Compared to the Sham group, participants who received OMT showed significant alterations in the functional connectivity of several regions belonging to the pain matrix. Specifically, OMT was associated with decreased connectivity of a parietal cluster that includes the somatosensory cortex and an increase of connectivity of the right anterior insula and ventral and dorsal anterolateral prefrontal areas. Crucially, the change in connectivity strength observed in the ventral anterolateral prefrontal cortex, a putative region of the affective-reappraisive layer of the pain matrix, correlates with the reduction in pain perception caused by the OMT.

CONCLUSIONS

This study offers insights into the brain mechanisms underlying the analgesic effect of OMT. Our findings support a link between OMT-driven functional cortical architecture alterations and improved clinical outcomes.

The Role and Applications of Artificial Intelligence in the Treatment of Chronic Pain

PURPOSE OF REVIEW

This review aims to explore the interface between artificial intelligence (AI) and chronic pain, seeking to identify areas of focus for enhancing current treatments and yielding novel therapies.

RECENT FINDINGS

In the United States, the prevalence of chronic pain is estimated to be upwards of 40%. Its impact extends to increased healthcare costs, reduced economic productivity, and strain on healthcare resources. Addressing this condition is particularly challenging due to its complexity and the significant variability in how patients respond to treatment. Current options often struggle to provide long-term relief, with their benefits rarely outweighing the risks, such as dependency or other side effects. Currently, AI has impacted four key areas of chronic pain treatment and research: (1) predicting outcomes based on clinical information; (2) extracting features from text, specifically clinical notes; (3) modeling 'omic data to identify meaningful patient subgroups with potential for personalized treatments and improved understanding of disease processes; and (4) disentangling complex neuronal signals responsible for pain, which current therapies attempt to modulate. As AI advances, leveraging state-of-the-art architectures will be essential for improving chronic pain treatment. Current efforts aim to extract meaningful representations from complex data, paving the way for personalized medicine. The identification of unique patient subgroups should reveal targets for tailored chronic pain treatments. Moreover, enhancing current treatment approaches is achievable by gaining a more profound understanding of patient physiology and responses. This can be realized by leveraging AI on the increasing volume of data linked to chronic pain.

Resting-state brain activity as a biomarker of chronic pain impairment and a mediator of its association with pain resilience

Past cross-sectional chronic pain studies have revealed aberrant resting-state brain activity in regions involved in pain processing and affect regulation. However, there is a paucity of longitudinal research examining links of resting-state activity and pain resilience with changes in chronic pain outcomes over time. In this prospective study, we assessed the status of baseline (T1) resting-state brain activity as a biomarker of later impairment from chronic pain and a mediator of the relation between pain resilience and impairment at follow-up. One hundred forty-two adults with chronic musculoskeletal pain completed a T1 assessment comprising a resting-state functional magnetic resonance imaging scan based on regional homogeneity (ReHo) and self-report measures of demographics, pain characteristics, psychological status, pain resilience, pain severity, and pain impairment. Subsequently, pain impairment was reassessed at a 6-month follow-up (T2). Hierarchical multiple regression and mediation analyses assessed relations of T1 ReHo and pain resilience scores with changes in pain impairment. Higher T1 ReHo values in the right caudate nucleus were associated with increased pain impairment at T2, after controlling for all other statistically significant self-report measures. ReHo also partially mediated associations of T1 pain resilience dimensions with T2 pain impairment. T1 right caudate nucleus ReHo emerged as a possible biomarker of later impairment from chronic musculoskeletal pain and a neural mechanism that may help to explain why pain resilience is related to lower levels of later chronic pain impairment. Findings provide empirical foundations for prospective extensions that assess the status of ReHo activity and self-reported pain resilience as markers for later impairment from chronic pain and targets for interventions to reduce impairment. PRACTITIONER POINTS: Resting-state markers of impairment: Higher baseline (T1) regional homogeneity (ReHo) values, localized in the right caudate nucleus, were associated with exacerbations in impairment from chronic musculoskeletal pain at a 6-month follow-up, independent of T1 demographics, pain experiences, and psychological factors. Mediating role of ReHo values: ReHo values in the right caudate nucleus also mediated the relationship between baseline pain resilience levels and later pain impairment among participants. Therapeutic implications: Findings provide empirical foundations for research extensions that evaluate (1) the use of resting-state activity in assessment to identify people at risk for later impairment from pain and (2) changes in resting-state activity as biomarkers for the efficacy of treatments designed to improve resilience and reduce impairment among those in need.

Noninvasive neuromodulation of subregions of the human insula differentially affect pain processing and heart-rate variability: a within-subjects pseudo-randomized trial

ABSTRACT

The insula is an intriguing target for pain modulation. Unfortunately, it lies deep to the cortex making spatially specific noninvasive access difficult. Here, we leverage the high spatial resolution and deep penetration depth of low-intensity focused ultrasound (LIFU) to nonsurgically modulate the anterior insula (AI) or posterior insula (PI) in humans for effect on subjective pain ratings, electroencephalographic (EEG) contact heat-evoked potentials, as well as autonomic measures including heart-rate variability (HRV). In a within-subjects, repeated-measures, pseudo-randomized trial design, 23 healthy volunteers received brief noxious heat pain stimuli to the dorsum of their right hand during continuous heart-rate, electrodermal, electrocardiography and EEG recording. Low-intensity focused ultrasound was delivered to the AI (anterior short gyrus), PI (posterior longus gyrus), or under an inert Sham condition. The primary outcome measure was pain rating. Low-intensity focused ultrasound to both AI and PI similarly reduced pain ratings but had differential effects on EEG activity. Low-intensity focused ultrasound to PI affected earlier EEG amplitudes, whereas LIFU to AI affected later EEG amplitudes. Only LIFU to the AI affected HRV as indexed by an increase in SD of N-N intervals and mean HRV low-frequency power. Taken together, LIFU is an effective noninvasive method to individually target subregions of the insula in humans for site-specific effects on brain biomarkers of pain processing and autonomic reactivity that translates to reduced perceived pain to a transient heat stimulus.

Impaired neuronal macroautophagy in the prelimbic cortex contributes to comorbid anxiety-like behaviors in rats with chronic neuropathic pain

A large proportion of patients with chronic pain experience co-morbid anxiety. The medial prefrontal cortex (mPFC) is proposed to underlie this comorbidity, but the molecular and neuronal mechanisms are not fully understood. Here, we reported that impaired neuronal macroautophagy in the prelimbic cortical (PrL) subregion of the mPFC paralleled the occurrence of anxiety-like behaviors in rats with chronic spared nerve injury (SNI). Intriguingly, such macroautophagy impairment was mainly observed in a FOS/c-Fos+ neuronal subpopulation in the PrL. Chemogenetic inactivation of this comorbid anxiety-related neuronal ensemble relieved pain-induced anxiety-like behaviors. Rescuing macroautophagy impairment in this neuronal ensemble relieved chronic pain-associated anxiety and mechanical allodynia and restored synaptic homeostasis at the molecular level. By contrast, artificial disruption of macroautophagy induced early-onset co-morbid anxiety in neuropathic rats, but not general anxiety in normal rats. Taken together, our work identifies causal linkage between PrL neuronal macroautophagy dysfunction and comorbid anxiety in neuropathic pain and provides novel insights into the role of PrL by differentiating its contribution in pain-induced comorbid anxiety from its modulation over general anxiety-like behaviors.Abbreviation: AAV: adeno-associated viruses; ACC: anterior cingulate cortex; ATG5: autophagy related 5; ATG7: autophagy related 7; ATG12: autophagy related 12; CAMK2/CaMKII: calcium/calmodulin-dependent protein kinase II; CNO: clozapine-N-oxide; CQ: chloroquine; DIA: data independent acquisition; DIO: double floxed inverse orf; DLG4/PSD-95: discs large MAGUK scaffold protein 4; Dox: doxycycline; GABA: γ-aminobutyric acid; GFP: green fluorescent protein; GO: gene ontology; Gi: inhibitory guanine nucleotide-binding proteins; HsCHRM4/M4D: human cholinergic receptor muscarinic 4; HsSYN: human synapsin; KEGG: Kyoto encyclopedia of genes and genomes; LAMP1: lysosomal-associated membrane protein 1; LC3-II: PE conjugated microtubule-associated protein 1 light chain3; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; mPFC: medial prefrontal cortex; P2A: 2A self-cleaving peptide; PPI: protein-protein interaction networks; PrL: prelimbic cortex; RBFOX3/NeuN: RNA binding protein, fox-1 homolog (C. elegans) 3; rtTA: reverse tetracycline-transactivator; SDS-PAGE: sodium dodecylsulfate-polyacrylamide gel electrophoresis; SHANK3: SH3 and multiple ankyrin repeat domains 3; SLC1A1/EAAC1: solute carrier family 1 (neuronal/epithelial high affinity glutamate transporter, systemXag), member 1; SNAP23: synaptosomal-associated protein 23; SNI:spared nerve injury; SQSTM1/p62: sequestosome 1; SYT3: synaptotagmin 3; TRE: tetracycline-responsive element; TRE3G: third-generation tetracycline-responsive element.

Altered Corticobrainstem Connectivity during Spontaneous Fluctuations in Pain Intensity in Painful Trigeminal Neuropathy

Chronic neuropathic pain can result from nervous system injury and can persist in the absence of external stimuli. Although ongoing pain characterizes the disorder, in many individuals, the intensity of this ongoing pain fluctuates dramatically. Previously, it was identified that functional magnetic resonance imaging signal covariations between the midbrain periaqueductal gray (PAG) matter, rostral ventromedial medulla (RVM), and spinal trigeminal nucleus are associated with moment-to-moment fluctuations in pain intensity in individuals with painful trigeminal neuropathy (PTN). Since this brainstem circuit is modulated by higher brain input, we sought to determine which cortical sites might be influencing this brainstem network during spontaneous fluctuations in pain intensity. Over 12 min, we recorded the ongoing pain intensity in 24 PTN participants and classified them as fluctuating (n = 13) or stable (n = 11). Using a PAG seed, we identified connections between the PAG and emotional-affective sites such as the hippocampal and posterior cingulate cortices, the sensory-discriminative posterior insula, and cognitive-affective sites such as the dorsolateral prefrontal (dlPFC) and subgenual anterior cingulate cortices that were altered dependent on spontaneous high and low pain intensity. Additionally, sliding-window functional connectivity analysis revealed that the dlPFC-PAG connection anticorrelated with perceived pain intensity over the entire 12 min period. These findings reveal cortical systems underlying moment-to-moment changes in perceived pain in PTN, which likely cause dysregulation in the brainstem circuits previously identified, and consequently alter the appraisal of pain across time.

Abnormalities of brain structure and function in cervical spondylosis: a multi-modal voxel-based meta-analysis

Background

Previous neuroimaging studies have revealed structural and functional brain abnormalities in patients with cervical spondylosis (CS). However, the results are divergent and inconsistent. Therefore, the present study conducted a multi-modal meta-analysis to investigate the consistent structural and functional brain alterations in CS patients.

Methods

A comprehensive literature search was conducted in five databases to retrieve relevant resting-state functional magnetic resonance imaging (rs-fMRI), structural MRI and diffusion tensor imaging (DTI) studies that measured brain functional and structural differences between CS patients and healthy controls (HCs). Separate and multimodal meta-analyses were implemented, respectively, by employing Anisotropic Effect-size Signed Differential Mapping software.

Results

13 rs-fMRI studies that used regional homogeneity, amplitude of low-frequency fluctuations (ALFF) and fractional ALFF, seven voxel-based morphometry (VBM) studies and one DTI study were finally included in the present research. However, no studies on surface-based morphometry (SBM) analysis were included in this research. Due to the insufficient number of SBM and DTI studies, only rs-fMRI and VBM meta-analyses were conducted. The results of rs-fMRI meta-analysis showed that compared to HCs, CS patients demonstrated decreased regional spontaneous brain activities in the right lingual gyrus, right middle temporal gyrus (MTG), left inferior parietal gyrus and right postcentral gyrus (PoCG), while increased activities in the right medial superior frontal gyrus, bilateral middle frontal gyrus and right precuneus. VBM meta-analysis detected increased GMV in the right superior temporal gyrus (STG) and right paracentral lobule (PCL), while decreased GMV in the left supplementary motor area and left MTG in CS patients. The multi-modal meta-analysis revealed increased GMV together with decreased regional spontaneous brain activity in the left PoCG, right STG and PCL among CS patients.

Conclusion

This meta-analysis revealed that compared to HCs, CS patients had significant alterations in GMV and regional spontaneous brain activity. The altered brain regions mainly included the primary visual cortex, the default mode network and the sensorimotor area, which may be associated with CS patients' symptoms of sensory deficits, blurred vision, cognitive impairment and motor dysfunction. The findings may contribute to understanding the underlying pathophysiology of brain dysfunction and provide references for early diagnosis and treatment of CS.

Systematic review registration

https://www.crd.york.ac.uk/PROSPERO/, CRD42022370967.

Modulating Neural Circuits of Pain in Preclinical Models: Recent Insights for Future Therapeutics

Chronic pain is a pathological state defined as daily pain sensation over three consecutive months. It affects up to 30% of the general population. Although significant research efforts have been made in the past 30 years, only a few and relatively low effective molecules have emerged to treat chronic pain, with a considerable translational failure rate. Most preclinical models have focused on sensory neurotransmission, with particular emphasis on the dorsal horn of the spinal cord as the first relay of nociceptive information. Beyond impaired nociceptive transmission, chronic pain is also accompanied by numerous comorbidities, such as anxiety-depressive disorders, anhedonia and motor and cognitive deficits gathered under the term "pain matrix". The emergence of cutting-edge techniques assessing specific neuronal circuits allow in-depth studies of the connections between "pain matrix" circuits and behavioural outputs. Pain behaviours are assessed not only by reflex-induced responses but also by various or more complex behaviours in order to obtain the most complete picture of an animal's pain state. This review summarises the latest findings on pain modulation by brain component of the pain matrix and proposes new opportunities to unravel the mechanisms of chronic pain.

An overview of diagnosis and assessment methods for neuropathic pain

Neuropathic pain, defined as pain arising as a consequence of a lesion or disease affecting the somatosensory nervous system, requires precise diagnostic assessment. Different diagnostic tools have been devised for the diagnosis of neuropathic pain. This review offers insights into the diagnostic accuracy of screening questionnaires and different tests that investigate the somatosensory nervous system, in patients with suspected neuropathic pain. Thus, it illustrates how these tools can aid clinicians in accurately diagnosing neuropathic pain.

Integrated pain care models and the importance of aligning stakeholder values

Sustained widespread deployment of clinically and cost-effective models of integrated pain care could be bolstered by optimally aligning shared stakeholder values.

Pain perception as hierarchical Bayesian inference: A test case for the theory of constructed emotion

An intriguing perspective about human emotion, the theory of constructed emotion considers emotions as generative models according to the Bayesian brain hypothesis. This theory brings fresh insight to existing findings, but its complexity renders it challenging to test experimentally. We argue that laboratory studies of pain could support the theory because although some may not consider pain to be a genuine emotion, the theory must at minimum be able to explain pain perception and its dysfunction in pathology. We review emerging evidence that bear on this question. We cover behavioral and neural laboratory findings, computational models, placebo hyperalgesia, and chronic pain. We conclude that there is substantial evidence for a predictive processing account of painful experience, paving the way for a better understanding of neuronal and computational mechanisms of other emotions.

Gray Matter Changes Following Mild COVID-19: An MR Morphometric Study in Healthy Young People

BACKGROUND

Although COVID-19 is primarily an acute respiratory infection, 5%-40% of patients develop late and prolonged symptoms with frequent neurological complaints, known as long COVID syndrome. The presentation of the disease suggests that COVID infection may cause functional and/or morphological central nervous system alterations, but studies published in the literature report contradictory findings.

PURPOSE

To investigate the chronic effects of COVID-19 on cerebral grey matter in a group of young patients without comorbidities, with mild course of COVID infection and no medical complaints at the time of examination.

STUDY TYPE

Prospective.

POPULATION

Thirty-eight young (age = 26.6 ± 5.0 years; male/female = 14/24), adult participants who recovered from mild COVID infection without a history of clinical long COVID and 37 healthy control subjects (age = 25.9 ± 2.8 years; male/female = 14/23).

FIELD STRENGTH/SEQUENCE

Three Tesla, 3D T1-weighted magnetization-prepared rapid gradient-echo, 2D T2-weighted turbo spin-echo.

ASSESSMENT

MRI-based morphometry and volumetry along with neuropsychological testing and self-assessed questionnaire.

STATISTICAL TESTS

Fisher's exact test, Mann-Whitney U-test, and multiple linear regression analyses were used to assess differences between COVID and healthy control groups. P < 0.05 was used as cutoff for significance.

RESULTS

In the COVID group, significantly lower bilateral mean cortical thickness (left/right-hemisphere: 2.51 ± 0.06 mm vs. 2.56 ± 0.07 mm, η2 p = 0.102/2.50 ± 0.06 mm vs. 2.54 ± 0.07 mm, η2 p = 0.101), lower subcortical gray matter (57881 ± 3998 mm3 vs. 60470 ± 5211 mm3, η2 p = 0.100) and lower right olfactory bulb volume (52.28 ± 13.55 mm3 vs. 60.98 ± 15.8 mm3, η2 p = 0.078) were found. In patients with moderate to severe anosmia, cortical thickness was significantly lower bilaterally, as compared to patients without olfactory function loss (left/right-hemisphere: 2.50 ± 0.06 mm vs. 2.56 ± 0.05 mm, η2 = 0.173/2.49 ± 0.06 mm vs. 2.55 ± 0.05 mm, η2 = 0.189). Using further exploratory analysis, significantly reduced cortical thickness was detected locally in the right lateral orbitofrontal cortex in the COVID group (2.53 ± 0.10 mm vs. 2.60 ± 0.09 mm, η2 p = 0.112).

DATA CONCLUSION

Even without any subjective or objective neurological complaints at the time of the MR scan, subjects in the COVID group showed gray matter alterations in cortical thickness and subcortical gray matter volume.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY: Stage 3.

[The EAN-NeuPSIG guideline on the diagnosis of neuropathic pain-a summary]

In this joint guideline of the scientific societies and working groups mentioned in the title, evidence-based recommendations for the use of screening questionnaires and diagnostic tests in patients with neuropathic pain were developed. The systematic literature search and meta-analysis yielded the following results: Of the screening questionnaires, Douleur Neuropathique en 4 Questions (DN4), I‑DN4 (self-administered DN4), and Leeds Assessment of Neuropathic Symptoms and Signs (LANSS) received a strong recommendation, while S‑LANSS (self-administered LANSS) and PainDETECT received weak recommendations for their use in the diagnostic workup of patients with possible neuropathic pain. There was a strong recommendation for the use of skin biopsy and a weak recommendation for quantitative sensory testing and nociceptive evoked potentials. The role of confocal corneal microscopy is still unclear. Functional imaging and peripheral nerve blocks are helpful in elucidating the pathophysiology, but current literature does not support their use in diagnosing neuropathic pain. In selected cases, genetic testing in specialized centers may be considered.

Insula→Amygdala and Insula→Thalamus Pathways Are Involved in Comorbid Chronic Pain and Depression-Like Behavior in Mice

The comorbidity of chronic pain and depression poses tremendous challenges for the treatment of either one because they exacerbate each other with unknown mechanisms. As the posterior insular cortex (PIC) integrates multiple somatosensory and emotional information and is implicated in either chronic pain or depression, we hypothesize that the PIC and its projections may contribute to the pathophysiology of comorbid chronic pain and depression. We show that PIC neurons were readily activated by mechanical, thermal, aversive, and stressful and appetitive stimulation in naive and neuropathic pain male mice subjected to spared nerve injury (SNI). Optogenetic activation of PIC neurons induced hyperalgesia and conditioned place aversion in naive mice, whereas inhibition of these neurons led to analgesia, conditioned place preference (CPP), and antidepressant effect in both naive and SNI mice. Combining neuronal tracing, optogenetics, and electrophysiological techniques, we found that the monosynaptic glutamatergic projections from the PIC to the basolateral amygdala (BLA) and the ventromedial nucleus (VM) of the thalamus mimicked PIC neurons in pain modulation in naive mice; in SNI mice, both projections were enhanced accompanied by hyperactivity of PIC, BLA, and VM neurons and inhibition of these projections led to analgesia, CPP, and antidepressant-like effect. The present study suggests that potentiation of the PIC→BLA and PIC→VM projections may be important pathophysiological bases for hyperalgesia and depression-like behavior in neuropathic pain and reversing the potentiation may be a promising therapeutic strategy for comorbid chronic pain and depression.

Acupuncture for radicular pain: a review of analgesic mechanism

Radicular pain, a common and complex form of neuropathic pain, presents significant challenges in treatment. Acupuncture, a therapy originating from ancient traditional Chinese medicine and widely utilized for various pain types, including radicular pain, has shown promising outcomes in the management of lumbar radicular pain, cervical radicular pain, and radicular pain due to spinal stenosis. Despite its efficacy, the exact mechanisms through which acupuncture achieves analgesia are not fully elucidated and are the subject of ongoing research. This review sheds light on the current understanding of the analgesic mechanisms of acupuncture for radicular pain, offering valuable perspectives for both clinical application and basic scientific research. Acupuncture is postulated to relieve radicular pain by several mechanisms: peripherally, it reduces muscle spasms, lessens mechanical pressure on nerve roots, and improves microcirculation; at the molecular level, it inhibits the HMGB1/RAGE and TLR4/NF-κB signaling pathways, thereby decreasing the release of pro-inflammatory cytokines; within the spinal cord, it influences synaptic plasticity; and centrally, it modulates brain function, particularly affecting the medial prefrontal cortex, anterior cingulate cortex, and thalamus within the default mode network. By acting across these diverse biological domains, acupuncture presents an effective treatment modality for radicular pain, and deepening our understanding of the underlying mechanisms regarding analgesia for radicular pain is crucial for enhancing its clinical efficacy and advancement in pain management.

Peripheral Pain Captured Centrally: Altered Brain Morphology on MRI in Small Fiber Neuropathy Patients With and Without an SCN9A Gene Variant

The current study aims to characterize brain morphology of pain as reported by small fiber neuropathy (SFN) patients with or without a gain-of-function variant involving the SCN9A gene and compare these with findings in healthy controls without pain. The Neuropathic Pain Scale was used in patients with idiopathic SFN (N = 20) and SCN9A-associated SFN (N = 12) to capture pain phenotype. T1-weighted, structural magnetic resonance imaging (MRI) data were collected in patients and healthy controls (N = 21) to 1) compare cortical thickness and subcortical volumes and 2) quantify the association between severity, quality, and duration of pain with morphological properties. SCN9A-associated SFN patients showed significant (P < .017, Bonferroni corrected) higher cortical thickness in sensorimotor regions, compared to idiopathic SFN patients, while lower cortical thickness was found in more functionally diverse regions (eg, posterior cingulate cortex). SFN patient groups combined demonstrated a significant (Spearman's ρ = .44-.55, P = .005-.049) correlation among itch sensations (Neuropathic Pain Scale-7) and thickness of the left precentral gyrus, and midcingulate cortices. Significant associations were found between thalamic volumes and duration of pain (left: ρ = -.37, P = .043; right: ρ = -.40, P = .025). No associations were found between morphological properties and other pain qualities. In conclusion, in SCN9A-associated SFN, profound morphological alterations anchored within the pain matrix are present. The association between itch sensations of pain and sensorimotor and midcingulate structures provides a novel basis for further examining neurobiological underpinnings of itch in SFN. PERSPECTIVE: Cortical thickness and subcortical volume alterations in SFN patients were found in pain hubs, more profound in SCN9A-associated neuropathy, and correlated with itch and durations of pain. These findings contribute to our understanding of the pathophysiological pathways underlying chronic neuropathic pain and symptoms of itch in SFN.

Distinctive cortical morphological patterns in primary trigeminal neuralgia: a cross-sectional clinical study

PURPOSE

The characteristics of surface-based morphological patterns to primary trigeminal neuralgia (PTN) are still not well understood. This study aims to screen the useful cortical indices for the prediction of PTN and the quantification of pain severity.

METHODS

Fifty PTN patients and 48 matched healthy subjects enrolled in the study from March 2016 to August 2021. High-resolution T1 data were performed at 3.0 Tesla scanner and were analyzed with FreeSurfer software to detect the abnormalities of cortical mean curve (CMC), cortical thickness (CT), surface area (SA), and cortical volume (CV) in PTN patients compared to healthy controls. Logistic regression analysis was conducted to determine whether certain morphological patterns could predict PTN disorder. Then, the relationships of cortical indices to the pain characteristics in patient group were examined using linear regression model.

RESULTS

Distinctive cortical alterations were discovered through surface-based analysis, including increased temporal CMC, decreased insular CT and fusiform SA, along with decreased CV in several temporal and occipital areas. Moreover, the difference of temporal CMC was greater than other cortical parameters between the two groups, and the combination of certain morphological indices was of good value in the diagnosis for PTN. Besides, CT of left insula was negatively associated with the pain intensity in PTN patients.

CONCLUSION

The patients with PTN demonstrate distinctive morphological patterns in several cortical regions, which may contribute to the imaging diagnosis of this refractory disorder and be useful for the quantification of the orofacial pain.

CLINICAL TRIALS

The registry name of this study in https://clinicaltrials.gov/ : Magnetic Resonance Imaging Study on Patients with Trigeminal Neuralgia (MRI-TN) https://clinicaltrials.gov/ ID: NCT02713646 A link to the full application: https://clinicaltrials.gov/ct2/results?cond=&term=NCT02713646&cntry=&state=&city=&dist= The first patient with primary trigeminal neuralgia was recruited on November 28, 2016.

Right posterior insular epidural stimulation in rats with neuropathic pain induces a frequency-dependent and opioid system-mediated reduction of pain and its comorbid anxiety and depression

Neuropathic pain (NP) is a sensory, emotional, and persistent disturbing experience caused by a lesion or disease of the somatosensory system which can lead when chronic to comorbidities such as anxiety and depression. Available treatments (pharmacotherapy, neurostimulation) have partial and unpredictable response; therefore, it seems necessary to find a new therapeutical approach that could alleviate most related symptoms and improve patients 'emotional state'. Posterior Insula seems to be a potential target of neurostimulation for pain relief. However, its effects on pain-related anxiety and depression remain unknown. Using rats with spared nerve injury (SNI), this study aims to elucidate the correlation between NP and anxio-depressive disorders, evaluate potential analgesic, anxiolytic, and antidepressant effects of right posterior insula stimulation (IS) using low (LF-IS, 50 Hz) or high (HF-IS, 150 Hz) frequency and assess endogenous opioid involvement in these effects. Results showed positive correlation between NP, anxiety, and depression. LF-IS reversed anhedonia and despair-like behavior through pain alleviation, whereas HF-IS only reduced anhedonia, all effects involving endogenous opioids. These findings support the link between NP and anxio-depressive disorders. Moreover, IS appears to have analgesic, anxiolytic and antidepressant effects mediated by the endogenous opioid system, making it a promising target for neurostimulation.

P2X4 signalling contributes to hyperactivity but not pain sensitization comorbidity in a mouse model of attention deficit/hyperactivity disorder

Introduction: Attention deficit/hyperactivity disorder (ADHD) is a common neurodevelopmental disorder characterized by hyperactivity, inattention, and impulsivity that often persist until adulthood. Frequent comorbid disorders accompany ADHD and two thirds of children diagnosed with ADHD also suffer from behavioural disorders and from alteration of sensory processing. We recently characterized the comorbidity between ADHD-like symptoms and pain sensitisation in a pharmacological mouse model of ADHD, and we demonstrated the implication of the anterior cingulate cortex and posterior insula. However, few studies have explored the causal mechanisms underlying the interactions between ADHD and pain. The implication of inflammatory mechanisms has been suggested but the signalling pathways involved have not been explored. Methods: We investigated the roles of purinergic signalling, at the crossroad of pain and neuroinflammatory pathways, by using a transgenic mouse line that carries a total deletion of the P2X4 receptor. Results: We demonstrated that P2X4 deletion prevents hyperactivity in the mouse model of ADHD. In contrast, the absence of P2X4 lowered thermal pain thresholds in sham conditions and did not affect pain sensitization in ADHD-like conditions. We further analysed microglia reactivity and the expression of inflammatory markers in wild type and P2X4KO mice. Our results revealed that P2X4 deletion limits microglia reactivity but at the same time exerts proinflammatory effects in the anterior cingulate cortex and posterior insula. Conclusion: This dual role of P2X4 could be responsible for the differential effects noted on ADHD-like symptoms and pain sensitization and calls for further studies to investigate the therapeutic benefit of targeting the P2X4 receptor in ADHD patients.

Fractal Similarity of Pain Brain Networks

The conscious perception of pain is the result of dynamic interactions of neural activities from local brain regions to distributed brain networks. Mapping out the networks of functional connections between brain regions that form and disperse when an experimental participant received nociceptive stimulations allow to characterize the pattern of network connections related to the pain experience.Although the pattern of intra- and inter-areal connections across the brain are incredibly complex, they appear also largely scale free, with "fractal" connectivity properties reproducing at short and long-time scales. Our results combining intracranial recordings and functional imaging in humans during pain indicate striking similarities in the activity and topological representation of networks at different orders of temporality, with reproduction of patterns of activation from the millisecond to the multisecond range. The connectivity analyzed using graph theory on fMRI data was organized in four sets of brain regions matching those identified through iEEG (i.e., sensorimotor, default mode, central executive, and amygdalo-hippocampal).Here, we discuss similarities in brain network organization at different scales or "orders," in participants as they feel pain. Description of this fractal-like organization may provide clues about how our brain regions work together to create the perception of pain and how pain becomes chronic when its organization is altered.

Interactive effects of pain and arousal state on heart rate and cortical activity in the mouse anterior cingulate and somatosensory cortices

Sensory disconnection is a hallmark of sleep, yet the cortex retains some ability to process sensory information. Acute noxious stimulation during sleep increases the heart rate and the likelihood of awakening, indicating that certain mechanisms for pain sensing and processing remain active. However, processing of somatosensory information, including pain, during sleep remains underexplored. To assess somatosensation in natural sleep, we simultaneously recorded heart rate and local field potentials in the anterior cingulate (ACC) and somatosensory (S1) cortices of naïve, adult male mice, while applying noxious and non-noxious stimuli to their hind paws throughout their sleep-wake cycle. Noxious stimuli evoked stronger heart rate increases in both wake and non-rapid eye movement sleep (NREMS), and resulted in larger awakening probability in NREMS, as compared to non-noxious stimulation, suggesting differential processing of noxious and non-noxious information during sleep. Somatosensory information differentially reached S1 and ACC in sleep, eliciting complex transient and sustained responses in the delta, alpha, and gamma frequency bands as well as somatosensory evoked potentials. These dynamics depended on sleep state, the behavioral response to the stimulation and stimulation intensity (non-noxious vs. noxious). Furthermore, we found a correlation of the heart rate with the gamma band in S1 in the absence of a reaction in wake and sleep for noxious stimulation. These findings confirm that somatosensory information, including nociception, is sensed and processed during sleep even in the absence of a behavioral response.

Neural Correlates of Pain-Autonomic Coupling in Patients With Complex Regional Pain Syndrome Treated by Repetitive Transcranial Magnetic Stimulation of the Motor Cortex

OBJECTIVES

Complex regional pain syndrome (CRPS) is a chronic pain condition involving autonomic dysregulation. In this study, we report the results of an ancillary study to a larger clinical trial investigating the treatment of CRPS by neuromodulation. This ancillary study, based on functional magnetic resonance imaging (fMRI), evaluated the neural correlates of pain in patients with CRPS in relation to the sympathetic nervous system and for its potential relief after repetitive transcranial magnetic stimulation of the motor cortex.

MATERIALS AND METHODS

Eleven patients with CRPS at one limb (six women, five men, aged 52.0 ± 9.6 years) were assessed before and one month after the end of a five-month repetitive transcranial magnetic stimulation (rTMS) therapy targeting the motor cortex contralateral to the painful limb, by means of electrochemical skin conductance (ESC) measurement, daily pain intensity scores on a visual numerical scale (VNS), and fMRI with motor tasks (alternation of finger movements and rest). The fMRI scans were analyzed voxelwise using ESC and VNS pain score as regressors to derive their neural correlates. The criterion of response to rTMS therapy was defined as ≥30% reduction in VNS pain score one month after treatment compared with baseline.

RESULTS

At baseline, ESC values were reduced in the affected limb vs the nonaffected limb. There was a covariance of VNS with brain activation in a small region of the primary somatosensory cortex (S1) contralateral to the painful side on fMRI investigation. After rTMS therapy on motor cortex related to the painful limb, the VNS pain scores significantly decreased by 22% on average. The criterion of response was met in six of 11 patients (55%). In these responders, at one month after treatment, ESC value increased and returned to normal in the CRPS-affected limb, and overall, the increase in ESC correlated with the decrease in VNS after motor cortex rTMS therapy. At one month after treatment, there also was a covariance of both variables (ESC and VNS) with fMRI activation of the S1 region previously mentioned. The fMRI activation of other brain regions (middle frontal gyrus and temporo-parietal junction) showed correlation with ESC values before and after treatment. Finally, we found a positive correlation at one month after treatment (not at baseline) between VNS pain score and fMRI activation in the temporo-parietal junction contralateral to painful side.

CONCLUSIONS

This study first shows a functional pain-autonomic coupling in patients with CRPS, which could involve a specific S1 region. However, the modulation of sympathetic sudomotor activities expressed by ESC changes was rather correlated with functional changes in other brain regions. Finally, the pain relief observed at one month after rTMS treatment was associated with a reduced activation of the temporo-parietal junction on the side in which rTMS was performed. These findings open perspectives to define new targets or biomarkers for using rTMS to treat CRPS-associated pain.

CLINICAL TRIAL REGISTRATION

The Clinicaltrials.gov registration number for the study is NCT02817880.

Multilevel Laser-Induced Pain Measurement with Wasserstein Generative Adversarial Network - Gradient Penalty Model

Pain is an experience of unpleasant sensations and emotions associated with actual or potential tissue damage. In the global context, billions of people are affected by pain disorders. There are particular challenges in the measurement and assessment of pain, and the commonly used pain measuring tools include traditional subjective scoring methods and biomarker-based measures. The main tools for biomarker-based analysis are electroencephalography (EEG), electrocardiography and functional magnetic resonance. The EEG-based quantitative pain measurements are of immense value in clinical pain management and can provide objective assessments of pain intensity. The assessment of pain is now primarily limited to the identification of the presence or absence of pain, with less research on multilevel pain. High power laser stimulation pain experimental paradigm and five pain level classification methods based on EEG data augmentation are presented. First, the EEG features are extracted using modified S-transform, and the time-frequency information of the features is retained. Based on the pain recognition effect, the 20-40[Formula: see text]Hz frequency band features are optimized. Afterwards the Wasserstein generative adversarial network with gradient penalty is used for feature data augmentation. It can be inferred from the good classification performance of features in the parietal region of the brain that the sensory function of the parietal lobe region is effectively activated during the occurrence of pain. By comparing the latest data augmentation methods and classification algorithms, the proposed method has significant advantages for the five-level pain dataset. This research provides new ways of thinking and research methods related to pain recognition, which is essential for the study of neural mechanisms and regulatory mechanisms of pain.

The temporal effect of uncertain context on the perceptual processing of painful and non-painful stimulation

Uncertainty has been demonstrated to influence the perception of noxious stimuli, but little is known about the effects of prolonged uncertain contexts on the perception of painful and non-painful stimuli. To address this knowledge gap, the present study utilized a cue-based NPU-threat task, where uncertain and certain trials were separated into distinct blocks. The objective was to investigate the impact of uncertain contexts on the temporal dynamics of electroencephalogram (EEG) activity during the processing of painful and non-painful stimuli. The results revealed that the influence of uncertain contexts on neural responses extends beyond painful trials and is also evident in non-painful trials. In uncertain contexts, it has been observed that painful stimuli elicit larger P2 amplitudes and late beta band (13-30 Hz) event-related desynchronization (ERD) around 500-700 ms. However, in certain contexts, painful stimuli evoke stronger late gamma band (50-70 Hz) event-related synchronization (ERS) around 600-700 ms. For non-painful trials, in uncertain contexts, significantly higher amplitudes of the late positive potential (LPP) component and delta-theta band (2-7 Hz) ERS were observed compared to certain non-painful stimuli. These findings demonstrate that uncertain contexts exert a significant impact on the processing of both painful and non-painful stimuli, and this influence is mediated by distinct neural mechanisms.

Central neuropathic pain

Central neuropathic pain arises from a lesion or disease of the central somatosensory nervous system such as brain injury, spinal cord injury, stroke, multiple sclerosis or related neuroinflammatory conditions. The incidence of central neuropathic pain differs based on its underlying cause. Individuals with spinal cord injury are at the highest risk; however, central post-stroke pain is the most prevalent form of central neuropathic pain worldwide. The mechanisms that underlie central neuropathic pain are not fully understood, but the pathophysiology likely involves intricate interactions and maladaptive plasticity within spinal circuits and brain circuits associated with nociception and antinociception coupled with neuronal hyperexcitability. Modulation of neuronal activity, neuron-glia and neuro-immune interactions and targeting pain-related alterations in brain connectivity, represent potential therapeutic approaches. Current evidence-based pharmacological treatments include antidepressants and gabapentinoids as first-line options. Non-pharmacological pain management options include self-management strategies, exercise and neuromodulation. A comprehensive pain history and clinical examination form the foundation of central neuropathic pain classification, identification of potential risk factors and stratification of patients for clinical trials. Advanced neurophysiological and neuroimaging techniques hold promise to improve the understanding of mechanisms that underlie central neuropathic pain and as predictive biomarkers of treatment outcome.