Secondary hyperalgesia and perceptual wind-up following intradermal injection of capsaicin in humans

Indication for spinal sensitization in chronic low back pain: mechanical hyperalgesia adjacent to but not within the most painful body area

Introduction

In 85% of patients with chronic low back pain (CLBP), no specific pathoanatomical cause can be identified. Besides primary peripheral drivers within the lower back, spinal or supraspinal sensitization processes might contribute to the patients' pain.

Objectives

The present study conceptualized the most painful area (MP) of patients with nonspecific CLBP as primarily affected area and assessed signs of peripheral, spinal, and supraspinal sensitization using quantitative sensory testing (QST) in MP, a pain-free area adjacent to MP (AD), and a remote, pain-free control area (CON).

Methods

Fifty-nine patients with CLBP (51 years, SD = 16.6, 22 female patients) and 35 pain-free control participants individually matched for age, sex, and testing areas (49 years, SD = 17.5, 19 female participants) underwent a full QST protocol in MP and a reduced QST protocol assessing sensory gain in AD and CON. Quantitative sensory testing measures, except paradoxical heat sensations and dynamic mechanical allodynia (DMA), were Z-transformed to the matched control participants and tested for significance using Z-tests (α = 0.001). Paradoxical heat sensations and DMA occurrence were compared between cohorts using Fisher's exact tests (α = 0.05). The same analyses were performed with a high-pain and a low-pain CLBP subsample (50% quantile).

Results

Patients showed cold and vibration hypoesthesia in MP (all Ps < 0.001) and mechanical hyperalgesia (P < 0.001) and more frequent DMA (P = 0.044) in AD. The results were mainly driven by the high-pain CLBP subsample. In CON, no sensory alterations were observed.

Conclusion

Mechanical hyperalgesia and DMA adjacent to but not within MP, the supposedly primarily affected area, might reflect secondary hyperalgesia originating from spinal sensitization in patients with CLBP.

Theta Burst Stimulation of the Human Motor Cortex Modulates Secondary Hyperalgesia to Punctate Mechanical Stimuli

OBJECTIVES

Many chronic pain conditions show evidence of dysregulated synaptic plasticity, including the development and maintenance of central sensitization. This provides a strong rationale for neuromodulation therapies for the relief of chronic pain. However, variability in responses and low fidelity across studies remain an issue for both clinical trials and pain management, demonstrating insufficient mechanistic understanding of effective treatment protocols.

MATERIALS AND METHODS

In a randomized counterbalanced crossover designed study, we evaluated two forms of patterned repetitive transcranial magnetic stimulation, known as continuous theta burst stimulation (TBS) and intermittent TBS, during normal and central sensitization states. Secondary hyperalgesia (a form of use-dependent central sensitization) was induced using a well-established injury-free pain model and assessed by standardized quantitative sensory testing involving light touch and pinprick pain thresholds in addition to stimulus-response functions.

RESULTS

We found that continuous TBS of the human motor cortex has a facilitatory (pronociceptive) effect on the magnitude of perceived pain to secondary hyperalgesia, which may rely on induction and expression of neural plasticity through heterosynaptic long-term potentiation-like mechanisms.

CONCLUSIONS

By defining the underlying mechanisms of TBS-driven synaptic plasticity in the nociceptive system, we offer new insight into disease mechanisms and provide targets for promoting functional recovery and repair in chronic pain. For clinical applications, this knowledge is critical for development of more efficacious and mechanisms-based neuromodulation protocols, which are urgently needed to address the chronic pain and opioid epidemics.

Reliability of quantitative sensory testing in the assessment of somatosensory function after high-frequency stimulation-induced sensitisation of central nociceptive pathways

ABSTRACT

The high frequency stimulation (HFS) model can be used alongside quantitative sensory testing (QST) to assess the sensitisation of central nociceptive pathways. However, the validity and between-session reliability of using QST z -score profiles to measure changes in mechanical and thermal afferent pathways in the HFS model are poorly understood. In this study, 32 healthy participants underwent QST before and after HFS (5× 100 Hz trains; 10× electrical detection threshold) in the same heterotopic skin area across 2 repeated sessions. The only mechanical QST z -score profiles that demonstrated a consistent gain of function across repeated test sessions were mechanical pain threshold (MPT) and mechanical pain sensitivity (MPS), which were associated with moderate and good reliability, respectively. There was no relationship between HFS intensity and MPT and MPS z -score profiles. There was no change in low intensity, but a consistent facilitation of high-intensity pin prick stimuli in the mechanical stimulus response function across repeated test sessions. There was no change in cold pain threshold (CPT) and heat pain threshold (HPT) z -score profiles across session 1 and 2, which were associated with moderate and good reliability, respectively. There were inconsistent changes in the sensitivity to innocuous thermal QST parameters, with cool detection threshold (CDT), warm detection threshold (WDT), and thermal sensory limen (TSL) all producing poor reliability. These data suggest that HFS-induced changes in MPS z -score profiles is a reliable way to assess experimentally induced central sensitisation and associated secondary mechanical hyperalgesia in healthy participants.

Sensory phenotypes in complex regional pain syndrome and chronic low back pain-indication of common underlying pathomechanisms

Introduction

First-line pain treatment is unsatisfactory in more than 50% of chronic pain patients, likely because of the heterogeneity of mechanisms underlying pain chronification.

Objectives

This cross-sectional study aimed to better understand pathomechanisms across different chronic pain cohorts, regardless of their diagnoses, by identifying distinct sensory phenotypes through a cluster analysis.

Methods

We recruited 81 chronic pain patients and 63 age-matched and sex-matched healthy controls (HC). Two distinct chronic pain cohorts were recruited, ie, complex regional pain syndrome (N = 20) and low back pain (N = 61). Quantitative sensory testing (QST) was performed in the most painful body area to investigate somatosensory changes related to clinical pain. Furthermore, QST was conducted in a pain-free area to identify remote sensory alterations, indicating more widespread changes in somatosensory processing.

Results

Two clusters were identified based on the QST measures in the painful area, which did not represent the 2 distinct pain diagnoses but contained patients from both cohorts. Cluster 1 showed increased pain sensitivities in the painful and control area, indicating central sensitization as a potential pathomechanism. Cluster 2 showed a similar sensory profile as HC in both tested areas. Hence, either QST was not sensitive enough and more objective measures are needed to detect sensitization within the nociceptive neuraxis or cluster 2 may not have pain primarily because of sensitization, but other factors such as psychosocial ones are involved.

Conclusion

These findings support the notion of shared pathomechanisms irrespective of the pain diagnosis. Conversely, different mechanisms might contribute to the pain of patients with the same diagnosis.

Modulation of the spinal N13 SEP component by high- and low-frequency electrical stimulation. Experimental pain models matter

OBJECTIVE

The N13 component of somatosensory evoked potential (N13 SEP) represents the segmental response of cervical dorsal horn neurons. Neurophysiological studies in healthy participants showed that capsaicin-induced central sensitization causes an increase of the N13 SEP amplitude. Consequently, in human research, this spinal component may serve as a valuable readout of central sensitization. In this study, we wanted to verify if the sensitivity of the N13 SEP for detecting central sensitization is consistent across different experimental pain models inducing central sensitization and secondary hyperalgesia, namely high and low-frequency electrical stimulation (HFS and LFS).

METHODS

In 18 healthy participants, we recorded SEP after bilateral ulnar nerve stimulation before and after secondary hyperalgesia was induced through HFS and LFS applied on the ulnar nerve territory of the hand of one side. The area of secondary hyperalgesia was mapped with a calibrated 128-mN pinprick probe, and the mechanical pain sensitivity with three calibrated 16-64-256-mN pinprick probes.

RESULTS

Although both HFS and LFS successfully induced secondary hyperalgesia only LFS increased the amplitude of the N13 SEP.

CONCLUSIONS

These findings suggest that the sensitivity of the N13 SEP for detecting dorsal horn excitability changes may critically depend on the different experimental pain models.

SIGNIFICANCE

Our results indicate that LFS and HFS could trigger central sensitization at the dorsal horn level through distinct mechanisms, however this still needs confirmation by replication studies.

Painful Cutaneous Laser Stimulation for Temporal Summation of Pain Assessment

Variability in pain sensitivity arises not only from the differences in peripheral sensory receptors but also from the differences in central nervous system (CNS) pain inhibition and facilitation mechanisms. Temporal summation of pain (TSP) is an experimental protocol commonly used in human studies of pain facilitation but is susceptible to confounding when elicited with the skin-contact thermode, which adds the responses of touch-related Aβ low-threshold mechanoreceptors to nociceptive receptors. In the present study, we evaluate an alternative method involving the use of a contactless cutaneous laser for TSP assessment. We show that repetitive laser stimulations with a one second inter-stimulus interval evoked reliable TSP responses in a significant proportion of healthy subjects (N = 36). Female subjects (N = 18) reported greater TSP responses than male subjects confirming earlier studies of sex differences in central nociceptive excitability. Furthermore, repetitive laser stimulations during TSP induction elicited increased time-frequency electroencephalography (EEG) responses. The present study demonstrates that repetitive laser stimulation may be an alternative to skin-contact methods for TSP assessment in patients and healthy controls. PERSPECTIVE: Temporal summation of pain (TSP) is an experimental protocol commonly used in human studies of pain facilitation. We show that contactless cutaneous laser stimulation is a reliable alternative to the skin contact approaches during TSP assessment.

Central sensitization in CRPS patients with widespread pain: a cross-sectional study

OBJECTIVE

Widespread pain hypersensitivity and enhanced temporal summation of pain (TSP) are commonly reported in patients with complex regional pain syndrome (CRPS) and discussed as proxies for central sensitization. This study aimed to directly relate such signs of neuronal hyperexcitability to the pain phenotype of CRPS patients.

METHODS

Twenty-one CRPS patients and 20 healthy controls (HC) were recruited. The pain phenotype including spatial pain extent (assessed in % body surface) and intensity were assessed and related to widespread pain hypersensitivity, TSP, and psychological factors. Quantitative sensory testing (QST) was performed in the affected, the contralateral and a remote (control) area.

RESULTS

CRPS patients showed decreased pressure pain thresholds in all tested areas (affected: t(34)  = 4.98, P < .001, contralateral: t(35) = 3.19, P = .005, control: t(31) = 2.65, P = .012). Additionally, patients showed increased TSP in the affected area (F(3,111) = 4.57, P = .009) compared to HC. TSP was even more enhanced in patients with a high compared to a low spatial pain extent (F(3,51) = 5.67, P = .008), suggesting pronounced spinal sensitization in patients with extended pain patterns. Furthermore, the spatial pain extent positively correlated with the Bath Body Perception Disturbance Scale (ρ = 0.491; P = .048).

CONCLUSIONS

Overall, we provide evidence that the pain phenotype in CRPS, that is, spatial pain extent, might be related to sensitization mechanism within the central nociceptive system. This study points towards central neuronal excitability as a potential therapeutic target in patients with more widespread CRPS.

Responders and nonresponders to topical capsaicin display distinct temporal summation of pain profiles

Introduction

Topical application of capsaicin can produce an ongoing pain state in healthy participants. However, approximately one-third report no pain response (ie, nonresponders), and the reasons for this are poorly understood.

Objectives

In this study, we investigated temporal summation of pain (TSP) profiles, pain ratings and secondary hyperalgesia responses in responders and nonresponders to 1% topical capsaicin cream.

Methods

Assessments were made at baseline and then during an early (ie, 15 minutes) and late (ie, 45 minutes) time points post-capsaicin in 37 healthy participants.

Results

Participants reporting a visual analogue scale (VAS) rating of >50 were defined as responders (n = 24) and those with <50 VAS rating were defined as nonresponders (n = 13). There was a facilitation of TSP during the transition from an early to the late time point post-capsaicin (P<0.001) and the development of secondary hyperalgesia (P<0.05) in the responder group. Nonresponders showed no changes in TSP or secondary hyperalgesia during the early and late time points. There was an association between baseline TSP scores and the later development of a responder or nonresponder phenotype (r = 0.36; P = 0.03). Receiver operating characteristic analysis revealed that baseline TSP works as a good response predictor at an individual level (area under the curve = 0.75).

Conclusion

These data suggest that responders and nonresponders have different facilitatory pain mechanisms. The assessment of TSP may help to identify participants with stronger endogenous pain facilitation who may be more likely to respond to topical capsaicin.

Sumatriptan prevents central sensitisation specifically in the trigeminal dermatome in humans

BACKGROUND

The exact mechanism and site of action of triptans in aborting migraine attacks remain under debate. We hypothesized that the clinical efficacy of triptans lies in aborting central sensitization and focused on the question of why triptans are headache-specific, i.e. highly effective in migraine and cluster headache and ineffective in extracephalic pain.

METHODS

Forty healthy participants were enrolled in this double-blinded, randomized, placebo-controlled study. The effect of sumatriptan (n=20) vs placebo (n=20) was investigated in a cephalic (V1) vs an extracephalic dermatome (forearm) using a topical capsaicin sensitization model. Capsaicin-induced primary and secondary hyperalgesia were evaluated using quantitative sensory testing.

RESULTS

After capsaicin application, primary hyperalgesia developed in both sumatriptan and placebo groups in both dermatomes. However, sumatriptan exclusively prevented secondary hyperalgesia in the V1 dermatome but not on the forearm. Placebo exerted no effects on secondary hyperalgesia in both trigeminal and extracephalic dermatomes. Additionally, sumatriptan reduced the flare size exclusively in the V1 dermatome.

CONCLUSIONS

Our data suggest that sumatriptan reduces central sensitization (secondary hyperalgesia) without modulating peripheral sensitization (primary hyperalgesia) in a human pain model of capsaicin-induced sensitization. Moreover, despite a systemic administration of sumatriptan, the modulatory effects are trigeminal-specific, echoing the clinical effect of triptans in aborting headaches, but not extracephalic pain.

Diabetic Neuropathy in Hands: An Endemic Complication Waiting to Unfold?

In this cross-sectional study we aimed to quantify the somatosensory dysfunction in the hand in people with diabetes with distal symmetrical polyneuropathy (DSPN) in hands and explore early signs of nerve dysfunction in people with diabetes without DSPN in hands. The clinical diagnosis of DSPN was confirmed with electrodiagnosis and corneal confocal microscopy. Thermal and mechanical nerve function in the hand was assessed with quantitative sensory tests. Measurements were compared between healthy participants (n = 31), individuals with diabetes without DSPN (n = 35), individuals with DSPN in feet but not hands (DSPNFEET ONLY) (n = 31), and individuals with DSPN in hands and feet (DSPNHANDS & FEET) (n = 28) with one-way between-group ANOVA. The somatosensory profile of the hand in people with DSPNHANDS & FEET showed widespread loss of thermal and mechanical detection. This profile in hands is comparable with the profile in the feet of people with DSPN in feet. Remarkably, individuals with DSPNFEET ONLY already showed a similar profile of widespread loss of nerve function in their hands. People with diabetes without DSPN in feet already had some nerve dysfunction in their hands. These findings suggest that nerve function assessment in hands should become more routine in people with diabetes.

When Differential Descending Control of Speed Matters: Descending Modulation of A- versus C-Fiber Evoked Spinal Nociception

Descending pain modulatory systems (DPMS) that originate within the brain and act to modulate spinal nociceptive transmission are a major determinant of the acute and chronic pain experience. Investigations of these systems in basic scientific research is critical to the development of therapeutic strategies for the relief of pain. Despite our best efforts, something is lost in translation. This article will explore whether this is due in part to a primary focus on sensory modality leading to a failure to differentiate between descending control of A- vs. C-fiber mediated spinal nociception.

Study protocol: an observational study of distress, immune function and persistent pain in HIV

INTRODUCTION

Many people with HIV report both distress and pain. The relationship between distress and pain is bidirectional, but the mechanisms by which distress exacerbates pain are unclear. The inflammatory response to challenge (inflammatory reactivity, IR) may be a partial mediator, given that neuroimmune interactions provide a substrate for IR to also influence neurological reactivity and, thus, pain-related neural signalling. This prospective, observational, case-control study will characterise the relationships between distress, IR, pain-related signalling as captured by induced secondary hyperalgesia (SH), and pain, in people with HIV who report persistent pain (PP) (cases) or no pain (controls).

METHODS AND ANALYSIS

One hundred people with suppressed HIV, reporting either PP or no pain, will be assessed two or four times over 6 months. The primary outcomes are distress (Hopkins 25-item symptom checklist), IR (multiplex assay after LPS challenge), and PP (Brief Pain Inventory), assessed at the baseline timepoint, although each will also be assessed at follow-up time points. Induced SH will be assessed in a subsample of 60 participants (baseline timepoint only). To test the hypothesis that IR partly mediates the relationship between distress and pain, mediation analysis will use the baseline data from the PP group to estimate direct and indirect contributions of distress and IR to pain. To test the hypothesis that IR is positively associated with SH, data from the subsample will be analysed with generalised mixed effects models to estimate the association between IR and group membership, with SH as the dependent variable.

ETHICS AND DISSEMINATION

Information obtained from this study will be published in peer-reviewed journals and presented at scientific meetings. The study has been approved by the Human Research Ethics Committee of the University of Cape Town (approval number: 764/2019) and the City of Cape Town (ref: 24699).

TRIAL REGISTRATION NUMBER

NCT04757987.

Characterization of Patients With and Without Painful Peripheral Neuropathy After Receiving Neurotoxic Chemotherapy: Traditional Quantitative Sensory Testing vs C-Fiber and Aδ-Fiber Selective Diode Laser Stimulation

Painful chemotherapy induced peripheral neuropathy (CIPN) is a common complication of chemotherapy with drugs such as taxanes and platinum compounds. Currently, no methods are available for early detection of sensory changes that are associated with painful CIPN, nor are there biomarkers that are specific to painful CIPN. This study aimed to compare Diode Laser fiber type-selective stimulator (DLss), a method to selectively stimulate cutaneous C and Aδ fibers, to traditional quantitative sensory testing (QST) in determining psychophysical differences between patients with painful CIPN and a control group. Sensory testing was performed on the dorsal mid-foot of 20 patients with painful neuropathy after taxane- or platinum-based chemotherapy, and 20 patients who received similar neurotoxic chemotherapy, without painful CIPN. In a multivariable analysis, C-fiber to Aδ fiber detection threshold ratio, measured by DLss, was significantly different between the groups (P <.05). While QST parameters such as warmth detection threshold were different between the groups in univariate analyses, these findings were likely attributable to group differences in patient age and cumulative chemotherapy dose. PERSPECTIVE: In this study, fiber-specific DLss test showed potential in identifying sensory changes that are specific for painful neuropathy, encouraging future testing of this approach as a biomarker for early detection of painful CIPN. TRIAL REGISTRATION: The study was approved by the Washington University Institutional Review Board (#201807162) and registered at ClinicalTrials.gov (NCT03687970).

Pain sensitivities predict prophylactic treatment outcomes of flunarizine in chronic migraine patients: A prospective study

BACKGROUND

We aimed to assess the differences in quantitative sensory testing between chronic migraine and healthy controls and to explore the association between pain sensitivities and outcomes in chronic migraine following preventive treatment.

METHODS

In this prospective open-label study, preventive-naïve chronic migraine and healthy controls were recruited, and cold, heat, mechanical punctate, and pressure pain thresholds over the dermatomes of first branch of trigeminal nerve and first thoracic nerve were measured by quantitative sensory testing at baseline. Chronic migraines were treated with flunarizine and treatment response was defined as ≥50% reduction in the number of monthly headache days over the 12-week treatment period.

RESULTS

Eighty-four chronic migraines and fifty age-and-sex-matched healthy controls were included in the analysis. The chronic migraine had higher cold pain thresholds over the dermatomes of the first branch of trigeminal nerve and the first thoracic nerve (p < 0.001 and < 0.001), lower pressure pain thresholds over the dermatomes of the first thoracic nerve (p = 0.003), heat pain thresholds over the dermatomes of the first branch of the trigeminal nerve and the first thoracic nerve (p < 0.001 and p = 0.015) than healthy controls. After treatment, 24/84 chronic migraine had treatment response. The responders with relatively normal pain sensitivity had higher heat pain thresholds over the dermatome of the first branch of the trigeminal nerve (p = 0.002), mechanical punctate pain thresholds over the dermatomes of the first branch of the trigeminal nerve (p = 0.023), and pressure pain thresholds over the dermatomes of the first branch of the trigeminal nerve (p = 0.026) than the hypersensitive non-responders. Decision tree analysis showed that patients with mechanical punctate pain threshold over the dermatomes of the first branch of the trigeminal nerve > 158 g (p = 0.020) or heat pain threshold over the dermatome of the first branch of the trigeminal nerve > 44.9°C (p = 0.002) were more likely to be responders.

CONCLUSIONS

Chronic migraine were generally more sensitive compared to healthy controls. Preventive treatment with flunarizine should be recommended particularly for chronic migraine who have relatively normal sensitivity to mechanical punctate or heat pain.Trial registration: This study was registered on ClinicalTrials.gov (Identifier: NCT02747940).

Dose-Dependent Pain and Pain Radiation after Chemical Stimulation of the Thoracolumbar Fascia and Multifidus Muscle: A Single-Blinded, Cross-Over Study Revealing a Higher Impact of Fascia Stimulation

Acute low back pain can be experimentally induced by injections of hypertonic saline into deep tissues of the back, such as fascia and muscle. The current study investigated the dose-dependency of peak-pain and spatial extent of concomitant radiating pain following 50, 200 and 800 μL bolus injections of hypertonic saline (5.8%) into the thoracolumbar fascia and multifidus muscle, since data on dose-dependency is lacking in humans. Sixteen healthy subjects rated (11 female, 5 male; 23.3 ± 3.1 years, mean ± SD) intensity and spatial extent of pain. Injections into the fascia resulted in significantly higher peak-pain (+86%, p < 0.001), longer pain durations (p < 0.05), and larger pain areas (+65%, p < 0.02) and were less variable than intramuscular injections. Peak-pain ratings and pain areas were 2−3-fold higher/larger for 200 μL vs. 50 μL. In contrast, peak pain increased only marginally at 800 μL by additional 20%, while pain areas did not increase further at all in both, fascia and muscle. Thus, higher injection volumes did also not compensate the lower sensitivity of muscle. Peak-pain ratings and pain areas correlated between fascia and muscle (r = 0.530, p < 0.001 and r = 0.337, p < 0.02, respectively). Peak-pain ratings and pain areas correlated overall (r = 0.490, p < 0.0001), but a weak correlation remained when the impact of between-tissue differences and different injection volumes were singled out (partial r = 0.261, p < 0.01). This study shows dose-dependent pain responses of deep tissues where an injection volume of 200 μL of hypertonic saline is deemed an adequate stimulus for tissue differentiation. We suggest that pain radiation is not simply an effect of increased peripheral input but may afford an individual disposition for the pain radiation response. Substantially higher pain-sensitivity and wider pain areas support fascia as an important contributor to non-specific low back pain.

Disentangling 'sciatica' to understand and characterise somatosensory profiles and potential pain mechanisms

OBJECTIVES

The study aimed to investigate if patients with lumbar radicular pain only and those with combined lumbar radicular pain + radiculopathy differ in their somatosensory profiles and pain experiences.

METHODS

Quantitative sensory testing (QST) was performed in 26 patients (mean age 47 ± 10 years, 10 females) with unilateral leg pain in the L5 or S1 distribution in their main pain area (MPA) and contralateral mirror side, in the relevant foot dermatome on the symptomatic side and in the hand dorsum. Pain experience was captured on the painDETECT.

RESULTS

Eight patients presented with lumbar radicular pain only and 18 patients with combined radicular pain + radiculopathy. Patients with radicular pain only demonstrated widespread loss of function (mechanical detection) bilaterally in the MPA (p<0.003) and hand (p=0.002), increased heat sensitivity in both legs (p<0.019) and cold/heat sensitivity in the hand (p<0.024). QST measurements in the dermatome did not differ compared to HCs and patients with radiculopathy. Patients with lumbar radiculopathy were characterised by a localised loss of function in the symptomatic leg in the MPA (warm, mechanical, vibration detection, mechanical pain threshold, mechanical pain sensitivity p<0.031) and dermatome (mechanical, vibration detection p<0.001), consistent with a nerve root lesion. Pain descriptors did not differ between the two groups with the exception of numbness (p<0.001). Patients with radicular pain did not report symptoms of numbness, while 78% of patients with radiculopathy did.

CONCLUSIONS

Distinct differences in somatosensory profiles and pain experiences were demonstrated for each patient group, suggesting differing underlying pain mechanisms.

Absolute and relative reliability of a comprehensive quantitative sensory testing protocol in women treated for breast cancer

OBJECTIVE

Quantitative sensory testing (QST) are non-invasive psychophysical assessment techniques to evaluate functioning of the somatosensory nervous system. Despite the importance of reliability for correct use of QST results in research and clinical practice, the relative and absolute intra-and inter-rater reliability of a comprehensive QST protocol to evaluate the functioning of both peripheral and central somatosensory nervous system in a breast cancer population, has not yet been investigated.

SETTING

University Hospitals, Leuven, Belgium.

SUBJECTS

Thirty women at least six months after unilateral breast cancer surgery.

METHODS

The protocol included nine static and dynamic QST methods (mechanical detection-pain thresholds, pressure pain thresholds, thermal detection-pain thresholds for heat and cold, temporal summation and conditioned pain modulation (CPM)) performed in the surgical area and more distant regions. Absolute and relative intra (60-minutes interval) and inter-rater (one-week interval) reliability was evaluated using intraclass correlation coefficients, standard error of measurement and Bland-Altman plots.

RESULTS

A moderate to excellent relative intra- and inter-rater reliability was found for the evaluation of mechanical thresholds, pressure pain thresholds and temporal summation. Reliability of the CPM paradigm was considered weak. Systematic bias between raters was noticed for detection of mechanical and cold stimuli at the non-affected trunk and CPM.

CONCLUSIONS

Except for the evaluation of CPM, the QST protocol was found suitable for identifying differences between subjects (relative reliability) and individual follow-up after breast cancer surgery (limited systematic bias) during a one-week timeframe. Additional research is required to determine measurement properties that influence CPM test stability in order to establish a more reliable CPM test paradigm.

Effects of neural mobilizations through movement representation techniques for the improvement of neural mechanosensitivity of the median nerve region: a randomized controlled trial

PURPOSE

The main objective was to compare the effects of neural mobilization (NM), NM performed through mirror therapy (MT), NM performed through action observation (AO) training and finally classic rehabilitation program (mobility and strength) exercises on neural mechanosensitivity, widespread of proximal and distal pain and pressure pain thresholds (PPT). The second objective was to assess the effects of these interventions on handgrip strength, conditioned pain modulation, motor imagery ability and temporal summation.

MATERIALS AND METHODS

Single-blinded randomized controlled trial. Fifty-four healthy subjects were randomly assigned to each group. Neural mechanosensitivity, widespread pain and PPT were the main variables. The secondary variables included handgrip strength, conditioned pain modulation, motor imagery ability and temporal summation.

RESULTS

All groups showed significant differences in time*factor for neural mechanosensitivity (p = 0.001), PPT in the dermatome of the median nerve (p = 0.007), PPT at carpal tunnel (p < 0.05) and proximal widespread (p = 0.01). No differences were found for distal widespread, conditioned pain modulation, handgrip strength motor imagery ability or temporal summation (p > 0.05). There is an absence of statistically significant differences between groups.

CONCLUSIONS

NM through movement representation techniques can reduce mechanosensitivity and mechanical hyperalgesia in the median nerve dermatome and forearm, although no differences were found between groups.

A novel computational technique for the quantification of temporal summation in healthy individuals

BACKGROUND

The pathophysiology of chronic musculoskeletal pain is linked to the neurophysiologic condition known as central sensitization. Developing reliable, sensitive and clinically feasible techniques for quantifying central sensitization is a timely priority for advancing the field of chronic pain diagnosis and management.

OBJECTIVE

To compare the sensitivity of the Windup Ratio, a commonly employed Quantitative Sensory Testing (QST) technique, to a novel approach, the Sumsquare method, for detecting changes in experimentally induced central sensitization.

DESIGN

Individual, randomized, controlled experimental study.

METHODS

A total of 37 subjects assigned to experimental (N = 18) and control (N = 19) groups. Central sensitization was experimentally induced in the C5-C6 spinal segments using topical capsaicin (0.075%); controls received a non-sensitizing placebo (Lubriderm). Windup (temporal summation) was assessed using weighted pinpricks (MRC Systems, Heidelberg, Germany) applied within regions of secondary hyperalgesia surrounding the topical capsaicin. A train of 10 stimuli was applied at baseline, 10, 20 and 30 min post-topical application and participants provided numeric pain ratings after each pinprick application. Sumsquare and Windup Ratio outcomes were calculated using the pain rating data.

RESULTS

Sumsquare outcome was significantly increased at all time points (10, 20, 30 min) post-sensitization (p < 0.05); in contrast, no differences in Windup Ratio from baseline were observed at any time point post-sensitization (p > 0.05).

CONCLUSIONS

Sumsquare outcome offers greater sensitivity than Windup Ratio for detecting changes in experimentally induced central sensitization. These findings introduce a novel method for assessing changes in central sensitization in patients presenting with chronic musculoskeletal pain hypersensitivity.

Human surrogate models of central sensitization: A critical review and practical guide

Background

As in other fields of medicine, development of new medications for management of neuropathic pain has been difficult since preclinical rodent models do not necessarily translate to the clinics. Aside from ongoing pain with burning or shock‐like qualities, neuropathic pain is often characterized by pain hypersensitivity (hyperalgesia and allodynia), most often towards mechanical stimuli, reflecting sensitization of neural transmission.

Data treatment

We therefore performed a systematic literature review (PubMed‐Medline, Cochrane, WoS, ClinicalTrials) and semi‐quantitative meta‐analysis of human pain models that aim to induce central sensitization, and generate hyperalgesia surrounding a real or simulated injury.

Results

From an initial set of 1569 reports, we identified and analysed 269 studies using more than a dozen human models of sensitization. Five of these models (intradermal or topical capsaicin, low‐ or high‐frequency electrical stimulation, thermode‐induced heat‐injury) were found to reliably induce secondary hyperalgesia to pinprick and have been implemented in multiple laboratories. The ability of these models to induce dynamic mechanical allodynia was however substantially lower. The proportion of subjects who developed hypersensitivity was rarely provided, giving rise to significant reporting bias. In four of these models pharmacological profiles allowed to verify similarity to some clinical conditions, and therefore may inform basic research for new drug development.

Conclusions

While there is no single “optimal” model of central sensitization, the range of validated and easy‐to‐use procedures in humans should be able to inform preclinical researchers on helpful potential biomarkers, thereby narrowing the translation gap between basic and clinical data.

Significance

Being able to mimic aspects of pathological pain directly in humans has a huge potential to understand pathophysiology and provide animal research with translatable biomarkers for drug development. One group of human surrogate models has proven to have excellent predictive validity: they respond to clinically active medications and do not respond to clinically inactive medications, including some that worked in animals but failed in the clinics. They should therefore inform basic research for new drug development.

Tenderness of the Skin after Chemical Stimulation of Underlying Temporal and Thoracolumbar Fasciae Reveals Somatosensory Crosstalk between Superficial and Deep Tissues

Musculoskeletal pain is often associated with pain referred to adjacent areas or skin. So far, no study has analyzed the somatosensory changes of the skin after the stimulation of different underlying fasciae. The current study aimed to investigate heterotopic somatosensory crosstalk between deep tissue (muscle or fascia) and superficial tissue (skin) using two established models of deep tissue pain (namely focal high frequency electrical stimulation (HFS) (100 pulses of constant current electrical stimulation at 10× detection threshold) or the injection of hypertonic saline in stimulus locations as verified using ultrasound). In a methodological pilot experiment in the TLF, different injection volumes of hypertonic saline (50–800 µL) revealed that small injection volumes were most suitable, as they elicited sufficient pain but avoided the complication of the numbing pinprick sensitivity encountered after the injection of a very large volume (800 µL), particularly following muscle injections. The testing of fascia at different body sites revealed that 100 µL of hypertonic saline in the temporal fascia and TLF elicited significant pinprick hyperalgesia in the overlying skin (–26.2% and –23.5% adjusted threshold reduction, p < 0.001 and p < 0.05, respectively), but not the trapezius fascia or iliotibial band. Notably, both estimates of hyperalgesia were significantly correlated (r = 0.61, p < 0.005). Comprehensive somatosensory testing (DFNS standard) revealed that no test parameter was changed significantly following electrical HFS. The experiments demonstrated that fascia stimulation at a sufficient stimulus intensity elicited significant across-tissue facilitation to pinprick stimulation (referred hyperalgesia), a hallmark sign of nociceptive central sensitization.

An intensity matched comparison of laser- and contact heat evoked potentials

Previous studies comparing laser (LEPs) and contact heat evoked potentials (CHEPs) consistently reported higher amplitudes following laser compared to contact heat stimulation. However, none of the studies matched the perceived pain intensity, questioning if the observed difference in amplitude is due to biophysical differences between the two methods or a mismatch in stimulation intensity. The aims of the current study were twofold: (1) to directly compare the brain potentials induced by intensity matched laser and contact heat stimulation and (2) investigate how capsaicin-induced secondary hyperalgesia modulates LEPs and CHEPs. Twenty-one healthy subjects were recruited and measured at four experimental sessions: (1) CHEPs + sham, (2) LEPs + sham, (3) CHEPs + capsaicin, and (4) LEPs + capsaicin. Baseline (sham) LEPs latency was significantly shorter and amplitude significantly larger compared to CHEPs, even when matched for perceived pain. Neither CHEPs nor LEPs was sensitive enough to detect secondary hyperalgesia. These differences provide evidence that a faster heating rate results in an earlier and more synchronized LEPs than CHEPs. To our knowledge, this was the first study to match perceived intensity of contact heat and laser stimulations, revealing distinct advantages associated with the acquisition of LEPs.

Pain perception and processing in individuals with posttraumatic stress disorder: a systematic review with meta-analysis

Supplemental Digital Content is Available in the Text.

Posttraumatic stress disorder encompasses latent subgroups of individuals with qualitative differences in pain perception.

Posttraumatic stress disorder (PTSD) is a known risk factor for the development of chronic pain conditions, and almost 1 in 5 individuals with chronic pain fulfills the criteria for PTSD. However, the relationship between PTSD and pain is poorly understood and studies on pain perception in patients with PTSD show inconsistent results suggesting that different sensory profiles exist among individuals with PTSD. Here, we (1) systematically summarize the current literature on experimentally evoked pain perception in patients with PTSD compared to subjects without PTSD, and (2) assess whether the nature of the traumatic event is associated with different patterns in pain perception. The main outcome measures were pain threshold, pain tolerance, and pain intensity ratings as well as measures of temporal summation of pain and conditioned pain modulation. A systematic search of MEDLINE, EMBASE, Web of Science, PsycINFO, and CINAHL identified 21 studies for the meta-analysis, including 422 individuals with PTSD and 496 PTSD-free controls. No main effect of PTSD on any outcome measure was found. However, stratification according to the nature of trauma revealed significant differences of small to medium effect sizes. Combat-related PTSD was associated with increased pain thresholds, whereas accident-related PTSD was associated with decreased pain thresholds. No clear relationship between PTSD and experimentally evoked pain perception exists. The type of trauma may affect pain thresholds differently indicating the presence of different subgroups with qualitative differences in pain processing.

The Mysteries of Capsaicin-Sensitive Afferents

A fundamental subdivision of nociceptive sensory neurons is named after their unique sensitivity to capsaicin, the pungent ingredient in hot chili peppers: these are the capsaicin-sensitive afferents. The initial excitation by capsaicin of these neurons manifested as burning pain sensation is followed by a lasting refractory state, traditionally referred to as "capsaicin desensitization," during which the previously excited neurons are unresponsive not only to capsaicin but a variety of unrelated stimuli including noxious heat. The long sought-after capsaicin receptor, now known as TRPV1 (transient receptor potential cation channel, subfamily V member 1), was cloned more than two decades ago. The substantial reduction of the inflammatory phenotype of Trpv1 knockout mice has spurred extensive efforts in the pharmaceutical industry to develop small molecule TRPV1 antagonists. However, adverse effects, most importantly hyperthermia and burn injuries, have so far prevented any compounds from progressing beyond Phase 2. There is increasing evidence that these limitations can be at least partially overcome by approaches outside of the mainstream pharmaceutical development, providing novel therapeutic options through TRPV1. Although ablation of the whole TRPV1-expressing nerve population by high dose capsaicin, or more selectively by intersectional genetics, has allowed researchers to investigate the functions of capsaicin-sensitive afferents in health and disease, several "mysteries" remain unsolved to date, including the molecular underpinnings of "capsaicin desensitization," and the exact role these nerves play in thermoregulation and heat sensation. This review tries to shed some light on these capsaicin mechanisms.

Modulation of Muscle Pain Is Not Somatotopically Restricted: An Experimental Model Using Concurrent Hypertonic-Normal Saline Infusions in Humans

We have previously shown that during muscle pain induced by infusion of hypertonic saline (HS), concurrent application of vibration and gentle brushing to overlying and adjacent skin regions increases the overall pain. In the current study, we focused on muscle-muscle interactions and tested whether HS-induced muscle pain can be modulated by innocuous/sub-perceptual stimulation of adjacent, contralateral, and remote muscles. Psychophysical observations were made in 23 healthy participants. HS (5%) was infused into a forearm muscle (flexor carpi ulnaris) to produce a stable baseline pain. In separate experiments, in each of the three test locations (n = 10 per site)—ipsilateral hand (abductor digiti minimi), contralateral forearm (flexor carpi ulnaris), and contralateral leg (tibialis anterior)—50 μl of 0.9% normal saline (NS) was infused (in triplicate) before, during, and upon cessation of HS-induced muscle pain in the forearm. In the absence of background pain, the infusion of NS was imperceptible to all participants. In the presence of HS-induced pain in the forearm, the concurrent infusion of NS into the ipsilateral hand, contralateral forearm, and contralateral leg increased the overall pain by 16, 12, and 15%, respectively. These effects were significant, reproducible, and time-locked to NS infusions. Further, the NS-evoked increase in pain was almost always ascribed to the forearm where HS was infused with no discernible percept attributed to the sites of NS infusion. Based on these observations, we conclude that intramuscular infusion of HS results in muscle hyperalgesia to sub-perceptual stimulation of muscle afferents in a somatotopically unrestricted manner, indicating the involvement of a central (likely supra-spinal) mechanism.

Mechanical detection and pain thresholds: comparability of devices using stepped and ramped stimuli

Nylon monofilaments may substitute for German Research Network on Neuropathic Pain standards in tactile but not pinprick testing. Ramped stimuli are faster but underestimate thresholds due to reaction time artefacts.

Quantitative sensory testing is used to assess somatosensory function in humans. The protocol of the German Research Network on Neuropathic Pain (DFNS) provides comprehensive normative values using defined tools; however, some of these may not be feasible in low-resource settings.

A Quantitative Sensory Testing Approach to Pain in Autism Spectrum Disorders

Sensory abnormalities in autism has been noted clinically, with pain insensitivity as a specified diagnostic criterion. However, there is limited research using psychophysically robust techniques. Thirteen adults with ASD and 13 matched controls completed an established quantitative sensory testing (QST) battery, supplemented with measures of pain tolerance and central modulation. The ASD group showed higher thresholds for light touch detection and mechanical pain. Notably, the ASD group had a greater range of extreme scores (the number of z-scores outside of the 95% CI > 2), dynamic mechanical allodynia and paradoxical heat sensation; phenomena not typically seen in neurotypical individuals. These data support the need for research examining central mechanisms for pain in ASD and greater consideration of individual difference.

Central sensitization of nociceptive pathways demonstrated by robot-controlled pinprick-evoked brain potentials

OBJECTIVE

The aim of this study was to assess the effect of central sensitization, induced by high frequency electrical stimulation of the skin (HFS), on pinprick-evoked brain potentials (PEPs) using robot-controlled mechanical pinprick stimulation and a stimulus evaluation task.

METHODS

In 16 healthy volunteers HFS was applied to the right volar forearm. Robot- controlled pinprick stimuli (64 mN) were applied before and 20 minutes after HFS to the skin surrounding the area onto which HFS was applied. During pinprick stimulation, the EEG was recorded and the quality of perception and perceived intensity of the pinprick stimuli was collected.

RESULTS

After HFS, the skin surrounding the site at which HFS was delivered showed increased mechanical pinprick sensitivity. Both the early-latency negative peak of PEPs and the later-latency peak were significantly increased after HFS.

CONCLUSIONS

This study shows increased PEPs after HFS when they are elicited by a robot-controlled mechanical pinprick stimulator and participants are engaged in a stimulus evaluation task during pinprick stimulation.

SIGNIFICANCE

This is the first study that shows a significant increase of both PEP peaks, and therefore, it provides a preferred setup for assessing the function of mechanical nociceptive pathways in the context of central sensitization.

Psychophysical characterisation of burning mouth syndrome-A systematic review and meta-analysis

BACKGROUND

Primary burning mouth syndrome (BMS) is an oro-facial disease with neuropathic characteristics. Psychophysics, such as quantitative sensory testing (QST), is used to sub-classify neuropathic pain syndromes, but their usefulness in characterising BMS is not yet clear.

OBJECTIVE

The aim of this study was to summarise and to quantitatively and qualitatively analyse the available information about QST findings in BMS, and to reflect on possible mechanisms of disease.

METHODS

In this systematic review and meta-analysis, different search strategies were used to screen for articles in PubMed, Embase, EBSCOhost, Cochrane Library, Web of Science, Google Scholar and two sources of conference abstracts. Primary clinical studies focused on QST assessment in patients with BMS were included. Data were synthesised qualitatively and quantitatively. Risk of bias was assessed following the AHRQ guidelines.

RESULTS

Thirteen articles with low to moderate risk of bias and one conference abstract were selected from 45 unique articles that were identified. Individually, the studies reported combinations of thermal and mechanical sensory impairments measured by QST. The meta-analysis showed significant sensory differences between patients and controls in warmth (effect size = 0.683; P < .05) and cold detection thresholds (effect size = -0.580; P < .001).

CONCLUSION

The results indicate that thermal sensitivity seems to be altered in patients with BMS compared to controls, suggesting a small-fibre neuropathy. However, study protocols were highly variable and heterogeneous. Therefore, studies with better designs and complete reporting of results should be performed to bring value to the use of psychophysics in the assessment of BMS.

Association of quantitative sensory testing parameters with clinical outcome in patients with lumbar radiculopathy undergoing microdiscectomy

Background/Aim

This study aimed to establish the somatosensory profile of patients with lumbar radiculopathy at pre‐and post‐microdiscectomy and to explore any association between pre‐surgical quantitative sensory test (QST) parameters and post‐surgical clinical outcomes.

Methods

A standardized QST protocol was performed in 53 patients (mean age 38 ± 11 years, 26 females) with unilateral L5/S1 radiculopathy in the main pain area (MPA), affected dermatome and contralateral mirror sites and in age‐ and gender‐,and body site‐matched healthy controls. Repeat measures at 3 months included QST, the Oswestry Disability Index (ODI) and numerous other clinical measures; at 12 months, only clinical measures were repeated. A change <30% on the ODI was defined as ‘no clinically meaningful improvement’.

Results

Patients showed a significant loss of function in their symptomatic leg both in the dermatome (thermal, mechanical, vibration detection p < .002), and MPA (thermal, mechanical, vibration detection, mechanical pain threshold, mechanical pain sensitivity p < .041) and increased cold sensitivity in the MPA (p < .001). Pre‐surgical altered QST parameters improved significantly post‐surgery in the dermatome (p < .018) in the symptomatic leg and in the MPA (p < .010), except for thermal detection thresholds and cold sensitivity. Clinical outcomes improved at 3 and 12 months (p < .001). Seven patients demonstrated <30% change on the ODI at 12 months. Baseline loss of function in mechanical detection in the MPA was associated with <30% change on the ODI at 12 months (OR 2.63, 95% CI 1.09–6.37, p = .032).

Conclusion

Microdiscectomy resulted in improvements in affected somatosensory parameters and clinical outcomes. Pre‐surgical mechanical detection thresholds may be predictive of clinical outcome.

Significance

This study documented quantitative sensory testing (QST) profiles in patients with lumbar radiculopathy in their main pain area (MPA) and dermatome pre‐ and post‐microdiscectomy and explored associations between QST parameters and clinical outcome. Lumbar radiculopathy was associated with loss of function in modalities mediated by large and small sensory fibres. Microdiscectomy resulted in significant improvements in loss of function and clinical outcomes in 85% of our cohort. Pre‐surgical mechanical detection thresholds in the MPA may be predictive of clinical outcome.

Within- and between-session reliability of secondary hyperalgesia induced by electrical high-frequency stimulation

BACKGROUND

An increasing number of studies are focusing on secondary hyperalgesia to better understand central sensitization, as this phenomenon may play an important role in persistent pain. Recent studies have shown that, compared to the classical high-frequency stimulation protocol (HFS) at 100 Hz, a protocol using 42 Hz stimulation induces a more intense and a larger area of secondary hyperalgesia (SH).

OBJECTIVES

The aim of this study was to investigate the within- and between-session reliability of SH induced by this optimized HFS protocol.

METHODS

Thirty-two healthy subjects received HFS to their volar forearm in two sessions, separated by at least 2 weeks. SH was assessed by measuring the area size of increased sensitivity to pinprick stimuli after applying HFS, the sensitivity to pinprick stimuli after applying HFS and the change in pinprick sensitivity after versus before HFS. Assessments were made before HFS, and 30, 35 and 40 min after HFS. Relative and absolute reliability were analysed using intraclass correlation coefficients (ICCs), coefficients of variation (CVs), standard error of means (SEMs) and the minimum detectable changes (MDCs).

RESULTS

The area of SH showed good to excellent within-session and between-session relative reliability (ICCs > 0.80), except for the change in pinprick sensitivity, which showed close to poor between-session relative reliability (ICC = 0.53). Furthermore, measures of absolute reliability generally demonstrated large between-subject variability and significant fluctuations across repeated measurements.

CONCLUSIONS

HFS-induced hyperalgesia is suitable to discriminate or compare individuals but it may not be sensitive to changes due to an intervention.

SIGNIFICANCE

It is crucial to evaluate central sensitization adequately in humans. This study formally establishes the reliability of secondary hyperalgesia induced by electrical high-frequency stimulation. The results of this study will improve future studies investigating secondary hyperalgesia in humans.

Capsaicin-Induced Changes in Electrical Pain Perception Threshold Can Be Used to Assess the Magnitude of Secondary Hyperalgesia in Humans

Objectives

Areas of secondary hyperalgesia can be assessed using quantitative sensory testing (QST). Delivering noxious electrocutaneous stimulation could provide added benefit by allowing multiple measurements of the magnitude of hyperalgesia. We aimed to characterize the use of electrical pain perception (EPP) thresholds alongside QST as a means by which to measure changes in pain thresholds within an area of secondary mechanical hyperalgesia.

Methods

EPP and heat pain thresholds (HPTs) were measured at five distinct points at baseline and following 1% capsaicin cream application, one within a central zone and four within a secondary zone. Areas of secondary mechanical hyperalgesia were mapped using QST. In a further 14 participants, capsaicin-induced reduction in EPP thresholds was mapped using a radial lines approach across 24 points.

Results

There was a reduction in EPP threshold measured at the four points within the secondary zone, which was within the mapped area of mechanical secondary hyperalgesia. The magnitude of secondary hyperalgesia could be split into a mild (∼4% reduction) and severe (∼21% reduction) area within an individual subject. There was no reduction in HPT within the secondary zone, but there was a reduction in both HPT and EPP threshold within the primary zone. EPP mapping revealed differences in the magnitude and spread of hyperalgesia across all subjects.

Conclusions

Measuring capsaicin-induced reduction in EPP thresholds can be used to map hyperalgesic areas in humans. This semi-automated approach allows rapid assessment of the magnitude of hyperalgesia, both within an individual subject and across a study population.

Sex differences in measures of central sensitization and pain sensitivity to experimental sleep disruption: implications for sex differences in chronic pain

Study Objectives

Females demonstrate heightened central sensitization (CS), a risk factor for chronic pain characterized by enhanced responsivity of central nervous system nociceptors to normal or subthreshold input. Sleep disruption increases pain sensitivity, but sex has rarely been evaluated as a moderator and few experiments have measured CS. We evaluated whether two nights of sleep disruption alter CS measures of secondary hyperalgesia and mechanical temporal summation in a sex-dependent manner. We also evaluated differences in measures of pain sensitivity.

Methods

Seventy-nine healthy adults (female n = 46) participated in a randomized crossover experiment comparing two consecutive nights of eight pseudorandomly distributed forced awakenings (FA [-200 min sleep time]) against two nights of undisturbed sleep (US). We conducted sensory testing the mornings following Night 2; the heat-capsaicin pain model was used to induce secondary hyperalgesia.

Results

FA reduced total sleep time (REM and NREM Stage 3) more profoundly in males. We observed divergent, sex-dependent effects of FA on secondary hyperalgesia and temporal summation. FA significantly increased secondary hyperalgesia in males and significantly increased temporal summation in females. Sex differences were not attributable to differential sleep loss in males. FA also significantly reduced heat-pain threshold and cold pressor pain tolerance, independently of sex.

Conclusions

Sleep disruption enhances different pain facilitatory measures of CS in males and females suggesting that sleep disturbance may increase risk for chronic pain in males and females via distinct pathways. Findings have implications for understanding sex differences in chronic pain and investigating sleep in chronic pain prevention efforts.

Central Sensitisation and functioning in patients with chronic low back pain: protocol for a cross-sectional and cohort study

INTRODUCTION

A relevant subsample of patients with chronic low back pain (CLBP) have manifested augmented central pain processing, central sensitisation (CS). Patients with CLBP have limited functioning and participation. Theoretically, physical functioning in patients with CLBP can plausibly be linked to CS; however, evidence to explain such association is scarce. Moreover, there is no gold standard for CS diagnosis. The objectives of the study are: (1) to analyse the association between instruments assessing reference symptoms and signs attributed to CS; (2) to analyse whether reference symptoms and signs attributed to CS are associated with functioning measurement outcomes; and (3) to analyse whether changes (between baseline and discharge) in reference symptoms and signs attributed to CS are related to changes in each of the functioning measurement outcomes.

METHODS AND ANALYSIS

A cross-sectional and longitudinal observational study is performed with measurements taken at baseline and discharge of an interdisciplinary rehabilitation programme. A sample size of 110 adult patients with CLBP has been calculated for the study. CS measurements are: Central Sensitisation Inventory, quantitative sensory testing and heart rate variability. Functioning measurements are: lifting capacity, maximal aerobic capacity, accelerometry and reported functioning. Statistical analyses to be performed are: (1) correlation between CS measurements, (2) multiple regression between functioning (dependent variable) and CS measurements (independent variable), and (3) multiple regression between changes in scores of functioning (dependent variable) and CS measurements (independent variable), and corrected for sex and age.

ETHICS AND DISSEMINATION

The study obtained the clearance to its implementation from the Medical Research Ethics Committee of the University Medical Center Groningen in July 2017. The results will be disseminated through scientific publications in peer-reviewed journals, presentations at relevant conferences, and reports to stakeholders.

TRIAL REGISTRATION NUMBER

NTR7167/NL6980.

Association between pain phenotype and disease activity in rheumatoid arthritis patients: a non-interventional, longitudinal cohort study

BACKGROUND

In well-controlled rheumatoid arthritis (RA) without significant joint damage, a substantial proportion of patients complain of persistent pain. Previous studies have identified different pain phenotypes in RA, in which non-nociceptive pain phenotypes are associated with higher concurrent disease activity scores. In this longitudinal study, we explored associations between pain phenotypes and long-term disease activity outcome in RA patients. Secondly, we explored whether pain phenotype is associated with comorbid conditions.

METHODS

One hundred eighty established RA patients were classified with a nociceptive (61%) or a non-nociceptive (39%) pain phenotype, based on their responses to the painDETECT-questionnaire. Two years of clinical follow-up data on disease activity outcomes were collected. Information on comorbid diseases was derived from electronic patient files.

RESULTS

Patients with a non-nociceptive pain phenotype showed higher mean disease activity scores (DAS28, 2.57; 95% CI, 2.37-2.77 vs. 2.11; 95% CI, 1.94-2.27; p < 0.001) and a twofold lower chance of achieving sustained DAS28 remission (OR = 0.49; 95% CI, 0.26-0.92; p = 0.020). Only the tender joint count and patient global health significantly differed between the pain phenotype groups. Patients with a non-nociceptive pain phenotype had more often been diagnosed with concurrent fibromyalgia (9.9% vs. 0.9%; p = 0.007) and other pain-associated comorbid diseases (52.1% vs. 35.8%; p = 0.030) compared with patients with a nociceptive pain phenotype.

CONCLUSION

This longitudinal study showed consistently worse long-term disease activity outcomes in RA patients with a non-nociceptive pain phenotype which appeared to be mainly due to differences in the subjective components of the disease activity score.

TRIAL REGISTRATION

The DREAM cohort study is registered in the Netherlands Trial Register: NTR578.

Attenuation of capsaicin-induced ongoing pain and secondary hyperalgesia during exposure to an immersive virtual reality environment

Introduction:

There is growing evidence that virtual reality (VR) can be used in the treatment of chronic pain conditions. However, further research is required to better understand the analgesic mechanisms during sensitised pain states.

Objectives:

We examined the effects of an immersive polar VR environment on capsaicin-induced ongoing pain and secondary hyperalgesia. We also investigated whether the degree of analgesia was related to baseline conditioned pain modulation (CPM) responses.

Methods:

Nineteen subjects had baseline CPM and electrical pain perception (EPP) thresholds measured before the topical application of capsaicin cream. Visual analogue scale ratings were measured to track the development of an ongoing pain state, and EPP thresholds were used to measure secondary hyperalgesia. The effects of a passive polar VR environment on ongoing pain and secondary hyperalgesia were compared with sham VR (ie, 2D monitor screen) in responders to capsaicin (n = 15).

Results:

Virtual reality was associated with a transient reduction in ongoing pain and an increase in EPP thresholds in an area of secondary hyperalgesia. Baseline CPM measurements showed a significant correlation with VR-induced changes in secondary hyperalgesia, but not with VR-induced changes in ongoing pain perception. There was no correlation between VR-induced changes in pain perception and VR-induced changes in secondary hyperalgesia.

Conclusion:

Virtual reality can reduce the perception of capsaicin-induced ongoing pain and secondary hyperalgesia. We also show that CPM may provide a means by which to identify individuals likely to respond to VR therapy.

Childhood Adversity and Pain Facilitation

OBJECTIVE

This study investigated whether childhood adversity would be associated with hypersensitivity on two measures of central pain facilitation: area of secondary allodynia and temporal summation of second pain (TSSP), and whether pain facilitation would be explained by adult posttraumatic stress disorder (PTSD) symptoms.

METHOD

Participants endorsing high (n = 31) and low (n = 31) childhood adversity underwent capsaicin-induced secondary allodynia and TSSP testing. The tests were conducted a week apart with test order counterbalanced.

RESULTS

Larger areas of secondary allodynia were observed in the high adversity group compared with the low adversity group (F(1,60) = 4.81, p = .032). This group difference was largely (62%) explained by greater PTSD symptoms in the high adversity group. Although no overall difference was found in TSSP slopes (p = .886), this was attributed to an order by group interaction (F(1,58) = 5.07, p = .028) and low power. Subsequent analyses revealed positive TSSP slopes in the high adversity group when TSSP testing was performed first, and this order effect was associated with blunted sympathetic responses to TSSP on the first visit. The two facilitation measures were unrelated (p = .631).

CONCLUSIONS

Larger areas of secondary allodynia were observed in the high adversity group, which was explained largely by PTSD symptoms. This suggests that adversity-related changes in pain facilitation may underlie the association between childhood adversity and generalized widespread pain. Although TSSP was affected by previous testing, adversity-related pain facilitation was observed when TSSP testing occurred first. Finally, adversity was not associated with a consistent pattern of hypersensitivity across the two measures of central pain facilitation.

A systematic review of experimental methods to manipulate secondary hyperalgesia in humans: protocol

BACKGROUND

Neuropathic pain affects 7-10% of people, but responds poorly to pharmacotherapy, indicating a need for better treatments. Mechanistic research on neuropathic pain frequently uses human surrogate models of the secondary hyperalgesia that is a common feature of neuropathic pain. Experimentally induced secondary hyperalgesia has been manipulated with pharmacological and non-pharmacological methods to clarify the relative contributions of different mechanisms to secondary hyperalgesia. However, this literature has not been systematically synthesised. The aim of this systematic review is to identify, describe, and compare methods that have been used to manipulate experimentally induced secondary hyperalgesia in healthy humans.

METHODS

A systematic search strategy will be supplemented by reference list checks and direct contact with identified laboratories to maximise the identification of data reporting the experimental manipulation of experimentally induced secondary hyperalgesia in healthy humans. Duplicated screening, risk of bias assessment, and data extraction procedures will be used. Authors will be asked to provide data as necessary. Data will be pooled and meta-analyses conducted where possible, with subgrouping according to manipulation method. Manipulation methods will be ranked for potency and risk.

DISCUSSION

The results of this review will provide a useful reference for researchers interested in using experimental methods to manipulate secondary hyperalgesia in humans and will help to clarify the relative contributions of different mechanisms to secondary hyperalgesia.

SYSTEMATIC REVIEW REGISTRATION

This protocol will be registered on PROSPERO before the review begins. Review records will be updated on PROSPERO once the review is complete. This review is intended for publication in a peer-reviewed journal. Analyses and scripts will be made publicly available.

Birth experience in newborn infants is associated with changes in nociceptive sensitivity

Vaginal birth prepares the fetus for postnatal life. It confers respiratory, cardiovascular and homeostatic advantages to the newborn infant compared with elective cesarean section, and is reported to provide neonatal analgesia. We hypothesize that infants born by vaginal delivery will show lower noxious-evoked brain activity a few hours after birth compared to those born by elective cesarean section. In the first few hours of neonatal life, we record electrophysiological measures of noxious-evoked brain activity following the application of a mildly noxious experimental stimulus in 41 infants born by either vaginal delivery or by elective cesarean section. We demonstrate that noxious-evoked brain activity is related to the mode of delivery and significantly lower in infants born by vaginal delivery compared with those born by elective cesarean section. Furthermore, we found that the magnitude of noxious-evoked brain activity is inversely correlated with fetal copeptin production, a surrogate marker of vasopressin, and dependent on the experience of birth-related distress. This suggests that nociceptive sensitivity in the first few hours of postnatal life is influenced by birth experience and endogenous hormonal production.

SIGMA-1 Receptor Gene Variants Affect the Somatosensory Phenotype in Neuropathic Pain Patients

Pain sensitivity is characterized by interindividual variability, determined by factors including genetic variation of nociceptive receptors and pathways. The sigma-1 receptor (SIGMAR1) is involved in pain modulation especially under pre-sensitized conditions. However, the contribution of SIGMAR1 genetic variants to pain generation and sensitivity is unknown yet. This study aimed to identify effects of 5 SIGMAR1 variants on the somatosensory phenotype of neuropathic pain patients (n = 228) characterized by standardized quantitative sensory testing. Principal component analysis revealed that the SIGMAR1 variants -297G>T (rs10814130) and 5A>C (rs1800866) significantly lowered thermal detection and heat/pressure nociception in particular in neuropathic pain patients with mainly preserved somatosensory function. Compared to wild-type, the variant allele -297T was associated with loss of warm detection (P = .049), lower heat-pain sensitivity (P = .027) and wind-up ratio (P = .023) as well as increased paradoxical heat sensation (P = .020). Likewise for 5A>C the strongest genotype-associated differences observed were reduced peripheral (less heat hyperalgesia; P = .026) and central sensitization (lower mechanical pain sensitivity; P = .026) in variant compared to wild-type carriers. This study indicates lack of association of SIGMAR1 -297G>T and 5A>C genetic variants to susceptibility to develop chronic pain, but significant modulation of somatosensory function in neuropathic pain patients. PERSPECTIVE: This article presents the first study indicating a modulation of somatosensory function in neuropathic pain patients by selected genetic variants in SIGMAR1. As our findings could contribute to the explanation of interindividual differences in drug response they might help to improve the treatment of neuropathic pain.

Enhanced facilitation and diminished inhibition characterizes the pronociceptive endogenous pain modulatory balance of persons living with HIV and chronic pain

Chronic pain in persons living with HIV (PLWH) may be related to alterations in endogenous pain modulatory processes (e.g., high facilitation and low inhibition of nociception) that promote exaggerated pain responses, known as hyperalgesia, and central nervous system (CNS) sensitization. This observational study examined differences in endogenous pain modulatory processes between 59 PLWH with chronic pain, 51 PLWH without chronic pain, and 50 controls without HIV or chronic pain. Quantitative sensory testing for temporal summation (TS) of mechanical and heat pain as well as conditioned pain modulation (CPM) were used to assess endogenous pain facilitatory and inhibitory processes, respectively. Associations among TS, CPM, and self-reported clinical pain severity were also examined in PLWH with chronic pain. Findings demonstrated significantly greater TS of mechanical and heat pain for PLWH with chronic pain compared to PLWH without chronic pain and controls. CPM effects were present in controls, but not in either PLWH with or without chronic pain. Among PLWH with chronic pain, greater TS of mechanical pain was significantly associated with greater average clinical pain severity. Results of this study suggest that enhanced facilitation and diminished inhibition characterizes the pronociceptive endogenous pain modulatory balance of persons living with HIV and chronic pain.

Validation of an index of Sensitivity to Movement-Evoked Pain in patients with whiplash injuries

Introduction: Sensitivity to Movement-Evoked Pain is a pain summation phenomenon identified in various chronic pain populations. Objectives: This study investigated the validity of a procedure used to assess pain summation in response to a repeated lifting task in individuals with whiplash injuries. Methods: Sixty-five participants completed measures of pain severity and duration, Temporal Summation (TS) of pinprick pain, pain catastrophizing and fear of movement, and work-related disability before lifting a series of 18 weighted canisters. An index of Sensitivity to Movement-Evoked Pain was computed as the increase in pain reported by participants over successive lifts of the weighted canisters. An index of TS was computed by dividing the pain reported in response to the final pinprick by the pain reported in response to the 1st pinprick in a train of 10 pinpricks. Results: Analyses replicated previous findings showing a repetitive lifting task-induced pain summation in approximately 20% to 25% of a sample of individuals with whiplash injuries. Analyses also revealed significant correlations between SMEP, TS, and pain-related psychological variables. Hierarchical regression analyses showed that TS and pain catastrophizing made significant unique contributions to the prediction of SMEP. These findings join a growing body of research on movement-evoked pain in persistent spinal pain conditions. Conclusion: The repeated lifting task used in this study successfully induced pain summation in a group of patients with whiplash injuries.

Novel capsaicin-induced parameters of microcirculation in migraine patients revealed by imaging photoplethysmography

Background

The non-invasive biomarkers of migraine can help to develop the personalized medication of this disorder. In testing of the antimigraine drugs the capsaicin-induced skin redness with activated TRPV1 receptors in sensory neurons associated with the release of the migraine mediator CGRP has already been widely used.

Methods

Fourteen migraine patients (mean age 34.6 ± 10.2 years) and 14 healthy volunteers (mean age 29.9 ± 9.7 years) participated in the experiment. A new arrangement of imaging photoplethysmography recently developed by us was used here to discover novel sensitive parameters of dermal blood flow during capsaicin applications in migraine patients.

Results

Blood pulsation amplitude (BPA) observed as optical-intensity waveform varying synchronously with heartbeat was used for detailed exploration of microcirculatory perfusion induced by capsicum patch application. The BPA signals, once having appeared after certain latent period, were progressively rising until being saturated. Capsaicin-induced high BPA areas were distributed unevenly under the patch, forming “hot spots.” Interestingly the hot spots were much more variable in migraine patients than in the control group. In contrast to BPA, a slow component of waveforms related to the skin redness changed significantly less than BPA highlighting the latter parameter as the potential sensitive biomarker of capsaicin-induced activation of the blood flow. Thus, in migraine patients, there is a non-uniform (both in space and in time) reaction to capsaicin, resulting in highly variable openings of skin capillaries.

Conclusion

BPA dynamics measured by imaging photoplethysmography could serve as a novel sensitive non-invasive biomarker of migraine-associated changes in microcirculation.

Reactive oxygen species affect spinal cell type-specific synaptic plasticity in a model of neuropathic pain

Spinal synaptic plasticity is believed to drive central sensitization that underlies the persistent nature of neuropathic pain. Our recent data showed that synaptic plasticity in the dorsal horn is cell type specific: intense afferent stimulation produced long-term potentiation (LTP) in excitatory spinothalamic tract neurons (STTn), whereas it produced long-term depression (LTD) in inhibitory GABAergic interneurons (GABAn). In addition, reactive oxygen species (ROS) were shown to be involved in LTP in STTn (STTn-LTP) and in LTD in GABAn (GABAn-LTD). This study examined the roles of 2 biologically important ROS--superoxide [·O2] and hydroxyl radicals [·OH]--in neuropathic mechanical hyperalgesia and cell type-specific spinal synaptic plasticity. The [·O2] donor induced stronger mechanical hyperalgesia than the [·OH] donor in naive mice. The [·O2] scavenger showed greater antihyperalgesic effect than [·OH] scavengers in the spinal nerve ligation (SNL) mouse model of neuropathic pain. In addition, the [·O2] donor induced both STTn-LTP and GABAn-LTD, but the [·OH] donor induced only GABAn-LTD. On the other hand, the [·O2] scavenger inhibited STTn-LTP and GABAn-LTD induction in naive mice and alleviated SNL-induced potentiation in STTn and depression in GABAn. The [·OH] scavenger, however, inhibited depression in GABAn but did not interfere with potentiation in STTn. These results indicate that mechanical hyperalgesia in SNL mice is the result of the combination of STTn-LTP and GABAn-LTD. Behavioral outcomes compliment electrophysiological results which suggest that [·O2] mediates both STTn-LTP and GABAn-LTD, whereas [·OH] is involved primarily in GABAn-LTD.