Peripheral sensitization reduces laser-evoked potential habituation

Assessing signs of central sensitization: A critical review of physiological measures in experimentally induced secondary hyperalgesia

BACKGROUND AND OBJECTIVES

Central sensitization (CS) is believed to play a role in many chronic pain conditions. Direct non-invasive recording from single nociceptive neurons is not feasible in humans, complicating CS establishment. This review discusses how secondary hyperalgesia (SHA), considered a manifestation of CS, affects physiological measures in healthy individuals and if these measures could indicate CS. It addresses controversies about heat sensitivity changes, the role of tactile afferents in mechanical hypersensitivity and detecting SHA through electrical stimuli. Additionally, it reviews the potential of neurophysiological measures to indicate CS presence.

DATABASES AND DATA TREATMENT

Four databases, PubMed, ScienceDirect, Scopus and Cochrane Library, were searched using terms linked to 'hyperalgesia'. The search was limited to research articles in English conducted in humans until 2023.

RESULTS

Evidence for heat hyperalgesia in the SHA area is sparse and seems to depend on the experimental method used. Minimal or no involvement of tactile afferents in SHA was found. At the spinal level, the threshold of the nociceptive withdrawal reflex (RIII) is consistently reduced during experimentally induced SHA. The RIII area and the spinal somatosensory potential (N13-SEP) amplitude are modulated only with long-lasting nociceptive input. At the brain level, pinprick-evoked potentials within the SHA area are increased.

CONCLUSIONS

Mechanical pinprick hyperalgesia is the most reliable behavioural readout for SHA, while the RIII threshold is the most sensitive neurophysiological readout. Due to scarce data on reliability, sensitivity and specificity, none of the revised neurophysiological methods is currently suitable for CS identification at the individual level.

SIGNIFICANCE

Gathering evidence for CS in humans is a crucial research focus, especially with the increasing interest in concepts such as 'central sensitization-like pain' or 'nociplastic pain'. This review clarifies which readouts, among the different behavioural and neurophysiological proxies tested in experimental settings, can be used to infer the presence of CS in humans.

Habituation to pain: self-report, electroencephalography, and functional magnetic resonance imaging in healthy individuals. A scoping review and future recommendations

ABSTRACT

Habituation to pain is a fundamental learning process and important adaption. Yet, a comprehensive review of the current state of the field is lacking. Through a systematic search, 63 studies were included. Results address habituation to pain in healthy individuals based on self-report, electroencephalography, or functional magnetic resonance imaging. Our findings indicate a large variety in methods, experimental settings, and contexts, making habituation a ubiquitous phenomenon. Habituation to pain based on self-report studies shows a large influence of expectations, as well as the presence of individual differences. Furthermore, widespread neural effects, with sometimes opposing effects in self-report measures, are noted. Electroencephalography studies showed habituation of the N2-P2 amplitude, whereas functional magnetic resonance imaging studies showed decreasing activity during painful repeated stimulation in several identified brain areas (cingulate cortex and somatosensory cortices). Important considerations for the use of terminology, methodology, statistics, and individual differences are discussed. This review will aid our understanding of habituation to pain in healthy individuals and may lead the way to improving methods and designs for personalized treatment approaches in chronic pain patients.

Evidence- and data-driven classification of low back pain via artificial intelligence: Protocol of the PREDICT-LBP study

In patients presenting with low back pain (LBP), once specific causes are excluded (fracture, infection, inflammatory arthritis, cancer, cauda equina and radiculopathy) many clinicians pose a diagnosis of non-specific LBP. Accordingly, current management of non-specific LBP is generic. There is a need for a classification of non-specific LBP that is both data- and evidence-based assessing multi-dimensional pain-related factors in a large sample size. The "PRedictive Evidence Driven Intelligent Classification Tool for Low Back Pain" (PREDICT-LBP) project is a prospective cross-sectional study which will compare 300 women and men with non-specific LBP (aged 18-55 years) with 100 matched referents without a history of LBP. Participants will be recruited from the general public and local medical facilities. Data will be collected on spinal tissue (intervertebral disc composition and morphology, vertebral fat fraction and paraspinal muscle size and composition via magnetic resonance imaging [MRI]), central nervous system adaptation (pain thresholds, temporal summation of pain, brain resting state functional connectivity, structural connectivity and regional volumes via MRI), psychosocial factors (e.g. depression, anxiety) and other musculoskeletal pain symptoms. Dimensionality reduction, cluster validation and fuzzy c-means clustering methods, classification models, and relevant sensitivity analyses, will classify non-specific LBP patients into sub-groups. This project represents a first personalised diagnostic approach to non-specific LBP, with potential for widespread uptake in clinical practice. This project will provide evidence to support clinical trials assessing specific treatments approaches for potential subgroups of patients with non-specific LBP. The classification tool may lead to better patient outcomes and reduction in economic costs.

Segmental Chiropractic Spinal Manipulation Does not Reduce Pain Amplification and the Associated Pain-Related Brain Activity in a Capsaicin-Heat Pain Model

Musculoskeletal injuries lead to sensitization of nociceptors and primary hyperalgesia (hypersensitivity to painful stimuli). This occurs with back injuries, which are associated with acute pain and increased pain sensitivity at the site of injury. In some cases, back pain persists and leads to central sensitization and chronic pain. Thus, reducing primary hyperalgesia to prevent central sensitization may limit the transition from acute to chronic back pain. It has been shown that spinal manipulation (SM) reduces experimental and clinical pain, but the effect of SM on primary hyperalgesia and hypersensitivity to painful stimuli remains unclear. The goal of the present study was to investigate the effect of SM on pain hypersensitivity using a capsaicin-heat pain model. Laser stimulation was used to evoke heat pain and the associated brain activity, which were measured to assess their modulation by SM. Eighty healthy participants were recruited and randomly assigned to one of the four experimental groups: inert cream and no intervention; capsaicin cream and no intervention; capsaicin cream and SM at T7; capsaicin cream and placebo. Inert or capsaicin cream (1%) was applied to the T9 area. SM or placebo were performed 25 min after cream application. A series of laser stimuli were delivered on the area of cream application (1) before cream application, (2) after cream application but before SM or placebo, and (3) after SM or placebo. Capsaicin cream induced a significant increase in laser pain (p < 0.001) and laser-evoked potential amplitude (p < 0.001). However, SM did not decrease the amplification of laser pain or laser-evoked potentials by capsaicin. These results indicate that segmental SM does not reduce pain hypersensitivity and the associated pain-related brain activity in a capsaicin-heat pain model.

Protective role of natural killer cells in neuropathic pain conditions

ABSTRACT

During the past few years, the research of chronic neuropathic pain has focused on neuroinflammation within the central nervous system and its impact on pain chronicity. As part of the ERA-Net NEURON consortium, we aimed to identify immune cell patterns in the cerebrospinal fluid (CSF) of patients with herpes zoster neuralgia and patients with polyneuropathy (PNP), which may contribute to pain chronicity in these neuropathic pain conditions. Cerebrospinal fluid of 41 patients (10 herpes zoster and 31 PNP) was analyzed by flow cytometry identifying lymphocyte subsets: CD4+ (T-helper cells), CD8+ (cytotoxic T cells), CD19+ (B cells), and CD56+ (natural killer [NK]) cells. At baseline and at follow-up, the somatosensory phenotype was assessed with quantitative sensory testing. In addition, the patients answered epidemiological questionnaires and the PainDETECT questionnaire. Immune cell profiles and somatosensory profiles, as well as painDETECT questionnaire scores, were analyzed and correlated to determine specific immune cell patterns, which contribute to chronic pain. We found a negative correlation (P = 0.004, r = -0.596) between the frequency of NK cells and mechanical pain sensitivity (MPS), one of the most relevant quantitative sensory testing markers for central sensitization; a high frequency of NK cells correlated with low MPS. The analysis of the individual follow-up showed a worsening of the pain condition if NK-cell frequency was low. Low NK-cell frequency is associated with signs of central sensitization (MPS), whereas high NK-cell frequency might prevent central sensitization. Therefore, NK cells seem to play a protective role within the neuroinflammatory cascade and may be used as a marker for pain chronicity.

Small nerve fiber selectivity of laser and intraepidermal electrical stimulation: A comparative study between glabrous and hairy skin

OBJECTIVES

In clinical neurophysiology practice, various methods of stimulation can be used to activate small-diameter nociceptive cutaneous afferents located in the epidermis. These methods include different types of laser and intraepidermal electrical stimulation techniques. The diffusion of the stimulation in the skin, inside or under the epidermis, depends on laser wavelength and electrode design, in particular. The aim of this study was to compare several of these techniques in their ability to selectively stimulate small nerve fibers.

METHODS

In 8 healthy subjects, laser stimulation (using a CO₂ or Nd:YAP laser) and intraepidermal electrical stimulation (using a micropatterned, concentric planar, or concentric needle electrode), were applied at increasing energy or intensity on the dorsal or volar aspect of the right hand or foot. The subjects were asked to define the perceived sensation (warm, pinprick, or electric shock sensation, corresponding to the activation of C fibers, Aδ fibers, or Aβ fibers, respectively) after each stimulation. Depending on the difference in the sensations perceived between dorsal (hairy skin with thin stratum corneum) and volar (glabrous skin with thick stratum corneum) stimulations, the diffusion of the stimulation inside or under the epidermis and the nature of the activated afferents were determined.

RESULTS

Regarding laser stimulation, the perceived sensations turned from warm to pinprick with increasing energies of stimulation, in particular with the Nd:YAP laser, of which pulse could penetrate deep in the skin according to its short wavelength. In contrast, CO₂ laser stimulation produced only warm sensations and no pricking sensation when applied to the glabrous skin, perhaps due to a thicker stratum corneum and the shallow penetration of the CO₂ laser pulse. Regarding intraepidermal electrical stimulation using concentric electrodes, the perceived sensations turned from pinprick to a combination of pinprick and electrical shocks with increasing intensities. Using the concentric planar electrode, the sensations perceived at high stimulation intensity even consisted of electric shocks without concomitant pinprick. In contrast, using the micropatterned electrode, only pinprick sensations were produced by the stimulation of the hairy skin, while the stimulation of the glabrous skin produced no sensation at all within the limits of stimulation intensities used in this study.

CONCLUSIONS

Using the CO₂ laser or the micropatterned electrode, pinprick sensations were selectively produced by the stimulation of hairy skin, while only warm sensation or no sensation at all were produced by the stimulation of glabrous skin. These two techniques appear to be more selective with a limited diffusion of the stimulation into the skin, restricting the activation of sensory afferents to the most superficial and smallest intraepidermal nerve fibers.

Chiropractic Spinal Manipulation Prevents Secondary Hyperalgesia Induced by Topical Capsaicin in Healthy Individuals

Background and Aims: Spinal manipulation (SM) is currently recommended for the management of back pain. Experimental studies indicate that the hypoalgesic mechanisms of SM may rely on inhibition of segmental processes related to temporal summation of pain and, possibly, on central sensitization, although this remains unclear. The aim of this study was to determine whether experimental back pain, secondary hyperalgesia, and pain-related brain activity induced by capsaicin are decreased by segmental SM. Methods: Seventy-three healthy volunteers were randomly allocated to one of four experimental groups: SM at T5 vertebral level (segmental), SM at T9 vertebral level (heterosegmental), placebo intervention at T5 vertebral level, or no intervention. Topical capsaicin was applied to the area of T5 vertebra for 40 min. After 20 min, the interventions were administered. Pressure pain thresholds (PPTs) were assessed outside the area of capsaicin application at 0 and 40 min to examine secondary hyperalgesia. Capsaicin pain intensity and unpleasantness were reported every 4 min. Frontal high-gamma oscillations were also measured with electroencephalography. Results: Pain ratings and brain activity were not significantly different between groups over time (p > 0.5). However, PPTs were significantly decreased in the placebo and control groups (p < 0.01), indicative of secondary hyperalgesia, while no hyperalgesia was observed for groups receiving SM (p = 1.0). This effect was independent of expectations and greater than placebo for segmental (p < 0.01) but not heterosegmental SM (p = 1.0). Conclusions: These results indicate that segmental SM can prevent secondary hyperalgesia, independently of expectations. This has implications for the management of back pain, particularly when central sensitization is involved.

Revealing the time course of laser-evoked potential habituation by high temporal resolution analysis

BACKGROUND AND OBJECTIVES

Reduced laser-evoked potential (LEP) habituation indicates abnormal central pain processing. But the paradigm (four stimulation blocks a 25 stimuli) is time consuming and potentially omits important information on the exact habituation time course. This study examined whether a high temporal resolution (HTR) analysis (dividing the four stimulation blocks into 12 analysis blocks) can answer the following questions: (a) After how many stimuli does LEP habituation occur? (b) Is there a difference in LEP habituation in younger versus older subjects? (c) Is HTR applicable on radiculopathy patients?

METHODS

EEG data of 129 subjects were included. Thirty-four young healthy and 28 advanced-aged healthy subjects were tested with LEPs on the hand dorsum. Thirty-seven radiculopathy patients and 30 controls were tested with LEPs on the L3 dermatome. The EEG data of the hand dorsa have been analysed conventionally and with HTR analysis. The applicability of HTR has been tested on radiculopathy patients and respective controls.

RESULTS

HTR was well feasible in young healthy subjects and revealed a strong habituation effect during the first 25 stimuli (i.e. within the first 5 min). After approximately 48 stimuli, no further significant habituation was detectable. LEP amplitudes were higher in young subjects. HTR was unsuitable for elderly subjects and middle-aged radiculopathy patients.

CONCLUSIONS

In young healthy subjects, HTR allows a shortening of the test protocol while providing a detailed information on the time course of LEP habituation. A shorter protocol might be useful for the applicability of the LEP paradigm for clinical and experimental settings as well as pharmacological studies.

SIGNIFICANCE

The usage of high temporal resolution (HTR) analysis in young healthy subjects enables a short test protocol and provides the exact time course of laser-evoked potential habituation. This can be useful for the examination of neurological conditions affecting younger patients and for pharmacological studies. HTR was inapplicable in advanced-aged subjects and patients with radiculopathy.

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.

Metabolite activity in the anterior cingulate cortex during a painful stimulus using functional MRS

To understand neurochemical brain responses to pain, proton magnetic resonance spectroscopy (1H-MRS) is used in humans in vivo to examine various metabolites. Recent MRS investigations have adopted a functional approach, where acquisitions of MRS are performed over time to track task-related changes. Previous studies suggest glutamate is of primary interest, as it may play a role during cortical processing of noxious stimuli. The objective of this study was to examine the metabolic effect (i.e., glutamate) in the anterior cingulate cortex during noxious stimulation using fMRS. The analysis addressed changes in glutamate and glutamate + glutamine (Glx) associated with the onset of pain, and the degree by which fluctuations in metabolites corresponded with continuous pain outcomes. Results suggest healthy participants undergoing tonic noxious stimulation demonstrated increased concentrations of glutamate and Glx at the onset of pain. Subsequent reports of pain were not accompanied by corresponding changes in glutamate of Glx concentrations. An exploratory analysis on sex revealed large effect size changes in glutamate at pain onset in female participants, compared with medium-sized effects in male participants. We propose a role for glutamate in the ACC related to the detection of a noxious stimulus.

Pain-autonomic interaction: A surrogate marker of central sensitization

BACKGROUND

Central sensitization represents a key pathophysiological mechanism underlying the development of neuropathic pain, often manifested clinically as mechanical allodynia and hyperalgesia. Adopting a mechanism-based treatment approach relies highly on the ability to assess the presence of central sensitization. The aim of the study was to investigate potential pain-autonomic readouts to operationalize experimentally induced central sensitization in the area of secondary hyperalgesia.

METHODS

Pinprick evoked potentials (PEPs) and sympathetic skin responses (SSRs) were recorded in 20 healthy individuals. Three blocks of PEP and SSR recordings were performed before and after heat-induced secondary hyperalgesia. All measurements were also performed before and after a control condition. Multivariate analyses were performed using linear mixed-effect regression models to examine the effect of experimentally induced central sensitization on PEP and SSR parameters (i.e. amplitudes, latencies and habituation) and on pinprick pain ratings.

RESULTS

The noxious heat stimulation induced robust mechanical hyperalgesia with a significant increase in PEP and SSR amplitudes (p < 0.001) in the area of secondary hyperalgesia. Furthermore, PEP and SSR habituation were reduced (p < 0.001) after experimentally induced central sensitization.

CONCLUSIONS

The findings demonstrate that combined recordings of PEPs and SSRs are sensitive to objectify experimentally induced central sensitization and may have a great potential to reveal its presence in clinical pain conditions. Corroborating current pain phenotyping with pain-autonomic markers has the potential to unravel central sensitization along the nociceptive neuraxis and might provide a framework for mechanistically founded therapies.

SIGNIFICANCE

Our findings provide evidence that combined recordings of sympathetic skin responses (SSRs) and pinprick evoked potentials (PEPs) might be able to unmask central sensitization induced through a well-established experimental pain model in healthy individuals. As such, these novel readouts of central sensitization might attain new insights towards complementing clinical pain phenotyping.

Clinical neurophysiology of pain

Clinical neurophysiologic investigation of pain pathways in humans is based on specific techniques and approaches, since conventional methods of nerve conduction studies and somatosensory evoked potentials do not explore these pathways. The proposed techniques use various types of painful stimuli (thermal, laser, mechanical, or electrical) and various types of assessments (measurement of sensory thresholds, study of nerve fiber excitability, or recording of electromyographic reflexes or cortical potentials). The two main tests used in clinical practice are quantitative sensory testing and pain-related evoked potentials (PREPs). In particular, PREPs offer the possibility of an objective assessment of nociceptive pathways. Three types of PREPs can be distinguished depending on the type of stimulation used to evoke pain: laser-evoked potentials, contact heat evoked potentials, and intraepidermal electrical stimulation evoked potentials (IEEPs). These three techniques investigate both small-diameter peripheral nociceptive afferents (mainly Aδ nerve fibers) and spinothalamic tracts without theoretically being able to differentiate the level of lesion in the case of abnormal results. In routine clinical practice, PREP recording is a reliable method of investigation for objectifying the existence of a peripheral or central lesion or loss of function concerning the nociceptive pathways, but not the existence of pain. Other methods, such as nerve fiber excitability studies using microneurography, more directly reflect the activities of nociceptive axons in response to provoked pain, but without detecting or quantifying the presence of spontaneous pain. These methods are more often used in research or experimental study design. Thus, it should be kept in mind that most of the results of neurophysiologic investigation performed in clinical practice assess small fiber or spinothalamic tract lesions rather than the neuronal mechanisms directly at the origin of pain and they do not provide objective quantification of pain.

Laser-evoked potentials in painful radiculopathy

OBJECTIVE

The aims of this exploratory study were (1) to develop a standardized objective electrophysiological technique with laser-evoked potentials to assess dorsal root damage quantitatively and (2) to correlate these LEP measures with clinical parameters and sensory abnormalities (QST) in the affected dermatome.

METHODS

Thirty-eight patients with painful radiculopathy and 20 healthy subjects were investigated with LEP recorded from the affected dermatome and control areas as well as with quantitative sensory testing. Questionnaires evaluating severity and functionality were applied.

RESULTS

On average, LEP amplitudes and latencies from the affected dermatomes did not differ from the contralateral control side. In patients with left L5 radiculopathy (more severely affected) the N2 latency was longer and the amplitudes reduced.

CONCLUSIONS

The N2P2 amplitude correlated with pinprick evoked sensations in QST. The N2 latency from the affected dermatome correlates with pain intensity, chronicity, clinical severity and with a decrease of physical function.

SIGNIFICANCE

An increase in N2-latency indicates a more pronounced nerve root damage, which is associated with a decrease of function and an increase of severity and pain. LEP amplitudes are associated with the functional status of the nociceptive system and may distinguish between degeneration of neuronal systems and central sensitization processes.

Exploring the Mechanisms of Exercise-Induced Hypoalgesia Using Somatosensory and Laser Evoked Potentials

Exercise-induced hypoalgesia is well described, but the underlying mechanisms are unclear. The aim of this study was to examine the effect of exercise on somatosensory evoked potentials, laser evoked potentials, pressure pain thresholds and heat pain thresholds. These were recorded before and after 3-min of isometric elbow flexion exercise at 40% of the participant's maximal voluntary force, or an equivalent period of rest. Exercise-induced hypoalgesia was confirmed in two experiments (Experiment 1–SEPs; Experiment 2–LEPs) by increased pressure pain thresholds at biceps brachii (24.3 and 20.6% increase in Experiment 1 and 2, respectively; both d > 0.84 and p < 0.001) and first dorsal interosseous (18.8 and 21.5% increase in Experiment 1 and 2, respectively; both d > 0.57 and p < 0.001). In contrast, heat pain thresholds were not significantly different after exercise (forearm: 10.8% increase, d = 0.35, p = 0.10; hand: 3.6% increase, d = 0.06, p = 0.74). Contrasting effects of exercise on the amplitude of laser evoked potentials (14.6% decrease, d = −0.42, p = 0.004) and somatosensory evoked potentials (10.9% increase, d = −0.02, p = 1) were also observed, while an equivalent period of rest showed similar habituation (laser evoked potential: 7.3% decrease, d = −0.25, p = 0.14; somatosensory evoked potential: 20.7% decrease, d = −0.32, p = 0.006). The differential response of pressure pain thresholds and heat pain thresholds to exercise is consistent with relative insensitivity of thermal nociception to the acute hypoalgesic effects of exercise. Conflicting effects of exercise on somatosensory evoked potentials and laser evoked potentials were observed. This may reflect non-nociceptive contributions to the somatosensory evoked potential, but could also indicate that peripheral nociceptors contribute to exercise-induced hypoalgesia.

Expectation to feel more pain disrupts the habituation of laser-pain rating and laser-evoked potential amplitudes

Increased pain perception due to the expectation to feel more pain is called nocebo effect. The present study aimed at investigating whether: (1) the mere expectation to feel more pain after the administration of an inert drug can affect the laser-pain rating and the laser-evoked potential (LEP) amplitude, and (2) the learning potentiates the nocebo effect. Eighteen healthy volunteers were told that an inert cream, applied on the right hand, would increase the laser pain and LEP amplitude to right hand stimulation. They were randomly assigned to either "verbal session" or "conditioning session". In the "verbal session", LEPs to both right and left hand stimulation were recorded at the same intensity before (baseline) and after cream application. In the "conditioning session", after an initial cream application the laser stimulus intensity was increased surreptitiously to make the subjects believe that the treatment really increased the pain sensation. Then, the cream was reapplied, and LEPs were recorded at the same stimulus intensity as at the baseline. It was found that the verbal suggestion to feel more pain disrupted the physiological habituation of the laser-pain rating and LEP amplitude to treated (right) hand stimulation. Unlike previously demonstrated for the placebo effect, the learning did not potentiate the nocebo effect.