Pinprick Evoked Potentials-Reliable Acquisition in Healthy Human Volunteers

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

BACKGROUND

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

NEW METHOD

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

RESULTS AND COMPARISON WITH EXISTING METHODS

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

CONCLUSIONS

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

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.

EEG Responses to Upper Limb Pinprick Stimulation in Acute and Early Subacute Motor and Sensorimotor Stroke: A Proof of Concept

Electroencephalogram (EEG) during pinprick stimulation has the potential to unveil neural mechanisms underlying sensorimotor impairments post-stroke. A proof-of-concept study explored event-related peak pinprick amplitude and oscillatory responses in healthy controls and in people with acute and subuacute motor and sensorimotor stroke, their relationship, and to what extent EEG somatosensory responses can predict sensorimotor impairment. In this study, 26 individuals participated, 10 people with an acute and early subacute sensorimotor stroke, 6 people with an acute and early subacute motor stroke, and 10 age-matched controls. Pinpricks were applied to the dorsa of the impaired hand to collect somatosensory evoked potentials. Time(-frequency) analyses of somatosensory evoked potential (SEP) data at electrodes C3 and C4 explored peak pinprick amplitude and oscillatory responses across the three groups. Also, in stroke, (sensori-)motor impairments were assessed with the Fugl Meyer Assessment Upper Extremity (FMA) and Erasmus modified Nottingham Sensory Assessment (EmNSA) at baseline and 7 to 14 days later. Mixed model analyses were used to address objectives. It was demonstrated that increased beta desynchronization magnitude correlated with milder motor impairments (R2adjusted = 0.213), whereas increased beta resynchronization and delta power were associated to milder somatosensory impairment (R2adjusted = 0.550). At the second session, larger peak-to-peak SEP amplitude and beta band resynchronization at baseline were related to greater improvements in EMNSA and FMA scores, respectively, in the sensorimotor stroke group. These findings highlight the potential of EEG combined with somatosensory stimuli to differentiate between sensorimotor and motor impairments in stroke, offering preliminary insights into both diagnostic and prognostic aspects of upper limb recovery.

Radiofrequency evoked potentials: A new window into the nociceptive system

OBJECTIVE

To describe the cortical evoked potentials in response to radiofrequency stimulation (RFEPs) in human volunteers.

METHODS

Seventeen healthy volunteers participated in an experimental session in which radiofrequency (RF) and electrical (ES) stimulation were applied to the dorsum of the hands and feet. EEG was recorded to evaluate evoked responses for each stimulus modality and stimulation site.

RESULTS

Electrophysiological results showed highly synchronous responses compatible with activation of heat-sensitive nociceptors. Latencies of the N2 and P2 peaks in the RFEPs were longer compared to EPs evoked by ES. Furthermore, the latency of the P2 peak was also longer after stimulation of the feet compared to the hand.

CONCLUSIONS

RF stimulation is capable of selective activation of nociceptive fibres by means of rapid skin heating. RFEPs showed the highest degree of synchronicity achieved to date for evoked cortical responses to thermal stimulation.

SIGNIFICANCE

RF stimulation represents a viable alternative in the experimental and clinical assessment of the nociceptive system.

Clinical neurophysiology of neuropathic pain

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

The Role of Pain Expectations in the Development of Secondary Pinprick Hypersensitivity: Behavioral-Neurophysiological Evidence and the Role of Pain-Related Fear

Secondary mechanical hypersensitivity, a common symptom of neuropathic pain, reflects increased responsiveness of nociceptive pathways and can be induced temporarily in healthy volunteers using high-frequency electrical stimulation of the skin. Expectations modulate acute pain perception and fear of pain has been shown to attenuate and amplify the placebo and nocebo effects, respectively. However, the role of expectations and fear in the development of mechanical secondary hypersensitivity remains unclear. The modulatory role of fear and expectations in the development of mechanical secondary hypersensitivity remains so far mainly correlational. Here, we randomly assigned healthy participants (women) to a placebo, nocebo, or control group. In the experimental groups, participants' expectations of pain were manipulated using verbal suggestions accompanied by an inert treatment. Fear of pain was evaluated both in terms of fear of pain and via questionnaires. Sensitivity to mechanical stimulation was assessed by self-reported pinprick ratings before and after high-frequency stimulation; pinprick-evoked potentials elicited by the stimulation were recorded. The placebo group developed the least mechanical secondary hypersensitivity (smaller proximal-distal spread), while the nocebo group developed the most, but only when outliers were excluded. Higher expectations of pain predicted a greater development of mechanical secondary hypersensitivity. Anticipatory pain-related fear only mediated the relationship between unpleasantness expectations and perceived pinprick unpleasantness. Dispositional fear of pain moderated the relationship between expectations and the perceived intensity and unpleasantness of pinpricks. No group differences were observed in pinprick-evoked potentials. We provide preliminary evidence that both expectations and fear impact the development of mechanical secondary hypersensitivity. PERSPECTIVE: Expectations of pain may influence the development of secondary mechanical hypersensitivity. This effect is moderated by dispositional fear of pain and partially mediated by situational fear of pain.

Nociceptive Processing of Elite Athletes Varies between Sport-Specific Loads: An EEG-Based Study Approach

INTRODUCTION

For the downstream nociceptive processing of elite athletes, recent studies indicate that athletes probably tolerate more pain as compared with a normally active population. Phenotyping the nociceptive processing of athletes in different types of endurance sports can provide insight into training-specific effects, which may help in understanding the long-term effects of specific exercise.

METHODS

Twenty-six elite endurance athletes from the disciplines of rowing, triathlon, and running and 26 age- and sex-matched, recreationally active control subjects who participated in the subjective pain perception and processing of standardized noxious stimuli were investigated by EEG. This included standardized heat pain thresholds (HPT) and contact heat-evoked potentials from heat stimulation, measured with EEG as well as pinprick-evoked potentials from mechanical stimulation.

RESULTS

After noxious stimulation, athletes showed a higher activation of the event-related spectral perturbation (ERSP) patterns in the N2P2 EEG response at the Cz Electrode compared with the controls. After noxious contact heat stimulation, triathletes had a higher ERSP activation compared with the controls, whereas the rowers had a higher ERSP activation after noxious mechanical stimulation. Also, HPT in triathletes were increased despite their increased central activation after thermal stimulation. We found a correlation between increased HPT and training hours and years, although athletes did not differ within these variables.

CONCLUSIONS

Although we were able to identify differences between athletes of different endurance sports, the reasons and implications of these differences remain unclear. The study of sport-specific somatosensory profiles may help to understand the mechanisms of exercise-related long-term effects on pain processing and perception. Furthermore, sport-specific somatosensory effects may support the personalization of exercise interventions and identify risk factors for chronic pain in elite athletes.

A Systematic Review and Meta-analysis of Non-pharmacological Methods to Manipulate Experimentally Induced Secondary Hypersensitivity

This systematic review and meta-analysis investigated the effects of non-pharmacological manipulations on experimentally induced secondary hypersensitivity in pain-free humans. We investigated the magnitude (change/difference in follow-up ratings from pre-manipulation ratings) of secondary hypersensitivity (primary outcome), and surface area of secondary hypersensitivity (secondary outcome), in 27 studies representing 847 participants. Risk of bias assessment concluded most studies (23 of 27) had an unclear or high risk of performance and detection bias. Further, 2 (of 27) studies had a high risk of measurement bias. Datasets were pooled by the method of manipulation and outcome. The magnitude of secondary hypersensitivity was decreased by diverting attention, anodal transcranial direct current stimulation, or emotional disclosure; increased by directing attention toward the induction site, nicotine deprivation, or negative suggestion; and unaffected by cathodal transcranial direct current stimulation or thermal change. Area of secondary hypersensitivity was decreased by anodal transcranial direct current stimulation, emotional disclosure, cognitive behavioral therapy, hyperbaric oxygen therapy, placebo analgesia, or spinal manipulation; increased by directing attention to the induction site, nicotine deprivation, or sleep disruption (in males only); and unaffected by cathodal transcranial direct current stimulation, thermal change, acupuncture, or electroacupuncture. Meta-analytical pooling was only appropriate for studies that used transcranial direct current stimulation or hyperbaric oxygen therapy, given the high clinical heterogeneity among the studies and unavailability of data. The evidence base for this question remains small. We discuss opportunities to improve methodological rigor including manipulation checks, structured blinding strategies, control conditions or time points, and public sharing of raw data. PERSPECTIVE: We described the effects of several non-pharmacological manipulations on experimentally induced secondary hypersensitivity in humans. By shedding light on the potential for non-pharmacological therapies to influence secondary hypersensitivity, it provides a foundation for the development and testing of targeted therapies for secondary hypersensitivity.

Assessment and management of pain/nociception in patients with disorders of consciousness or locked-in syndrome: A narrative review

The assessment and management of pain and nociception is very challenging in patients unable to communicate functionally such as patients with disorders of consciousness (DoC) or in locked-in syndrome (LIS). In a clinical setting, the detection of signs of pain and nociception by the medical staff is therefore essential for the wellbeing and management of these patients. However, there is still a lot unknown and a lack of clear guidelines regarding the assessment, management and treatment of pain and nociception in these populations. The purpose of this narrative review is to examine the current knowledge regarding this issue by covering different topics such as: the neurophysiology of pain and nociception (in healthy subjects and patients), the source and impact of nociception and pain in DoC and LIS and, finally, the assessment and treatment of pain and nociception in these populations. In this review we will also give possible research directions that could help to improve the management of this specific population of severely brain damaged patients.

Reliability of Upper Limb Pin-Prick Stimulation With Electroencephalography: Evoked Potentials, Spectra and Source Localization

In order for electroencephalography (EEG) with sensory stimuli measures to be used in research and neurological clinical practice, demonstration of reliability is needed. However, this is rarely examined. Here we studied the test-retest reliability of the EEG latency and amplitude of evoked potentials and spectra as well as identifying the sources during pin-prick stimulation. We recorded EEG in 23 healthy older adults who underwent a protocol of pin-prick stimulation on the dominant and non-dominant hand. EEG was recorded in a second session with rest intervals of 1 week. For EEG electrodes Fz, Cz, and Pz peak amplitude, latency and frequency spectra for pin-prick evoked potentials was determined and test-retest reliability was assessed. Substantial reliability ICC scores (0.76-0.79) were identified for evoked potential negative-positive amplitude from the left hand at C4 channel and positive peak latency when stimulating the right hand at Cz channel. Frequency spectra showed consistent increase of low-frequency band activity (< 5 Hz) and also in theta and alpha bands in first 0.25 s. Almost perfect reliability scores were found for activity at both low-frequency and theta bands (ICC scores: 0.81-0.98). Sources were identified in the primary somatosensory and motor cortices in relation to the positive peak using s-LORETA analysis. Measuring the frequency response from the pin-prick evoked potentials may allow the reliable assessment of central somatosensory impairment in the clinical setting.

A modality-specific somatosensory evoked potential test protocol for clinical evaluation: A feasibility study

OBJECTIVE

We aimed to establish an objective neurophysiological test protocol that can be used to assess the somatosensory nervous system.

METHODS

In order to assess most fiber subtypes of the somatosensory nervous system, repetitive stimuli of seven different modalities (touch, vibration, pinprick, cold, contact heat, laser, and warmth) were synchronized with the electroencephalogram (EEG) and applied on the cheek and dorsum of the hand and dorsum of the foot in 21 healthy subjects and three polyneuropathy (PNP) patients. Latencies and amplitudes of the modalities were assessed and compared. Patients received quantitative sensory testing (QST) as reference.

RESULTS

We found reproducible evoked potentials recordings for touch, vibration, pinprick, contact-heat, and laser stimuli. The recording of warm-evoked potentials was challenging in young healthy subjects and not applicable in patients. Latencies were shortest within Aβ-fiber-mediated signals and longest within C-fibers. The test protocol detected function loss within the Aβ-fiber and Aδ-fiber-range in PNP patients. This function loss corresponded with QST findings.

CONCLUSION

In this pilot study, we developed a neurophysiological test protocol that can specifically assess most of the somatosensory modalities. Despite technical challenges, initial patient data appear promising regarding a possible future clinical application.

SIGNIFICANCE

Established and custom-made stimulators were combined to assess different fiber subtypes of the somatosensory nervous system using modality-specific evoked potentials.

Modulation of the N13 component of the somatosensory evoked potentials in an experimental model of central sensitization in humans

The N13 component of somatosensory evoked potential (N13 SEP) represents the segmental response of dorsal horn neurons. In this neurophysiological study, we aimed to verify whether N13 SEP might reflect excitability changes of dorsal horn neurons during central sensitization. In 22 healthy participants, we investigated how central sensitization induced by application of topical capsaicin to the ulnar nerve territory of the hand dorsum modulated N13 SEP elicited by ulnar nerve stimulation. Using a double-blind placebo-controlled crossover design, we also tested whether pregabalin, an analgesic drug with proven efficacy on the dorsal horn, influenced capsaicin-induced N13 SEP modulation. Topical application of capsaicin produced an area of secondary mechanical hyperalgesia, a sign of central sensitization, and increased the N13 SEP amplitude but not the peripheral N9 nor the cortical N20-P25 amplitude. This increase in N13 SEP amplitude paralleled the mechanical hyperalgesia and persisted for 120 min. Pregabalin prevented the N13 SEP modulation associated with capsaicin-induced central sensitization, whereas capsaicin application still increased N13 SEP amplitude in the placebo treatment session. Our neurophysiological study showed that capsaicin application specifically modulates N13 SEP and that this modulation is prevented by pregabalin, thus suggesting that N13 SEP may reflect changes in dorsal horn excitability and represent a useful biomarker of central sensitization in human studies.

Tests for central sensitization in general practice: a Delphi study

INTRODUCTION

Central sensitization (CS) may explain the persistence of symptoms in patients with chronic pain and persistent physical symptoms (PPS). There is a need for assessing CS in the consultation room. In a recently published systematic review, we made an inventory of tests for CS. In this study we aimed to assess which tests might have added value, might be feasible and thus be suitable for use in general practice.

METHODS

We conducted a Delphi study consisting of two e-mail rounds to reach consensus among experts in chronic pain and PPS. We invited 40 national and international experts on chronic pain and PPS, 27 agreed to participate. We selected 12 tests from our systematic review and additional searches; panellists added three more tests in the first round. We asked the panellists, both clinicians and researchers, to rate these 15 tests on technical feasibility for use in general practice, added value and to provide an overall judgement for suitability in general practice.

RESULTS

In two rounds the panellists reached consensus on 14 of the 15 tests: three were included, eleven excluded. Included were the Central Sensitization Inventory (CSI), pressure pain thresholds (PPTs) and monofilaments. No consensus was reached on the Sensory Hypersensitivity Scale.

CONCLUSION

In a Delphi study among an international panel of experts, three tests for measuring CS were considered to be suitable for use in general practice: the Central Sensitization Inventory (CSI), pressure pain thresholds (PPTs) and monofilaments.

Combined Neurophysiologic and Neuroimaging Approach to Reveal the Structure-Function Paradox in Cervical Myelopathy

BACKGROUND AND OBJECTIVES

To explore the so-called structure-function paradox in individuals with focal spinal lesions by means of tract-specific MRI coupled with multimodal evoked potentials and quantitative sensory testing.

METHODS

Individuals with signs and symptoms attributable to cervical myelopathy (i.e., no evidence of competing neurologic diagnoses) were recruited at the Balgrist University Hospital, Zurich, Switzerland, between February 2018 and March 2019. We evaluated the relationship between the extent of structural damage within spinal nociceptive pathways (i.e., dorsal horn, spinothalamic tract, anterior commissure) assessed with atlas-based MRI and (1) the functional integrity of spinal nociceptive pathways measured with contact heat-, cold-, and pinprick-evoked potentials and (2) clinical somatosensory phenotypes assessed with quantitative sensory testing.

RESULTS

Sixteen individuals (mean age 61 years) with either degenerative (n = 13) or posttraumatic (n = 3) cervical myelopathy participated in the study. Most individuals presented with mild myelopathy (modified Japanese Orthopaedic Association score >15; n = 13). A total of 71% of individuals presented with structural damage within spinal nociceptive pathways on MRI. However, 50% of these individuals presented with complete functional sparing (i.e., normal contact heat-, cold-, and pinprick-evoked potentials). The extent of structural damage within spinal nociceptive pathways was not associated with functional integrity of thermal (heat: p = 0.57; cold: p = 0.49) and mechano-nociceptive pathways (p = 0.83) or with the clinical somatosensory phenotype (heat: p = 0.16; cold: p = 0.37; mechanical: p = 0.73). The amount of structural damage to the spinothalamic tract did not correlate with spinothalamic conduction velocity (p > 0.05; ρ = -0.11).

DISCUSSION

Our findings provide neurophysiologic evidence to substantiate that structural damage in the spinal cord does not equate to functional somatosensory deficits. This study recognizes the pronounced structure-function paradox in cervical myelopathies and underlines the inevitable need for a multimodal phenotyping approach to reveal the eloquence of lesions within somatosensory pathways.

Revisiting the Examination of Sharp/Dull Discrimination as Clinical Measure of Spinothalamic Tract Integrity

Objective: Revisiting the sharp/dull discrimination as clinical measure of spinothalamic tract function considering the International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI). Three clinically relevant factors were evaluated as to their impact on reliability: (1) the localization of dermatomes in relation to the sensory level, (2) the examination tool, and (3) the threshold of correct answers for grading of a preserved sharp/dull discrimination.

Design: Prospective monocentric psychometric study.

Setting: Spinal Cord Injury Center, Heidelberg University Hospital, Germany.

Participants: Convenient sample of 21 individuals with subacute spinal cord injury (age: 31–82 years) and 20 individuals without spinal cord injury (age: 24–63 years).

Assessment: All participants underwent three assessments for sharp/dull discrimination, applying five commonly used examination tools in seven dermatomes, performed by three trained examiners under conditions in accordance with ISNCSCI.

Main Outcome Measures: Assessment of interrater reliability by determining both the Fleiss kappa (κ) coefficient and the percentage agreement between raters. Data were dichotomized regarding the ISNCSCI threshold.

Results: Interrater reliability in individuals with SCI was overall substantial (κ = 0.68; CI 0.679–0.681) and moderate (κ = 0.54; CI 0.539–0.543) in dermatomes below the sensory level. All applied tools led to at least moderate reliability below the sensory level (lowest κ = 0.44; CI 0.432–0.440), with the officially endorsed safety pin achieving the highest (substantial) reliability (κ = 0.64; CI 0.638–0.646). Percentage agreement differed between non-SCI (97.3%) and formally intact above level dermatomes in SCI (89.2%).

Conclusions: Sharp/dull discrimination as a common clinical examination technique for spinothalamic tract function is a reliable assessment. Independent from the used examination tools, reliability was substantial, with the medium-sized safety pin delivering the most favorable results. Notwithstanding this, all other tools could be considered if a safety pin is not available. Regarding interrater reliability and guessing probability, a threshold of 80% correct responses for preserved sharp/dull discrimination appears to be most suitable, which is in line with current clinical approaches and ISNCSCI. The causal attribution of the identified differences in sharp/dull discrimination between clinically intact dermatomes of individuals with SCI and unaffected dermatomes of individuals without SCI requires future work.

Clinical Trial Registration Number (German Clinical Trials Register): DRKS00015334 (https://www.drks.de).