Methodological Considerations for the Temporal Summation of Second Pain

Temporal Summation of the Thermal Grill Illusion is Comparable to That Observed Following Noxious Heat

The thermal grill illusion (TGI) describes a peculiar or even painful percept caused by non-noxious, interlaced warm and cold stimuli. It involves the glutamatergic system and is affected in putatively nociplastic syndromes such as fibromyalgia. The glutamatergic system is also involved in wind-up, that is, the increased activation of spinal neurons following repeated noxious stimulation leading to a temporal summation of perceived stimulus intensity. Here we combined both stimulation methods to further investigate whether non-noxious stimuli as employed in the TGI can lead to a similar summation of perceived stimulus intensity. In an experiment using a full crossover within-subjects design, 35 healthy volunteers received repeated stimuli, either in a thermal grill configuration or simply noxious heat. Both modalities were presented as sequences of 1 lead-in contact, followed by 11 consecutive contacts (each between 1.5 and 3 seconds), with either fast repetition ("wind-up" condition), or 2 slow-repeating control conditions. The main analyses concerned the relative pre-to-post sequence changes to quantify putatively wind-up-related effects. Pain ratings and skin conductance level (SCL) increased more strongly in "wind-up" than in control conditions. Interestingly, wind-up-related effects were of the same magnitude in TGI as compared to the pain control modality. Further, contact-by-contact SCL tracked how the effect emerged over time. These results indicate that although TGI does not involve noxious stimuli it is amenable to temporal summation and wind-up-like processes. Since both phenomena involve the glutamatergic system, the combination of wind-up with the TGI could yield a promising tool for the investigation of chronic pain conditions. PERSPECTIVE: Using thermal stimuli in an experimental protocol to combine 1) the TGI (painful or peculiar percept from simultaneous cold/warm stimulation) and 2) wind-up (increase in stimulus intensity after repeated exposure) holds promise to investigate pain and thermoceptive mechanisms, and chronic pain conditions.

The Impact of Sleep Disturbances on Endogenous Pain Modulation: A Systematic Review and Meta-Analysis

The bidirectional relationship between sleep and pain problems has been extensively demonstrated but despite all the accumulating evidence, their shared mechanisms are currently not fully understood. This review examined the association between sleep disturbances, defined as a broad array of sleep-related outcomes (eg, poor quality, short duration, insomnia), and endogenous pain modulation (EPM) in healthy and clinical populations. Our search yielded 6,151 references, and 37 studies met the eligibility criteria. Qualitative results showed mixed findings regarding the association between sleep disturbances and temporal summation of pain (TSP) and conditioned pain modulation (CPM), with poor sleep more commonly associated with decreased pain inhibition in both populations. Quantitative results indicated that such associations were not statistically significant, neither in healthy populations when EPM outcomes were assessed for changes pre-/post-sleep intervention (TSP: .31 [95%CI: -.30 to .92]; P = .321; CPM: .40 [95%CI: -.06 to .85] P = .088) nor in clinical populations when such association was assessed via correlation (TSP: -.00 [95%CI: -.22 to .21] P = .970; CPM: .12 [95%CI: -.05 to .29]; P = .181). For studies that reported results by sex, meta-analysis showed that experimental sleep disturbances impaired pain inhibition in females (1.43 [95%CI: .98-1.88]; P < .001) but not in males (-.30 [95%CI: -2.69 to 1.60]; P = .760). Only one study investigating the association between sleep disturbances and offset analgesia was identified, while no studies assessing spatial summation of pain were found. Overall, this review provides a comprehensive overview of the association between sleep disturbances and EPM function, emphasizing the need for further investigation to clarify specific mechanisms and phenotypic subtypes. PERSPECTIVE: This review shines a light on the association between sleep disturbances and endogenous pain modulation function. Qualitatively, we found a frequent association between reduced sleep quality and impaired pain inhibition. However, quantitatively such an association was not corroborated. Sex-specific effects were observed, with females presenting sleep-related impaired pain inhibition but not males.

Optimizing Temporal Summation of Heat Pain Using a Constant Contact Heat Stimulator

Purpose

Temporal summation (TS) of pain occurs when pain increases over repeated presentations of identical noxious stimuli. TS paradigms can model central sensitization, a state of hyperexcitability in nociceptive pathways that promotes chronic pain onset and maintenance. Many experimenters use painful heat stimuli to measure TS (TS-heat); yet, TS-heat research faces unresolved challenges, including difficulty evoking summation in up to 30-50% of participants. Moreover, substantial variability exists between laboratories regarding the methods for evoking and calculating TS-heat.

Patients and Methods

To address these limitations, this study sought to identify optimal parameters for evoking TS-heat in healthy participants with a commercially available constant contact heat stimulator, the Medoc TSA-II. Working within constraints of the TSA-II, stimulus trains with varying parameters (eg, stimulus frequency, baseline temp, peak temp, peak duration, testing site) were tested in a sample of 32 healthy, chronic pain-free participants to determine which combination best evoked TS-heat. To determine whether TS scoring method altered results, TS-heat was scored using three common methods.

Results

Across all methods, only two trains successfully evoked group-level TS-heat. These trains shared the following parameters: site (palmar hand), baseline and peak temperatures (44°C and 50°C, respectively), and peak duration (0.5 s). Both produced summation that peaked at moderate pain (~50 out of 100 rating).

Conclusion

Future TS-heat investigations using constant contact thermodes and fixed protocols may benefit from adopting stimulus parameters that include testing on the palmar hand, using 44°C baseline and 50°C peak temperatures, at ≥0.33 Hz stimulus frequency, and peak pulse durations of at least 0.5 seconds.

Concurrent validity of dynamic bedside quantitative sensory testing paradigms in breast cancer survivors with persistent pain

BACKGROUND

Studies on the concurrent validity of clinically applicable testing protocols for conditioned pain modulation (CPM) and temporal summation of pain (TSP) in breast cancer survivors (BCS) with persistent pain are lacking.

OBJECTIVES

This study investigated the concurrent validity of two bedside protocols for CPM and TSP in comparison to a respective reference protocol. The participants' preferences for bedside CPM and TSP protocols were assessed.

METHODS

Thirty BCS experiencing persistent pain were included in this study. Each participant underwent a reference test along with two bedside alternatives for assessing both TSP and CPM. For CPM, a cold pressor test (CPT) and blood pressure cuff (BPC) were used as conditioning stimulus. The test stimulus was elicited in parallel by pressure pain threshold after 45 and 90 s of conditioning at the lower limb. The CPM reference test consisted of parallel heat stimuli at the forearms using a two-thermode system. TSP was elicited using a von Frey monofilament (256 mN) and an algometer (98 kPa) at the affected site and opposite lower limb. The TSP reference test consisted of heat stimuli at the affected site and opposite lower limb. Participants' testing preference was examined using a purpose-designed questionnaire. Spearman's rank test examined the correlation between protocols.

RESULTS

The two bedside CPM protocols were strongly correlated (r = 0.787-0.939, p < 0.005). A strong correlation was found between the BPC protocol and reference test using the relative effect magnitude (r = 0.541-0.555, p < 0.005). The bedside TSP protocols were moderately correlated with each other only at the lower limb using absolute change scores (r = 0.455, p = 0.012). No significant correlation was found between the bedside and reference TSP protocols.

CONCLUSION

The significantly moderate to very strong correlations between the bedside protocols validate their interchangeability. Researchers and clinicians should be able to choose which bedside protocol they utilize; however, participants favored the use of a BPC and algometer for the evaluation of CPM and TSP, respectively.

Lifestyle factors and psychological factors are associated with central pain processing in service members with persistent low-back pain: A cross-sectional exploratory study

Persistent low-back pain (LBP) is highly prevalent in the military. Altered central pain processing is one of the mechanisms found to underlie persistent LBP. Our aim was to explore which factors are associated with altered pain processing in Dutch service members with persistent LBP. This knowledge may guide clinicians in what factors to address in the treatment of dysfunctional pain processing in service members with persistent LBP. Twenty-one service members with persistent LBP (mean age 34.0 years, 18 males) were included in this cross-sectional exploratory study. Participants completed questionnaires regarding lifestyle and psychological factors. Altered central pain processing was measured by temporal summation of pain to examine the function of the pain facilitatory system and by conditioned pain modulation to examine the pain inhibitory function. Univariable and multivariable linear regression analyses were performed. A higher local temporal summation of pain was associated with a longer sitting time, a higher level of physical activity and a higher level of pain catastrophizing. A higher local conditioned pain modulation was associated with a higher level of pain catastrophizing, anxiety and depression symptoms, and with a lower sleep quality. A higher remote conditioned pain modulation effect was associated with a higher level of physical activity, a higher body mass index and a shorter sitting time. This study succeeded in identifying lifestyle and psychological factors associated with altered pain processing in service members with persistent LBP. Prospective studies are needed to examine causality in these relationships.

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.

Investigation of Correlations Between Pain Modulation Paradigms

OBJECTIVE

Endogenous pain modulation can be quantified through the use of various paradigms. Commonly used paradigms include conditioned pain modulation (CPM), offset analgesia (OA), spatial summation of pain (SSP), and temporal summation of pain (TSP), which reflect spatial and temporal aspects of pro- and antinociceptive processing. Although these paradigms are regularly used and are of high clinical relevance, the underlying physiological mechanisms are not fully understood.

DESIGN

The aim of this study is therefore to assess the association between these paradigms by using comparable protocols and methodological approaches.

SETTING

University campus.

SUBJECTS

Healthy and pain-free volunteers (n = 48) underwent psychophysical assessment of CPM, OA, SSP, and TSP (random order) at the same body area (volar nondominant forearm) with individualized noxious stimuli.

METHODS

CPM included heat stimuli before, during, and after a noxious cold-water bath, whereas for OA, three heat stimuli were applied: baseline trial, offset trial, and constant trial. For the SSP paradigm, two differently sized heat stimulation areas were evaluated, whereas for TSP, the first and last stimulus of 10 consecutive short heat stimuli were assessed. A computerized visual analog scale was used to continuously evaluate pain intensity. The magnitudes of all associations between all paradigm pairs were analyzed with Spearman's correlation, and individual influencing factors were assessed with a multivariate linear regression model.

RESULTS

Weak to moderate correlations among all four paradigms were found (P > 0.05), and no distinct influencing factors were identified.

CONCLUSIONS

A limited association between pain modulation paradigms suggests that CPM, OA, SSP, and TSP assess distinct aspects of endogenous analgesia with different underlying physiological mechanisms.

Transcutaneous electrical nerve stimulation (TENS): towards the development of a clinic-friendly method for the evaluation of excitatory and inhibitory pain mechanisms

Background: Temporal summation and conditioned pain modulation (CPM) can be measured using a thermode and cold pressor test (CPTest). Unfortunately, these complex and expensive tools are ill-suited for routine clinical assessments.

Aims: We aimed to compare the temporal summation and CPM obtained with the thermode + CPTest paradigm to those obtained with a novel paradigm using transcutaneous electrical nerve stimulation (TENS).

Methods: We assessed temporal summation and CPM in 29 healthy participants, using two paradigms (random order): TENS, and thermode + CPTest. In the TENS paradigm, both the conditioning stimulus (CS) and the test stimulus (TS) were delivered using TENS; in the thermode + CPTest paradigm, the CS consisted of a CPTest and the TS was delivered using a thermode. We compared the average temporal summation and CPM evoked by the two paradigms.

Results: Average temporal summation was similar for both modalities (P = 0.90), and the number of participants showing temporal summation was similar in both paradigms (19 with thermode vs. 18 with TENS; P = 1.00). Average CPM response was larger following the thermode + CPTest than following the TENS (P = 0.005), and more participants showed CPM with the thermode + CPTest paradigm compared to the TENS paradigm (24 vs. 14; P = 0.01).

Conclusions: Both paradigms were roughly equivalent in the ability to evoke temporal summation (although response to one modality did not predict response to the other), but the TENS paradigm appeared to be less apt to induce a CPM response than the thermode + CPTest paradigm.

Development and Validation of a Predictive Model of Pain Modulation Profile to Guide Chronic Pain Treatment: A Study Protocol

Introduction: Quantitative sensory testing is frequently used in research to assess endogenous pain modulation mechanisms, such as Temporal Summation (TS) and Conditioned Pain Modulation (CPM), reflecting excitatory and inhibitory mechanisms, respectively. Numerous studies found that a dysregulation of these mechanisms is associated with chronic pain conditions. In turn, such a patient's “profile” (increased TS and/or weakened CPM) could be used to recommend different pharmacological treatments. However, the procedure to evaluate these mechanisms is time-consuming and requires expensive equipment that is not available in the clinical setting. In this study, we aim to identify psychological, physiological and socio-demographic markers that could serve as proxies to allow healthcare professionals to identify these pain phenotypes in clinic, and consequently optimize pharmacological treatments.

Method: We aim to recruit a healthy participant cohort (n = 360) and a chronic pain patient cohort (n = 108). Independent variables will include psychological questionnaires, pain measurements, physiological measures and sociodemographic characteristics. Dependent variables will include TS and CPM, which will be measured using quantitative sensory testing in a single session. We will evaluate one prediction model and two validation models (for healthy and chronic pain participants) using multiple regression analysis between TS/CPM and our independent variables. The significance thresholds will be set at p = 0.05, respectively.

Perspectives: This study will allow us to develop a predictive model to compute the pain modulation profile of individual patients based on their biopsychosocial characteristics. The development of the predictive model is the first step toward the overarching goal of providing clinicians with a set of quick and cheap tests, easily applicable in clinical practice to orient pharmacological treatments.

Pressure pain threshold and temporal summation in adults with episodic and persistent low back pain trajectories: a secondary analysis at baseline and after lumbar manipulation or sham

Background

People with chronic low back pain (LBP) typically have increased pain sensitivity compared to healthy controls, however its unknown if pain sensitivity differs based on LBP trajectory at baseline or after manual therapy interventions. We aimed to compare baseline pressure pain threshold (PPT) and temporal summation (TS) between people without LBP, with episodic LBP, and with persistent LBP, and to compare changes over time in PPT and TS after a lumbar spinal manipulation or sham manipulation in those with LBP.

Methods

Participants were aged 18–59, with or without LBP. Those with LBP were categorised as having either episodic or persistent LBP. PPT and TS were tested at baseline. LBP participants then received a lumbar spinal manipulation or sham, after which PPT and TS were re-tested three times over 30 min. Generalised linear mixed models were used to analyse data.

Results

One hundred participants (49 female) were included and analysed. There were 20 non-LBP participants (mean age 31 yrs), 23 episodic LBP (mean age 35 yrs), and 57 persistent LBP (mean age 37 yrs). There were no significant differences in PPT or TS between groups at baseline. There was a non-significant pattern of lower PPT (higher sensitivity) from the non-LBP group to the persistent LBP group at baseline, and high variability. Changes in PPT and TS after the interventions did not differ between the two LBP groups.

Discussion

We found no differences between people with no LBP, episodic LBP, or persistent LBP in baseline PPT or TS. Changes in PPT and TS following a lumbar manual therapy intervention do not appear to differ between LBP trajectories.

Trial registration

The trial was prospectively registered with ANZCTR (ACTRN12617001094369).

Considerations for multi-centre conditioned pain modulation (CPM) research; an investigation of the inter-rater reliability, level of agreement and confounders for the Achilles tendon and Triceps Surae

Objective

This study aimed to investigate the inter-rater reliability of the conditioned pain modulation (CPM) effect.

Methods

The reliability between two examiners assessing the CPM effect via pressure pain thresholds and induced using the cold pressor test of 28 healthy volunteers at the mid-portion Achilles tendon (AT) and Triceps Surae musculotendinous junction was performed. Reliability was calculated using intraclass correlation coefficient (ICC). Confounders were assessed using multivariable generalised estimating equations (GEEs). Bias in the level of agreement was assumed if the confidence intervals (CIs) of the mean difference in Bland-Altman plots did not cross the line of equality.

Results

The inter-rater reliability of the CPM effect was poor to moderate in the AT (ICC 95% CI = 0.00-0.66) and Triceps Surae (ICC 95% CI = 0.00-0.69). However, when accounting for confounders within the GEE, there were no differences between testers and Bland-Altman plots reported good agreement between testers. Habitual completion of running-related physical activity was a confounder for both the AT parallel-paradigm (p = 0.017) and sequential-paradigm (p = 0.029). Testing order was a confounder for the AT (p = 0.023) and Triceps Surae (p = 0.014) parallel-paradigm.

Conclusion

This study suggests the CPM effect may be site specific (i.e. differences between the AT and Triceps Surae exist). In addition, differences in the reliability between examiners are likely due to the influence of confounders and not examiner technique and therefore appropriate analysis should be used in research investigating the CPM effect.

Mechanisms of Below-Level Pain Following Spinal Cord Injury (SCI)

Mechanisms of below-level pain are discoverable as neural adaptations rostral to spinal injury. Accordingly, the strategy of investigations summarized here has been to characterize behavioral and neural responses to below-level stimulation over time following selective lesions of spinal gray and/or white matter. Assessments of human pain and the pain sensitivity of humans and laboratory animals following spinal injury have revealed common disruptions of pain processing. Interruption of the spinothalamic pathway partially deafferents nocireceptive cerebral neurons, rendering them spontaneously active and hypersensitive to remaining inputs. The spontaneous activity among these neurons is disorganized and unlikely to generate pain. However, activation of these neurons by their remaining inputs can result in pain. Also, injury to spinal gray matter results in a cascade of secondary events, including excitotoxicity, with rostral propagation of excitatory influences that contribute to chronic pain. Establishment and maintenance of below-level pain results from combined influences of injured and spared axons in the spinal white matter and injured neurons in spinal gray matter on processing of nociception by hyperexcitable cerebral neurons that are partially deafferented. A model of spinal stenosis suggests that ischemic injury to the core spinal region can generate below-level pain. Additional questions are raised about demyelination, epileptic discharge, autonomic activation, prolonged activity of C nocireceptive neurons, and thalamocortical plasticity in the generation of below-level pain. PERSPECTIVE: An understanding of mechanisms can direct therapeutic approaches to prevent development of below-level pain or arrest it following spinal cord injury. Among the possibilities covered here are surgical and other means of attenuating gray matter excitotoxicity and ascending propagation of excitatory influences from spinal lesions to thalamocortical systems involved in pain encoding and arousal.

Effects of manipulating the interstimulus interval on heat-evoked temporal summation of second pain across the age span

This study examined the effects of interstimulus interval (ISI) on heat-evoked temporal summation of second pain (TSSP) and tested whether greatest maintenance of TSSP would occur at longer ISIs in older adults. Several lines of evidence support that TSSP is associated with central sensitization and is centrally mediated. The participants were 198 community-dwelling adults divided into 3 age cohorts (18-39, 40-59, and 60-78 years of age). Six TSSP trials used a train of 10 contacts with a preheated probe that made repetitive contact with the volar forearm. Participants completed 2 trials at each ISI of 2.5, 3.5, and 4.5 seconds. The intraclass correlations for each pair of trials support the reliability of the current methodology. Temporal summation of second pain scores declined in a time-dependent manner across ISI. In addition, greater maintenance of TSSP at longer ISIs was observed in middle-aged and older age groups compared with the younger cohort. Significant associations were found between TSSP and measures of recent pain. Greater summation at longer ISIs in older adults would suggest slower decay of excitability in spinal neurons and infer increased risk for central sensitization with advancing age.

Reliability of measurements for sub-painful and painful perception on artificial electrical stimulations

Artificial electrical stimulation is a common type of stimulus to induce sub-painful and painful sensation in clinical or neuroscience experiments. The Numerical Rating Scale (NRS) is often used to evaluate subjective perception due to external stimulations. Yet the relationship between the intensity levels of electrical stimulations and self-perception has seldom been examined. The aim of the study was to obtain evidence on the reliability and accuracy of sub-painful and painful perceptions of healthy participants using the NRS under different levels of electrical stimulus. A total of 72 pain-free healthy volunteers (female=44) were recruited. In the first experiment, each participant was given different levels of a non-nociceptive or nociceptive electrical stimulus and then asked to give a perception rating based on an 11-point NRS. In the second experiment, each participant was asked to memorize 5 levels of sub-nociceptive or nociceptive stimuli and to recognize the level of stimulus given each time. For the NRS rating task, intraclass coefficients (ICCs) reached satisfactory level for sub-nociceptive (0.85<ICC<0.93) and nociceptive stimulation (0.90<ICC<0.96). The ICCs were the highest for the weakest sub-nociceptive and nociceptive stimuli. For the stimulus recognition task, accuracy was also found to be highest for the weakest sub-nociceptive stimulus (κ=0.67) and lowest for the strongest nociceptive stimulus (κ=0.34). The results suggest that, with adequate training, NRS can be a reliable measurement tool for both sub-painful and painful rating due to electrical stimulation.