Within-session sensitization and between-session habituation: A robust physiological response to repetitive painful heat stimulation

The roles of neural adaptation and sensitization in contact lens discomfort

PURPOSE

To investigate the roles of neural adaptation and sensitization in contact lens discomfort (CLD).

METHODS

Cooling stimuli (20 °C) were applied to the cornea in a group comprising 24 symptomatic and 25 asymptomatic contact lens (CL) wearers as well as 15 non-CL wearing controls, using a computerized Belmonte esthesiometer. The adaptation paradigm consisted of 20 repetitive stimuli at threshold, sub- and supra-threshold levels. The sensitization paradigm involved five levels of suprathreshold stimuli ranging between 1x to 2x threshold. Following each stimulus, participants rated the sensation magnitude regarding intensity, coolness and irritation. Measurements were taken with habitual CL (BL_CL), after 2 weeks of no-CL (No_CL) and after restarting habitual CL wear (ReSt_CL).

RESULTS

The symptomatic subjects exhibited a lower threshold but reported enhanced sensations during the adaptation and sensitization paradigm, compared to the asymptomatic and control groups (all p ≤ 0.021). At the BL_CL and ReSt_CL visits, they showed increased ratings to repeated subthreshold stimuli (p = 0.025) and greater irritation during the sensitization paradigm (p ≤ 0.032). Ratings in asymptomatic and control groups were relatively unchanged over time (p ≥ 0.181). Logistic regression revealed a link between the augmented sensory responses and increased likelihood with CLD.

CONCLUSION

The maladaptive sensory responses seen in CLD subjects, with reduced adaptation and heightened sensitization to ocular surface stimulation, suggest an imbalance between sensitization and adaptation in CLD. As CLD may represent a reversible subcategory of dry eye, it can serve as a human dry eye model for studying the neurosensory effect of ocular surface stimulation.

Adversity type and timing predict temporal summation of pain in African-American adults

African Americans are disproportionately exposed to adversity across the lifespan, which includes both stressful and traumatic events. Adversity, in turn, is associated with alterations in pain responsiveness. Racial differences in pain responsiveness among healthy adults are well established. However, the extent to which adversity type and timing are associated with alterations in pain responsiveness among healthy African-American adults is not well understood. The present study included 160 healthy African-American adults (98 women), ages 18 to 45. Outcome measures included pain tolerance and temporal summation of pain to evoked thermal pain. Composite scores were created for early-life adversity (childhood trauma, family adversity) and recent adversity (perceived stress, chronic stress burden). A measure of lifetime racial discrimination was also included. Higher levels of recent adversity were associated with higher temporal summation of pain, controlling for gender, age, and education. Neither early-life adversity nor lifetime racial discrimination were associated with temporal summation of pain. The present findings suggest that heightened temporal summation of pain among healthy African-American adults is associated with exposure to recent adversity events. Improved understanding of how recent adversity contributes to heightened temporal summation of pain in African Americans could help to mitigate racial disparities in pain experiences by identifying at-risk individuals who could benefit from early interventions.

Habituation to Pain in Patients with Chronic Pain: Clinical Implications and Future Directions

In this review, the latest insights into habituation to pain in chronic pain are summarized. Using a systematic search, results of studies on the evidence of habituation to (experimental) pain in migraine, chronic low back pain, fibromyalgia, and a variety of chronic pain indications are presented. In migraine, reduced habituation based on self-report and the EEG-based N1 and N2-P2 amplitude is reported, but the presence of contradictory results demands further replication in larger, well-designed studies. Habituation to pain in chronic low back pain seems not to differ from controls, with the exception of EEG measures. In fibromyalgia patients, there is some evidence for reduced habituation of the N2-P2 amplitude. Our analysis shows that the variability between outcomes of studies on habituation to pain is high. As the mechanisms underlying habituation to pain are still not fully understood and likely involve several pathways, it is now too early to conclude that habituation to pain is related to clinical outcomes and can be used as a diagnostic marker. The review ends with a discussion on future directions for research including the use of standard outcome measures to improve comparisons of habituation to pain in patients and controls, as well as a focus on individual differences.

Single Session Anodal Transcranial Direct Current Stimulation on Different Cortical Areas

Abstract. Transcranial direct current stimulation (tDCS) studies in healthy volunteers have shown conflicting results in terms of modulation in pain thresholds. The aim of this study was to investigate how single session anodal tDCS and modulated tDCS (mtDCS) of distinct cortical areas affected pain and perception thresholds in healthy participants. Five different stimulation conditions were applied at different cortical sites to 20 healthy volunteers to investigate the effects of tDCS and mtDCS (20 Hz) on pain and perception thresholds. TDCS over the motor cortex (M1), mtDCS over the motor cortex, tDCS over the dorsolateral prefrontal cortex (DLPFC), mtDCS of the DLPFC, and mtDCS over the occipital cortex were the stimulation conditions. All of the stimulations were anodal. The stimulations were given in a randomized order at 20-minute intervals. For comparison, electrical pain and perception thresholds were obtained from the right middle finger before and during the tDCS. After each measurement, participants were asked to give a score to their pain. In repeated measures analysis of variance (RM-ANOVA) test, the Condition × Time interaction showed no significant influence on changes in pain, perception thresholds, and pain scores ( p = .48, p = .89, and p = .50, respectively). However, regardless of the condition types, there was a significant difference in pain and perceptual thresholds during tDCS ( p = .01, p = .025, respectively). Our findings did not support difference in pain and perception modulation by a single session anodal tDCS over M1 and DLPFC compared to the occipital cortex in healthy volunteers. The increase in all thresholds during tDCS, irrespective of conditions, and peripheral sensations, including an active control group, taken together, suggest a placebo effect of active tDCS. Future studies about pain and perception in healthy subjects should consider the level of experimental pain and a strong placebo effect.

Attentional interference by pain in a dishabituation procedure: an experimental investigation

ABSTRACT

The ability to habituate to pain may be adaptive, and it may enable us to pursue valuable goals despite the pain. In this study, we experimentally investigated this idea using the primary task paradigm in which participants had to identify the color of a circle (blue or yellow) as quickly as possible while ignoring painful or tactile distractors that are presented on some of the trials. In the first experiment, we were interested whether the attentional interference effect because of the presentation of the distractors and its habituation would differ between painful and tactile distractor stimuli. In the second experiment, we investigated dishabituation (ie, the phenomenon that the introduction of a different stimulus results in an increase of the decremented response to the original stimulus). We expected habituation of the attentional interference to occur both in the tactile and the painful distractor condition, but to be less complete in the painful condition. Moreover, we hypothesized that the dishabituation would be stronger for the painful than for the tactile distractors. We did find evidence for an interference effect because of the presence of distractors. We also found habituation of attentional interference. However, the interference and its habituation were no different for tactile and painful distractors. Moreover, we did not find evidence for dishabituation. These are the first studies of their kind. Implications and guidelines for future research are formulated.

Hypohydration but not Menstrual Phase Influences Pain Perception in Healthy Women

Chronic pain is a pervasive health problem and is associated with tremendous socioeconomic costs. However, current pain treatments are often ineffective due, in part, to the multi-factorial nature of pain. Mild hypohydration was shown to increase experimental pain sensitivity in men, but whether this also occurs in women has not been examined. Fluctuations in ovarian hormones (i.e., 17ß-oestradiol and progesterone) throughout the menstrual cycle may influence a woman's pain sensitivity, as well as hydration levels, suggesting possible interactions between hypohydration and menstrual phase on pain. We investigated the effects of mild hypohydration (HYPO, 24 hr of fluid restriction) on ischaemic pain sensitivity in 14 eumenorrheic women during the early follicular (EF) and mid-luteal (ML) phases of their menstrual cycle. We also examined whether acute water ingestion could reverse the negative effects of hypohydration. Elevated serum osmolality, plasma copeptin, and urine specific gravity indicated mild hypohydration. Compared to euhydration, HYPO reduced pain tolerance (by 34 ± 46 s; P = 0.02, ηp2 = 0.37) and increased ratings of pain intensity (by 0.7 ± 0.7 cm; P = 0.004; ηp2 = 0.55) and unpleasantness (by 0.7 ± 0.9 cm; P = 0.02; ηp2 = 0.40); these results were not influenced by menstrual phase. Water ingestion reduced thirst perception (Visual Analogue Scale, by 2.3 ± 0.9 cm; P < 0.001, ηp2 = 0.88) but did not reduce pain sensitivity. Therefore, hypohydration increases pain sensitivity in women with no influence of menstrual phase.

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.

Signaling via the FLP-14/FRPR-19 neuropeptide pathway sustains nociceptive response to repeated noxious stimuli in C. elegans

In order to thrive in constantly changing environments, animals must adaptively respond to threatening events. Noxious stimuli are not only processed according to their absolute intensity, but also to their context. Adaptation processes can cause animals to habituate at different rates and degrees in response to permanent or repeated stimuli. Here, we used a forward genetic approach in Caenorhabditis elegans to identify a neuropeptidergic pathway, essential to prevent fast habituation and maintain robust withdrawal responses to repeated noxious stimuli. This pathway involves the FRPR-19A and FRPR-19B G-protein coupled receptor isoforms produced from the frpr-19 gene by alternative splicing. Loss or overexpression of each or both isoforms can impair withdrawal responses caused by the optogenetic activation of the polymodal FLP nociceptor neuron. Furthermore, we identified FLP-8 and FLP-14 as FRPR-19 ligands in vitro. flp-14, but not flp-8, was essential to promote withdrawal response and is part of the same genetic pathway as frpr-19 in vivo. Expression and cell-specific rescue analyses suggest that FRPR-19 acts both in the FLP nociceptive neurons and downstream interneurons, whereas FLP-14 acts from interneurons. Importantly, genetic impairment of the FLP-14/FRPR-19 pathway accelerated the habituation to repeated FLP-specific optogenetic activation, as well as to repeated noxious heat and harsh touch stimuli. Collectively, our data suggest that well-adjusted neuromodulation via the FLP-14/FRPR-19 pathway contributes to promote nociceptive signals in C. elegans and counteracts habituation processes that otherwise tend to rapidly reduce aversive responses to repeated noxious stimuli.

Beyond Pain: A Study on the Variance of Pain Thresholds Within BDSM Interactions in Dominants and Submissives

BACKGROUND

BDSM is an acronym describing bondage and discipline, dominance and submission, and sadism and masochism. Afflicting or receiving pain is usually an important part of the BDSM interaction.

AIM

This research will focus on better understanding the aspect of pain within a BDSM interaction.

METHODS

Submissive and dominant counterparts of 35 couples were recruited to participate in a BDSM interaction, of which 34 dominants and 33 submissives were included in the analyses. A non-BDSM interested control group (n = 27) was included to control for social interaction, of which 24 were included in the analyses.

OUTCOMES

This research investigates the differences in (i) baseline pain thresholds, (ii) the impact of a BDSM interaction on those thresholds, and (iii) threshold moderating factors like pain cognition between submissive and dominant BDSM participants and control individuals.

RESULTS

BDSM practitioners have a higher pain threshold overall and a BSDM interaction will result in a temporary elevation of pain thresholds for submissives. Additionally, pain thresholds in dominants will be dependent upon their fear of pain and tendency to catastrophize pain and submissives will experience less fear of pain than the control group.

CLINICAL IMPLICATIONS

By further enhancing our understanding of the mechanisms behind a BDSM interaction in this way, we aspire to relieve the stigma these practitioners still endure.

STRENGTHS & LIMITATIONS

This is one of the first studies of its kind with a large sample size compared to similar research, which makes it a significant contribution to the field. It must be mentioned that there is a possible selection bias because recruitment was only done through the Flemish BDSM community and specifically those who visit clubs. Additionally, pain threshold remains a subjective measurement, which must be taken into account.

CONCLUSION

This study helps shed further light on the biological processes behind a BDSM interaction through pain threshold measurements. Wuyts E, De Neef N, Coppens V, et al. Beyond Pain: A Study on the Variance of Pain Thresholds Within BDSM Interactions in Dominants and Submissives. J Sex Med 2021;18:556-564.

The effect of acute-experimental pain models on offset analgesia

BACKGROUND

Offset analgesia (OA) is characterized by a disproportionately large decrease in pain perception after a slight decrease in noxious stimulation. In patients with ongoing pain, this response is reduced. The effect is pronounced in painful body areas. The influence of acute pain has not been sufficiently investigated. The aim of this study was to investigate the influence of two experimental acute pain models, measured within the area of acute pain and on the non-affected opposite side, thereby considering the possible somatotopic nature of OA.

METHODS

Healthy, pain-free volunteers (n = 75) were randomly assigned to one of three groups (cold water, exercise and control group). The 'cold water group' immersed one hand into cold water for 3 min (Cold Pressor Task), while the 'exercise group' performed an isometric grip exercise for 3 min. There was no manipulation in the control group. Each experimental pain stimulus was performed at both (dominant, non-dominant) forearms. The individualized OA paradigm consisted of offset and constant temperature trials. Offset analgesia was measured immediately before, during and after the experimental pain stimuli.

RESULTS

A significant difference in OA was shown during experimental pain when compared to the control condition (exercise vs. control: p < 0.001, cold vs. control: p = 0.001), with no difference between the experimental conditions (p > 0.05). Immediately following the pain stimulation, results were marginally non-significant (p = 0.05).

CONCLUSIONS

Experimental painful stimulation reduced OA. This result should be interpreted with caution due to potential influences of conditioned pain modulation or exercise-induced hypoalgesia as well as possible floor effects.

SIGNIFICANCE

Temporal contrast of pain perception is inhibited in acute pain states. This study showed that reduced offset analgesia is observed when pain is experimentally induced using noxious cold and exercise stimuli.

Observation of Nociceptive Processing: Effect of Intra-Epidermal Electric Stimulus Properties on Detection Probability and Evoked Potentials

Monitoring nociceptive processing is a current challenge due to a lack of objective measures. Recently, we developed a method for simultaneous tracking of psychophysical detection probability and brain evoked potentials in response to intra-epidermal stimulation. An exploratory investigation showed that we could quantify nociceptive system behavior by estimating the effect of stimulus properties on the evoked potential (EP). The goal in this work was to accurately measure nociceptive system behavior using this method in a large group of healthy subjects to identify the locations and latencies of EP components and the effect of single- and double-pulse stimuli with an inter-pulse interval of 10 or 40 ms on these EP components and detection probability. First, we observed the effect of filter settings and channel selection on the EP. Subsequently, we compared statistical models to assess correlation of EP and detection probability with stimulus properties, and quantified the effect of stimulus properties on both outcome measures through linear mixed regression. We observed lateral and central EP components in response to intra-epidermal stimulation. Detection probability and central EP components were positively correlated to the amplitude of each pulse, regardless of the inter-pulse interval, and negatively correlated to the trial number. Both central and lateral EP components also showed strong correlation with detection. These results show that both the observed EP and the detection probability reflect the various steps of processing of a nociceptive stimulus, including peripheral nerve fiber recruitment, central synaptic summation, and habituation to a repeated stimulus.

The effect of mindful attention training for pain modulation capacity: Exploring the mindfulness-pain link

OBJECTIVE

Mindfulness has been shown to be beneficial for chronic pain. The underlying mechanisms of the mindfulness-pain link, however, are yet to be established. Particularly, the effects of mindfulness on pain modulation, which is shown to be dysfunctional among chronic pain patients, barely has been tested. This study investigated whether a short mindful attention training based on Langerian mindfulness mitigates reductions in pain modulation.

METHOD

Systemic quantitative-somatosensory testing of conditioned pain modulation (CPM) was conducted in 60 undergraduates, who were randomly assigned to one of three groups: (1) Pain-specific mindful attention training; (2) nonspecific mindful attention training; and (3) no mindful attention training. CPM was tested before and after the intervention.

RESULTS

As hypothesized, a reduction in CPM magnitude was observed only in the control group, whereas this reduction was abolished in the two mindfulness groups.

CONCLUSIONS

Langerian mindfulness may mitigate pain modulation reduction as observed in chronic pain, thus shedding light on its potential advantages.

Reliability of temporal summation, thermal and pressure pain thresholds in a healthy cohort and musculoskeletal trauma population

Traumatic injuries affect approximately 978 million people worldwide with 56.2 million requiring inpatient care. Quantitative sensory testing (QST) can be useful in predicting outcome following trauma, however the reliability of multiple QST including temporal summation (TS), heat and cold pain thresholds (HPT, CPT) and pressure pain thresholds (PPT) is unknown. We assessed intra (between day) and inter-rater (within day) reliability of QST in asymptomatic participants (n = 21), and inter-rater (within day) reliability in participants presenting with acute musculoskeletal trauma (n = 25). Intra-class correlations with 95% confidence intervals (ICC 3,2), standard error of measurement (SEM) and Bland Altman Plots for limits of agreement were calculated. For asymptomatic participants, reliability was good to excellent for HPT (ICC range 0.76–0.95), moderate to good for PPT (ICC range 0.52–0.93), with one site rated poor (ICC 0.41), and poor to excellent for TS scores (ICC range 0.20–0.91). For musculoskeletal trauma participants reliability was good to excellent for HPT and PPT (ICC range 0.76–0.86), and moderate to good reliability for TS (ICC range 0.69–0.91). SEM for HPT for both sets of participants was ~1°C and an average of 7N for asymptomatic participants and less than 8N for acute musculoskeletal trauma participants for PPT. This study demonstrates moderate to excellent intra and inter-rater reliability for HPT and PPT in asymptomatic participants and good to excellent inter-rater reliability for acute musculoskeletal trauma participants, with TS showing more variability for both sets of participants. This study provides foundations for future work evaluating the sensory function over time following acute musculoskeletal trauma.

Boundary effects of expectation in human pain perception

Perception of sensory stimulation is influenced by numerous psychological variables. One example is placebo analgesia, where expecting low pain causes a painful stimulus to feel less painful. Yet, because pain evolved to signal threats to survival, it should be maladaptive for highly-erroneous expectations to yield unrealistic pain experiences. Therefore, we hypothesised that a cue followed by a highly discrepant stimulus intensity, which generates a large prediction error, will have a weaker influence on the perception of that stimulus. To test this hypothesis we collected two independent pain-cueing datasets. The second dataset and the analysis plan were preregistered ( https://osf.io/5r6z7/ ). Regression modelling revealed that reported pain intensities were best explained by a quartic polynomial model of the prediction error. The results indicated that the influence of cues on perceived pain decreased when stimulus intensity was very different from expectations, suggesting that prediction error size has an immediate functional role in pain perception.

Deep continuous theta burst stimulation of the operculo-insular cortex selectively affects Aδ-fibre heat pain

KEY POINTS

Deep continuous theta burst stimulation (cTBS) of the right operculo-insular cortex delivered with a double cone coil selectively impairs the ability to perceive thermonociceptive input conveyed by Aδ-fibre thermonociceptors without concomitantly affecting the ability to perceive innocuous warm, cold or vibrotactile sensations. Unlike deep cTBS, superficial cTBS of the right operculum delivered with a figure-of-eight coil does not affect the ability to perceive thermonociceptive input conveyed by Aδ-fibre thermonociceptors. The effect of deep operculo-insular cTBS on the perception of Aδ-fibre input was present at both the contralateral and the ipsilateral hand. The magnitude of the increase in Aδ-heat detection threshold induced by the deep cTBS was significantly correlated with the intensity of the cTBS pulses. Deep cTBS delivered over the operculo-insular cortex is associated with a risk of transcranial magnetic stimulation-induced seizure.

ABSTRACT

Previous studies have suggested a pivotal role of the insular cortex in nociception and pain perception. Using a double-cone coil designed for deep transcranial magnetic stimulation, our objective was to assess (1) whether continuous theta burst stimulation (cTBS) of the operculo-insular cortex affects differentially the perception of different types of thermal and mechanical somatosensory inputs, (2) whether the induced after-effects are lateralized relative to the stimulated hemisphere, and (3) whether the after-effects are due to neuromodulation of the insula or neuromodulation of the more superficial opercular cortex. Seventeen participants took part in two experiments. In Experiment 1, thresholds and perceived intensity of Aδ- and C-fibre heat pain elicited by laser stimulation, non-painful cool sensations elicited by contact cold stimulation and mechanical vibrotactile sensations were assessed at the left hand before, immediately after and 20 min after deep cTBS delivered over the right operculo-insular cortex. In Experiment 2, Aδ-fibre heat pain and vibrotactile sensations elicited by stimulating the contralateral and ipsilateral hands were evaluated before and after deep cTBS or superficial cTBS delivered using a flat figure-of-eight coil. Only the threshold to detect Aδ-fibre heat pain was significantly increased 20 min after deep cTBS. This effect was present at both hands. No effect was observed after superficial cTBS. Neuromodulation of the operculo-insular cortex using deep cTBS induces a bilateral reduction of the ability to perceive Aδ-fibre heat pain, without concomitantly affecting the ability to perceive innocuous warm, cold or vibrotactile sensations.

Neural Habituation to Painful Stimuli Is Modulated by Dopamine: Evidence from a Pharmacological fMRI Study

In constantly changing environments, it is crucial to adaptively respond to threatening events. In particular, painful stimuli are not only processed in terms of their absolute intensity, but also with respect to their context. While contextual pain processing can simply entail the repeated processing of information (i.e., habituation), it can, in a more complex form, be expressed through predictions of magnitude before the delivery of nociceptive information (i.e., adaptive coding). Here, we investigated the brain regions involved in the adaptation to nociceptive electrical stimulation as well as their link to dopaminergic neurotransmission (placebo/haloperidol). The main finding is that haloperidol changed the habituation to the absolute pain intensity over time. More precisely, in the placebo condition, activity in left postcentral gyrus and midcingulate cortex increased linearly with pain intensity only in the beginning of the experiment and subsequently habituated. In contrast, when the dopaminergic system was blocked by haloperidol, a linear increase with pain intensity was present throughout the entire experiment. Finally, there were no adaptive coding effects in any brain regions. Together, our findings provide novel insights into the nature of pain processing by suggesting that dopaminergic neurotransmission plays a specific role for the habituation to painful stimuli over time.

Transcranial static magnetic field stimulation (tSMS) of the visual cortex decreases experimental photophobia

Background

Transcranial static magnetic field stimulation (tSMS) reduces cortical excitability in humans.

Methods

The objective of this study was to determine whether tSMS over the occipital cortex is effective in reducing experimental photophobia. In a sham-controlled double-blind crossover study, tSMS (or sham) was applied for 10 minutes with a cylindrical magnet on the occiput of 20 healthy subjects. We assessed subjective discomfort induced by low-intensity and high-intensity visual stimuli presented in a dark room before, during and after tSMS (or sham).

Results

Compared to sham, tSMS significantly reduced the discomfort induced by high-intensity light stimuli.

Conclusions

The visual cortex may contribute to visual discomfort in experimental photophobia, providing a rationale for investigating tSMS as a possible treatment for photophobia in migraine.

Behavioral Inhibition and Behavioral Activation are Related to Habituation of Nociceptive Flexion Reflex, but Not Pain Ratings

Habituation (ie, decreases in responding) and sensitization (ie, increases in responding) after prolonged or repeated exposures to a fixed stimulus have been identified as important in adaptation to repeated or prolonged noxious stimulation. Determinants of habituation or sensitization are poorly understood, and experimental investigation of habituation of pain ratings have generally relied on pain reports and statistical techniques that average responses across a group of participants. Using a cross-sectional design, the current study used multilevel growth curve analyses to examine changes in the nociceptive flexion reflex (NFR), a spinal nociceptive withdrawal reflex, and pain ratings in response to 12 repeated, constant intensity, noxious electrocutaneous stimuli. Unconditional growth curve models indicated that, on average, participants evidenced habituation of the NFR and sensitization of pain ratings. However, a substantial subgroup of participants exhibited the opposite pattern of change. In conditional models, behavioral inhibition, b = .10, P = .003, and behavioral activation, b = -.07, P = .07, independently interacted with the growth curve to predict changes in NFR, but not pain ratings, across the 12 stimuli. These findings provide preliminary experimental support for Jensen and colleagues' 2-factor model of pain experience and implicate a role for approach and avoidance motivations in descending modulation of NFR.

PERSPECTIVE

Using repeated NFR stimulation, this study showed that most participants exhibited NFR habituation and pain sensitization; however, a substantial subgroup showed an opposite pattern of pain habituation (25.0%) and NFR sensitization (31.4%). Further, NFR habituation was moderated by individual differences in behavioral activation and behavioral inhibition.

MRI scanner environment increases pain perception in a standardized nociceptive paradigm

Functional magnetic resonance imaging (MRI) has been widely used in neuroscientific studies to investigate neural correlates of perception and higher cognitive functions. Early on, the MR-scanning procedure itself has been identified to create discomfort and anxiety in some individuals, which may influence task performance and perception. The present study analyzed behavioral differences in pain intensity ratings obtained in two distinct situations: MR environment and laboratory setting. Within our longitudinal study design twenty healthy volunteers were exposed daily to an identical paradigm consisting of 60 repeated noxious heat stimuli (46 °C) on 21 consecutive days. After each block of ten stimuli, participants were prompted to rate pain intensity on a visual analog scale (VAS). On days 1, 8, 14, and 21 ratings scores were obtained during a functional imaging scan, whereas on the remaining days the sessions were conducted in a laboratory. It has come to our attention that pain intensity ratings acquired in MR environment were significantly higher than behavioral data collected in the lab setting. Given that the stimuli were standardized and no task or distraction confounded the ratings, it is likely that the attentional focus on noxious stimulation was identical in both conditions. It seems that the highly artificial scanner environment as such is sufficient to increase awareness/alertness. Given that salience rather than pure nociceptive input has been suggested to explain functional imaging results in painful conditions, these findings highlight concerns regarding the comparability of behavioral data assembled across inconsistent settings.

Relationship between adaptation and cardiovascular response to tonic cold and heat pain Adaptability to tonic pain and cardiovascular responses

BACKGROUND

The mechanisms of adaptation to tonic pain are not elucidated. We hypothesized that the adaptability to tonic pain is related to the cardiovascular system.

METHODS

Twenty-six subjects received over two sessions in a random order: tonic cold (7 ± 0.2 °C) and heat pain (47.5 ± 0.5 °C) on the hand for 5 min. Pain intensity, blood pressure (BP), and heart rate (HR) were continuously monitored.

RESULTS

Pain experience during the heat (HIT) and cold (CIT) immersion tests exhibited different average time courses, being approximated with a linear and cubic function, respectively. In each test, two groups of participants could be identified based on the time course of their tonic thermal pain: one-third of participants were pain adaptive and two-thirds non adaptive. The adaptive group exhibited higher initial pain, lower last pain, and shorter latency to peak pain than the non-adaptive one. Interestingly, some participants were adaptive to both pain stimuli, most were not. HIT as well as CIT produced a stable elevation of BP. However, BP was higher during CIT than HIT (p = 0.034). HR was also increased during CIT and HIT, but the two tests differed with respect to the time course of responses. Finally, the intensity and time course of pain rating to both HIT and CIT correlated with neither BP nor HR responses.

CONCLUSIONS

These results suggest that individual sensitivity and adaptability to tonic thermal pain is related to the intensity of initial pain rating and the latency to peak pain but not to cardiovascular responses.

Effects of Different Anesthetics on Pain Processing in an Experimental Human Pain Model

OBJECTIVE

After surgical procedures, anesthesia itself may affect pain perception. Particularly, there is increasing evidence that opioids not only have analgesic effects but also provoke pronociceptive changes, that is, opioid-induced hyperalgesia. We investigated the effect of different anesthetic regimens on pain processing in volunteers using a transdermal electrical pain model. In this model, stimulation of epidermal nerve fibers representing mainly peptidergic C-nociceptors leads to secondary hyperalgesia and habituation to the stimulus.

METHODS

Forty-eight healthy volunteers underwent conditioning noxious stimulation (CS) over 5 days. On day 2, the volunteers were randomized into 4 groups: control group (no anesthesia) and 3 groups receiving anesthesia before CS in anesthetic doses: propofol (P), propofol/remifentanil (PR), and propofol/remifentanil/S-ketamine (PRK). Quantitative sensory testing was performed on days 1 through 5 and on day 22.

RESULTS

In every group, CS was associated with short- and long-term habituation to the electrical stimulus. Repetitive CS resulted in unmodified short-term sensitization with stable areas of hyperalgesia. Although the PR group showed a trend toward increased areas of hyperalgesia on day 2, no significant differences were detectable between the groups. In contrast, anesthesia resulted in decreased intensity of the electrically evoked pain on day 2. Finally, the mechanical pain threshold before CS on day 5 was increased in all groups and remained elevated 3 weeks after the first CS, consistent with a long-term antinociceptive effect after CS.

CONCLUSIONS

The results suggest a short-term analgesic effect of general anesthesia. Furthermore, the conditioning stimulation over several days induced differential modulation of pro- and antinociceptive systems.

Enhanced short-term sensitization of facial compared with limb heat pain

Habituation and sensitization are important features of individual sensitivity to repetitive noxious stimulation and have been investigated in numerous studies. However, it is unclear whether these phenomena vary depending on the site of stimulation. Here we compared short-term and long-term effects of painful heat stimulation on the forehead and limb using an established longitudinal heat pain paradigm performed over 8 consecutive days in 36 healthy volunteers. Participants were randomized into 2 groups; participants received repetitive heat pain stimulation either on the left volar forearm or on the left side of the forehead. Our data show a comparable degree of habituation over the course of 8 days in both groups. However, participants in the trigeminal stimulation group exhibited stronger within-session sensitization (indexed by a higher within-session increase in pain intensity ratings) than those who received the forearm stimulation. Furthermore, over the course of the experiment we found a correlation between habituation and anxiety, showing less habituation in participants with higher trait anxiety scores. Our findings are in line with somatotopic differences in response to painful stimulation and a higher proneness of trigeminal pain to sensitization processes, which might be explained by the biological relevance of the head and facial area for vital functions. The contribution of this sensitivity to the development and maintenance of clinical facial pain and headache disorders warrants further investigation.

PERSPECTIVE

This study uses psychophysical methods to evaluate the differences in long-term habituation and short-term sensitization to heat pain between the trigeminal and spinal systems. We found stronger sensitization for trigeminal compared with nociceptive stimuli on the forearm. The contribution of this sensitivity to clinical pain states warrants further investigation.

An improved model of heat-induced hyperalgesia--repetitive phasic heat pain causing primary hyperalgesia to heat and secondary hyperalgesia to pinprick and light touch

This study tested a modified experimental model of heat-induced hyperalgesia, which improves the efficacy to induce primary and secondary hyperalgesia and the efficacy-to-safety ratio reducing the risk of tissue damage seen in other heat pain models. Quantitative sensory testing was done in eighteen healthy volunteers before and after repetitive heat pain stimuli (60 stimuli of 48°C for 6 s) to assess the impact of repetitive heat on somatosensory function in conditioned skin (primary hyperalgesia area) and in adjacent skin (secondary hyperalgesia area) as compared to an unconditioned mirror image control site. Additionally, areas of flare and secondary hyperalgesia were mapped, and time course of hyperalgesia determined. After repetitive heat pain conditioning we found significant primary hyperalgesia to heat, and primary and secondary hyperalgesia to pinprick and to light touch (dynamic mechanical allodynia). Acetaminophen (800 mg) reduced pain to heat or pinpricks only marginally by 11% and 8%, respectively (n.s.), and had no effect on heat hyperalgesia. In contrast, the areas of flare (−31%) and in particular of secondary hyperalgesia (−59%) as well as the magnitude of hyperalgesia (−59%) were significantly reduced (all p<0.001). Thus, repetitive heat pain induces significant peripheral sensitization (primary hyperalgesia to heat) and central sensitization (punctate hyperalgesia and dynamic mechanical allodynia). These findings are relevant to further studies using this model of experimental heat pain as it combines pronounced peripheral and central sensitization, which makes a convenient model for combined pharmacological testing of analgesia and anti-hyperalgesia mechanisms related to thermal and mechanical input.

The dynamics of pain: evidence for simultaneous site-specific habituation and site-nonspecific sensitization in thermal pain

Repeated exposure to noxious stimuli changes their painfulness, due to multiple adaptive processes in the peripheral and central nervous systems. Somewhat paradoxically, repeated stimulation can produce an increase (sensitization) or a decrease (habituation) in pain. Adaptation processes may also be body-site-specific or operate across body sites, and considering this distinction may help explain the conditions under which habituation versus sensitization occurs. To dissociate the effects of site-specific and site-nonspecific adaptation processes, we examined reported pain in 100 participants during counterbalanced sequences of noxious thermal stimulation on multiple skin sites. Analysis of pain ratings revealed 2 opposing sequential effects: repeated stimulations of the same skin site produced temperature-dependent habituation, whereas repeated stimulations across different sites produced sensitization. Stimulation trials were separated by ∼20 seconds, and sensitization was unrelated to the distance between successively stimulated sites, suggesting that neither temporal nor spatial summation occurred. To explain these effects, we propose a dynamic model with 2 adaptation processes, one site-specific and the other site-nonspecific. The model explains 93% of the variance in the group-mean pain ratings after controlling for current stimulation temperature, with its estimated parameters showing evidence for habituation for the site-specific process and sensitization for the site-nonspecific process. The 2 pain adaptation processes revealed in this study, and the ability to disentangle them, may hold keys to understanding multiple pain-regulatory mechanisms and their disturbance in chronic pain syndromes.

PERSPECTIVE

This article presents novel evidence for simultaneous site-specific habituation and site-nonspecific sensitization in thermal pain, which can be disentangled (and the direction and strength of each process estimated) by a dynamic model. The dissociation of site-specific and site-nonspecific adaptation processes may hold keys to understanding multiple pain-regulatory mechanisms in both healthy and patient populations.

Can quantitative sensory testing move us closer to mechanism-based pain management?

OBJECTIVE

This review summarizes the scientific literature relating to the use of quantitative sensory testing (QST) for mechanism-based pain management.

DESIGN

A literature search was undertaken using PubMed and search terms including quantitative sensory testing, pain, chronic pain, response to treatment, outcome measure.

SETTINGS AND PATIENTS

Studies including QST in healthy individuals and those with painful disorders were reviewed.

MEASURES

Publications reported on QST methodological issues including associations among measures and reliability. We also included publications on the use of QST measures in case-control studies, their associations with biopsychosocial mechanisms, QST measures predicting clinical pain, as well as predicting and reflecting treatment responses.

RESULTS

Although evidence suggests that QST may be useful in a mechanism-based classification of pain, there are gaps in our current understanding that need to be addressed including making QST more applicable in clinical settings. There is a need for developing shorter QST protocols that are clinically predictive of various pain subtypes and treatment responses without requiring expensive equipment. Future studies are needed, examining the clinical predictive value of QST including sensitivity and specificity for pain classification or outcome prediction. These findings could enable third-party payers' reimbursement, which would facilitate clinical implementation of QST.

CONCLUSIONS

With some developments, QST could become a cost-effective and clinically useful component of pain assessment and diagnosis, which can further our progress toward the goal of mechanism-based personalized pain management.

Neuronal differences between chronic low back pain and depression regarding long-term habituation to pain

BACKGROUND

Longitudinal studies of experimental pain are rare and little is known about the differences regarding sensitization and habituation over longer periods in patients with chronic pain or depression compared with controls.

METHODS

We used a standardized longitudinal painful heat paradigm that was designed to induce long-term habituation in 19 patients with chronic low back pain (CLBP), 21 patients with depression (DEP) and 21 healthy participants (controls) over a time course of eight consecutive days. We applied functional magnetic resonance imaging on the first and last day of this period and after 3 months.

RESULTS

Although the pain paradigm was standardized, patients with DEP exhibited significantly higher pain thresholds and a trend to higher pain ratings and, in functional imaging, showed less activation of the operculum and the secondary somatosensory cortex (S2) as compared to patients with CLBP and controls. Conversely, patients with CLBP showed increased activation in the anterior insula and parietal operculum as compared to patients with DEP and controls. Within session, all participants sensitized to pain, which was associated with higher activation levels in the thalamus, amygdala, midcingulate cortex, and sensory and motor areas. However, patients with depression showed significantly less activation in midbrain and brainstem areas.

CONCLUSION

Given that pain and depression potentiate each other clinically, our data suggest that this may involve different cortical pain areas.

Comparing neural response to painful electrical stimulation with functional MRI at 3 and 7 T

Progressing from 3T to 7 T functional MRI enables marked improvements of human brain imaging in vivo. Although direct comparisons demonstrated advantages concerning blood oxygen level dependent (BOLD) signal response and spatial specificity, these mostly focused on single brain regions with rather simple tasks. Considering that physiological noise also increases with higher field strength, it is not entirely clear whether the advantages of 7T translate equally to the entire brain during tasks which elicit more complex neuronal processing. Therefore, we investigated the difference between 3T and 7 T in response to transcutaneous electrical painful and non-painful stimulation in 22 healthy subjects. For painful stimuli vs. baseline, stronger activations were observed at 7 T in several brain regions including the insula and supplementary motor area, but not the secondary somatosensory cortex (p<0.05 FWE-corrected). Contrasting painful vs. non-painful stimulation limited the differences between the field strengths to the periaqueductal gray (PAG, p<0.001 uncorrected) due to a similar signal increase at 7 T for both the target and specific control condition in most brain regions. This regional specificity obtained for the PAG at higher field strengths was confirmed by an additional spatial normalization strategy optimized for the brainstem. Here, robust BOLD responses were obtained in the dorsal PAG at 7 T (p<0.05 FWE-corrected), whereas at 3T activation was completely missing for the contrast against non-painful stimuli. To summarize, our findings support previously reported benefits obtained at ultra-high field strengths also for complex activation patterns elicited by painful electrical stimulation. However, this advantage depends on the region and even more on the contrast of interest. The greatest gain at 7 T was observed within the small brainstem region of the PAG, where the increased field strength offered marked improvement for the localization of activation foci with high spatial specificity.