Individual Differences in Pain Sensitivity: Measurement, Causation, and Consequences

A practical guide and perspectives on the use of experimental pain modalities with children and adolescents

Use of experimental pain is vital for addressing research questions that would otherwise be impossible to examine in the real world. Experimental induction of pain in children is highly scrutinized given the potential for harm and lack of direct benefit to a vulnerable population. However, its use has critically advanced our understanding of the mechanisms, assessment and treatment of pain in both healthy and chronically ill children. This article introduces various experimental pain modalities, including the cold pressor task, the water load symptom provocation test, thermal pain, pressure pain and conditioned pain modulation, and discusses their application for use with children and adolescents. It addresses practical implementation and ethical issues, as well as the advantages and disadvantages offered by each task. The incredible potential for future research is discussed given the array of experimental pain modalities now available to pediatric researchers.

Conditioned pain modulation and situational pain catastrophizing as preoperative predictors of pain following chest wall surgery: a prospective observational cohort study

BACKGROUND

Variability in patients' postoperative pain experience and response to treatment challenges effective pain management. Variability in pain reflects individual differences in inhibitory pain modulation and psychological sensitivity, which in turn may be clinically relevant for the disposition to acquire pain. The aim of this study was to investigate the effects of conditioned pain modulation and situational pain catastrophizing on postoperative pain and pain persistency.

METHODS

Preoperatively, 42 healthy males undergoing funnel chest surgery completed the Spielberger's State-Trait Anxiety Inventory and Beck's Depression Inventory before undergoing a sequential conditioned pain modulation paradigm. Subsequently, the Pain Catastrophizing Scale was introduced and patients were instructed to reference the conditioning pain while answering. Ratings of movement-evoked pain and consumption of morphine equivalents were obtained during postoperative days 2-5. Pain was reevaluated at six months postoperatively.

RESULTS

Patients reporting persistent pain at six months follow-up (n = 15) were not significantly different from pain-free patients (n = 16) concerning preoperative conditioned pain modulation response (Z = 1.0, P = 0.3) or level of catastrophizing (Z = 0.4, P = 1.0). In the acute postoperative phase, situational pain catastrophizing predicted movement-evoked pain, independently of anxiety and depression (β = 1.0, P = 0.007) whereas conditioned pain modulation predicted morphine consumption (β = -0.005, P = 0.001).

CONCLUSIONS

Preoperative conditioned pain modulation and situational pain catastrophizing were not associated with the development of persistent postoperative pain following funnel chest repair. Secondary outcome analyses indicated that conditioned pain modulation predicted morphine consumption and situational pain catastrophizing predicted movement-evoked pain intensity in the acute postoperative phase. These findings may have important implications for developing strategies to treat or prevent acute postoperative pain in selected patients. Pain may be predicted and the malfunctioning pain inhibition mechanism as tested with CPM may be treated with suitable drugs augmenting descending inhibition.

Slow temporal summation of pain for assessment of central pain sensitivity and clinical pain of fibromyalgia patients

BACKGROUND

In healthy individuals slow temporal summation of pain or wind-up (WU) can be evoked by repetitive heat-pulses at frequencies of ≥.33 Hz. Previous WU studies have used various stimulus frequencies and intensities to characterize central sensitization of human subjects including fibromyalgia (FM) patients. However, many trials demonstrated considerable WU-variability including zero WU or even wind-down (WD) at stimulus intensities sufficient for activating C-nociceptors. Additionally, few WU-protocols have controlled for contributions of individual pain sensitivity to WU-magnitude, which is critical for WU-comparisons. We hypothesized that integration of 3 different WU-trains into a single WU-response function (WU-RF) would not only control for individuals' pain sensitivity but also better characterize their central pain responding including WU and WD.

METHODS

33 normal controls (NC) and 38 FM patients participated in a study of heat-WU. We systematically varied stimulus intensities of.4 Hz heat-pulse trains applied to the hands. Pain summation was calculated as difference scores of 1st and 5th heat-pulse ratings. WU-difference (WU-Δ) scores related to 3 heat-pulse trains (44°C, 46°C, 48°C) were integrated into WU-response functions whose slopes were used to assess group differences in central pain sensitivity. WU-aftersensations (WU-AS) at 15 s and 30 s were used to predict clinical FM pain intensity.

RESULTS

WU-Δ scores linearly accelerated with increasing stimulus intensity (p<.001) in both groups of subjects (FM>NC) from WD to WU. Slope of WU-RF, which is representative of central pain sensitivity, was significantly steeper in FM patients than NC (p<.003). WU-AS predicted clinical FM pain intensity (Pearson's r = .4; p<.04).

CONCLUSIONS

Compared to single WU series, WU-RFs integrate individuals' pain sensitivity as well as WU and WD. Slope of WU-RFs was significantly different between FM patients and NC. Therefore WU-RF may be useful for assessing central sensitization of chronic pain patients in research and clinical practice.

Adaptability to pain is associated with potency of local pain inhibition, but not conditioned pain modulation: a healthy human study

This study investigated the relationship between pain sensitivity, adaptability, and potency of endogenous pain inhibition, including conditioned pain modulation (CPM) and local pain inhibition. Forty-one healthy volunteers (20 male, 21 female) received conditioning stimulation (CS) over 2 sessions in a random order: tonic heat pain (46 °C) on the right leg for 7 minutes and cold pressor pain (1 °C to 4 °C) on the left hand for 5 minutes. Participants rated the intensity of pain continuously using a 0 to 10 electronic visual analogue scale. The primary outcome measures were pressure pain thresholds (PPT) measured at the heterotopic and homotopic location to the CS sites before, during, and 20 minutes after CS. Two groups of participants, pain adaptive and pain nonadaptive, were identified based on their response to pain in the cold pressor test. Pain-adaptive participants showed a pain reduction between peak pain and pain at end of the test by at least 2 of 10 (n=16); whereas the pain-nonadaptive participants reported unchanged peak pain during 5-minute CS (n=25). Heterotopic PPTs during the CS did not differ between the 2 groups. However, increased homotopic PPTs measured 20 minutes after CS correlated with the amount of pain reduction during CS. These results suggest that individual sensitivity and adaptability to pain does not correlate with the potency of CPM. Adaptability to pain is associated with longer-lasting local pain inhibition.

Factors associated with pain induced by orthodontic separators

Pain resulting from the application of orthodontic forces varies markedly across individuals. The reasons of this variability are still largely unknown. To investigate factors that may be associated with orthodontic pain following the application of orthodontic separators. One hundred and seven participants were screened for pain response over 48 h following placement of orthodontic elastomeric separators. The highest (n = 10) and lowest (n = 10) pain responders were identified, and data collected on tooth pain sensitivity to electrical stimulation in conjunction with using the Pain Catastrophising Scale (PCS), Dental Anxiety Scale (DAS) and cold pressor test (CPT). There were statistically significant differences between high- and low-pain responders in catastrophising score (P ≤ 0.023). For every PCS magnification score of 1 unit higher, the relative risk of being a high-pain responder was 1.6 (P = 0.002); those scoring higher on helplessness had a lower risk of being so. DAS scores of high-pain responders were twice as high as those of low-pain responder (P = 0.043). During the first 2 min of CPT, the high-pain responders experienced more pain than the low-pain responders (P ≤ 0.029). Tooth pain thresholds did not differ between the two different pain responder groups. Pain catastrophising, dental anxiety and cold sensitivity appear to modify the pain experienced following placement of orthodontic separators. Further research is needed to determine the validity of screening questions to identify at-risk patients prior to commencing orthodontic treatment.

Attachment styles, pain, and the consumption of analgesics during labor: a prospective observational study

Individuals with less secure attachment styles have been shown to experience more pain than people with more secure attachment styles; however, attachment styles have not yet been examined in the context of labor pain and analgesic consumption. The purpose of this prospective observational study was to assess the influence of the mother's attachment style on the perception of labor pain, as assessed by a visual analog scale and analgesic consumption. Eighty-one pregnant women with a mean age of 32 years (standard deviation = 5.1) were assessed during the third trimester of pregnancy and during labor. The physical predictors of labor pain were recorded, and the adult attachment style was assessed with the Adult Attachment Scale-Revised. For labor analgesia, a low dose of patient-controlled epidural analgesia protocol (ropivacaine .6 mg/mL plus sufentanil .5 μg/mL) was used. Women with a secure attachment style reported significantly less labor pain (P < .001) and a significantly lower analgesic consumption during labor (P < .001) than insecurely attached women. These findings suggest that women's attachment style was associated with labor pain and analgesic consumption and support the relevance of the attachment theory as a promising conceptual framework for understanding labor pain.

PERSPECTIVE

This study showed that women with an insecure attachment style were more likely to report higher pain before patient-controlled epidural analgesia and higher analgesic consumption and to request supplemental analgesia during labor. The assessment of adult attachment has the potential to identify women at high risk of poorly coping with pain during childbirth.

Genetics of disc-related disorders: current findings and lessons from other complex diseases

Disc-related disorders are highly genetic conditions with heritability estimates of up to 75 % and yet, few genomic locations have been moderately associated with the disorders. Candidate gene studies have shown possible disease associations on loci and genes of 1p21.1 (COL11A1), 6q27 (THBS2), 9q22.31 (ASPN), 10p12.31 (SKT), 20q11.2 (GDF5) and 20q13.12 (MMP9). More recently, in 2012, the first genome-wide association study revealed variants on loci and genes of 3p26.2, 6p21.32 (HLA region) and 6q26 (PARK2) that associate with disc-related disorders. In many other complex diseases, large meta-analyses of hundreds of thousands of study subjects and loci have revealed remarkable pathways. As methodology is evolving rapidly, we have already stepped into the era of routinely sequencing all bases in all human exons and we are approaching the era of sequencing the entire genome of study subjects with common diseases. The past decade has taught us that the common variants seen throughout populations seem to have low effects in many common diseases, explain relatively little of the overall heritability of the diseases and demand thousands of study subjects to identify associations. It seems that familial rare variants play an important role in many common diseases leading us back to valuing studies with large families and isolated populations. Moreover, careful characterization of environmental conditions are needed to explore and determine gene-environment interactions as genes that increase disease risk in one context may not do so under another context.

Acute stress contributes to individual differences in pain and pain-related brain activity in healthy and chronic pain patients

Individual differences in pain sensitivity and reactivity are well recognized but the underlying mechanisms are likely to be diverse. The phenomenon of stress-induced analgesia is well documented in animal research and individual variability in the stress response in humans may produce corresponding changes in pain. We assessed the magnitude of the acute stress response of 16 chronic back pain (CBP) patients and 18 healthy individuals exposed to noxious thermal stimulations administered in a functional magnetic resonance imaging experiment and tested its possible contribution to individual differences in pain perception. The temperature of the noxious stimulations was determined individually to control for differences in pain sensitivity. The two groups showed similar significant increases in reactive cortisol across the scanning session when compared with their basal levels collected over 7 consecutive days, suggesting normal hypothalamic-pituitary-adrenal axis reactivity to painful stressors in CBP patients. Critically, after controlling for any effect of group and stimulus temperature, individuals with stronger cortisol responses reported less pain unpleasantness and showed reduced blood oxygenation level-dependent activation in nucleus accumbens at the stimulus onset and in the anterior mid-cingulate cortex (aMCC), the primary somatosensory cortex, and the posterior insula. Mediation analyses indicated that pain-related activity in the aMCC mediated the relationship between the reactive cortisol response and the pain unpleasantness. Psychophysiological interaction analysis further revealed that higher stress reactivity was associated with reduced functional connectivity between the aMCC and the brainstem. These findings suggest that acute stress modulates pain in humans and contributes to individual variability in pain affect and pain-related brain activity.

The influence of pain sensitivity on the treatment outcome of transforaminal epidural steroid injection in patients with lumbar spinal stenosis

The aim of this study was to investigate the effect of individual pain sensitivity on the results of transforaminal epidural steroid injection (TFESI) for the patients with lumbar spinal stenosis (LSS). Seventy-seven patients with LSS were included in this study. Prospectively planned evaluations were performed twice consecutively before and 2 months after TFESI. These included a detailed medical history, a physical examination, and completion of a series of questionnaires, including pain sensitivity questionnaire (PSQ), Oswestry disability index (ODI), and visual analog scale (VAS) for back and leg pain. The correlations were analyzed among variables between total PSQ/PSQ-moderate/PSQ-minor and pain and disability level measured by VAS for back/leg pain and ODI both before and 2 months after TFESI. Two months after TFESI, there were significant decreases in VAS for back/leg pain and ODI compared with those before injection. Before injection, VAS for back pain and leg pain was highly associated with the PSQ scores including total PSQ and PSQ subscores after adjustment for age, BMI, and grade of canal stenosis. However, any subscores of PSQ and total PSQ scores were not correlated with either VAS for back pain/leg pain or ODI 2 months after TFESI with adjustment made to age, BMI, gender, and grade of canal stenosis. This study highlights that individual pain sensitivity does not influence the outcomes of TFESI treatment in patients with LSS, even though pain sensitivity has a significant negative correlation with symptom severity of LSS.

Reduced supraspinal nociceptive responses and distinct gene expression profile in CXBH recombinant inbred mice

CXBH mice, known as an "opioid receptor-rich" strain, are a recombinant inbred mouse strain established by crossing the C57BL/6By and BALB/cBy strains. In the present study, we investigated nociceptive and antinociceptive sensitivity in CXBH mice and elucidated the underlying molecular mechanisms. CXBH mice exhibited slightly higher morphine-induced antinociception compared with C57BL/6J and BALB/cBy mice in the hot-plate test but not tail-flick test. CXBH mice exhibited a marked reduction of nociceptive sensitivity, regardless of the type of nociceptive stimulus, with the exception of tail stimulation. Changes in gene expression that corresponded to reduced nociceptive sensitivity in the brains of CXBH mice were observed in 62 transcripts, including pain- and analgesia-related transcripts, in a whole-genome expression assay. The total mRNA expression of opioid receptors was higher in CXBH mice than in C57BL/6J and BALB/cBy mice. However, the expression levels of MOR-1 mRNA, a major transcript of the μ opioid receptor gene, were not different among the C57BL/6J, BALB/cBy, and CXBH strains. In conclusion, supraspinal nociceptive responses were reduced in the CXBH mouse strain, and the expression levels of transcripts were altered in the brain of this strain.

PERSPECTIVE

This article presents the nociceptive and antinociceptive properties of CXBH recombinant inbred mice and gene expression differences that may underlie nociceptive tolerance in the strain. The CXBH mouse strain may be a useful animal model to investigate the molecular basis of individual differences in supraspinal pain sensitivity.

Morphine- and buprenorphine-induced analgesia and antihyperalgesia in a human inflammatory pain model: a double-blind, randomized, placebo-controlled, five-arm crossover study

PURPOSE

Opioid therapy is associated with the development of tolerance and paradoxically increased sensitivity to pain. It has been suggested that buprenorphine is associated with a higher antihyperalgesia/analgesia ratio than μ-opioid receptor agonists. The primary outcome of this study was therefore to investigate relative differences in antihyperalgesia and analgesia effects between morphine and buprenorphine in an inflammatory pain model in volunteers. The secondary outcome was to examine the relationship between pain sensitivity and opioid-induced effects on analgesia, antihyperalgesia, and descending pain modulation.

SUBJECTS AND METHODS

Twenty-eight healthy subjects were included. The study was a double-blind, randomized, placebo-controlled, five-arm crossover study with a multimodal (electrical, mechanical, and thermal stimuli) testing technique. After baseline assessments, intravenous infusions of morphine (10/20 mg), buprenorphine (0.3/0.6 mg), or placebo (normal saline) were administered over a 210-minute period, during which a cold pressor test, heat injury (47°C, 7 minutes, 12.5 cm(2)), and the first postburn assessment were done. After completion of the drug infusions, two additional postburn assessments were done. The subjects were monitored during each 8-hour session by an anesthesiologist.

RESULTS

For nearly all tested variables, significant dose-dependent analgesic effects were demonstrated. The median antihyperalgesia/analgesia ratio (secondary hyperalgesia/heat injury relative to placebo) for low-dose morphine was 0.01 (interquartile range: -6.2; 9.9), 0.00 (-2.4; 2.1) for high-dose morphine, 0.03 (-1.8; 2.1) for low-dose buprenorphine, and 0.00 (-3.2; 1.1) for high-dose buprenorphine (P > 0.466). There were no significant differences in opioid responses between high and low pain-sensitive subjects (P > 0.286). High-dose buprenorphine, compared to placebo, was associated with a significantly enhanced action of the descending inhibitory pain control system (P = 0.004).

CONCLUSION

The present study, using multimodal testing technique, could not demonstrate any significant differences between morphine and buprenorphine in the profiles of antihyperalgesia and analgesia. Only high-dose buprenorphine was associated with a significant effect on the descending inhibitory pain control system.

Is the pain-reducing effect of opioid medication reliable? A psychophysical study of morphine and pentazocine analgesia

A number of laboratory studies have confirmed the efficacy of opioid medication in reducing pain generated by a number of psychophysical modalities. However, one implicit assumption of clinical and experimental pain testing of analgesics is that the analgesic response is stable and will be comparable across repeated administrations. In the current study, the repeatability of opioid analgesia was assessed in a randomized, double-blinded study using 3 psychophysical pain modalities (e.g., thermal, pressure, and ischemic) over 4 medication sessions (2 with active drug, 2 with placebo). Psychophysical responses were evaluated before and after intravenous administration of either morphine (0.08mg/kg; n=52) or pentazocine (0.5mg/kg; n=49). To determine the ability of a drug to reduce pain, 4 analytic methods (i.e., absolute change, percent change, ratio, and residualized change scores) were calculated to generate separate analgesic index scores for each measure and drug condition. All analgesic index scores demonstrated a greater analgesic response compared to saline for both medications, but stability (i.e., test-retest correlations) of the opioid analgesic indices depended on the pain measurement. Ischemic pain outcomes were moderately stable across sessions for both opioid medications; however, heat and pressure analgesic index scores were moderately stable for only morphine and pentazocine, respectively. Finally, within stimulus modalities, analgesic index scores were highly correlated with each other, suggesting that the different methods for computing analgesic responses provided comparable results. These results suggest that analgesic measures are able to distinguish between active drugs. In addition, analgesic responses to morphine and pentazocine demonstrate at least moderate reliability.

Quality of life following third molar removal under conscious sedation

AIM

The aim of this study was to assess quality of life (QoL) and degree of satisfaction among outpatients subjected to surgical extraction of all four third molars under conscious sedation. A second objective was to describe the evolution of self-reported pain measured in a visual analogue scale (VAS) in the 7 days after extraction.

STUDY DESIGN

Fifty patients received a questionnaire assessing social isolation, working isolation, eating and speaking ability, diet modifications, sleep impairment, changes in physical appearance, discomfort at suture removal and overall satisfaction at days 4 and 7 after surgery. Pain was recorded by patients on a 100-mm pain visual analogue scale (VAS) every day after extraction until day 7.

RESULTS

Thirty-nine patients fulfilled correctly the questionnaire. Postoperative pain values suffered small fluctuations until day 5 (range: 23 to 33 mm in a 100-mm VAS), when dicreased significantly. A positive association was observed between difficult ranked surgeries and higher postoperative pain levels. The average number of days for which the patient stopped working was 4.9.

CONCLUSION

The removal of all third molars in a single appointment causes an important deterioration of the patient's QoL during the first postoperative week, especially due to local pain and eating discomfort.

The effect of emotional distress on persistent pelvic girdle pain after delivery: a longitudinal population study

OBJECTIVE

To study the prognosis for pelvic girdle pain, and to explore the association between presence of emotional distress during pregnancy and pelvic girdle syndrome 6 months after delivery.

DESIGN

Longitudinal population study.

SETTING

Norway, for the period 1999-2008.

POPULATION

A follow-up of 41 421 women in the Norwegian Mother and Child Cohort who reported pelvic girdle pain at 30 weeks of gestation.

METHODS

Data were obtained by self-administered questionnaires in weeks 17 and 30 of gestation, and 6 months after delivery.

MAIN OUTCOME MEASURE

Pelvic girdle syndrome 6 months after delivery, defined as pain in the anterior pelvis and on both sides in the posterior pelvis.

RESULTS

Six months after delivery, 78.0% of the women had recovered, 18.5% reported persistent pain in one or two pelvic locations, 3.0% reported pelvic girdle syndrome and 0.5% reported severe pelvic girdle syndrome. The recovery rates decreased with increasing levels of pain severity in pregnancy. Being emotionally distressed at two time points during pregnancy was associated with the presence of pelvic girdle syndrome (adjusted OR 1.5, 95% CI 1.2-1.9) and severe pelvic girdle syndrome (adjusted OR 1.9, 95% CI 1.1-3.1), after adjustment for pain severity in pregnancy, other medical conditions, body mass index, age at menarche, previous low back pain, and smoking during pregnancy.

CONCLUSIONS

In this follow-up of women with pelvic girdle pain in pregnancy, the recovery rates after delivery were high. Our findings suggest that the presence of emotional distress during pregnancy is independently associated with the persistence of pelvic girdle pain after delivery.

Abnormal endogenous pain modulation is a shared characteristic of many chronic pain conditions

The intensity of acute and chronic pain depends on interactions between peripheral impulse input and CNS pain mechanisms, including facilitation and inhibition. Whereas tonic pain inhibition is a characteristic of most pain-free individuals, pain facilitation can be detected in many chronic pain patients. The capability to inhibit pain is normally distributed along a wide continuum in the general population and can be used to predict chronic pain. Accumulating evidence suggests that endogenous pain inhibition depends on activation of the prefrontal cortex, periaqueductal gray and rostral ventral medulla. Quantitative sensory test paradigms have been designed to acquire detailed information regarding each individual's endogenous pain inhibition and facilitation. Such tests include: temporal summation of pain, which is mostly used to assess facilitatory pain modulation by measuring the change in pain perception during a series of identical nociceptive stimuli; and conditioned pain modulation, which tests pain inhibition by utilizing two simultaneously applied painful stimuli (the 'pain inhibits pain' paradigm). Considerable indirect evidence seems to indicate that not only increased pain facilitation but also ineffective pain inhibition represents a predisposition for chronic pain. This view is supported by the fact that many chronic pain syndromes (e.g., fibromyalgia, temporomandibular joint disorder, irritable bowel syndrome, headache and chronic fatigue syndrome) are associated with hypersensitivity to painful stimuli and reduced endogenous pain inhibition. However, future prospective studies will be necessary to provide definitive evidence for this relationship. Such research would not only provide important information about mechanisms relevant to chronic pain but would also permit identification of individuals at high risk for future chronic pain.

Pharmacogenomic considerations in opioid analgesia

Translating pharmacogenetics to clinical practice has been particularly challenging in the context of pain, due to the complexity of this multifaceted phenotype and the overall subjective nature of pain perception and response to analgesia. Overall, numerous genes involved with the pharmacokinetics and dynamics of opioids response are candidate genes in the context of opioid analgesia. The clinical relevance of CYP2D6 genotyping to predict analgesic outcomes is still relatively unknown; the two extremes in CYP2D6 genotype (ultrarapid and poor metabolism) seem to predict pain response and/or adverse effects. Overall, the level of evidence linking genetic variability (CYP2D6 and CYP3A4) to oxycodone response and phenotype (altered biotransformation of oxycodone into oxymorphone and overall clearance of oxycodone and oxymorphone) is strong; however, there has been no randomized clinical trial on the benefits of genetic testing prior to oxycodone therapy. On the other hand, predicting the analgesic response to morphine based on pharmacogenetic testing is more complex; though there was hope that simple genetic testing would allow tailoring morphine doses to provide optimal analgesia, this is unlikely to occur. A variety of polymorphisms clearly influence pain perception and behavior in response to pain. However, the response to analgesics also differs depending on the pain modality and the potential for repeated noxious stimuli, the opioid prescribed, and even its route of administration.

Twin studies of pain

Twin studies provide a method for estimating the heritability of phenotypes and for examining genetic and environmental relationships between phenotypes. We conducted a systematic review of twin studies of pain, including both clinical and experimental pain phenotypes. Fifty-six papers were included, whereof 52 addressed clinical phenotypes. Of the most comprehensively studied phenotypes, available data indicates heritability around 50% for migraine, tension-type headache and chronic widespread pain, around 35% for back and neck pain, and around 25% for irritable bowel syndrome. However, differences in phenotype definitions make these results somewhat uncertain. All clinical studies relied on dichotomous outcomes and none used pain intensity as continuous phenotype. This is a major weakness of the reviewed studies and gives reason to question their validity with respect to pain mechanisms. Experimental pain studies indicate large differences in heritability across pain modalities. Whereas there is evidence for substantial common genetic risk across many clinical pain conditions, different experimental pain phenotypes appear to be associated with different genetic factors. Recommendations for future research include inclusion of pain intensity scaling and number of pain sites in phenotyping. Furthermore, studies examining the genetic relationships between pain phenotypes, in particular between clinical and experimental phenotypes, should be prioritized.

Validation of the pain sensitivity questionnaire in chronic pain patients

Recently, a self-rating measure for pain perception based on imagined painful daily life situations, the Pain Sensitivity Questionnaire (PSQ), has been developed and shown to correlate with experimentally obtained pain intensity ratings in healthy subjects. Here, we assessed the validity of the PSQ for investigation of general pain perception (ie, pain perception outside the site of clinical pain) in chronic pain patients. PSQ scores were obtained in 134 chronic pain patients and compared to those of 185 healthy control subjects. In a subgroup of 46 chronic pain patients, we performed experimental pain testing outside the clinical pain site, including different modalities (heat, cold, pressure, and pinprick) and different measures (pain thresholds, pain intensity ratings). Results show that PSQ scores were significantly correlated with both experimental pain intensity ratings (Pearson's r=0.71, P<.001) and experimental pain thresholds (r=-0.52, P<.001). In addition, chronic pain patients exhibited significantly elevated PSQ scores as compared to healthy controls, consistent with the generalized increase of experimentally determined pain perception that has repeatedly been reported in chronic pain patients. These results demonstrate that the PSQ constitutes a valid self-rating measure of pain perception outside the clinical pain site in chronic pain patients and might serve as an alternative to experimental assessment of pain perception outside the clinical pain site in situations where experimental pain testing is not feasible.

Hyperalgesia in heroin dependent patients and the effects of opioid substitution therapy

Evidence suggests that patients on opiate maintenance therapy for the treatment of addiction present with opioid-induced hyperalgesia. This study compared the experimental (cold-pressor, electrical stimulation) pain responses of 82 treatment-seeking heroin-dependent adults, randomized to methadone (METH, n = 11) or buprenorphine (BUP, n = 64) therapy, with matched drug free controls (n = 21). Heroin-dependent participants were evaluated at baseline (treatment entry), medication (METH or BUP) stabilization (4-8 weeks), and chronic administration (12-18 weeks), at trough (just prior to dosing) and peak (3 hours after dosing) plasma levels. Collection of the control group's pain responses occurred twice during a single session, 3 hours apart. Baseline comparisons indicate that heroin-dependent individuals demonstrate significantly shorter latencies to threshold and tolerance for cold-pressor pain than the control group. Across pain stimuli and time points, little change in pain responses was found over time, the exception being cold pressor pain tolerance, for which hyperalgesia significantly increased at trough METH/BUP levels in both groups as they stabilized in treatment. We conclude that heroin-dependent individuals are hyperalgesic, and that once stabilized in treatment, are not different in pain responses regardless of treatment agent. The effects of nonpharmacologic therapy and previous heroin use may explain increased hyperalgesia found with treatment.

PERSPECTIVE

To better understand the clinical phenomenon of opioid-induced hyperalgesia, this article describes experimental pain responses of heroin-dependent participants both prior to and over the course of maintenance therapy with methadone or buprenorphine. Hyperalgesia is present with illicit and treatment opioid use, and does not appear to appreciably improve over the course of treatment.

A quantitative review of ethnic group differences in experimental pain response: do biology, psychology, and culture matter?

OBJECTIVE

Pain is a subjectively complex and universal experience. We examine research investigating ethnic group differences in experimental pain response and factors contributing to group differences.

METHOD

We conducted a systematic literature review and analysis of studies using experimental pain stimuli to assess pain sensitivity across multiple ethnic groups. Our search covered the period from 1944 to 2011, and used the PubMed bibliographic database; a reference source containing over 17 million citations. We calculated effect sizes; identified ethnic/racial group categories, pain stimuli, and measures; and examined findings regarding biopsychosociocultural factors contributing to ethnic/racial group differences.

RESULTS

We found 472 studies investigating ethnic group differences and pain. Twenty-six of these met our review inclusion criteria of investigating ethnic group differences in experimental pain. The majority of studies included comparisons between African Americans (AA) and non-Hispanic Whites (NHW). There were consistently moderate to large effect sizes for pain tolerance across multiple stimulus modalities; AA demonstrated lower pain tolerance. For pain threshold, findings were generally in the same direction, but effect sizes were small to moderate across ethnic groups. Limited data were available for suprathreshold pain ratings. A subset of studies comparing NHW and other ethnic groups showed a variable range of effect sizes for pain threshold and tolerance.

CONCLUSION

There are potentially important ethnic/racial group differences in experimental pain perception. Elucidating ethnic group differences has translational merit for culturally competent clinical care and for addressing and reducing pain treatment disparities among ethnically/racially diverse groups.

A systematic literature review of 10 years of research on sex/gender and experimental pain perception - part 1: are there really differences between women and men?

The purpose of this systematic review was to summarize and critically appraise the results of 10 years of human laboratory research on pain and sex/gender. An electronic search strategy was designed by a medical librarian and conducted in multiple databases. A total of 172 articles published between 1998 and 2008 were retrieved, analyzed, and synthesized. The first set of results (122 articles), which is presented in this paper, examined sex difference in the perception of laboratory-induced thermal, pressure, ischemic, muscle, electrical, chemical, and visceral pain in healthy subjects. This review suggests that females (F) and males (M) have comparable thresholds for cold and ischemic pain, while pressure pain thresholds are lower in F than M. There is strong evidence that F tolerate less thermal (heat, cold) and pressure pain than M but it is not the case for tolerance to ischemic pain, which is comparable in both sexes. The majority of the studies that measured pain intensity and unpleasantness showed no sex difference in many pain modalities. In summary, 10 years of laboratory research have not been successful in producing a clear and consistent pattern of sex differences in human pain sensitivity, even with the use of deep, tonic, long-lasting stimuli, which are known to better mimic clinical pain. Whether laboratory studies in healthy subjects are the best paradigm to investigate sex differences in pain perception is open to question and should be discussed with a view to enhancing the clinical relevance of these experiments and developing new research avenues.

A systematic literature review of 10 years of research on sex/gender and pain perception - part 2: do biopsychosocial factors alter pain sensitivity differently in women and men?

This systematic review summarizes the results of 10 years of laboratory research on pain and sex/gender. An electronic search strategy was designed by a medical librarian to access multiple databases. A total of 172 articles published between 1998 and 2008 were retrieved, analyzed, and synthesized. The second set of results presented in this review (129 articles) examined various biopsychosocial factors that may contribute to differences in pain sensitivity between healthy women and men. The results revealed that the involvement of hormonal and physiological factors is either inconsistent or absent. Some studies suggest that temporal summation, allodynia, and secondary hyperalgesia may be more pronounced in women than in men. The evidence to support less efficient endogenous pain inhibitory systems in women is mixed and does not necessarily apply to all pain modalities. With regard to psychological factors, depression may not mediate sex differences in pain perception, while the role of anxiety is ambiguous. Cognitive and social factors appear to partly explain some sex-related differences. Finally, past individual history may be influential in female pain responses. However, these conclusions must be treated with much circumspection for various methodological reasons. Furthermore, some factors/mechanisms remain understudied in the field. There is also a need to assess and improve the ecological validity of findings from laboratory studies on healthy subjects, and perhaps a change of paradigm needs to be considered at this point in time to better understand the factors that influence the experience of women and men who suffer from acute or chronic pain.

Decoding an individual's sensitivity to pain from the multivariate analysis of EEG data

The perception of pain is characterized by its tremendous intra- and interindividual variability. Different individuals perceive the very same painful event largely differently. Here, we aimed to predict the individual pain sensitivity from brain activity. We repeatedly applied identical painful stimuli to healthy human subjects and recorded brain activity by using electroencephalography (EEG). We applied a multivariate pattern analysis to the time-frequency transformed single-trial EEG responses. Our results show that a classifier trained on a group of healthy individuals can predict another individual's pain sensitivity with an accuracy of 83%. Classification accuracy depended on pain-evoked responses at about 8 Hz and pain-induced gamma oscillations at about 80 Hz. These results reveal that the temporal-spectral pattern of pain-related neuronal responses provides valuable information about the perception of pain. Beyond, our approach may help to establish an objective neuronal marker of pain sensitivity which can potentially be recorded from a single EEG electrode.

The cost and benefit of juvenile training on adult perceptual skill

Sensory experience during development can modify the CNS and alter adult perceptual skills. While this principle draws support from deprivation or chronic stimulus exposure studies, the effect of training is addressed only in adults. Here, we asked whether a brief period of training during development can exert a unique impact on adult perceptual skills. Juvenile gerbils were trained to detect amplitude modulation (AM), a stimulus feature elemental to animal communication sounds. When the performance of these juvenile-trained animals was subsequently assessed in adulthood, it was superior to a control group that received an identical regimen of training as adults. The juvenile-trained animals displayed significantly better AM detection thresholds. This was not observed in an adult group that received only exposure to AM stimuli as juveniles. To determine whether enhanced adult performance was due solely to learning the conditioned avoidance procedure, juveniles were trained on frequency modulation (FM) detection, and subsequently assessed on AM detection as adults. These animals displayed significantly poorer AM detection thresholds than all other groups. Thus, training on a specific auditory task (AM detection) during development provided a benefit to performance on that task in adulthood, whereas an identical training regimen in adulthood did not bring about this enhancement. In contrast, there was a cost, in adulthood, following developmental training on a different task (FM detection). Together, the results demonstrate a period of heightened sensitivity in the developing CNS such that behavioral training in juveniles has a unique impact on adult behavioral capabilities.

Dental anxiety in relation to neuroticism and pain sensitivity. A twin study

Predisposing personality traits as well as heightened pain sensitivity and fear of pain have been hypothesized as central factors in the development of dental anxiety. The aim of the study was to estimate the heritability of dental anxiety, and to investigate the genetic and environmental sources of covariance between dental anxiety on one hand, and pain sensitivity and the neuroticism trait on the other. A sample comprising 188 twins, aged 23-35 years (53 monozygotic and 39 dizygotic twin pairs, and 4 single twins whose co-twin did not participate), was included in the study. Measures of dental anxiety and personality were obtained using Corah's Dental Anxiety Scale and the NEO Personality Inventory Revised, respectively. Heat pain and cold pressor pain sensitivity were assessed using standard pain testing procedures. Bivariate Cholesky models were employed to decompose the correlations between phenotypes into genetic and environmental factors. Using models with common additive genetic (A) and individual-specific environmental (E) factors, moderate heritability (i.e., .41) for dental anxiety was demonstrated. Virtually all of the phenotypic correlation between neuroticism and dental anxiety could be accounted for by A. Furthermore, a substantial part of the variance in dental anxiety was due to specific genetic and individual environmental influences unrelated to neuroticism. The phenotypic correlations between dental anxiety and the pain sensitivity indices were close to zero. Thus, while neuroticism and dental anxiety share a sizeable proportion of genetic (but not environmental) risk factors, the results also suggest that these two attributes are distinct entities with overlapping, but not identical, etiologies.

Pain during topical photodynamic therapy: uncomfortable and unpredictable

BACKGROUND

The major drawback of the widely used photodynamic therapy (PDT) is treatment-related pain.

OBJECTIVE

Gain insight into the intensity of and predictive factors for painful burning sensation associated with PDT.

METHODS

A prospective cohort study was performed at the department of Dermatology in the Maastricht University Medical Centre in Maastricht, a reference centre for dermatological oncology in The Netherlands. A total of 141 lesions in 108 patients were included, treated from November 2008 until June 2009 with PDT for superficial basal cell carcinoma, Bowen's disease (BD) or actinic keratosis (AK). Painful burning sensation was scored based on an 11-point pain intensity numeric rating scale (PI-NRS) (0=no pain; 10=worst possible pain).

RESULTS

The percentage of patients with a PI-NRS score over six was 32.6% and 37.9% during the primary and follow-up PDT session respectively. A total of 76.6% (95/124) of the patients was consistent in pain intensity score reporting. Factors associated with higher PI-NRS scores were treatment of AK or BD, tumour localization in the head/neck region, patient's age over 70, Fitzpatrick skintype I/II, photosensitizer 5-aminolevulinic acid and use of oral analgesics. After mutual adjustment of these factors, Fitzpatrick skintype remained the only independent predictor of PI-NRS scores during PDT.

CONCLUSION

It remains difficult to decide which patients should be considered for pain relieving measures. The solution remains to support all patients treated with PDT with pain relieving techniques or to let the support of pain relieving measures depend on the reported pain score for the primary session.

Are psychoactive substance (opioid)-dependent chronic pain patients hyperalgesic?

OBJECTIVES

One indirect line of evidence for opioid-induced hyperalgesia (OIH) in humans is decreased pain thresholds (PTREs) and tolerances (PTOLs) in opioid addicts on opioids. There are a number of such studies in opioid maintained addicts, but no such studies in chronic pain patients (CPPs) with current opioid addiction. The objective of this study was to determine if this group demonstrates hyperalgesia.

METHODS

CPPs were subdivided into those with psychoactive substance (opioid dependence) (PSOD) (n=38) (addicted) and those taking opioids but without PSOD (n=198) (not addicted). A group of opioid-free non-CPPs served as a control group (n=36). PTREs/PTOLs were determined in each group by pressure pain stimulation in both the right and left index fingers. PTREs/PTOLs were compared by analysis of variance among the three groups controlling for sex/age and by analysis of covariance between the PSOD CPPs and non-PSOD CPPs controlling for duration of pain and visual analog scale pain level over the last 24 hours.

RESULTS

The PSOD and non-PSOD CPPs had significantly lower PTREs and PTOLs vs. the control group, ie, were hyperalgesic. However, they were not significantly different from each other.

CONCLUSIONS

This study contributes to the human OIH literature. However, because of the potential confounders in this study, the issue of OIH in humans remains unresolved.

Prolonged maturation of auditory perception and learning in gerbils

In humans, auditory perception reaches maturity over a broad age range, extending through adolescence. Despite this slow maturation, children are considered to be outstanding learners, suggesting that immature perceptual skills might actually be advantageous to improvement on an acoustic task as a result of training (perceptual learning). Previous non-human studies have not employed an identical task when comparing perceptual performance of young and mature subjects, making it difficult to assess learning. Here, we used an identical procedure on juvenile and adult gerbils to examine the perception of amplitude modulation (AM), a stimulus feature that is an important component of most natural sounds. On average, Adult animals could detect smaller fluctuations in amplitude (i.e., smaller modulation depths) than Juveniles, indicating immature perceptual skills in Juveniles. However, the population variance was much greater for Juveniles, a few animals displaying adult-like AM detection. To determine whether immature perceptual skills facilitated learning, we compared naïve performance on the AM detection task with the amount of improvement following additional training. The amount of improvement in Adults correlated with naïve performance: those with the poorest naïve performance improved the most. In contrast, the naïve performance of Juveniles did not predict the amount of learning. Those Juveniles with immature AM detection thresholds did not display greater learning than Adults. Furthermore, for several of the Juveniles with adult-like thresholds, AM detection deteriorated with repeated testing. Thus, immature perceptual skills in young animals were not associated with greater learning.