Reference values of mechanical and thermal pain tests in a pain-free population

Vitamin D and central hypersensitivity in patients with chronic pain

BACKGROUND

Low vitamin D is implicated in various chronic pain conditions with, however, inconclusive findings. Vitamin D might play an important role in mechanisms being involved in central processing of evoked pain stimuli but less so for spontaneous clinical pain.

OBJECTIVE

This study aims to examine the relation between low serum levels of 25-hydroxyvitamin D3 (25-OH D) and mechanical pain sensitivity.

DESIGN

We studied 174 patients (mean age 48 years, 53% women) with chronic pain. A standardized pain provocation test was applied, and pain intensity was rated on a numerical analogue scale (0-10). The widespread pain index and symptom severity score (including fatigue, waking unrefreshed, and cognitive symptoms) following the 2010 American College of Rheumatology preliminary diagnostic criteria for fibromyalgia were also assessed. Serum 25-OH D levels were measured with a chemiluminescent immunoassay.

RESULTS

Vitamin deficiency (25-OH D < 50 nmol/L) was present in 71% of chronic pain patients; another 21% had insufficient vitamin D (25-OH D < 75 nmol/L). After adjustment for demographic and clinical variables, there was a mean ± standard error of the mean increase in pain intensity of 0.61 ± 0.25 for each 25 nmol/L decrease in 25-OH D (P = 0.011). Lower 25-OH D levels were also related to greater symptom severity (r = -0.21, P = 0.008) but not to the widespread pain index (P = 0.83) and fibromyalgia (P = 0.51).

CONCLUSIONS

The findings suggest a role of low vitamin D levels for heightened central sensitivity, particularly augmented pain processing upon mechanical stimulation in chronic pain patients. Vitamin D seems comparably less important for self-reports of spontaneous chronic pain.

Gender differences in blood pressure-related hypoalgesia in a general population: the Tromsø Study

An inverse association between resting blood pressure (BP) and acute pain sensitivity is well documented. Whether BP-related hypoalgesia differs by gender is unclear from prior work. Whether it increases proportionally with BP throughout the full BP range is also unknown. We examined BP-related hypoalgesia in a general population sample (n = 10,371, aged 30-87) of equal gender distribution reflecting the extremely low through hypertensive BP range. Resting BP was assessed and individuals participated in a standardized cold pressor test, providing pain ratings every 9 seconds. For systolic BP (SBP), a significant SBP × Gender interaction was observed on mean pain ratings (P < .001). Females displayed significant BP-related hypoalgesia (P < .001), with males showing a 38% smaller effect (P < .001). A similar DBP × Gender interaction was also observed (P < .05). Spline regression indicated a significant (P < .001) change in slope of the SBP-pain association at 140 mmHg. Among individuals with lower resting SBP (<140/90), increasing hypoalgesia with increasing SBP levels was observed (P < .001), with no further increases in those with higher BP (≥140/90; P > .10). This is the first large-scale study to confirm past results suggesting that BP-related hypoalgesia differs by gender; that is, females exhibited greater hypoalgesia. BP-related hypoalgesia appears subject to ceiling effects in the hypertensive BP range.

PERSPECTIVE

Females show greater BP-related hypoalgesia than males, highlighting gender differences in endogenous antinociceptive systems. Extent of BP-related hypoalgesia does not increase further once resting pressures reach the hypertensive range, suggesting persistent maximal demands on these antinociceptive systems among hypertensive individuals.

Ranking of tests for pain hypersensitivity according to their discriminative ability in chronic neck pain

BACKGROUND AND OBJECTIVES

Quantitative sensory testing (QST) is widely used to investigate peripheral and central sensitization. However, the comparative performance of different QST for diagnostic or prognostic purposes is unclear. We explored the discriminative ability of different quantitative sensory tests in distinguishing between patients with chronic neck pain and pain-free control subjects and ranked these tests according to the extent of their association with pain hypersensitivity.

METHODS

We performed a case-control study in 40 patients and 300 control subjects. Twenty-six tests, including different modalities of pressure, heat, cold, and electrical stimulation, were used. As measures of discrimination, we estimated receiver operating characteristic curves and likelihood ratios.

RESULTS

The following quantitative sensory tests displayed the best discriminative value: (1) pressure pain threshold at the site of the most severe neck pain (fitted area under the receiver operating characteristic curve, 0.92), (2) reflex threshold to single electrical stimulation (0.90), (3) pain threshold to single electrical stimulation (0.89), (4) pain threshold to repeated electrical stimulation (0.87), and (5) pressure pain tolerance threshold at the site of the most severe neck pain (0.86). Only the first 3 could be used for both ruling in and out pain hypersensitivity.

CONCLUSIONS

Pressure stimulation at the site of the most severe pain and parameters of electrical stimulation were the most appropriate QST to distinguish between patients with chronic neck pain and asymptomatic control subjects. These findings may be used to select the tests in future diagnostic and longitudinal prognostic studies on patients with neck pain and to optimize the assessment of localized and spreading sensitization in chronic pain patients.

Do central hypersensitivity and altered pain modulation predict the course of chronic low back and neck pain?

OBJECTIVES

Widespread central hypersensitivity and altered conditioned pain modulation (CPM) have been documented in chronic pain conditions. Information on their prognostic values is limited. This study tested the hypothesis that widespread central hypersensitivity (WCH) and altered CPM, assessed during the chronic phase of low back and neck pain, predict poor outcome.

METHODS

A total of 169 consecutive patients with chronic low back or neck pain, referred to the pain clinic during 1 year, were analyzed. Pressure pain tolerance threshold at the second toe and tolerance time during cold pressor test at the hand assessed WCH. CPM was measured by the change in pressure pain tolerance threshold (test stimulus) after cold pressor test (conditioning stimulus). A structured telephone interview was performed 12 to 15 months after testing to record outcome parameters. Linear regression models were used, with average and maximum pain intensity of the last 24 hours at follow-up as endpoints. Multivariable analyses included sex, age, catastrophizing scale, Beck Depression Inventory, pain duration, intake of opioids, and type of pain syndrome.

RESULTS

Statistically significant reductions from baseline to follow-up were observed in pain intensity (P<0.001). No evidence for an association between the measures of WCH or CPM and intensity of chronic pain at follow-up was found.

DISCUSSION

A major predictive value of the measures that we used is unlikely. Future studies adopting other assessment modalities and possibly standardized treatments are needed to further elucidate the prognostic value of WCH and altered CPM in chronic pain.

Pain in 1,000 women treated for breast cancer: a prospective study of pain sensitivity and postoperative pain

BACKGROUND

This article describes the methods and results of the early part (experimental pain tests and postoperative analgesia) of a study that assesses genetic and other factors related to acute pain and persistent pain after treatment of breast cancer in a prospective cohort of 1,000 women.

METHODS

One thousand consenting patients were recruited to the study. Before surgery (breast resection or mastectomy with axillary surgery), the patients filled in questionnaires about health, life style, depression (Beck Depression Inventory), and anxiety (State-Trait Anxiety Inventory). They were also exposed to experimental tests measuring heat (43° and 48°C, 5 s) and cold (2-4°C) pain intensity and tolerance. Anesthesia was standardized with propofol and remifentanil, and postoperative analgesia was optimized with i.v. oxycodone.

RESULTS

The patients showed significant interindividual variation in heat and cold pain sensitivity and cold pain tolerance. There was a strong correlation between the experimental pain measures across the tests. Presence of chronic pain, the number of previous operations, and particularly state anxiety were related to increased pain sensitivity. Previous smoking correlated with decreased heat pain sensitivity. These factors explained 4-5% of the total variance in pain sensitivity in these tests. Oxycodone consumption during 20 h was significantly higher in patients who had axillary clearance. Oxycodone consumption had only a weak correlation with the experimental pain measures.

CONCLUSIONS

Contact heat and cold pressure tests identify variability in pain sensitivity which is modified by factors such as anxiety, chronic pain, previous surgery, and smoking. High levels of anxiety are connected to increased pain sensitivity in experimental and acute postoperative pain.

Translational pain biomarkers in the early development of new neurotherapeutics for pain management

Translation of the analgesic efficacy of investigational neurotherapeutics from pre-clinical pain models into clinical trial phases is associated with a high risk of failure. Application of human pain biomarkers in early stages of clinical trials can potentially enhance the rate of successful translation, which would eventually reduce both length and costs of drug development after the pre-clinical stage. Human pain biomarkers are based on the standardized activation of pain pathways followed by the assessment of ongoing and paroxysmal pain, plus evoked responses which can be applied to healthy individuals and patients prior to and after pharmacological interventions. This review discusses the rationality and feasibility of advanced human pain biomarkers in early phases of drug development for pain management which is still an unmet medical need.

Prolactin regulates TRPV1, TRPA1, and TRPM8 in sensory neurons in a sex-dependent manner: Contribution of prolactin receptor to inflammatory pain

Prolactin (PRL) is a hormone produced in the anterior pituitary but also synthesized extrapituitary where it can influence diverse cellular processes, including inflammatory responses. Females experience greater pain in certain inflammatory conditions, but the contribution of the PRL system to sex-dependent inflammatory pain is unknown. We found that PRL regulates transient receptor potential (TRP) channels in a sex-dependent manner in sensory neurons. At >20 ng/ml, PRL sensitizes TRPV1 in female, but not male, neurons. This effect is mediated by PRL receptor (PRL-R). Likewise, TRPA1 and TRPM8 were sensitized by 100 ng/ml PRL only in female neurons. We showed that complete Freund adjuvant (CFA) upregulated PRL levels in the inflamed paw of both male and female rats, but levels were higher in females. In contrast, CFA did not change mRNA levels of long and short PRL-R in the dorsal root ganglion or spinal cord. Analysis of PRL and PRL-R knockout (KO) mice demonstrated that basal responses to cold stimuli were only altered in females, and with no significant effects on heat and mechanical responses in both sexes. CFA-induced heat and cold hyperalgesia were not changed in PRL and PRL-R KO compared with wild-type (WT) males, whereas significant reduction of heat and cold post-CFA hyperalgesia was detected in PRL and PRL-R KO females. Attenuation of CFA-induced mechanical allodynia was observed in both PRL and PRL-R KO females and males. Thermal hyperalgesia in PRL KO females was restored by administration of PRL into hindpaws. Overall, we demonstrate a sex-dependent regulation of peripheral inflammatory hyperalgesia by the PRL system.

Neural and physiological responses to a cold pressor challenge in healthy adolescents

Abnormal autonomic function is common in pediatric diseases. Assessment of central mechanisms underlying autonomic challenges may reveal vulnerabilities antecedent to system failure. Our objective was to characterize central markers and physiological responses to a cold pressor challenge in normal children as a critical step for establishing such screening. We performed functional magnetic resonance imaging (fMRI) and collected physiological measures during cold application to the foot in 24 healthy adolescents (15.5 ± 0.4 years, 13 male). The protocol included a 120-sec baseline, 120-sec right-foot cold water immersion (4°C), and 120-sec recovery. Analyses included heart rate (HR) cross-correlations with fMRI signals. Cold application increased HR 13% 5-7 sec after onset, which remained elevated throughout the challenge. Respiratory rate transiently increased (peak 22%), then declined (nadir 12% below baseline), before normalizing at 75 sec. Cold onset rapidly increased somatosensory cortex and medullary signals, which fell after 25 sec. Right anterior insular cortex signals increased early, followed after 20 sec by the left anterior insula, with HR declining 8 sec later. Amygdalae signals also rose, but signals declined in the posterior cingulate cortex, caudate nucleus, hippocampus, and hypothalamus. Declining signals appeared late in the cerebellar fastigial nuclei (60-120 sec), and in the pons and thalamus. Somatosensory cortex, fastigial nuclei, and hypothalamic responses were principally left-sided, with bilateral responses elsewhere. Late left anterior insula responses likely underlie the HR decline; the late cerebellar pattern may modulate recovery. The laterality, timing, and amplitude of normative responses and rostral response differentiation indicate the complex integration of adolescent autonomic processing and provide indices for pathological comparisons.

Experimental pain phenotype profiles in a racially and ethnically diverse sample of healthy adults

OBJECTIVE

To examine patterns of interindividual variability in experimental pain responses emerging from multiple experimental pain measures in a racially/ethnically diverse sample of healthy adults and to examine the association between the derived phenotype profiles with demographic, psychological, and health-related measures.

METHODS

Two hundred and ninety-one participants underwent heat, cold, pressure, and ischemic pain assessments, and completed several psychological and health-related assessments. The experimental pain measures were subjected to a principal component analysis and factor scores were used to compute Pain Sensitivity Index scores. The scores were subsequently submitted to a cluster analysis to identify patterns of pain sensitivity across experimental pain modalities.

RESULTS

The sample was equally composed of non-Hispanic whites, African Americans, and Hispanic whites. Sensitivity scores were computed for heat pain, pressure pain, cold pain, ischemic pain, and temporal summation of heat pain. Five distinct clusters were characterized by high heat pain sensitivity, low ischemic pain sensitivity, low cold pain sensitivity, low pressure pain sensitivity, and high temporal summation. Cluster membership was significantly different by sex as well as somatic reactivity and catastrophizing, although cluster differences were most pronounced between the heat pain-sensitive individuals vs the cold pain-insensitive individuals.

CONCLUSIONS

Our findings highlight the importance of phenotyping individuals to account for interindividual differences in pain responses. Our findings also replicate previously reported pain phenotypes, which are not solely related to demographic, psychosocial, or health-related factors in our healthy participants. Future studies designed to elucidate the biological underpinnings of pain sensitivity profiles would be of substantial value.

Morphine versus oxycodone analgesia after percutaneous kidney stone surgery: a randomised double blinded study

According to previous studies oxycodone might have some advantages over morphine in the treatment of visceral pain. This study investigated the opioid consumption (primary outcome), pain relief and side effects (secondary outcomes) of morphine versus oxycodone after percutaneous nephrolithotomy using a method where the somatic pain component was minimized. Forty-four adult patients were studied. The patients were randomised to receive either morphine or oxycodone intravenously as postoperative pain treatment. During the first 4 h after surgery the opioid consumption, pain scores and side effects (nausea, dizziness, sedation, respiratory effects and itching) were registered. The postoperative opioid consumption varied considerably between the patients but the mean opioid consumption in the morphine and oxycodone group was comparable (18.93 mg versus 16.15 mg, P = 0.7). Nausea was significantly less frequent with morphine (P = 0.03). In this study morphine and oxycodone produced similar analgesia the first 4 h after surgery but the frequency of nausea was significantly less patient-reported with morphine. The hypothesis that oxycodone would be superior in the treatment of visceral pain after percutaneous kidney stone operation was not confirmed.

Pain sensitivity and pericranial tenderness in children with tension-type headache: a controlled study

Purpose

To compare tenderness and pain sensitivity in children (aged 7–17 years) with tension-type headache (TTH) and healthy controls using total tenderness score (TTS), pressure pain threshold (PPT), and pain perceived at suprapressure pain threshold (supraPPT).

Patients and methods

Twenty-three children with frequent episodic TTH, 36 with chronic TTH, and 57 healthy controls were included. TTS was measured bilaterally at seven pericranial myofascial structures. PPT and supraPPT were assessed in the finger, m. temporalis, and m. trapezius by a Somedic® algometer. SupraPPT was defined as the pain perceived at a stimulus calculated as the individual site-specific PPT + 50%.

Statistics

The effect of group, sex, age, headache frequency, intensity, and years on TTS, PPT, and supraPPT was analyzed by general linear models. Confirmatory factor analysis was analyzed for mutual relations between measurements.

Results and conclusion

Tenderness increased uniformly in both frequent episodic TTH (median 14; interquartile range [IQR] 10–18; P < 0.001) and chronic TTH (median 13; IQR 9–20; P < 0.001) compared to controls (median 5, IQR 3–11). However, the children with frequent episodic TTH and chronic TTH did not show significantly increased sensitivity when measured by PPT or supraPPT. Factor analysis confirmed that the site-specific measurements depended on general latent variables. Consequently, the PPT and supraPPT tests can be assumed to measure central pain-processing levels.

The prevalence of widespread central hypersensitivity in chronic pain patients

BACKGROUND

Chronic pain is associated with generalized hypersensitivity and impaired endogenous pain modulation (conditioned pain modulation; CPM). Despite extensive research, their prevalence in chronic pain patients is unknown. This study investigated the prevalence and potential determinants of widespread central hypersensitivity and described the distribution of CPM in chronic pain patients.

METHODS

We examined 464 consecutive chronic pain patients for generalized hypersensitivity and CPM using pressure algometry at the second toe and cold pressor test. Potential determinants of generalized central hypersensitivity were studied using uni- and multivariate regression analyses. Prevalence of generalized central hypersensitivity was calculated for the 5th, 10th and 25th percentile of normative values for pressure algometry obtained by a previous large study on healthy volunteers. CPM was addressed on a descriptive basis, since normative values are not available.

RESULTS

Depending on the percentile of normative values considered, generalized central hypersensitivity affected 17.5-35.3% of patients. 23.7% of patients showed no increase in pressure pain threshold after cold pressor test. Generalized central hypersensitivity was more frequent and CPM less effective in women than in men. Unclearly classifiable pain syndromes showed higher frequencies of generalized central hypersensitivity than other pain syndromes.

CONCLUSIONS

Although prevalent in chronic pain, generalized central hypersensitivity is not present in every patient. An individual assessment is therefore required in order to detect altered pain processing. The broad basic knowledge about central hypersensitivity now needs to be translated into concrete clinical consequences, so that patients can be offered an individually tailored mechanism-based treatment.

Percentile normative values of parameters of electrical pain and reflex thresholds

Background and purpose

Central hypersensitivity, defined as an increased excitability of the central nervous system, is considered as the main factor behind facilitation of central pain processes and is probably a very important factor in the induction and maintenance of chronic pain. Widespread hyposensitivity is less studied than hypersensitivity states, but recent work indicates that hypoesthesia may be present in chronic non-neuropathic pain conditions and could have negative prognostic value. Electrical pain and reflex thresholds are well established measures of central pain sensitivity in human pain research. One potential application of these assessments in clinical practice is the detection of central hyper- or hyposensitivity in individual patients. In order to identify these disturbances in the central pain processing of individual patients, knowledge of reference values is essential. We computed percentile normative values of nociceptive withdrawal reflex (NWR) and pain thresholds to different electrical stimulation paradigms. The aim was to provide reference values for the assessment of widespread central hyper- and hyposensitivity in individual patients.

Methods

300 pain-free subjects (150 males and 150 females, 18-80 years) were studied. Pain and reflex thresholds to single and repeated electrical stimulation (applied on the innervations area of the sural nerve), and the area of reflex receptive fields (RRF) were recorded. The RRF area was defined as the skin area of the sole of the foot from which a NWR could be evoked in the tibialis anterior muscle, expressed as proportion of the foot sole. For the threshold assessments, quantile regressions were performed to compute critical normative values for widespread central hypersensitivity (5th, 10th and 25th percentiles) and hyposensitivity (75th, 90th and 95th percentiles). For the RRF the opposite applied, computing normative values for widespread central hypersensitivity as 75th, 90th and 95th percentiles, and normative values for hyposensitivity as 5th, 10th and 25th percentiles. The following covariates were included in the regression analyses: gender, age, height, weight, body mass index, body side of testing, depression (Beck Depression Inventory), anxiety (State-Trait-Anxiety-Inventory), catastrophizing (Catastrophizing Scale of the Coping Strategies Questionnaire) and Short-Form 36.

Results

Age had a significant impact on the subjective pain threshold to single electrical stimuli. The reflex threshold to single electrical stimuli was lower on the dominant compared to the non-dominant side. Therefore, the percentiles for single stimulus pain threshold have been stratified by age and the percentiles for single stimulus reflex threshold by body side (dominant vs. non-dominant). Critical normative values of all tests were computed for widespread central hypersensitivity (5th, 10th and 25th percentiles) and hyposensitivity (75th, 90th and 95th percentiles). The values are provided in the table of the manuscript.

Conclusions and implications

The computed estimates of critical normative values for the electrical pain test can be used in clinical practice for the assessment of widespread central hypersensitivity and hyposensitivity in individual patients, and in future research on mechanism-based treatment of chronic pain.

Sex differences in pain and pain inhibition: multiple explanations of a controversial phenomenon

A clear majority of patients with chronic pain are women; however, it has been surprisingly difficult to determine whether this sex bias corresponds to actual sex differences in pain sensitivity. A survey of the currently available epidemiological and laboratory data indicates that the evidence for clinical and experimental sex differences in pain is overwhelming. Various explanations for this phenomenon have been given, ranging from experiential and sociocultural differences in pain experience between men and women to hormonally and genetically driven sex differences in brain neurochemistry.

Basic aspects of musculoskeletal pain: from acute to chronic pain

The transition from acute to chronic musculoskeletal pain is not well understood. To understand this transition, it is important to know how peripheral and central sensitization are manifested and how they can be assessed. A variety of human pain biomarkers have been developed to quantify localized and widespread musculoskeletal pain. In addition, human surrogate models may be used to induce sensitization in otherwise healthy volunteers. Pain can arise from different musculoskeletal structures (e.g. muscles, joints, ligaments, or tendons), and differentiating the origin of pain from those different structures is a challenge. Tissue specific pain biomarkers can be used to tease these different aspects. Chronic musculoskeletal pain patients in general show signs of local/central sensitization and spread of pain to degrees which correlate to pain intensity and duration. From a management perspective, it is therefore highly important to reduce pain intensity and try to minimize the duration of pain.

Hypertension prevalence and diminished blood pressure-related hypoalgesia in individuals reporting chronic pain in a general population: the Tromsø study

Resting blood pressure (BP) is inversely related to pain sensitivity in individuals free of chronic pain, reflecting homeostatic interactions between cardiovascular and pain modulatory systems. Several laboratory studies indicate that BP-related hypoalgesia is diminished in chronic pain patients, suggesting dysfunction in these interacting systems. Separate epidemiological findings reveal elevated hypertension prevalence in the chronic pain population. This study for the first time simultaneously evaluated both hypertension prevalence and BP-related hypoalgesia as they relate to chronic pain in the same sample. Resting BP and pain sensitivity were evaluated in a large general population sample (n=10,135, aged 30-87years). Subjects participated in a standardized 106s cold pressor test, providing pain ratings at 9s intervals. Self-reported presence of chronic pain and history of hypertension and use of antihypertensive medication were assessed. Significant interactions between chronic pain status and resting systolic (P<.001) and diastolic BP (P<.001) on mean pain ratings were observed. These interactions were due to significant (P<.001) BP-related hypoalgesia in individuals free of chronic pain that was twice the magnitude of the hypoalgesia observed in the group reporting chronic pain. Presence of chronic pain was associated with significantly increased odds of comorbid hypertension (P<.001). Within the chronic pain group, higher chronic pain intensity was a significant predictor of positive hypertension status beyond the effects of traditional demographic risk factors (P<.05). Results are consistent with the hypothesis that increased hypertension risk in the chronic pain population might be linked in part to chronic pain-related dysfunction in interacting cardiovascular-pain modulatory systems.

Effects of ethnicity and gender role expectations of pain on experimental pain: a cross-cultural study

BACKGROUND

Gender role expectations of pain (GREP) have been shown to mediate sex differences in experimental pain. Few studies have investigated the role of ethnicity in shaping GREP. The aim of this study was to examine interactions between ethnicity and GREP on experimentally induced pressure and ischaemic pain in Libyan and white British students in their respective countries.

METHODS

Libyan (n = 124) and white British (n = 51) students completed a GREP questionnaire and their response to experimental pain was measured. Blunt pressure pain threshold (PPT) was measured over the 1st interosseous muscle using algometry. Pain intensity and pain unpleasantness (100 mm visual analogue scale) were measured at 1-min intervals during a submaximal effort tourniquet test on the forearm.

RESULTS

Multivariate analysis of variance detected significant effects for Sex and Ethnicity on pain measurements. Men had higher PPTs than women (p < 0.001). Libyans had higher PPTs than white British participants (p < 0.001). There were significant effects for Sex and Ethnicity for pain intensity ratings (p < 0.01) but no significant differences between the sexes in pain unpleasantness (p > 0.05). Libyan participants had higher pain intensity (p < 0.01) and pain unpleasantness (p < 0.05) ratings compared with white British participants. There were effects for Sex and Ethnicity for all GREP dimensions. Libyan participants exhibited stronger stereotypical views in GREP than white British participants (p < 0.001).

CONCLUSIONS

GREP was the mediator of sex but not ethnic differences in pain report, suggesting that gender stereotypical attitudes to pain account for differences in pain expression between men and women.

Serotonin-1A receptor polymorphism (rs6295) associated with thermal pain perception

Background

Serotonin (5-HT) is highly involved in pain regulation and serotonin-1A (5-HT1A) receptors are important in determining central 5-HT tone. Accordingly, variation in the 5-HT1A receptor gene (HTR1A) may contribute to inter-individual differences in human pain sensitivity. The minor G-allele of the HTR1A single nucleotide polymorphism (SNP) rs6295 attenuates firing of serotonergic neurons and reduces postsynaptic expression of the receptor. Experiments in rodents suggest that 5-HT1A-agonism modulates pain in opposite directions at mild compared to high noxious intensities. Based upon this and several other similar observations, we hypothesized that G-carriers would exhibit a relative hypoalgesia at mild thermal stimuli but tend towards hyperalgesia at higher noxious intensities.

Methods

Fourty-nine healthy individuals were selectively genotyped for rs6295. Heat- and cold-pain thresholds were assessed along with VAS-ratings of a range of suprathreshold noxious heat intensities (45°C–49°C). Nociceptive-flexion reflex (NFR) thresholds were also assessed.

Results

Volunteers did not deviate significantly from Hardy-Weinberg equilibrium. G-carriers were less sensitive to threshold-level thermal pain. This relative hypoalgesia was abolished at suprathreshold noxious intensities where G-carriers instead increased their ratings of heat-pain significantly more than C-homozygotes. No differences with regard to NFR-thresholds emerged.

Conclusion/Significance

To the best of our knowledge this is the first study of human pain perception on the basis of variation in HTR1A. The results illustrate the importance of including a range of stimulus intensities in assessments of pain sensitivity. In speculation, we propose that an attenuated serotonergic tone may be related to a ‘hypo- to hyperalgesic’ response-pattern. The involved mechanisms could be of clinical interest as variation in pain regulation is known to influence the risk of developing pain pathologies. Further investigations are therefore warranted.

Prediction of pain sensitivity in healthy volunteers

PURPOSE

The primary objective of the present study was to evaluate predictive parameters of the acute pain score during induction of an inflammatory heat injury.

PATIENTS AND METHODS

Healthy volunteers (50 females/50 males) were included in the study. The predictive potential of gender, anthropometric (body surface area, body mass index), psychological (anxiety, depression, vulnerability), and psychophysical (quantitative sensory testing, conditioned pain modulation) variables in estimating the pain response to a validated heat injury (47°C, 7 minutes, area 12.5 cm(2)) were investigated. All assessments were made in duplicate sessions separated by 21 days (median).

RESULTS

There were three main findings in this study. First, a predictive model of pain sensitivity during the heat injury, including both genders and using multiple regression technique, could account for 28% of the variance (P < 0.0001), but gender-related differences in the final model could not be demonstrated. Second, the results confirmed significant gender-related differences in perception of electrical, pressure, and cold pressor stimuli (P < 0.002). Third, positive correlations between anthropometric data and pain perception during electrical and pressure stimuli were demonstrated (P < 0.001 and P < 0.005, respectively).

CONCLUSION

The study demonstrated predictability of acute pain sensitivity, and although gender-related differences in pain perception were demonstrated, no gender-related differences in pain sensitivity could be shown. Interestingly, positive correlations between anthropometric data and pain perception were shown for the first time.

Early pain after laparoscopic inguinal hernia repair. A qualitative systematic review

BACKGROUND

Early post-operative pain after laparoscopic groin hernia repair may, as in other laparoscopic operations, have its own individual pain pattern and patient-related predictors of early pain. The purpose of this review was to characterise pain within the first post-operative week after transabdominal pre-peritoneal repair (TAPP) and total extraperitoneal repair (TEP), and to identify patient-related predictors of early pain.

METHODS

A qualitative systematic review was conducted. Pubmed, Embase, CINAHL, and the Cochrane database were searched for studies on early pain (first week) after TAPP or TEP.

RESULTS

We included 71 eligible studies with 14,023 patients. Post-operative pain is most severe on day 0 and mainly on a level of 13-58 mm on a visual analogue scale and decreases to low levels on day 3. There seems to be no difference in pain intensity and duration when TEP and TAPP are compared. Deep abdominal pain (i.e. groin pain/visceral pain) dominates over superficial pain (i.e. somatic pain) and shoulder pain (i.e. referred pain) after TAPP. Predictors of early pain are young age and pre-operative high pain response to experimental heat stimulation. Furthermore, evidence supported early pain intensity as a predictive risk factor of chronic pain after laparoscopic groin hernia repair.

CONCLUSION

Early pain within the first week after TAPP and TEP is most severe on the first post-operative day, and the pain pattern is dominated by deep abdominal pain. Early post-operative pain is most intense in younger patients and can be predicted by pre-operative high pain response to experimental heat stimulation.

Ageing Challenges the Results of any Outcome Study: How to Address the Effects of Ageing on Activities of Daily Living

OBJECTIVES: Clinical study results might be substantially biased by the effects of ageing, resulting in considerable underestimation of treatment efficacy, especially when assessing activities of daily living (ADL) in elderly people. This study aimed to define age- and comorbidity-related normative values in ADL. METHODS: In cross-sectional sampling, 23 763 German-speaking Swiss individuals were contacted. Valid questionnaires from 16 191 (68%) eligible individuals ≥ 18 years of age were included in the study. Age-related ADL reference values were calculated using questionnaire data. RESULTS: The sample was representative of the German-speaking Swiss population as determined by 2003 census data. Age- and pain-related ADL reference values were plotted and confirmed that disability increased with increasing age. CONCLUSIONS: The use of ADL scales that provide age- and comorbidity-related reference values for outcome studies or studies focusing on the elderly is recommended in order to reduce the effect of bias.

Gender role affects experimental pain responses: a systematic review with meta-analysis

Gender role refers to the culturally and socially constructed meanings that describe how women and men should behave in certain situations according to feminine and masculine roles learned throughout life. The aim of this meta-analysis was to evaluate the relationship between gender role and experimental pain responses in healthy human participants. We searched computerized databases for studies published between January 1950 and May 2011 that had measured gender role in healthy human adults and pain response to noxious stimuli. Studies were entered into a meta-analysis if they calculated a correlation coefficient (r) for gender role and experimental pain. Searches yielded 4465 'hits' and 13 studies were eligible for review. Sample sizes were 67-235 participants and the proportion of female participants was 45-67%. Eight types of gender role instrument were used. Meta-analysis of six studies (406 men and 539 women) found a significant positive correlation between masculine and feminine personality traits and pain threshold and tolerance, with a small effect size (r = 0.17, p = 0.01). Meta-analysis of four studies (263 men and 297 women) found a significant negative correlation between gender stereotypes specific to pain and pain threshold and tolerance, with a moderate effect size (r = -0.41, p < 0.001). In conclusion, individuals who considered themselves more masculine and less sensitive to pain than the typical man showed higher pain thresholds and tolerances. Gender stereotypes specific to pain scales showed stronger associations with sex differences in pain sensitivity response than masculine and feminine personality trait scales.

Diagnosis of altered central pain processing

STUDY DESIGN

Nonsystematic review.

OBJECTIVE

To review the current knowledge on detecting altered central pain processing in individual patients with chronic pain.

SUMMARY OF BACKGROUND DATA

Alterations in central pain processing are mainly characterized by hyperexcitability of the central nervous system and disturbances in endogenous pain modulation. Although these mechanisms are widely recognized as important determinants of pain and disability, there is need for translation of this knowledge into benefits for patients. To this purpose, the first step is the detection of abnormalities in pain processing in individual patients. Quantitative sensory tests (QST) explore aspects of nociception and pain perception, and are therefore potentially useful for diagnostic purposes.

METHODS

Nonsystematic review of the reliability, validity and reference values of QST for the assessment of altered central pain processing in chronic pain patients.

RESULTS

The reliability of QST is generally high. However, most studies have been performed on healthy volunteers, and few reliability data in patients are available. Furthermore, little is known on the reliability of measures of endogenous pain modulation. The face validity of QST is acceptable. The construct validity cannot be tested, because there is no gold standard for the detection of altered central pain processing in humans. Reference values of different types of QST for applications in neuropathic and musculoskeletal pain have been determined in large samples of pain-free subjects.

CONCLUSION

QST can be used in clinical practice to assess the presence of sensory abnormalities in individual patients. Because information on the reliability and validity of the tests is incomplete, the findings should be interpreted with caution. It is still unclear to what extent disturbances in central pain processing are relevant for the determination of symptoms in individual patients. Furthermore, the therapeutic consequences of these assessments remain undetermined. These are challenges of future translational research.

Algometry with a clothes peg compared to an electronic pressure algometer: a randomized cross-sectional study in pain patients

BACKGROUND

Hypersensitivity of the central nervous system is widely present in pain patients and recognized as one of the determinants of chronic pain and disability. Electronic pressure algometry is often used to explore aspects of central hypersensitivity. We hypothesized that a simple pain provocation test with a clothes peg provides information on pain sensitivity that compares meaningfully to that obtained by a well-established electronic pressure algometer. "Clinically meaningful" was defined as a medium (r = 0.3-0.5) or high (r > 0.5) correlation coefficient according to Cohen's conventions.

METHODS

We tested 157 in-patients with different pain types. A calibrated clothes peg was applied for 10 seconds and patients rated the pain intensity on a 0 to 10 numerical rating scale. Pressure pain detection threshold (PPdt) and pressure pain tolerance threshold (PPtt) were measured with a standard electronic algometer. Both methods were performed on both middle fingers and ear lobes. In a subgroup of 47 patients repeatability (test-retest reliability) was calculated.

RESULTS

Clothes peg values correlated with PPdt values for finger testing with r = -0.54 and for earlobe testing with r = -0.55 (all p-values < 0.001). Clothes peg values also correlated with PPtt values for finger testing with r = -0.55 (p < 0.001). Test-retest reliability (repeatability) showed equally stable results for clothes peg algometry and the electronic algometer (all r-values > 0.89, all p-values < 0.001).

CONCLUSIONS

Information on pain sensitivity provided by a calibrated clothes peg and an established algometer correlate at a clinically meaningful level.

Perception of thermal pain and the thermal grill illusion is associated with polymorphisms in the serotonin transporter gene

Aim

The main aim of this study was to assess if the perception of thermal pain thresholds is associated with genetically inferred levels of expression of the 5-HT transporter (5-HTT). Additionally, the perception of the so-called thermal grill illusion (TGI) was assessed. Forty-four healthy individuals (27 females, 17 males) were selected a-priori based on their 5-HTTLPR/rs25531 (‘tri-allelic 5-HTTLPR’) genotype, with inferred high or low 5-HTT expression. Thresholds for heat- and cold-pain were determined along with the sensory and affective dimensions of the TGI.

Results

Thresholds to heat- and cold-pain correlated strongly (rho  = −0.58, p<0.001). Individuals in the low 5-HTT-expressing group were significantly less sensitive to heat-pain (p = 0.02) and cold-pain (p = 0.03), compared to the high-expressing group. A significant gender-by-genotype interaction also emerged for cold-pain perception (p = 0.02); low 5-HTT-expressing females were less sensitive. The TGI was rated as significantly more unpleasant (affective-motivational dimension) than painful (sensory-discriminatory dimension), (p<0.001). Females in the low 5-HTT expressing group rated the TGI as significantly less unpleasant than high 5-HTT expressing females (p<0.05), with no such differences among men.

Conclusion/Significance

We demonstrate an association between inferred low 5-HTT expression and elevated thresholds to thermal pain in healthy non-depressed individuals. Despite the fact that reduced 5-HTT expression is a risk factor for chronic pain we found it to be related to hypoalgesia for threshold thermal pain. Low 5-HTT expression is, however, also a risk factor for depression where thermal insensitivity is often seen. Our results may thus contribute to a better understanding of the molecular underpinnings of such paradoxical hypoalgesia. The results point to a differential regulation of thermoafferent-information along the neuraxis on the basis of 5-HTT expression and gender. The TGI, suggested to rely on the central integration of thermoafferent-information, may prove a valuable tool in probing the affective-motivational dimension of these putative mechanisms.