Sex differences in heat pain thresholds as a function of assessment method and rate of rise

Age-, gender- and body site-specific reference values of thermal Quantitative Sensory Testing in the Italian population using the Q-sense device

BACKGROUND

Age-, gender- and body site-specific values of thermal Quantitative Sensory Testing (QST) measures have not yet been reported using the novel and cheap device 'Q-sense'. Here, we aimed to assess normative values of Q-sense-derived parameters in a representative Italian population.

METHODS

QST parameters were measured in 84 healthy participants (42 males; aged 20-76 years) equally distributed into three age groups (18-39, 40-59 and 60-80 years). We explored the Warm and the Cold Detection Thresholds (WDT and CDT, respectively) with the method of limits (MLI) and the method of levels (MLE), and the Heat Pain Threshold (HPT) with the MLI. We tested the trigeminal supraorbital region, the hand thenar, and the foot dorsum on the right body side.

RESULTS

We calculated non-parametric reference limits (2.5-97.5th) according to age, gender and tested site. All QST measures were affected by age, gender and tested site. In the extra-trigeminal body sites, females showed lower WDT and higher CDT, while males had higher HPT. Worse sensory discriminative abilities and increased HPT values were found in people aged over 40 on the foot. Age-related differences were more evident with the reaction time-dependent MLI vs. MLE paradigm.

CONCLUSIONS

Demographic characteristics must be considered when QST is used in the clinical setting. The definition of reference limits for sensory testing with the Q-sense herein provided can pave the way towards a more widespread use of thermal QST for diagnosing small fiber neuropathy and for identifying patients' profiles in different chronic pain syndromes.

Cannabidiol has therapeutic potential for myofascial pain in female and male parkinsonian rats

The musculoskeletal orofacial pain is a complex symptom of Parkinson's disease (PD) resulting in stomatognathic system dysfunctions aggravated by the disease rigidity and postural instability. We tested the effect of cannabidiol (CBD), a non-psychotomimetic constituent of Cannabis sativa, in PD-related myofascial pain. Wistar adult female and male rats orofacial allodynic and hyperalgesic responses were tested by Von Frey and formalin tests, before and 21 days past 6-OHDA lesion. Algesic response was tested after masseter muscle injection of CBD (10, 50, 100 μg in 10 μL) or vehicle. Males compared to females in all estrous cycles' phases presented reduced orofacial allodynia and hyperalgesia. According to the estrous cycle's phases, females presented distinct orofacial nociceptive responses, being the estrus phase well-chosen for nociceptive analysis after 6-OHDA lesion (phase with fewer hormone alterations and adequate length). Dopaminergic neuron lesion decreased mechanical and inflammatory nociceptive thresholds in females and males in a higher proportion in females. CBD local treatment reduced the increased orofacial allodynia and hyperalgesia, in males and females. The female rats were more sensitive to CBD effect considering allodynia, responding to the lowest dose. Although females and males respond to the effect of three doses of CBD in the formalin test, males showed a superior reduction in the hyperalgesic response. These results indicate that hemiparkinsonian female in the estrus phase and male answer differently to the different doses of CBD therapy and nociceptive tests. CBD therapy is effective for parkinsonism-induced orofacial nociception.

Optimizing temperature threshold testing in small-fiber neuropathy

INTRODUCTION

We examined optimization of a temperature threshold testing (TTT) protocol for patients with suspected small-fiber neuropathy (SFN) to lessen the burden for both patients and technicians, without sacrificing accuracy.

METHODS

Data from 81 patients with SFN (skin biopsy and TTT abnormal) and 81 without SFN (skin biopsy and TTT normal) were used. Warm, cold, and heat pain sensation thresholds were determined bilaterally on the thenar eminence and foot dorsum by methods of limits and levels. Diagnostic accuracy was determined for various sensory modality combinations through comparative corresponding area under the receiver-operator characteristic curves.

RESULTS

Assessment of warm and cold thresholds in all extremities by the method of levels showed the best discriminatory ability (area under the curve 0.95, sensitivity 84.2%, specificity 93.8%).

CONCLUSIONS

These assessments are suggested for TTT examination in possible SFN patients. By applying this combination, the time needed for TTT can be reduced, maintaining diagnostic accuracy.

Temperature threshold testing: a systematic review

The diagnosis of small fiber neuropathy (SFN) has been recently defined as typical symptoms due to small nerve fiber dysfunction accompanied by reduced intra-epidermal nerve fiber density (IENFD) or abnormal temperature threshold testing (TTT). Guidelines have been published for the assessment of IENFD. However, international guidelines for TTT are lacking. This paper presents a systematic literature review on reported TTT methods and provides recommendations for its future use in studies evaluating patients. A total of 164 papers fulfilled pre-defined requirements and were selected for review. Over 15 types of instruments are currently being used with a variety of methodological approaches for location, stimulus application, and sensation qualities examined. Consensus is needed to standardize the use of TTT as a diagnostic and follow-up tool in patients.

Sex differences in the stability of conditioned pain modulation (CPM) among patients with chronic pain

OBJECTIVES

To examine the temporal stability of conditioned pain modulation (CPM), formerly termed diffuse noxious inhibitory controls, among a sample of patients with chronic pain. The study also examined the factors that might be responsible for the stability of CPM.

DESIGN, SUBJECTS, AND METHODS

In this test-retest study, patients underwent a series of standardized psychophysical pain-testing procedures designed to assess CPM on two separate occasions (i.e., baseline and follow up). Patients also completed self-report measures of catastrophizing (Pain Catastrophizing Scale [PCS] and negative affect [NA]).

RESULTS

Overall, results provided evidence for the stability of CPM among patients with chronic pain. Results, however, revealed considerable sex differences in the stability of CPM. For women, results revealed a significant test-retest correlation between baseline and follow-up CPM scores. For men, however, the test-retest correlation between baseline and follow-up CPM scores was not significant. Results of a Fisher's Z-test revealed that the stability of CPM was significantly greater for women than for men. Follow-up analyses revealed that the difference between men and women in the stability of CPM could not be accounted for by any demographic (e.g., age) and/or psychological factors (PCS and NA).

CONCLUSIONS

Our findings suggest that CPM paradigms possess sufficient reliability to be incorporated into bedside clinical evaluation of patients with chronic pain, but only among women. The lack of CPM reproducibility/stability observed among men places limits on the potential use of CPM paradigms in clinical settings for the assessment of men's endogenous pain-inhibitory function.

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.

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.

Inert gas narcosis has no influence on thermo-tactile sensation

Contribution of skin thermal sensors under inert gas narcosis to the raising hypothermia is not known. Such information is vital for understanding the impact of narcosis on behavioural thermoregulation, diver safety and judgment of thermal (dis)comfort in the hyperbaric environment. So this study aimed at establishing the effects of normoxic concentration of 30% nitrous oxide (N(2)O) on thermo-tactile threshold sensation by studying 16 subjects [eight females and eight males; eight sensitive (S) and eight non-sensitive (NS) to N(2)O]. Their mean (SD) age was 22.1 (1.8) years, weight 72.8 (15.3) kg, height 1.75 (0.10) m and body mass index 23.8 (3.8) kg m(-2). Quantitative thermo-tactile sensory testing was performed on forearm, upper arm and thigh under two experimental conditions: breathing air (air trial) and breathing normoxic mixture of 30% N(2)O (N(2)O trial) in the mixed sequence. Difference in thermo-tactile sensitivity thresholds between two groups of subjects in two experimental conditions was analysed by 3-way mixed-model analysis of covariance. There were no statistically significant differences in thermo-tactile thresholds either between the Air and N(2)O trials, or between S and NS groups, or between females and males, or with respect to body mass index. Some clinically insignificant lowering of thermo-tactile thresholds occurred only for warm thermo-tactile thresholds on upper arm and thigh. The results indicated that normoxic mixture of 30% N(2)O had no influence on thermo-tactile sensation in normothermia.

Test-retest and interobserver reliability of quantitative sensory testing according to the protocol of the German Research Network on Neuropathic Pain (DFNS): a multi-centre study

Quantitative sensory testing (QST) is an instrument to assess positive and negative sensory signs, helping to identify mechanisms underlying pathologic pain conditions. In this study, we evaluated the test-retest reliability (TR-R) and the interobserver reliability (IO-R) of QST in patients with sensory disturbances of different etiologies. In 4 centres, 60 patients (37 male and 23 female, 56.4±1.9years) with lesions or diseases of the somatosensory system were included. QST comprised 13 parameters including detection and pain thresholds for thermal and mechanical stimuli. QST was performed in the clinically most affected test area and a less or unaffected control area in a morning and an afternoon session on 2 consecutive days by examiner pairs (4 QSTs/patient). For both, TR-R and IO-R, there were high correlations (r=0.80-0.93) at the affected test area, except for wind-up ratio (TR-R: r=0.67; IO-R: r=0.56) and paradoxical heat sensations (TR-R: r=0.35; IO-R: r=0.44). Mean IO-R (r=0.83, 31% unexplained variance) was slightly lower than TR-R (r=0.86, 26% unexplained variance, P<.05); the difference in variance amounted to 5%. There were no differences between study centres. In a subgroup with an unaffected control area (n=43), reliabilities were significantly better in the test area (TR-R: r=0.86; IO-R: r=0.83) than in the control area (TR-R: r=0.79; IO-R: r=0.71, each P<.01), suggesting that disease-related systematic variance enhances reliability of QST. We conclude that standardized QST performed by trained examiners is a valuable diagnostic instrument with good test-retest and interobserver reliability within 2days. With standardized training, observer bias is much lower than random variance. Quantitative sensory testing performed by trained examiners is a valuable diagnostic instrument with good interobserver and test-retest reliability for use in patients with sensory disturbances of different etiologies to help identify mechanisms of neuropathic and non-neuropathic pain.

Thermoreception and nociception of the skin: a classic paper of Bessou and Perl and analyses of thermal sensitivity during a student laboratory exercise

About four decades ago, Perl and collaborators were the first ones who unambiguously identified specifically nociceptive neurons in the periphery. In their classic work, they recorded action potentials from single C-fibers of a cutaneous nerve in cats while applying carefully graded stimuli to the skin (Bessou P, Perl ER. Response of cutaneous sensory units with unmyelinated fibers to noxious stimuli. J Neurophysiol 32: 1025-1043, 1969). They discovered polymodal nociceptors, which responded to mechanical, thermal, and chemical stimuli in the noxious range, and differentiated them from low-threshold thermoreceptors. Their classic findings form the basis of the present method that undergraduate medical students experience during laboratory exercises of sensory physiology, namely, quantitative testing of the thermal detection and pain thresholds. This diagnostic method examines the function of thin afferent nerve fibers. We collected data from nearly 300 students that showed that 1) women are more sensitive to thermal detection and thermal pain at the thenar than men, 2) habituation shifts thermal pain thresholds during repetititve testing, 3) the cold pain threshold is rather variable and lower when tested after heat pain than in the reverse case (order effect), and 4) ratings of pain intensity on a visual analog scale are correlated with the threshold temperature for heat pain but not for cold pain. Median group results could be reproduced in a retest. Quantitative sensory testing of thermal thresholds is feasible and instructive in the setting of a laboratory exercise and is appreciated by the students as a relevant and interesting technique.

Repeatability of cold pain and heat pain thresholds: The application of sensory testing in migraine research

Normal heat pain threshold (HPT) and cold pain threshold (CPT) repeatability should be estimated in order to identify thermal allodynia in longitudinal studies, but such data are scarce in the literature. The aim of our study was to estimate normal HPT and CPT repeatability in the face, forehead, neck and hand. In addition, we reviewed briefly normative studies of thermal pain thresholds relevant for headache research. Thermal pain thresholds were measured on three different days in 31 healthy headache-free subjects. Coefficients of repeatability and normal limits were calculated. HPT and CPT were lowest in the face. Pooled across regions, the lower repeatability limit for the test/retest ratio was 63% for HPT and 55% for CPT. The upper normal CPT limit varied between 24.5°C and 29.7°C. Lower HPT limits ranged between 35.5°C and 40.8°C. Quantitative sensory methods provide useful information about headache and pain pathophysiology, and it is important to estimate the normal test/retest repeatability range in follow-up studies.

Thermal perception thresholds recorded using method of limits change over brief time intervals

Quantitative Sensory Testing (QST) of thermal perception thresholds assesses small afferent nerve function. QST has also been widely used to investigate the effects of interventions on the perception of activity within these nerve fibres, often over brief time periods. The natural variation in perception thresholds over brief time periods has not been determined, however, complicating accurate identification of induced changes. The present study therefore investigated changes in thermal perception threshold values within a 1-h period. Twenty-four healthy women volunteers aged 18-28 years (mean 20.6, SD 2.8) undertook cold sensation (CS), warm sensation (WS), cold pain (CP), and hot pain (HP) perception threshold measurements on the thenar eminence of the dominant hand during six 8-min experimental cycles. The order of stimulus presentation was randomized within pre-selected criteria. An adaptation temperature of 32 degrees C, a rate of temperature change of 0.5 degrees C/s, a 3 cm x 3 cm thermode, and a method of limits algorithm were used. Separate two-way ANOVAs with repeated measures showed statistically significant changes over time for WS, CS, and HP (p < 0.05), but not for CP (p = 0.232). The results indicate that WS, CS, and HP perception thresholds change significantly with repeated testing over a 1-h period. These results should be carefully considered when assessing the importance of observed changes in thermal perception thresholds. In research trials exclusion of a control group would be a fundamental flaw.

Sex, gender, and pain: a review of recent clinical and experimental findings

Sex-related influences on pain and analgesia have become a topic of tremendous scientific and clinical interest, especially in the last 10 to 15 years. Members of our research group published reviews of this literature more than a decade ago, and the intervening time period has witnessed robust growth in research regarding sex, gender, and pain. Therefore, it seems timely to revisit this literature. Abundant evidence from recent epidemiologic studies clearly demonstrates that women are at substantially greater risk for many clinical pain conditions, and there is some suggestion that postoperative and procedural pain may be more severe among women than men. Consistent with our previous reviews, current human findings regarding sex differences in experimental pain indicate greater pain sensitivity among females compared with males for most pain modalities, including more recently implemented clinically relevant pain models such as temporal summation of pain and intramuscular injection of algesic substances. The evidence regarding sex differences in laboratory measures of endogenous pain modulation is mixed, as are findings from studies using functional brain imaging to ascertain sex differences in pain-related cerebral activation. Also inconsistent are findings regarding sex differences in responses to pharmacologic and non-pharmacologic pain treatments. The article concludes with a discussion of potential biopsychosocial mechanisms that may underlie sex differences in pain, and considerations for future research are discussed.

PERSPECTIVE

This article reviews the recent literature regarding sex, gender, and pain. The growing body of evidence that has accumulated in the past 10 to 15 years continues to indicate substantial sex differences in clinical and experimental pain responses, and some evidence suggests that pain treatment responses may differ for women versus men.

Sex-based differences in pain perception and treatment

OBJECTIVE

This review highlights research on sex-based differences in pain perception and treatment. We sought to illuminate the complex factors contributing to differences in pain and analgesic responses between males and females, ranging from psychosocial to biological processes.

DESIGN

We reviewed published studies of pain induction by chemical, electric, heat, surgical, or psychological means, and opioid and nonopioid analgesia comparing responses in men and women.

RESULTS

A substantial body of research indicates that women experience greater clinical pain, suffer greater pain-related distress, and show heightened sensitivity to experimentally induced pain compared with men. Research on sex-based differences in the pain experience and treatment is beginning to uncover patterns that may enable tailoring of pain treatment to individual characteristics. The factors underpinning sex differences in the experience of pain are multifactorial and complex; for example, psychosocial factors such as pain-related catastrophizing may explain sex-based differences in reporting certain types of pain, as women tend to use catastrophizing to a greater degree. Gonadal hormone levels in cycling women also have a substantial impact on pain perception and analgesic response. Women perceive more pain during the luteal phase, and estrogen antagonists provide long-term pain relief in certain situations.

CONCLUSIONS

Collectively, greater understanding of the factors that commonly and differentially affect the disparity in pain perception, as well as analgesic response, are beginning to illuminate research targets and promising areas of therapeutic intervention for improved pain management.

Sex, gender, and pain: an overview of a complex field

Traditionally, biomedical research in the field of pain has been conducted with male animals and subjects. Over the past 20-30 yr, it has been increasingly recognized that this narrow approach has missed an important variable: sex. An ever-increasing number of studies have established sex differences in response to pain and analgesics. These studies have demonstrated that the differences between the sexes appear to have a biological and psychological basis. We will provide brief review of the epidemiology, rodent, and human experimental findings. The controversies and widespread disagreement in the literature highlight the need for a progressive approach to the questions involving collaborative efforts between those trained in the basic and clinical biomedical sciences and those in the epidemiological and social sciences. In order for patients suffering from acute and/or chronic pain to benefit from this work, the approach has to involve the use or development of clinically relevant models of nociception or pain to answer the basic, but complex, question. The present state of the literature allows no translation of the work to our clinical decision-making.

Abnormal heat and pain perception in remitted heroin dependence months after detoxification from methadone-maintenance

Patients receiving methadone maintenance therapy (MMT) for opiate dependence have altered nociception, complicating analgesic treatment. Increasing numbers of patients are choosing opiate-free treatment programs, yet data on the course of this abnormality months after detoxification from methadone is contradictory and based exclusively on cold pressor experiments. Heat and pain thresholds were measured by quantitative sensory testing (QST) in 23 subjects with heroin dependence in full, sustained remission months after detoxification from methadone and 27 healthy non-drug using controls. Self reports of pain intensity and unpleasantness were also collected. Test scores were compared across groups and correlated with measures of drug use history. There were significant differences between remitted opiate-dependent subjects and controls on the measures of heat threshold (38.83 vs. 35.96; Mann-Whitney U=177.5, p=0.006), and the measure of pain threshold (48.73 vs. 47.62; Mann-Whitney U=217.5, p=0.043). There was no correlation of any measure of drug use history with the heat or pain experience. Abstinent, formerly opioid-dependent patients continue to demonstrate abnormal noxious perception months after detoxification from methadone.

Sex differences in muscle pain: self-care behaviors and effects on daily activities

Women have a higher prevalence of fibromyalgia and myofascial pain than men, but sex differences in muscle pain are inconsistently detected. We examined sex differences in ratings and effects of recalled and experimentally-induced muscle pain. In study 1 (n = 188), participants completed a questionnaire about recalled muscle pain. In study 2 (n = 55), participants described muscle pain from an exercise stimulus across 3 days by telephone. Muscle pain ratings, self-care behaviors for muscle pain, and effects of muscle pain on activities were measured. No significant sex differences were found except that women tended to view exercise as more effective for decreasing muscle pain than men (F (1, 187) = 5.43, P = .02, eta(2) = .03), fewer women performed exercise for induced muscle pain than men, and women's activity interference was significantly higher than men's at the third day after exercise (F (2, 42) = 6.54, P = .01, eta(2) = .14). These findings support the absence of meaningful sex differences in muscle pain ratings. However, additional investigations are needed that consider the daily activities completed by people and the prevalence and incidence of performing a wide range of self-care behaviors for pain.

PERSPECTIVE

These studies support that sex differences are not present in recalled and experimentally-induced muscle pain ratings. Therefore, we must be cautious about generalizing the musculoskeletal pain literature to muscle pain. Additional research is needed to interpret potential sex differences in self-care behaviors for muscle pain and activity interference from muscle pain.

Estrogen Amplifies Pain Responses to Uterine Cervical Distension in Rats by Altering Transient Receptor Potential-1 Function

INTRODUCTION

Estrogen sensitizes responses to painful stimuli, but its contribution to acute and chronic pain from the uterine cervix is unknown. Previous studies link the excitatory transient receptor potiential-1 channel (TRPV-1) to sensitization in viscera, and show that estrogen increases TRPV-1 expression in afferents from the uterine cervix. Here, we tested whether estrogen enhanced responses to uterine cervical distension in rats, and whether this involved TRPV-1 channels.

METHODS

Ovariectomized rats, with or without estrogen replacement, were anesthetized and hypogastric nerve and abdominal muscle contraction reflex responses to graded uterine cervical distension were recorded. Single unit hypogastric nerve fiber firing was measured before and after acute treatment with the TRPV-1 antagonist, capsaizepine, or vehicle.

RESULTS

Abdominal muscle contraction reflex responses to uterine cervical distension were enhanced in estrogen-treated rats. Hypogastric afferent responses to cervical distension were reduced by capsaizepine in estrogen-treated animals, but were unaffected in ovariectomized animals without estrogen replacement.

CONCLUSIONS

These data suggest that the TRPV-1 channel is unimportant for normal mechanosensation in the cervix in the absence of estrogen, since capsaizepine failed to reduce responses to uterine cervical distension in rats without estrogen replacement. In contrast, TRPV-1 function is important for estrogen-induced sensitization. These data raise the possibility that acute and chronic pain coming from the cervix, such as labor or cancer, may be enhanced by estrogen and might be reduced by antagonists of TRPV-1.

Differences in the sensations of cold on the anterior torso of control and spinal-cord-transected individuals

Application of cold stimuli to the sentient portion of the anterior torso of 12 spinal-cord-transected individuals (patients) and to comparable sites of 11 control participants showed that thresholds are lower for women than for men and that the difference between the sexes is maintained following spinal-cord transection. Patients of both sexes were more sensitive to cold stimuli than were controls. Estimates of stimulus intensity showed that participants reliably distinguished the 3 cool stimuli but that control women offered significantly larger estimates than control men. Spinal-cord transection produced an increase in the intensity of the sensations in women and a reduction in men. The changes in sensory perception that follow spinal-cord injury extend throughout the somatosensory system and involve all modalities. These changes cannot be explained as a simple release from inhibition.

Quantitative Somatosensory Testing of Warm and Heat-Pain Thresholds: The Effect of Body Region and Testing Method

OBJECTIVES

To study whether the sensitivity to noxious and innocuous heat varies across the body.

METHODS

Heat-pain threshold and warm sensation threshold were measured in 20 patients, using the Method of Limits (MLI) and the Method of Levels (MLE) in 5 regions: the chest, forearm, mid thigh, and the dorsal surface of the hand and foot.

RESULTS

With the Method of Limits, heat-pain threshold increased gradually from the lowest level in the chest (mean 42 degrees C) to peak level in the foot (44.5 degrees C, P < 0.001). With the Method of Levels, heat-pain threshold did not differ between body regions. Warm sensation thresholds measured with both Method of Limits and the Method of Levels was higher in the chest (36.2 degrees C) and foot (36.8 degrees C) compared to the other regions (mean of 35.3 degrees C, P < 0.01). The correlation between heat-pain threshold and warm sensation threshold was low to moderate, depending on the tested region. Differences in heat-pain threshold and warm sensation threshold between the methods were highest in the legs and smallest in the chest. The correlation between the Method of Limits and Method of Levels was moderate for heat-pain threshold (0.57) and good for warm sensation threshold (0.71).

DISCUSSION

The sensitivity to noxious heat is uniform across the body when measured with a reaction-time-free method (Method of Levels), but is greater in proximal than in distal regions, when measured with a reaction-time-dependent method (Method of Limits). Regardless of measuring method, the sensitivity to innocuous heat is not uniform across the body. It is concluded that the Method of Levels is preferred when heat-pain threshold is to be compared between body regions. For heat-pain threshold, within-patient comparisons can be made between each pair of regions tested. However, for warm sensation threshold, within-patient comparisons should be conducted between contralateral symmetrical regions.

Characterizing individual differences in heat-pain sensitivity

Heat induced pain has been shown to follow a positively accelerating power function for groups of subjects, yet the extent to which this applies to individual subjects is unknown. Statistical methods were developed for assessing the goodness of fit and reliability of the power function for data from individual subjects with the aim of using such functions for characterizing individual differences in heat-pain sensitivity. 175 subjects rated ascending and random series of contact heat stimuli with visual analogue scales for pain intensity (VAS-I) and unpleasantness (VAS-A). Curve fitting showed excellent model fit. Substitution of model estimates in place of observed VAS scores produced minimal bias in group means, about 0.3 VAS units in the ascending series and 1.0 in the random series, on a 0-100 scale. Individual power function exponents were considerably higher for the ascending than for the random series and somewhat higher for VAS-A than for VAS-I (means: ascending VAS-I=9.04, VAS-A=9.80; random VAS-I=4.95, VAS-A=5.67). The reliability of VAS estimates was high (>==.93), and for the ascending series it remained so when extrapolating 4 degrees C beyond the empirical range. Exponent reliability was high for the ascending series (VAS-I=.92; VAS-A=.91), but considerably lower for the random series (VAS-I=.69; VAS-A=.71). Individual differences constituted 60% of the total variance in pain ratings, whereas stimulus temperature accounted for only 40%. This finding underscores the importance of taking individual differences into account when performing pain studies.

The A118G single nucleotide polymorphism of the mu-opioid receptor gene (OPRM1) is associated with pressure pain sensitivity in humans

Responses to painful stimuli are characterized by tremendous interindividual variability, and genetic factors likely account for some proportion of this variability. However, few studies have identified genetic contributions to experimental pain perception in humans. This experiment investigated whether the A118G single nucleotide polymorphism of the mu-opioid receptor gene ( OPRM1 ) was associated with responses to three different experimental pain modalities in a sample of 167 healthy volunteers (96 female, 71 male). Responses to thermal, mechanical, and ischemic pain were assessed in all subjects, and genotyping of OPRM1 was performed, which revealed that the rare A118G allele occurred in 24 females (25%) and 12 males (17%). Statistical analyses indicated that subjects with a rare allele had significantly higher pressure pain thresholds than those homozygous for the common allele. Also, a sex by genotype interaction emerged for heat pain ratings at 49 degrees C, such that the rare allele was associated with lower pain ratings among men but higher pain ratings among women. These data indicate an association of a common single nucleotide polymorphism of OPRM1 with mechanical pain responses and that this genotype may be associated with heat pain perception in a sex-dependent manner. This study examines the association of the A118G SNP of OPRM1 to experimental pain sensitivity. The results indicate that the rare allele is associated with higher pressure pain thresholds. These results support previous contentions that OPRM1 may be a pain-relevant gene; however, replication of these findings is needed.

Site-dependent and subject-related variations in perioral thermal sensitivity

The Marstock method of limits was used to obtain thresholds for detection of cooling, warming, cold pain and heat pain for 34 young adults, upon eight spatially matched sites on the left and right sides of the face, the right ventral forearm and the scalp. Male and female subjects were tested by both a male and a female experimenter. Neither the experimenter nor the gender of the subject individually influenced the thresholds. The thermal thresholds varied greatly across facial sites: sixfold and tenfold for cool and warmth, respectively, from the most sensitive sites on the vermilion to the least sensitive facial site, the preauricular skin. Warm thresholds were 68% higher than cool thresholds, on average, and 12% higher on the left compared to the right side of the face. The mean cold pain threshold increased from 21.0 degrees C on the hairy upper lip to 17.8 degrees C on the preauricular skin. Sites on the upper lip were also most sensitive to noxious heat with pain thresholds of 42-43 degrees C. The scalp was notably insensitive to innocuous and noxious changes in temperature. For the sensations of nonpainful cool and warmth, the more sensitive a site, the less the estimates of the thresholds differed between subjects. In contrast, for heat pain, the more sensitive a site, the more the estimates differed between subjects. Subjects who were relatively more sensitive to cool tended to be relatively more sensitive to warmth. Subjects' sensitivities to nonpainful cool and warmth were less predictive of their sensitivities to painful cold and heat, respectively. Short-term within-subject variability increased with the magnitude of the thresholds. The lower the threshold, the more similar were repeated measurements of it, within a 5-25 s period.

Sex differences in the long-term stability of forehead cold pressor pain

The purpose of this study was to examine sex differences in the stability of experimental pain responding across time. Stability was assessed by using 2 forehead cold pressor applications separated by 9 months. Twenty-eight men and 20 women completed both Session 1 and Session 2. Repeated measures analysis of variance showed a main effect for Session on maximum pain level. Women reported significantly more pain at Session 2, whereas men showed no difference between sessions. There were no differences on pain report between men and women at Session 1. A significant Session by Sex interaction was associated with perceived chronic stress and trait anxiety levels. At Session 2 but not Session 1, women endorsed a significantly greater expectation than men to experience unpleasant aftereffects from the cold pressor task. Additional analysis showed that chronic stress and trait anxiety were significantly associated with sex-specific pain responding. We propose that the influence of a prior painful incident on an identical repeated painful experience differs between men and women. We speculate that this influence is related to sex differences in psychological mechanisms used to interpret painful stimuli within the context of remembered experiences. To our knowledge, this is the first report of sex differences in the long-term stability of an experimental laboratory pain stimulus, controlling for follicular phase of the female menstrual cycle.

PERSPECTIVE

This study examines sex differences in the stability of experimental pain responding across a 9-month period. We speculate that psychological mechanisms influence one's interpretation of a prior painful incident and that this interpretation facilitates increased pain reporting in response to an identical repeated exposure, as was observed for women.

A quantitative somatosensory testing of pain threshold in individuals with mental retardation

The commonly held view, mainly based on behavioral observations, is that individuals with mental retardation (MR) have a decreased sensitivity to pain. However, the sensitivity to noxious stimuli was not systematically measured in these individuals. For this purpose we developed an experimental protocol with which we trained individuals with mild MR (unspecified MR and Down's syndrome) in heat-pain threshold (HPT) measurement on the hand, and then performed the measurement using both the method of limits (MLI) which relies on reaction time (RT) and the method of levels (MLE) which is RT-free. This allowed for an indirect assessment of the RT and conduction velocity (CV) of these individuals. We found that HPT in individuals with unspecified MR (41.23+/-1.86 degrees C) and Down's syndrome (40.96+/-2.93 degrees C) was significantly lower than that of controls (42.86+/-2.42 degrees C) when measured with the MLE (P < 0.05). With the MLI no significant differences in HPT were found between the groups. However, the RT and CV values of individuals with unspecified MR and Down's syndrome were significantly lower compared to controls (e.g. mean RT of 1.86 and 2.55 compared to 1.2 s, respectively, P < 0.01). From this work it would appear that individuals with MR are not only pain-sensitive, but also more sensitive to heat-pain than normal. It is suggested that computerized quantitative testing of pain threshold is feasible in individuals with MR preferably by using RT-free methods (e.g. the MLE) due to the low RT and CV values exhibited by them.

Spatial summation of pressure pain: effect of body region

The characteristics of spatial summation of pressure pain are not clear. Pressure pain threshold (PPT) and perceived pressure pain intensity were measured in the hand, painfree back and myofascial trigger points (MTPs) in the back, using three different stimulus areas (0.5, 1 and 2 cm(2)). PPT decreased and perceived pain increased significantly with an increase in stimulation area in all the regions (e.g. PPT in the back, from 406+/-168 to 205+/-102kPa, P<0.0001). The magnitude of spatial summation of pressure pain was not significantly different between the regions. However, PPT in the back was significantly higher compared to the hand and MTPs (e.g. for 2 cm(2): mean of 205+/-102 vs 175+/-75 and 159+/-72kPa, P<0.01, respectively). Irrespective of body region, the quality of pain evoked with the large areas (1 and 2 cm(2)) was of pressure whereas in the small area (0.5 cm(2)) it was perceived as a prick. In conclusion, both PPT and perceived pressure pain intensity are subject to a considerable spatial summation in all the regions tested. The quality of pressure-evoked pain is probably determined by this spatial summation. Body region significantly affects the PPT level for a fixed stimulation area but not the magnitude of its spatial summation for areas up to 2 cm(2), which are probably within the receptive field of single spinal nociceptive neurons.

Gender and laterality differences in thermosensation throughout the perceptible range

Several studies suggest that females exhibit greater sensitivity to experimentally induced thermal pain than males. These investigations have focused mainly on the sensory-discriminative rather than the affective aspect of pain. Moreover, potential gender differences for the affective components of innocuous thermal sensations have yet to be examined. The primary aim of the present study was to evaluate gender differences in the sensory and the affective dimensions of the entire thermosensory system, including warmth, coolness, heat pain and cold pain. The secondary aim was to evaluate laterality differences in these same perceptual dimensions and ranges. Twenty healthy females and 20 healthy males immersed their hands in water baths maintained at temperatures ranging from 10 to 47 degrees C, and rated their perceived thermal intensity, (un)pleasantness, and pain intensity. There was a progressive growth in the thermal intensity ratings as bath temperatures either increased or decreased from the adapting temperature of 33 degrees C. No gender differences emerged for these thermal intensity ratings. However, a significant sex effect emerged for the pain intensity ratings (P<0.01), and a significant sex x temperature interaction for the affective ratings (P<0.01). Females provided higher unpleasantness and pain intensity ratings for the more extreme temperatures (10, 15 and 47 degrees C), compared to males. Moreover, women perceived the milder temperature baths as more pleasant than men did. For a given painful temperature, unpleasantness ratings were higher than pain intensity ratings. This relationship between unpleasantness ratings and pain ratings was not significantly different between the sexes. No laterality differences emerged for the thermal intensity ratings. However, perceived pain intensity was significantly higher for the left as compared to the right hand (P<0.01). Ratings of unpleasantness also tended to be higher for the left vs. right hand, but this difference fell just short of statistical significance (P=0.06). These findings indicate that sex differences in thermosensory perception are not general, but occur only for the painful and affective components. Of particular note is the sex difference for affective but not intensive ratings of innocuous temperatures, revealing sex differences in thermal perception outside the nociceptive system.

Effects of electric stimulation on C and A delta fiber-mediated thermal perception thresholds 11No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the authors(s) or upon any organization with which the author(s) is/are associated

OBJECTIVE

To determine if interferential current (IFC) or transcutaneous electric nerve stimulation (TENS) alters C and A delta fiber-mediated thermal perception thresholds.

DESIGN

Single-blind, randomized controlled trial.

SETTING

Laboratory.

PARTICIPANTS

One hundred forty healthy women volunteers (mean age +/- standard deviation, 20.6+/-2.7 y).

INTERVENTIONS

Subjects were randomly and exclusively assigned to 1 of 7 groups (n=20 in each): 0, 5, and 100 Hz of IFC; 5 and 100 Hz of TENS; placebo and control stimulation. Stimulation was applied through 2 electrodes placed over the median nerve. Warm sensation, cold sensation, hot pain, and cold pain perception thresholds were measured from the thenar eminence by using a quantitative sensory testing device and a method of limits algorithm.

MAIN OUTCOME MEASURES

Warm sensation, cold sensation, hot pain, and cold pain thresholds (degrees C) before, during, and after stimulation.

RESULTS

There was a statistically significant effect of time for all 4 thermal perception thresholds (separate 2-way analyses of variance with repeated measures, all P<.001). There were no statistically significant differences between experimental groups, nor any interaction effects (all P>.05).

CONCLUSIONS

Neither IFC nor TENS altered C and A delta fiber-mediated thermal perception thresholds. The results suggest that any analgesic mechanisms with these modalities are likely to be complex.

Gender differences in patterns of cerebral activation during equal experience of painful laser stimulation

A previous functional imaging study demonstrated greater female response in the anterior insula and thalamus and left prefrontal activation in men and right prefrontal activation in women during equal heat intensity but unequal pain experience. For the current study, subjective intensities of noxious heat delivered to the back of the right hand were equalized across subjects, and regional cerebral blood flow was recorded by using positron emission tomography. The female subjects required less laser energy before reporting pain, but the difference was not significant. Correlation of regional cerebral blood flow with subjective pain experience in the whole group showed significant bilateral responses in the parietal, lateral premotor, prefrontal, secondary somatosensory, anterior cingulate and insula cortices, as well as the thalamus. There was significantly greater activation in the left, contralateral, prefrontal, primary and secondary somatosensory, parietal, and insula cortices in the male subjects compared with the female subjects and greater response in the perigenual cingulate cortex in the female subjects. Our study is the first to associate consistent pain experience with gender differences in central response. These differences may relate to differential processing of acute pain with implications for clinical disorders that show a female dominance. The subtle behavioral differences and inconsistent findings across studies, however, suggest the need for caution and further experimentation before speculating further.

Coping strategies and life satisfaction in subgrouped fibromyalgia patients

The present study describes pain- and stress-coping strategies and life satisfaction in subgroups of fibromyalgia patients. Thirty-two females with fibromyalgia syndrome (FMS) and 21 healthy pain-free women were studied. Those with FMS were classified as thermal (both heat and cold) pain sensitive or slightly cold pain sensitive based on pain thresholds determined using a Thermotest device. Global stress-coping styles, life satisfaction, and specific pain-coping strategies were measured. Patients classified as thermal pain sensitive were affected by physical symptoms to a greater extent than were those classified as slightly cold pain sensitive. The thermal pain sensitive group used more diverting attention coping strategies than the slightly cold pain sensitive group did. Separating fibromyalgia patients into subgroups might increase the potential for improving nursing care of these patients. Through the use of effective coping strategies in dealing with stress and pain, life satisfaction may also be enhanced.

Catastrophizing and health-related quality of life: a 6-year follow-up of patients with chronic low back pain

A pain rehabilitation model that focused on emotions was implemented to influence catastrophizing by, and health-related quality of life (HRQL) for, persons with chronic low back pain. Twelve individuals, 7 men and 5 women (aged 33 to 57 years), all with long-term pain despite treatment, were included in the study and a single case research experimental design (SCRED) was used to follow the patterns of coping with pain for 6 years. The HRQL was measured before and 6 years after the intervention. Coping strategies and HRQL were evaluated with the Coping Strategy Questionnaire (CSQ) and the SF-36, respectively. The evaluation of pain coping strategies after 3 years found decreased catastrophizing, a decrease that had continued 3 years later. HRQL showed significantly improved mental health and impaired physical capacity at the 6-year follow-up. Changes in catastrophizing or in HRQL did not appear to influence self-scored bodily pain. Altered catastrophizing appeared to be a long-term process. This research indicates the need for rehabilitation programs to assess and evaluate patients' pain and their need for improved quality of life, rather than focusing only on the elimination of pain.

Sex, gender, and pain: women and men really are different

Sex-related differences in the experience of both clinical and experimentally induced pain have been widely reported. Specifically, females are at greater risk for developing several chronic pain disorders, and women exhibit greater sensitivity to noxious stimuli in the laboratory compared with men. Several mechanisms have been proposed to account for these sex differences. Psychosocial factors such as sex role beliefs, pain coping strategies, mood, and pain-related expectancies may underlie these effects. In addition, there is evidence that familial factors can alter pain responses, and these intergenerational influences may differ as a function of sex. Sex hormones are also known to affect pain responses, which may mediate the sex differences. Although the magnitude of these effects has not been well characterized, there are potentially important practical implications of sex differences in pain responses. These implications are discussed, and directions for future research are delineated.

Stress coping strategies in thermal pain sensitive and insensitive healthy subjects

The aim of this study was to investigate stress coping strategies used in relation to heat and cold pain thresholds in healthy subjects. After using the Jalowiec Coping Scale, cold and heat pain thresholds were examined using the Quantitative Somatosensory Test in 47 healthy subjects. The participants were separated into thermal pain sensitive and insensitive groups, based on thermal pain perception. The results showed that subjects sensitive to thermal pain tended to adopt an emotive stress coping style significantly more commonly than the insensitive subjects. Furthermore, women displayed a marked preference for this style compared to men. The conclusion is that emotional stress coping did play a role in the perception of thermal pain in this group of healthy subjects and that clinical nursing interventions need to focus on the relationship between emotion and coping.