Gender differences in pain modulation evoked by repeated injections of glutamate into the human trapezius muscle

Incidence and predictors of chronic pain after inguinal hernia surgery: a systematic review and meta-analysis

PURPOSE

The mesh is currently the preferred treatment option for hernia repair surgery. Chronic postoperative inguinal pain (CPIP), lasting more than 3 months after surgery, is a complication that significantly impacts patients' quality of life. Currently, there is a lack of evidence-based information describing the incidence and independent predictive factors of chronic pain, posing a serious challenge in clinical practice for devising personalized prevention strategies. Hence, we conducted this systematic review and meta-analysis to investigate the incidence and predictive factors, aiming to provide a reference for developing plans to prevent chronic pain.

METHODS

We conducted a systematic search of PubMed, Cochrane, Embase, and Web of Science, with the retrieval cutoff date set at December 17, 2022. The included studies underwent assessment using the NOS scale, and subgroup analysis for the incidence was carried out based on different regions.

RESULTS

Ultimately, 18 studies were included, involving 29,466 patients. Meta-analysis showed that the pooled incidence of chronic pain was 17.01% (95%CI 12.78% ~ 21.71%). The incidence was 18.65% (95%CI 13.59% ~ 24.29%) in Europe, 14.70% (95%CI 7.87% ~ 23.17%) in Asia, and 6.04%(95%CI 4.62 ~ 7.64) in North America. Furthermore, We also found that the risk factors for CPIP are younger age [OR = 2.261 (95%CI 1.126 ~ 4.549)], presence of other postoperative complications [OR = 1.849 (95%CI 1.034 ~ 3.305)], hernial sac defect < 3 cm [OR = 1.370 (95%CI 1.012 ~ 1.853)], being female [OR = 1.885 (95%CI 1.024 ~ 3.472)], postoperative pain [OR = 1.553 (95%CI 1.276 ~ 1.889)], preoperative pain [OR = 2.321 (95%CI 1.354 ~ 3.979)], and having a history of ipsilateral inguinal hernia repair [OR = 2.706 (95% CI 1.445 ~ 5.069)].

CONCLUSIONS

The incidence of persistent pain following hernia repair surgery is high in current clinical practice, a concern that should not be overlooked. Stratified assessment tools need to be established for patients experiencing early chronic pain, and personalized follow-up strategies and preventive interventions should be developed for those with potentially high risks. These measures aim to enhance the quality of life for patients after hernia repair.

Spatiotemporal characterization of an experimental model of muscle pain in humans based on short-wave diathermy

BACKGROUND

Commonly used models for eliciting muscle pain involve the injection of algesic substances or the induction of delayed onset muscle soreness. The former require invasive procedures, and the time frame for pain induction and subsidence in the latter can be inconvenient. This study presents a detailed spatiotemporal characterization of a new experimental model of muscle pain based on short-wave diathermy (SWD), developed to overcome the limitations of existing models.

METHODS

The shoulder was selected as target site and the effects of the model were tested in two sessions to assess its reliability. Pain intensity profiles were recorded during the application of SWD, and changes in pressure pain threshold (PPT) in the infraspinatus muscle, together with pain intensity, duration, and quality were assessed 30 min after induction.

RESULTS

SWD-induced pain intensity scores averaged 4 points on a visual analogue scale, whereas PPT showed a consistent decrease of about 25% relative to baseline values. Pain was localized in the shoulder area, and was described as continuous, dull, well-delimited, heavy, and bearable. Pain lasted for an average of 145 min without requiring reinduction and was reliably elicited in both experimental sessions.

CONCLUSION

SWD can be used to elicit experimental muscle pain in a non-invasive, long-lasting, and reliable way and allows for repeated within- and between-session testing in the shoulder.

SIGNIFICANCE STATEMENT

SWD produces deep heating in muscles by converting electromagnetic energy to thermal energy. It was previously shown that it can be used to elicit experimental pain in the forearm muscles, and the present study demonstrates that this can be reliably generalized to other body sites, such as the shoulder. Furthermore, SWD application is non-invasive and presents a convenient time frame for pain induction and subsidence, thus overcoming limitations associated with traditional muscle pain models.

Factors associated with frozen shoulder in adults: a retrospective study

OBJECTIVE

This study aims to explore the risk factors associated with frozen shoulder patients and further analyze the relationship between gender and diabetes with frozen shoulder.

METHOD

We have reviewed the data of 1205 frozen shoulder patients in China's Xinjiang region from 2018 to 2023. The collected information included patients' gender, occupation, place of origin, marital status, age, the season of disease onset, duration of illness, etiology, surgical history, hypertension, diabetes, respiratory diseases, knee joint disease, hyperlipidemia, cardiovascular diseases, cervical spondylosis, lumbar disc herniation, rheumatoid arthritis, hyperuricemia, sleep quality, smoking and alcohol consumption, and constipation. We have used multifactor logistic regression analysis to identify the risk factors for a frozen shoulder.

RESULTS

Single-factor logistic regression analysis showed that the number of females, patients with diabetes, knee joint disease, constipation, and patients with poor sleep quality in the observation group are higher than in the control group (P < 0.05). There were no statistically significant differences between the two groups in terms of occupation, place of origin, marital status, age, season of disease onset, duration of illness, etiology, surgical history, hypertension, respiratory diseases, hyperlipidemia, cardiovascular diseases, cervical spondylosis, lumbar disc herniation, rheumatoid arthritis, hyperuricemia, smoking, and alcohol consumption history (P > 0.05). Multivariate analysis showed that the final model included four variables: gender, diabetes history, sleep, and constipation. Among them, the OR values of gender and diabetes history were more significant than 1, indicating that they were independent risk factors for frozen shoulder, while the OR values of sleep and constipation were less than 1, suggesting that they were negatively associated with the occurrence of frozen shoulder.

CONCLUSION

The results of this study suggest that gender and diabetes are independent risk factors for frozen shoulder. Additionally, poor sleep quality and constipation also can be correlated with the occurrence of a frozen shoulder.

Sex differences in descending control of nociception (DCN) responses after chronic orofacial pain induction in rats and the contribution of kappa opioid receptors

Descending control of nociception (DCN), a measure of efficiency of descending pain inhibition, can be assessed in animals by the combined application of test and conditioning noxious stimuli. Evidence from pre-clinical and clinical studies indicates that this mechanism of pain control may differ between sexes and might be impaired in many chronic pain states. However, little is known about sex differences in DCN efficiency in models of acute and chronic orofacial pain. Herein, we first evaluated DCN responses in male and female rats by the applying formalin into the upper lip or capsaicin into the forepaw as the conditioning stimulus, followed by mechanical stimulation (Randall-Selitto) of the hind paw as the test stimulus. The same protocol (i.e., capsaicin in the forepaw followed by mechanical stimulation of the hind paw) was evaluated in male and female rats on day 3 after intraoral incision and on day 15 and 30 after chronic constriction injury of the infraorbital nerve (CCI-ION). Additionally, we assessed the effect of the kappa opioid receptor (KOR) antagonist Norbinaltorphimine (nor-BNI) on DCN responses of female nerve-injured rats. This study shows that naïve female rats exhibit less efficient DCN compared to males. Postoperative pain did not alter DCN responses in female and male rats, but CCI-ION induced loss of DCN responses in females but not in males. Systemic pretreatment with nor-BNI prevented the loss of DCN induced by CCI-ION in female rats. The results reveal sex differences in DCN responses and female-specific impairment of DCN following chronic orofacial pain. Moreover, the findings suggest that, at least for females, blocking KOR could be a promising therapeutic approach to prevent maladaptive changes in chronic orofacial pain.

Body Mass Index and Its Influence on Chronic Low Back Pain in the Spanish Population: A Secondary Analysis from the European Health Survey (2020)

Risk factors such as obesity and a sedentary lifestyle contribute to the development of chronic low back pain.

PURPOSE

To describe how body mass index (BMI) influences the prevalence of chronic low back pain in the general Spanish population and assess this influence given other factors as sex, age, physical occupational demands, and recreational physical activity.

METHODS

An analytical cross-sectional design was performed based on secondary data from the European Health Survey in Spain (2020). Data on age, gender, physical occupational demands, physical activity, BMI, and presence of chronic low back pain were extracted.

RESULTS

A total of 19,716 (52.2% women) subjects with a median age of 53 years old were analyzed. Of these, 18.3% had chronic low back pain, 39% were overweight, and 16.1% were obese. The adjusted generalized linear model showed that being obese increased the odds of chronic low back pain by 1.719 times (p < 0.001). Being a woman increased the odds by 1.683 times (p < 0.001). Performing occupational tasks requiring high physical demands increased the odds by 1.403 times (p < 0.001). Performing physical activity in leisure time several times a week decreased the odds by 0.598 times (p < 0.001). For every additional year of age, the odds of chronic low back pain increased by 3.3% (p < 0.001).

CONCLUSIONS

Obesity is related with the presence of chronic low back pain in the general Spanish population. This association persists, being the more relevant factor, after adjusting the association of BMI and chronic low back pain with other factors such as sex, age, physical occupational demands, and recreational physical activity.

Prevalence/Incidence of Low Back Pain and Associated Risk Factors Among Nursing and Medical Students: A Systematic Review and Meta-Analysis

OBJECTIVE

To summarize evidence regarding the prevalence and incidence of low back pain and associated risk factors in nursing and medical students. TYPE: Systematic review and meta-analysis.

LITERATURE SURVEY

The protocol was registered with PROSPERO (CRD42015029729). Its reporting followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Seven databases were searched until August 2020 to identify relevant studies.

METHODOLOGY

Two independent reviewers screened, extracted, and evaluated the risk of bias of the selected studies. Meta-analyses were used to estimate 12-month prevalence/incidence rates of low back pain and associated risk factors in these students. Levels of evidence for risk factors were determined by the updated Guidelines for Systematic Reviews in the Cochrane Collaboration Back Review Group.

SYNTHESIS

Sixteen studies involving 7072 students were included. The pooled 12-month prevalence rates of low back pain for nursing and medical students were 44% (95% confidence interval [95% CI]: 27%-61%) and 53% (95% CI: 44%-62%), respectively. The 12-month incidence of low back pain in nursing students ranged from 29% to 67%. No incidence rate was reported in medical students. Strong/moderate-quality evidence supported that final year of study (pooled odds ratio [OR] from five studies, 1.96, 95% CI: 1.13-3.40), anxiety (OR ranging from 3.12 to 4.61), or high mental pressure or psychological distress (OR ranging from 1.37 to 4.52) was associated with a higher 12-month low back pain prevalence in both student groups. Moderate-quality evidence suggested that prior history of low back pain (pooled OR from two studies: 3.46, 95% CI: 1.88-6.36) was associated with a higher 12-month low back pain incidence in nursing students. Similarly, moderate-quality evidence suggested that female medical students (pooled OR from two studies: 1.77, 95% CI: 1.09-2.86) demonstrated a higher 12-month low back pain prevalence than male counterparts.

CONCLUSIONS

Although it is impossible to alter nonmodifiable risk factors for low back pain, universities may develop and implement proper strategies to mitigate modifiable risk factors in these students.

Differential Analgesia From Vibratory Stimulation During Local Injection of Anesthetic: A Randomized Clinical Trial

BACKGROUND

Inadequate pain reduction during anesthetic injection is a significant medical and surgical problem. Vibratory distraction reduces this pain; however, there are minimal data identifying those who respond best.

OBJECTIVE

To quantify analgesia from vibration before anesthetic injection.

MATERIALS AND METHODS

In this partially blinded, single-institution trial, adult participants were randomized to intervention (vibratory anesthetic device, VAD ON) or placebo (VAD OFF). Pain was assessed using the 11-point numeric rating scale (NRS). Relative reduction in NRS between VAD OFF and ON was used to identify minimum clinically important and substantially clinically important difference in pain.

RESULTS

One hundred one tested sites from 87 subjects were assessed. Sixty-three percent were men with a median age of 66 years. From univariate analysis, males, subjects aged <60, and head and neck (HN) treated subjects had a significant reduction in NRS (p < .05). Multivariate analysis identified NRS reductions in females <60 (p = .012), males ≥70 (p = .002), females and males treated on HN (p = .048 and p = .035, respectively), and males ≥70 treated on HN (p = .012). Substantially clinically important difference (≥57% NRS reduction) included subjects <60, females <70, HN treatment aged 60 to 69, males ≥70, and females treated on HN.

CONCLUSION

Vibratory anesthetic device reduces pain during anesthetic injection, primarily for HN treatments and older male subjects.

The impact of anxiety and catastrophizing on interleukin-6 responses to acute painful stress

Objective

To examine the influence of anxiety and pain-related catastrophizing on the time course of acute interleukin-6 (IL-6) responses to standardized noxious stimulation among patients with chronic pain.

Methods

Data were collected from 48 participants in the following demographically matched groups: patients with chronic pain (n=36) and healthy controls (n=12). Participants underwent a series of Quantitative Sensory Testing (QST) procedures assessing responses to mechanical and thermal stimuli during two separate visits, in a randomized order. One visit consisted of standard, moderately painful QST procedures, while the other visit involved nonpainful analogs to these testing procedures. Blood samples were taken at baseline, and then for up to 2 hours after QST in order to study the time course of IL-6 responses.

Results

Results of multilevel analyses revealed that IL-6 responses increased across assessment time points in both visits (p<0.001). While patients with chronic pain and healthy controls did not differ in the magnitude of IL-6 responses, psychological factors influenced IL-6 trajectories only in the chronic pain group. Among patients, increases in catastrophizing over the course of the QST session were associated with elevated IL-6 responses only during the painful QST session (p<0.05). When controlling for anxiety, results indicated that the main multilevel model among patients remained significant (p<0.05).

Conclusion

Under specific conditions (eg, application of a painful stressor), catastrophizing may be associated with amplified proinflammatory responses in patients with persistent pain. These findings suggest that psychosocial interventions that reduce negative pain-related cognitions may benefit patients’ inflammatory profiles.

Analysis of Pressure Pain Hypersensitivity, Ultrasound Image, and Quality of Life in Patients with Chronic Plantar Pain: A Preliminary Study

OBJECTIVES

To evaluate widespread pressure pain in patients with chronic plantar heel pain compared with that in healthy controls and to investigate the differences in ultrasound imaging and quality of life between these two groups.

METHODS

A total of 22 patients (11 female) with chronic plantar heel pain and the same number of healthy patients, matched according to age and gender, were included in this pilot study. Pressure pain thresholds (PPTs) were bilaterally assessed over the calcaneus bone, the plantar fascia, the first and fifth metatarsals, the soleus muscle, the second metacarpal, and the zygapophyseal joint of C5-C6. Plantar fascia thickness was measured via ultrasound imaging. In addition, quality of life and physical function were assessed using the Short-Form 36 (SF-36) questionnaire and the Foot and Ankle Ability Measure (FAAM) questionnaire, respectively.

RESULTS

Analysis of covariance (ANCOVA) results showed significant differences in the PPTs at all points between the groups (P < 0.001), but not between sides. The PPTs were significantly lower in the patients than in the controls at all sites (P < 0.05). The results showed significant increases in fascia thickness at the calcaneus insertion (group: F = 74.172, P ≤ 0.001; side: F = 8.920, P ≤ 0.001) and the middle fascia point (group: F = 133.685, P = <0.001; side: F = 11.414, P = <0.001) on ultrasound in the patient group compared with the matched control group. The analysis also revealed that the patient group had a significantly lower score on every subscale of the SF-36 and FAAM questionnaires (all P < 0.001), except for the mental component, compared with the matched control group.

DISCUSSION

Patients suffering from chronic plantar heel pain showed widespread and bilateral hypersensitivity, increased thickness of the plantar fascia in the affected foot, and deterioration in quality of life and physical functioning compared with matched controls.

Specificity of hemodynamic brain responses to painful stimuli: a functional near-infrared spectroscopy study

Assessing pain in individuals not able to communicate (e.g. infants, under surgery, or following stroke) is difficult due to the lack of non-verbal objective measures of pain. Near-infrared spectroscopy (NIRS) being a portable, non-invasive and inexpensive method of monitoring cerebral hemodynamic activity has the potential to provide such a measure. Here we used functional NIRS to evaluate brain activation to an innocuous and a noxious electrical stimulus on healthy human subjects (n = 11). For both innocuous and noxious stimuli, we observed a signal change in the primary somatosensory cortex contralateral to the stimulus. The painful and non-painful stimuli can be differentiated based on their signal size and profile. We also observed that repetitive noxious stimuli resulted in adaptation of the signal. Furthermore, the signal was distinguishable from a skin sympathetic response to pain that tended to mask it. Our results support the notion that functional NIRS has a potential utility as an objective measure of pain.

Intradermal glutamate and capsaicin injections: intra- and interindividual variability of provoked hyperalgesia and allodynia

Intradermal injections of glutamate and capsaicin are attractive to use in human experimental pain models because hyperalgesia and allodynia mimic isolated aspects of clinical pain disorders. The aim of the present study was to investigate the reproducibility of these models. Twenty healthy male volunteers (mean age 24 years; range 18-38 years) received intradermal injections of glutamate and capsaicin in the volar forearm. Magnitudes of secondary pinprick hyperalgesia and brush-evoked allodynia were investigated using von Frey filaments (gauges 10, 15, 60 and 100 g) and brush strokes. Areas of secondary hyperalgesia and allodynia were quantified immediately after injection and after 15, 30 and 60 min. Two identical experiments separated by at least 7 days were performed. Reproducibility across and within volunteers (inter- and intra-individual variation, respectively) was assessed using intraclass correlation coefficient (ICC) and coefficient of variation (CV). Secondary pinprick hyperalgesia was observed as a marked increase in the visual analogue scale (VAS) response to von Frey gauges 60 and 100 g (P < 0.001) after glutamate injection. For capsaicin, secondary pinprick hyperalgesia was detected with all von Frey gauges (P < 0.001). Glutamate evoked reproducible VAS response to all von Frey gauges (ICC > 0.60) and brush strokes (ICC > 0.83). Capsaicin injection was reproducible for secondary hyperalgesia (ICC > 0.70) and allodynia (ICC > 0.71). Intra-individual variability was generally lower for the VAS response to von Frey and brush compared with areas of secondary hyperalgesia and allodynia. In conclusion, glutamate and capsaicin yield reproducible hyperalgesic and allodynic responses, and the present model is well suited for basic research, as well as for assessing the modulation of central phenomena.

Painful heat attenuates electrically induced muscle pain in men and women

Background and purpose

Women exhibit higher prevalence of most painful disorders. Several explanations have been proposed for this discrepancy, one being that endogenous pain modulatory pathways, which affect incoming nociceptive signals, act differently in men and women. A less efficient pain inhibitory system has been proposed as a contributing factor to explain why women exhibit higher prevalence of most painful disorders. The present study determined whether muscle pain, induced experimentally by electrical stimulation, is inhibited by a painful heat stimulus. This conditioned pain modulation (CPM) paradigm was used to determine whether women show signs of reduced inhibition compared to men.

Methods

Forty self-reported healthy individuals (20 female, 20 male) participated in a cross-over design with painful and non-painful heat as a conditioning stimulus. Test stimuli were painful intramuscular electrical stimulation of the tibialis anterior muscle at two intensities; low (1.1 × pain threshold) and high (1.6 × pain threshold). Painful conditioning was contact heat (45–49 ° C) to the contralateral forearm. Nonpainful conditioning was contact heat at 35 °C. Ten test stimuli were delivered in three blocks (before, during and after conditioning) in two sessions (painful and non-painful conditioning). The women were tested during days 12-14 of the menstrual cycle. This interval corresponds to the ovulatory phase of the menstrual cycle, the interval during which women are reported to show the largest inhibitory effects.

Results

Test stimuli were rated significantly lower during painful conditioning, compared with before conditioning. This was found for both low and high test stimulus intensities. Anonspecific attenuation was seen during non-painful conditioning for the low test stimulus intensity. Test stimuli were rated significantly lower also 3 min after conditioning, compared with before conditioning. The inhibitory effects were not different between men and women. Similar findings were obtained also if six non-CPM-responders (subjects rating test stimuli higher during conditioning than before conditioning) were excluded.

Conclusions and implications

The present findings indicate that painful contact heat inhibits electrically induced muscle pain and that inhibition was not different between men and women, when women were tested in the interval 12-14 days after their last menstruation. Some inhibition of muscle pain was seen during non-painful conditioning, indicating that nonspecific inhibitory effects were triggered. Also the nonspecific inhibitory effects were similar in men and women.

Neural and psychosocial contributions to sex differences in knee osteoarthritic pain

People with osteoarthritis (OA) can have significant pain that interferes with function and quality of life. Women with knee OA have greater pain and greater reductions in function and quality of life than men. In many cases, OA pain is directly related to sensitization and activation of nociceptors in the injured joint and correlates with the degree of joint effusion and synovial thickening. In some patients, however, the pain does not match the degree of injury and continues after removal of the nociceptors with a total joint replacement. Growth of new nociceptors, activation of nociceptors in the subchondral bone exposed after cartilage degradation, and nociceptors innervating synovium sensitized by inflammatory mediators could all augment the peripheral input to the central nervous system and result in pain. Enhanced central excitability and reduced central inhibition could lead to prolonged and enhanced pain that does not directly match the degree of injury. Psychosocial variables can influence pain and contribute to pain variability. This review explores the neural and psychosocial factors that contribute to knee OA pain with an emphasis on differences between the sexes and gaps in knowledge.

Spatial pain propagation over time following painful glutamate activation of latent myofascial trigger points in humans

The aim of this present study was to test the hypothesis that tonic nociceptive stimulation of latent myofascial trigger points (MTPs) may induce a spatially enlarged area of pressure pain hyperalgesia. Painful glutamate (.2 mL, 1M) stimulation of latent MTPs and non-MTPs in the forearm was achieved by an electromyography-guided procedure. Pain intensity (as rated on the visual analog scale [VAS]) and referred pain area following glutamate injections were recorded. Pressure pain threshold (PPT) was measured over 12 points in the forearm muscles and at the mid-point of tibialis anterior muscle before and at .5 hour, 1 hour, and 24 hours after glutamate injections. The results showed that maximal pain intensity, the area under the VAS curve, and referred pain area were significantly higher and larger following glutamate injection into latent MTPs than non-MTPs (all, P < .05). A significantly lower PPT level was detected over time after glutamate injection into latent MTPs at .5 hour (at 4 points), 1 hour (at 7 points), and 24 hours (at 6 points) in the forearm muscles. However, a significantly lower PPT was observed only at 24 hours after glutamate injection into non-MTPs in the forearm muscles (at 4 points, P < .05) when compared to the pre-injection PPT. PPT at the mid-point of the tibialis anterior was significantly decreased at 1 hour only as compared to the pre-injection PPT in both groups (< .05). The results of the present study indicate that nociceptive stimulation of latent MTPs is associated with an early onset of locally enlarged area of mechanical hyperalgesia.

PERSPECTIVE

This study shows that MTPs are associated with an early occurrence of a locally enlarged area of pressure hyperalgesia associated with spreading central sensitization. Inactivation of MTPs may prevent spatial pain propagation.

Sex and pain perception and analgesia

Sex is an important factor in painful experience modulation. Large volume of evidence shows that experience is different for males and females, as well as the answer to some classes of analgesics. Laboratory experiments suggest that women have a lower pain threshold than men related to pain from noxious stimuli such as heat, cold, pressure and electrical stimulation. Pain is a dynamic phenomenon under the influence of various mechanisms of excitatory and inhibitory control. The differences in pain perception related to sex may be associated with hyperalgesia in women, but also to the hypoactivity of the inhibitory system of pain in females. The purpose of this review besides showing some relationship for gonadal hormones, central nervous system and pain is to provide reference points for the discussion of one of the most intriguing aspects of the pathophysiology of pain: the differences in the presence of painful stimuli related to gender.

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

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

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

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

Involvement of peripheral ionotropic glutamate receptors in orofacial thermal hyperalgesia in rats

Background

The purpose of the present study was to elucidate the mechanisms that may underlie the sensitization of trigeminal spinal subnucleus caudalis (Vc) and upper cervical spinal cord (C1-C2) neurons to heat or cold stimulation of the orofacial region following glutamate (Glu) injection.

Results

Glu application to the tongue or whisker pad skin caused an enhancement of head-withdrawal reflex and extracellular signal-regulated kinase (ERK) phosphorylation in Vc-C2 neurons. Head-withdrawal reflex and ERK phosphorylation were also enhanced following cold stimulation of the tongue but not whisker pad skin in Glu-injected rats, and the head-withdrawal reflex and ERK phosphorylation were enhanced following heat stimulation of the tongue or whisker pad skin. The enhanced head-withdrawal reflex and ERK phosphorylation after heat stimulation of the tongue or whisker pad skin, and those following cold stimulation of the tongue but not whisker pad skin were suppressed following ionotropic glutamate receptor antagonists administration into the tongue or whisker pad skin. Furthermore, intrathecal administration of MEK1/2 inhibitor PD98059 caused significant suppression of enhanced head-withdrawal reflex in Glu-injected rats, heat head-withdrawal reflex in the rats with Glu injection into the tongue or whisker pad skin and cold head-withdrawal reflex in the rats with Glu injection into the tongue.

Conclusions

The present findings suggest that peripheral Glu receptor mechanisms may contribute to cold hyperalgesia in the tongue but not in the facial skin, and also contribute to heat hyperalgesia in the tongue and facial skin, and that the mitogen-activated protein kinase cascade in Vc-C2 neurons may be involved in these Glu-evoked hyperalgesic effects.

Pressure pain sensitivity maps of the neck-shoulder and the low back regions in men and women

Background

Musculoskeletal pain in the low back and neck-shoulder regions is a major problem among the working population all over the world. The prevalence of musculoskeletal pain is found to be higher among women. Women also have lower pressure pain thresholds (PPTs) than men. Pressure pain topography aims at mapping the spatial distribution of PPT within a muscle in an attempt to track changes in mechanical sensitivity. In order to assess gender differences in the pain topography, it is necessary to map the distribution in both healthy men and women. The aim of this study was to assess PPT maps from the cervico-thoracic and lumbar regions in men and women.

Methods

Eleven men and eleven women without any known musculoskeletal disorders participated in the study. PPT was measured twice at 36 points over the trapezius muscle of the dominant arm, at 36 points over the trapezius muscle on the contralateral side and at 12 points over the spine between the left and right trapezius. Further, 11 points were measured over the erector spinae muscle on the left side of the spine between the first and the fifth lumbar vertebrae, 11 on the right side and 5 points on the spine itself. The measurements on each trapezius muscle were divided according to anatomical subdivisions. Three-way and two-way ANOVAs were used to analyse the differences in PPTs with the following factors: gender, locations and sub-divisions (only for cervico-thoracic region).

Results

There were no differences between left and right side in neither the cervico-thoracic nor the lumbar region, but there were (large effect) differences between the subdivisions in the trapezius with the lowest values in the upper part (P < 0.001; partial η² = 0.19). Women had (small effect) lower PPT in both cervico-thoracic and lumbar regions (P ≤ 0.001; partial η² = 0.02 for both regions), but gender had no effect on neither location nor subdivisions.

Conclusions

The pain topography was not found to be different between genders in the cervico-thoracic and lumbar regions. This study can be used as basis for further clinical studies on musculoskeletal disorders.

Pain following the repair of an abdominal hernia

Pain and other types of discomfort are frequent symptoms following the repair of an abdominal hernia. After 1 year, the incidence of light to moderate pain following inguinal hernia repair is as high as 10% and 2% for severe disabling chronic pain. Postoperative chronic pain not only affects the individual patient, but may also have a great impact on relatives and society, and may be a cause of concern for the responsible surgeon. This paper provides an overview of the anatomy, surgical procedures, and disposing factors (age, gender, ethnicity, genotype, previous hernia repair, pain prior to surgery, psychosocial characteristics, and surgical procedures) related to the postoperative pain conditions. Furthermore, the mechanisms for both acute and chronic pain are presented. We focus on inguinal hernia repair, which is the most frequent type of abdominal hernia surgery that leads to chronic pain. Finally, the paper provides an update on the diagnostic and treatment routines for postoperative pain.

Reliability of temporal summation and diffuse noxious inhibitory control

BACKGROUND

The test-retest reliability of temporal summation (TS) and diffuse noxious inhibitory control (DNIC) has not been reported to date. Establishing such reliability would support the possibility of future experimental studies examining factors affecting TS and DNIC. Similarly, the use of manual algometry to induce TS, or an occlusion cuff to induce DNIC of TS to mechanical stimuli, has not been reported to date. Such devices may offer a simpler method than current techniques for inducing TS and DNIC, affording assessment at more anatomical locations and in more varied research settings.

METHOD

The present study assessed the test-retest reliability of TS and DNIC using the above techniques. Sex differences on these measures were also investigated.

RESULTS

Repeated measures ANOVA indicated successful induction of TS and DNIC, with no significant differences across test-retest occasions. Sex effects were not significant for any measure or interaction. Intraclass correlations indicated high test-retest reliability for all measures; however, there was large interindividual variation between test and retest measurements.

CONCLUSION

The present results indicate acceptable within-session test-retest reliability of TS and DNIC. The results support the possibility of future experimental studies examining factors affecting TS and DNIC.

Noxious inhibition of temporal summation is impaired in chronic tension-type headache

OBJECTIVE

To examine effects of stress on noxious inhibition and temporal summation (TS) in tension-type headache.

BACKGROUND

Stress is the most commonly reported trigger of a chronic tension-type headache (CTH) episode; however, the mechanisms underlying this are unclear. Stress affects pain processing throughout the central nervous system, including, potentially, mechanisms of TS and diffuse noxious inhibitory controls (DNIC), both of which may be abnormal in CTH sufferers (CTH-S). No studies have examined TS of pressure pain or DNIC of TS in CTH-S to date. Similarly, effects of stress on TS or DNIC of TS have not been reported in healthy subjects or CTH-S to date.

METHODS

The present study measured TS and DNIC of TS in CTH-S and healthy controls (CNT) exposed to an hour-long stressful mental task, and in CTH-S exposed to an hour-long neutral condition. TS was elicited at finger and shoulder via 10 pulses from a pressure algometer, applied before and during stimulation from an occlusion cuff at painful intensity.

RESULTS

Algometer pain ratings increased more in the CTH compared with the CNT group, and were inhibited during occlusion cuff more in the CNT compared with CTH groups. Task effects on TS or DNIC were not significant.

CONCLUSIONS

The results indicate increased TS to pressure pain and impaired DNIC of TS in CTH-S. Stress does not appear to aggravate abnormal TS or DNIC mechanisms in CTH-S.

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.

Effect of mental stress on cold pain in chronic tension-type headache sufferers

Mental stress is a noted contributing factor in chronic tension-type headache (CTH), however the mechanisms underlying this are not clearly understood. One proposition is that stress aggravates already increased pain sensitivity in CTH sufferers. This hypothesis could be partially tested by examining effects of mental stress on threshold and supra-threshold experimental pain processing in CTH sufferers. Such studies have not been reported to date. The present study measured pain detection and tolerance thresholds and ratings of supra-threshold pain stimulation from cold pressor test in CTH sufferers (CTH-S) and healthy Control (CNT) subjects exposed to a 60-min stressful mental task, and in CTH sufferers exposed to a 60-min neutral condition (CTH-N). Headache sufferers had lower pain tolerance thresholds and increased pain intensity ratings compared to controls. Pain detection and tolerance thresholds decreased and pain intensity ratings increased during the stress task, with a greater reduction in pain detection threshold and increase in pain intensity ratings in the CTH-S compared to CNT group. The results support the hypothesis that mental stress contributes to CTH through aggravating already increased pain sensitivity in CTH sufferers.

The role of resilience and purpose in life in habituation to heat and cold pain

This study examined the role of resilience in habituation to heat and cold pain in healthy women (n = 47). Heat and cold pain thresholds were each assessed across 5 equally spaced trials. Resilience, purpose in life, optimism, social support, and neuroticism were assessed using self-report measures. The hypothesis was that the resilience and the associated resilience factors would be positively related to habituation to heat and cold pain while controlling for neuroticism. Multilevel modeling was used to test the hypothesis. When considering each characteristic separately, resilience and purpose in life predicted greater habituation to heat pain while resilience, purpose in life, optimism, and social support predicted greater habituation to cold pain. When controlling for the other characteristics, both resilience and purpose in life predicted greater habituation to heat and cold pain. Resilience and associated characteristics such as a sense of purpose in life may be related to enhanced habituation to painful stimuli. Future research should further examine the relationship between resilience, purpose in life, and habituation to pain and determine whether psychosocial interventions that target resilience and purpose in life improve habituation and reduce vulnerability to chronic pain.

PERSPECTIVE

This article showed that resilience and a sense of purpose in life were both related to the ability to habituate to heat and cold pain in healthy women. These personal characteristics may enhance habituation to pain by providing the confidence and motivation to persist in the face of painful stimuli.

Glutamate and capsaicin-induced pain, hyperalgesia and modulatory interactions in human tendon tissue

Experimental glutamate and capsaicin-induced pain has not been described in tendon tissue despite the implications of addressing these receptors in pain management strategies. This study investigated pain induction and modulatory interactions by injecting glutamate (0.5 ml, 1 M) and capsaicin (0.5 ml, 5 microg, 33 microM) to human tendon tissue. Following the initial glutamate or capsaicin injection, a second injection of either glutamate (following capsaicin), capsaicin (following glutamate) or hypertonic saline (after both glutamate and capsaicin) was given. Twelve male volunteers participated. Subjects had four sequences of injections to tibialis anterior tendon over two sessions 1 week apart. Pain intensity responses were scored on a visual analogue scale (VAS). Pressure pain thresholds (PPTs) were assessed before, during and after pain induction. Capsaicin caused significantly higher peak pain scores compared to glutamate (P < 0.003) whilst glutamate pain was of significantly longer duration (P < 0.0003). Capsaicin following glutamate resulted in significantly higher average VAS scores 180-450 s after injection compared to capsaicin as primary injection (P < 0.05). PPTs were significantly reduced during capsaicin pain (72 +/- 5 and 80 +/- 6% of pre-pain values at the injection site and 2 cm proximal, P < 0.002). Following capsaicin, hypertonic saline and glutamate showed significant reductions in PPT at the same sites and to a similar degree compared to baseline (P < 0.002). The results indicate in tendon tissue a facilitation of response to capsaicin injection following glutamate injection. PPTs were only reliably reduced by capsaicin injection. These results emphasize the possible importance of peripheral glutamate receptor antagonists in pain management in musculoskeletal conditions.

Algesic agents exciting muscle nociceptors

Morphologically, muscle nociceptors are free nerve endings connected to the CNS by thin myelinated (group III) or unmyelinated (group IV) afferent fibers. Not all of these endings are nociceptive; approximately 40% have a low mechanical threshold and likely fulfill non-nociceptive functions. Two chemical stimuli are particularly relevant as causes of muscle pain. The first is a drop in tissue pH, i.e. an increase in proton (H+) concentration. A large number of painful patho(physio)logical alterations of muscle tissue are associated with an acidic interstitial pH (e.g. tonic contractions, spasm, inflammation). The second important cause of muscle pain is a release of adenosine triphosphate (ATP). ATP is present in all body cells, but in muscle its concentration is particularly high. Any damage of muscle cells (trauma, necrotic myositis) is accompanied by a release of ATP from the cells. Therefore, ATP is considered a general pain stimulus by some. ATP and protons are relatively specific stimuli for muscle pain; in cutaneous pain they play a less important role. The numerous agents that are released in pathologically altered muscle include substances that desensitize mechanosensitive group IV receptors. Capsaicin has a long-lasting desensitizing action, brain-derived neurotrophic factor, and tumor necrosis factor-alpha, a short-lasting one. Most of the agents exciting group IV units (e.g. low pH, ATP, capsaicin) activate not only nociceptive endings but also non-nociceptive ones. The only substance encountered that excites exclusively nociceptive group IV receptors is nerve growth factor (NGF). In rat muscle chronically inflamed with complete Freund's adjuvant, most group IV endings are sensitized to mechanical (and to some) chemical stimuli. However, stimulants such as ATP, NGF, and solutions of low pH were found to be less effective in inflamed muscle. A possible explanation for this surprising finding is that in inflamed muscle the concentrations of ATP and NGF and H+ are increased. Therefore, experimental administration of these agents is a less effective stimulus.

Association of catastrophizing with interleukin-6 responses to acute pain

Catastrophizing exerts its deleterious effects on pain via multiple pathways, and some researchers have reported that high levels of catastrophizing are associated with enhanced physiological reactivity to painful stimulation. In this project, 42 generally healthy adults underwent a series of psychophysical pain testing procedures assessing responses to noxious mechanical, heat, and cold stimuli. Pain catastrophizing cognitions were assessed prior to and then immediately after the various pain induction procedures. Blood samples were taken at baseline and then at several time points from the end of the procedures to 1h post-testing. Samples were assayed for serum levels of cortisol and interleukin-6 (IL-6). Both cortisol and IL-6 increased from baseline during the post-testing period (p's<.05), with cortisol returning to baseline by 1h post-testing and IL-6 remaining elevated. Pain catastrophizing, measured immediately after the pain procedures, was unrelated to cortisol reactivity, but was strongly related to IL-6 reactivity (p<.01), with higher levels of catastrophizing predicting greater IL-6 reactivity. In multivariate analyses, the relationship between catastrophizing and IL-6 reactivity was independent of pain ratings. Collectively, these findings suggest that cognitive and emotional responses during the experience of pain can shape pro-inflammatory immune system responses to noxious stimulation. This pathway may represent one important mechanism by which catastrophizing and other psychosocial factors shape the experience of both acute and chronic pain in a variety of settings.

Effect of heterotopic noxious conditioning stimulation on electrical and pressure pain thresholds in two different anatomical regions

OBJECTIVES

The aims of the study were to investigate the influence of heterotopic noxious conditioning stimulation (HNCS) on pain thresholds in the orofacial and spinal regions and to find out whether there are gender differences in this respect.

MATERIAL AND METHODS

Thirty healthy subjects (15 of each sex) with a mean (SD) age of 25.1 (4.4) years participated. Pain thresholds to electrical (EPT) and pressure stimuli (PPT) were recorded in the masseter muscle and 1st upper incisor (tooth), as well as in the fingertip, before, during, and 5 and 15 min after a cold pressor task to the contralateral hand immersed in ice-cold water for a maximum of 5 min.

RESULTS

With the exception of the EPT in the orofacial region, all pain thresholds increased during the HNCS and then returned to baseline during the 15 min follow-up. The significant changes in EPT were greater in the finger than in the tooth, while the changes in PPT were greater in the masseter muscle than in the finger. Electrical stimuli in the finger induced greater significant changes of pain thresholds than pressure. In the orofacial region, pressure induced greater significant changes in pain thresholds during HNCS than electrical stimuli did. The HNCS induced pain of high intensity and unpleasantness, i.e. varying between 5 and 10 on the numeric rating scale (NRS). There were no gender differences in the response to the HNCS.

CONCLUSION

We conclude that, in general, HNCS induced by cold pressor stimulation increases pain thresholds, but the magnitude of the effect differs between the orofacial region and the finger and is influenced by the tissue and type of test stimuli.