The association between proportion of night shifts and musculoskeletal pain and headaches in nurses: a cross-sectional study.

BACKGROUND AND PURPOSE: Shift work is associated with musculoskeletal pain and headaches, but little is known about how the intensity of shift work exposure is related to musculoskeletal pain and headaches. This study aimed to investigate whether a higher proportion of night shifts is associated with a higher occurrence of musculoskeletal pain and headaches. Furthermore, to investigate whether sleep duration can mediate this potential association. METHOD: The study included 684 nurses in rotating shift work who responded to a daily questionnaire about working hours, sleep, and pain for 28 consecutive days. The data were treated cross-sectionally. RESULTS: A negative binomial regression analysis adjusted for age and BMI revealed that working a higher proportion of night shifts is not associated with a higher occurrence of musculoskeletal pain and headaches. On the contrary, those working ≥ 50% night shifts had a significantly lower occurrence of pain in the lower extremities than those who worked < 25% night shifts (IRR 0.69 95% CI 0.51, 0.94). There was no indication of a mediation effect with total sleep time (TST). CONCLUSION: The results of this study indicate that working a higher proportion of night shifts is not associated with a higher occurrence of musculoskeletal pain and headaches.

Effects of insufficient sleep on sensorimotor processing in migraine: A randomised, blinded crossover study of event related beta oscillations.

BACKGROUND: Migraine has a largely unexplained connection with sleep and is possibly related to a dysfunction of thalamocortical systems and cortical inhibition. In this study we investigate the effect of insufficient sleep on cortical sensorimotor processing in migraine. METHODS: We recorded electroencephalography during a sensorimotor task from 46 interictal migraineurs and 28 controls after two nights of eight-hour habitual sleep and after two nights of four-hour restricted sleep. We compared changes in beta oscillations of the sensorimotor cortex after the two sleep conditions between migraineurs, controls and subgroups differentiating migraine subjects usually having attacks starting during sleep and not during sleep. We included preictal and postictal recordings in a secondary analysis of temporal changes in relation to attacks. RESULTS: Interictally, we discovered lower beta synchronisation after sleep restriction in sleep related migraine compared to non-sleep related migraine (p=0.006) and controls (p=0.01). No differences were seen between controls and the total migraine group in the interictal phase. After migraine attacks, we observed lower beta synchronisation (p<0.001) and higher beta desynchronisation (p=0.002) after sleep restriction closer to the end of the attack compared to later after the attack. CONCLUSION: The subgroup with sleep related migraine had lower sensorimotor beta synchronisation after sleep restriction, possibly related to dysfunctional GABAergic inhibitory systems. Sufficient sleep during or immediately after migraine attacks may be of importance for maintaining normal cortical excitability.

Pain thresholds and suprathreshold pain after sleep restriction in migraine - A blinded crossover study.

OBJECTIVE: There is an unexplained association between disturbed sleep and migraine. In this blinded crossover study, we investigate if experimental sleep restriction has a different effect on pain thresholds and suprathreshold pain in interictal migraineurs and controls. METHODS: Forearm heat pain thresholds and tolerance thresholds, and trapezius pressure pain thresholds and suprathreshold pain were measured in 39 interictal migraineurs and 31 healthy controls after two consecutive nights of partial sleep restriction and after habitual sleep. RESULTS: The effect of sleep restriction was not significantly different between interictal migraineurs and controls in the primary analyses. Pressure pain thresholds tended to be lower (i.e., increased pain sensitivity) after sleep restriction in interictal migraineurs compared to controls with a 48-hour preictal-interictal cut-off (p = 0.061). We found decreased pain thresholds after sleep restriction in two of seven migraine subgroup comparisons: heat pain thresholds decreased in migraineurs with lower pain intensity during attacks (p = 0.005) and pressure pain thresholds decreased in migraineurs with higher severity of photophobia during attacks (p = 0.031). Heat pain thresholds tended to decrease after sleep restriction in sleep-related migraine (p = 0.060). Sleep restriction did not affect suprathreshold pain measurements in either group. CONCLUSION: This study could not provide strong evidence for an increased effect of sleep restriction on pain sensitivity in migraineurs compared to healthy controls. There might be a slightly increased effect of sleep restriction in migraineurs, detectable using large samples or more pronounced in certain migraine subgroups.

Shift work, low-grade inflammation, and chronic pain: a 7-year prospective study.

OBJECTIVES: We investigated prospective associations of shift work with chronic pain and C-reactive protein (CRP), an indicator of inflammation. Furthermore, we elucidated CRP as a possible mediator and/or moderator of effects of shift work on pain. METHODS: Data from a 7 years follow-up study were analyzed (N = 2323). Shift work and chronic pain of "neck/shoulder", "arm/hand", "upper back", "low back", "hip/leg/feet", and "other regions" were measured by questionnaires. "Chronic widespread pain", "number of chronic pain sites", and "any chronic pain" were computed. CRP was measured in serum samples. Logistic and Poisson regressions were conducted. Mediation was assessed by casual mediation analyses and moderation by the Relative Excess Risk due to Interaction (RERI). RESULTS: Shift work was not associated with any chronic pain variable and no mediation was detected. CRP was associated with low back pain, hip/leg pain, and "number of pain sites", and also with the combination of shift work and CRP of 1-2.99 mg/L (compared to: no shiftwork and CRP < 1). Additionally, shiftwork and CRP 1-2.99 mg/L was associated with risk of "any chronic pain" (OR: 1.76, 95% CI: 1.12, 2.85), which was not associated with CRP alone. Moderation analyses suggested the risks for "any chronic pain" and "number of pain regions" increased when individuals with elevated CRP worked shifts-beyond what the separate effects of CRP and shift would suggest. CONCLUSIONS: We found no evidence of shift work in general affecting CRP or chronic pain. However, shift work and elevated CRP combined may influence chronic pain.

Pain complaints are associated with quick returns and insomnia among Norwegian nurses, but do not differ between shift workers and day only workers.

PURPOSE: To determine whether common work schedule characteristics among Norwegian nurses were associated with subjective pain complaints. METHODS: A cross-sectional study in a sample of 1585 nurses, part of the longitudinal questionnaire-based cohort project 'Survey of Shift work, Sleep and Health' (SUSSH). Pain from six regions were assessed: 'headache', 'neck/shoulder/upper back', 'upper extremities', 'lower back', 'lower extremities', and 'abdomen'. Logistic and negative binomial regression (adjusted for age, sex, percentage of full-time equivalent, marital status and children living at home) were conducted where work schedule, number of night shifts last year, number of quick returns (QR) last year (< 11 h between shifts) and insomnia were predictors of localized pain, widespread pain and number of pain sites. RESULTS: Localized pain, widespread pain and number of pain sites were associated with insomnia (OR 2.06, 95% CI 1.66-2.55, OR 2.14, 95% CI 1.47-3.09, IRR 1.70, 95% CI 1.51-1.91, respectively). Work schedule and number of night shifts worked last year were not associated with any of the three pain measures. Number of QRs worked last year tended to be associated with number of pain sites. CONCLUSION: The study did not support the hypothesis that non-daytime work schedules are associated with pain complaints. Neither was there support for the hypothesis linking number of night shifts, or the number of QRs, to pain complaints. Future studies should aim to determine the association between QRs and pain in more detail. Pain complaints were associated with insomnia.

Sleep duration mediates abdominal and lower-extremity pain after night work in nurses.

OBJECTIVES: The aim of this study was to investigate the association between different working shifts (i.e. morning, evening, night shifts) and headache, musculoskeletal and abdominal pain, and the extent to which reduced sleep duration could account for these associations. METHODS: Nurses (N = 679, 649 female, aged 22-53 years) were followed up for a period of 28 consecutive days, responding to a diary about sleep, shift type and pain complaints (measured on a Likert-type scale ranging from 0 to 3). Generalised structural equation modelling mediation analysis (GSEM) was performed to test whether shift type was associated with higher incidence or higher intensity of pain (headache, pain in neck/shoulders/upper back, upper extremity, low back, lower extremity and abdominal pain), and if this effect was mediated by sleep duration (continuous variable), after controlling for age, work and lifestyle factors. RESULTS: Pain scores in lower extremities were decreased following night shifts in general. However, when night shifts were followed by short sleep duration, the risk of pain in the lower extremities and abdominal pain were increased. Headache and pain in the upper extremity were increased after night shifts, but were not associated with sleep duration. Pain in the neck/shoulder/upper back and lower back was not related to shift work. CONCLUSIONS: Among nurses in a three-shift rotating schedule, night shifts increased the risk of pain in several regions, but only pain in the lower extremities and abdomen was related to reduced sleep duration.

Evoked potentials in the Atlantic cod following putatively innocuous and putatively noxious electrical stimulation: a minimally invasive approach.

Aspects of peripheral and central nociception have previously been studied through recording of somatosensory evoked potentials (SEPs) to putative noxious stimuli in specific brain regions in a few freshwater fish species. In the present study, we describe a novel, minimally invasive method for recording SEPs from the central nervous system of the Atlantic cod (Gadus morhua). Cutaneous electric stimulation of the tail in 15 fish elicited SEPs at all stimulus intensities (2, 5, 10 and 20 mA) with quantitative properties corresponding to stimulus intensity. In contrast to previous fish studies, the methodological approach used in Atlantic cod in the current study uncovered a number of additional responses that could originate from multiple brain regions. Several of these responses were specific to stimulation at the highest stimulus intensities, possibly representing qualitative differences in central processing between somatosensory and nociceptive stimuli.

Night work, season and alertness as occupational safety hazards in the Arctic: protocol for the Noralert observational crossover study among Norwegian process operators.

INTRODUCTION: The objective of this study is to determine the effects of night work, Arctic seasonal factors and cold working environments on human functions relevant to safety. The study aims to quantify the contribution of (1) several consecutive night shifts, (2) seasonal variation on sleepiness, alertness and circadian rhythm and (3) whether a computational model of sleep, circadian rhythms and cognitive performance can accurately predict the observed sleepiness and alertness. METHODS AND ANALYSIS: In an observational crossover study of outdoor and indoor workers (n=120) on a three-shift schedule from an industrial plant in Norway (70 °N), measurements will be conducted during the summer and winter. Sleep duration and quality will be measured daily by smartphone questionnaire, aided by actigraphy and heart rate measurements. Sleepiness and alertness will be assessed at regular intervals by the Karolinska Sleepiness Scale and the psychomotor vigilance test, respectively. Saliva samples will assess melatonin levels, and a blood sample will measure circadian time. Thermal exposures and responses will be measured by sensors and by thermography. ETHICS AND DISSEMINATION: All participants will give written informed consent to participate in the study, which will be conducted in accordance with the Declaration of Helsinki. The Norwegian Regional Committee for Medical Research Ethics South-East D waivered the need for ethics approval (reference 495816). Dissemination plans include academic and lay publications, and partnerships with national and regional policymakers.

Influence of Shift Work on Arterial Stiffness and Systemic Inflammation: A 3-Year Follow-up Study in Industry.

OBJECTIVE: To assess changes in cardiovascular disease risk factors during a 3-year follow-up among 57 rotating shift workers and 29 day workers in industry. METHODS: We collected demographics by questionnaire, examined blood pressure, heart rate, pulse wave velocity, carotid media thickness, and maximal oxygen uptake. We assessed blood samples for determination of lipids, glycosylated hemoglobin, C-reactive protein, markers of inflammation, and particle concentrations/respirable dust. Baseline comparisons were analyzed using logistic regression (plaque) and linear regression for all other outcomes. We applied mixed models to assess differences in change in health outcomes between the shift workers and the day workers. RESULTS: At baseline, the adhesion molecules soluble vascular cell adhesion molecule 1 and soluble P-selectin were elevated among the shift workers compared with that of the day workers. There was a significant difference in change in pulse wave velocity between shift workers (1.29-m/s increase) and day workers (0.11-m/s increase) over the 3-year follow-up. Respirable dust levels were below the Norwegian occupational exposure limit. CONCLUSIONS: Shift work in industry is associated with arterial stiffening reflecting increased risk for future cardiovascular disease. More uncertainly, we found some support for systemic inflammation.

Extended Work Shifts Increase Subjective Pain Complaints Among Cabin Crewmembers, but Not Among Airline Pilots or Healthcare Workers: A Repeated-Measures Study.

OBJECTIVE: To determine whether four consecutive extended work shifts are associated with an increased risk of subjective pain complaints, sleep duration, and sleep disturbances. METHODS: Forty-three healthcare workers, 41 cabin crewmembers, and 18 airline pilots working 4 consecutive extended workdays reported subjective pain complaints and sleep after the 1st and 4th workday. RESULTS: The risk of headache (odds ratio [OR] 21.4, 95% confidence interval [CI] 1.85 to 246.5) and pain in the hands, arms, or wrists (OR 3.78, 95% CI 1.84 to 7.76) increased after workday 4 versus workday 1 in cabin crewmembers. Sleep duration was longer (0.6 to 1.1 hours), and sleep disturbances fewer, the night before the fourth extended workday, compared with before the first workday, in all occupations. CONCLUSIONS: We found no general support for an association between extended work shifts and subjective pain, whereas sleep duration was improved, and sleep disturbances reduced after 4 consecutive extended workdays.

Sleep Duration, Number of Awakenings and Arterial Stiffness in Industrial Shift Workers: A Five-Week Follow-Up Study.

Shift work may increase the risk for hypertension and arterial stiffness, potentially a consequence of disturbed sleep. The aim of this study was to investigate possible correlations between sleep length and spontaneous awakenings with selected cardiovascular risk factors in shift workers at an industrial plant. We examined 19 shift workers by means of blood pressure and arterial stiffness, measured as pulse wave velocity (PWV), prior to and after a 5-week shift period. Sleep patterns were monitored on a daily basis with the assistance of a smartphone-based sleep diary (the entire test period) and by actigraphy (limited to 2 weeks). The number of awakenings and total sleep time were calculated. Shorter sleep duration was associated with higher blood pressure and partly with higher PWV, indicating an increased risk of cardiovascular disease (CVD) with reduced sleep duration. Unexpectedly, a lower number of awakenings was associated with an increase in blood pressure, indicating a reduced risk of CVD. No other significant associations were determined. The results from the present study among shift workers in Norway could support the hypothesis that short sleep duration is associated with elevated blood pressure and arterial stiffness.

A Cease in Shift Work Reverses Arterial Stiffness but Increases Weight and Glycosylated Hemoglobin A 5-Month Follow-Up in Industry.

BACKGROUND: Literature suggests an association between shift work and cardiovascular disease (CVD). Limited evidence is available on how a cessation of shift work affects CVD risk factors. AIM: We investigated whether a five-month plant shutdown affected CVD risk factors in 30 industrial shift workers. METHODS: We collected demographic data, self-reported data on physical activity (PA) and medical history by questionnaire. Pre- and post-plant shutdown, we measured blood pressure (BP), heart rate, lipids, glycosylated hemoglobin (HbA1c) and C-reactive protein (CRP). Additionally, we collected markers of inflammation, Matrix metalloproteinase-9 (MMP-9), Interleukin-6 (IL-6), Monocyte chemoattractant protein-1 (MCP-1), Tumor necrosis factor-alpha (TNF-α), P-selectin, Interleukin-1 beta (IL-1ß), and Interleukin-23 (IL-23). We also examined arterial stiffness (central blood pressure, augmentation pressure, and pulse wave velocity) by means of SphygmoCor® (AtCor Medical Pty Ltd., Sydney, Australia). We monitored sleep by actigraphy prior to and after plant shutdown, with additional registration of sleep quality and assessment of insomnia symptoms. RESULTS: After five months of plant shutdown, we found that HbA1c increased by 1.9 mmol/mol, weight by 1 kg and MCP-1 by 27.3 pg/mL, all unexpectedly. The other markers of inflammation did not change during shutdown, but CRP decreased close to significant levels. There were no changes in lipids during follow-up. Pulse-wave velocity (PWV) was reduced from 8.1 m/s (SD = 1.5) to 7.6 m/s (SD = 1.5), p = 0.03. The workers reported fewer signs of insomnia after shutdown. CONCLUSIONS: Our findings suggest that a five-month cessation in shift work increases weight and HbA1c, but also improves insomnia symptoms and reverses arterial stiffening.

Patterns of pain complaints and insomnia symptoms are associated with abusive supervision in the Norwegian working population: a latent class analysis.

OBJECTIVES: Previous findings suggest that abusive supervision, i.e., subordinates' perceptions of their supervisor's behaviours as hostile (excluding physical aggression), may increase the risk of health complaints. In addition, recent data suggest that the FKBP5 genotype rs9470080 important in the regulation of cortisol release, influences the same outcome. Adding to this complexity, different health complaints often co-occur. The present study aimed to (1) uncover patterns of pain complaints and insomnia symptoms by using latent class analysis, (2) determine whether abusive supervision or FKBP5 rs9470080 was associated with these patterns, and (3) examine the interaction between abusive supervision and FKBP5 genotype regarding pain and insomnia symptoms. METHODS: The data was collected through a national probability survey of 5,000 employees drawn from the National Central Employee Register by Statistics Norway. Abusive supervision was measured by a 5-item version of the Tepper's 2000 scale. Pain and insomnia symptoms were measured by 5 items reflecting pain and 3 items reflecting insomnia. The FKBP5 rs9470080 genotyping was carried out using TaqMan assay. RESULTS: A total of 1,226 participants returned the questionnaire and the saliva kit sample. Based on these the latent class analyses revealed four classes based on response patterns of pain and insomnia symptoms. In the regression analysis, abusive supervision was a significant predictor for the response patterns. However, neither the FKBP5 nor the interaction between abusive supervision and FKBP5 showed significant contributions. CONCLUSIONS: In conclusion, awareness of the association between abusive supervision and the revealed four pain- and insomnia subgroups, and what separates them, may be important for prognosis and an optimal follow-up for those affected.

Adherence to self-managed exercises for patients with persistent subacromial pain: the Ad-Shoulder feasibility study.

BACKGROUND: Exercise is recommended for patients with subacromial pain. It has been suggested that good exercise adherence improves clinical outcomes. Despite this, little attention has been paid to the need for behavioural frameworks to enhance adherence to home exercise programmes for patients with subacromial pain. METHODS: A feasibility study with pre-post design was used. Participants aged > 18 years, with subacromial pain, who had received conservative treatment during the past 6 months, were recruited. The Ad-Shoulder intervention consisted of 1-5 individual sessions provided over 3 months and was based on 5 self-management skills, which aimed to enhance the patients' self-efficacy and adherence to self-managed exercises. The primary objectives were assessed according to predefined progression criteria: (1) the recruitment rate (10 patients enrolled within 12 weeks), (2) follow-up rate (≥ 80% on all self-reported measures), (3) objective physical activity measures (≥ 80% of participants would contribute valid data at each time point), (4) adherence with the self-managed exercises (≥ 80% of the participants would adhere to ≥ 80% of the assigned home exercise programme), (5) fidelity of the delivery of the intervention (the therapists delivered the intervention according to the protocol) and (6) adverse events (< 30% would report adverse events (including mild)). The results were reported using descriptive statistics. RESULTS: Eleven patients were recruited during 16 weeks. Ten patients completed the self-reported measures at baseline and week 12. Objective physical activity measures were successfully obtained for 100% (11/11) at baseline, 64% (7/11) at week six and 82% at week 12. Fifty-five percent (6/11) of the participants satisfactorily completed at least 80% of their home exercise programme. All sessions were delivered according to the protocol. None of the patients reported any adverse events. CONCLUSIONS: Objective physical activity data measures at baseline and week 12, follow-up, the physiotherapists' fidelity to the intervention and adverse events met our pre-specified progression criteria. Recruitment and adherence to the self-managed exercise programme were both below the anticipated level. Further intervention development is necessary to understand whether adherence to the self-managed exercises could be enhanced and additional methods of recruitment would need to be considered, including additional recruitment sites, in any planning for a future main trial. TRIAL REGISTRATION: ClinicalTrials.gov, NCT04190836 , Registered December 9, 2019-retrospectively registered.

Safety incidents associated with extended working hours. A systematic review and meta-analysis.

OBJECTIVE: We performed a systematic review to assess potential consequences of extended working hours on accidents, near-accidents, safety incidents and injuries (incidents) by considering the overall certainty of evidence. METHODS: We searched five databases systematically (Medline, Embase, PsycINFO, Web of Science, and Proquest Health and safety Science Abstract) and identified 10072 studies published until December 2020. Twenty-two studies met the inclusion criteria. We followed a systematic approach to evaluate risk of bias and synthesize results in a meta-analysis. The certainty of evidence was determined by a modified version of The Grading of Recommendations Assessment, Development and Evaluation (GRADE). RESULTS: Our analyses indicated an association between working >12 hours/day (RR: 1.24, 95%CI: 1.11, 1.40), or working >55 hours/week (RR: 1.24, 95%CI: 0.98, 1.57), and elevated risk of incidents. The certainty of evidence evaluated as low. Weak or no associations were observed for other exposure contrasts: working >8 hours/day (RR: 0.93, 95%CI: 0.72, 1.19), or working overtime (RR: 1.08, 95%CI: 0.75, 1.55), working 41-48 hours/week (RR: 1.02, 95%CI: 0.92, 1.13) or 49-54 hours/week (RR: 1.02, 95%CI: 0.97, 1.07). The certainty of evidence was evaluated as low (very low for 41-48 hours/week). CONCLUSIONS: Daily working hours >12 hours and weekly working hours exceeding 55 hours was associated and increased risk of incidents. The level of evidence was low. Hence, further high-quality research is warranted to elucidate these associations.

The association between selected genetic variants and individual differences in experimental pain.

OBJECTIVES: The underlying mechanisms for individual differences in experimental pain are not fully understood, but genetic susceptibility is hypothesized to explain some of these differences. In the present study we focus on three genetic variants important for modulating experimental pain related to serotonin (SLC6A4 5-HTTLPR/rs25531 A>G), catecholamine (COMT rs4680 Val158Met) and opioid (OPRM1 rs1799971 A118G) signaling. We aimed to investigate associations between each of the selected genetic variants and individual differences in experimental pain. METHODS: In total 356 subjects (232 low back pain patients and 124 healthy volunteers) were genotyped and assessed with tests of heat pain threshold, pressure pain thresholds, heat pain tolerance, conditioned pain modulation (CPM), offset analgesia, temporal summation and secondary hyperalgesia. Low back pain patients and healthy volunteers did not differ in regards to experimental test results or allelic frequencies, and were therefore analyzed as one group. The associations were tested using analysis of variance and the Kruskal-Wallis test. RESULTS: No significant associations were observed between the genetic variants (SLC6A4 5-HTTLPR/rs25531 A>G, COMT rs4680 Val158Met and OPRM1 rs1799971 A118G) and individual differences in experimental pain (heat pain threshold, pressure pain threshold, heat pain tolerance, CPM, offset analgesia, temporal summation and secondary hyperalgesia). CONCLUSIONS: The selected pain-associated genetic variants were not associated with individual differences in experimental pain. Genetic variants well known for playing central roles in pain perception failed to explain individual differences in experimental pain in 356 subjects. The finding is an important contribution to the literature, which often consists of studies with lower sample size and one or few experimental pain assessments.

Low Back Pain With Persistent Radiculopathy; the Clinical Role of Genetic Variants in the Genes SOX5, CCDC26/GSDMC and DCC.

In a recently published genome-wide association study (GWAS) chronic back pain was associated with three loci; SOX5, CCDC26/GSDMC and DCC. This GWAS was based on a heterogeneous sample of back pain disorders, and it is unknown whether these loci are of clinical relevance for low back pain (LBP) with persistent radiculopathy. Thus, we examine if LBP with radiculopathy 12 months after an acute episode of LBP with radiculopathy is associated with the selected single nucleotide polymorphisms (SNPs); SOX5 rs34616559, CCDC26/GSDMC rs7833174 and DCC rs4384683. In this prospective cohort study, subjects admitted to a secondary health care institution due to an acute episode of LBP with radiculopathy, reported back pain, leg pain, and Oswestry Disability Index (ODI), were genotyped and followed up at 12 months (n = 338). Kruskal-Wallis H test showed no association between the SNPs and back pain, leg pain or ODI. In conclusion, LBP with radiculopathy 12 months after an acute episode of LBP with radiculopathy, is not associated with the selected SNPs; SOX5 rs34616559, CCDC26/GSDMC rs7833174 and DCC rs4384683. This absent or weak association suggests that the SNPs previously associated with chronic back pain are not useful as prognostic biomarkers for LBP with persistent radiculopathy.

"First pain" in humans: convergent and specific forebrain responses.

BACKGROUND: Brief heat stimuli that excite nociceptors innervated by finely myelinated (Aδ) fibers evoke an initial, sharp, well-localized pain ("first pain") that is distinguishable from the delayed, less intense, more prolonged dull pain attributed to nociceptors innervated by unmyelinated (C) fibers ("second pain"). In the present study, we address the question of whether a brief, noxious heat stimulus that excites cutaneous Aδ fibers activates a distinct set of forebrain structures preferentially in addition to those with similar responses to converging input from C fibers. Heat stimuli at two temperatures were applied to the dorsum of the left hand of healthy volunteers in a functional brain imaging (fMRI) paradigm and responses analyzed in a set of volumes of interest (VOI). RESULTS: Brief 41°C stimuli were painless and evoked only C fiber responses, but 51°C stimuli were at pain threshold and preferentially evoked Aδ fiber responses. Most VOI responded to both intensities of stimulation. However, within volumes of interest, a contrast analysis and comparison of BOLD response latencies showed that the bilateral anterior insulae, the contralateral hippocampus, and the ipsilateral posterior insula were preferentially activated by painful heat stimulation that excited Aδ fibers. CONCLUSIONS: These findings show that two sets of forebrain structures mediate the initial sharp pain evoked by brief cutaneous heat stimulation: those responding preferentially to the brief stimulation of Aδ heat nociceptors and those with similar responses to converging inputs from the painless stimulation of C fibers. Our results suggest a unique and specific physiological basis, at the forebrain level, for the "first pain" sensation that has long been attributed to Aδ fiber stimulation and support the concept that both specific and convergent mechanisms act concurrently to mediate pain.

[Mechanisms for pain inhibiton in the central nervous system]. / Sentralnervesystemets mekanismer for smertehemming.

BACKGROUND: Several endogenous factors regulate the perception of pain. Understanding of pain-alleviating mechanisms is increasing, which is useful both for doctors who treat pain-ridden patients and for researchers interested in the physiology of pain. This article provides an overview of such mechanisms. MATERIAL AND METHODS: This review article is based on literature identified through a non-systematic search in PubMed. RESULTS: Endogenous pain-alleviating mechanisms are mainly controlled by different parts of the reticular substance, and are normally activated by painful stimulation. Expectation of pain reduction (placebo analgesic effect), painful stimulation in other sites and high blood pressure are examples of factors which may increase the body's endogenous pain-alleviating mechanisms. Opioid-sensitive cells in the brain stem are important for endogenous pain alleviation. Reduced endogenous pain alleviation is found in a number of painful conditions, but it has not been clarified whether reduced endogenous pain inhibition is a cause of or a result of chronic pain. INTERPRETATION: Strengthening of the body's own mechanisms for pain alleviation is possible and potentially useful in treatment of pain-ridden patients.

Sleep restriction does not potentiate nocebo-induced changes in pain and cortical potentials.

BACKGROUND: The increased pain sensitivity following reduced sleep may be related to changes in cortical processing of nociceptive stimuli. Expectations shape pain perception and can inhibit (placebo) or enhance (nocebo) pain. Sleep restriction appears to enhance placebo responses; however, whether sleep restriction also affects nocebo responses remains unknown. The aim of the present study was to determine whether sleep restriction facilitates nocebo-induced changes in pain and pain-evoked cortical potentials. METHODS: In an experimental study with a crossover design, the sensitivity to electrically induced pain was determined in 53 nurses under two sleep conditions, after habitual sleep and after two consecutive nights at work. Nocebo was induced by conditioning one-third of the pain stimuli. Pain-elicited cortical event-related potentials were recorded by electroencephalography (EEG). Data were analysed both in the time domain (N2P2 amplitude) and in the time-frequency domain (ERP magnitude). Sleepiness and vigilance were also assessed. RESULTS: Both nocebo alone and sleep restriction alone increased the sensitivity to electrically induced pain. However, no interaction effect was found. Moreover, the magnitude of the pain-elicited responses increased after sleep restriction and decreased after nocebo expectation, suggesting that nocebo is probably not an underlying mechanism for the commonly observed hyperalgesia induced by sleep restriction. CONCLUSIONS: The present work addresses whether sleep restriction, known to increase the sensitivity of the pain system, facilitates nocebo-induced hyperalgesia. Our findings suggest that this is not the case, indicating that the increased sensitivity of the pain system following nocebo and sleep restriction are mediated by different cortical mechanisms.