Heterotopic ischemic pain attenuates somatosensory evoked potentials induced by electrical tooth stimulation: Diffuse noxious inhibitory controls in the trigeminal nerve territory

Impact of low-load blood flow restriction training on knee osteoarthritis pain and muscle strength: a systematic review and meta-analysis of randomized controlled trials

Objectives

The effectiveness of low-load blood flow restriction training (LL-BFRT) in alleviating symptoms in patients with knee osteoarthritis (KOA) remains inconclusive. This systematic review and meta-analysis aim to comprehensively assess the effects of LL-BFRT compared to conventional resistance training on pain, muscle strength, and functional capacity in individuals with KOA.

Data sources

PubMed, Embase, Web of Science, EBSCO, Scopus, and Cochrane trails were searched.

Study selection

We included randomized controlled trials involving patients with KOA, in which the intervention group underwent LL-BFRT.

Data extraction

Literature quality and risk of bias were assessed using the Physiotherapy Evidence Database (PEDro) scale and the Cochrane Risk-of-Bias Tool (ROB 2). Data were extracted using a predefined table, including outcomes such as pain, quadriceps muscle strength, 30-s sit-to-stand test (30STS) and Timed Up and Go test (TUG).

Result

Ten studies were included in the meta-analysis. The pooled results indicated that, compared to conventional resistance training, LL-BFRT significantly improved knee joint pain [SMD = 0.25, 95%CI (0.02, 0.48), P = 0.03], increased quadriceps muscle strength [SMD = 0.46, 95%CI (0.04, 0.88), P = 0.03], and enhanced performance on the 30s sit-to-stand test (30STS) [WMD = 1.71, 95%CI (0.30, 3.11), P = 0.02]. However, no significant difference was observed in the improvement of the Timed Up and Go test (TUG) [WMD = -0.13, 95%CI (-0.51, 0.24), P = 0.49]. Subgroup analysis revealed that interventions with an occlusion pressure >100 mmHg and a duration ≤6 weeks had a significant impact on pain relief, quadriceps muscle strength, and the 30STS performance. For patients with KOA aged >65 years, LL-BFRT was more effective in alleviating pain, while for patients aged ≤65 years, it demonstrated more significant improvements in quadriceps strength and 30STS performance.

Conclusion

Limited evidence suggests that LL-BFRT may be more effective than conventional resistance training in improving pain, quadriceps muscle strength, and 30STS performance in patients with KOA, while exhibiting a comparable effect on TUG test.

Systematic Review Registration

https://www.crd.york.ac.uk/prospero/#myprospero, identifier CRD42024603542.

Mechanical and thermal thresholds before and after application of a conditioning stimulus in healthy Göttingen Minipigs

Minipigs are widely used in biomedical research for translational studies. However, information about pain elicited by experimental procedures is lacking. Non-invasive methods as quantitative sensory testing and conditioned pain modulation are particularly attractive. Our overarching aim was to explore and refine these methods for assessing post-operative pain in minipigs after myocardial infarction. As first step, we aimed at defining mechanical and thermal thresholds in healthy adults Göttingen Minipigs, evaluating their reliability, and testing their modifications after the application of a conditioning stimulus. Thresholds were assessed at different body sites before and after a painful conditioning stimulus (CS) (cuffed tourniquet) and sham CS (uncuffed tourniquet) in eleven animals. Thresholds' reliability was assessed using interclass correlation coefficient (ICC). The effect of the CS was assessed calculating absolute change, percentage change of the thresholds and standard error of measurement. Baseline mechanical thresholds (Newton) were: left hindlimb 81 [73; 81]; left forearm 81 [72.1; 81]; right forearm 81 [76; 81]; left chest 80.5 [68; 81]; right chest 81 [76.5; 81]; left neck 81 [70.3; 81]; right neck 74.8 [62.3; 80.5]. Reliability of mechanical thresholds was good at right chest (ICC = 0.835) and moderate at left chest (ICC = 0.591), left hindlimb (ICC = 0.606) and left neck (ICC = 0.518). Thermal thresholds showed poor reliability in all the tested sites. A modulatory effect was present at right chest, but it was seen when both a painful CS and a sham CS was applied. Minipigs tendentially showed a pro-nociceptive profile (i.e. conditioning pain facilitation). The measured thresholds are a reference for future trials in this species. Mechanical thresholds showed to be more reliable and, therefore, more useful, than thermal ones. The pain facilitation might be explained by the phenomenon of stress induced hyperalgesia, but this finding needs to be further investigated with a stricter paradigm.

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.

Blood flow restriction training promotes functional recovery of knee joint in patients after arthroscopic partial meniscectomy: A randomized clinical trial

Purpose: To explore the effect of blood flow restriction training (BFRT) on the recovery of knee function in patients after arthroscopic partial meniscectomy (APM). Methods: Forty patients undergoing APM surgery were included in this parallel group, two-arm, single-assessor blinded, randomized clinical trial. The subjects were randomly divided into two groups: routine rehabilitation group (RR Group, n = 20) and routine rehabilitation + blood flow restriction training group (RR + BFRT Group, n = 20). One subject in each group dropped out during the experiment. All patients received 8 weeks of routine rehabilitation starting from the second day after APM. In addition, patients in the RR + BFRT group required additional BFRT twice a week. Visual analogue scale (VAS) score, range of motion (ROM), one-leg standing test (OLST) score, Lysholm knee score, quadriceps muscle strength, quadriceps thickness, and thigh circumference were evaluated at preoperative, postoperative, 4 and 8 weeks after surgery. SPSS 25.0 software was used for statistical analysis of the data. Repeated measures ANOVA was used if the data were normally distributed and had homogeneity of variance. Generalized estimating equations were chosen if the data were not normally distributed or had homogeneity of variance. Results: There were no significant differences in VAS score, ROM, OLST score, Lysholm knee score, quadriceps muscle strength, quadriceps thickness, and thigh circumference between the two groups before surgery (p > 0.05). Compared with postoperative, VAS score, ROM, OLST score, Lysholm knee score, and thigh circumference were significantly improved in the RR group (p < 0.05), while quadriceps muscle strength and quadriceps thickness were not significantly enhanced at 8 weeks postoperatively (p > 0.05). However, VAS score, ROM, OLST score, Lysholm knee score, quadriceps muscle strength, quadriceps thickness, and thigh circumference were all significantly improved in the RR + BFRT group at 8 weeks postoperatively (p < 0.05). Furthermore, compared with the RR group, VAS score (50% vs. 86%), ROM (7.9% vs. 16.0%), OLST score (57.3% vs. 130.1%), Lysholm knee score (38.4% vs. 55.7%), relative peak torque (11.0% vs. 84.7%), mean power (20.6% vs. 88.1%), rectus femoris thickness (0.40% vs. 13.0%), vastus medialis (0.29% vs. 5.32%), vastus lateralis (0% vs. 6.2%), vastus internus (0% vs. 5.8%), and thigh circumference (2.7% vs. 5.8%) in the RR + BFRT group were significantly improved at 4 and 8 weeks postoperatively (p < 0.05). Conclusion: BFRT combined with routine rehabilitation training can better promote the recovery of knee joint function in patients after APM, especially the improvement of quadriceps muscle strength and thickness.

Symptomatic but not Asymptomatic COVID-19 Impairs Conditioned Pain Modulation in Young Adults

Pain is a common symptom reported in COVID-19 patients. Impaired endogenous pain-modulatory mechanisms such as conditioned pain modulation (CPM), and exercise-induced hypoalgesia (EIH) have been found in chronic pain conditions but is often overlooked in acute conditions that evoke painful symptoms, such as COVID-19. The purpose was to compare pressure-pain sensitivity, CPM, and EIH function among individuals who previously had COVID-19, both symptomatically and asymptomatically, and a healthy control group. Pressure pain thresholds of 59 participants were assessed in the forearm and leg using a pressure algometer before and after 1) submersion of their dominant foot in cold water (2°C) for 1min; and 2) isometric knee extension performed to task-failure at 25% of their maximal contraction. The CPM response was attenuated in individuals who were infected with symptomatic COVID-19 (N = 26) compared to asymptomatic COVID-19 (N = 13) in arm (-1.0% ± 20.3 vs 33.3% ± 26.2; P < .001) and leg (12.8% ± 22.0 vs 33.8% ± 28.2; P = .014) and compared to controls (N = 20) in arm only (-1.0% ± 26.2 vs 23.4% ± 28.2; P = .004). The EIH response was not different between groups. CPM was impaired in individuals who had symptomatic COVID-19, which may have long-term implications on pain modulation.

Perspective

This study reveals that CPM was impaired in individuals who had symptomatic COVID-19 during the first wave of COVID-19, pre vaccine. These findings present a preliminary motive to study the long-term implications of COVID-19 and its effects on pain modulation.

Conditioned Pain Modulation Is Not Impaired in Individuals with Frozen Shoulder: A Case-Control Study

Frozen shoulder (FS) is a poorly understood condition resulting in substantial shoulder pain and mobility deficits. The mechanisms behind FS are not yet fully understood, but, similar to other persistent pain states, central pain mechanisms may contribute to ongoing symptoms in this population. The objective of this research was to investigate conditioned pain modulation (CPM) in people with FS compared with pain-free individuals. A total of 64 individuals with FS and 64 healthy volunteers participated in this cross-sectional study. CPM was assessed by using the pressure pain threshold (PPT) and an occlusion cuff (tourniquet test) as the test and conditioning stimulus, respectively. The absolute and percentage of change in PPT (CPM effect) as well as pain profiles (pro-nociceptive vs. anti-nociceptive) of individuals with FS and healthy controls were calculated. No significant differences in the absolute change in the PPT or CPM effect were found in people with FS compared to pain-free controls. Moreover, no between-group differences in the percentage of subjects with pro-nociceptive and anti-nociceptive pain profiles were observed. These results suggest that endogenous pain inhibition is normally functioning in people with FS. Altered central pain-processing mechanisms may thus not be a characteristic of this population.

Conditioned pain modulation-A comprehensive review

Conditioned pain modulation (CPM) is a centrally processed measure of the net effect of the descending pain pathway. This comprises both the facilitatory as well as the inhibitory effect. In the past, CPM or similar effects have been previously described using different terminologies such as diffuse noxious inhibitory control (DNIC), heterotopic noxious conditioning stimulation (HNCS) or endogenous analgesia (EA). A variety of patient-related factors such as age, gender, hormones, race, genetic and psychological factors have been thought to influence the CPM paradigms. CPM paradigms have also been associated with a wide range of methodological variables including the mode of application of the 'test' as well as the 'conditioning' stimuli. Despite all these variabilities, CPM seems to reliably lend itself to the pain modulation profile concept and could in future become one of the phenotypic biomarkers for pain and also a guide for mechanism-based treatment in chronic pain. Future research should focus on establishing consistent methodologies for measuring CPM and thereby enhancing the robustness of this emerging biomarker for pain.

The effect of mindful attention training for pain modulation capacity: Exploring the mindfulness-pain link

OBJECTIVE

Mindfulness has been shown to be beneficial for chronic pain. The underlying mechanisms of the mindfulness-pain link, however, are yet to be established. Particularly, the effects of mindfulness on pain modulation, which is shown to be dysfunctional among chronic pain patients, barely has been tested. This study investigated whether a short mindful attention training based on Langerian mindfulness mitigates reductions in pain modulation.

METHOD

Systemic quantitative-somatosensory testing of conditioned pain modulation (CPM) was conducted in 60 undergraduates, who were randomly assigned to one of three groups: (1) Pain-specific mindful attention training; (2) nonspecific mindful attention training; and (3) no mindful attention training. CPM was tested before and after the intervention.

RESULTS

As hypothesized, a reduction in CPM magnitude was observed only in the control group, whereas this reduction was abolished in the two mindfulness groups.

CONCLUSIONS

Langerian mindfulness may mitigate pain modulation reduction as observed in chronic pain, thus shedding light on its potential advantages.

Painful Cutaneous Electrical Stimulation vs. Heat Pain as Test Stimuli in Conditioned Pain Modulation

Different paradigms can assess the effect of conditioned pain modulation (CPM). The aim of the present study was to compare heat pain, as an often used test stimulus (TS), to painful cutaneous electrical stimulation (PCES), having the advantage of the additional recording of PCES-related evoked potentials. In 28 healthy subjects we applied heat and PCES at the dominant hand as test stimulus (TS) to compare the CPM-effect elicited by hand immersion into cold water (10 °C) as conditioning stimulus (CS). Subjects rated the pain intensity of TS at baseline, during and 5 min after CS application and additionally of CS, on a numerical rating scale (NRS) (0–100). The ‘early’ (during CS–before CS) and ‘late’ (after CS–before CS) CPM-effects were analyzed. Parallel to the PCES, the related evoked potentials were recorded via Cz to evaluate any changes in PCES-amplitudes. CS reduced significantly the pain intensity of both PCES and heat pain as TS. On a group level, the CPM-effect did not differ significantly between both paradigms. Both early and late CPM-effect based on PCES correlated significantly with the CS pain intensity (r = −0.630 and −0.503, respectively), whereas using heat pain the correlation was not significant. We found a significant reduction of PCES-amplitudes during CS, but this did not correlate with the PCES-induced pain intensity. Correlation with the CS painfulness (r = −0.464) did not achieve the significance level after Bonferroni correction. The extent of the CPM effects was similar in both testing paradigms at group level, despite intraindividual differences. Future studies should further elicit the exact mechanisms explaining the modality of these specific differences.

Individual variability and sex differences in conditioned pain modulation and the impact of resilience, and conditioning stimulus pain unpleasantness and salience

Distinct pain experiences are shaped both by personal attributes and characteristics of noxious stimuli. An Individual's capacity for endogenous pain inhibition (reflected by conditioned pain modulation [CPM]), their resilience, and the pain unpleasantness and salience of painful stimuli can impact their pain perception. Here, we aimed to determine how individual variability in CPM relates to sex and resilience as personal attributes, and pain unpleasantness and salience of the CPM conditioning stimulus (CS). We evaluated CPM in 106 healthy participants (51 female and 55 male) based on the change in test stimulus pain applied concurrently with a painful CS, both delivered by painful heat. The CS reduced test stimulus pain in only half of the participants (CPM subgroup), but did not do so for the other half (no-CPM subgroup), many who exhibited pain facilitation. A regression model explained CPM effects after accounting for sex, resilience, CS pain unpleasantness and salience. In the CPM subgroup regression model, the CPM effect was positively related to CS pain unpleasantness, while the CPM effect was not related to any variable in the no-CPM subgroup model. Correlation analyses revealed that the CPM effect was anticorrelated with resilience in males with no-CPM. The CPM effect was correlated with CS pain unpleasantness in males with CPM and in females with no-CPM. The CPM effect and CS salience were correlated in the whole group more strongly than in the subgroups. These data reveal that the complexity of contributors to CPM variability include both personal attributes and attributes of the CS.

Acute alcohol effects on conditioned pain modulation, but not temporal summation of pain

Although pain reduction after alcohol administration has repeatedly been demonstrated, alcohol effects on advanced and clinically relevant dynamic pain paradigms are still unknown. As such, temporal summation of pain (TSP) and conditioned pain modulation (CPM) indicate mechanisms of endogenous pain modulation and involve certain neurotransmitter systems crucially influenced by alcohol. Our study is the first to investigate acute alcohol effects on TSP and CPM. We investigated 39 healthy subjects in a placebo-controlled within-subject design and targeted alcohol levels of 0.06% (dose 1) and 0.08% (dose 2). Pain threshold, TSP, and CPM were evaluated before and after an alcoholic or placebo drink. Temporal summation of pain was assessed as enhanced pain response to 5 repetitive contact heat stimuli (threshold +3°C). Conditioned pain modulation was tested as pain inhibition when a conditioning stimulus (46°C hot water) was applied concurrently to a test stimulus (contact heat; threshold + 3°C). Both alcohol doses boosted CPM, with a greater effect size for the higher dose. Conditioning stimulus ratings increased after alcohol intake but were not correlated with CPM, suggesting independence of these effects. Temporal summation of pain was not affected by alcohol, and alcohol effects on pain threshold were small and limited to the higher dose. Our findings suggest that analgesic alcohol effects might be mainly driven by an enhancement of endogenous pain inhibition. The frequent use of alcohol as self-medication in chronic pain might be motivated by alcohol temporarily restoring deficient CPM, thus leading to pain relief in the short run and alcohol-related problems in the long run.

New method for postoperative pain relief using a combination of noxious and non-noxious stimuli after impacted wisdom tooth extraction

Although in clinical dentistry the major method used for pain relief is oral administration of analgesics, alternative methods are available, such as transcutaneous electrical nerve stimulation (TENS), acupuncture, vibration and conditioned pain modulation (CPM), formerly termed diffuse noxious inhibitory control. The aim of the present study was to investigate the combined effects of non-noxious (TENS) and noxious (CPM) stimuli on postoperative pain after extraction of an impacted wisdom tooth. The study involved 44 patients who were scheduled to undergo impacted wisdom tooth extraction. The patients were randomly allocated into four groups: noxious stimuli, non-noxious stimuli, combined noxious and non-noxious stimuli, and a sham group. On the day after tooth extraction, stimulation procedures for pain relief were performed and changes in the level of perceived pain were scored using a visual analog scale (VAS). The combination of non-noxious and noxious stimuli decreased the VAS scores by 63.7%, indicating a more potent analgesic effect than that in the non-noxious, noxious, and sham groups. This method of analgesia using a combination of non-noxious and noxious stimuli can be applied to patients who are unable to tolerate analgesics, such as those with allergy, hypersensitivity or digestive disorders, and those who are pregnant.

Blood Flow Restriction induces hypoalgesia in recreationally active adult male anterior knee pain patients allowing therapeutic exercise loading

OBJECTIVE

To evaluate if a single blood flow restriction (BFR)-exercise bout would induce hypoalgaesia in patients with anterior knee pain (AKP) and allow painless application of therapeutic exercise.

DESIGN

Cross-sectional repeated measures design.

SETTING

Institutional out-patients physiotherapy clinic.

PATIENTS

Convenience sample of 30 AKP patients.

INTERVENTION

BFR was applied at 80% of complete vascular occlusion. Four sets of low-load open kinetic chain knee extensions were implemented using a pain monitoring model.

MAIN OUTCOME MEASUREMENTS

Pain (0-10) was assessed immediately after BFR application and after a physiotherapy session (45 min) during shallow and deep single-leg squat (SSLS, DSLS), and step-down test (SDT). To estimate the patient rating of clinical effectiveness, previously described thresholds for pain change (≥40%) were used, with appropriate adjustments for baseline pain levels.

RESULTS

Significant effects were found with greater pain relief immediate after BFR in SSLS (d = 0.61, p < 0.001), DSLS (d = 0.61, p < 0.001), and SDT (d = 0.60, p < 0.001). Time analysis revealed that pain reduction was sustained after the physiotherapy session for all tests (d_(SSLS) = 0.60, d_(DSLS) = 0.60, d_(SDT) = 0.58, all p < 0.001). The reduction in pain effect size was found to be clinically significant in both post-BFR assessments.

CONCLUSION

A single BFR-exercise bout immediately reduced AKP with the effect sustained for at least 45 min.

Pain Adaptability in Individuals With Chronic Musculoskeletal Pain Is Not Associated With Conditioned Pain Modulation

Healthy humans can be divided into the pain adaptive (PA) and the pain nonadaptive (PNA) groups; PA showed a greater decrease in pain rating to a cold pressor test (CPT) than PNA. This study examined if the dichotomy of pain adaptability existed in individuals with chronic musculoskeletal pain. CPTs at 2°C and 7°C were used to assess the status of pain adaptability in participants with either chronic nonspecific low back pain or knee osteoarthritis. The participants' potency of conditioned pain modulation (CPM) and local inhibition were measured. The strengths of pain adaptability at both CPTs were highly correlated. PA and PNA did not differ in their demographic characteristics, pain thresholds from thermal and pressure stimuli, or potency of local inhibition or CPM. PA reached their maximum pain faster than PNA (t₄₁ = -2.76, P < .01), and had a gradual reduction of pain unpleasantness over 7 days whereas PNA did not (F_6,246 = 3.01, P = .01). The dichotomy of pain adaptability exists in musculoskeletal pain patients. Consistent with the healthy human study, the strength of pain adaptability and potency of CPM are not related. Pain adaptability could be another form of endogenous pain inhibition of which clinical implication is yet to be understood.

PERSPECTIVE

The dichotomy of pain adaptability was identified in healthy humans. The current study confirms that this dichotomy also exists in individuals with chronic musculoskeletal pain, and could be reliably assessed with CPTs at 2°C and 7°C. Similar to the healthy human study, pain adaptability is not associated with CPM, and may reflect the temporal aspect of pain inhibition.

Nonpainful wide-area compression inhibits experimental pain

Compression therapy, a well-recognized treatment for lymphoedema and venous disorders, pressurizes limbs and generates massive non-noxious afferent sensory barrages. The aim of this study was to study whether such afferent activity has an analgesic effect when applied on the lower limbs, hypothesizing that larger compression areas will induce stronger analgesic effects, and whether this effect correlates with conditioned pain modulation (CPM). Thirty young healthy subjects received painful heat and pressure stimuli (47°C for 30 seconds, forearm; 300 kPa for 15 seconds, wrist) before and during 3 compression protocols of either SMALL (up to ankles), MEDIUM (up to knees), or LARGE (up to hips) compression areas. Conditioned pain modulation (heat pain conditioned by noxious cold water) was tested before and after each compression protocol. The LARGE protocol induced more analgesia for heat than the SMALL protocol (P < 0.001). The analgesic effect interacted with gender (P = 0.015). The LARGE protocol was more efficient for females, whereas the MEDIUM protocol was more efficient for males. Pressure pain was reduced by all protocols (P < 0.001) with no differences between protocols and no gender effect. Conditioned pain modulation was more efficient than the compression-induced analgesia. For the LARGE protocol, precompression CPM efficiency positively correlated with compression-induced analgesia. Large body area compression exerts an area-dependent analgesic effect on experimental pain stimuli. The observed correlation with pain inhibition in response to robust non-noxious sensory stimulation may suggest that compression therapy shares similar mechanisms with inhibitory pain modulation assessed through CPM.

Short-term test-retest-reliability of conditioned pain modulation using the cold-heat-pain method in healthy subjects and its correlation to parameters of standardized quantitative sensory testing

BACKGROUND

Conditioned Pain Modulation (CPM) is often used to assess human descending pain inhibition. Nine different studies on the test-retest-reliability of different CPM paradigms have been published, but none of them has investigated the commonly used heat-cold-pain method. The results vary widely and therefore, reliability measures cannot be extrapolated from one CPM paradigm to another. Aim of the present study was to analyse the test-retest-reliability of the common heat-cold-pain method and its correlation to pain thresholds.

METHODS

We tested the short-term test-retest-reliability within 40 ± 19.9 h using a cold-water immersion (10 °C, left hand) as conditioning stimulus (CS) and heat pain (43-49 °C, pain intensity 60 ± 5 on the 101-point numeric rating scale, right forearm) as test stimulus (TS) in 25 healthy right-handed subjects (12females, 31.6 ± 14.1 years). The TS was applied 30s before (TSbefore), during (TSduring) and after (TSafter) the 60s CS. The difference between the pain ratings for TSbefore and TSduring represents the early CPM-effect, between TSbefore and TSafter the late CPM-effect. Quantitative sensory testing (QST, DFNS protocol) was performed on both sessions before the CPM assessment.

STATISTICS

paired t-tests, Intraclass correlation coefficient (ICC), standard error of measurement (SEM), smallest real difference (SRD), Pearson's correlation, Bland-Altman analysis, significance level p < 0.05 with Bonferroni correction for multiple comparisons, when necessary.

RESULTS

Pain ratings during CPM correlated significantly (ICC: 0.411…0.962) between both days, though ratings for TSafter were lower on day 2 (p < 0.005). The early (day 1: 16.7 ± 11.7; day 2: 19.5 ± 11.9; ICC: 0.618, SRD: 20.2) and late (day 1: 1.7 ± 9.2; day 2: 7.6 ± 11.5; ICC: 0.178, SRD: 27.0) CPM effect did not differ significantly between both days. Both early and late CPM-effects did not correlate with the pain thresholds.

CONCLUSIONS

The short-term test-retest-reliability of the early CPM-effect using the heat-cold-pain method in healthy subjects achieved satisfying results in terms of the ICC. The SRD of the early CPM effect showed that an individual change of > 20 NRS can be attributed to a real change rather than chance. The late CPM-effect was weaker and not reliable.

Enhancement of Analgesic Effect by Combination of Non-Noxious Stimulation and Noxious Stimulation in Humans

The aim of the this study was to investigate the combined effects of heterosegmental non-noxious and noxious stimulation on electrically induced tooth pain. The late component of somatosensory-evoked potentials (SEP), induced by electrical tooth stimulation and pain intensity, were examined under electrical stimulation to forearms. Noxious, non-noxious, and combined non-noxious and noxious electrical stimulation were applied to median nerves on the forearms. Four experimental sessions (ie, control session, combined non-noxious and noxious stimulation session, non-noxious stimulation session, and noxious stimulation session were performed for each subject at each 10-minute interval for 30 minutes. The amplitudes of the SEP and VAS scores in the combined stimulation session decreased significantly compared with those in the control session and the reduction rates were 51.1% (13.4 μV) and 41.0% (23.5 mm), respectively. These results show that the combined stimulation has a more potent analgesic effect than that of either the non-noxious or the noxious stimulation. It is suggested that a potent analgesia was produced by an activated central mechanism, including endogenous opioid and descending pain inhibitory systems due to combined non-noxious and noxious stimulation.

Dysfunctional endogenous pain modulation in patients with functional dyspepsia

BACKGROUND

Endogenous pain modulation (EPM) is central to the processing of sensory information. Visceral and somatic EPM are abnormal in irritable bowel syndrome, but have not been studied in functional dyspepsia (FD).

METHODS

Visceral EPM was assessed in 34 FD patients and 42 healthy controls. Gastric pain was induced with oral capsaicin and EPM was studied by adding heterotopic thermal foot stimulation or distraction by STROOP test. Somatic EPM was assessed using foot heat stimulation with heterotopic hand electrical stimulation.

KEY RESULTS

Endogenous pain modulation by distraction reduced mean gastric pain by 11.9 on the 0-100 visual analog scale (95% CI: 3.8-20.1) in controls (p = 0.006) and by 2.0 (-6.18 to 10.44) in FD (p = 0.6), with greater EPM in controls than in FD (difference -13.3 [-26.1 to -0.5]; p = 0.04). Endogenous pain modulation by heterotopic foot stimulation reduced gastric pain by 6.5 (-0.7 to 13.6) in controls (p = 0.07) and by 7.1 (-2.29 to 16.47) in FD (p = 0.1), with no significant difference in EPM between controls and FD (-2.0 [-14.5 to 10.5]; p = 0.75). In patients with prominent FD pain, greater pain correlated with decreased visceral EPM by distraction (r = 0.51, p = 0.04). Somatic EPM by heterotopic stimulation significantly decreased foot pain in controls (p = 0.004), but not in FD (p = 0.80).

CONCLUSIONS & INFERENCES

In FD, visceral pain modulation by distraction was dysfunctional compared to controls. Somatic pain modulation was also decreased in FD. These data and the correlation of abnormal pain modulation by distraction with clinical pain in pain-predominant FD suggest a potential pathophysiological significance of abnormal pain modulation in FD.

A large conditioned pain modulation response is not related to a large blood pressure response: a study in healthy men

BACKGROUND

Endogenous pain modulation has been studied with the conditioned pain modulation (CPM) paradigm with large differences in the magnitude of the CPM effect. We hypothesized that differences in CPM effects might be associated with differences in blood pressure responses to the conditioning stimulus when comparing the CPM effects using two different conditioning stimuli.

METHODS

A single-blind repeated-measures design with block-randomization was applied on 25 healthy male subjects. The test stimulus (TS; tonic heat pain for 120 s) was first presented alone, thereafter in parallel with a conditioning stimulus (CS). Conditioning stimuli were either a cold pressor test (CPT) or equally painful ischaemic muscle pain (ISC), both lasting 120 s. Finger blood pressure and heart rate were recorded continuously. Data were analysed in a linear mixed model framework with CS type (CPT or ISC) and conditioning (TS or TS + CS) as independent factors.

RESULTS

An inhibitory CPM effect was found for both types of conditioning (p < 0.001). The CPM effect was larger during CPT conditioning compared with ISC conditioning (p = 0.001). No association with the concomitant cardiovascular response (blood pressure and heart rate) was found (p > 0.34).

CONCLUSION

Cold pressor pain CS induces larger CPM effects than ischaemic pain CS. The larger CPM effect is, however, not associated with a larger blood pressure response. Other factors related to the CS should be investigated to understand why different CS modalities give different CPM effects.

The influence of intensity and duration of a painful conditioning stimulation on conditioned pain modulation in volunteers

BACKGROUND AND METHOD

The aim was to investigate influence from variations in intensity of a painful conditioning stimulation (CS) on early (0-6 min) and prolonged (6-12 min) conditioned pain modulation (CPM) in volunteers during concurrent exposure to test stimuli (TS). CS was applied to either forearm using painful heat with an intensity of 2/10 and 5/10, respectively, rated on a 0-10 numerical pain rating scale. At a second session, CS with an intensity of 7/10 was applied to the arm using a tourniquet. Threshold and suprathreshold painful heat and pressure as well as painful repeated monofilament pricking (RMP) were assessed as TS.

RESULTS

Regardless of TS, there was no significant difference in the magnitude of CPM within the same stimulus modality during the various intensities and phases of the CS. Significant modulation of heat pain thresholds (HPTs) was found during the early phase at 5/10 and 7/10, but not at 2/10. Only at 5/10 the prolonged CS resulted in a significant additional increase in HPT. During the early CS phase, CPM of suprathreshold heat pain was found at 2/10 and 5/10. The prolonged CS resulted in a significant additional temperature increase at 5/10. Only during the early phase significant CPM of pressure pain thresholds were found for all three pain intensities in conjunction with a significant CPM of suprathreshold pressure pain at 5/10. There was no CPM of RMP.

CONCLUSION

The CS intensity and the duration of CPM modulated pain sensitivity differentially across TS modalities.

Understanding central mechanisms of acupuncture analgesia using dynamic quantitative sensory testing: a review

We discuss the emerging translational tools for the study of acupuncture analgesia with a focus on psychophysical methods. The gap between animal mechanistic studies and human clinical trials of acupuncture analgesia calls for effective translational tools that bridge neurophysiological data with meaningful clinical outcomes. Temporal summation (TS) and conditioned pain modulation (CPM) are two promising tools yet to be widely utilized. These psychophysical measures capture the state of the ascending facilitation and the descending inhibition of nociceptive transmission, respectively. We review the basic concepts and current methodologies underlying these measures in clinical pain research, and illustrate their application to research on acupuncture analgesia. Finally, we highlight the strengths and limitations of these research methods and make recommendations on future directions. The appropriate addition of TS and CPM to our current research armamentarium will facilitate our efforts to elucidate the central analgesic mechanisms of acupuncture in clinical populations.

Expectations of increased and decreased pain explain the effect of conditioned pain modulation in females

Objective

Chronic pain is believed to be related to a dysfunction of descending pain modulatory mechanisms. Functioning of descending pain modulation can be assessed by various methods, including conditioned pain modulation (CPM). CPM refers to the inhibition of one source of pain by a second noxious stimulus, termed the conditioning stimulus. This procedure can activate an endogenous pain inhibitory mechanism that inhibits early nociceptive processing. Chronic pain and anxiety disorders are more prevalent among females and it has been hypothesized that females react with more negative emotions towards unpleasant stimuli and this might be part of the explanation of greater pain sensitivity in females. The present study investigated whether expectations modulate the effect of conditioning stimulation on pain, subjective stress, and heart rate. In addition, we investigated whether the modulation of CPM by expectations differed between males and females.

Methods

Seventy-two subjects (including 36 women) received six noxious heat stimuli to the forearm. During three of these stimuli, a conditioning stimulus (cold-water bath) was applied to the contralateral arm in order to activate CPM. One third of the subjects were told that this would reduce pain (analgesia group), one-third that it would increase pain (hyperalgesia group), and one third received no information about its effect (no info group).

Results

Information that conditioning stimulation decreased or enhanced pain had the corresponding effect in females, but not in males. Conditioning stimulation increased stress, but not heart rate in females in the hyperalgesia group. A higher expectation of analgesia and lower stress during conditioning stimulation was associated with larger inhibitory CPM.

Conclusion

These results suggest that reduced inhibitory CPM can be due to contextually induced cognitive and emotional factors and not necessarily a dysfunction of descending inhibitory pathways.

Effects of dexmedetomidine on conditioned pain modulation in humans

BACKGROUND

Systemic administration of dexmedetomidine (DEX; selective α(2) -adrenoceptor agonist) is found to inhibit diffuse noxious inhibitory control in rats, now referred to as conditioned pain modulation (CPM) in humans. The present study was designed to investigate the effect of intravenous administration of DEX on CPM in humans.

METHODS

There were two sequential sessions in this double blind, randomized study. The first session was the control with normal saline infusion (N(1st), L(1st), H(1st)). During the second session, three types of agents were infused: normal saline (N(2nd)); a low plasma concentration of DEX (0.04 ng/mL; L(2nd)); and a high plasma concentration of DEX (0.08 ng/mL; H(2nd)). The amplitude of somatosensory evoked potentials (ampSEP)s and the visual analogue scale of tooth pain (VASt) induced by electrical tooth stimulation were evaluated with and without conditioning CO(2) laser stimulation of the hand. The inhibition rate (% inhibition) was calculated [= (1-[ampSEP or VASt with conditioning stimuli]/[ampSEP or VASt without conditioning stimuli]) × 100] to compare the magnitude of the DEX effects on CPM.

RESULTS

The inhibition rates of ampSEPs and VASt in Types N, L and H varied significantly, demonstrating a dose-dependent reduction of CPM effects of ampSEP and VASt during randomized DEX administration, consistent with results from animal studies.

CONCLUSIONS

The present study shows that systemic administration of an α(2) -adrenoceptor agonist (DEX), less than the clinical dose, inhibited CPM in humans. These results may provide some mechanistic insight into why many chronic pain patients show impaired CPM.

Cognitive manipulation targeted at decreasing the conditioning pain perception reduces the efficacy of conditioned pain modulation

Although painfulness of the conditioning stimulus (CS) is required for the activation of conditioned pain modulation (CPM), it is still unclear whether CPM expression depends on the objective physical intensity of the CS or the subjective perception of its pain. Accordingly, we cognitively manipulated the perceived CS pain, rendering the physical aspects of the CPM paradigm untouched. Baseline CPM was measured among 48 young healthy male subjects using the parallel paradigm with contact heat as test pain and hand immersion in hot water as CS. Subjects were then randomized into 4 groups, all of which were cognitively manipulated as to the CS-induced pain: group 1, placebo (CS less painful); group 2, nocebo (CS more painful); and groups 3 and 4, the informed control groups for groups 1 and 2, respectively. CPM was reassessed after the manipulation. Comparing the groups by MANCOVA (multivariate analysis of covariance) revealed that placebo exerted decreased CS pain and consequent attenuation of CPM magnitudes, while nocebo elicited increased CS pain, but without CPM elevation (P<.0001). Within the placebo group, the reduction in CS pain was associated with diminished CPM responses (r=0.767; P=.001); however, no such relationship characterized the nocebo group. Pain inhibition under CPM seems to depend on the perceived level of the CS pain rather than solely its physical intensity. Cognitively decreasing the perceived CS pain attenuates CPM magnitude, although a ceiling effect may limit CPM enhancement after cognitively increased CS pain. These findings emphasize the relevance of cognitive mechanisms in determining endogenous analgesia processes in humans.

The inter- and intra-individual variance in descending pain modulation evoked by different conditioning stimuli in healthy men

Background and purpose

Conditioned pain modulation (CPM) is a phenomenon in which pain is inhibited by heterotopic noxious stimulation. It is not known how the experimental condition affects the magnitude of the CPM response and the inter-and intra-individual variations. It is important to get the information of the test–retest reliability and inter–individual variations of CPM to apply CPM as a diagnostic tool or for screening analgesic compounds. This study evaluated (1) the magnitude of CPM, (2) the inter-individual coefficient of variation (inter-CV) and (3) the intra-individual coefficient of variation (intra-CV) to (A) different stimulus modalities to evoke CPM and (B) different assessment sites.

Methods

Twelve healthy men (age 19–38 years) participated in this study. Cold pressor pain (CPP) (immersing the hand into cold water), tourniquet pain (cuff around the upper arm) and mechanical pressure pain (craniofacial region) were used in randomized order as conditioning stimuli (CS). The test stimulus (TS) was pressure pain applied to the right masseter muscle, left forearm and leg (bilateral tibialis anterior: TA). The responses were pressure pain thresholds (PPT), pressure pain tolerance (PPTol) thresholds and the pain intensity which was assessed on a visual analogue scale (VAS, 0–10 cm) following 1.4 and 1.6 × PPT applied to TA. The TS was applied before, during and 10 min after the CS. The intra-individual CV was estimated between different days.

Results

CPP induced the most powerful CPM on PPT (66.3 ± 10.0% increase), VAS ratings (41.5 ± 5.3% reduction) and PPTol (32.6±4.6% increase), especially at TA, and resulted in the smallest inter-CV (41.4–60.1%). Independently of the CS, the inter-CV in general showed that the recordings from the orofacial region and the forearm had smaller values than from the leg. The smallest intra-CV value was obtained in pain ratings with CPP (27.0%).

Conclusions

This study suggests that (1) the CPP evokes the largest CPM, (2) the leg as the assessment site results in the largest CPM responses and (3) the CPP causes the smallest inter- and intra-CV.

Implication

The present investigation implicates that the CPP is the most efficient conditioning stimulus to induce CPM when assessed by pressure pain thresholds.

Cutaneous limb inflammation produces analgesia to pressure pain in the ipsilateral forehead of healthy volunteers

To investigate the pain-modulatory effects of a local inflammatory stimulus on pain elsewhere in the body, capsaicin was applied topically to the forearm of 14 healthy female volunteers. Pressure-pain thresholds and sensitivity to sharpness were assessed on each side of the forehead twice per day during 48 hours of capsaicin treatment, and in the treated and contralateral forearm before and at the end of treatment. Heat was applied to the treated area to rekindle pain at times of forehead assessment. Hyperalgesia to sharpness, but not pressure pain, developed in the treated area whereas sensations remained stable in the contralateral forearm. Sharpness ratings decreased bilaterally in the forehead after 6 hours of treatment, and ipsilateral analgesia to pressure pain developed in the forehead when the capsaicin site was heated after 48 hours of treatment. These findings suggest that pain modulation involves unilateral regulatory mechanisms in addition to local and generalized pain control. The dissociated changes to sharpness and pressure pain indicate distinct cutaneous and deep central pain pathways.

PERSPECTIVE

The findings lend support to an increasing body of research which demonstrates that pain modulation involves hemilateral mechanisms in addition to local and generalized controls. Elucidation of mechanisms that modulate ipsilateral pain processing may help to clarify the pathophysiology of complex regional pain syndrome, which is characterized by hemilateral hyperalgesia.

DNIC-mediated analgesia produced by a supramaximal electrical or a high-dose formalin conditioning stimulus: roles of opioid and alpha2-adrenergic receptors

Background

Diffuse noxious inhibitory controls (DNIC) can be produced by different types of conditioning stimuli, but the analgesic properties and underlying mechanisms remain unclear. The aim of this study was to differentiate the induction of DNIC analgesia between noxious electrical and inflammatory conditioning stimuli.

Methods

First, rats subjected to either a supramaximal electrical stimulation or an injection of high-dose formalin in the hind limb were identified to have pain responses with behavioral evidence and spinal Fos-immunoreactive profiles. Second, suppression of tail-flick latencies by the two noxious stimuli was assessed to confirm the presence of DNIC. Third, an opioid receptor antagonist (naloxone) and an α2-adrenoreceptor antagonist (yohimbine) were injected, intraperitoneally and intrathecally respectively, before conditioning noxious stimuli to test the involvement of descending inhibitory pathways in DNIC-mediated analgesia.

Results

An intramuscular injection of 100 μl of 5% formalin produced noxious behaviors with cumulative pain scores similar to those of 50 μl of 2% formalin in the paw. Both electrical and chemical stimulation significantly increased Fos expression in the superficial dorsal horns, but possessed characteristic distribution patterns individually. Both conditioning stimuli prolonged the tail-flick latencies indicating a DNIC response. However, the electrical stimulation-induced DNIC was reversed by yohimbine, but not by naloxone; whereas noxious formalin-induced analgesia was both naloxone- and yohimbine-reversible.

Conclusions

It is demonstrated that DNIC produced by different types of conditioning stimuli can be mediated by different descending inhibitory controls, indicating the organization within the central nervous circuit is complex and possibly exhibits particular clinical manifestations.

Tonic pain abolishes cortical habituation of visual evoked potentials in healthy subjects

We investigated changes in visual cortex excitability by analyzing visual evoked potential (VEP) habituation in healthy subjects during tonic pain evoked by the cold-pressor test (CPT). We tested VEP amplitude habituation (slope of the linear regression line for N1-P1 amplitude from the 1st to 6th block of 100 sweeps) in 19 healthy volunteers during 4 experimental conditions: baseline; no-pain (hand held in warm water, 25 degrees C); pain (hand held in cold water, 2-4 degrees C); and the after-effects of tonic pain. During baseline and no-pain sessions, VEPs habituated normally across the 6 consecutive blocks (mean slope -.28 and -.18%), whereas during pain and its after-effects they failed to decrease (0%, and -.11%). Tonic pain induced by the CPT abolishes normal VEP habituation and the lack of habituation persists after the CPT is stopped. Tonic pain probably abolishes VEP habituation by acting on brainstem neural structures which modulate thalamo-cortical activation thereby changing visual cortex excitability.

PERSPECTIVE

This study shows that tonic pain alters visual cortex excitability, a brain region unrelated to pain processing. These changes probably reflect defensive strategies against pain. Extending the study from healthy volunteers to patients with migraine between attacks would offer the opportunity to investigate visual cortical excitability under conditions when baseline habituation is absent.

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.

Cold-induced limb pain decreases sensitivity to pressure-pain sensations in the ipsilateral forehead

The aim of this study was to investigate the effect of unilateral limb pain on sensitivity to pain on each side of the forehead. In the first experiment, pressure-pain thresholds and sharpness sensations were assessed on each side of the forehead in 45 healthy volunteers before and after a 10 degrees C cold pressor of the hand and in 18 controls who were not subjected to the cold pressor. In a second experiment, forehead sensitivity was assessed in 32 healthy volunteers before and after a 2 degrees C cold pressor. The assessments were repeated without the cold pressor, and before and after six successive 4 degrees C cold pressor tests. The 10 degrees C cold pressor did not influence forehead sensitivity, whereas the 2 degrees C cold pressor and the 4 degrees C cold pressor tests resulted in bilateral analgesia to sharpness and pressure. The analgesia to pressure was greater in the ipsilateral forehead. Stress-induced analgesia and diffuse noxious inhibitory controls may have contributed to the analgesia to pressure-pain and sharpness sensations bilaterally after the most painful cold pressor tests. The locus coeruleus inhibits ipsilateral nociceptive activity in dorsal horn neurons during limb inflammation, and thus may have mediated the ipsilateral component of analgesia. Pain-evoked changes in forehead sensitivity differed for sharpness and pressure, possibly due to separate thalamic or cortical representations of cutaneous and deep tissue sensibility. These findings suggest that several mechanisms act concurrently to influence pain sensitivity at sites distant from a primary site of painful stimulation.

Experimental muscle pain impairs descending inhibition

In chronic musculoskeletal pain conditions, the balance between supraspinal facilitation and inhibition of pain shifts towards an overall decrease in inhibition. Application of a tonic painful stimulus results in activation of diffuse noxious inhibitory controls (DNIC). The aims of the present experimental human study were (1) to compare DNIC, evoked separately, by hypertonic saline (6%)-induced muscle pain (tibialis anterior) or cold pressor pain; (2) to investigate DNIC evoked by concomitant experimental muscle pain and cold pressor pain, and (3) to analyze for gender differences. Ten males and 10 age matched females participated in two sessions. In the first session unilateral muscle pain or unilateral cold pressor pain were induced separately; in the second session unilateral muscle pain and unilateral cold pressor pain were induced concomitantly. Pressure pain thresholds (PPT) were measured around the knee joint before, during, and after DNIC induction. Cold pressor pain increased PPT in both males and females with greater increases in males. Hypertonic saline-evoked muscle pain significantly increased PPT in males but not in females. When cold pressor and muscle pain were applied concomitantly the PPT increases were smaller when compared to the individual sessions. This study showed for the first time that two concurrent conditioning tonic pain stimuli (muscle pain and cold pressor pain) cause less DNIC compared with either of the conditioning stimuli given alone; and males showed greater DNIC than females. This may explain why patients with chronic musculoskeletal pain have impaired DNIC.

Activating endogenous visceral pain modulation: a comparison of heterotopic stimulation methods in healthy controls

All sensory input underlies modulation by endogenous central nervous system pathways. Dysfunctional endogenous pain modulation has been demonstrated in central sensitization and in several pain syndromes, including Irritable Bowel Syndrome (IBS) Activation of endogenous visceral pain modulation by heterotopic stimulation was compared using different methods. Rectal electrical or distension pain alone or with simultaneous (i.e. heterotopic) noxious hand or foot cold stimulation were investigated in randomized sequence in 14 male and 1 female healthy subjects. Mean pain intensities on a visual analogue scale of 0-100 (95% CI) during tonic rectal electrical and distension stimulation alone were 64 (52-76) and 55 (39-71), respectively. Rectal distension pain decreased by 36% (18-55) with simultaneous hand and by 45% (24-66) with simultaneous foot cold pain. Rectal electrical pain decreased by 45% (29-61) during hand and by 46% (28-64) during foot cold pain. Facilitation, i.e. increased rectal pain during heterotopic stimulation was observed in only 1 of 60 stimulation runs. Potent and consistent activation of endogenous visceral pain inhibition was achieved with heterotopic cold pain limb stimulation. Somato-visceral convergence did not affect the effectiveness of induction of endogenous visceral pain inhibition in healthy subjects, as hand and foot heterotopic stimulation resulted in similar pain inhibition. Pain facilitation, as shown earlier in IBS patients, was not evident in healthy controls.

Supraspinal modulation of trigeminal nociception and pain

OBJECTIVE

This study examined modulation of trigeminal pain/nociception by 2 supraspinal mechanisms: emotional controls of nociception and diffuse noxious inhibitory controls.

BACKGROUND

Prior research suggests emotional picture viewing (emotional controls) and tonic noxious stimuli (diffuse noxious inhibitory controls) engage supraspinal mechanisms to modulate pain and nociceptive processes. It is currently unknown, however, whether emotional controls modulate trigeminal pain and nociception. Additionally, the influences of emotional controls and diffuse noxious inhibitory controls have not been compared in the same group of participants.

METHODS

Noxious electrodermal stimuli were delivered to the trigeminal nerve using a concentric electrode designed to selectively activate nociceptive fibers. Trigeminal nociception and pain were assessed (34 participants) from the nociceptive blink reflex and pain ratings, respectively. Emotional controls were engaged by presentation of standardized picture stimuli (pleasant, neutral, and unpleasant) shown to reliably evoke pleasure-induced inhibition and displeasure-induced facilitation of pain and nociception. Diffuse noxious inhibitory controls were engaged with a forearm ischemia task.

RESULTS

Trigeminal pain (self-report ratings) and nociception (blinks) were facilitated by unpleasant pictures and inhibited by pleasant pictures. Emotion induction (as assessed from trend analysis) explained 51% of the variance in trigeminal pain and 25% of the variance in trigeminal nociception. Additionally, forearm ischemia inhibited trigeminal pain but not nociception. The baseline vs ischemia comparison explained 17% of the variance in pain report and 0.1% of the variance in blinks. Supraspinal modulation by emotional controls and diffuse noxious inhibitory controls were uncorrelated.

CONCLUSIONS

Emotional controls and diffuse noxious inhibitory controls modulated trigeminal pain and emotional controls modulated trigeminal nociception. These procedures can be used to study supraspinal modulation of nociceptive processing in disorders of the trigeminal pain system, including headache.

Diffuse noxious inhibitory controls triggered by heterotopic CO2 laser conditioning stimulation decreased the SEP amplitudes induced by electrical tooth stimulation with different intensity at an equally inhibitory rate

The purpose of this study was to investigate (1) whether selective Adelta-fiber stimulation with CO(2) laser produces a diffuse noxious inhibitory controls (DNIC) effect in the trigeminal nerve territory; and (2) whether the DNIC effect differs depending on test stimulus intensities under constant conditioning stimuli. To examine whether the CO(2) laser radiation on the dorsum of the hand selectively stimulates Adelta-fibers, laser evoked potentials (LEP) were recorded. The mean peak latency of LEP was 381.4 ms. The findings revealed that the CO(2) laser selectively stimulated Adelta-fibers. Electrical tooth stimuli with 3 levels of intensities (1.2, 1.4, 1.6 times the pain threshold) were applied to subjects as test stimulation in randomized order, with a CO(2) laser stimulus of 18 mJ/mm(2) applied to the dorsum of the hand for 4 min as the noxious conditioning stimulus. Somatosensory evoked potentials (SEP) induced by electrical tooth stimulation were recorded and tooth pain intensity was evaluated using a visual analogue scale (VAS). The amplitudes of the SEP late component and VAS values were significantly decreased only during the conditioning stimuli without aftereffect. The inhibitory rates of the amplitudes ranged from 31.3% to 34.6% and the VAS values from 29.0% to 31.2%. There were no significant differences in their inhibitory rates between the 3 test stimulus intensities. The result indicated that selective Adelta-fiber stimulation with the CO(2) laser produces a DNIC effect in the trigeminal nerve territory and suggested that the DNIC effect does not depend on the intensity of the test stimuli.