Disrupted offset analgesia distinguishes patients with chronic pain from healthy controls

A potential role for the middle temporal gyrus in mediating pain rumination in patients with chronic pain

Perception of pain offset is essential to free one from any aftereffects of nociception. Because patients with chronic pain show impaired temporal contrast of pain, we tested whether they show slower perception of pain offset after the end of a nociceptive stimulus compared with pain-free subjects. We sought for cerebral correlates for offset and aftereffects of pain using functional magnetic resonance imaging (fMRI). In 19 patients with chronic pain and 15 pain-free subjects, we gave 10 blocks of pain stimuli by a Peltier-type thermal stimulator on the left volar forearm and recorded pain intensity continuously by a visual analogue scale (VAS), while performing whole-brain fMRI. We measured latency to reach "no pain" after stimulus offset (VAS End Latency). We analyzed fMRI signal changes during cessation of pain and made a between-group comparison. Individual parameters were sought for correlations with the rumination score from the Pain Catastrophizing Scale. Patients showed a longer VAS End Latency, and a larger after-pain activity in the left middle temporal gyrus, the right parietal lobule and, after small-volume correction, the right dorsolateral prefrontal cortex than pain-free subjects. Both VAS End Latency and after-pain activity were positively correlated with the rumination score. Psychophysiological interaction analysis revealed that higher rumination scores were associated with weaker coupling between the middle temporal and prefrontal cortices in patients and pain-free subjects with positive rumination scores. We suggest a possible role for the middle temporal gyrus in mediating pain aftereffects and rumination, and its modulation by the prefrontal-temporal network.

Interhemispheric and Corticothalamic White-Matter Dysfunction Underlies Affective Morbidity and Impaired Pain Modulation in Chronic Pain

BACKGROUND

Although patients with chronic pain show behavioral signs of impaired endogenous pain modulation, responsible cerebral networks have yet to be anatomically delineated. We used diffusion tensor imaging (DTI) to examine the white-matter alterations in patients with chronic pain compared with healthy subjects. We further measured thermal pain modulatory responses using the offset analgesia (OA) paradigm. We tested whether the white-matter indices be associated with psychophysical parameters reflecting morbidity and modulatory responses of pain in patients, and whether they could serve as diagnostic biomarkers of chronic pain.

METHODS

Twenty-six patients with chronic pain and 18 age- and gender-matched healthy controls were enrolled. After completing psychophysical questionnaires, they underwent OA measurement and whole-brain DTI in a 3 Tesla magnetic resonance imaging scanner. Fractional anisotropy (FA) and radial diffusivity (RD) of the white-matter were computed and compared between the groups with tract-based spatial statistics using the FMRIB Software Library (FSL) software. Correlations were sought among white-matter indices, thermal pain responses, and psychophysical parameters. The white-matter indices and OA-related parameters were tested whether they distinguish patients from controls by receiver operating characteristic analysis.

RESULTS

During OA, patients showed a shorter latency to the maximum (maximum visual analog scale [VAS] latency, 16.0 ± 3.7 vs 18.9 ± 3.1 second [mean ± standard deviation, SD]; P = .032) but a longer latency to the minimum pain (OA latency, 15.6 ± 3.5 vs 11.1 ± 4.2 seconds; P = .004) than controls. They showed a smaller mean FA (0.44 ± 0.12 vs 0.45 ± 0.11; P = .012) and a larger mean RD of the global white-matter (0.00057 ± 0.00002 vs 0.00056 ± 0.00002; P = .038) than controls, at specific areas including the corpus callosum, anterior thalamic radiation, and forceps major. FA of the splenium of the corpus callosum was associated with maximum VAS latency (r = 0.493) and OA latency (r = -0.552). The Pain Catastrophizing Scale scores showed strong negative correlations with FA across those specific areas (r = -0.405). Those latencies during OA and white-matter metrics distinguished patients from controls ( P < .05).

CONCLUSIONS

Patients with chronic pain showed dysfunction of the white matter concerned with interhemispheric communication of sensorimotor information as well as descending corticothalamic modulation of pain in association with affective morbidity and altered temporal dynamics of pain perception. We suggest that an impaired interhemispheric modulation of pain, through the corpus callosum, might be a novel cerebral mechanism in chronification of pain.

Can Offset Analgesia Magnitude Provide Additional Information About Endogenous Pain Modulation in People With Knee Osteoarthritis?: An Experimental Study

OBJECTIVES

To investigate the relationship between offset analgesia magnitude and the responsiveness to conditioned pain modulation (CPM), temporal summation of (second) pain (TSP), and clinical pain severity in people with knee osteoarthritis (KOA).

METHODS

Electrical stimuli were applied to 88 participants with KOA to measure offset analgesia at the volar forearm of the dominant hand, and CPM and TSP at the most symptomatic knee and ipsilateral volar wrist. Clinical pain severity was assessed using the pain subscale of the Knee injury and Osteoarthritis Outcome Score (KOOS PAIN ). Linear mixed-effects models evaluated pain modulatory effects across all tests, and Spearman partial correlations assessed associations between offset analgesia, CPM, TSP, and KOOS PAIN while accounting for covariates of interest. Participants unable to validly finish all psychophysical tests were excluded from effect and correlation analyses but were evaluated for predictors of nonvalid completion using bivariate Stochastic Search Variable Selection.

RESULTS

Significant pain modulation was observed across all psychophysical tests ( P <0.05), and no meaningful predictors of nonvalid test completion were found. Offset analgesia magnitude did not significantly correlate with CPM, TSP, or KOOS PAIN ( P ≥0.05), with a maximum partial correlation coefficient of ρ =0.21.

DISCUSSION

Offset analgesia was not associated with CPM, TSP, or KOOS PAIN in people with KOA. Despite the lack of case-control studies comparing offset analgesia between people with KOA and healthy controls, these findings suggest that offset analgesia may provide information about endogenous pain modulation beyond CPM and TSP, though its clinical translation remains uncertain.

Effect of Catastrophic Thinking on the Analgesic Effect of Electroacupuncture

Introduction

Patients with chronic pain and high-level catastrophic thoughts often do not respond to acupuncture. This may be related to hypofunctioning of the dorsolateral prefrontal cortex and the descending pain inhibitory system. Therefore, we examined the relationship between the level of catastrophic thinking and the analgesic effect of electroacupuncture using the pain catastrophizing scale (PCS). We also evaluated the descending pain inhibitory system using conditioned pain modulation (CPM) and offset analgesia (OA). The relationship between catastrophic thinking and the descending pain inhibitory system was also examined.

Materials and Methods

After testing the hospital anxiety and depression scale and the PCS in 14 healthy adults, the current pain threshold (CPT), CPM, and OA were measured, in order, before the intervention. Thereafter, electroacupuncture was applied to 3 limbs (the dominant hand and both lower extremities) at 4 Hz, and to the scalp at 100 Hz, for 30 minutes, and the CPT was measured again immediately after the intervention. The difference in the CPT before and after the intervention was taken as the analgesic effect.

Results

The participants were divided into 2 groups, the H-PCS group (≥16 points) and the L-PCS group (≤15 points), according to the PCS score, and the analgesic effects of electroacupuncture were significantly different (P = 0.04). However, no relationship was found between the PCS score and the CPM (r = -0.02, P = 0.94) and OA effects (r = -0.19, P = 0.49).

Conclusion

It was suggested that people with high-level catastrophic thinking may find it difficult to obtain the analgesic effects of electroacupuncture.

Preoperatively assessed offset analgesia predicts acute postoperative pain following orthognathic surgery

OBJECTIVES

High intensity and longer duration of acute postoperative pain are generally associated with a higher risk of developing chronic postoperative pain. Therefore, it is important to identify the preoperative predictors for acute postoperative pain. Preoperative evaluation of offset analgesia (OA) and the Pain Catastrophising Scale (PCS) may be potential predictors for acute postoperative pain. This study aimed to investigate the relationship between preoperative OA, PCS, and acute postoperative pain following orthognathic surgery.

METHODS

Thirty patients (19 females) scheduled to undergo orthognathic surgery were included in this study. OA and PCS were evaluated preoperatively, and the patients reported their postoperative pain intensity using the visual analogue scale [0-100 mm] until it reached zero (number of days with pain). OA was induced on the dominant forearm via three consecutive painful heat pulses delivered for 5 s (T1=46 °C), 5 s (T2=47 °C), and 20 s (T3=46 °C). Subsequently, the associations between OA, PCS, and the number of days with pain were analysed.

RESULTS

The median duration of postoperative pain was 10.3 days. Multiple linear regression analysis showed a significant (p=0.0019) predictive value of OA (p=0.008) for the number of days with pain. The PCS-magnification component was positively correlated with the number of days with pain (R=0.369, p=0.045), with no predictive values of PCS-total and PCS-subscale scores observed.

CONCLUSIONS

Preoperative evaluation of OA may be a new individualised, predictive tool for the number of days with acute postoperative pain following orthognathic surgery; hence, a possible biomarker for the patient's vulnerability to developing chronic postoperative pain.

ETHICAL COMMITTEE NUMBER

The study was approved by the Ethics Committee of Meikai University (A1624, A2113).

TRIAL REGISTRY NUMBER

This study was registered in the University Hospital Medical Information Network Clinical Trials Registry (UMIN-CTR) Clinical Trial (Unique ID: UMIN000026719, UMIN000046957).

Offset analgesia is increased intra-orally

BACKGROUND

Offset analgesia (OA) is commonly used to quantify endogenous pain inhibition. However, the potential role of afferent inputs and the subsequent peripheral factors from different body areas on the underlying mechanisms are still unclear.

OBJECTIVES

The aim of this cross-sectional study was to compare the magnitude of OA in four different body areas representing a) glabrous and non-glabrous skin, b) trigeminal and extra-trigeminal areas, and c) intra- and extra-oral tissue.

METHODS

OA was assessed at the oral mucosa of the lower lip, at the skin of the cheek, the forearm and the palm of the hand in 32 healthy and pain-free participants. OA testing included two trials: (1) a constant trial (30 seconds of constant heat stimulation at an individualized temperature of Pain₅₀ (pain intensity of 50 out of 100)), and (2) an offset trial (10 seconds of individualized Pain₅₀ , followed by 5 seconds at Pain₅₀ +1°C and 15 seconds at Pain₅₀ ). Participants continuously rated their pain during each trial with a computerized visual analog scale.

RESULTS

A significant OA response was recorded at the oral mucosa (p<0.001, d=1.24), the cheek (p<0.001, d=0.84) and the forearm (p<0.001, d=1.04), but not at the palm (p=0.19, d=0.24). Significant differences were shown for OA recorded at the cheek versus the mucosa (p=0.02), and between palm and mucosa (p=0.007), but not between the remaining areas (p>0.05).

CONCLUSION

This study suggests that intra-oral endogenous pain inhibition assessed with OA is enhanced and supports the role of peripheral mechanisms contributing to the OA response.

Temporal properties of pain contrast enhancement using repetitive stimulation

BACKGROUND

Offset analgesia (OA) is characterized by a disproportionately large reduction in pain following a small decrease in noxious stimulation and is based on temporal pain contrast enhancement (TPCE). The underlying mechanisms of this phenomenon are still poorly understood. This study is aiming to investigate whether TPCE can also be induced by repetitive stimulation, i.e., by stimuli clearly separated in time.

METHODS

A repetitive TPCE paradigm was induced in healthy, pain-free subjects (n = 33) using heat stimuli. Three different interstimulus intervals (ISIs) were used: 5, 15, and 25 seconds. All paradigms were contrasted with a control paradigm without temperature change. Participants continuously rated perceived pain intensity. In addition, electrodermal activity (EDA) was recorded as a surrogate measure of autonomic arousal.

RESULTS

Temporal pain contrast enhancement was confirmed for ISI 5 seconds (p < 0.001) and ISI 15 seconds (p = 0.005) but not for ISI 25 seconds (p = 0.07), however, the magnitude of TPCE did not differ between ISIs (p = 0.11). A TPCE-like effect was also detected with increased EDA values.

CONCLUSIONS

TPCE can be induced by repetitive stimulation. This finding may be explained by a combination of the mechanisms underlying the OA and a facilitated pain habituation.

Functional connectivity modulations during offset analgesia in chronic pain patients: an fMRI study

Patients with neuropathic pain and fibromyalgia showed reduced or absent offset analgesia (OA) response and attenuated cerebral activity in descending pain modulatory and reward systems in patients. However, neural network modifications of OA in chronic pain have not been determined. We enrolled 23 patients with various chronic pain and 17 age- and gender- matched healthy controls. All participants were given OA-related noxious thermal stimuli, including 3 repeats of offset analgesia paradigm at 46-47-46 °C and constant paradigm at 46 °C on the left volar forearm under whole-brain functional magnitude resonance imaging (fMRI). We evaluated magnitude of OA, examined OA modulated functional connectivity using psychophysiological interaction analysis and resting-state functional connectivity analysis and explored their behavioral correlations in patients compared with controls.Compared to controls, chronic pain patients showed smaller magnitude of OA (P = 0.047). OA modulated connectivity decreased between posterior cingulate cortex (PCC) and right medial prefrontal cortex (MPFC) in proportion to current chronic pain (P = 0.018); decreased between right pallidum and right thalamus, and increased between right caudate nucleus and left primary somatosensory cortex (P _FDR < 0.05).The impaired PCC-MPFC connectivity might play an important role in dysfunction of OA and contribute to pain chronification.

Offset analgesia identifies impaired endogenous pain modulation in pediatric chronic pain disorders

Offset analgesia (OA), a psychophysical test of endogenous pain inhibition, is diminished in many adult chronic pain disorders but OA has not been investigated in youth with chronic pain disorders. This study assessed OA responses in 30 youth with chronic primary and secondary pain disorders and 32 healthy controls. The OA, control, and constant thermal tests were evoked with an individualized noxious heat stimulus of approximately 50/100 mm on a visual analogue scale followed by 1°C offset temperature. This study also examined the association of OA responses with 2 self-report measures of pain sensitivity, the Central Sensitization Inventory (CSI) and Pain Sensitivity Questionnaire. Patients exhibited diminished capacity to activate OA with a reduction in ΔeVASc of 53 ± 29% vs controls 74 ± 24% (P = 0.003) even after multivariate regression adjusting for age, sex, and body mass index. Patients also showed decreased ability to habituate to a constant noxious heat stimulus compared to controls (P = 0.021). Central Sensitization Inventory scores showed excellent predictive accuracy in differentiating patients from controls (area under the curve = 0.95; 95% CI: 0.91-0.99) and CSI score ≥30 was identified as an optimal cutoff value. Pain Sensitivity Questionnaire scores did not differentiate patients from controls nor correlate with OA. In this study, 60% of youth with chronic pain showed reduced capacity for endogenous pain inhibition.

The effect of acute-experimental pain models on offset analgesia

BACKGROUND

Offset analgesia (OA) is characterized by a disproportionately large decrease in pain perception after a slight decrease in noxious stimulation. In patients with ongoing pain, this response is reduced. The effect is pronounced in painful body areas. The influence of acute pain has not been sufficiently investigated. The aim of this study was to investigate the influence of two experimental acute pain models, measured within the area of acute pain and on the non-affected opposite side, thereby considering the possible somatotopic nature of OA.

METHODS

Healthy, pain-free volunteers (n = 75) were randomly assigned to one of three groups (cold water, exercise and control group). The 'cold water group' immersed one hand into cold water for 3 min (Cold Pressor Task), while the 'exercise group' performed an isometric grip exercise for 3 min. There was no manipulation in the control group. Each experimental pain stimulus was performed at both (dominant, non-dominant) forearms. The individualized OA paradigm consisted of offset and constant temperature trials. Offset analgesia was measured immediately before, during and after the experimental pain stimuli.

RESULTS

A significant difference in OA was shown during experimental pain when compared to the control condition (exercise vs. control: p < 0.001, cold vs. control: p = 0.001), with no difference between the experimental conditions (p > 0.05). Immediately following the pain stimulation, results were marginally non-significant (p = 0.05).

CONCLUSIONS

Experimental painful stimulation reduced OA. This result should be interpreted with caution due to potential influences of conditioned pain modulation or exercise-induced hypoalgesia as well as possible floor effects.

SIGNIFICANCE

Temporal contrast of pain perception is inhibited in acute pain states. This study showed that reduced offset analgesia is observed when pain is experimentally induced using noxious cold and exercise stimuli.

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.

Tapentadol treatment results in long-term pain relief in patients with chronic low back pain and associates with reduced segmental sensitization

The endogenous pain system may be used as a biomarker in the pharmacological treatment of patients with CLBP, enabling an individualized, mechanism-based treatment approach.

Introduction:

Chronic low back pain (CLBP) is one of the most common chronic pain conditions in pain practice.

Objectives:

In the current study, we describe phenotypes of patients with CLBP based on the status of their endogenous pain modulatory system.

Methods:

Conditioned pain modulation (a measure of central pain inhibition), temporal summation (TS, a measure of pain facilitation), and offset analgesia (a measure of temporal filtering of nociception) were evaluated in 53 patients with CLBP at painful and nonpainful sites. Next, in a double-blind, randomized, placebo-controlled trial, 40 patients with defective conditioned pain modulation responses received treatment with tapentadol prolonged-release or placebo for 3 months.

Results:

The majority of patients (87%) demonstrated loss of central pain inhibition combined with segmentally increased TS and reduced offset analgesia at the lower back region. During treatment, tapentadol reduced pain intensity more than placebo (tapentadol −19.5 ± 2.1 mm versus placebo −7.1 ± 1.8 mm, P = 0.025). Furthermore, tapentadol significantly decreased pain facilitation by reduction of TS responses at the lower back (tapentadol −0.94 ± 1.9 versus placebo 0.01 ± 1.5, P = 0.020), which correlated with pain reduction (P < 0.001).

Conclusion:

Patients with CLBP demonstrated different phenotypes of endogenous pain modulation. In patients with reduced conditioned pain modulation, tapentadol produced long-term pain relief that coincided with reduction of signs of pain facilitation. These data indicate that the endogenous pain system may be used as a biomarker in the pharmacological treatment of CLBP, enabling an individualized, mechanism-based treatment approach.

Offset analgesia is reduced on the palm and increases with stimulus duration

BACKGROUND

A noxious stimulus following a more intense stimulus often feels less painful than continuous noxious stimulation. This effect, known as offset analgesia (OA), may be due to descending inhibitory control, to changes in peripheral neural transmission or both. The timing and location of noxious thermal stimulation were manipulated to better understand the peripheral and central contributions to OA.

METHODS

In a first experiment, participants (n = 29) provided continuous pain ratings as stimuli were delivered to the palm or dorsum of each hand. Offset trials included 44°C (T1), 45°C (T2) and 44°C (T3) stimulation periods. Baseline trials were identical except the T3 temperature fell to 35°C. Constant trials were 44°C throughout. The duration of T1 and T2 was either 1 s or 6 s, whereas T3 was always 12 s. In a second experiment, participants (n = 43) rated pain levels of noxious stimuli presented to the forearms with varying T1 and T2 durations (3, 6, 10 or 13 s) and a 20 s T3 period.

RESULTS

OA effects became stronger with increasing inducing durations. OA, however, was not found on the palm even at longer durations.

CONCLUSIONS

The increase in OA with duration suggests that accumulated nociceptive signalling is more important to triggering OA than is a decrease in nociceptors' instantaneous firing rates. The lack of OA on the palm, however, implies a key role for the rapidly adapting Type II AMH fibres that may be absent or not readily activated on the palm. Unravelling the relative central and peripheral contribution to OA requires further investigation.

SIGNIFICANCE

Offset analgesia (OA) is a fundamentally temporal phenomenon dependent on dynamic changes in stimulus intensity. Here we demonstrate increased OA with increased stimulus duration. This finding implies the more slowly-responding AMH-I peripheral mechanoreceptors contribute to OA. The more rapidly responding AMH-II peripheral mechanoreceptors, however, may be absent or more difficult to activate in the palm where we did not observe OA. This finding implies that the AMH-II receptors are necessary for OA. Our studies suggest methods to unravel the different peripheral and central contributions to OA.

Onset hyperalgesia and offset analgesia: Transient increases or decreases of noxious thermal stimulus intensity robustly modulate subsequent perceived pain intensity

Reported pain intensity depends not only on stimulus intensity but also on previously experienced pain. A painfully hot temperature applied to the skin evokes a lower subjective pain intensity if immediately preceded by a higher temperature, a phenomenon called offset analgesia. Previous work indicated that prior pain experience can also increase subsequent perceived pain intensity. Therefore, we examined whether a given noxious stimulus is experienced as more intense when it is preceded by an increase from a lower temperature. Using healthy volunteer subjects, we observed a disproportionate increase in pain intensity at a given stimulus intensity when this intensity is preceded by a rise from a lower intensity. This disproportionate increase is similar in magnitude to that of offset analgesia. We call this effect onset hyperalgesia. Control stimuli, in which a noxious temperature is held constant, demonstrate that onset hyperalgesia is distinct from receptor or central sensitization. The absolute magnitudes of offset analgesia and onset hyperalgesia correlate with each other but not with the noxious stimulus temperature. Finally, the magnitude of both offset analgesia and onset hyperalgesia depends on preceding temperature changes. Overall, this study demonstrates that the perceptual effect of a noxious thermal stimulus is influenced in a bidirectional manner depending upon both the intensity and direction of change of the immediately preceding thermal stimulus.

Commentary: Novel Use of Offset Analgesia to Assess Adolescents and Adults with Treatment Resistant Endometriosis-Associated Pain

Background and Objective

Endometriosis, affecting approximately 176 million adults and adolescents worldwide, is a debilitating condition in which uterine tissue grows outside the uterus. The condition costs the US economy approximately $78 billion annually in pain-related disability. By understanding the neural underpinnings of endometriosis-associated pain (EAP) and risk factors for chronification, translational research methods could lessen diagnostic delays and maximize successful pain remediation. This can be accomplished by the novel use of a known method, offset analgesia (OA), to better elucidate the neural mechanisms that may contribute to and maintain EAP. This commentary will provide justification and rationale for the use of OA in the study of EAP.

Conclusion

Utilizing an OA paradigm in patients with endometriosis, especially adolescents, may (1) provide insight into neural mechanisms contributing to pain maintenance, which could capture those at-risk for the transition to chronic pelvic pain, (2) provide a metric for the development of future centrally mediated treatment options for this population, and (3) elucidate the brain changes that result in resistance to treatment and pain chronification.

Best Evidence Rehabilitation for Chronic Pain Part 5: Osteoarthritis

Osteoarthritis (OA) is a leading cause of chronic pain and disability in older adults, which most commonly affects the joints of the knee, hip, and hand. To date, there are no established disease modifying interventions that can halt or reverse OA progression. Therefore, treatment is focused on alleviating pain and maintaining or improving physical and psychological function. Rehabilitation is widely recommended as first-line treatment for OA as, in many cases, it is safer and more effective than the best-established pharmacological interventions. In this article, we describe the presentation of OA pain and give an overview of its peripheral and central mechanisms. We then provide a state-of-the-art review of rehabilitation for OA pain—including self-management programs, exercise, weight loss, cognitive behavioral therapy, adjunct therapies, and the use of aids and devices. Next, we explore several promising directions for clinical practice, including novel education strategies to target unhelpful illness and treatment beliefs, methods to enhance the efficacy of exercise interventions, and innovative, brain-directed treatments. Finally, we discuss potential future research in areas, such as treatment adherence and personalized rehabilitation for OA pain.

Stepwise increasing sequential offsets cannot be used to deliver high thermal intensities with little or no perception of pain

Offset analgesia (OA) is the disproportionate decrease in pain experience following a slight decrease in noxious heat stimulus intensity. We tested whether sequential offsets would allow noxious temperatures to be reached with little or no perception of pain. Forty-eight participants continuously rated their pain experience during trials containing trains of heat stimuli delivered by Peltier thermode. Stimuli were adjusted through either stepwise sequential increases of 2°C and decreases of 1°C or direct step increases of 1°C up to a maximum of 46°C. Step durations (1, 2, 3, or 6 s) varied by trial. Pain ratings generally followed presented temperature, regardless of step condition or duration. For 6-s steps, OA was observed after each decrease, but the overall pain trajectory was unchanged. We found no evidence that sequential offsets could allow for little pain perception during noxious temperature presentation.NEW & NOTEWORTHY Offset analgesia is the disproportionate decrease in pain experience following a slight decrease in noxious heat stimulus intensity. We tested whether sequential offsets would allow noxious temperatures to be reached with little or no perception of pain. We found little evidence of such overall analgesia. In contrast, we observed analgesic effects after each offset with long-duration stimuli, even with relatively low-temperature noxious stimuli.

Offset analgesia is not affected by cold pressor induced analgesia

Background and aims Offset analgesia (OA) is a pain modulating mechanism described as a disproportionately large decrease in pain intensity evoked by a minor decrease in stimulus intensity. Precise mechanisms of OA are still not elucidated and studies are needed to evaluate factors modulating OA. The aim of this study was to investigate OA before and during tonic cold pain (thought to induce descending inhibition), in a group of healthy volunteers. Methods A randomized, crossover study was performed in 17 healthy participants (8 males and 9 females). The OA paradigm lasted 35 s and was induced by the traditional method using thermal stimulation applied to the forearm. A constant control heat stimulus (CTL) paradigm was used as control to assess adaptation. Pain intensity was assessed continuously. For induction of tonic cold pain, the participants immersed their hand into 2°C water for 2 min. After 1 min and 25 s, the heat stimulation (OA or CTL paradigm) was repeated to assess the modulatory effect of the cold pressor test. Results It was possible to induce OA both before and during the cold pressor test. Tonic cold pain modulated the peak pain reported during both the OA (p=0.015) and CTL paradigms (p=0.001) reflecting endogenous pain modulation. However, the magnitude of OA was not modulated by tonic cold pain (p>0.05). Conclusions The offset analgesia magnitude was not modulated by simultaneously tonic cold pain, thought to reflect another endogenous pain modulation mechanism. Implications Neither offset analgesia magnitude nor adaptation were modulated by cold pressor induced endogenous analgesia. This could be explained by the fact, that offset analgesia was already at maximum in healthy participants. Hence, offset analgesia may not be a suitable assessment tool to investigate modulation induced by experimental methods or pharmacology in healthy participants.

Attenuation of offset analgesia is associated with suppression of descending pain modulatory and reward systems in patients with chronic pain

Background

Offset analgesia is a disproportionate decrease of pain perception following a slight decrease of noxious thermal stimulus and attenuated in patients with neuropathic pain. We examined offset analgesia in patients with heterogeneous chronic pain disorders and used functional magnetic resonance imaging to explore modification of cerebral analgesic responses in comparison with healthy controls.

Results

We recruited seventeen patients with chronic pain and seventeen age-, sex-matched healthy controls. We gave a noxious thermal stimulation paradigm including offset analgesia and control stimuli on the left volar forearm, while we obtained a real-time continuous pain rating and a whole-brain functional magnetic resonance imaging. Baseline, first plateau (5 s), increment (5 s), and second plateau (20 s) temperatures of offset analgesia stimulus were set at 32°C, 46°C, 47°C, and 46°C, respectively. Control stimulus included 30-s 46°C stimulus or only the first 10 s of offset analgesia stimulus. We evaluated magnitude of offset analgesia, analyzed cerebral activation by thermal stimulation, and further compared offset analgesia-related activation between the groups. Magnitude of offset analgesia was larger in controls than in patients (median: 28.9% (interquartile range: 11.0–56.0%) vs. 19.0% (4.2–48.7%), p = 0.047). During the second plateau, controls showed a larger blood oxygenation level-dependent activation than patients at the putamen, anterior cingulate, dorsolateral prefrontal cortices, nucleus accumbens, brainstem, and medial prefrontal cortex (p < 0.05), which are known to mediate either of descending pain modulation or reward responses. Offset analgesia-related activity at the anterior cingulate cortex was negatively correlated with neuropathic component of pain in patients with chronic pain (p = 0.004).

Conclusions

Attenuation of offset analgesia was associated with suppressed activation of the descending pain modulatory and reward systems in patients with chronic pain, at least in the studied cohort. The present findings might implicate both behavioral and cerebral plastic alterations contributing to chronification of pain.

Clinical trial registry: The Japanese clinical trials registry (UMIN-CTR, No. UMIN000011253; http://www.umin.ac.jp/ctr/)

The influence of isometric exercise on endogenous pain modulation: comparing exercise-induced hypoalgesia and offset analgesia in young, active adults

Background and aims

Impairment of endogenous analgesia has been associated with the development, maintenance and persistence of pain. Endogenous analgesia can be evaluated using exercise-induced hypoalgesia (EIH) and offset analgesia (OffA) paradigms, which measure temporal filtering of sensory information. It is not clear if these paradigms are underpinned by common mechanisms, as EIH and OffA have not previously been directly compared. A further understanding of the processes responsible for these clinically relevant phenomena may have future diagnostic and therapeutic utility in management of individuals with persistent pain conditions. The primary aim of this study was to investigate if there is a correlation between the magnitudes of EIH and OffA. The secondary aim of the study was to examine whether exercise influences OffA.

Methods

Thirty-six healthy, pain-free participants were recruited. EIH was evaluated using pressure pain thresholds (PPT) and pain ratings to suprathreshold pressure stimuli over tibialis anterior and the cervical spine. OffA evaluation utilised a three-step protocol, whereby individualised heat pain thermal stimuli [Numerical Rating Scale (NRS)=50/100] were applied (T1), before increasing 1 °C (T2), followed by 1 °C reduction (T3). The magnitude of OffA was calculated as the percentage reduction in the NRS from T2 to T3. PPT/suprathreshold pain ratings and OffA measures were recorded, before and after 5 min of isometric quadriceps exercise performed at 20–25% maximum voluntary contraction (MVC); and following a 15 min rest period. Data were analysed using repeated measures (RM) ANCOVA and correlational analyses.

Results

There was no correlation between EIH measures (PPTs or pain ratings to suprathreshold pressure stimuli over tibialis anterior or the cervical spine) and OffA (p>0.11 for all). OffA was induced and not modulated by exercise (p=0.28).

Conclusions

Five minutes of 20–25% MVC lower limb isometric exercise provided non-pharmacological pain modulation in young, active adults. Magnitude of EIH was not correlated with that of OffA, and exercise did not influence magnitude of OffA.

Implications

These results suggest that in young, pain-free individuals, separate testing of these two paradigms is required to comprehensively evaluate efficacy of endogenous analgesia. If these results are replicated in patient populations, alternative or complementary methods to exercise interventions may be required to modulate impaired OffA.

Abstracts of the 7th Asian Pain Symposium

The Asian Pain Symposium (APS) is a main pain research meeting in Asia. Since established in 2000 in Kyoto, five other APSs have been held in different Asian regions including Seoul of Korea in 2004, Fukuoka of Japan in 2008, Shanghai of China in 2011, Okazaki of Japan in 2013, and Suzhou of China in 2015. The 7th Asian Pain Symposium (APS 2017) was held in Taipei of Taiwan during October 26th to October 29th, 2017. The APS 2017 was sponsored by The Ministry of Science and Technology of Taiwan and Institute of Biomedical Science and Neuroscience Program of Academia Sinica and Taiwan Pain Society. The president of the APS 2017 was Dr. Bai Chuang Shyu, Institute of Biomedical Sciences, Academia Sinica, Taiwan. Local organizing committee also include Dr. Jen-Chuen Hsieh, Institute of Brain Science, National Yang-Ming University and Veteran General Hospital, Taiwan, Dr. Wei-Zen Sun, Department of Anesthesiology, National Taiwan University Hospital, Taiwan, and Dr. Chih-Cheng Chen, Institute of Biomedical Sciences, Academia Sinica, Taiwan. Main topics of the APS 2017 included the latest progress of pain research and novel strategies of pain treatments. Symposium attendees presented their interesting and exciting research findings in the areas of 1) basic sensory and nociceptive functions, 2) ion channels and their functions in somatosensory physiology and pain, 3) brain functions and regulations in pain, 4) spinal cord mechanisms of nociception and pain, 5) analgesia and pain regulations, 6) chronic pain mechanisms and treatment, and 7) brain circuits underlying the physiological and pathological pain. There were a total of 29 oral presentations and 23 poster presentations at the 7th APS. A council meeting was held during the 7th APS, and at this council meeting Dr. Seog Bae OH (Seoul National University) was elected as the president of 8th Asian Pain Symposium to organize the next symposium in Seoul, Korea in 2019. In order to keep a permanent record and to help promote pain research in Asia, we have collected abstracts of oral presentations and posted them below in the order when the presentations were given at the 7th Asian Pain Symposium.

Neural Mechanisms of Offset Analgesia

One feels a disproportionately large decrease of pain sensation on a slight decrease of thermal pain stimulus. Such phenomenon is termed offset analgesia and considered mediated by endogenous analgesic mechanisms. Offset analgesia was found attenuated in patients with neuropathic pain. We further found that such attenuation occurred in a more heterogeneous population of patients with chronic pain. By functional magnetic resonance imaging, we also found negative blood oxygenation level-dependent signals at those areas concerned with descending pain modulatory and reward systems during offset analgesia in the same cohort of patients. We propose that dysfunction of those systems, as revealed by attenuation of offset analgesia, might well be part of neural mechanisms of pain chronification.