Tapentadol potentiates descending pain inhibition in chronic pain patients with diabetic polyneuropathy

Assessing the influence of non-ischaemic A-fiber conduction blockade on offset analgesia: an experimental study

Offset analgesia (OA) is believed to reflect the efficiency of the endogenous pain modulatory system. However, the underlying mechanisms are still being debated. Previous research suggested both, central and peripheral mechanisms, with the latter involving the influence of specific A-delta-fibers. Therefore, this study aimed to investigate the influence of a non-ischaemic A-fiber conduction blockade on the OA response in healthy participants. A total of 52 participants were recruited for an A-fiber conduction blockade via compression of the superficial radial nerve. To monitor fiber-specific peripheral nerve conduction capacity, quantitative sensory testing was performed continuously. Before, during and after the A-fiber block, an individualized OA-paradigm was applied to the dorsum of both hands (blocked and control side were randomized). Pain intensity of each heat stimulus was evaluated by an electronic visual analogue scale. A successful A-fiber conduction blockade was achieved in thirty participants. Offset analgesia has been verified within time (before, during, after blockade), and condition (blocked and control side) (p < 0.01, d > 0.5). Repeated measurements ANOVA showed no significant interaction effects between OA within condition and time (p = 0.24, η²p = 0.05). Hence, no significant effect of A-fiber blockade was detected on OA during noxious heat stimulation. The results suggest that peripheral A-fiber afferents may play a minor role in OA compared to alternative central mechanisms or other fibers. However, further studies are needed to substantiate a central rather than peripheral influence on OA. PERSPECTIVE: This article presents the observation of offset analgesia before, during and after a successful A-fiber conduction blockade in healthy volunteers. A better understanding of the mechanisms of offset analgesia and endogenous pain modulation in general may help to explain the underlying aspects of pain disorders.

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.

Prediction of duloxetine efficacy in addition to self-management in painful temporomandibular disorders: A randomised, placebo-controlled clinical trial

BACKGROUND

Conditioned pain modulation (CPM) is a potential predictor of treatment response that has not been studied in temporomandibular disorders (TMD).

OBJECTIVES

We conducted a randomised, double-blind, placebo-controlled trial (RCT) of duloxetine in addition to self-management (SM) strategies to investigate its efficacy to reduce pain intensity in painful TMD patients. Moreover, we investigated whether baseline CPM would predict the duloxetine efficacy to reduce TMD pain intensity.

METHODS

Eighty participants were randomised to duloxetine 60 mg or placebo for 12 weeks. The primary outcomes were the change in the pain intensity from baseline to week-12 and CPM-sequential paradigm at baseline. Safety, physical and emotional functioning outcomes were also evaluated.

RESULTS

Of 80 participants randomised, 78 were included in intention-to-treat analysis. Pain intensity decreased for SM-duloxetine and SM-placebo but did not differ between groups (p = .82). A more efficient CPM was associated with a greater pain intensity reduction regardless of the treatment group (p = .035). Physical and emotional functioning did not differ between groups, but adverse events (p = .014), sleep impairment (p = .003) and catastrophizing symptoms (p = .001) were more prevalent in SM-duloxetine group.

CONCLUSION

This study failed to provide evidence of a beneficial effect of adding duloxetine to SM strategies for treatment of painful TMD. Nonetheless, this RCT has shown the feasibility of applying pain modulation assessment to predict short-term treatment response in painful TMD patients, which confirms previous finds that CPM evaluation may serve a step forward in individualising pain treatment.

Differential Effects of Thermal Stimuli in Eliciting Temporal Contrast Enhancement: A Psychophysical Study

Offset analgesia (OA) is observed when pain relief is disproportional to the reduction of noxious input and is based on temporal contrast enhancement (TCE). This phenomenon is believed to reflect the function of the inhibitory pain modulatory system. However, the mechanisms contributing to this phenomenon remain poorly understood, with previous research focusing primarily on painful stimuli and not generalizing to nonpainful stimuli. Therefore, the aim of this study was to investigate whether TCE can be induced by noxious as well as innocuous heat and cold stimuli. Asymptomatic subjects (n = 50) were recruited to participate in 2 consecutive experiments. In the first pilot study (n = 17), the parameters of noxious and innocuous heat and cold stimuli were investigated in order to implement them in the main study. In the second (main) experiment, subjects (n = 33) participated in TCE paradigms consisting of 4 different modalities, including noxious heat (NH), innocuous heat (IH), noxious cold (NC), and innocuous cold (IC). The intensity of the sensations of each thermal modality was assessed using an electronic visual analog scale. TCE was confirmed for NH (P < .001), NC (P = .034), and IC (P = .002). Conversely, TCE could not be shown for IH (P = 1.00). No significant correlation between TCE modalities was found (r < .3, P > .05). The results suggest that TCE can be induced by both painful and nonpainful thermal stimulation but not by innocuous warm temperature. The exact underlying mechanisms need to be clarified. However, among other potential mechanisms, this may be explained by a thermo-specific activation of C-fiber afferents by IH and of A-fiber afferents by IC, suggesting the involvement of A-fibers rather than C-fibers in TCE. More research is needed to confirm a peripheral influence. PERSPECTIVE: This psychophysical study presents the observation of temporal contrast enhancement during NH, NC, and innocuous cold stimuli but not during stimulation with innocuous warm temperatures in healthy volunteers. A better understanding of endogenous pain modulation mechanisms might be helpful in explaining the underlying aspects of pain disorders.

Oxaliplatin causes increased offset analgesia during chemotherapy - a feasibility study

OBJECTIVES

Offset analgesia (OA) is the phenomenon where the perceived pain intensity to heat stimulation disproportionally decreases after a slight decrease in stimulation temperature. The neural mechanisms of OA are not fully understood, but it appears that both peripheral and central temporal filtering properties are involved. Chemotherapy with oxaliplatin often causes acute peripheral sensory neuropathy, and manifests primarily as a cold induced allodynia. The aim of this exploratory patient study was to investigate if OA was affected by the neurotoxic effects of adjuvant oxaliplatin treatment.

METHODS

OA was assessed in 17 colon cancer patients during 12 cycles of adjuvant oxaliplatin treatment. The OA response was estimated as the decrease in pain intensity caused by a temperature decrease from 46 °C to 45 °C. Changes in the OA during the treatment period was estimated using a mixed linear model and corrected for multiple comparisons by Sidak's test.

RESULTS

OA was increased significantly when assessed before the 2nd, 3rd, 5th, 6th, 9th, and 10th treatment cycle compared to the first (baseline) treatment (p<0.05).

CONCLUSIONS

OA is generally decreased in persons suffering from chronic pain or peripheral neuropathy as compared to healthy controls. But in the present study, OA increased during chemotherapy with oxaliplatin. The underlying mechanism of this unexpected increase should be further explored.

Does the painDETECT questionnaire identify impaired conditioned pain modulation in people with musculoskeletal pain? - a diagnostic accuracy study

BACKGROUND

People with neuropathic-like symptoms had more unfavourable pain features than people with nociceptive. Moreover, deficient conditioned pain modulation is common in people with neuropathic-like symptoms. PainDETECT questionnaire have been used to assess the central sensitisation sign and symptoms. However, whether the painDETECT questionnaire can identify the conditioned pain modulation's impairment is still unknown. Therefore, the current study aimed to evaluate the diagnostic accuracy of the painDETECT questionnaire in detecting the impairment of conditioned pain modulation in people with musculoskeletal pain.

METHODS

We conducted a diagnostic accuracy comparing the painDETECT questionnaire (index method) with the cold pressor test, the psychophysical test used to assess the conditioned pain modulation (reference standard). We determined diagnostic accuracy by calculating sensitivity, specificity, predictive values, and likely hood ratios.

RESULTS

We retrospectively enrolled 308 people with musculoskeletal pain in outpatient departments. Most participants were female (n 20 = 220, 71.4%) and had a mean age of 52.2 (± 15.0) years. One hundred seventy-three (56.1%) participants were classified as nociceptive pain, 69 (22.4%) as unclear, and 66 (21.4%) as neuropathic-like symptoms. According to the cold pressor test, 60 (19.4%) participants presented impairment of conditioned pain modulation. The cutoff point of 12 of the painDETECT questionnaire showed values of diagnostic accuracy below 70% compared to the cold pressor test, except for a negative predictive value [76.9 95% Confidence Interval (CI) 71.7 to 81.5]. The cutoff point 19 showed high specificity (78.6%, 95% CI 73.0 to 83.5), high negative predictive value (80.5%, 95% CI 78.1 to 82.7), and accuracy of 67.5% compared to the cold pressor test.

CONCLUSION

The painDETECT questionnaire seems valuable for ruling out people with musculoskeletal pain and impairment of conditioned pain modulation.

Harnessing the conditioned pain modulation response in migraine diagnosis, outcome prediction, and treatment-A narrative review

OBJECTIVE

To present the potential use and relevance of the conditioned pain modulation (CPM) response to migraine diagnosis, outcome prediction, and treatment.

BACKGROUND

The CPM response is a widely used laboratory test to examine inhibitory pain modulation capabilities.

METHODS

This narrative review summarizes and synthesizes the findings on the CPM response in patients with migraine.

RESULTS

For diagnosis, we summarized the studies comparing CPM responses between patients with migraine and individuals without migraine or with other headache syndromes, as well as between patients with subtypes of migraine. For prediction, we summarized the studies utilizing the CPM response to predict migraine outcome, such as response to interventions. For treatment, we described a device that utilizes the CPM response for acute and preventative migraine treatment. In addition, we suggest the requirements needed for the CPM response to be used for migraine diagnosis, outcome prediction, and treatment.

CONCLUSIONS

Although more research is needed, the CPM response could be a useful tool for improving migraine management.

Painful Diabetic Peripheral Neuropathy: Practical Guidance and Challenges for Clinical Management

Painful diabetic peripheral neuropathy (PDPN) is present in nearly a quarter of people with diabetes. It is estimated to affect over 100 million people worldwide. PDPN is associated with impaired daily functioning, depression, sleep disturbance, financial instability, and a decreased quality of life. Despite its high prevalence and significant health burden, it remains an underdiagnosed and undertreated condition. PDPN is a complex pain phenomenon with the experience of pain associated with and exacerbated by poor sleep and low mood. A holistic approach to patient-centred care alongside the pharmacological therapy is required to maximise benefit. A key treatment challenge is managing patient expectation, as a good outcome from treatment is defined as a reduction in pain of 30-50%, with a complete pain-free outcome being rare. The future for the treatment of PDPN holds promise, despite a 20-year void in the licensing of new analgesic agents for neuropathic pain. There are over 50 new molecular entities reaching clinical development and several demonstrating benefit in early-stage clinical trials. We review the current approaches to its diagnosis, the tools, and questionnaires available to clinicians, international guidance on PDPN management, and existing pharmacological and non-pharmacological treatment options. We synthesise evidence and the guidance from the American Association of Clinical Endocrinology, American Academy of Neurology, American Diabetes Association, Diabetes Canada, German Diabetes Association, and the International Diabetes Federation into a practical guide to the treatment of PDPN and highlight the need for future research into mechanistic-based treatments in order to prioritise the development of personalised medicine.

Pain Mechanisms Associated With Disease Activity in Patients With Rheumatoid Arthritis Treated With Disease-Modifying Antirheumatic Drugs: A Regression Tree Analysis

OBJECTIVE

Although pain affects the assessment of disease activity in patients with rheumatoid arthritis (RA), pain is not always directly related to peripheral joint inflammation. Peripheral and central nervous system regulatory mechanisms also affect pain perception. We used regression tree methodology to identify mechanisms most predictive of disease activity after disease-modifying antirheumatic drug (DMARD) treatment.

METHODS

Disease activity was evaluated using the Disease Activity Score in 28 joints (DAS28) in 176 patients with RA, before and after starting a DMARD. Quantitative sensory testing (QST), including pressure pain thresholds (PPTs), temporal summation, and conditioned pain modulation (CPM), were used to assess pain mechanisms. Regression tree methodology was used to determine the QST modalities most predictive of DAS28 after DMARD treatment.

RESULTS

This analysis identified 4 groups defined by baseline DAS28 category and either knee PPT (a combined measure of peripheral and central nervous system dysregulation) or CPM (a measure of descending pain inhibition). Among patients starting with low/moderate disease activity, lower knee PPT (PPT ≤ 4.65 kgf) most strongly predicted higher posttreatment disease activity (group 1 mean DAS28 2.8 [SD 1.0] vs group 2 mean DAS28 3.5 [SD 1.0]). Among patients starting with high baseline disease activity, less efficient descending pain modulation (CPM ≤ 1.55) most strongly predicted higher posttreatment disease activity (group 3 mean DAS28 3.4 [SD 1.4] vs group 4 mean DAS28 4.6 [SD 1.1]).

CONCLUSION

These results highlight the importance of identifying and treating aberrant peripheral and central pain regulation in patients with RA starting or switching DMARD therapy.

Optimizing and Accelerating the Development of Precision Pain Treatments for Chronic Pain: IMMPACT Review and Recommendations

Large variability in the individual response to even the most-efficacious pain treatments is observed clinically, which has led to calls for a more personalized, tailored approach to treating patients with pain (ie, "precision pain medicine"). Precision pain medicine, currently an aspirational goal, would consist of empirically based algorithms that determine the optimal treatments, or treatment combinations, for specific patients (ie, targeting the right treatment, in the right dose, to the right patient, at the right time). Answering this question of "what works for whom" will certainly improve the clinical care of patients with pain. It may also support the success of novel drug development in pain, making it easier to identify novel treatments that work for certain patients and more accurately identify the magnitude of the treatment effect for those subgroups. Significant preliminary work has been done in this area, and analgesic trials are beginning to utilize precision pain medicine approaches such as stratified allocation on the basis of prespecified patient phenotypes using assessment methodologies such as quantitative sensory testing. Current major challenges within the field include: 1) identifying optimal measurement approaches to assessing patient characteristics that are most robustly and consistently predictive of inter-patient variation in specific analgesic treatment outcomes, 2) designing clinical trials that can identify treatment-by-phenotype interactions, and 3) selecting the most promising therapeutics to be tested in this way. This review surveys the current state of precision pain medicine, with a focus on drug treatments (which have been most-studied in a precision pain medicine context). It further presents a set of evidence-based recommendations for accelerating the application of precision pain methods in chronic pain research. PERSPECTIVE: Given the considerable variability in treatment outcomes for chronic pain, progress in precision pain treatment is critical for the field. An array of phenotypes and mechanisms contribute to chronic pain; this review summarizes current knowledge regarding which treatments are most effective for patients with specific biopsychosocial characteristics.

Neuropathic Pain Was Associated with Central Sensitivity Syndrome in Patients with Preoperative Lumbar Spinal Stenosis Using the painDETECT and Central Sensitization Inventory Questionnaires: A Cross-Sectional Study

Background

Lumbar spinal stenosis (LSS) patients have been reported to have neuropathic pain and central sensitivity syndrome (CSS). These associations have been reported in other diseases but are unknown in preoperative LSS patients. We aimed to investigate the association between neuropathic pain and CSS in preoperative LSS patients using the painDETECT and the Central Sensitization Inventory (CSI) questionnaires.

Methods

This cross-sectional study was conducted from November 2021 to March 2022. The data were collected regarding demographics and pain, including neuropathic pain, numbness, LSS severity, physical function, quality of life, and CSS. Patients were divided into two groups, patients with acute and chronic pain, and further classified into three categories based on the clinical phenotype of patients in each group. Independent variables included age, gender, type of LSS (bilateral or unilateral symptoms), Numerical Rating Scale of leg pain, CSI, and the Zurich Claudication Questionnaire (ZCQ) for symptom severity and physical function. The dependent variable was painDETECT. Multiple regression analysis using the forced entry method examined the association between painDETECT and CSI.

Results

Of the 119 patients with preoperative LSS, 106 were included. The mean age of the participants was 69.9 years, and 45.3% were female. Neuropathic pain was present in 19.8%, and CSS was present in 10.4%. The CSI (β = 0.468, p < 0.001) and ZCQ for symptom severity (β = 0.304, p < 0.01) were significantly associated with the painDETECT, explaining 47.8% of the variance in the painDETECT score.

Conclusions

There is an association between neuropathic pain and CSS in patients with preoperative LSS using the painDETECT and CSI questionnaires.

Diffuse noxious inhibitory controls in chronic joint inflammatory Pain: Study of the descending serotonergic modulation mediated through 5HT3 receptors

The loss of diffuse noxious inhibitory controls (DNIC) is recognized as a predictor of chronic pain. Mechanistically, DNIC produces analgesia by a heterotopically applied conditioning-noxious stimulus (CS) and yet underexplored descending modulatory inputs. Here, we aimed at studying DNIC in monoarthritis (MA) by exploring the spinal component of the descending serotonergic system, specifically 5-hydroxytryptamine 3 receptors (5-HT3R). MA was induced in male Wistar rats by tibiotarsal injection of complete Freund's adjuvant. Mechanical hyperalgesia and DNIC were assessed weekly by the Randall-Selitto test. Immunohistochemistry was used to quantify spinal 5-HT3R, and tryptophan hydroxylase (TPH) colocalization with phosphorylated extracellular signal-regulated protein kinases 1/2 at the rostroventromedial medulla (RVM). Spinal serotonin (5-HT) was quantified by HPLC. The effects of intrathecal ondansetron, a 5-HT3R antagonist, were assessed on mechanical hyperalgesia and DNIC. MA resulted in a prolonged steady-state mechanical hyperalgesia. In contrast, DNIC peaked after 28 days, decreasing afterwards until extinction at 42 days. At this later timepoint, MA rats showed increased: (i) spinal 5-HT3R and 5-HT levels, (ii) neuronal serotonergic activation and TPH expression at the RVM. Ondansetron reversed mechanical hyperalgesia and restored DNIC, regardless of being administered before or after CS. However, data variability was higher upon administration before CS in MA-animals. Prolonged MA upregulates the descending serotonergic modulation, which simultaneously results in increased nociception and DNIC extinction, through 5-HT3R. Our data suggest a role for spinal 5-HT3R in the top-down modulation of DNIC. Additionally, these receptors may also be involved in the bottom-up circuitry implicated in the trigger of DNIC.

High Blood Glucose and Excess Body fat Enhance Pain Sensitivity and Weaken Pain Inhibition in Healthy Adults: A Single-blind Cross-over Randomized Controlled Trial

To investigate links between blood glucose, body fat mass and pain, the effects of acute hyperglycaemia on pain sensitivity and pain inhibition were examined in healthy adults with normal (n = 24) or excess body fat (n = 20) determined by dual-energy X-ray absorptiometry. Effects of hyperglycaemia on heart rate variability and reactive hyperaemia were also explored. For the overall sample, ingesting 75-g glucose enhanced pain sensitivity during 1-minute cold-water immersion of both feet (conditioning stimulus) and weakened the pain inhibitory effect of cold water on pressure pain thresholds (test stimulus). Exploratory subgroup analyses not adjusted for multiple comparisons suggested that this effect was limited to people with excess fat mass. In addition, acute hyperglycaemia suppressed resting heart rate variability only in people with excess fat mass. Furthermore, regardless of blood glucose levels, people with excess fat mass had weaker pain inhibition for pinprick after cold water and reported more pain during 5-minutes of static blood flow occlusion. Neither high blood glucose nor excess body fat affected pinprick-temporal summation of pain or reactive hyperaemia. Together, these findings suggest that hyperglycaemia and excess fat mass interfere with pain processing and autonomic function. PERSPECTIVE: Ingesting 75-g glucose (equivalent to approximately 2 standard cans of soft drink) interfered with pain-processing and autonomic function, particularly in people with excess body fat mass. As both hyperglycaemia and overweight are risk factors for diabetes, whether these are sources of pain in people with diabetes should be further explored.

Appropriate use of tapentadol: focus on the optimal tapering strategy

OBJECTIVE

Due to its opioid and non-opioid mechanism of action, tapentadol is considered an atypical opioid with improved gastrointestinal tolerability versus traditional opioids. As for all opioid analgesics it is important to understand how to discontinue a treatment when it is not needed anymore. The aim of this article was to provide an overview of opioid therapy in non-cancer pain, with a specific focus on tapering of tapentadol in patients with chronic non-cancer pain, and suggestions on how to achieve tapering.

METHODS

Studies for this narrative review were identified via PubMed using a structured search strategy, focusing on management of chronic non-cancer pain with opioids, and the efficacy, tolerability, and pharmacology of tapentadol prolonged release. Publications were limited to English-language articles published within the last ∼10 years.

RESULTS

The review discusses the use and discontinuation of opioids in general, as well clinical data on discontinuation of tapentadol specifically. We provide a flow chart, which can be used by clinicians in the context of their own clinical experience to appropriately taper tapentadol in patients with chronic non-cancer pain. The flow chart can be easily tailored to individual patient characteristics, duration of tapentadol treatment, response to progressive dosage reduction, and likelihood of withdrawal symptom occurrence.

CONCLUSIONS

While tapentadol is associated with a low frequency of opioid withdrawal symptoms after abrupt discontinuation, use of a tapering strategy is prudent. Tapering strategies developed for opioids in general can potentially be safely individualized in tapentadol-treated patients, although research on tapering strategies for tapentadol is required.

Influence of tapentadol and oxycodone on the spinal cord and brain using electrophysiology: a randomized, placebo-controlled trial

AIMS

The aim of this study was to investigate the effects of tapentadol and oxycodone using the nociceptive withdrawal reflex and sensory evoked potentials.

METHODS

Twenty-one healthy volunteers completed a cross-over trial with oxycodone (10 mg), tapentadol (50 mg) extended-release tablets, or placebo treatment administered orally BID for 14 days. Electrical stimulations were delivered on the plantar side of the foot to evoke a nociceptive withdrawal reflex at baseline and post-interventions. Electromyography, recorded at tibialis anterior, and electroencephalography were recorded for analysis of: number of reflexes, latencies, and area under the curve of the nociceptive withdrawal reflex as well as latencies, amplitudes and dipole sources of the sensory-evoked potential.

RESULTS

Tapentadol decreased the odds ratio of eliciting nociceptive withdrawal reflex by -0.89 (P = .001, 95% confidence interval [CI] -1.46, -0.32), whereas oxycodone increased the latency of the N1 component of the sensory-evoked potential at the vertex by 12.5 ms (P = .003, 95% CI 3.35, 21.69). Dipole sources revealed that the anterior cingulate component moved caudally for all three interventions (all P < .02), and the insula components moved caudally in both the oxycodone and tapentadol arms (all P < .03).

CONCLUSION

A decrease in the number of nociceptive withdrawal reflex was observed during tapentadol treatment, possibly relating to the noradrenaline reuptake inhibition effects on the spinal cord. Both oxycodone and tapentadol affected cortical measures possible due to μ-opioid receptor agonistic effects evident in the dipole sources, with the strongest effect being mediated by oxycodone. These findings could support the dual effect analgesic mechanisms of tapentadol in humans as previously shown in preclinical studies.

Monoaminergic mediation of hyperalgesic and analgesic descending control of nociception in mice

ABSTRACT

Descending control of nociception (DCN; also known as conditioned pain modulation [CPM], the behavioral correlate of diffuse noxious inhibitory controls) is the phenomenon whereby pain inhibits pain in another part of the body and is the subject of increasing study because it may represent a biomarker of chronic pain. We recently discovered that pain modulation upon application of a DCN paradigm involving low-intensity test stimuli occurs in the direction of hyperalgesia in healthy mice and rats, whereas the use of high-intensity stimuli produces analgesia. To elucidate the physiological mechanisms underlying hyperalgesic DCN, we administered agonists and antagonists of norepinephrine (NE) and serotonin (5-HT) receptors, key neurochemical players in the production of analgesic DCN. We find that three different monoamine reuptake inhibitors-the NE-selective reboxetine, the 5-HT-selective fluoxetine, and the dual NE/5-HT agonist duloxetine-all abolish hyperalgesic DCN when administered into the spinal cord (but not systemically), with no effect on heat or mechanical pain sensitivity. Reboxetine's attenuation of hyperalgesic DCN is mediated by α 2 -adrenergic receptors (i.e., blocked by atipamezole), and fluoxetine's effect is mediated by 5-HT 7 receptors (i.e., blocked by SB269970). In contrast, analgesic DCN was found to be reversed by atipamezole and SB269970 themselves, with no effect of reboxetine or fluoxetine. Thus, hyperalgesic DCN appears to be the neurochemical opposite to analgesic DCN. These data further validate and help elucidate a pre-clinical paradigm that mimics dysfunctional CPM, and thus may form the basis of translational experiments that aim to reveal preventative pharmacological strategies for individuals predisposed to persistent pain.

Cannabidiol enhances the antinociceptive effects of morphine and attenuates opioid-induced tolerance in the chronic constriction injury model

Neuropathic pain (NP) is a complex health problem that includes sensorial manifestations such as evoked and ongoing pain. Cannabidiol (CBD) has shown potential in the treatment of NP and the combination between opioids and cannabinoids has provided promising results on pain relief. Thus, our study aimed to investigate the effect of treatment combination between CBD and morphine on evoked and ongoing pain, and the effect of CBD on morphine-induced tolerance in the model of chronic constriction injury (CCI) of the sciatic nerve in rats. Mechanical thresholds (i.e., evoked pain) were evaluated before and 7 days after surgery. We also employed a 4-day conditioned place preference (CPP) protocol, to evaluate relief of ongoing pain (6-9 days after surgery). Treatment with morphine (2 and 4 mg/kg) or CBD (30 mg/kg) induced a significant antinociceptive effect on evoked pain. The combination of CBD (30 mg/kg) and morphine (1 mg/kg) produced an enhanced antinociceptive effect, when compared to morphine alone (1 mg/Kg). Treatment with morphine (1 and 2 mg/kg) or CBD (30 mg/kg) alone failed to induce significant scores in the CPP test. However, combined treatment of CBD (30 mg/kg) and morphine (1 mg/kg) provided significant positive scores, increased the number of entrances in the drug-paired chamber in the CPP test and did not alter locomotor activity in rats. Lastly, treatment with CBD partially attenuated morphine-induced tolerance. In summary, our results support the indication of CBD as an adjuvant to opioid therapy for the attenuation of NP and opioid-induced analgesic tolerance.

Prevention and Management Strategies for Diabetic Neuropathy

Diabetic neuropathy (DN) is a common complication of diabetes that is becoming an increasing concern as the prevalence of diabetes rapidly rises. There are several types of DN, but the most prevalent and studied type is distal symmetrical polyneuropathy, which is the focus of this review and is simply referred to as DN. It can lead to a wide range of sensorimotor and psychosocial symptoms and is a major risk factor for diabetic foot ulceration and Charcot neuropathic osteoarthropathy, which are associated with high rates of lower limb amputation and mortality. The prevention and management of DN are thus critical, and clinical guidelines recommend several strategies for these based on the best available evidence. This article aims to provide a narrative review of DN prevention and management strategies by discussing these guidelines and the evidence that supports them. First, the epidemiology and diverse clinical manifestations of DN are summarized. Then, prevention strategies such as glycemic control, lifestyle modifications and footcare are discussed, as well as the importance of early diagnosis. Finally, neuropathic pain management strategies and promising novel therapies under investigation such as neuromodulation devices and nutraceuticals are reviewed.

Conditioned pain modulation is more efficient in patients with painful diabetic polyneuropathy than those with nonpainful diabetic polyneuropathy

ABSTRACT

Endogenous pain modulation, as tested by the conditioned pain modulation (CPM) protocol, is typically less efficient in patients with chronic pain compared with healthy controls. We aimed to assess whether CPM is less efficient in patients with painful diabetic polyneuropathy (DPN) compared with those with nonpainful DPN. Characterization of the differences in central pain processing between these 2 groups might provide a central nervous system explanation to the presence or absence of pain in diabetic neuropathy in addition to the peripheral one. Two hundred seventy-one patients with DPN underwent CPM testing and clinical assessment, including quantitative sensory testing. Two modalities of the test stimuli (heat and pressure) conditioned to cold noxious water were assessed and compared between patients with painful and nonpainful DPN. No significant difference was found between the groups for pressure pain CPM; however, patients with painful DPN demonstrated unexpectedly more efficient CPMHEAT (-7.4 ± 1.0 vs -2.3 ± 1.6; P = 0.008). Efficient CPMHEAT was associated with higher clinical pain experienced in the 24 hours before testing (r = -0.15; P = 0.029) and greater loss of mechanical sensation (r = -0.135; P = 0.042). Moreover, patients who had mechanical hypoesthesia demonstrated more efficient CPMHEAT (P = 0.005). More efficient CPM among patients with painful DPN might result from not only central changes in pain modulation but also from altered sensory messages coming from tested affected body sites. This calls for the use of intact sites for proper assessment of pain modulation in patients with neuropathy.

Family History of Alcohol Use Disorder as a Predictor of Endogenous Pain Modulation Among Moderate to Heavy Drinkers

Family history of alcohol use disorder (AUD) is frequently endorsed by persons with chronic pain. Although individuals with a family history of AUD have demonstrated enhanced sensitivity to painful stimulation, previous research has not examined endogenous pain modulation in this population. The goal of this study was to test family history of AUD as a predictor of conditioned pain modulation, offset analgesia, and temporal summation among a sample of moderate and heavy drinkers. Adults with no current pain (N = 235; 58.3% male; M_age = 34.3; 91.9% non-Hispanic; 60% white) were evaluated for family history of AUD at baseline and pain modulatory outcomes were assessed via quantitative sensory testing. Participants with a family history of AUD (relative to those without) evinced a pro-nociceptive pain modulation profile in response to experimental pain. Specifically, family history of AUD was associated with deficits in pain-inhibitory processes. Approximately 4% of the variance in endogenous pain modulation was accounted for by family history, and exploratory analyses suggested these effects may be driven by paternal AUD. PERSPECTIVE: The current findings suggest individuals with a family history of AUD demonstrate pain modulatory function that may predispose them to the development of chronic pain. Clinically, these data may inform pain management approaches for individuals with a family history of AUD.

Impaired conditioned pain modulation was restored after a single exercise session in individuals with and without fibromyalgia

Supplemental Digital Content is Available in the Text.

Submaximal isometric exercise improved impaired conditioned pain modulation acutely in individuals with and without fibromyalgia, regardless of health status.

Introduction:

Exercise is an effective nonpharmacological intervention for individuals with fibromyalgia syndrome (FMS); however, considerable variability is observed in their pain response after a single exercise session that could be due to differences in baseline central pain inhibition (ie, conditioned pain modulation [CPM]).

Objectives:

This study examined the effect of isometric exercise on CPM in people with FMS and control participants. A subaim was to identify whether pain inhibition after exercise was due to differences in baseline CPM.

Methods:

Twenty-one individuals with FMS (50.5 ± 14.9 years) and 22 age-matched and sex-matched controls (49.2 ± 13.3 years) participated in a familiarization session and 2 randomized experimental sessions: (1) low-intensity isometric exercise and (2) quiet rest control. Conditioned pain modulation was measured before and after each experimental session. In addition, body composition and physical activity levels were collected to determine potential group differences.

Results:

Both groups had comparable body composition and physical activity levels and reported similar exercise-induced hypoalgesia (increase in pressure pain thresholds) at the exercising muscle (quadriceps muscle) and systemically (deltoid muscle). Both groups had a decrease in CPM after exercise and quiet rest; however, in both FMS and control participants with impaired baseline CPM, there was an increase in CPM at the deltoid muscle after exercise.

Conclusion:

In persons with low CPM, irrespective of health status, isometric exercise enhanced CPM at a site distal from the exercising muscle. Our results support the use of isometric exercise when initiating an exercise program especially for individuals with impaired CPM.

Reliability of conditioned pain modulation in healthy individuals and chronic pain patients: a systematic review and meta-analysis

OBJECTIVES

Conditioned pain modulation (CPM) is a psychophysical parameter that is used to reflect the efficacy of endogenous pain inhibition. CPM reliability is important for research and potential clinical applications. The aim of this systematic review and meta-analysis was to evaluate the reliability of CPM tests in healthy individuals and chronic pain patients.

METHODS

We searched three databases for peer-reviewed studies published from inception to October 2020: EMBASE, Web of Science and NCBI. Risk of bias and the quality of the included studies were assessed. A meta-analysis with a random effects model was conducted to estimate intraclass correlation coefficients (ICCs).

RESULTS

Meta-analysis was performed on 25 papers that examined healthy participants (k=21) or chronic pain patients (k=4). The highest CPM intra-session reliability was with pressure as test stimulus (TS) and ischemic pressure (IP) or cold pressor test (CPT) as conditioning stimulus (CS) in healthy individuals (ICC 0.64, 95% CI 0.45-0.77), and pressure as TS with CPT as CS in patients (ICC 0.77, 95% CI 0.70-0.82). The highest inter-session ICC was with IP as TS and IP or CPT as CS (ICC 0.51, 95% CI 0.42-0.59) in healthy subjects. The only data available in patients for inter-session reliability were with pressure as TS and CPT as CS (ICC 0.44, 95% CI 0.11-0.69). Quality ranged from very good to excellent using the QACMRR checklist. The majority of the studies (24 out of 25) scored inadequate in Kappa coefficient reporting item of the COSMIN-ROB checklist.

CONCLUSIONS

Pressure and CPT were the TS and CS most consistently associated with good to excellent intra-session reliability in healthy volunteers and chronic pain patients. The inter-session reliability was fair or less for all modalities, both in healthy volunteers and chronic pain patients.

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.

Simple Quantitative Sensory Testing Reveals Paradoxical Co-existence of Hypoesthesia and Hyperalgesia in Diabetes

Background: Diabetic neuropathy is characterized by the paradoxical co-existence of hypo- and hyperalgesia to sensory stimuli. The literature shows consistently sensory differences between healthy and participants with diabetes. We hypothesized that due to differences in pathophysiology, advanced quantitative sensory testing (QST) might reveal sensory discrepancies between type 1 (T1D) and type 2 diabetes (T2D). Furthermore, we investigated whether vibration detection thresholds (VDT) were associated with sensory response.

Method: Fifty-six adults with T1D [43 years (28–58)], 99 adults with T2D [65 years (57–71)], and 122 healthy individuals [51 years (34–64)] were included. VDT, pressure pain detection thresholds (pPDT) and tolerance (pPTT), tonic cold pain (hand-immersion in iced water), and central pain mechanisms (temporal summation and conditioned pain modulation) were tested and compared between T1D and T2D. VDT was categorized into normal (< 18 V), intermediary (18–25 V), or high (> 25 V).

Results: In comparison to healthy, analysis adjusted for age, BMI, and gender revealed hypoalgesia to tibial (pPDT): p = 0.01, hyperalgesia to tonic cold pain: p < 0.01, and diminished temporal summation (arm: p < 0.01; abdomen: p < 0.01). In comparison to participants with T2D, participants with T1D were hypoalgesic to tibial pPDT: p < 0.01 and pPTT: p < 0.01, and lower VDT: p = 0.02. VDT was not associated with QST responses.

Conclusion: Participants with T1D were more hypoalgesic to bone pPDT and pPTT independent of lower VDT, indicating neuronal health toward normalization. Improved understanding of differentiated sensory profiles in T1D and T2D may identify improved clinical endpoints in future trials.

Opioids and Ocular Surface Pathology; A Literature Review of New Treatments Horizons

This review discusses the role of opioids in the corneal surface and the different pathways and therapeutic methods of management. A literature review was performed using PubMed database. For the database search, the main searching words “opioid” and “topical opioid treatment” were used with the descriptors “cornea”, “ocular surface”, “neuropathic corneal pain”, “corneal sensitivity” and “naltrexone”; original scientific articles and reviews were included to achieve the purpose of the review. The endogenous opioid system has relevant functions in the organism, and in daily use, opioids are used as painkillers. However, these drugs may be employed for other indications as opioid pathways have a wide spectrum. The corneal surface for topical treatment is easily accessible, hence sparing the side effects of systemic opioids. Instillation of opioid antagonist substances, such as naltrexone, increases corneal healing rates and stimulates the division of corneal epithelium cells without deleterious effects. The natural modulation of endogenous opioids controls different forms of pain, including inflammatory and neuropathic pain, both in the ocular surface and in the central nervous system. There are diverse methods in controlling pain using opioids, especially in refractory forms. This review attempts to collect the literature about corneal surface and opioid pathways to provide an overview image and a possible direction of the news treatments.

Current Strategies for the Management of Painful Diabetic Neuropathy

The development of painful diabetic neuropathy (PDN) is a common complication of chronic diabetes that can be associated with significant disability and healthcare costs. Prompt symptom identification and aggressive glycemic control is essential in controlling the development of neuropathic complications; however, adequate pain relief remains challenging and there are considerable unmet needs in this patient population. Although guidelines have been established regarding the pharmacological management of PDN, pain control is inadequate or refractory in a high proportion of patients. Pharmacotherapy with anticonvulsants (pregabalin, gabapentin) and antidepressants (duloxetine) are common first-line agents. The use of oral opioids is associated with considerable morbidity and mortality and can also lead to opioid-induced hyperalgesia. Their use is therefore discouraged. There is an emerging role for neuromodulation treatment modalities including intrathecal drug delivery, spinal cord stimulation, and dorsal root ganglion stimulation. Furthermore, consideration of holistic alternative therapies such as yoga and acupuncture may augment a multidisciplinary treatment approach. This aim of this review is to focus on the current management strategies for the treatment of PDN, with a discussion of treatment rationale and practical considerations for their implementation.

Modulation of offset analgesia in patients with chronic pain and healthy subjects - a systematic review and meta-analysis

OBJECTIVES

Offset analgesia (OA) induces a brief pain inhibition and studies suggest OA impairment in patients with chronic pain when compared to healthy subjects. Conditioned pain modulation remains the most studied descending pain inhibitory control mechanism and is modulated by centrally-acting analgesics. Since OA may be mediated by similar neural substrates as conditioned pain modulation, understanding if OA is a peripheral or central proxy of pain modulation is important. The modulatory effect of centrally-acting drugs on OA in healthy and chronic pain populations has not yet been systematically reviewed and meta-analyzed, and this systematic review and meta-analysis aimed to identify studies employing interventions for modulating OA magnitude.

METHODS

A systematic search of PubMed, Embase, Web of Science, and the Cochrane Library yielded 146 records of which 11 (172 healthy pain-free subjects, 106 chronic pain patients) were eligible for qualitative synthesis, and 10 for meta-analysis on overall modulatory effect of interventions on OA, and subgroup analysis of patients and healthy pain-free subjects.

RESULTS

Risk of bias was evident for study participation and study confounding in the included studies. Several different methods for assessing and calculating OA magnitude were identified, which may affect interpretability of findings and warrants standardization. The meta-analysis showed no modulatory effects on OA overall (standardized mean difference (SMD) [95%CI]: 0.04 [-0.22, 0.30], Z=0.29, p=0.77), or in the subgroup analysis for patients (SMD [95%CI]: -0.04 [-0.63, 0.71], Z=0.13, p=0.90) or healthy pain-free subjects (SMD [95%CI]: 0.01 [-0.21, 0.24], Z=0.11, p=0.91). Moderate to substantial heterogeneity was found for the overall analysis (I2=47%, p=0.03) and patient subgroup analysis (I2=75%, p=0.003).

CONCLUSIONS

The current systematic review and meta-analysis conclude that centrally-acting drugs and exercise do not influence OA. Evidence on the peripheral contribution to OA response requires further investigations. Preclinical models of OA should be established to identify the neurophysiology and -biology behind OA.

The descending pain modulation system predicts short term efficacy of multimodal pain therapy - an observational prospective cohort study

OBJECTIVES

Treating chronic pain patients with multimodal pain therapy (MMPT) alters perception, awareness, and processing of pain at multiple therapeutic levels. Several clinical observations suggest that the effects of therapy may go beyond the possible sum of each level of therapy and may be due to a central descending inhibitory effect measurable by conditioned pain modulation (CPM). Thus, we investigated whether CPM is able to identify a group of patients that benefit particularly from MMPT.

METHODS

This was an observational prospective cohort study. Patients were hospitalized on a special pain medicine ward with specially trained staff for 10 days. The patients were questioned and had investigations before and shortly after MMPT and were followed-up on 3 months post discharge. Before and after treatment, subjects were investigated via CPM and quantitative sensory testing (QST) as well as completing questionnaires. The study was registered in the German Clinical Trials Register (DRKS00006850).

RESULTS

During the study period of 24 months, 224 chronic pain patients were recruited. 51 percent of patients completed the study period. There was an improvement in overall groups regarding all domains assessed, lasting beyond the end of the intervention. Patients with a sufficient CPM effect, defined as a reduction in pain during the conditioning stimulus, at baseline did show a more pronounced reduction in mean pain ratings than those without. This was not the case 3 months after therapy. Furthermore, sufficient CPM was identified as a predictor for pain reduction using a linear regression model.

CONCLUSION

In conclusion, this study shows that while a heterogeneous group of patients with chronic pain disorders does sustainably benefit from MMPT in general, patients with a sufficient CPM effect do show a more pronounced decrease in pain ratings directly after therapy in comparison to those without.

Painful diabetic peripheral neuropathy: Role of oxidative stress and central sensitisation

AIMS

Diabetic peripheral neuropathy (DPN) occurs in about half of people with diabetes, of whom a quarter may develop chronic pain. Pain may remain for years yet be difficult to treat because the underlying mechanisms remain unclear. There is consensus that processing excessive glucose leads to oxidative stress, interfering with normal metabolism. In this narrative review, we argue that oxidative stress may also contribute to pain.

METHODS

We reviewed literature in PubMed published between January 2005 and August 2021.

RESULTS AND CONCLUSIONS

In diabetes, hyperglycaemia and associated production of reactive species can directly increase pain signalling and activate sensory neurons; or the effects can be indirect, mediated by mitochondrial damage and enhanced inflammation. Furthermore, pain processing in the central nervous system is compromised in painful DPN. This is implicated in central sensitisation and dysfunctional pain modulation. However, central pain modulatory function is understudied in diabetes. Future research is required to clarify whether central sensitisation and/or disturbances in central pain modulation contribute to painful DPN. Positive results would facilitate early detection and future treatment.

The Treatment of Painful Diabetic Neuropathy

Painful diabetic peripheral neuropathy (painful-DPN) is a highly prevalent and disabling condition, affecting up to one-third of patients with diabetes. This condition can have a profound impact resulting in a poor quality of life, disruption of employment, impaired sleep, and poor mental health with an excess of depression and anxiety. The management of painful-DPN poses a great challenge. Unfortunately, currently there are no Food and Drug Administration (USA) approved disease-modifying treatments for diabetic peripheral neuropathy (DPN) as trials of putative pathogenetic treatments have failed at phase 3 clinical trial stage. Therefore, the focus of managing painful- DPN other than improving glycaemic control and cardiovascular risk factor modification is treating symptoms. The recommended treatments based on expert international consensus for painful- DPN have remained essentially unchanged for the last decade. Both the serotonin re-uptake inhibitor (SNRI) duloxetine and α2δ ligand pregabalin have the most robust evidence for treating painful-DPN. The weak opioids (e.g. tapentadol and tramadol, both of which have an SNRI effect), tricyclic antidepressants such as amitriptyline and α2δ ligand gabapentin are also widely recommended and prescribed agents. Opioids (except tramadol and tapentadol), should be prescribed with caution in view of the lack of definitive data surrounding efficacy, concerns surrounding addiction and adverse events. Recently, emerging therapies have gained local licenses, including the α2δ ligand mirogabalin (Japan) and the high dose 8% capsaicin patch (FDA and Europe). The management of refractory painful-DPN is difficult; specialist pain services may offer off-label therapies (e.g. botulinum toxin, intravenous lidocaine and spinal cord stimulation), although there is limited clinical trial evidence supporting their use. Additionally, despite combination therapy being commonly used clinically, there is little evidence supporting this practise. There is a need for further clinical trials to assess novel therapeutic agents, optimal combination therapy and existing agents to determine which are the most effective for the treatment of painful-DPN. This article reviews the evidence for the treatment of painful-DPN, including emerging treatment strategies such as novel compounds and stratification of patients according to individual characteristics (e.g. pain phenotype, neuroimaging and genotype) to improve treatment responses.

Successful Pain Relief With Duloxetine in a Patient With Chronic Postsurgical Pain After Open Reduction and Internal Fixation: A Case Report

Chronic postsurgical pain (CPSP) is a common complication of surgery. We report that a patient with CPSP after open reduction and internal fixation (ORIF) had pain relief with duloxetine, and that the conditioned pain modulation (CPM) efficiency may predict the efficacy of duloxetine. A 54-year-old woman with CPSP after ORIF due to proximal humeral fracture was presented to our orthopedic clinic one month after surgery. Despite several analgesics, she still had pain three months after surgery, pain during activity was 74 on the visual analogue scale (VAS), 16 on the American Shoulder and Elbow Surgeons Standardized Shoulder Assessment Form (ASES), 18 on the PainDETECT questionnaire, and CPM efficiency was -5.7%. The patient was treated with duloxetine, starting at 20mg/day and increasing every week. Three months after starting duloxetine, pain on the VAS was 18, ASES was 61, PainDETECT questionnaire was 6, and CPM efficiency was -39.8%. The dose of duloxetine was decreased every week and then withdrawn. Neuropathic pain may be involved even in patients with CPSP after ORIF, and duloxetine may be efficacious in such cases. CPM testing may provide useful information for clinicians in selecting appropriate drugs and in determining when to withdraw drugs.

Different Drugs for the Treatment of Painful Diabetic Peripheral Neuropathy: A Meta-Analysis

Objective: To systematically evaluate the effects of different drugs for the treatment of painful diabetic peripheral neuropathy.

Methods: All literature from PubMed, Embase, and Cochrane Central Register of Controlled Trials published over the past 12 years (from January 1, 2008 to June 1, 2020) was searched, and two reviewers independently assessed study eligibility, continuous data extraction, independent assessment of bias risk, and graded strength of evidence. The pain score was used as the main result, and 30 and 50% pain reduction and adverse events were used as secondary results.

Results: A total of 37 studies were included. Pregabalin, duloxetine, tapentadol, lacosamide, mirogabalin, and capsaicin were all more effective than placebo in alleviating the pain associated with diabetic peripheral neuropathy, while ABT-894 and gabapentin showed no significant effect. In addition, the efficacy of buprenorphine, tanezumab, fulranumab and others could not be concluded due to insufficient studies.

Conclusion: Pregabalin and duloxetine showed good therapeutic effects on painful DPN, but adverse events were also significant. The analgesic effects of ABT-894 and gabapentin need to be further studied with longer and larger RCTs. As an opioid drug, tapentadol has a good analgesic effect, but due to its addiction, it needs to be very cautious in clinical use. Although lacosamide, mirogabalin, and capsaicin are more effective than placebo, the therapeutic effect is weaker than pregabalin. For the results of our meta-analysis, long-term studies are still needed to verify their efficacy and safety in the future.

Systematic Review Registration: PROSPERO, identifier: CRD42020197397.

Neuropharmacological basis for multimodal analgesia in chronic pain

Managing chronic pain remains a major unmet clinical challenge. Patients can be treated with a range of interventions, but pharmacotherapy is the most common. These include opioids, antidepressants, calcium channel modulators, sodium channel blockers, and nonsteroidal anti-inflammatory drugs. Many of these drugs target a particular mechanism; however, chronic pain in many diseases is multifactorial and induces plasticity throughout the sensory neuroaxis. Furthermore, comorbidities such as depression, anxiety, and sleep disturbances worsen quality of life. Given the complexity of mechanisms and symptoms in patients, it is unsurprising that many fail to achieve adequate pain relief from a single agent. The efforts to develop novel drug classes with better efficacy have not always proved successful; a multimodal or combination approach to analgesia is an important strategy in pain control. Many patients frequently take more than one medication, but high-quality evidence to support various combinations is often sparse. Ideally, combining drugs would produce synergistic action to maximize analgesia and reduce side effects, although sub-additive and additive analgesia is still advantageous if additive side-effects can be avoided. In this review, we discuss pain mechanisms, drug actions, and the rationale for mechanism-led treatment selection.Abbreviations: COX - cyclooxygenase, CGRP - calcitonin gene-related peptide, CPM - conditioned pain modulation, NGF - nerve growth factor, NNT - number needed to treat, NMDA - N-methyl-d-aspartate, NSAID - nonsteroidal anti-inflammatory drugs, TCA - tricyclic antidepressant, SNRI - serotonin-noradrenaline reuptake inhibitor, QST - quantitative sensory testing.

Assessment of pain associated with chronic pancreatitis: An international consensus guideline

Pain is the most common symptom in chronic pancreatitis (CP) with a major impact on quality of life. Few validated questionnaires to assess pain in CP exist, and the lack of consensus negatively impacts clinical management, research and meta-analysis. This guideline aims to review generic pain questionnaires for their usability in CP, to outline how pain assessment can be modified by confounding factors and pain types, to assess the value of additional measures such as quality of life, mental health and quantitative sensory testing, and finally to review pain assessment questionnaires used specifically in CP. A systematic review was done to answer 27 questions that followed the PICO (Population; Intervention; Comparator; Outcome) template. Quality of evidence of the statements was judged by Grades of Recommendation, Assessment, Development and Evaluation (GRADE) criteria. The manuscript was sent for review to 36 experts from various disciplines and continents in a multi-stage Delphi process, and finally reviewed by patient representatives. Main findings were that generic pain instruments are valid in most settings, but aspects of pain are specific for CP (including in children), and instruments have to account for the wide phenotypic variability and development of sensitization of the central nervous system. Side effects to treatment and placebo effects shall also be considered. Some multidimensional questionnaires are validated for CP and are recommended together with assessment of quality of life and psychiatric co-morbidities. This guideline will result in more homogeneous and comprehensive pain assessment to potentially improve management of painful CP.

Investigation of Correlations Between Pain Modulation Paradigms

OBJECTIVE

Endogenous pain modulation can be quantified through the use of various paradigms. Commonly used paradigms include conditioned pain modulation (CPM), offset analgesia (OA), spatial summation of pain (SSP), and temporal summation of pain (TSP), which reflect spatial and temporal aspects of pro- and antinociceptive processing. Although these paradigms are regularly used and are of high clinical relevance, the underlying physiological mechanisms are not fully understood.

DESIGN

The aim of this study is therefore to assess the association between these paradigms by using comparable protocols and methodological approaches.

SETTING

University campus.

SUBJECTS

Healthy and pain-free volunteers (n = 48) underwent psychophysical assessment of CPM, OA, SSP, and TSP (random order) at the same body area (volar nondominant forearm) with individualized noxious stimuli.

METHODS

CPM included heat stimuli before, during, and after a noxious cold-water bath, whereas for OA, three heat stimuli were applied: baseline trial, offset trial, and constant trial. For the SSP paradigm, two differently sized heat stimulation areas were evaluated, whereas for TSP, the first and last stimulus of 10 consecutive short heat stimuli were assessed. A computerized visual analog scale was used to continuously evaluate pain intensity. The magnitudes of all associations between all paradigm pairs were analyzed with Spearman's correlation, and individual influencing factors were assessed with a multivariate linear regression model.

RESULTS

Weak to moderate correlations among all four paradigms were found (P > 0.05), and no distinct influencing factors were identified.

CONCLUSIONS

A limited association between pain modulation paradigms suggests that CPM, OA, SSP, and TSP assess distinct aspects of endogenous analgesia with different underlying physiological mechanisms.

Tapentadol and oxycodone reduce cingulate glutamate in healthy volunteers

Tapentadol and oxycodone are commonly used analgesics. Preclinical studies have shown that oxycodone modulates brain metabolites related to opioid pathways, whereas tapentadol also affects noradrenergic activity. However, knowledge about the function of the medications in the human brain is limited. The aim was to investigate effects of tapentadol and oxycodone on brain glutamate, the most important neurotransmitter in pain processing. Magnetic resonance spectroscopy was obtained in 21 healthy subjects from the anterior cingulate cortex, prefrontal cortex, and insula at baseline and after 14 days of treatment with either 50 mg tapentadol, 10 mg oxycodone (equipotent dose, both extended release) or placebo twice daily in a randomized double-blind cross-over study. Compared to baseline, decreased glutamate/creatine levels were identified in anterior cingulate cortex after tapentadol (1.26 ± 0.14 vs. 1.35 ± 0.18, P = .04) and oxycodone (1.26 ± 0.10 vs. 1.35 ± 0.12, P = .05) treatments, both with 7% reduction. This indicates that both analgesics modulate the glutamatergic system at the supraspinal level in humans.

Spinal Inhibitory Dysfunction in Patients With Painful or Painless Diabetic Neuropathy

OBJECTIVE

Impaired rate-dependent depression of the Hoffman reflex (HRDD) is a marker of spinal inhibitory dysfunction and has previously been associated with painful neuropathy in a proof-of-concept study in patients with type 1 diabetes. We have now undertaken an assessment of HRDD in patients with type 1 or type 2 diabetes.

RESEARCH DESIGN AND METHODS

A total of 148 participants, including 34 healthy control subjects, 42 patients with painful diabetic neuropathy, and 62 patients with diabetic neuropathy without pain, underwent an assessment of HRDD and a detailed assessment of peripheral neuropathy, including nerve conduction studies, corneal confocal microscopy, and thermal threshold testing.

RESULTS

Compared with healthy control subjects (P < 0.001) and patients without pain (P < 0.001), we found that HRDD is impaired in patients with type 1 or type 2 diabetes with neuropathic pain. These impairments are unrelated to diabetes type and the presence or severity of neuropathy. In contrast, patients without neuropathic pain (P < 0.05) exhibited enhanced HRDD compared with control subjects.

CONCLUSIONS

We suggest that loss or impairment of HRDD may help to identify a subpopulation of patients with painful diabetic neuropathy mediated by impaired spinal inhibitory systems who may respond optimally to therapies that target spinal or supraspinal mechanisms. Enhanced RDD in patients without pain may reflect engagement of spinal pain-suppressing mechanisms.

Brainstem Pain-Modulation Circuitry and Its Plasticity in Neuropathic Pain: Insights From Human Brain Imaging Investigations

Acute pain serves as a protective mechanism that alerts us to potential tissue damage and drives a behavioural response that removes us from danger. The neural circuitry critical for mounting this behavioural response is situated within the brainstem and is also crucial for producing analgesic and hyperalgesic responses. In particular, the periaqueductal grey, rostral ventromedial medulla, locus coeruleus and subnucleus reticularis dorsalis are important structures that directly or indirectly modulate nociceptive transmission at the primary nociceptive synapse. Substantial evidence from experimental animal studies suggests that plasticity within this system contributes to the initiation and/or maintenance of chronic neuropathic pain, and may even predispose individuals to developing chronic pain. Indeed, overwhelming evidence indicates that plasticity within this circuitry favours pro-nociception at the primary synapse in neuropathic pain conditions, a process that ultimately contributes to a hyperalgesic state. Although experimental animal investigations have been crucial in our understanding of the anatomy and function of the brainstem pain-modulation circuitry, it is vital to understand this system in acute and chronic pain states in humans so that more effective treatments can be developed. Recent functional MRI studies have identified a key role of this system during various analgesic and hyperalgesic responses including placebo analgesia, offset analgesia, attentional analgesia, conditioned pain modulation, central sensitisation and temporal summation. Moreover, recent MRI investigations have begun to explore brainstem pain-modulation circuitry plasticity in chronic neuropathic pain conditions and have identified altered grey matter volumes and functioning throughout the circuitry. Considering the findings from animal investigations, it is likely that these changes reflect a shift towards pro-nociception that ultimately contributes to the maintenance of neuropathic pain. The purpose of this review is to provide an overview of the human brain imaging investigations that have improved our understanding of the pain-modulation system in acute pain states and in neuropathic conditions. Our interpretation of the findings from these studies is often guided by the existing body of experimental animal literature, in addition to evidence from psychophysical investigations. Overall, understanding the plasticity of this system in human neuropathic pain conditions alongside the existing experimental animal literature will ultimately improve treatment options.

Monoaminergic and Opioidergic Modulation of Brainstem Circuits: New Insights Into the Clinical Challenges of Pain Treatment?

The treatment of neuropathic pain remains a clinical challenge. Analgesic drugs and antidepressants are frequently ineffective, and opioids may induce side effects, including hyperalgesia. Recent results on brainstem pain modulatory circuits may explain those clinical challenges. The dual action of noradrenergic (NA) modulation was demonstrated in animal models of neuropathic pain. Besides the well-established antinociception due to spinal effects, the NA system may induce pronociception by directly acting on brainstem pain modulatory circuits, namely, at the locus coeruleus (LC) and medullary dorsal reticular nucleus (DRt). The serotoninergic system also has a dual action depending on the targeted spinal receptor, with an exacerbated activity of the excitatory 5-hydroxytryptamine 3 (5-HT3) receptors in neuropathic pain models. Opioids are involved in the modulation of descending modulatory circuits. During neuropathic pain, the opioidergic modulation of brainstem pain control areas is altered, with the release of enhanced local opioids along with reduced expression and desensitization of μ-opioid receptors (MOR). In the DRt, the installation of neuropathic pain increases the levels of enkephalins (ENKs) and induces desensitization of MOR, which may enhance descending facilitation (DF) from the DRt and impact the efficacy of exogenous opioids. On the whole, the data discussed in this review indicate the high plasticity of brainstem pain control circuits involving monoaminergic and opioidergic control. The data from studies of these neurochemical systems in neuropathic models indicate the importance of designing drugs that target multiple neurochemical systems, namely, maximizing the antinociceptive effects of antidepressants that inhibit the reuptake of serotonin and noradrenaline and preventing desensitization and tolerance of MOR at the brainstem.

Evidence for 5-HT1A receptor-mediated antiallodynic and antihyperalgesic effects of apigenin in mice suffering from mononeuropathy

BACKGROUND AND PURPOSE

Neuropathic pain places a devastating health burden, with very few effective therapies. We investigated the potential antiallodynic and antihyperalgesic effects of apigenin, a natural flavonoid with momoamine oxidase (MAO) inhibitory activity, against neuropathic pain and investigated the mechanism(s).

EXPERIMENTAL APPROACH

The neuropathic pain model was produced by chronic constriction injury of sciatic nerves in male C57BL/6J mice, with pain-related behaviours being assayed by von Frey test and Hargreaves test. In this model the role of 5-HT and 5-HT_1A receptor-related mechanisms were investigated in vivo/in vitro.

KEY RESULTS

Apigenin repeated treatment (p.o., once per day for 2 weeks), in a dose-related manner (3, 10 and 30 mg·kg^-1 ), ameliorated the allodynia and hyperalgesia in chronic nerve constriction injury in mice. These effects seem dependent on neuronal 5-hydroxytryptamine, because (i) the antihyperalgesia and antiallodynia were attenuated by depletion of 5-HT with p-chlorophenylalanine and potentiated by 5-hydroxytryptophan and (ii), apigenin-treated chronic constriction injury mice caused an increased level of spinal 5-HT, associated with diminished MAO activity. In vivo administration, spinally or systematically, of the 5-HT_1A antagonist WAY-100635 inhibited the apigenin-induced antiallodynia and antihyperalgesia. In vitro, apigenin acted as a positive allosteric modulator to increase the efficacy (stimulation of [³⁵ S]GTPγS binding) of the 5-HT_1A agonist 8-OH-DPAT. Apigenin attenuated neuronal changes caused by chronic constriction of the sciatic nerve in mice, without causing a hypertensive crisis.

CONCLUSION AND IMPLICATIONS

Apigenin antiallodynic and antihyperalgesic actions against neuropathic pain crucially involve spinal 5-HT_1A receptors and indicate it could be used to treat neuropathic pain.

Pain in Parkinson's disease: Mechanism-based treatment strategies

PURPOSE OF REVIEW

Chronic pain, highly prevalent throughout the course of Parkinson's disease (PD), has been ranked as one of the top ten most bothersome symptoms people with Parkinson's (PwP) are experiencing. Yet, robust evidence-based treatment strategies are lacking. This unmet need is partly attributable to the multifaceted nature of PD-related pain, which results in part from a complex and poorly understood interplay involving a range of neurotransmitter pathways. Degeneration of nigrostriatal dopaminergic pathways and alterations of central nervous system extra-striatal dopaminergic, noradrenergic, serotoninergic, glutamatergic, opioidergic and endocannabinoid circuits may all promote a heightened experience of pain in PwP. Thus, the potential targets for mechanism-based pain-relieving strategies in PwP are several. These targets are discussed herein.

RECENT FINDINGS

An increasing number of clinical trials and experimental studies in animal models of PD are being designed with the aim of addressing the pathophysiological mechanism(s) underlying PD-related pain. Overall, recent research findings highlight the analgesic effects of dopaminergic and opioidergic medication for certain subtypes of pain in PwP, whereas proposing novel strategies that involve targeting other neurotransmitter pathways.

SUMMARY

The origin of pain in PwP remains under investigation. Although our understanding of the mechanisms underpinning persistent pain in PD has improved in recent years, this has not yet translated to clinical alleviation of this most troublesome nonmotor symptom. Patient stratification linked with evidence-based personalized pain-treatment plans for optimal analgesic relief will rely on advances in our understanding of the dopaminergic and nondopaminergic targets outlined in this review.

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.

Therapeutic Potential of Polyphenols in the Management of Diabetic Neuropathy

Diabetic neuropathy (DN) is a common and serious diabetes-associated complication that primarily takes place because of neuronal dysfunction in patients with diabetes. Use of current therapeutic agents in DN treatment is quite challenging because of their severe adverse effects. Therefore, there is an increased need of identifying new safe and effective therapeutic agents. DN complications are associated with poor glycemic control and metabolic imbalances, primarily oxidative stress (OS) and inflammation. Various mediators and signaling pathways such as glutamate pathway, activation of channels, trophic factors, inflammation, OS, advanced glycation end products, and polyol pathway have a significant contribution to the progression and pathogenesis of DN. It has been indicated that polyphenols have the potential to affect DN pathogenesis and could be used as potential alternative therapy. Several polyphenols including kolaviron, resveratrol, naringenin, quercetin, kaempferol, and curcumin have been administered in patients with DN. Furthermore, chlorogenic acid can provide protection against glutamate neurotoxicity via its hydrolysate, caffeoyl acid group, and caffeic acid through regulating the entry of calcium into neurons. Epigallocatechin-3-gallate treatment can protect motor neurons by regulating the glutamate level. It has been demonstrated that these polyphenols can be promising in combating DN-associated damaging pathways. In this article, we have summarized DN-associated metabolic pathways and clinical manifestations. Finally, we have also focused on the roles of polyphenols in the treatment of DN.

Although tapentadol and oxycodone both increase colonic volume, tapentadol treatment resulted in softer stools and less constipation: a mechanistic study in healthy volunteers

OBJECTIVES

Opioids are often used in treatment of severe pain, although many patients experience gastrointestinal side-effects like constipation. The aim of the current study was to investigate changes in colonic volume, as the result of both colonic motility and fluid transport, in healthy volunteers during opioid treatment with tapentadol as compared with oxycodone and placebo.

METHODS

In a randomized, double-blind, cross-over study, 21 healthy male volunteers were administered equianalgesic dosages of oral tapentadol (50 mg bid), oxycodone (10 mg bid) or corresponding placebo for 14 days. Segmental colonic volumes were quantified using T2-weighted magnetic resonance images, and gastrointestinal side-effects were assessed with questionnaires.

RESULTS

Total colonic volume increase during treatment was higher during tapentadol and oxycodone treatment (median 48 and 58 mL) compared to placebo (median -14 mL, both p≤0.003). Tapentadol (and placebo) treatment resulted in more bowel movements (both p<0.05) and softer stool consistency as compared with oxycodone (both p<0.01). Only oxycodone treatment was associated with increased constipation, straining during defecation, and tiredness (all p≤0.01). The colonic volume increase during treatment was directly associated with softer stools during tapentadol treatment (p=0.019).

CONCLUSIONS

Tapentadol treatment increased colonic volume without leading to harder stools, likely as the opioid sparing effects result in less water absorption from the gut lumen. Oxycodone treatment also increased colonic volume, but with a simultaneous increase in stool dryness and gastrointestinal and central nervous system side-effects. The results confirm that tapentadol treatment may be advantageous to oxycodone regarding tolerability to pain treatment.

Offset analgesia and onset hyperalgesia with different stimulus ranges

A comparison between the effects of offset analgesia and onset hyperalgesia and how these effects relate to the stimulus range of thermal stimulation.

Introduction:

Offset analgesia (OA), a large reduction in pain after a brief increase in intensity of an otherwise stable painful stimulus, has been established by a large body of research. But the opposite effect, onset hyperalgesia (OH), a disproportional hyperalgesic response after a briefly decreased intensity of a painful stimulus, has only been investigated in one previous study.

Objectives:

The aim of this study was to induce OA and OH in healthy participants and explore the effects of different stimulus ranges (increase/decrease of temperature) on OA and OH.

Methods:

A total of 62 participants were tested in 2 identical experiments. Offset analgesia and OH conditions included 2 different temperature deviations (±1°C/±2°C) from initial temperature and were compared with a constant temperature (control).

Results:

Offset analgesia was successfully elicited in OA_1°C in experiment 1, and in OA_1°C and OA_2°C in experiment 2. Results indicate a continuous stimulus–response relationship between the stimulus range and the resulting hypoalgesic response. Onset hyperalgesia was only elicited in OH_2°C in experiment 1. Exploratory analysis showed that the lack of OH response in experiment 2 could be explained by sex differences, and that OA and OH responses were only weakly correlated.

Conclusions:

The asymmetry between pain responses after a brief temperature increase and decrease suggests that different mechanisms are involved in the pain responses to increasing and decreasing temperature. This asymmetry may also be explained by high temperatures in OA condition (+1°C/+2°C above baseline) that could be seen as salient “learning signals,” which augment the response to following changes in temperature.