Opioid-induced Hyperalgesia in Humans: Molecular Mechanisms and Clinical Considerations

Influence of psychological factors and pain sensitivity on the efficacy of opioid-free anesthesia: A randomized clinical trial

OBJECTIVE

This study aimed to investigate the effects of opioid-free anesthesia (OFA) in laparoscopic gastrectomy and identify the psychological factors that could influence the efficacy of OFA.

METHOD

120 patients undergoing laparoscopic gastrectomy were allocated to either the opioid-based anesthesia group (OA) (n = 60) or the OFA (n = 60) group. Remifentanil was administered to the OA group intraoperatively, whereas dexmedetomidine and lidocaine were administered to the OFA group. The interaction effect of the psychological factors on OFA was analyzed using the aligned rank transform for nonparametric factorial analyses.

RESULTS

The opioid requirement for 24 h after surgery was lower in the OFA group than in the OA group (fentanyl equivalent dose 727 vs. 650 μg, p = 0.036). The effect of OFA was influenced by the pain catastrophizing scale (p = 0.041), temporal pain summation (p = 0.046), and pressure pain tolerance (p = 0.034). This indicates that patients with pain catastrophizing or high pain sensitivity significantly benefited from OFA, whereas patients without these characteristics did not.

CONCLUSIONS

This study demonstrated that OFA with dexmedetomidine and lidocaine effectively reduced the postoperative 24-h opioid requirements following laparoscopic gastrectomy, which was modified by baseline pain catastrophizing and pain sensitivity.

CLINICAL TRIAL REGISTRY

The study protocol was approved by the Institutional Review Board of Yonsei University Health System Gangnam Severance Hospital (#3-2021-0295) and registered at ClinicalTrials.gov (NCT05076903).

Small G-Protein Rheb Gates Mammalian Target of Rapamycin Signaling to Regulate Morphine Tolerance in Mice

BACKGROUND

Analgesic tolerance due to long-term use of morphine remains a challenge for pain management. Morphine acts on μ-opioid receptors and downstream of the phosphatidylinositol 3-kinase signaling pathway to activate the mammalian target of rapamycin (mTOR) pathway. Rheb is an important regulator of growth and cell-cycle progression in the central nervous system owing to its critical role in the activation of mTOR. The hypothesis was that signaling via the GTP-binding protein Rheb in the dorsal horn of the spinal cord is involved in morphine-induced tolerance.

METHODS

Male and female wild-type C57BL/6J mice or transgenic mice (6 to 8 weeks old) were injected intrathecally with saline or morphine twice daily at 12-h intervals for 5 consecutive days to establish a tolerance model. Analgesia was assessed 60 min later using the tail-flick assay. After 5 days, the spine was harvested for Western blot or immunofluorescence analysis.

RESULTS

Chronic morphine administration resulted in the upregulation of spinal Rheb by 4.27 ± 0.195-fold (P = 0.0036, n = 6), in turn activating mTOR by targeting rapamycin complex 1 (mTORC1). Genetic overexpression of Rheb impaired morphine analgesia, resulting in a tail-flick latency of 4.65 ± 1.10 s (P < 0.0001, n = 7) in Rheb knock-in mice compared to 10 s in control mice (10 ± 0 s). Additionally, Rheb overexpression in spinal excitatory neurons led to mTORC1 signaling overactivation. Genetic knockout of Rheb or inhibition of mTORC1 signaling by rapamycin potentiated morphine-induced tolerance (maximum possible effect, 52.60 ± 9.56% in the morphine + rapamycin group vs. 16.60 ± 8.54% in the morphine group; P < 0.0001). Moreover, activation of endogenous adenosine 5'-monophosphate-activated protein kinase inhibited Rheb upregulation and retarded the development of morphine-dependent tolerance (maximum possible effect, 39.51 ± 7.40% in morphine + metformin group vs. 15.58 ± 5.79% in morphine group; P < 0.0001).

CONCLUSIONS

This study suggests spinal Rheb as a key molecular factor for regulating mammalian target of rapamycin signaling.

EDITOR’S PERSPECTIVE

Characterization of Opioid Use in the Intensive Care Unit and Its Impact Across Care Transitions: A Prospective Study

Purpose: The objective of this study is to characterize opioid intensity in the intensive care unit (ICU) and its association with opioid utilization across care transitions. Methods: This is a prospective cohort study. Medically ill ICU patients with complete medication histories who survived to discharge were included. Opioid intensity was characterized based on IV morphine milligram equivalents (IV MME). Primary outcomes were opioid prescribing upon ICU and hospital discharge. Results: Opioids were prescribed to 34.1% and 31.1% of patients upon ICU and hospital discharge. Within the ≥50 mean IV MME/ICU day cohort, 64.7% of patients received opioids after ICU discharge compared to 45.8% and 13.6% in the 1-49 mean IV MME/ICU day and no opioid groups (P < .05). Within the ≥50 mean IV MME/ICU day cohort, 70.6% of patients were prescribed opioids after hospitalization compared to 37.3% and 13.6% of patients who received less or no opioids. (P < .05). Within the ≥50 mean IV MME/ICU day cohort, 29.4% of patients were opioid naïve and discharged with an opioid, which is over double compared to patients with lower opioid requirements (P < .05). Conclusion: Patients with higher mean daily ICU opioid requirements had increased opioid prescribing across care transitions despite preadmission opioid use.

Patient-level patterns in daily prescribed opioid dosage in single level lumbar fusion are associated with postoperative opioid dosage and adverse events: a retrospective analysis of claims data

BACKGROUND

Patients undergoing lumbar spine surgery have high rates of preoperative opioid use, which is associated with inferior outcomes and higher risks for opioid dependency postoperatively.

PURPOSE

Determine whether there are identifiable subgroups of patients that follow distinct patterns in pre- and postoperative opioid dosing. Examine how preoperative patterns in opioid dosing relate to postoperative opioid patterns, opioid cessation, and the risk for adverse events.

STUDY DESIGN/SETTING

Retrospective analysis of an administrative claims database (MeritiveTM Marketscan® Research Databases 2007-2015).

PATIENT SAMPLE

The 9,768 patients undergoing primary single level lumbar fusion.

OUTCOME MEASURES

Primary: daily morphine milligram equivalent (MME) opioid dosing calculated from prescriptions dispensed for 1 year before and after surgery; secondary: 90-day all-cause readmission and complications, 90-day acute postoperative pain, 90-day and 1-year reoperation, surgical costs, length of stay, and discharge disposition.

METHODS

Distinct patient subgroups defined by patterns of daily MME pre- and postoperatively were identified via group-based trajectory modeling. Associations between these groups and outcomes were assessed with multivariable logistic regression with risk adjustment for patient and surgical factors.

RESULTS

Among primary single level lumbar fusion patients, 59.5% filled an opioid prescription in the 3 months preceding surgery, whereas 40.5% were opioid naïve (Naïve). Five distinct subgroups of daily MME were identified among those filling opioids preoperatively: (1) Naïve to 3m (21.2% of patients): no opioids until 3 months preoperatively, escalating to 15 MME/day; (2) Low to 3m (11.4%): very low or as needed dose until 3 months preoperatively, escalating to 15 MME/day; (3) 6m Rise (6.9%): no opioids until 6 months preoperatively, escalating to >30 MME/day; (4) Medium (9.8%): increased linearly from 10 to 25 MME/day across the year before surgery; (5) High (10.0%): increased linearly from 60 to >80 MME/day across the year before surgery. These five preoperative opioid groups were related to postoperative opioids filled in a dose-response manner. The two preoperative patient groups with chronic Medium to High-dose opioid dosing were associated with increased adverse events, including all-cause readmission, reoperation, and pneumonia, whereas a low baseline group with a large, earlier preoperative rise in opioid dosing (6m Rise) had increased encounters for acute postoperative pain. Postoperatively, only 9.5% of patients did not fill an opioid prescription. Five distinct postoperative subgroups were identified based on their patterns in daily MME: Two groups ceased filling opioids within the year following surgery (33.6% of patients), and three groups declined in opioid dosage following surgery but plateaued at low (0-5 MME/day, 29.1%), medium (10-15 MME/day, 12.0%), or high (70-75 MME/day), 13.1%) doses by 1 year. Patients within the higher preoperative opioid groups were more likely to belong to the postoperative groups that were unable to cease filling opioids.

CONCLUSIONS

Identification of a patient's pre-operative time trend in daily opioid use may provide significant prognostic value and help guide pain management and risk reduction efforts.

LEVEL OF EVIDENCE

III.

Nociceptive Flexion Reflex Threshold is No Suitable Marker for Diagnosing Opioid-Induced Hyperalgesia

Background

Opioid induced hyperalgesia (OIH) describes a state of altered pain sensation due to opioid exposure. It often occurs among persons with opioid use disorder receiving substitution therapy.

Methods

The purpose of this study was to find out, whether OIH diagnosis could be facilitated by an objective pain indicating marker: the Nociceptive Flexion Reflex (NFR). Forty persons with opioid use disorder, 20 of them maintained on methadone and 20 treated with buprenorphine, as well as a control group of 20 opioid-free subjects, were examined. It was aimed to find out whether and in which way these opioid agonists alter reflex threshold (NFR-T). A cold-pressor test was performed to investigate the prevalence of OIH. Furthermore, electrical stimulation and electromyography analyzation were used for NFR-T measurement. Subjective pain ratings were evaluated with a numeric rating scale.

Results

Significantly increased sensitivity to cold pressor pain was found in both maintenance groups when compared to their opioid-free counterparts (p < 0.001). Neither methadone nor buprenorphine showed any effect on NFR-T. This might be explained by the reflex approaching at the wrong location in the central nervous system. Consequently, NFR-T is not a suitable marker for diagnosing OIH.

Conclusion

Although methadone and buprenorphine have been proven to cause OIH, no effect on NFR-T was observed. A statistically significant effect could have been observed with a larger number of participants. Further research, with special focus on patients' adjuvant medication, should be conducted in the future, to facilitate diagnosis of OIH and provide appropriate pain management for maintenance patients.

Mortality and health-related quality of life in older adults with long-term use of opioids, z-hypnotics or benzodiazepines: a prospective observational study at 5 years follow-up

OBJECTIVES

Disease and medication use in older age is a consequence of age-related declining health. Multimorbidity followed by polypharmacy is common. Central nervous system depressing (CNSD) drugs such as opioids, benzodiazepines and z-hypnotics are not recommended for long-term use in older adults but are in use by many. We aimed to assess mortality and change in health-related quality of life (HRQoL) in older adults with long-term use of CNSDs.

METHOD

A prospective observational study was conducted at Akershus University Hospital, Norway, 2017-2019, with follow-up in 2021-2022, including 246 participants aged 65-90. At 5-year follow-up, 78 (32%) participants had passed away. Mortality data were collected from patient electronic health records. Of the surviving 168 (68%), we collected further follow-up data from 38 (16%) participants. Follow-up included demographic and clinical data. The EuroQuol Group EQ-5D-5L questionnaire was used to measure HRQoL. Analysis include Cox regression model for survival data and linear mixed model for change in HRQoL over time.

RESULTS

At follow-up, 78 (31.7%) were deceased. Mean survival time was 3.3 years. Total time for survival data was 4.7 years. Mortality was higher among participants with long-term use of CNSD (HR 1.9 95% CI (1.2 to 3.2), p=0.01). The multivariable analysis found being older (HR 1.1 95% CI (1.0 to 1.1), p=0.020) and male sex (HR 2.1 95% CI (1.2 to 3.5), p=0.008) to be associated with increased risk of mortality. According to the linear mixed model (n=38), there was no significant difference between surviving users and non-users in change in HRQoL EQ-5D-5L index from baseline to follow-up.

CONCLUSION

Mortality was higher for long-term users of CNSDs at 5-year follow-up. Being older and male sex were associated with mortality. Among survivors, there was no significant difference between the groups in change of HRQoL over time.

TRIAL REGISTRATION NUMBER

NCT03162081; 22 May 2017.

Management of Critically Ill Patients Receiving Medications for Opioid Use Disorder

TOPIC IMPORTANCE

Critical care clinicians are likely to see an increasing number of patients admitted to the ICU who are receiving US Food and Drug Administration-approved medications for opioid use disorder (MOUDs) given the well-documented benefits of these agents. Oral methadone, multiple formulations of buprenorphine, and extended-release naltrexone are the three types of MOUD most likely to be encountered by ICU clinicians; however, these drugs vary with respect to formulations, pharmacokinetics, and adverse effects.

REVIEW FINDINGS

No published clinical practice guidelines or consensus statements are available to guide decision-making in patients admitted to the ICU setting who are receiving MOUDs before admission. Additionally, no randomized trials and limited observational studies have evaluated issues related to MOUD use in the ICU. Therefore, ICU clinicians caring for patients admitted who are taking MOUDs must base their decision-making on data extrapolation from pharmacokinetic, pharmacologic, and clinical studies performed in non-ICU settings.

SUMMARY

Despite the challenges in administering MOUDs in critically ill patients, extrapolation of data from other hospital settings suggests that the benefits of continuing MOUD therapy outweigh the risks in patients able to continue therapy. This article provides guidance for critical care clinicians caring for patients admitted to the ICU already receiving methadone, buprenorphine, or extended-release naltrexone. The guidance includes algorithms to aid clinicians in the clinical decision-making process, recognizing the inherent limitations of the existing evidence on which the algorithms are based and the need to account for patient-specific considerations.

The Roles of Endogenous Opioids in Placebo and Nocebo Effects: From Pain to Performance to Prozac

Placebo and nocebo effects have been well documented for nearly two centuries. However, research has only relatively recently begun to explicate the neurobiological underpinnings of these phenomena. Similarly, research on the broader social implications of placebo/nocebo effects, especially within healthcare delivery settings, is in a nascent stage. Biological and psychosocial outcomes of placebo/nocebo effects are of equal relevance. A common pathway for such outcomes is the endogenous opioid system. This chapter describes the history of placebo/nocebo in medicine; delineates the current state of the literature related to placebo/nocebo in relation to pain modulation; summarizes research findings related to human performance in sports and exercise; discusses the implications of placebo/nocebo effects among diverse patient populations; and describes placebo/nocebo influences in research related to psychopharmacology, including the relevance of endogenous opioids to new lines of research on antidepressant pharmacotherapies.

Opioid infusions at different times of the day produce varying degrees of opioid-induced hyperalgesia

BACKGROUND

Opioids are metabolised by enzymes the activities of which vary with the circadian rhythm. We examined whether opioid infusions administered at different times of the day produce varying degrees of opioid-induced hyperalgesia (OIH) in animal experiments and clinical studies.

METHODS

Male Sprague-Dawley rats received remifentanil infusions (1 μg kg-1·min-1 for 1 h) at Zeitgeber times (ZT) 0, 4, 8, 12, 16, or 20 h. Rhythmicity of mechanical hypersensitivity was assayed after the infusion. Mechanical hypersensitivity, drug concentration, and metabolic enzyme activity of Wistar rats that received sufentanil (10 μg kg-1; four consecutive i.p. injections at 15-min intervals) or remifentanil infusion at ZT0 or ZT8 were assayed. Sixty patients who underwent abdominal laparoscopic surgery under general anaesthesia received remifentanil infusion (0.15 μg kg-1 min-1) and sufentanil injection (0.2 μg kg-1) at induction and skin incision, respectively. Postoperative pressure pain sensitivity, pain Numeric Rating Scale (NRS), drug concentrations, and nonspecific esterase activity were assessed.

RESULTS

Sprague-Dawley rats that received remifentanil infusion exhibited a robust rhythmic paw withdrawal threshold (JTK_CYCLE: P=0.001, Q=0.001, Phase=26). Wistar rats infused with remifentanil or sufentanil at ZT8 exhibited greater OIH (P<0.001) than those infused at ZT0, with higher blood concentrations (P<0.001) and lower metabolic enzyme activities (P=0.026 and P=0.028, respectively). Patients in the afternoon group exhibited higher pressure pain sensitivity at forearm (P=0.002), higher NRS (P<0.05), higher drug concentrations (sufentanil: P=0.037, remifentanil: P=0.005), and lower nonspecific esterase activity (P=0.024) than the morning group.

CONCLUSIONS

Opioid infusions administered at different times of day produced varying degrees of OIH, possibly related to circadian rhythms of metabolic enzyme activities.

CLINICAL TRIAL REGISTRATION

NCT05234697.

Use of Psychedelics for Pain: A Scoping Review

Chronic pain is a public health concern that affects approximately 1.5 billion people globally. Conventional therapeutic agents including opioid and non-opioid analgesics have been associated with adverse side effects, issues with addiction, and ineffective analgesia. Novel agents repurposed to treat pain via different mechanisms are needed to fill the therapeutic gap in chronic pain management. Psychedelics such as lysergic acid diethylamide and psilocybin (the active ingredient in psychedelic mushrooms) are thought to alter pain perception through direct serotonin receptor agonism, anti-inflammatory effects, and synaptic remodeling. This scoping review was conducted to identify human studies in which psychedelic agents were used for the treatment of pain. Twenty-one articles that assessed the effects of psychedelics in treating various pain states were included. The present scarcity of clinical trials and small sample sizes limit their application for clinical use. Overall, psychedelics appear to show promise for analgesia in patients with certain headache disorders and cancer pain diagnoses. Future studies must aim to examine the combined effects of psychotherapy and psychedelics on chronic pain.

Epigenetic regulation in opioid induced hyperalgesia

About 25 million American adults experience pain daily and one of the most commonly prescribed drugs to treat pain are opioids. Prolonged opioid usage and dose escalations can cause a paradoxical response where patients experience enhanced pain sensitivity. This opioid induced hyperalgesia (OIH) is a major hurdle when treating pain in the clinic because its underlying mechanisms are still not fully understood. OIH is also commonly overlooked and lacks guidelines to prevent its onset. Research on pain disorders and opioid usage have recognized potential epigenetic drivers of disease including DNA methylation, histone modifications, miRNA regulation, but their involvement in OIH has not been well studied. This article discusses epigenetic changes that may contribute to pathogenesis, with an emphasis on miRNA alterations in OIH. There is a crucial gap in knowledge including how multiple epigenetic modulators contribute to OIH. Elucidating the epigenetic changes underlying OIH and the crosstalk among these mechanisms could lead to the development of novel targets for the prevention and treatment of this painful phenomena.

EEG-derived pain threshold index-guided versus standard care during propofol-remifentanil anesthesia: A randomized controlled trial

Purpose

The pain threshold index (PTI), a novel index of nociception based on spontaneous EEG wavelet analysis, has been reported to provide reliable accuracy for predicting postoperative pain and hemodynamic reactivity. The present study is aimed to investigate whether PTI-guided analgesia reduces the pain intensity and rate of remedial analgesia in the post-anesthesia care unit (PACU).

Methods

A total of 122 females undergoing elective gynecologic surgeries had been randomized to receive either PTI-guided analgesia (PTI group) or standard clinical care (control group). Remifentanil administration in the PTI group was guided by PTI to maintain the value between 40 and 65, while that in the control group was guided by hemodynamic changes. The primary outcome was remedial analgesia rate in the PACU. The postoperative pain scores, intraoperative remifentanil requirements, opioid-related adverse events and perioperative serum stress hormone concentrations between the two groups were also compared.

Findings

It was found that 23 of 58 patients (40%) in the control group and 8 of 58 patients (14%) in the PTI group needed remedial analgesia. The relative risk of receiving remedial analgesia was 2.88 (95% CI, 1.40-5.89, P = 0.002) in the control group. Sufentanil consumption in the PACU (μg) was lower in the PTI group (P = 0.002) than in the control group. Remifentanil and propofol consumption, opioid-related adverse events between these two groups were comparable.

Implications

PTI-guided analgesia during gynaecologic operations resulted in 25.87% less remedial analgesia. However, studies with different PTI thresholds and larger, more diverse populations should be conducted to further demonstrate the clinical effectiveness of PTI.

Perioperative opioid-minimization approach as a useful protocol in the management of patients with Ehlers-Danlos syndrome-hypermobility type, craniocervical instability and severe chronic pain who are to undergo occipito-cervical fixation

Patients suffering from connective tissue disorders like Ehlers-Danlos syndrome hypermobility type/joint hypermobility syndrome (EDS-HT/JHS) may be affected by craniocervical instability (CCI). These patients experience myalgic encephalomyelitis, chronic fatigue, depression, extreme occipital-cervical pain, and severe widespread pain that is difficult to relieve with opioids. This complex and painful condition can be explained by the development of chronic neuroinflammation, opioid-induced hyperalgesia, and central sensitization. Given the challenges in treating such severe physical pain, we evaluated all the analgesic methods previously used in the perioperative setting, and updated information was presented. It covers important physiopathological aspects for the perioperative care of patients with EDS-HT/JHS and CCI undergoing occipital-cervical/thoracic fixation/fusion. Moreover, a change of paradigm from the current opioid-based management of anesthesia/analgesia in these patients to the perioperative opioid minimization strategies used by the authors was analyzed and proposed as follow-up considerations from our previous case series. These strategies are based on total-intravenous opioid-free anesthesia, multimodal analgesia, and a postoperative combination of anti-hyperalgesic coadjuvants (lidocaine, ketamine, and dexmedetomidine) with an opioid-sparing effect.

The mechanisms and management of persistent postsurgical pain

An estimated 10%-50% of patients undergoing a surgical intervention will develop persistent postsurgical pain (PPP) lasting more than 3 months despite adequate acute pain management and the availability of minimally invasive procedures. The link between early and late pain outcomes for surgical procedures remains unclear-some patients improve while others develop persistent pain. The elective nature of a surgical procedure offers a unique opportunity for prophylactic or early intervention to prevent the development of PPP and improve our understanding of its associated risk factors, such as pre-operative anxiety and the duration of severe acute postoperative pain. Current perioperative pain management strategies often include opioids, but long-term consumption can lead to tolerance, addiction, opioid-induced hyperalgesia, and death. Pre-clinical models provide the opportunity to dissect mechanisms underpinning the transition from acute to chronic, or persistent, postsurgical pain. This review highlights putative mechanisms of PPP, including sensitisation of peripheral sensory neurons, neuroplasticity in the central nervous system and nociceptive signalling along the neuro-immune axis.

Preclinical evidence for the use of the atypical antipsychotic, brexpiprazole, for opioid use disorder

Opioid addiction is characterized by adaptations in the mesolimbic dopamine system that occur during chronic opioid use. Alterations in dopaminergic transmission contribute to pathological drug-seeking behavior and other symptoms associated with opioid withdrawal following drug discontinuation, making drug abstinence challenging and contributing to high rates of relapse among those suffering from substance use disorder. Recently, the use of dopamine partial agonists has been proposed as a potential strategy to restore dopaminergic signalling during drug withdrawal, while avoiding the adverse side effects associated with stronger modulators of dopaminergic transmission. We investigated the effects of the atypical antipsychotic brexpiprazole, which is a partial agonist at dopamine D2 and D3 receptors, in a mouse model of opioid dependence. The development of opioid dependence in mice is characterized by locomotor sensitization, analgesic tolerance, opioid-induced hyperalgesia, and drug-seeking behavior. We set up four paradigms to model the effects of brexpiprazole on each of these adaptations that occur during chronic opioid use in male and female C57BL/6J mice. Concomitant treatment of brexpiprazole during chronic morphine administration attenuated the development of locomotor sensitization. Brexpiprazole treatment abolished morphine place preference and blocked reinstatement of this behavior following extinction. Brexpiprazole treatment did not alter morphine analgesia, nor did it impact the development of morphine tolerance. However, brexpiprazole treatment did prevent the expression of opioid-induced hyperalgesia in a tail-withdrawal assay, while failing to improve somatic withdrawal symptoms. Altogether, these results provide preclinical evidence for the efficacy of brexpiprazole as a modulator of dopamine-dependent behaviors during opioid use and withdrawal.

Engineering human spinal microphysiological systems to model opioid-induced tolerance

pioids are commonly used for treating chronic pain. However, with continued use, they may induce tolerance and/or hyperalgesia, which limits therapeutic efficacy. The human mechanisms of opioid-induced tolerance and hyperalgesia are significantly understudied, in part, because current models cannot fully recapitulate human pathology. Here, we engineered novel human spinal microphysiological systems (MPSs) integrated with plug-and-play neural activity sensing for modeling human nociception and opioid-induced tolerance. Each spinal MPS consists of a flattened human spinal cord organoid derived from human stem cells and a 3D printed organoid holder device for plug-and-play neural activity measurement. We found that the flattened organoid design of MPSs not only reduces hypoxia and necrosis in the organoids, but also promotes their neuron maturation, neural activity, and functional development. We further demonstrated that prolonged opioid exposure resulted in neurochemical correlates of opioid tolerance and hyperalgesia, as measured by altered neural activity, and downregulation of μ-opioid receptor expression of human spinal MPSs. The MPSs are scalable, cost-effective, easy-to-use, and compatible with commonly-used well-plates, thus allowing plug-and-play measurements of neural activity. We believe the MPSs hold a promising translational potential for studying human pain etiology, screening new treatments, and validating novel therapeutics for human pain medicine.

Preclinical study in a postoperative pain model to investigate the action of ketamine, lidocaine, and ascorbic acid in reversing fentanyl-induced, non-glutamate-dependent hyperalgesia

Introduction

Opioid-induced hyperalgesia (OIH) is a paradoxical phenomenon in which exposure to opioids can increase sensitivity to painful stimuli. Currently, several drugs have been used in an attempt to prevent OIH. We design this study to address the effect of preemptive treatment with ketamine, lidocaine, and ascorbic acid in a rat preclinical model of perioperative opioid-induced hyperalgesia.

Methods

To reproduce OIH in a model of postoperative pain, rats received successive doses of fentanyl subcutaneously and underwent an incision in the paw. In an attempt to prevent OIH, ketamine, lidocaine, and ascorbic acid were administered before treatment with fentanyl. The von Frey test and the hot-plate test were used to evaluate mechanical allodynia and thermal hyperalgesia, respectively, with a follow-up period from 1 hour up to 7 days after surgery. Spinal cord nerve terminals (synaptosomes) were used to assess glutamate release under our experimental conditions.

Results

Consecutive fentanyl injections increased the postoperative pain as indicated by increased thermal hyperalgesia and allodynia 48 hours after incision. Ketamine, lidocaine, and the combination of ketamine + lidocaine were able to prevent thermal hyperalgesia but not mechanical allodynia. Ascorbic acid did not prevent the hyperalgesia induced by fentanyl. We found no correlation between spinal glutamate release and the pharmacological treatments.

Conclusion

Fentanyl induced a hyperalgesic effect that last few days in a postoperative model of pain. Hyperalgesic effect was not totally inhibited by ketamine and lidocaine in rats. Increased glutamate release was not the main molecular mechanism of fentanyl-induced hyperalgesia.

Changes in Psychological Outcomes after Cessation of Full Mu Agonist Long-Term Opioid Therapy for Chronic Pain

Improved understanding of psychological features associated with full mu agonist long-term opioid therapy (LTOT) cessation may offer advantages for clinicians. This preliminary study presents changes in psychological outcomes in patients with chronic, non-cancer pain (CNCP) after LTOT cessation via a 10-week multidisciplinary program which included treatment with buprenorphine. Paired t-tests pre- and post-LTOT cessation were compared in this retrospective cohort review of data from electronic medical records of 98 patients who successfully ceased LTOT between the dates of October 2017 to December 2019. Indicators of quality of life, depression, catastrophizing, and fear avoidance, as measured by the 36-Item Short Form Survey, the Patient Health Questionnaire-9-Item Scale, the Pain Catastrophizing Scale, and the Fear Avoidance Belief Questionnaires revealed significant improvement. Scores did not significantly improve for daytime sleepiness, generalized anxiety, and kinesiophobia, as measured by the Epworth Sleepiness Scale, the Generalized Anxiety Disorder 7-Item Scale, and the Tampa Scale of Kinesiophobia. The results suggest that successful LTOT cessation may be interconnected with improvements in specific psychological states.

Development of opioid-induced hyperalgesia depends on reactive astrocytes controlled by Wnt5a signaling

Opioids are the frontline analgesics for managing various types of pain. Paradoxically, repeated use of opioid analgesics may cause an exacerbated pain state known as opioid-induced hyperalgesia (OIH), which significantly contributes to dose escalation and consequently opioid overdose. Neuronal malplasticity in pain circuits has been the predominant proposed mechanism of OIH expression. Although glial cells are known to become reactive in OIH animal models, their biological contribution to OIH remains to be defined and their activation mechanism remains to be elucidated. Here, we show that reactive astrocytes (a.k.a. astrogliosis) are critical for OIH development in both male and female mice. Genetic reduction of astrogliosis inhibited the expression of OIH and morphine-induced neural circuit polarization (NCP) in the spinal dorsal horn (SDH). We found that Wnt5a is a neuron-to-astrocyte signal that is required for morphine-induced astrogliosis. Conditional knock-out of Wnt5a in neurons or its co-receptor ROR2 in astrocytes blocked not only morphine-induced astrogliosis but also OIH and NCP. Furthermore, we showed that the Wnt5a-ROR2 signaling-dependent astrogliosis contributes to OIH via inflammasome-regulated IL-1β. Our results reveal an important role of morphine-induced astrogliosis in OIH pathogenesis and elucidate a neuron-to-astrocyte intercellular Wnt signaling pathway that controls the astrogliosis.

Integrating mechanistic-based and classification-based concepts into perioperative pain management: an educational guide for acute pain physicians

Chronic pain begins with acute pain. Physicians tend to classify pain by duration (acute vs chronic) and mechanism (nociceptive, neuropathic and nociplastic). Although this taxonomy may facilitate diagnosis and documentation, such categories are to some degree arbitrary constructs, with significant overlap in terms of mechanisms and treatments. In clinical practice, there are myriad different definitions for chronic pain and a substantial portion of chronic pain involves mixed phenotypes. Classification of pain based on acuity and mechanisms informs management at all levels and constitutes a critical part of guidelines and treatment for chronic pain care. Yet specialty care is often siloed, with advances in understanding lagging years behind in some areas in which these developments should be at the forefront of clinical practice. For example, in perioperative pain management, enhanced recovery protocols are not standardized and tend to drive treatment without consideration of mechanisms, which in many cases may be incongruent with personalized medicine and mechanism-based treatment. In this educational document, we discuss mechanisms and classification of pain as it pertains to commonly performed surgical procedures. Our goal is to provide a clinical reference for the acute pain physician to facilitate pain management decision-making (both diagnosis and therapy) in the perioperative period.

Intrathecal minocycline does not block the adverse effects of repeated, intravenous morphine administration on recovery of function after SCI

Opioids are among the most effective analgesics for the management of pain in the acute phase of a spinal cord injury (SCI), and approximately 80% of patients are treated with morphine in the first 24 h following SCI. We have found that morphine treatment in the first 7 days after SCI increases symptoms of pain at 42 days post-injury and undermines the recovery of locomotor function in a rodent model. Prior research has implicated microglia/macrophages in opioid-induced hyperalgesia and the development of neuropathic pain. We hypothesized that glial activation may also underlie the development of morphine-induced pain and cell death after SCI. Supporting this hypothesis, our previous studies found that intrathecal and intravenous morphine increase the number of activated microglia and macrophages present at the spinal lesion site, and that the adverse effects of intrathecal morphine can be blocked with intrathecal minocycline. Recognizing that the cellular expression of opioid receptors, and the intracellular signaling pathways engaged, can change with repeated administration of opioids, the current study tested whether minocycline was also protective with repeated intravenous morphine administration, more closely simulating clinical treatment. Using a rat model of SCI, we co-administered intravenous morphine and intrathecal minocycline for the first 7 days post injury and monitored sensory and locomotor recovery. Contrary to our hypothesis and previous findings with intrathecal morphine, we found that minocycline did not prevent the negative effects of morphine. Surprisingly, we also found that intrathecal minocycline alone is detrimental for locomotor recovery after SCI. Using ex vivo cell cultures, we investigated how minocycline and morphine altered microglia/macrophage function. Commensurate with published studies, we found that minocycline blocked the effects of morphine on the release of pro-inflammatory cytokines but, like morphine, it increased glial phagocytosis. While phagocytosis is critical for the removal of cellular and extracellular debris at the spinal injury site, increased phagocytosis after injury has been linked to the clearance of stressed but viable neurons and protracted inflammation. In sum, our data suggest that both morphine and minocycline alter the acute immune response, and reduce locomotor recovery after SCI.

The YTHDF1-TRAF6 pathway regulates the neuroinflammatory response and contributes to morphine tolerance and hyperalgesia in the periaqueductal gray

Long-term use of opioids such as morphine has negative side effects, such as morphine analgesic tolerance and morphine-induced hyperalgesia (MIH). These side effects limit the clinical use and analgesic efficacy of morphine. Elucidation of the mechanisms and identification of feasible and effective methods or treatment targets to solve this clinical phenomenon are important. Here, we discovered that YTHDF1 and TNF receptor-associated factor 6 (TRAF6) are crucial for morphine analgesic tolerance and MIH. The m6A reader YTHDF1 positively regulated the translation of TRAF6 mRNA, and chronic morphine treatments enhanced the m6A modification of TRAF6 mRNA. TRAF6 protein expression was drastically reduced by YTHDF1 knockdown, although TRAF6 mRNA levels were unaffected. By reducing inflammatory markers such as IL-1β, IL-6, TNF-α and NF-κB, targeted reduction of YTHDF1 or suppression of TRAF6 activity in ventrolateral periaqueductal gray (vlPAG) slows the development of morphine analgesic tolerance and MIH. Our findings provide new insights into the mechanism of morphine analgesic tolerance and MIH indicating that YTHDF1 regulates inflammatory factors such as IL-1β, IL-6, TNF-α and NF-κB by enhancing TRAF6 protein expression.

Opioid Dose, Pain, and Recovery following Abdominal Surgery: A Retrospective Cohort Study

Background: The optimal dosage for opioids given to patients after surgery for pain management remains controversial. We examined the association of higher post-surgical opioid use with pain relief and recovery. Methods: We retrospectively enrolled adult patients who underwent elective abdominal surgery at our hospital between August 2021 and April 2022. Patients were divided into the “high-intensity” or “low-intensity” groups based on their post-surgical opioid use. Generalized estimating equation models were used to assess the associations between pain scores at rest and during movement on days 1, 2, 3, and 5 after surgery as primary outcomes. The self-reported recovery and incidence of adverse events were analyzed as secondary outcomes. Results: Among the 1170 patients in the final analysis, 293 were in the high-intensity group. Patients in the high-intensity group received nearly double the amount of oral morphine equivalents per day compared to those in the low-intensity group (84.52 vs. 43.80), with a mean difference of 40.72 (95% confidence interval (CI0 38.96−42.48, p < 0.001) oral morphine equivalents per day. At all timepoints, the high-intensity group reported significantly higher pain scores at rest (difference in means 0.45; 95% CI, 0.32 to 0.58; p < 0.001) and during movement (difference in means 0.56; 95% CI, 0.41 to 0.71; p < 0.001) as well as significantly lower recovery scores (mean difference (MD) −8.65; 95% CI, −10.55 to −6.67; p < 0.001). A post hoc analysis found that patients with moderate to severe pain during movement were more likely to receive postoperative high-intensity opioid use. Furthermore, patients in the non-high-intensity group got out of bed sooner (MD 4.31 h; p = 0.001), required urine catheters for shorter periods of time (MD 12.26 h; p < 0.001), and were hospitalized for shorter periods (MD 1.17 days; p < 0.001). The high-intensity group was at a higher risk of chronic postsurgical pain (odds ratio 1.54; 95% CI, 1.14 to 2.08, p = 0.005). Conclusions: High-intensity opioid use after elective abdominal surgery may not be sufficient for improving pain management or the quality of recovery compared to non-high-intensity use. Our results strengthen the argument for a multimodal approach that does not rely so heavily on opioids.

Endogenous opioid systems alterations in pain and opioid use disorder

Decades of research advances have established a central role for endogenous opioid systems in regulating reward processing, mood, motivation, learning and memory, gastrointestinal function, and pain relief. Endogenous opioid systems are present ubiquitously throughout the central and peripheral nervous system. They are composed of four families, namely the μ (MOPR), κ (KOPR), δ (DOPR), and nociceptin/orphanin FQ (NOPR) opioid receptors systems. These receptors signal through the action of their endogenous opioid peptides β-endorphins, dynorphins, enkephalins, and nociceptins, respectfully, to maintain homeostasis under normal physiological states. Due to their prominent role in pain regulation, exogenous opioids-primarily targeting the MOPR, have been historically used in medicine as analgesics, but their ability to produce euphoric effects also present high risks for abuse. The ability of pain and opioid use to perturb endogenous opioid system function, particularly within the central nervous system, may increase the likelihood of developing opioid use disorder (OUD). Today, the opioid crisis represents a major social, economic, and public health concern. In this review, we summarize the current state of the literature on the function, expression, pharmacology, and regulation of endogenous opioid systems in pain. Additionally, we discuss the adaptations in the endogenous opioid systems upon use of exogenous opioids which contribute to the development of OUD. Finally, we describe the intricate relationship between pain, endogenous opioid systems, and the proclivity for opioid misuse, as well as potential advances in generating safer and more efficient pain therapies.

Non-Surgical Management of Chronic Pelvic Pain in Females

PURPOSE OF REVIEW

The purpose of this paper is to review the most recent literature on non-surgical therapeutic options for chronic pelvic pain in females.

RECENT FINDINGS

Chronic pelvic pain can arise from virtually any organ system in the human body. If a precise etiology is identified, the management of chronic pelvic pain can be tailored accordingly. In some cases, patients with chronic pelvic pain can remain without a specific diagnosis. In these circumstances, adequate symptom control can still be achieved even if no underlying disorder is found. Although chronic pelvic pain is often a difficult disorder to manage, several non-surgical management options exist. Employing a multidisciplinary approach, most patients can achieve adequate symptom relief, usually without the need for surgical intervention.

Prescribed opioid use is associated with adverse cardiovascular outcomes in community-dwelling older persons

AIMS

Prescribed opioids are commonly used in the older community-dwelling population for the treatment of chronic pain. Although the harmful effects of opioid abuse and overdose are well understood, little is known about the long-term cardiovascular (CV) effects of prescribed opioids. The aim of this study was to investigate the CV effects associated with prescribed opioid use.

METHODS AND RESULTS

A post hoc analysis of participants in the Aspirin in Reducing Events in the Elderly (ASPREE) trial was conducted. Participants in the ASPREE trial included community-dwelling older adults without a prior history of CV disease (CVD). Prescribed opioid use was defined as opioid use at baseline and/or at the first annual visit (AV1). Cox proportional hazards regression was used to calculate hazard ratios and 95% confidence intervals (95% CI) for associations between opioid use and CVD events following AV1. Of the 17 701 participants included (mean age 75.2 years, 58.2% female), 813 took opioids either at baseline or at AV1. Over a median follow-up period of 3.58 years (IQR 2.50-4.62), CVD events, most notably heart failure hospitalization, occurred in 7% (n = 57) amongst opioid users and 4% (n = 680) amongst non-opioid users. After adjustment for multiple covariates, opiate use was associated with a 1.67-fold (CI 1.26-2.23, P < 0.001) increase in the hazard ratio for CVD events.

CONCLUSIONS

These findings identify opioid use as a non-traditional risk factor for CVD events in community-dwelling older adults.

Race, ethnicity, and the use of regional anesthesia in cancer patients undergoing open abdominal surgery: A single-center retrospective cohort study

Background

Where applicable, regional anesthesia has been shown to be superior to opioid or non-opioid analgesic modalities alone. However, some studies have shown ethnic-based disparities in the use of regional anesthesia in patients undergoing surgical procedures. In this study of patients who had undergone major oncologic surgery, our main objective was to compare the use of regional anesthesia between patients of different ethnicities.

Methods

A retrospective review of adults who had undergone major open abdominal surgical procedures between 2016 and 2021 was performed. Logistic regression models were used to assess the association between baseline patient characteristics and the use of regional anesthesia.

Results

A total of 4,791 patients were included in the analysis. The median age was 60.5 years [interquartile range, 49, 69], the majority were female (65%), and of American Society of Anesthesiologists Physical Status Class (ASA) 3 (94.7%). Regional anesthesia was used in 2,652 patients (55.4%) and was not associated with race or ethnicity (p = 0.287). Compared to White patients, the odds of regional anesthesia use in other racial/ethnic groups were: Asian {odds ratio (OR) 0.851 [95% confidence interval (CI), 0.660–1.097]; p = 0.2125}, Black/African American [OR 0.807 (95% CI, 0.651–1.001); p = 0.0508], Hispanic/Latino [OR 0.957 (95% CI, 0.824–1.154); p = 0.7676], Other race [OR 0.957 (95% CI, 0.627–1.461); p = 0.8376]. In the multivariable analysis, age [OR 0.995 (95% CI, 0.991–1.000); p = 0.0309] and female gender [OR 1.231 (95% CI, 1.090–1.390); p = 0.0008] were associated with the use of regional anesthesia.

Conclusion

In this single-institution retrospective study of adults who had undergone major open abdominal surgery, the use of regional anesthesia was not associated with race or ethnicity. In the multivariable analysis, age and female gender were associated with the use of regional anesthesia.

Trends in opioid and non-opioid treatment for chronic non-cancer pain and cancer pain among privately insured adults in the United States, 2012–2019

Recent clinical guidelines have emphasized non-opioid treatments in lieu of prescription opioids for chronic non-cancer pain, exempting cancer patients from these recommendations. In this study, we determine trends in opioid and non-opioid treatment among privately insured adults with chronic non-cancer pain (CNCP) or cancer. Using administrative claims data from IBM MarketScan Research Databases, we identified privately-insured adults who were continuously enrolled in insurance for at least one calendar year from 2012 to 2019. We identified individuals with CNCP diagnosis, defined as a diagnosis of arthritis, headache, low back pain, and/or neuropathic pain, and a individuals with cancer diagnosis in a calendar year. Outcomes included receipt of any opioid, non-opioid medication, or non-pharmacologic CNCP therapy and opioid prescribing volume, MME-per-day, and days’ supply. Estimates were regression-adjusted for age, sex, and region. Between 2012 and 2019, the proportion of patients who received any opioid decreased across both groups (CNCP: 49.7 to 30.5%, p<0.01; cancer: 86.0 to 78.7%, p<0.01). Non-opioid pain medication receipt remained steady for individuals with CNCP (66.7 to 66.4%, p<0.01) and increased for individuals with cancer (74.4 to 78.8%, p<0.01), while non-pharmacologic therapy use rose among individuals with CNCP (62.4 to 66.1%, p<0.01). Among those prescribed opioids, there was a decrease in the receipt of at least one prescription with >90 MME/day (CNCP: 13.9% in 2012 to 4.9% in 2019, p<0.01; Cancer: 26.2% to 7.6%, p<0.01); >7 days of supply (CNCP: 56.3% to 30.7%, p <0.01; Cancer: 47.5% to 22.7%, p<0.01), the mean number of opioid prescriptions (CNCP: 5.2 to 3.9, p<0.01; Cancer: 4.0 to 2.7, p<0.01) and mean MME/day (CNCP: 49.9 to 38.0, p<0.01; Cancer: 62.4 to 44.7, p<0.01). Overall, from 2012–2019, opioid prescribing declined for CNCP and cancer, with larger reductions for patients with CNCP. For both groups, reductions in prescribed opioids outpaced increases in non-opioid alternatives.

The association between pain and central nervous system depressing medication among hospitalised Norwegian older adults

OBJECTIVES

Central nervous system depressant medications (CNSD) including benzodiazepines, z-hypnotics and opioids are regularly prescribed for the older patient. These medications are linked to dependence and associated with severe side effects in some older patients. Consensus recommendations for this group suggest limiting their use. We have recently described a high proportion of long-term CNSD use and dependence among older in-hospital patients. In this study, we aim to investigate factors associated with pain intensity and presentation of pain among older adults with long-term use of CNSDs compared to non-users.

METHODS

Two hundred and forty six elderly hospitalised patients were recruited consecutively in a cross-sectional study. Data was collected from patients and electronic health records (EHR). Independent variables were sex, age, education, emotional symptoms (hospital anxiety and depression scale [HADS]), cognitive function (Mini-mental State Examination test [MMSE]), comorbidity (cumulative illness rating score - geriatrics [CIRS-G]), loneliness (the six-item De Jong Gierveld Loneliness Scale) and prolonged (≥4 weeks) use of any CNSDs or prolonged use of opioids (≥4 weeks). All variables, including pain intensity, were collected at one time point consistent with the cross-sectional study design. Statistical analyses included descriptive statistics and linear regression models using the above mentioned variables and pain intensity (visual analogue scale for pain intensity [VAS] pain 0-100) as outcome. Additional information regarding pain presentation was extracted from the patients' EHR.

RESULTS

Mean pain intensity VAS (SD) was 35.2 (30.4) and 18.1 (24.2) respectively, for patients with vs. without prolonged use of CNSDs. In the multivariable linear regression analysis, prolonged use of CNSDs and opioids were positively associated with pain intensity (VAS) (regression coefficient (95% CI) 20.7 (11.0; 30.3), p<0.001, and 19.8 (5.7; 33.8), p=0.006, respectively), while sex, age, education, MMSE, HADS, CIRS-G and loneliness scores were not. Pain related to back (23.2%) and lower extremities (23.2%) were most common pain sites, and those with one or more pain sites reported overall higher pain intensity compared to those with no reported pain sites (p<0.006).

CONCLUSIONS

Prolonged use of CNSD medications as well as prolonged use of opioids are both positively associated with pain intensity. The results may have implications for treatment and long-term pain management for older patients.

Non-pharmacological integrative therapies for chronic cancer pain

Objective

Chronic pain is one of the most detrimental symptoms exhibited by cancer patients, being an indication for opioid therapy in up to half of the patients’ receiving chemotherapy and in 90% of advanced cases. Various successful non-pharmacological integrative therapy options have been explored and implemented to improve the quality of life in these patients. This review aims to highlight the mechanisms implicated; assessment tools used for cancer pain and summarize current evidence on non-pharmacological approaches in the treatment of chronic cancer pain.

Data sources

A review of the literature was conducted using a combination of MeSH keywords including “Chronic cancer pain,” “Assessment,” “Non-pharmacological management,” and “Integrative therapy.”

Data summary

Data on the approach and assessment of chronic cancer pain as well as non-pharmacological integrative options have been displayed with the help of figures and tables. Of note, non-pharmacological integrative management was divided into three subcategories; physical therapy (involving exercise, acupuncture, massage, and transcutaneous electric nerve stimulation), psychosocial therapy (e.g. mindful practices, supportive therapy), and herbal supplementation.

Conclusions

The use of non-pharmacological integrative therapy in the management of chronic cancer pain has been grossly underestimated and must be considered before or as an adjuvant of other treatment regimens to ensure appropriate care.

Effects of Preoperative Electroacupuncture on Remifentanil-Induced Post-Infusion Hyperalgesia in Patients Undergoing Thyroidectomy: A Double-Blind Randomized Controlled Trial

Objective

Electroacupuncture (EA) delivered one day before surgery could reduce postoperative pain. Remifentanil-induced post-infusion hyperalgesia (RPH) was occurred after exposure to high-dose remifentanil. This study aimed to investigate the effects of preoperative EA on RPH in patients undergoing thyroidectomy.

Methods

A total of 80 patients who were scheduled to undergo elective thyroidectomy were randomly assigned to two groups: an EA group and a sham EA (SEA) group. EA was delivered at the Zusanli (ST36) and Neiguan (PC6) acupoints 24 h before the surgery. To ensure uniformity across all patients, remifentanil was administered at the same set rate (0.3 μg/kg/min) to all patients. Mechanical pain thresholds were recorded by an electronic von Frey device around the skin incision and on the arm before surgery as well as at 30 min and 6, 24, and 48 h after surgery.

Results

At 30 min and 6 h after surgery, the EA group showed considerably greater mechanical pain thresholds surrounding the surgical site compared with the SEA group. At 30 min and 6 h after surgery, the patients in the SEA group showed a greater incidence of postoperative hyperalgesia surrounding the surgical site than those in the EA group. At 24 and 48 h after surgery, no significant differences were found between the two groups, although the pain intensity of the EA group was less than that of the SEA group. There were also no substantial differences between the two groups in the frequency of postoperative adverse reactions and rescue analgesia needed.

Conclusion

EA administered 24 h before surgery could alleviate RPH in patients undergoing thyroidectomy.

Pharmacodynamics, toxicology and toxicokinetics of ropivacaine oil delivery depot

BACKGROUND

Ropivacaine oil delivery depot (RODD) can be used to treat postoperative incision pain. The aim was to study pharmacodynamics, toxicity and toxicokinetics of RODD.

METHODS

The base research of RODD were conducted. Thirty rabbits were randomly divided into saline, solvent, ropivacaine aqueous injection (RAI) 0.9 mg, RODD 0.9 mg and RODD 3 mg groups. The sciatic nerve of rabbits were isolated, dripped with RODD and the effect of nerve block were observed. In toxicity study, the rats were divided into saline, solvent and RODD 75, 150 and 300 mg/kg groups, 30 rats per group. In toxicokinetics, rats were divided into RODD 75, 150 and 300 mg/kg groups, 18 rats per group. The rats were subcutaneously injected drugs.

RESULTS

The analgesic duration of RODD 3 mg and RAI 0.9 mg blocking ischiadic nerve lasted about 20 h and 2 h, respectively, and their blocking intensity was similar. The rats in RODD 75 mg/kg did not show any toxicity. Compared with saline group, in RODD 150 mg/kg group neutrophils and mononuclear cells increased, lymphocytes decreased and albumin decreased(P < 0.05), and pathological examination showed some abnormals. In RODD 300 mg/kg group, 10 rats died and showed some abnormalities in central nerve system, hematologic indexes, part of biochemical indexes, and the weights of spleen, liver, and thymus. However, these abnormal was largely recovered on 14 days after the dosing. The results of toxicokinetics of RODD 75 mg/kg group showed that the Cmax was 1.24 ± 0.59 µg/mL and the AUC(0-24 h) was 11.65 ± 1.58 h·µg/mL.

CONCLUSIONS

Subcutaneous injection RODD releases ropivacaine slowly, and shows a stable and longer analgesic effect with a large safety range.

Complex Persistent Opioid Dependence-an Opioid-induced Chronic Pain Syndrome

OPINION STATEMENT

Long-term opioid therapy (LTOT) for chronic cancer and non-cancer pain is commonly ineffective in providing its stated goal of improving function through good control of pain. Opioid tapering (slow dose reduction and/or discontinuation), the logical solution, also appears to be ineffective among many patients on LTOT as it often leads to even worse pain control and function, leaving the patients and providers managing LTOT in a clinical conundrum with little treatment choices. Complex persistent opioid dependence (CPOD) was recently offered as a heuristic to explain this clinical conundrum exemplified by the ineffectiveness of both LTOT and opioid tapering. This manuscript provides a detailed description of the neurobehavioral underpinnings of CPOD, explaining how long-term opioid use can lead to more pain even while experiencing relief with each opioid dose. CPOD is characterized by the allostatic opponent mechanisms of neuroadaptations related to the progression of opioid dependence and tolerance involving nociceptive/anti-nociceptive brain systems causing opioid-induced hyperalgesia and reward/anti-reward systems causing hyperkatefia or suffering that induces pain experience through the cognitive/emotional component of pain mechanisms. "Opioid Induced Chronic Pain syndrome" (OICP) is offered as an alternate clinical diagnostic term instead of CPOD that has several limitations as a diagnosis term including poor patient acceptance due to stigma towards addiction and clinical confounding with opioid use disorder, a related but separate clinical entity. OICP with LTOT is conceptualized as a recoverable iatrogenic problem that can be managed by pain providers. Broad guidance on management of OICP is also provided.

Preoperative Opioid Dose and Surgical Outcomes in Colorectal Surgery

BACKGROUND

The worsening opioid epidemic has led to an increased number of surgical patients with chronic preoperative opioid use. However, the impact of opioids on perioperative outcomes has yet to be fully elucidated. The purpose of this study was to assess the association between preoperative opioid dose and surgical outcomes among colectomy patients.

METHODS

Adult colectomy patients in the IBM MarketScan database (2010-2017) were stratified based on preoperative opioid dose, calculated as the average opioid dose in morphine milligram equivalents (MME) in the 90 days prior to surgery: 0 MME, 1 to 49 MME, and 50 or more MME. The association between preoperative opioid dose and anastomotic leak, the primary outcome of interest, as well as other postoperative complications, was assessed using multivariable regression.

RESULTS

Among 45,515 adult colectomy patients, 71.4% did not use opioids (0 MME), 27.4% had an opioid dose between 1 and 49 MME, and 1.2% had an opioid dose at or above 50 MME. Patients with preoperative opioid use exhibited a higher incidence of anastomotic leak (0 MME: 4.8%, 1-49 MME: 5.5%, ≥50 MME: 8.3%; p trend = 0.001). Multivariable analysis demonstrated a dose-response relationship between preoperative opioids and surgical outcomes, as the odds of anastomotic leak worsened with increasing opioid dose (1-49 MME: OR 1.19, 95% CI 1.08-1.31, p < 0.001; ≥50 MME: OR 1.64, 95% CI 1.20-2.24, p = 0.002). Similar dose-response relationships were seen after risk-adjustment for lung complications, pneumonia, delirium, and 30-day readmission (p < 0.05 for all).

CONCLUSIONS

Providers should exercise caution when prescribing opioids preoperatively, as increasing doses of preoperative opioids were associated with worse surgical outcomes and higher 30-day readmission among adult colectomy patients.

Microglial ablation does not affect opioid-induced hyperalgesia in rodents

ABSTRACT

Opioids are the frontline analgesics in pain management. However, chronic use of opioid analgesics causes paradoxical pain that contributes to the decrease of their efficacy in pain control and the escalation of dose in long-term management of pain. The underling pathogenic mechanism is not well understood. Microglia have been commonly believed to play a critical role in the expression of opioid-induced hyperalgesia in animal models. We performed microglial ablation experiments using either genetic (CD11b-diphtheria toxin receptor transgenic mouse) or pharmacological (colony-stimulating factor-1 receptor inhibitor PLX5622) approaches. Surprisingly, ablating microglia using these specific and effective approaches did not cause detectable impairment in the expression of hyperalgesia induced by morphine. We confirmed this conclusion with a behavioral test of mechanical and thermal hyperalgesia, in male and female mice, and with different species (mouse and rat). These findings raise caution about the widely assumed contribution of microglia to the development of opioid-induced hyperalgesia.

Microinduction to Buprenorphine from Methadone for Chronic Pain: Outpatient Protocol with Case Examples

The negative sequelae of full mu agonist chronic opioid analgesic therapy (COAT) are numerous and well documented. One safer alternative to COAT use in chronic, non-cancer pain (CNCP) is a transition to buprenorphine. However, transitioning patients from methadone COAT regimens can be challenging due to the pharmacodynamics of buprenorphine, as well as to the limited commercial formulations of buprenorphine available, and their restrictive instructions for use. Presented here are clinical cases transitioned to buprenorphine from methadone via a novel microinduction protocol during enrollment in an outpatient, group, integrative, multidisciplinary program. The protocol was successful to promote satisfactory and sustained COAT cessation for patients with CNCP and is arguably safer than current conventional practices.

Pharmacological blockade of neurokinin1 receptor restricts morphine-induced tolerance and hyperalgesia in the rat

OBJECTIVES

The most notable adverse side effects of chronic morphine administration include tolerance and hyperalgesia. This study investigated the involvement of dorsal root ganglion (DRG) protein kinase Cɛ (PKCɛ) expression during chronic morphine administration and also considered the relationship between DRG PKCɛ expression and the substance P- neurokinin1 receptor (SP- NK1R) activity.

METHODS

Thirty-six animals were divided into six groups (n=6) in this study. In the morphine and sham groups, rats received 10 µg intrathecal (i.t.) morphine or saline for eight consecutive days, respectively. Behavioral tests were performed on days 1 and 8 before and after the first injections and then 48 h after the last injection (day 10). In the treatment groups, rats received NK1R antagonist (L-732,138, 25 µg) daily, either alone or 10 min before a morphine injection, Sham groups received DMSO alone or 10 min before a morphine injection. Animals were sacrificed on days 8 and 10, and DRG PKCɛ and SP expression were analyzed by western blot and immunohistochemistry techniques, respectively.

RESULTS

Behavioral tests indicated that tolerance developed following eight days of chronic morphine injection. Hyperalgesia was induced 48 h after the last morphine injection. Expression of SP and PKCɛ in DRG significantly increased in rats that developed morphine tolerance on day 8 and hyperalgesia on day 10, respectively. NK1R antagonist (L-732,138) not only blocked the development of hyperalgesia and the increase of PKCɛ expression but also alleviated morphine tolerance.

CONCLUSIONS

Our results provide evidence that DRG PKCɛ and SP-NK1R most likely participated in the generation of morphine tolerance and hyperalgesia. Pharmacological inhibition of SP-NK1R activity in the spinal cord suggests a role for NK1R and in restricting some side effects of chronic morphine. All experiments were performed by the National Institute of Health (NIH) Guidelines for the Care and Use of Laboratory Animals (NIH Publication No. 80-23, revised1996) and were approved by the Animal Ethics Committee of Shahid Beheshti University of Medical Sciences, Tehran, Iran (IR.SBMU.MSP.REC.1396.130).

Neuron Type-Dependent Synaptic Activity in the Spinal Dorsal Horn of Opioid-Induced Hyperalgesia Mouse Model

Opioids are widely used for pain relief; however, chronic opioid use causes a paradoxical state of enhanced pain sensitivity, termed “Opioid-induced hyperalgesia (OIH).” Despite the clinical importance of OIH, the detailed mechanism by which it enhances pain sensitivity remains unclear. In this study, we tested whether repeated morphine induces a neuronal circuit polarization in the mouse spinal dorsal horn (SDH). Transgenic mice expressing GFP to neurokinin 1 receptor-expressing neurons (sNK1Rn) and GABAergic interneurons (sGABAn) that received morphine [20 mg/kg, once daily for four consecutive days (i.p.)] developed mechanical hypersensitivity. Repeated morphine altered synaptic strengths in the SDH as a specific cell-type but not in a gender-dependent manner. In sNK1Rn and non-tonic firing neurons, repeated morphine treatment significantly increased frequency of spontaneous excitatory postsynaptic current (sEPSC) and evoked EPSC (eEPSC). In addition, repeated morphine treatment significantly decreased evoked inhibitory postsynaptic current (eIPSC) in sNK1Rn. Conversely, in sGABAn and tonic firing neurons, repeated morphine treatment significantly decreased sEPSC frequency and eEPSC, but had no change of eIPSC in sGABAn. Interestingly, repeated morphine treatment significantly decreased neuronal rheobase of sNK1Rn but had no effect on sGABAn. These findings suggest that spinal neuronal circuit polarization maybe the mechanism of OIH and identify a potential therapeutic mechanism to prevent or treat opioid-induced pain.

Opioid-induced Hyperalgesia in Patients With Chronic Pain: A Systematic Review of Published Cases

INTRODUCTION

Opioid-induced hyperalgesia (OIH) remains an issue in patients with chronic pain. Multiple cases of OIH in patients with chronic pain exposed to opioids have been reported worldwide. The objective of this systematic review was to summarize the evidence of OIH from clinical reports.

METHODS

We searched the PubMed, Cochrane, EMBASE, and LILACS databases for case reports and case series of OIH published up to December 2020, with the aim to summarize the evidence for OIH in patients with chronic pain from clinical reports and to discuss issues relevant to the clinical diagnosis and management of OIH.

RESULTS

We retrieved and reviewed 41 articles describing 72 cases. Clinical features of OIH were observed in patients of both sexes, all ages, and with various types of pain treated with different classes of opioids. OIH was reported at all doses, but most published studies reported a pattern of OIH following treatment with very high daily doses of opioids (median oral morphine equivalent dose of 850 mg). OIH was diagnosed clinically in all cases. Three different strategies for OIH management were described: opioid rotation, opioid cessation, and the use of adjuvant pharmacotherapies. All had statistically similar success rates for OIH treatment: 72%, 57%, and 79%, respectively. The decrease in pain was achieved rapidly (mean: 8 d; range: 1 to 28 d). Adjuvant therapies resulted in the largest decrease in dose. Ketamine and dexmedetomidine were the most widely used adjuvant drugs.

CONCLUSION

The key finding is that clinical symptoms of OIH can be resolved when this condition is diagnosed and managed.

The association between perioperative pupillary parameters and postoperative acute pain: A pilot cross-sectional study

OBJECTIVE

We aim to explore the capacity of perioperative pupillary variables to predict acute pain in the post-anesthesia care unit (PACU).

METHODS

Patients scheduled to undergo thoracic or abdominal surgery under general anesthesia between April 2021 and June 2021 were enrolled. We measured the pupil diameter, pupillary light reflex (PLR), and pupillary reflex dilatation 5 min before anesthesia induction (T1), 5 min after intubation (T2), at the end of anesthesia (T3), immediately before extubation (T4), and 5 min after extubation (T5). We assessed the early postoperative pain intensity in the PACU using Numeric Rating Scales (NRS) at recovery, 5 min after recovery, and 10 min after recovery. Logistic regression models were used to evaluate the association between perioperative pupillary variables and postoperative pain intensity.

RESULTS

Fifty-one patients were enrolled, 50 of whom were included in the final analysis. A total of 13 patients (26%) needed remedial analgesia in the PACU. Pupil parameters at T1, T2, T3, and T5 were not associated with NRS in the PACU. Multiple logistic regression models and receiver operating characteristic (ROC) curves indicated that only latency of PLR at T4 can predict postoperative acute pain. The ROC analysis showed that the cutoff value for latency of PLR at T4 was 0.29 s to discriminate between no pain and pain, and the area under the curve was 0.778 (95% CI 0.634-0.922, p = 0.002) with sensitivity 50.0% and specificity 91.7%.

CONCLUSION

The latency of PLR immediately before extubation may be a useful predictor for postoperative acute pain in the PACU.

Alternative Splicing Mechanisms Underlying Opioid-Induced Hyperalgesia

Prolonged use of opioids can cause opioid-induced hyperalgesia (OIH). The impact of alternative splicing on OIH remains partially characterized. A study of the absolute and relative modes of action of alternative splicing further the understanding of the molecular mechanisms underlying OIH. Differential absolute and relative isoform profiles were detected in the trigeminal ganglia and nucleus accumbens of mice presenting OIH behaviors elicited by chronic morphine administration relative to control mice. Genes that participate in glutamatergic synapse (e.g., Grip1, Grin1, Wnk3), myelin protein processes (e.g., Mbp, Mpz), and axon guidance presented absolute and relative splicing associated with OIH. Splicing of genes in the gonadotropin-releasing hormone receptor pathway was detected in the nucleus accumbens while splicing in the vascular endothelial growth factor, endogenous cannabinoid signaling, circadian clock system, and metabotropic glutamate receptor pathways was detected in the trigeminal ganglia. A notable finding was the prevalence of alternatively spliced transcription factors and regulators (e.g., Ciart, Ablim2, Pbx1, Arntl2) in the trigeminal ganglia. Insights into the nociceptive and antinociceptive modulatory action of Hnrnpk were gained. The results from our study highlight the impact of alternative splicing and transcriptional regulators on OIH and expose the need for isoform-level research to advance the understanding of morphine-associated hyperalgesia.

Analgesic effect of magnesium sulfate during total intravenous anesthesia: randomized clinical study

Introduction and objective

: Opioids have usually been used as intraoperative analgesic components, regardless of the many adverse effects they are associated with, such as nausea, vomiting, respiratory depression, and hyperalgesia. Several approaches have been investigated to reduce doses used, and magnesium sulfate has been shown to be a valuable analgesic adjunct. The main objective of the present trial was to evaluate the effectiveness of magnesium sulfate as the chief intraoperative analgesic, and the secondary objectives were to assess propofol consumption, postoperative analgesia, and intraoperative hemodynamic stability.

Methods

In this prospective, double-blind trial, 50 patients scheduled to undergo post-bariatric abdominoplasty under general intravenous anesthesia were divided into two groups, to receive remifentanil or magnesium sulfate as intraoperative analgesic. Fentanyl 1 µg kg^-1 was the rescue analgesic.

Results

Among the patients in the group receiving Magnesium Sulfate (MSG), 64% did not need supplemental analgesia and none of the patients in the Remifentanil Group (RG) required fentanyl. MSG patients showed propofol consumption 36.6% higher (guided by the Bispectral Index – BIS). MSG patients consumed significantly less ephedrine (mean ± SD) than RG patients, respectively 1.52 ± 4.38 mg and 10 ± 10.39 mg, p < 0.001. Mean values of blood concentrations of magnesium were comparable to values previously described in the literature.

Conclusion

Magnesium sulfate is a safe and effective option for intraoperative analgesia, when avoiding or decreasing opioid use is required.

Postoperative Drip-Infusion of Remifentanil Reduces Postoperative Pain-A Retrospective Observative Study

Development of remifentanil-induced hyperalgesia (RIH) postoperatively is an unpleasant experience that requires further treatment. This study assessed the effects of gradual withdrawal combined with drip infusion of remifentanil on postoperative pain and the requirement for rescue analgesics. A total of 559 patients receiving total intravenous anesthesia with propofol and remifentanil were enrolled. All patients either underwent gradual withdrawal of remifentanil (GWR) or gradual withdrawal combined with drip infusion (GWDR) with a dose of 1 mcg·kg⁻¹ for 30 min after extubation. The numeric rating scale (NRS) and the requirement of rescue analgesics were assessed. The requirement for rescue analgesics was significantly lower in the GWDR group than in the GWR group (13.2% vs. 35.7%; p < 0.001). At the post-anesthetic care unit (PACU), patients in the GWDR group had a lower NRS pain score (p < 0.001). In addition, in the postoperative 2nd hour, patients in the GWDR group had a significantly lower NRS than the GWR group (beta, −0.31; p = 0.003). No remifentanil-related adverse effects were observed. We found that gradual withdrawal combined with drip infusion of remifentanil required less rescue analgesics and reduced pain scores. The new way of remifentanil administration may be effective to prevent RIH.