Patient engagement in designing, conducting, and disseminating clinical pain research: IMMPACT recommended considerations

ABSTRACT

In the traditional clinical research model, patients are typically involved only as participants. However, there has been a shift in recent years highlighting the value and contributions that patients bring as members of the research team, across the clinical research lifecycle. It is becoming increasingly evident that to develop research that is both meaningful to people who have the targeted condition and is feasible, there are important benefits of involving patients in the planning, conduct, and dissemination of research from its earliest stages. In fact, research funders and regulatory agencies are now explicitly encouraging, and sometimes requiring, that patients are engaged as partners in research. Although this approach has become commonplace in some fields of clinical research, it remains the exception in clinical pain research. As such, the Initiative on Methods, Measurement, and Pain Assessment in Clinical Trials convened a meeting with patient partners and international representatives from academia, patient advocacy groups, government regulatory agencies, research funding organizations, academic journals, and the biopharmaceutical industry to develop consensus recommendations for advancing patient engagement in all stages of clinical pain research in an effective and purposeful manner. This article summarizes the results of this meeting and offers considerations for meaningful and authentic engagement of patient partners in clinical pain research, including recommendations for representation, timing, continuous engagement, measurement, reporting, and research dissemination.

Smallest Clinically Meaningful Improvement in Amputation-Related Pain and Brief Pain Inventory Scores as Defined by Patient Reports of Global Improvement After Cryoneurolysis: a Retrospective Analysis of a Randomized, Controlled Clinical Trial

BACKGROUND

The smallest meaningful improvement in pain scores (minimal clinically important difference [MCID]) after an analgesic intervention is essential information when both interpreting published data and designing a clinical trial. However, limited information is available for patients with chronic pain conditions, and what is published is derived from studies involving pharmacologic and psychological interventions. We here calculate these values based on data collected from 144 participants of a previously published multicenter clinical trial investigating the effects of a single treatment with percutaneous cryoneurolysis.

METHODS

In the original trial, we enrolled patients with a lower-limb amputation and established phantom pain. Each received a single-injection femoral and sciatic nerve block with lidocaine and was subsequently randomized to receive either ultrasound-guided percutaneous cryoneurolysis or sham treatment at these same locations. Investigators, participants, and clinical staff were masked to treatment group assignment with the exception of the treating physician performing the cryoneurolysis, who had no subsequent participant interaction. At both baseline and 4 months (primary end point), participants rated their phantom limb pain based on a numeric rating scale (NRS) and their interference of pain on physical and emotional functioning as measured with the Brief Pain Inventory's interference subscale. They subsequently qualitatively defined the change using the 7-point ordinal Patient Global Impression of Change (PGIC). The smallest clinically meaningful improvements in phantom limb pain and Brief Pain Inventory scores were calculated using an anchor-based method based on the PGIC.

RESULTS

The median (interquartile range [IQR]) phantom pain NRS (0-10) improvements at 4 months considered small, medium, and large were 1 [1-1], 3 [3-4], and 4 [3-6], respectively. The median improvements in the Brief Pain Inventory interference subscale (0-70) associated with a small, medium, and large analgesic changes were 16 [6-18], 24 [22-31], and 34 [22-46]. The proportions of patients that experienced PGIC ≥5 were 33% and 36% in the active and placebo groups, respectively. The relative risk of a patient experiencing PGIC ≥5 in the active group compared to the sham group with 95% confidence interval was 0.9 (0.6-1.4), P = .667.

CONCLUSIONS

Amputees with phantom limb pain treated with percutaneous cryoneurolysis rate analgesic improvements as clinically meaningful similar to pharmacologic treatments, although their MCID for the Brief Pain Inventory was somewhat larger than previously published values. This information on patient-defined clinically meaningful improvements will facilitate interpretation of available studies and guide future trial design.

Effects of systemic oxytocin administration on ultraviolet B-induced nociceptive hypersensitivity and tactile hyposensitivity in mice

Ultraviolet B (UVB) radiation induces cutaneous inflammation, leading to thermal and mechanical hypersensitivity. Here, we examine the mechanical properties and profile of tactile and nociceptive peripheral afferents functionally disrupted by this injury and the role of oxytocin (OXT) as a modulator of this disruption. We recorded intracellularly from L4 afferents innervating the irradiated area (5.1 J/cm2) in 4-6 old week male mice (C57BL/6J) after administering OXT intraperitoneally, 6 mg/Kg. The distribution of recorded neurons was shifted by UVB radiation to a pattern observed after acute and chronic injuries and reduced mechanical thresholds of A and C- high threshold mechanoreceptors while reducing tactile sensitivity. UVB radiation did not change somatic membrane electrical properties or fiber conduction velocity. OXT systemic administration rapidly reversed these peripheral changes toward normal in both low and high-threshold mechanoreceptors and shifted recorded neuron distribution toward normal. OXT and V1aR receptors were present on the terminals of myelinated and unmyelinated afferents innervating the skin. We conclude that UVB radiation, similar to local tissue surgical injury, cancer metastasis, and peripheral nerve injury, alters the distribution of low and high threshold mechanoreceptors afferents and sensitizes nociceptors while desensitizing tactile units. Acute systemic OXT administration partially returns all of those effects to normal.

Patient-centered results from a multicenter study of continuous peripheral nerve blocks and postamputation phantom and residual limb pain: secondary outcomes from a randomized, clinical trial

INTRODUCTION

We previously reported that a 6-day continuous peripheral nerve block reduces established postamputation phantom pain. To provide patients and providers with the information to best inform treatment decisions, here we reanalyze the data and present the results in a more patient-centered format. We also provide information on patient-defined clinically relevant benefits to facilitate evaluation of available studies and guide future trial design.

METHODS

The original trial enrolled participants with a limb amputation and phantom pain who were randomized to receive a 6-day continuous peripheral nerve block(s) of either ropivacaine (n=71) or saline (n=73) in a double-masked fashion. Here we calculate the percentage of each treatment group that experienced a clinically relevant improvement as defined by previous studies as well as present what the participants of our study defined as small, medium, and large analgesic improvements using the 7-point ordinal Patient Global Impression of Change scale.

RESULTS

Among patients who were given a 6-day ropivacaine infusion, 57% experienced at least a 2-point improvement on the 11-point numeric rating scale in their average and worst phantom pain 4 weeks postbaseline as compared with 26% (p<0.001) for average and 25% (p<0.001) for worst pain in patients given a placebo infusion. At 4 weeks, the percentage of participants rating their pain as improved was 53% for the active vs 30% for the placebo groups (95% CI 1.7 (1.1, 2.7), p=0.008). For all patients combined, the median (IQR) phantom pain Numeric Rating Scale improvements at 4 weeks considered small, medium, and large were 2 (0-2), 3 (2-5), and 5 (3-7), respectively. The median improvements in the Brief Pain Inventory interference subscale (0-70) associated with small, medium, and large analgesic changes were 8 (1-18), 22 (14-31), and 39 (26-47).

CONCLUSIONS

Among patients with postamputation phantom pain, a continuous peripheral nerve block more than doubles the chance of a clinically relevant improvement in pain intensity. Amputees with phantom and/or residual limb pain rate analgesic improvements as clinically relevant similarly to other chronic pain etiologies, although their smallest relevant improvement in the Brief Pain Inventory was significantly larger than previously published values.

TRIAL REGISTRATION NUMBER

NCT01824082.

Preliminary results from a randomized, controlled, cross-over trial of intrathecal oxytocin for neuropathic pain

OBJECTIVE

Compare intrathecal oxytocin, 100 µg to placebo on ongoing neuropathic pain and mechanical hyperalgesia and allodynia.

STUDY DESIGN

Randomized, controlled, double-blind cross-over.

SETTING

Clinical research unit.

SUBJECTS

Individuals aged 18 to 70 years with neuropathic pain for at least 6 months.

METHODS

Individuals received intrathecal injections of oxytocin and saline, separated by at least 7 days, and ongoing pain in neuropathic area (VAS [visual analog scale]) and areas of hypersensitivity to von Frey filament and cotton wisp brushing were measured for 4 hours. Primary outcome was VAS pain in the first 4 hours after injection, analyzed by linear mixed effects model. Secondary outcomes were verbal pain intensity scores at daily intervals for 7 days and areas of hypersensitivity and elicited pain for 4 hours after injections.

RESULTS

The study was stopped early after completion of 5 of 40 subjects planned due to slow recruitment and funding limitations. Pain intensity prior to injection was 4.75 ± 0.99 and modeled pain intensity decreased more after oxytocin than placebo to 1.61 ± 0.87 and 2.49 ± 0.87, respectively (P = .003). Daily pain scores were lower in the week following injection of oxytocin than saline (2.53 ± 0.89 vs 3.66 ± 0.89; P = .001). Allodynic area decreased by 11%, but hyperalgesic area increased by 18% after oxytocin compared to placebo. There were no study drug related adverse effects.

CONCLUSION

Although limited by the small number of subjects studied, oxytocin reduced pain more than placebo in all subjects. Further study of spinal oxytocin in this population is warranted.

TRIAL REGISTRATION

This study was registered at ClinicalTrials.gov on 03/27/2014 (NCT02100956). The first subject was studied on 06/25/2014.

Hypotension duration and vasopressor requirements following intrathecal oxytocin for Total hip arthroplasty: Secondary analysis of a randomized controlled trial

INTRODUCTION

A recent publication investigating intrathecal oxytocin, 100 μg, administered immediately prior to a spinal anesthetic in patients undergoing primary total hip arthroplasty surgery demonstrated a reduction in disability for 3-weeks, increased walking distance at 8-weeks, and earlier opioid cessation. This secondary analysis study was undertaken to assess the acute cardiovascular safety and analgesic efficacy of intrathecal oxytocin in this study population.

METHODS

90 patients were included in the analysis (44 randomized to spinal oxytocin and 46 to placebo [saline]). Data collected prospectively during the previously published study were supplemented with additional retrospectively collected data. The primary outcomes were comparisons of the duration of hypotension (minutes with mean arterial pressure < 65 mmHg) and cumulative vasopressor requirements during the initial 60 min following spinal placement. Secondary outcomes included hypotension durations and vasopressor requirements at later time points, perioperative fluid administration, physical therapy metrics, time to first opioid administration, cumulative opioid consumption through 24 h, and verbal pain scores through 24 h.

RESULTS

The duration of hypotension during the first 60 min following spinal placement did not differ between intrathecal oxytocin and placebo groups (12.2 ± 10.7 vs 14.0 ± 13.0 min, respectively; p = 0.476). There was also no difference in cumulative vasopressor requirements (1303 ± 883 vs 1156 ± 818 μg [phenylephrine equivalents]; p = 0.413) during that time period. No group differences were found for any of the investigated secondary outcomes.

CONCLUSION

The administration of 100 μg of intrathecal oxytocin does not significantly impact the duration of hypotension or the need for vasopressor agents when given as a component of a spinal anesthetic. The oxytocin and placebo groups also did not differ in regards to physical therapy related metrics, time to first opioid administration, cumulative opioids at 24-h, or pain scores through 24-h. What is already known on this topic: Rapid intravenous oxytocin causes hypotension after cesarean delivery, but intrathecal oxytocin does not cause hypotension in healthy volunteers.

WHAT THIS STUDY ADDS

Compared to saline control, intrathecal oxytocin, 100 μg did not increase the duration of hypotension or vasopressor requirements in patients during total hip arthroplasty. How this study might affect research, practice, or policy: Lack of hypotension from intrathecal oxytocin in this study supports future investigations to further explore its potential benefits, in terms of both analgesia and functional recovery following surgery.

Randomized controlled trial of intrathecal oxytocin on speed of recovery after hip arthroplasty

ABSTRACT

Recovery from surgery is quicker in the postpartum period, and this may reflect oxytocin action in the spinal cord. We hypothesized that intrathecal injection of oxytocin would speed recovery from pain and disability after major surgery. Ninety-eight individuals undergoing elective total hip arthroplasty were randomized to receive either intrathecal oxytocin (100 μg) or saline. Participants completed diaries assessing pain and opioid use daily and disability weekly, and they wore an accelerometer beginning 2 weeks before surgery until 8 weeks after. Groups were compared using modelled, adjusted trajectories of these measures. The study was stopped early due to the lack of funding. Ninety patients received intrathecal oxytocin (n = 44) or saline (n = 46) and were included in the analysis. There were no study drug-related adverse effects. Modelled pain trajectory, the primary analysis, did not differ between the groups, either in pain on day of hospital discharge (intercept: -0.1 [95% CI: -0.8 to 0.6], P = 0.746) or in reductions over time (slope: 0.1 pain units per log of time [95% CI: 0-0.2], P = 0.057). In planned secondary analyses, postoperative opioid use ended earlier in the oxytocin group and oxytocin-treated patients walked nearly 1000 more steps daily at 8 weeks ( P < 0.001) and exhibited a clinically meaningful reduction in disability for the first 21 postoperative days ( P = 0.007) compared with saline placebo. Intrathecal oxytocin before hip replacement surgery does not speed recovery from worst daily pain. Secondary analyses suggest that further study of intrathecal oxytocin to speed functional recovery without worsening pain after surgery is warranted.

Ultrasound-guided Percutaneous Cryoneurolysis to Treat Chronic Postamputation Phantom Limb Pain: A Multicenter Randomized Controlled Trial

BACKGROUND

Postamputation phantom pain is notoriously persistent with few validated treatments. Cryoneurolysis involves the application of low temperatures to reversibly ablate peripheral nerves. The authors tested the hypothesis that a single cryoneurolysis treatment would decrease phantom pain 4 months later.

METHODS

The authors enrolled patients with a lower-limb amputation and established phantom pain. Each received a single-injection femoral and sciatic nerve block with lidocaine and was subsequently randomized to receive either ultrasound-guided percutaneous cryoneurolysis or sham treatment at these same locations. The primary outcome was the change in average phantom pain intensity between baseline and 4 months as measured with a numeric rating scale (0 to 10), after which an optional crossover treatment was offered. Investigators, participants, and clinical staff were masked to treatment group assignment with the exception of the treating physician performing the cryoneurolysis, who had no subsequent participant interaction.

RESULTS

Pretreatment phantom pain scores were similar in both groups, with a median [quartiles] of 5.0 [4.0, 6.0] for active treatment and 5.0 [4.0, 7.0] for sham. After 4 months, pain intensity decreased by 0.5 [-0.5, 3.0] in patients given cryoneurolysis (n = 71) versus 0 [0, 3] in patients given sham (n = 73), with an estimated difference (95% CI) of -0.1 (-1.0 to 0.7), P = 0.759. Following their statistical gatekeeping protocol, the authors did not make inferences or draw conclusions on secondary endpoints. One serious adverse event occurred after a protocol deviation in which a femoral nerve cryolesion was induced just below the inguinal ligament-instead of the sensory-only saphenous nerve-which resulted in quadriceps weakness, and possibly a fall and clavicle fracture.

CONCLUSIONS

Percutaneous cryoneurolysis did not decrease chronic lower extremity phantom limb pain 4 months after treatment. However, these results were based upon the authors' specific study protocol, and since the optimal cryoneurolysis treatment parameters such as freeze duration and anatomic treatment location remain unknown, further research is warranted.

EDITOR’S PERSPECTIVE

Improving Preclinical Development of Novel Interventions to Treat Pain: Insanity Is Doing the Same Thing Over and Over and Expecting Different Results

Preclinical pain research has applied state-of-the-art methods over the past 40 years to describe, characterize, and image molecules, cells, and circuits in rodents to understand the pathophysiology of chronic pain. Despite generating a plethora of novel analgesic targets, pharmaceuticals for chronic pain treatment remain largely limited to the same 6 drug classes as present 40 years ago. It is possible that 40 years of effort has brought us to the verge of a paradigm shift and an explosion of novel analgesic drug classes with remarkable safety, efficacy, and tolerability. We think it more likely that advances will not occur until we follow the description of exciting discoveries with hypothesis testing using clinically relevant preclinical animal models and ethologically relevant outcome measures, which better reflect the clinical characteristics of chronic pain syndromes. Furthermore, to be valuable, experiments using such models must be conducted to the highest levels of internal validity, rigor, and reproducibility. Efforts by funders, most recently the Helping End Addiction Long-Term by the National Institutes of Health, aim to address some of these challenges and enhance communication and collaboration between preclinical and clinical investigators. However, the greater problem is a culture that emphasizes novelty and number of publications over scientific rigor and robust replication leading to a high likelihood of false-positive results. A path forward is provided by the evolution of clinical research beginning 50 years ago that resulted in methods to reduce bias and enhance transparency and ethics of reporting, moving from case reports to randomized controlled trials to innovative study designs with a focus on rigor, generalizability, and reproducibility. We argue that culture changed in clinical science in part because powerful forces outside the peer review system, especially from federal regulators that approve new drugs and human studies committees that addressed ethical failures of earlier research, mandated change in studies within their purview. Whether an external force will affect change in peclinical pain research is unclear.

Research approaches for evaluating opioid sparing in clinical trials of acute and chronic pain treatments: Initiative on Methods, Measurement, and Pain Assessment in Clinical Trials recommendations

ABSTRACT

Randomized clinical trials have demonstrated the efficacy of opioid analgesics for the treatment of acute and chronic pain conditions, and for some patients, these medications may be the only effective treatment available. Unfortunately, opioid analgesics are also associated with major risks (eg, opioid use disorder) and adverse outcomes (eg, respiratory depression and falls). The risks and adverse outcomes associated with opioid analgesics have prompted efforts to reduce their use in the treatment of both acute and chronic pain. This article presents Initiative on Methods, Measurement, and Pain Assessment in Clinical Trials (IMMPACT) consensus recommendations for the design of opioid-sparing clinical trials. The recommendations presented in this article are based on the following definition of an opioid-sparing intervention: any intervention that (1) prevents the initiation of treatment with opioid analgesics, (2) decreases the duration of such treatment, (3) reduces the total dosages of opioids that are prescribed for or used by patients, or (4) reduces opioid-related adverse outcomes (without increasing opioid dosages), all without causing an unacceptable increase in pain. These recommendations are based on the results of a background review, presentations and discussions at an IMMPACT consensus meeting, and iterative drafts of this article modified to accommodate input from the co-authors. We discuss opioid sparing definitions, study objectives, outcome measures, the assessment of opioid-related adverse events, incorporation of adequate pain control in trial design, interpretation of research findings, and future research priorities to inform opioid-sparing trial methods. The considerations and recommendations presented in this article are meant to help guide the design, conduct, analysis, and interpretation of future trials.

Percutaneous Neuromodulation of the Brachial Plexus and Sciatic Nerve for the Treatment of Acute Pain Following Surgery: Secondary Outcomes From a Multicenter, Randomized, Controlled Pilot Study

OBJECTIVES

We recently reported that percutaneous peripheral nerve stimulation (PNS or "neuromodulation") decreased pain and opioid consumption within the first two weeks following ambulatory surgery. However, the anatomic lead locations were combined for the analysis, and benefits for each location remain unknown. We therefore now report the effects of percutaneous PNS for brachial plexus and sciatic nerve leads separately.

MATERIALS AND METHODS

Before surgery, leads were implanted percutaneously to target the brachial plexus (N = 21) for rotator cuff repair or sciatic nerve (N = 40) for foot/ankle surgery, followed by a single injection of local anesthetic. Postoperatively, subjects were randomized in a double masked fashion to 14 days of electrical stimulation (N = 30) or sham/placebo (N = 31) using an external pulse generator. The primary outcome of interest was opioid consumption and pain scores evaluated jointly. Thus, stimulation was deemed effective if superior on either outcome and at least noninferior on the other.

RESULTS

For brachial plexus leads, during the first seven postoperative days pain measured with the numeric rating scale in participants given active stimulation was a median [interquartile range ] of 0.8 [0.5, 1.6] versus 3.2 [2.7, 3.5] in patients given sham (p < 0.001). For this same group, opioid consumption in participants given active stimulation was 10 mg [5, 20] versus 71 mg [35, 125] in patients given sham (p = 0.043). For sciatic nerve leads, pain scores for the active treatment group were 0.7 [0, 1.4] versus 2.8 [1.6, 4.6] in patients given sham (p < 0.001). During this same period, participants given active stimulation consumed 5 mg [0, 30] of opioids versus 40 mg [20, 105] in patients given sham (p = 0.004). Treatment effects did not differ statistically between the two locations.

CONCLUSIONS

Ambulatory percutaneous PNS of both the brachial plexus and sciatic nerve is an effective treatment for acute pain free of systemic side-effects following painful orthopedic surgery.

Heterogeneity in patterns of pain development after nerve injury in rats and the influence of sex

The genesis of neuropathic pain is complex, as sensory abnormalities may differ between patients with different or similar etiologies, suggesting mechanistic heterogeneity, a concept that is largely unexplored. Yet, data are usually grouped for analysis based on the assumption that they share the same underlying pathogenesis. Sex is a factor that may contribute to differences in pain responses. Neuropathic pain is more prevalent in female patients, but pre-clinical studies that can examine pain development in a controlled environment have typically failed to include female subjects. This study explored patterns of development of hyperalgesia-like behavior (HLB) induced by noxious mechanical stimulation in a neuropathic pain model (spared nerve injury, SNI) in both male and female rats, and autonomic dysfunction that is associated with chronic pain. HLB was analyzed across time, using both discrete mixture modeling and rules-based longitudinal clustering. Both methods identified similar groupings of hyperalgesia trajectories after SNI that were not evident when data were combined into groups by sex only. Within the same hyperalgesia development group, mixed models showed that development of HLB in females was delayed relative to males and reached a magnitude similar to or higher than males. The data also indicate that sympathetic tone (as indicated by heart rate variability) drops below pre-SNI level before or at the onset of development of HLB. This study classifies heterogeneity in individual development of HLB and identifies sexual dimorphism in the time course of development of neuropathic pain after nerve injury. Future studies addressing mechanisms underlying these differences could facilitate appropriate pain treatments.

Immediate Effects of a Continuous Peripheral Nerve Block on Postamputation Phantom and Residual Limb Pain: Secondary Outcomes From a Multicenter Randomized Controlled Clinical Trial

BACKGROUND

We recently reported that a 6-day continuous peripheral nerve block reduced established postamputation phantom pain 3 weeks after treatment ended. However, the immediate effects of perineural infusion (secondary outcomes) have yet to be reported.

METHODS

Participants from 5 enrolling academic centers with an upper or lower limb amputation and established phantom pain received a single-injection ropivacaine peripheral nerve block(s) and perineural catheter insertion(s). They were subsequently randomized to receive a 6-day ambulatory perineural infusion of either ropivacaine 0.5% or normal saline in a double-masked fashion. Participants were contacted by telephone 1, 7, 14, 21, and 28 days after the infusion started, with pain measured using the Numeric Rating Scale. Treatment effects were assessed using the Wilcoxon rank-sum test at each time point. Adjusting for 4 time points (days 1, 7, 14, and 21), P < .0125 was deemed statistically significant. Significance at 28 days was reported using methods from the original, previously published article.

RESULTS

Pretreatment average phantom and residual pain scores were balanced between the groups. The day after infusion initiation (day 1), average phantom, and residual limb pain intensity was lower in patients receiving local anesthetic (n = 71) versus placebo (n = 73): median [quartiles] of 0 [0-2.5] vs 3.3 [0-5.0], median difference (98.75% confidence interval [CI]) of -1.0 (-3.0 to 0) for phantom pain (P = .001) and 0 [0-0] vs 0 [0-4.3], and median difference 0.0 (-2.0 to 0.0) for residual limb pain (P < .001). Pain's interference with physical and emotional functioning as measured with the interference domain of the Brief Pain Inventory improved during the infusion on day 1 for patients receiving local anesthetic versus placebo: 0 [0-10] vs 10 [0-40], median difference (98.75% CI) of 0.0 (-16.0 to 0.0), P = .002. Following infusion discontinuation (day 6), a few differences were found between the active and placebo treatment groups between days 7 and 21. In general, sample medians for average phantom and residual limb pain scores gradually increased after catheter removal for both treatments, but to a greater degree in the control group until day 28, at which time the differences between the groups returned to statistical significance.

CONCLUSIONS

This secondary analysis suggests that a continuous peripheral nerve block decreases phantom and residual limb pain during the infusion, although few improvements were again detected until day 28, 3 weeks following catheter removal.

Seeding of breast cancer cell line (MDA-MB-231LUC+) to the mandible induces overexpression of substance P and CGRP throughout the trigeminal ganglion and widespread peripheral sensory neuropathy throughout all three of its divisions

Some types of cancer are commonly associated with intense pain even at the early stages of the disease. The mandible is particularly vulnerable to metastasis from breast cancer, and this process has been studied using a bioluminescent human breast cancer cell line (MDA-MB-231^LUC+). Using this cell line and anatomic and neurophysiologic methods in the trigeminal ganglion (TG), we examined the impact of cancer seeding in the mandible on behavioral evidence of hypersensitivity and on trigeminal sensory neurons. Growth of cancer cells seeded to the mandible after arterial injection of the breast cancer cell line in Foxn1 animals (allogeneic model) induced behavioral hypersensitivity to mechanical stimulation of the whisker pad and desensitization of tactile and sensitization of nociceptive mechanically sensitive afferents. These changes were not restricted to the site of metastasis but extended to sensory afferents in all three divisions of the TG, accompanied by widespread overexpression of substance P and CGRP in neurons through the ganglion. Subcutaneous injection of supernatant from the MDA-MB-231^LUC+ cell culture in normal animals mimicked some of the changes in mechanically responsive afferents observed with mandibular metastasis. We conclude that released products from these cancer cells in the mandible are critical for the development of cancer-induced pain and that the overall response of the system greatly surpasses these local effects, consistent with the widespread distribution of pain in patients. The mechanisms of neuronal plasticity likely occur in the TG itself and are not restricted to afferents exposed to the metastatic cancer microenvironment.

Ultrasound-Guided Percutaneous Peripheral Nerve Stimulation: A Pragmatic Effectiveness Trial of a Nonpharmacologic Alternative for the Treatment of Postoperative Pain

BACKGROUND

Percutaneous peripheral nerve stimulation (PNS) is an analgesic modality involving the insertion of a lead through an introducer needle followed by the delivery of electric current after needle withdrawal. This modality has been used extensively to treat chronic pain, but only small series have been published involving postoperative pain. The ultimate objective of this study is to determine the postoperative effects of percutaneous PNS following moderately to severely painful ambulatory surgery within a real-world clinical practice setting. The primary hypothesis is that surgical pain and opioid consumption during the initial 7 days after surgery will be reduced by percutaneous PNS compared with usual and customary analgesia (dual primary outcome measures).

DESIGN

A multicenter pragmatic effectiveness trial. We are randomizing participants having painful orthopedic surgical procedures of the upper and lower extremity to receive 14 days of either 1) electrical stimulation or 2) sham in a double-masked fashion. End points are being assessed at various time points over 12 postoperative months.

SUMMARY

The postoperative experience will be much improved if percutaneous PNS provides potent analgesia while concurrently decreasing opioid requirements following painful surgery. Because this modality can be administered for up to 60 days at home, it may provide postoperative analgesia that outlasts surgical pain yet has relatively few risks and, unlike opioids, has no systemic side effects or potential for abuse, addiction, and overdose. Percutaneous PNS has the potential to revolutionize postoperative analgesia as it has been practiced for the past century. This study will inform key stakeholders regarding an evidence-based nonpharmacologic approach to the management of postoperative pain.

A Painful Beginning: Early Life Surgery Produces Long-Term Behavioral Disruption in the Rat

Early life surgery produces peripheral nociceptive activation, inflammation, and stress. Early life nociceptive input and inflammation have been shown to produce long-term processing changes that are not restricted to the dermatome of injury. Additionally stress has shown long-term effects on anxiety, depression, learning, and maladaptive behaviors including substance abuse disorder and we hypothesized that early life surgery would have long-term effects on theses complex behaviors in later life. In this study surgery in the rat hindpaw was performed to determine if there are long-term effects on anxiety, depression, audiovisual attention, and opioid reward behaviors. Male animals received paw incision surgery and anesthesia or anesthesia alone (sham) at postnatal day 6. At 10 weeks after surgery, open field center zone entries were decreased, a measure of anxiety (n = 20) (P = 0.03) (effect size, Cohen's d = 0.80). No difference was found in the tail suspension test as a measure of depression. At 16-20 weeks, attentional performance in an operant task was similar between groups at baseline and decreased with audiovisual distraction in both groups (P < 0.001) (effect size, η² = 0.25), but distraction revealed a persistent impairment in performance in the surgery group (n = 8) (P = 0.04) (effect size, η² = 0.13). Opioid reward was measured using heroin self-administration at 16-24 weeks. Heroin intake increased over time in both groups during 24-h free access (P < 0.001), but was greater in the surgery group (P = 0.045), with a significant interaction between time and treatment (P < 0.001) (effect size, Cohen f ² = 0.36). These results demonstrate long-term disruptions in complex behaviors from surgical incision under anesthesia. Future studies to explore sex differences in early life surgery and the attendant peripheral neuronal input, stress, and inflammation will be valuable to understand emerging learning deficits, anxiety, attentional dysfunction, and opioid reward and their mechanisms. This will be valuable to develop optimal approaches to mitigate the long-term effects of surgery in early life.

Systemic administration of a β2-adrenergic receptor agonist reduces mechanical allodynia and suppresses the immune response to surgery in a rat model of persistent post-incisional hypersensitivity

Beta 2 adrenergic receptor (β2 AR) activation in the central and peripheral nervous system has been implicated in nociceptive processing in acute and chronic pain settings with anti-inflammatory and anti-allodynic effects of β2-AR mimetics reported in several pain states. In the current study, we examined the therapeutic efficacy of the β2-AR agonist clenbuterol in a rat model of persistent postsurgical hypersensitivity induced by disruption of descending noradrenergic signaling in rats with plantar incision. We used growth curve modeling of ipsilateral mechanical paw withdrawal thresholds following incision to examine effects of treatment on postoperative trajectories. Depletion of spinal noradrenergic neurons delayed recovery of hypersensitivity following incision evident as a flattened slope compared to non-depleted rats (-1.8 g/day with 95% CI -2.4 to -1.085, p < 0.0001). Chronic administration of clenbuterol reduced mechanical hypersensitivity evident as a greater initial intercept in noradrenergic depleted (6.2 g with 95% CI 1.6 to 10.8, p = 0.013) and non-depleted rats (5.4 g with 95% CI 1.2 to 9.6, p = 0.018) with plantar incision compared to vehicle treated rats. Despite a persistent reduction in mechanical hypersensitivity, clenbuterol did not alter the slope of recovery when modeled over several days (p = 0.053) or five weeks in depleted rats (p = 0.64). Systemic clenbuterol suppressed the enhanced microglial activation in depleted rats and reduced the density of macrophage at the site of incision. Direct spinal infusion of clenbuterol failed to reduce mechanical hypersensitivity in depleted rats with incision suggesting that beneficial effects of β2-AR stimulation in this model are largely peripherally mediated. Lastly, we examined β2-AR distribution in the spinal cord and skin using in-situ hybridization and IHC. These data add to our understanding of the role of β2-ARs in the nervous system on hypersensitivity after surgical incision and extend previously observed anti-inflammatory actions of β2-AR agonists to models of surgical injury.

Ambulatory continuous peripheral nerve blocks to treat postamputation phantom limb pain: a multicenter, randomized, quadruple-masked, placebo-controlled clinical trial

Phantom limb pain is thought to be sustained by reentrant neural pathways, which provoke dysfunctional reorganization in the somatosensory cortex. We hypothesized that disrupting reentrant pathways with a 6-day-long continuous peripheral nerve block reduces phantom pain 4 weeks after treatment. We enrolled patients who had an upper- or lower-limb amputation and established phantom pain. Each was randomized to receive a 6-day perineural infusion of either ropivacaine or normal saline. The primary outcome was the average phantom pain severity as measured with a Numeric Rating Scale (0-10) at 4 weeks, after which an optional crossover treatment was offered within the following 0 to 12 weeks. Pretreatment pain scores were similar in both groups, with a median (interquartile range) of 5.0 (4.0, 7.0) for each. After 4 weeks, average phantom limb pain intensity was a mean (SD) of 3.0 (2.9) in patients given local anesthetic vs 4.5 (2.6) in those given placebo (difference [95% confidence interval] 1.3 [0.4, 2.2], P = 0.003). Patients given local anesthetic had improved global impression of change and less pain-induced physical and emotional dysfunction, but did not differ on depression scores. For subjects who received only the first infusion (no self-selected crossover), the median decrease in phantom limb pain at 6 months for treated subjects was 3.0 (0, 5.0) vs 1.5 (0, 5.0) for the placebo group; there seemed to be little residual benefit at 12 months. We conclude that a 6-day continuous peripheral nerve block reduces phantom limb pain as well as physical and emotional dysfunction for at least 1 month.

Peripheral nerve injury in rats induces alternations in choice behavior associated with food reinforcement

Operant methods that allow animals to avoid painful stimuli are interpreted to assess the aversive quality of pain; however, such measurements require investigator-initiated stimuli to animals. Here we developed a shuttle maze test to repeatedly assess activity associated nociception without forced stimulation. Rats ambulate back and forth between two treat feeders by taking either a short route with a prickly surfaced arch or a longer route with a smooth floor. L5-L6 spinal nerve ligation (SNL) reduced the preference for the short route with the arch, correlated with hypersensitivity in the hind paw. Oral gabapentin restored the short route preference and reduced hypersensitivity in SNL rats, and blockade of spinal α2-adrenoceptors reduced gabapentin's effects on hypersensitivity but not on preference index. These results suggest that SNL injury alters behavior in the shuttle maze test and that the shuttle maze test shows comparable results to reflexive hypersensitivity after SNL in magnitude and response to gabapentin.

Incisional Nociceptive Input Impairs Attention-related Behavior and Is Associated with Reduced Neuronal Activity in the Prefrontal Cortex in Rats

WHAT WE ALREADY KNOW ABOUT THIS TOPIC

WHAT THIS ARTICLE TELLS US THAT IS NEW: BACKGROUND:: Cognitive capacity may be reduced from inflammation, surgery, anesthesia, and pain. In this study, we hypothesized that incision-induced nociceptive input impairs attentional performance and alters neuronal activity in the prefrontal cortex.

METHODS

Attentional performance was measured in rats by using the titration variant of the 5-choice serial reaction time to determine the effect of surgical incision and anesthesia in a visual attention task. Neuronal activity (single spike and local field potentials) was measured in the medial prefrontal cortex in animals during the task.

RESULTS

Incision significantly impaired attention postoperatively (area under curve of median cue duration-time 97.2 ± 56.8 [n = 9] vs. anesthesia control 25.5 ± 14.5 s-days [n = 9], P = 0.002; effect size, η = 0.456). Morphine (1 mg/kg) reduced impairment after incision (area under curve of median cue duration-time 31.6 ± 36.7 [n = 11] vs. saline 110 ± 64.7 s-days [n = 10], P < 0.001; η = 0.378). Incision also decreased cell activity (n = 24; 1.48 ± 0.58 vs. control, 2.93 ± 2.02 bursts/min; P = 0.002; η = 0.098) and local field potentials (n = 28; η = 0.111) in the medial prefrontal cortex.

CONCLUSIONS

These results show that acute postoperative nociceptive input from incision reduces attention-related task performance and decreases neuronal activity in the medial prefrontal cortex. Decreased neuronal activity suggests nociceptive input is more than just a distraction because neuronal activity increases during audiovisual distraction with similar behavioral impairment. This suggests that nociceptive input and the medial prefrontal cortex may contribute to attentional impairment and mild cognitive dysfunction postoperatively. In this regard, pain may affect postoperative recovery and return to normal activities through attentional impairment by contributing to lapses in concentration for routine and complex tasks.

Tachykinins modulate nociceptive responsiveness and sensitization: In vivo electrical characterization of primary sensory neurons in tachykinin knockout (Tac1 KO) mice

Since the failure of specific substance P antagonists to induce analgesia, the role of tachykinins in the development of neuropathic pain states has been discounted. This conclusion was reached without studies on the role of tachykinins in normal patterns of primary afferents response and sensitization or the consequences of their absence on the modulation of primary mechanonociceptive afferents after injury. Nociceptive afferents from animals lacking tachykinins (Tac1 knockout) showed a disrupted pattern of activation to tonic suprathreshold mechanical stimulation. These nociceptors failed to encode the duration and magnitude of natural pronociceptive stimuli or to develop mechanical sensitization as consequence of this stimulation. Moreover, paw edema, hypersensitivity, and weight bearing were also reduced in Tac1 knockout mice 24 h after paw incision surgery. At this time, nociceptive afferents from these animals did not show the normal sensitization to mechanical stimulation or altered membrane electrical hyperexcitability as observed in wild-type animals. These changes occurred despite a similar increase in calcitonin gene-related peptide immunoreactivity in sensory neurons in Tac1 knockout and normal mice. Based on these observations, we conclude that tachykinins are critical modulators of primary nociceptive afferents, with a preeminent role in the electrical control of their excitability with sustained activation or injury.

Capsaicin-induced pain and sensitisation in the postpartum period

BACKGROUND

Recovery from Caesarean delivery in women and surgical nerve injury in animals after delivery is more rapid than expected, an effect reversed in animals by spinal injection of an oxytocin receptor antagonist. We hypothesised that endogenous modulation of acute pain is altered postpartum.

METHODS

Endogenous inhibition of acute pain in a conditioned pain modulation paradigm or endogenous sensitisation by topical capsaicin was tested in women who were breastfeeding 10-14 days after Caesarean delivery and age-matched controls (n=80 total: 20 per group and 20 per test). The study was powered to detect a difference in area of hyperalgesia after capsaicin of 33%. Capsaicin-evoked pain was recorded in women, and capsaicin-evoked mechanical hypersensitivity was measured in rats 48 h after delivery and in age-matched female and male animals.

RESULTS

There was no effect of the postpartum period in the endogenous sensitisation assay in women, and the conditioned pain modulation assay failed to produce analgesia in either group. Postpartum women, however, reported less intense pain than controls at the end of topical capsaicin exposure (1.3 [1.4] vs 2.0 [2.0] on 0-10 verbal scale), and acute hypersensitivity after capsaicin was less in postpartum than control rats (withdrawal threshold 25 [15] vs 3.6 [1] g).

CONCLUSIONS

These results agree with a recent report that oxytocin may desensitise the transient receptor potential for vanilloid-1 channel, although other explanations, including hormone effects, are possible. These results do not, however, support the inhibition of capsaicin-evoked spinal sensitisation in the postpartum period.

CLINICAL TRIAL REGISTRATION

NCT01843517.

Plasticity and Function of Spinal Oxytocin and Vasopressin Signaling during Recovery from Surgery with Nerve Injury

WHAT WE ALREADY KNOW ABOUT THIS TOPIC

WHAT THIS ARTICLE TELLS US THAT IS NEW: BACKGROUND:: Recovery from pain after surgery is faster after cesarean delivery than after other abdominal procedures. The authors hypothesized that recovery in rats after surgery could be reversed by antagonism of spinal oxytocin or vasopressin receptors, that there may be a sex difference, and that spinal oxytocin innervation could change after surgery.

METHODS

Male and female rats underwent partial spinal nerve ligation surgery. Effects of nonselective and selective oxytocin and vasopressin 1A receptor antagonists on mechanical hypersensitivity during partial recovery were assessed (n = 8 to 14/group). Oxytocin immunoreactivity in the dorsal horn of the spinal cord (n = 7 to 8/group) and messenger RNA (mRNA) expression for oxytocin-binding receptors in dorsal root ganglia and spinal cord (n = 8/group) were measured.

RESULTS

Intrathecal injection of oxytocin and vasopressin receptor antagonists were similarly effective at reducing withdrawal threshold (in all experiments from 22 [19, 26] median [first quartile, third quartile]) g to 8.3 [6.4, 12] g after injection) in both sexes, while having no or minimal effects in animals without surgery. Oxytocin fiber immunoreactivity was 3- to 5-fold greater in lumbar than other regions of the spinal cord and was increased more than 2-fold in lumbar cord ipsilateral to surgery. Injury was also associated with a 6.5-fold increase in oxytocin receptor and a 2-fold increase in vasopressin 1A receptor messenger RNA expression in the L4 dorsal root ganglion ipsilateral to surgery.

CONCLUSIONS

These findings suggest that the capacity for oxytocin signaling in the spinal cord increases after surgery and that spinal oxytocin signaling plays ongoing roles in both sexes in recovery from mechanical hypersensitivity after surgery with known nerve injury.

Blockade of α2-adrenergic or metabotropic glutamate receptors induces glutamate release in the locus coeruleus to activate descending inhibition in rats with chronic neuropathic hypersensitivity

Locus coeruleus (LC)-spinal noradrenergic projections are important to endogenous analgesic mechanisms and can be activated by local glutamate signaling in the LC. The current study examined the local glutamatergic, GABAergic, and noradrenergic influences on glutamate release in the LC and noradrenergic descending inhibition in rats 6 weeks after spinal nerve ligation (SNL). Intra-LC injection of the α2 adrenoceptor antagonist idazoxan or the group 2 metabotropic glutamate receptor (mGluR) antagonist (RS)-α-Methyl-4-tetrazolylphenylglycine (MTPG) increased withdrawal thresholds in SNL animals and this was reversed by the blockade of α-amino-3-hydroxy-5-methyl- 4-isoxazolepropionic acid (AMPA) receptors in the LC or α2-adrenoceptors in the spinal cord, but not in normal animals. Neither blockade of GABA-A nor GABA-B receptors in the LC affected withdrawal thresholds in normal and SNL animals. Intra-LC perfusion of idazoxan increased extracellular glutamate in the LC in SNL animals but not in normal animals. Intra-LC perfusion of MTPG increased extracellular glutamate in the LC in both normal and SNL animals. These results suggest that local noradrenaline and glutamate tonically inhibit glutamate release in the LC after peripheral nerve injury and this may contribute to reduced descending inhibition in response to noxious input during chronic neuropathic pain.

Post-discharge hyperpolarization is an endogenous modulatory factor limiting input from fast-conducting nociceptors (AHTMRs)

Peripheral somatosensory neurons are frequently exposed to mechanical forces. Strong stimuli result in neuronal activation of high-threshold mechanosensory afferent neurons, even in the absence of tissue damage. Among these neurons, fast-conducting nociceptors (A-fiber high-threshold mechanoreceptors (AHTMRs)) are normally resistant to sustained activation, transiently encoding the mechanical stimulus intensity but not its full duration. This rapidly adapting response seems to depend on changes in the electrical excitability of the membrane of these afferent neurons during sustained stimulation, a restraint mechanism that disappears following sensitization. Here, we examine the mechanism by which strong peripheral activation of mechanoreceptors elicits this control process in the absence of tissue injury and temporally silences afferent neurons despite ongoing stimulation. To study this, mechanoreceptors in Sprague-Dawley rats were accessed at the soma in the dorsal root ganglia from T11 and L4/L5. Neuronal classification was performed using receptive field characteristics and passive and active electrical properties. Sustained mechanical nociceptive stimulation in the absence of tissue damage of AHTMRs induces a rapid membrane hyperpolarization and a period of reduced responsiveness to the stimuli. Moreover, this phenomenon appears to be unique to this subset of afferent neurons and is absent in slow-conducting C-mechanonociceptors (C-fiber high-threshold mechanoreceptors) and rapidly adapting fast-conducting low-threshold mechanoreceptors. Furthermore, this mechanism for rapid adaptation and reducing ongoing input is ablated by repeated strong stimuli and in sensitized AHTMRs after chronic neuropathic injury. Further studies to understand the underling molecular mechanisms behind this phenomenon and their modulation during the development of pathological conditions may provide new targets to control nociceptive hyperexcitability and chronic pain.

Psychosocial Stress Delays Recovery of Postoperative Pain Following Incisional Surgery in the Rat

Psychosocial factors such as anxiety, depression and catastrophizing, commonly associated with established chronic pain, also may be associated with an increased risk of chronic postsurgical pain (CPSP) when present preoperatively. We used a repeat social defeat (RSD) paradigm to induce psychosocial stress in rodents prior to incisional surgery of the paw. Mixed effects growth curve models were utilized to examine resolution of mechanical hypersensitivity in rats for four weeks following surgery. Eight days following surgery, immunohistochemistry was conducted to examine glial activation as well as evoked neuronal activation in the spinal cord. Here we document that RSD resulted in reduced weight gain and increased depressive symptoms prior to surgery. Rats exposed to RSD displayed delayed resolution of mechanical hypersensitivity in the ipsilateral paw following surgery compared to non-defeated rats. Prior exposure to RSD significantly increased microglial activation and neuronal sensitization (pERK-IR) within the ipsilateral spinal cord. In conclusion, we found that chronic social stress alters the neurobiological response to surgical injury, resulting in slowed recovery. This model maybe useful for future interventional studies examining the mechanistic interactions between depression and risk of CPSP.

Gestational Obstructive Sleep Apnea: Biomarker Screening Models and Lack of Postpartum Resolution

STUDY OBJECTIVES

To measure prevalence and severity of third trimester obstructive sleep apnea and evaluate postpartum resolution. To assess a novel biomarker for screening for obstructive sleep apnea in pregnancy.

METHODS

This prospective observational study was performed at Wake Forest School of Medicine obstetrics clinics between April 2014 and December 2015. Fractional exhaled nitric oxide measurements and sleep studies were obtained and compared at 32 0/7 to 35 6/7 weeks gestation and postpartum. Exhaled nitric oxide and risk factors for the development of gestational sleep apnea were evaluated for predictive ability independently and in screening models.

RESULTS

Of 76 women enrolled, 73 performed valid sleep studies in pregnancy and 65 had an additional valid study 6 to 15 weeks postpartum. Twenty-four women (37%) had gestational sleep apnea compared with 23 (35%) with postpartum sleep apnea (P > .99). Eight of 11 women (73%) retested 6 to 8 months postpartum had persistent sleep apnea. Exhaled nitric oxide had moderate discrimination screening for sleep apnea in pregnancy (area under the receiver operating characteristic curve = 0.64). A model utilizing exhaled nitric oxide, pregnancy-specific screening, and Mallampati score improved ability to identify women at risk for gestational sleep apnea (sensitivity = 46%, specificity = 91% and likelihood ratio = 5.11, area under receiver operating characteristic curve = 0.75).

CONCLUSIONS

Obstructive sleep apnea is common in the early postpartum period and often persisted at least 6 months. Exhaled nitric oxide as a sole biomarker to screen for sleep apnea in pregnancy has only modest discrimination. Combined with additional parameters sensitivity and specificity improved.

CLINICAL TRIAL REGISTRATION

Registry: ClinicalTrials.gov, Identifier: NCT02100943, Title: Exhaled Nitric Oxide as a Biomarker of Gestational Obstructive Sleep Apnea and Persistence Postpartum, URL: https://clinicaltrials.gov/ct2/show/NCT02100943.

Pupil responses and pain ratings to heat stimuli: Reliability and effects of expectations and a conditioning pain stimulus

BACKGROUND

The locus coeruleus (LC) signals salience to sensory stimuli and these responses can modulate the experience of pain stimuli. The pupil dilation response (PDR) to noxious stimuli is thought to be a surrogate for LC responses, but PDR response to Peltier-controlled noxious heat stimuli, the most commonly used method in experimental pain research, has not been described.

NEW METHOD

Healthy volunteers were presented with randomly presented heat stimuli of 5 sec duration and provided pain intensity ratings to each stimulus. Pupillometry was performed and a method developed to quantify the PDR relevant to these stimuli. The stimulus response, reliability, and effect of commonly used manipulations on pain experience were explored.

RESULTS

A method of artifact removal and adjusting for lag from stimulus initiation to PDR response was developed, resulting in a close correlation between pain intensity rating and PDR across a large range of heat stimuli. A reliable assessment of PDR within an individual was achieved with fewer presentations as heat stimulus intensity increased. The correlation between pain rating and PDR was disrupted when cognitive load is increased by manipulating expectations or presenting a second pain stimulus.

COMPARISON WITH EXISTING METHODS

The PDR began later after skin heating than electrical stimuli and this is the first examination of the PDR using standard nociceptive testing and manipulations of expectations and competing noxious stimulation.

CONCLUSIONS

A method is described applying PDR to standard heat nociceptive testing, demonstrating stimulus response, reliability, and disruption by cognitive manipulation.

A Randomized Controlled Trial Comparing Two Multimodal Analgesic Techniques in Patients Predicted to Have Severe Pain After Cesarean Delivery

BACKGROUND

Improved pain control after cesarean delivery remains a challenging objective. Poorly treated acute pain following delivery is associated with an increased risk of chronic pain and depression. This study was conducted to determine whether the addition of systemic acetaminophen and an increased dose of intrathecal morphine would further reduce acute pain. The primary outcome was pain intensity with movement at 24 hours postoperatively. Secondary measures included persistent pain and depression at 8 weeks.

METHODS

Seventy-four parturients scheduled for elective cesarean delivery under spinal anesthesia that were predicted to be above the 80th percentile for evoked pain intensity based on a 3-item preoperative screening questionnaire were enrolled. Patients in the intervention group received 300 mcg spinal morphine and 1 gram acetaminophen every 6 hours for 24 hours postoperatively. Patients in the control group received 150 mcg spinal morphine and placebo tablets. All patients received scheduled ibuprofen by mouth and IV morphine patient-controlled analgesia. At 24 hours, patients rated their pain intensity with movement, at rest, on average, and worst score using a visual analog scale for pain (100-mm unmarked line). The presence of persistent pain and depression was assessed at 8 weeks using the Edinburgh postpartum depression survey.

RESULTS

Providing a higher dose of spinal morphine combined with systemic acetaminophen to patients predicted to be at high risk for severe post-cesarean delivery pain significantly reduced evoked pain scores with movement at 24 hours (mean ± SD: 46 ± 25 mm in control group versus 31 ±17 mm in intervention group, P = 0.009; 95% confidence interval for the difference between means: 4 mm, 26 mm). There was no difference in the incidence of persistent pain (13% (4/30) in control group versus 10% (3/30) in intervention group, P > 0.99), or depression at 8 weeks postoperatively (10% (3/30) in control group versus 13% (4/30) in intervention group, P > 0.99).

CONCLUSIONS

Adding a higher dose of intrathecal morphine and oral acetaminophen to a multimodal pain regimen in patients predicted to be at risk for high acute postpartum pain after cesarean delivery results in a significant reduction of acute postoperative pain scores at 24 hours.

Mechanical sensibility of nociceptive and non-nociceptive fast-conducting afferents is modulated by skin temperature

The ability to distinguish mechanical from thermal input is a critical component of peripheral somatosensory function. Polymodal C fibers respond to both stimuli. However, mechanosensitive, modality-specific fast-conducting tactile and nociceptor afferents theoretically carry information only about mechanical forces independent of the thermal environment. We hypothesize that the thermal environment can nonetheless modulate mechanical force sensibility in fibers that do not respond directly to change in temperature. To study this, fast-conducting mechanosensitive peripheral sensory fibers in male Sprague-Dawley rats were accessed at the soma in the dorsal root ganglia from T11 or L4/L5. Neuronal identification was performed using receptive field characteristics and passive and active electrical properties. Neurons responded to mechanical stimuli but failed to generate action potentials in response to changes in temperature alone, except for the tactile mechanical and cold sensitive neurons. Heat and cold ramps were utilized to determine temperature-induced modulation of response to mechanical stimuli. Mechanically evoked electrical activity in non-nociceptive, low-threshold mechanoreceptors (tactile afferents) decreased in response to changes in temperature while mechanically induced activity was increased in nociceptive, fast-conducting, high-threshold mechanoreceptors in response to the same changes in temperature. These data suggest that mechanical activation does not occur in isolation but rather that temperature changes appear to alter mechanical afferent activity and input to the central nervous system in a dynamic fashion. Further studies to understand the psychophysiological implications of thermal modulation of fast-conducting mechanical input to the spinal cord will provide greater insight into the implications of these findings.

Disruption of Spinal Noradrenergic Activation Delays Recovery of Acute Incision-Induced Hypersensitivity and Increases Spinal Glial Activation in the Rat

Results of clinical studies suggest that descending inhibitory controls from the brainstem are important for speeding recovery from pain after surgery. We examined the effects of destroying spinally projecting noradrenergic neurons via intrathecally administered antibody to dopamine β-hydroxylase conjugated to saporin (DβH-saporin) on recovery in an acute incisional pain model. Mechanical and thermal paw withdrawal thresholds and nonevoked spontaneous guarding scores were tested for several weeks postoperatively and analyzed using mixed effects growth curve modeling. DβH-saporin treatment resulted in a significant prolongation in the duration of mechanical and to a lesser degree thermal hypersensitivity in the ipsilateral paw of incised rats but did not increase the duration of spontaneous guarding. DβH-saporin treatment was also associated with increased microglial and astrocyte activation in the ipsilateral spinal cord 21 days after incision compared with immunoglobulin G-saporin treated controls. Chronic intrathecal administration of the α2 adrenergic receptor antagonist atipamezole (50-200 μg/d) produced similar effects. These data suggest that spinally projecting noradrenergic pathways and spinal α2 adrenergic receptor activation are important for speeding recovery from hypersensitivity after surgical incision possibly by reducing spinal glial activation. Interventions that augment the noradrenergic system might be important to speed recovery from pain after surgery.

PERSPECTIVE

Endogenous descending spinal noradrenergic activation promotes resolution of incision-induced hypersensitivity and inhibits spinal microglial and astrocyte activation in part through α2 adrenergic receptors.

Failure of intrathecal ketorolac to reduce remifentanil-induced postinfusion hyperalgesia in humans

In rodents, acute exposure to opioids results in transient antinociception followed by longer lasting hypersensitivity to tactile or thermal stimuli, a phenomenon termed opioid-induced hyperalgesia. This hypersensitivity can be blocked or reversed by intrathecally administered cyclooxygenase inhibitors, including ketorolac, suggesting a role for spinal prostaglandins. In surgical patients, the dose of intraoperative opioid, particularly the short-acting drug, remifentanil, is directly related to increased pain and opioid requirements for many hours postoperatively. In addition, experimentally induced tactile hypersensitivity in humans is exaggerated after cessation of remifentanil infusions. The degree of this experimental opioid-induced hyperalgesia is reduced by systemic treatment with cyclooxygenase inhibitors, and investigators have speculated that this reduction reflects the actions in the central nervous system, most likely in the spinal cord. To test this hypothesis, we measured cerebrospinal fluid prostaglandin E2 concentrations during and after remifentanil infusion in 30 volunteers. These volunteers received intrathecal ketorolac or saline in a random, blinded manner during intravenous remifentanil infusion after generation of hypersensitivity by topical capsaicin. Remifentanil reduced pain to noxious heat stimuli and reduced areas of capsaicin-induced hypersensitivity similarly in those receiving intrathecal ketorolac or saline. The primary outcome measure, area of capsaicin-induced hypersensitivity after stopping remifentanil, showed a similar increase in those receiving ketorolac as in those receiving saline. Cerebrospinal fluid prostaglandin E2 concentrations did not increase during postinfusion hyperalgesia compared with those during infusion, and they were not increased during infusion compared with those in historical controls. These data fail to support the hypothesis that acute opioid-induced hyperalgesia reflects spinal cyclooxygenase activation causing central sensitization.

Intrathecal clonidine and adenosine: effects on pain and sensory processing in patients with chronic regional pain syndrome

Chronic pain may be accompanied by hyperalgesia and allodynia, and analgesic interventions may reduce these hypersensitivity phenomena. Preclinical data suggest that intrathecal clonidine and adenosine reduce hypersensitivity, but only clonidine reduces pain; therefore, we tested the effects of these interventions in patients with chronic pain. Twenty-two subjects with pain and hyperalgesia in a lower extremity from complex regional pain syndrome were recruited in a double-blind crossover study to receive intrathecal clonidine, 100 μg, or adenosine, 2 mg. Primary outcome measure was proportion with ≥30% reduction in pain 2 hours after injection, and secondary measures were pain report, areas of hypersensitivity, and temporal summation to heat stimuli. Treatments did not differ in the primary outcome measure (10 met success criterion after clonidine administration and 5 after adenosine administration), although they did differ in pain scores over time, with clonidine having a 3-fold greater effect (P = 0.014). Both drugs similarly reduced areas of hyperalgesia and allodynia by approximately 30% and also inhibited temporal summation. The percentage change in pain report did not correlate with the percentage change in areas of hyperalgesia (P = 0.09, r = 0.08) or allodynia (P = 0.24, r = 0.24) after drug treatment. Both intrathecal clonidine and adenosine acutely inhibit experimentally induced and clinical hypersensitivity in patients with chronic regional pain syndrome. Although these drugs do not differ in analgesia by the primary outcome measure, their difference in effect on pain scores over time and lack of correlation between effect on pain and hypersensitivity suggest that analgesia does not parallel antihyperalgesia with these treatments.

Assessment of attention threshold in rats by titration of visual cue duration during the five choice serial reaction time task

BACKGROUND

The 5 choice serial reaction time task (5CSRTT) is commonly used to assess attention in rodents. We sought to develop a variant of the 5CSRTT that would speed training to objective success criteria, and to test whether this variant could determine attention capability in each subject.

NEW METHOD

Fisher 344 rats were trained to perform a variant of the 5CSRTT in which the duration of visual cue presentation (cue duration) was titrated between trials based upon performance. The cue duration was decreased when the subject made a correct response, or increased with incorrect responses or omissions. Additionally, test day challenges were provided consisting of lengthening the intertrial interval and inclusion of a visual distracting stimulus.

RESULTS

Rats readily titrated the cue duration to less than 1s in 25 training sessions or less (mean±SEM, 22.9±0.7), and the median cue duration (MCD) was calculated as a measure of attention threshold. Increasing the intertrial interval increased premature responses, decreased the number of trials completed, and increased the MCD. Decreasing the intertrial interval and time allotted for consuming the food reward demonstrated that a minimum of 3.5s is required for rats to consume two food pellets and successfully attend to the next trial. Visual distraction in the form of a 3Hz flashing light increased the MCD and both premature and time out responses.

COMPARISON WITH EXISTING METHOD

The titration variant of the 5CSRTT is a useful method that dynamically measures attention threshold across a wide range of subject performance, and significantly decreases the time required for training. Task challenges produce similar effects in the titration method as reported for the classical procedure.

CONCLUSIONS

The titration 5CSRTT method is an efficient training procedure for assessing attention and can be utilized to assess the limit in performance ability across subjects and various schedule manipulations.

Peripheral nerve injury and gabapentin, but not their combination, impair attentional behavior via direct effects on noradrenergic signaling in the brain

Chronic pain after peripheral nerve damage is often accompanied by a reduction in prefrontal cortex (PFC)-related cognitive functions, which are regulated by noradrenaline, released from efferents originating in the locus coeruleus (LC). L5 to L6 spinal nerve ligation (SNL) in rats increased tissue content and extracellular concentrations of noradrenaline in microdialysates from the PFC, and impaired attentional level in the novel object recognition test. Systemic gabapentin, commonly used to treat chronic pain, impaired the novel object recognition task in normal but not SNL animals. Accordingly, gabapentin increased c-fos expression in LC neurons and noradrenaline release in the PFC in normal animals, but in SNL animals, gabapentin failed to increase c-fos expression in LC neurons projecting to the PFC and failed to increase noradrenaline release in the PFC. In contrast, locally perfused gabapentin reduced noradrenaline release in the PFC in vivo and in PFC synaptosomes in vitro. SNL- and gabapentin-induced impairment of novel object recognition task were reversed by intraperitoneal injection of the α1-adrenoceptor antagonist prazosin. These results suggest that increase in noradrenergic tone, induced by nerve injury or gabapentin, impairs PFC functions possibly via α1-adrenoceptor-mediated mechanisms; that the net effect of gabapentin on noradrenaline release in the PFC would depend on sometimes opposing actions at different sites; and that nerve injury selectively impairs the response to gabapentin in PFC-projecting neurons in the LC.

Nerve injury induces a new profile of tactile and mechanical nociceptor input from undamaged peripheral afferents

Chronic pain after nerve injury is often accompanied by hypersensitivity to mechanical stimuli, yet whether this reflects altered input, altered processing, or both remains unclear. Spinal nerve ligation or transection results in hypersensitivity to mechanical stimuli in skin innervated by adjacent dorsal root ganglia, but no previous study has quantified the changes in receptive field properties of these neurons in vivo. To address this, we recorded intracellularly from L4 dorsal root ganglion neurons of anesthetized young adult rats, 1 wk after L5 partial spinal nerve ligation (pSNL) or sham surgery. One week after pSNL, hindpaw mechanical withdrawal threshold in awake, freely behaving animals was decreased in the L4 distribution on the nerve-injured side compared with sham controls. Electrophysiology revealed that high-threshold mechanoreceptive cells of A-fiber conduction velocity in L4 were sensitized, with a seven-fold reduction in mechanical threshold, a seven-fold increase in receptive field area, and doubling of maximum instantaneous frequency in response to peripheral stimuli, accompanied by reductions in after-hyperpolarization amplitude and duration. Only a reduction in mechanical threshold (minimum von Frey hair producing neuronal activity) was observed in C-fiber conduction velocity high-threshold mechanoreceptive cells. In contrast, low-threshold mechanoreceptive cells were desensitized, with a 13-fold increase in mechanical threshold, a 60% reduction in receptive field area, and a 40% reduction in instantaneous frequency to stimulation. No spontaneous activity was observed in L4 ganglia, and the likelihood of recording from neurons without a mechanical receptive field was increased after pSNL. These data suggest massively altered input from undamaged sensory afferents innervating areas of hypersensitivity after nerve injury, with reduced tactile and increased nociceptive afferent response. These findings differ importantly from previous preclinical studies, but are consistent with clinical findings in most patients with chronic neuropathic pain.

Fast-conducting mechanoreceptors contribute to withdrawal behavior in normal and nerve injured rats

Fast-conducting myelinated high-threshold mechanoreceptors (AHTMR) are largely thought to transmit acute nociception from the periphery. However, their roles in normal withdrawal and in nerve injury-induced hyperalgesia are less well accepted. Modulation of this subpopulation of peripheral neurons would help define their roles in withdrawal behaviors. The optically active proton pump, ArchT, was placed in an adeno-associated virus-type 8 viral vector with the CAG promoter and was administered by intrathecal injection resulting in expression in myelinated neurons. Optical inhibition of peripheral neurons at the soma and transcutaneously was possible in the neurons expressing ArchT, but not in neurons from control animals. Receptive field characteristics and electrophysiology determined that inhibition was neuronal subtype-specific with only AHTMR neurons being inhibited. One week after nerve injury the AHTMR are hyperexcitable, but can still be inhibited at the soma and transcutaneously. Withdrawal thresholds to mechanical stimuli in normal and in hyperalgesic nerve-injured animals also were increased by transcutaneous light to the affected hindpaw. This suggests that AHTMR neurons play a role not only in threshold-related withdrawal behavior in the normal animal, but also in sensitized states after nerve injury. This is the first time this subpopulation of neurons has been reversibly modulated to test their contribution to withdrawal-related behaviors before and after nerve injury. This technique may prove useful to define the role of selective neuronal populations in different pain states.

Intrathecal ketorolac does not improve acute or chronic pain after hip arthroplasty: a randomized controlled trial

Hypersensitivity to mechanical stimuli following surgery has been reported in patients who subsequently develop chronic pain after surgery. In animals, peripheral injury increases prostaglandin production in the spinal cord, and spinal cyclooxygenase inhibitors reduce hypersensitivity after injury. We therefore tested the hypothesis that spinal ketorolac reduces hypersensitivity and acute and chronic pain after hip arthroplasty ( www.clinicaltrials.gov NCT 00621530). Sixty-two patients undergoing total hip arthroplasty with spinal anesthesia were randomized to receive 13.5 mg hyperbaric bupivacaine with spinal saline or 13.5 mg hyperbaric bupivacaine with 2 mg preservative-free ketorolac. The primary outcome was area of hypersensitivity surrounding the wound 48 h after surgery, but this only occurred in 4 patients, precluding assessment of this outcome. The groups did not differ in acute pain, acute opioid use, or pain incidence or severity at 2 and 6 months after surgery. There were no serious adverse events. Our results suggest that a single spinal dose of ketorolac does not substantially reduce acute surgical pain and is thus unlikely to reduce the risk of persistent incisional pain.

Gabapentin increases extracellular glutamatergic level in the locus coeruleus via astroglial glutamate transporter-dependent mechanisms

Gabapentin has shown to be effective in animals and humans with acute postoperative and chronic pain. Yet the mechanisms by which gabapentin reduces pain have not been fully addressed. The current study performed in vivo microdialysis in the locus coeruleus (LC) in normal and spinal nerve ligated (SNL) rats to examine the effect of gabapentin on extracellular glutamate concentration and its mechanisms of action with focus on presynaptic GABA-B receptors, astroglial glutamate transporter-1 (GLT-1), and interactions with α2δ subunits of voltage-gated Ca(2+) channels and endogenous noradrenaline. Basal extracellular concentration and tissue content of glutamate in the LC were greater in SNL rats than normal ones. Intravenously administered and LC-perfused gabapentin increased extracellular glutamate concentration in the LC. The net amount of glutamate increased by gabapentin is larger in SNL rats compared with normal ones, although the percentage increases from the baseline did not differ. The gabapentin-related α2δ ligand pregabalin increased extracellular glutamate concentration in the LC, whereas another α2δ ligand, 3-exo-aminobicyclo [2.2.1] heptane-2-exo-carboxylic acid (ABHCA), did not. Selective blockade by the dihydrokainic acid or knock-down of GLT-1 by the small interfering RNA abolished the gabapentin-induced glutamate increase in the LC, whereas blockade of GABA-B receptors by the CGP-35348 and depletion of noradrenalin by the dopamine-β-hydroxylase antibody conjugated to saporin did not. These results suggest that gabapentin induces glutamate release from astrocytes in the LC via GLT-1-dependent mechanisms to stimulate descending inhibition. The present study also demonstrates that this target of gabapentin in astrocytes does not require interaction with α2δ subunits in neurons.

Valproate prevents dysregulation of spinal glutamate and reduces the development of hypersensitivity in rats after peripheral nerve injury

The present study examined whether the histone deacetylase inhibitor valproate prevents downregulation of glutamate transporters in the primary cultured astrocytes and in the spinal cord after L5-L6 spinal nerve ligation (SNL) and whether this action of valproate on spinal glutamate transporters prevents spinal glutamate dysregulation and development of hypersensitivity after SNL. In cultured astrocytes, valproate prevented downregulation of glutamate transporter-1 (GLT-1) and glutamate-aspartate transporter in a concentration-dependent manner. Repeated oral administration of valproate reduced the development of hypersensitivity and prevented the downregulation of spinal GLT-1 and glutamate-aspartate transporter expression in rats after SNL, but did not affect mechanical nociception and expression of those transporters in normal rats. Valproate's effects on hypersensitivity and spinal GLT-1 expression in SNL rats were blocked by intrathecal administration of the selective GLT-1 blocker dihydrokainic acid or the GLT-1 selective small interfering RNA (siRNA). Extracellular glutamate concentration in the spinal cord, measured by microdialysis, was increased in animals with SNL or after GLT-1 selective siRNA treatment, and valproate prevented the SNL-induced glutamate increase. These results suggest that valproate reduces the development of chronic pain after nerve injury in part by preventing downregulation of glutamate transporters, especially GLT-1, to maintain normal extracellular glutamate concentrations in the spinal cord.

PERSPECTIVE

This study demonstrates that valproate prevents the downregulation of glutamate transporters in the spinal cord, which contributes in part to the development of chronic pain after nerve injury. Given clinical availability and established safety profiles, perioperative use of valproate should be tested to prevent chronic pain after surgery.

The puerperium alters spinal cord plasticity following peripheral nerve injury

Tissue and nerve damage can result in chronic pain. Yet, chronic pain after cesarean delivery is remarkably rare in women and hypersensitivity from peripheral nerve injury in rats resolves rapidly if the injury occurs in the puerperium. Little is known regarding the mechanisms of this protection except for a reliance on central nervous system oxytocin signaling. Here we show that the density of inhibitory noradrenergic fibers in the spinal cord is greater when nerve injury is performed in rats during the puerperium, whereas the expression of the excitatory regulators dynorphin A and neuregulin-1 in the spinal cord is reduced. The puerperium did not alter spinal cord microgial and astrocyte activation. Astrocyte activation, as measured by glial fibrillary acidic protein (GFAP) expression, was not evident in female rats with injury, regardless of delivery status suggesting sex differences in spinal astrocyte activation after injury. These results suggest a change in the descending inhibitory/facilitating balance on spinal nociception neurotransmission during the puerperium, as mechanisms for its protective effect against injury-induced hypersensitivity.

Resolution of pain after childbirth

BACKGROUND

Chronic pain after surgery occurs in 10-40% of individuals, including 5-20% of women after cesarean delivery in previous reports. Pain and depression 2 months after childbirth are independently associated with more severe acute post-delivery pain. Here we examine other predictors of pain at 2 months and determine the incidence of pain at 6 and 12 months after childbirth.

METHODS

Following Institutional Review Board approval, 1228 women were interviewed within 36 h of delivery. Of these, 937 (76%) were successfully contacted by telephone at 2 months, and, if they had pain, at 6 and 12 months after delivery. The primary outcome measure was presence of pain which began at the time of delivery. We also generated a model of severity of acute post-delivery pain and 2 month pain and depression.

RESULTS

Pain which began at the time of delivery was remarkably rare 6 and 12 months later (1.8% and 0.3% [upper 95% confidence limit, 1.2%], respectively). Past history of pain and degree of tissue damage at delivery accounted for 7.0% and 16.7%, respectively, of one aspect in the variability in acute post-delivery pain. Neither of these factors was associated with incidence of pain 2 months later.

CONCLUSIONS

Using a definition of new onset pain from delivery, we show a remarkably low incidence of pain 1 yr after childbirth, including those with surgical delivery. Additionally, degree of tissue trauma and history of chronic pain, risk factors for pain 2 months after other surgery, were unimportant to pain 2 months after cesarean or vaginal delivery.

Skin incision-induced receptive field responses of mechanosensitive peripheral neurons are developmentally regulated in the rat

Maturation of the nervous system results in changes in both central and peripheral processing. To better understand responses to injury in the young, developmental differences in the acute response to incision were investigated in both tactile and nociceptive myelinated peripheral mechanosensitive afferent neurons in vivo. Neuronal intrasomal recordings were performed in juvenile and infant rats in 34 L5 dorsal root ganglia, and each neuron was phenotypically defined. Neurons had a mechanosensitive receptive field in the glabrous skin on the plantar surface of the hind paw, which was characterized at baseline and for up to 45 min after incision. Fundamental maturational differences in the effect of incision were clear: in high-threshold nociceptive mechanoreceptors, the mechanical threshold decreased immediately and the receptive field size increased rapidly in juvenile rats but not in infant rats. Additionally, a divergence in changes in the instantaneous response frequency of tactile afferents occurred between the two ages. These differences may help explain maturational differences in responses to peripheral injury and suggest that differences in central nervous system responses may be partially mitigated by spatially confined and frequency-dependent differences resulting from tactile and nociceptive mechanosensitive input.

Predictors of headache before, during, and after pregnancy: a cohort study

OBJECTIVE

The present study endeavored to identify predictors of headache during pregnancy, shortly after delivery, and at 8-week follow-up.

BACKGROUND

Many women suffer from headaches during pregnancy and the post-partum period. However, little is known about factors that predict headache surrounding childbirth.

METHODS

Secondary analysis of longitudinal cohort study of 2434 parturients hospitalized for cesarean or vaginal delivery in 4 university hospitals in the United States and Europe. Data were gathered from interviews and review of medical records shortly after delivery; 972 of the women were contacted 8 weeks later to assess persistent headache. The primary outcome measures were experiencing headache during pregnancy, headache within 72 hours after delivery, and headache at 8 weeks after delivery.

RESULTS

Of the parturients, 10% experienced headache during pregnancy, 3.7% within 72 hours after delivery, and 3.6% at 8 weeks postdelivery. Compared to those without a history of headache, a history of headache prior to pregnancy was the strongest predictor of headache during pregnancy (9.8% vs 23.5%; risk ratio 2.4; 95% confidence interval [CI]: 1.4 to 4.0). Experiencing headache during pregnancy (adjusted hazard ratio HR 3.8; 95% CI: 2.4 to 6.2) and receiving needle-based regional anesthesia for pain treatment (adjusted hazard ratio 2.2; 95% CI: 1.1 to 4.5) were independently associated with headache within 72 hours after delivery with event rates of 11.1% and 10.5%, respectively. Compared to those without such a history, headache before pregnancy was significantly associated with experiencing headache 8 weeks after delivery (4.0% vs 23.8%; risk ratio = 6.0; 95% CI: 2.0 to 8.0), but headache during pregnancy or shortly after delivery was not. Several other psychosocial predictors (eg, somatization, smoking before pregnancy) were statistically associated with at least 1 headache outcome.

CONCLUSIONS

A history of headache prior to pregnancy is a strong predictor of headache during and after pregnancy, the latter independent of but compounded by spinal injection. Physicians should attend to prior headache history when making decisions about pain management during and after childbirth. As the lack of formal International Classification of Headache Disorders, 2nd Edition (ICHD-II), headache diagnoses is a limitation of this study, future longitudinal studies should replicate the present design while including headache subtyping consistent with ICHD-II nosology.