Demarcation of secondary hyperalgesia zones: Punctate stimulation pressure matters

Dose safety and pharmacodynamics of subcutaneous bupivacaine in a novel extended-release microparticle formulation: A phase 1, dose-ascending study in male volunteers

A novel microparticle-based extended-release local anaesthetic containing a bupivacaine/poly-lactic-co-glycolic acid (PLGA; LIQ865A) or plain bupivacaine (LIQ865B) was examined in a first-in-human trial. The objectives were to examine the dose safety/tolerability and pharmacodynamics. Randomized subcutaneous injections of LIQ865A (n = 16) or LIQ865B (n = 12) and diluent, contralaterally, were administered in a dose-ascending manner (150- to 600-mg bupivacaine). Subjects were admitted 24 h post-injection and followed for 30 days post-injection. The risk ratios (RRs; 95% CI) of erythematous reactions for LIQ865A versus diluent was 9.00 (1.81-52.23; P = 0.006) and for LIQ865B versus diluent 2.50 (0.69-9.94; P = 0.37). The RR for the development of hematomas (LIQ865A versus diluent) were 3.25 (1.52-8.16; P = 0.004) and 4.00 (0.72-24.89; P = 0.32) (LIQ865B versus diluent). Subcutaneous indurations persisting for 4-13 weeks were seen in 6/16 subjects receiving LIQ865A. One subject receiving LIQ865A (600-mg bupivacaine) developed intermittent central nervous system (CNS) symptoms of local anaesthetic systemic toxicity (85 min to 51 h post-injection) coinciding with plasma peak bupivacaine concentrations (490-533 ng/ml). Both LIQ865 formulations demonstrated dose-dependent hypoesthesia and hypoalgesia. The duration of analgesia ranged between 37 and 86 h. The overall number of local adverse events, however, prohibits clinical application without further pharmacological modifications.

Low-dose naltrexone for treatment of pain in patients with fibromyalgia: a randomized, double-blind, placebo-controlled, crossover study

Introduction

Fibromyalgia (FM) is a chronic fluctuating, nociplastic pain condition. Naltrexone is a µ-opioid-receptor antagonist; preliminary studies have indicated a pain-relieving effect of low-dose naltrexone (LDN) in patients with FM. The impetus for studying LDN is the assumption of analgesic efficacy and thus reduction of adverse effects seen from conventional pharmacotherapy.

Objectives

First, to examine if LDN is associated with analgesic efficacy compared with control in the treatment of patients with FM. Second, to ascertain the analgesic efficacy of LDN in an experimental pain model in patients with FM evaluating the competence of the descending inhibitory pathways compared with controls. Third, to examine the pharmacokinetics of LDN.

Methods

The study used a randomized, double-blind, placebo-controlled, crossover design and had a 3-phase setup. The first phase included baseline assessment and a treatment period (days -3 to 21), the second phase a washout period (days 22-32), and the third phase a baseline assessment followed by a treatment period (days 33-56). Treatment was with either LDN 4.5 mg or an inactive placebo given orally once daily. The primary outcomes were Fibromyalgia Impact Questionnaire revised (FIQR) scores and summed pain intensity ratings (SPIR).

Results

Fifty-eight patients with FM were randomized. The median difference (IQR) for FIQR scores between LDN and placebo treatment was -1.65 (18.55; effect size = 0.15; P = 0.3). The median difference for SPIR scores was -0.33 (6.33; effect size = 0.13; P = 0.4).

Conclusion

Outcome data did not indicate any clinically relevant analgesic efficacy of the LDN treatment in patients with FM.

Somatosensory Outcomes Following Re-Surgery in Persistent Severe Pain After Groin Hernia Repair: A Prospective Observational Study

Purpose

After groin hernia repair (globally more than 20 million/year) 2-4% will develop persistent severe pain (PSPG). Pain management is challenging and may require multimodal interventions, including re-surgery. Quantitative somatosensory testing (QST) is an investigational psychophysiological tool with the potential to uncover the pathophysiological mechanisms behind the pain, ie, revealing neuropathic or inflammatory components. The primary objective was to examine and describe the underlying pathophysiological changes in the groin areas by QST before and after re-surgery with mesh removal and selective neurectomy.

Patients and Methods

Sixty patients with PSPG scheduled for re-surgery and with an inflammatory "component" indicated by blunt pressure algometry were examined in median (95% CI) 7.9 (5.8-11.5) months before and 4.0 (3.5-4.6) months after re-surgery. The QST-analyses included standardized assessments of cutaneous mechanical/thermal detection and pain thresholds. Suprathreshold heat stimuli were applied. Deep tissue sensitivity was tested by pressure algometry. Testing sites were the groin areas and the lower arm. Before/after QST data were z-transformed.

Results

Re-surgery resulted in median changes in rest, average, and maximal pain intensity scores of -2.0, -2.5, and -2.0 NRS (0/10) units, respectively (P = 0.0001), and proportional increases in various standardized functional scores (P = 0.0001). Compared with the control sites, the cutaneous somatosensory detection thresholds of the painful groin were increased before re-surgery and increased further after re-surgery (median difference: 1.28 z-values; P = 0.001), indicating a successive post-surgical loss of nerve fiber function ("deafferentation"). Pressure algometry thresholds increased after re-surgery (median difference: 0.30 z-values; P = 0.001).

Conclusion

In this subset of patients with PSPG who underwent re-surgery, the procedure was associated with improved pain and functional outcomes. While the increase in somatosensory detection thresholds mirrors the surgery-induced cutaneous deafferentation, the increase in pressure algometry thresholds mirrors the removal of the deep "pain generator". The QST-analyses are useful adjuncts in mechanism-based somatosensory research.

Methodology and applicability of the human contact burn injury model: A systematic review

The contact burn injury model is an experimental contact thermode-based physiological pain model primarily applied in research of drug efficacy in humans. The employment of the contact burn injury model across studies has been inconsistent regarding essential methodological variables, challenging the validity of the model. This systematic review analyzes methodologies, outcomes, and research applications of the contact burn injury model. Based on these results, we propose an improved contact burn injury testing paradigm. A literature search was conducted (15-JUL-2020) using PubMed, EMBASE, Web of Science, and Google Scholar. Sixty-four studies were included. The contact burn injury model induced consistent levels of primary and secondary hyperalgesia. However, the analyses revealed variations in the methodology of the contact burn injury heating paradigm and the post-burn application of test stimuli. The contact burn injury model had limited testing sensitivity in demonstrating analgesic efficacy. There was a weak correlation between experimental and clinical pain intensity variables. The data analysis was limited by the methodological heterogenicity of the different studies and a high risk of bias across the studies. In conclusion, although the contact burn injury model provides robust hyperalgesia, it has limited efficacy in testing analgesic drug response. Recommendations for future use of the model are being provided, but further research is needed to improve the sensitivity of the contact burn injury method. The protocol for this review has been published in PROSPERO (ID: CRD42019133734).

Human surrogate models of central sensitization: A critical review and practical guide

Background

As in other fields of medicine, development of new medications for management of neuropathic pain has been difficult since preclinical rodent models do not necessarily translate to the clinics. Aside from ongoing pain with burning or shock‐like qualities, neuropathic pain is often characterized by pain hypersensitivity (hyperalgesia and allodynia), most often towards mechanical stimuli, reflecting sensitization of neural transmission.

Data treatment

We therefore performed a systematic literature review (PubMed‐Medline, Cochrane, WoS, ClinicalTrials) and semi‐quantitative meta‐analysis of human pain models that aim to induce central sensitization, and generate hyperalgesia surrounding a real or simulated injury.

Results

From an initial set of 1569 reports, we identified and analysed 269 studies using more than a dozen human models of sensitization. Five of these models (intradermal or topical capsaicin, low‐ or high‐frequency electrical stimulation, thermode‐induced heat‐injury) were found to reliably induce secondary hyperalgesia to pinprick and have been implemented in multiple laboratories. The ability of these models to induce dynamic mechanical allodynia was however substantially lower. The proportion of subjects who developed hypersensitivity was rarely provided, giving rise to significant reporting bias. In four of these models pharmacological profiles allowed to verify similarity to some clinical conditions, and therefore may inform basic research for new drug development.

Conclusions

While there is no single “optimal” model of central sensitization, the range of validated and easy‐to‐use procedures in humans should be able to inform preclinical researchers on helpful potential biomarkers, thereby narrowing the translation gap between basic and clinical data.

Significance

Being able to mimic aspects of pathological pain directly in humans has a huge potential to understand pathophysiology and provide animal research with translatable biomarkers for drug development. One group of human surrogate models has proven to have excellent predictive validity: they respond to clinically active medications and do not respond to clinically inactive medications, including some that worked in animals but failed in the clinics. They should therefore inform basic research for new drug development.

High-dose naloxone: Effects by late administration on pain and hyperalgesia following a human heat injury model. A randomized, double-blind, placebo-controlled, crossover trial with an enriched enrollment design

Severe chronic postsurgical pain has a prevalence of 4–10% in the surgical population. The underlying nociceptive mechanisms have not been well characterized. Following the late resolution phase of an inflammatory injury, high-dose μ-opioid-receptor inverse agonists reinstate hypersensitivity to nociceptive stimuli. This unmasking of latent pain sensitization has been a consistent finding in rodents while only observed in a limited number of human volunteers. Latent sensitization could be a potential triggering venue in chronic postsurgical pain. The objective of the present trial was in detail to examine the association between injury-induced secondary hyperalgesia and naloxone-induced unmasking of latent sensitization. Healthy volunteers (n = 80) received a cutaneous heat injury (47°C, 420 s, 12.5 cm²). Baseline secondary hyperalgesia areas were assessed 1 h post-injury. Utilizing an enriched enrollment design, subjects with a magnitude of secondary hyperalgesia areas in the upper quartile (‘high-sensitizers’ [n = 20]) and the lower quartile (‘low-sensitizers’ [n = 20]) were selected for further study. In four consecutive experimental sessions (Sessions 1 to 4), the subjects at two sessions (Sessions 1 and 3) received a cutaneous heat injury followed 168 h later (Sessions 2 and 4) by a three-step target-controlled intravenous infusion of naloxone (3.25 mg/kg), or normal saline. Assessments of secondary hyperalgesia areas were made immediately before and stepwise during the infusions. Simple univariate statistics revealed no significant differences in secondary hyperalgesia areas between naloxone and placebo treatments (P = 0.215), or between ‘high-sensitizers’ and ‘low-sensitizers’ (P = 0.757). In a mixed-effects model, secondary hyperalgesia areas were significantly larger following naloxone as compared to placebo for ‘high-sensitizers’ (P < 0.001), but not ‘low-sensitizers’ (P = 0.651). Although we could not unequivocally demonstrate naloxone-induced reinstatement of heat injury-induced hyperalgesia, further studies in clinical postsurgical pain models are warranted.

Within- and between-session reliability of secondary hyperalgesia induced by electrical high-frequency stimulation

BACKGROUND

An increasing number of studies are focusing on secondary hyperalgesia to better understand central sensitization, as this phenomenon may play an important role in persistent pain. Recent studies have shown that, compared to the classical high-frequency stimulation protocol (HFS) at 100 Hz, a protocol using 42 Hz stimulation induces a more intense and a larger area of secondary hyperalgesia (SH).

OBJECTIVES

The aim of this study was to investigate the within- and between-session reliability of SH induced by this optimized HFS protocol.

METHODS

Thirty-two healthy subjects received HFS to their volar forearm in two sessions, separated by at least 2 weeks. SH was assessed by measuring the area size of increased sensitivity to pinprick stimuli after applying HFS, the sensitivity to pinprick stimuli after applying HFS and the change in pinprick sensitivity after versus before HFS. Assessments were made before HFS, and 30, 35 and 40 min after HFS. Relative and absolute reliability were analysed using intraclass correlation coefficients (ICCs), coefficients of variation (CVs), standard error of means (SEMs) and the minimum detectable changes (MDCs).

RESULTS

The area of SH showed good to excellent within-session and between-session relative reliability (ICCs > 0.80), except for the change in pinprick sensitivity, which showed close to poor between-session relative reliability (ICC = 0.53). Furthermore, measures of absolute reliability generally demonstrated large between-subject variability and significant fluctuations across repeated measurements.

CONCLUSIONS

HFS-induced hyperalgesia is suitable to discriminate or compare individuals but it may not be sensitive to changes due to an intervention.

SIGNIFICANCE

It is crucial to evaluate central sensitization adequately in humans. This study formally establishes the reliability of secondary hyperalgesia induced by electrical high-frequency stimulation. The results of this study will improve future studies investigating secondary hyperalgesia in humans.

A single session of hyperbaric oxygen therapy demonstrates acute and long-lasting neuroplasticity effects in humans: a replicated, randomized controlled clinical trial

Purpose

Animal studies have demonstrated anti-inflammatory, and anti-nociceptive properties of hyperbaric oxygen therapy (HBOT). However, physiological data are scarce in humans. In a recent experimental study, the authors used the burn injury (BI) model observing a decrease in secondary hyperalgesia areas (SHA) in the HBOT-group compared to a control-group. Surprisingly, a long-lasting neuroplasticity effect mitigating the BI-induced SHA-response was seen in the HBOT-preconditioned group. The objective of the present study, therefore, was to confirm our previous findings using an examiner-blinded, block-randomized, controlled, crossover study design.

Patients and methods

Nineteen healthy subjects attended two BI-sessions with an inter-session interval of ≥28 days. The BIs were induced on the lower legs by a contact thermode (12.5 cm², 47C°, 420 s). The subjects were block-randomized to receive HBOT (2.4 ATA, 100% O₂, 90 min) or ambient conditions ([AC]; 1 ATA, 21% O₂), dividing cohorts equally into two sequence allocations: HBOT-AC or AC-HBOT. All sensory assessments performed during baseline, BI, and post-intervention phases were at homologous time points irrespective of sequence allocation. The primary outcome was SHA, comparing interventions and sequence allocations.

Results

Data are mean (95% CI). During HBOT-sessions a mitigating effect on SHA was demonstrated compared to AC-sessions, ie, 18.8 (10.5–27.0) cm² vs 32.0 (20.1–43.9) cm² (P=0.021), respectively. In subjects allocated to the sequence AC-HBOT a significantly larger mean difference in SHA in the AC-session vs the HBOT-session was seen 25.0 (5.4–44.7) cm² (P=0.019). In subjects allocated to the reverse sequence, HBOT-AC, no difference in SHA between sessions was observed (P=0.55), confirming a preconditioning, long-lasting (≥28 days) effect of HBOT.

Conclusion

Our data demonstrate that a single HBOT-session compared to control is associated with both acute and long-lasting mitigating effects on BI-induced SHA, confirming central anti-inflammatory, neuroplasticity effects of hyperbaric oxygen therapy.

Aδ and not C fibers mediate thermal hyperalgesia to short laser stimuli after burn injury in man

It remains unclear which nerve fibers are responsible for mediating hyperalgesia after skin injury. Here, we examined the role of Aδ and C fibers in inflammatory hyperalgesia after a first-degree burn injury. A CO2 laser delivered ultrafast short constant-temperature heat pulses to the upper part of the lower leg to stimulate selectively the relatively fast-conducting thinly myelinated Aδ and the slowly conducting unmyelinated C fibers. Participants were asked to respond as fast as possible whenever they detected a thermal stimulus. Thresholds and reaction times to selective Aδ and C fiber activations were measured in the conditioned and the surrounding intact skin, at pre-injury, and 1 hour and 24 hours after injury. First-degree burn injury caused a significant decrease in Aδ fiber detection thresholds and a significant increase in the proportion of Aδ-fiber-mediated responses in the inflamed area 24 hours, but not 1 hour, after burn injury. No changes in heat perception were observed in the intact skin surrounding the injury. No group differences in C-fiber-mediated sensations were observed. Our findings indicate that quickly adapting Aδ fibers but not quickly adapting C fibers are sensitized when activated by short and ultrafast heat stimuli after skin burn injury. Our results further show that this change occurs between 1 hour and 24 hours after injury and that it does not extend to the skin surrounding the injury.

Effects of target-controlled infusion of high-dose naloxone on pain and hyperalgesia in a human thermal injury model: a study protocol: A randomized, double-blind, placebo-controlled, crossover trial with an enriched design

Mu-opioid-receptor antagonists have been extensively studied in experimental research as pharmacological tools uncovering mechanisms of pain modulation by the endogenous opioid system. In rodents, administration of high doses of mu-opioid-receptor antagonists after the resolution of an inflammatory injury has demonstrated reinstatement of nociceptive hypersensitivity indicating unmasking of latent sensitization. In a recent human study, pain hypersensitivity assessed as secondary hyperalgesia area (SHA), was reinstated 7 days after a mild thermal injury, in 4 out of 12 subjects after a naloxone infusion.The aims of the present study are first, to replicate our previous findings in a larger-sized study; second, to examine if high sensitizers (subjects presenting with large SHA after a thermal injury) develop a higher degree of hypersensitivity after naloxone challenge than low sensitizers (subjects presenting with restricted SHA after a thermal injury); and third to examine a dose-response relationship between 3 stable naloxone concentrations controlled by target-controlled infusion, and the unmasking of latent sensitization.Healthy participants (n = 80) underwent a screening day (day 0) with induction of a thermal skin injury (47°C, 420 seconds, 12.5 cm). Assessment of SHA was performed 1 and 2 hours after the injury. Using an enriched design, only participants belonging to the upper quartile of SHA (Q4, high sensitizers; n = 20) and the lower quartile of SHA (Q1, low sensitizers; n = 20) continued the study, comprising 4 consecutive days-days 1 to 4. Thermal skin injuries were repeated on day 1 and day 3, whereas day 2 and day 4 (7 days after day 1 and day 3, respectively) were target-controlled infusion days in which the subjects were randomly allocated to receive either naloxone (3.25 mg/kg, 4 mg/mL) or placebo (normal saline) intravenous. The primary outcome was SHA assessed by weighted-pin instrument (128 mN) 0, 1, 2, and 165 to 169 hours after the thermal injury (day 1-4). The secondary outcomes were pin-prick pain thresholds assessed by weighted-pin instrument (8-512 mN) at primary and secondary hyperalgesia areas (days 1-4).The naloxone-induced unmasking of latent sensitization is an interesting model for exploring the transition from acute to chronic pain. The results from the present study may provide valuable information regarding future research in persistent postsurgical pain states.