A simple contact heat experimental pain model for evaluation of analgesic agents in healthy volunteers

Tramadol Effects on Brain Activity During Cognitive and Emotional Empathy for Pain: A Randomized Controlled Study

Pain is perceived not only by personal experience but also vicariously. Pain empathy is the ability to share and understand other's intentions and emotions in their painful conditions, which can be divided into cognitive and emotional empathy. It remains unclear how centrally acting analgesics would modulate brain activity related to pain empathy and which component of pain empathy would be altered by analgesics. In this study, we examined the effects of the analgesic tramadol on the brain activity for pain empathy in healthy adults. We used 2 tasks to assess brain activity for pain empathy. In experiment 1, we used a well-established picture-based pain empathy task involving passive observation of other's pain. In experiment 2, we developed a novel pain empathy task to assess brain activity during cognitive and emotional empathy for pain separately in a single task. We conducted a double-blind, placebo-controlled within-subject crossover study with functional magnetic resonance imaging for 33 participants in experiment 1 and 31 participants in experiment 2, respectively. In experiment 1, we found that tramadol decreased activation in the supramarginal gyrus during observation of other's pain compared with placebo. Supramarginal gyrus activation correlated negatively with the thermal pain threshold. In experiment 2, we found that tramadol decreased activation in angular gyrus in cognitive empathy for pain compared with placebo but did not change brain activity in emotional empathy for pain. PERSPECTIVE: Centrally acting analgesics such as tramadol may have not only analgesic effects on self-experienced pain but also on the complex neural processing of pain empathy.

Heat pain detection threshold is associated with the area of secondary hyperalgesia following brief thermal sensitization: a study of healthy male volunteers

Introduction

The area of secondary hyperalgesia following brief thermal sensitization (BTS) of the skin and heat pain detection thresholds (HPDT) may both have predictive abilities in regards to pain sensitivity and clinical pain states. The association between HPDT and secondary hyperalgesia, however, remains unsettled, and the dissimilarities in physiologic properties suggest that they may represent 2 distinctively different pain entities. The aim of this study was to investigate the association between HPDT and BTS-induced secondary hyperalgesia.

Methods

A sample of 121 healthy male participants was included and tested on 2 separate study days with BTS (45°C, 3 minutes), HPDT, and pain during thermal stimulation (45°C, 1 minute). Areas of secondary hyperalgesia were quantified after monofilament pinprick stimulation. The pain catastrophizing scale (PCS) and hospital anxiety and depression scale (HADS) were also applied.

Results

A significant association between HPDT and the size of the area of secondary hyperalgesia (p<0.0001) was found. The expected change in area of secondary hyperalgesia due to a 1-degree increase in HPDT was estimated to be −27.38 cm², 95% confidence interval (CI) of −37.77 to −16.98 cm², with an R² of 0.19. Likewise, a significant association between HADS-depression subscore and area of secondary hyperalgesia (p=0.046) was found, with an estimated expected change in secondary hyperalgesia to a 1-point increase in HADS-depression subscore of 11 cm², 95% CI (0.19–21.82), and with R² of 0.03. We found no significant associations between secondary hyperalgesia area and PCS score or pain during thermal stimulation.

Conclusion

HPDT and the area of secondary hyperalgesia after BTS are significantly associated; however, with an R² of only 19%, HPDT only offers a modest explanation of the inter-participant variation in the size of the secondary hyperalgesia area elicited by BTS.

Is heat pain detection threshold associated with the area of secondary hyperalgesia following brief thermal sensitization? A study of healthy volunteers - design and detailed plan of analysis

Background

Several factors are believed to influence the development and experience of pain.

Human clinical pain models are central tools, in the investigation of basic physiologic pain responses, and can be applied in patients as well as in healthy volunteers. Each clinical pain model investigates different aspects of the human pain response.

Brief thermal sensitization induces a mild burn injury, resulting in development of primary hyperalgesia at the site of stimulation, and secondary hyperalgesia surrounding the site of stimulation. Central sensitization is believed to play an important role in the development of secondary hyperalgesia; however, a possible association of secondary hyperalgesia following brief thermal sensitization and other heat pain models remains unknown. Our aim with this study is to investigate how close the heat pain detection threshold is associated with the size of the area of secondary hyperalgesia induced by the clinical heat pain model: Brief thermal sensitization.

Methods and design

We aim to include 120 healthy participants. The participants will be tested on two separate study days with the following procedures: i) Brief thermal sensitization, ii) heat pain detection threshold and iii) pain during thermal stimulation. Additionally, the participants will be tested with the Pain Catastrophizing Scale and Hospital Anxiety and Depression Scale questionnaires.

We conducted statistical simulations based on data from our previous study, to estimate an empirical power of 99.9 % with α of 0.05. We define that an R² < 0.25 and predictive intervals larger than +/−150 cm² are indications of a weak association.

Discussion

The area of secondary hyperalgesia may serve as a quantitative measure of the central sensitization induced by cutaneous heat stimulation, and thus may be a biomarker of an individual’s pain sensitivity. The number of studies investigating secondary hyperalgesia is growing; however basic knowledge of the physiologic aspects of secondary hyperalgesia in humans is still incomplete. We therefore find it interesting to investigate if HPDT, a known quantitative sensory test, is associated with areas of secondary hyperalgesia following brief thermal sensitization

Trial registration

Clinicaltrials.gov (Identifier: NCT02527395). Danish Research Ethics Committee (Identifier: H-8-2014-012). Danish Data Protection Agency (Identifier: 30-1436).

Electronic supplementary material

The online version of this article (doi:10.1186/s12871-016-0193-2) contains supplementary material, which is available to authorized users.

The Area of Secondary Hyperalgesia following Heat Stimulation in Healthy Male Volunteers: Inter- and Intra-Individual Variance and Reproducibility

INTRODUCTION

Clinical pain models can be applied when investigating basic physiologic pain responses in healthy volunteers. Several pain models exist; however, only few have been adequately validated. Our primary aim with this prospective study was to investigate the intra- and inter-individual variation in secondary hyperalgesia elicited by brief thermal sensitization (45°C for 3 min) in healthy volunteers.

MATERIAL AND METHODS

Fifty healthy volunteers were included. Areas of secondary hyperalgesia following brief thermal sensitization were investigated by 2 observers on 4 experimental days, with a minimum interval of 7 days. Additionally, heat pain detection threshold and pain during thermal stimulation (45°C for 1 min.), and the psychological tests Pain Catastrophizing Scale and Hospital Anxiety and Depression Score were applied.

RESULTS

For areas of secondary hyperalgesia, an intra-observer intra-person correlation of 0.85, 95% CI [0.78, 0.90], an intra-observer inter-person correlation of 0.03, 95% CI [0.00, 0.16], and a coefficient of variation of 0.17, 95% CI [0.14, 0.21] was demonstrated. Four percent of the study population had areas of secondary hyperalgesia both below the 1st and above the 3rd quartile considering all included participants. Heat pain detection threshold predicted area of secondary hyperalgesia with an adjusted R2 of 0.20 (P = 0.0006).

CONCLUSIONS

We have demonstrated a low intra-individual, and a high inter-individual variation in thermally induced secondary hyperalgesia. We conclude that brief thermal sensitization produce secondary hyperalgesia with a high level of reproducibility, which can be applied to investigate different phenotypes related to secondary hyperalgesia in healthy volunteers.

TRIAL REGISTRATION

clinicaltrials.gov NCT02166164.