Placebo effects on cutaneous pain and itch: a systematic review and meta-analysis of experimental results and methodology

A narrative review of the placebo effect: historical roots, current applications, and emerging insights

PURPOSE

Placebo is a term to define physiologically inactive compounds used in treatment that causes physical and emotional changes. The placebo effect, driven by expectation and conditioning, plays a significant role in various conditions like pain, depression, and Parkinson's disease, while the nocebo effect can hinder treatment outcomes. Understanding mechanisms such as neuromodulation and genetics has gained importance in modern medicine. This review aims to explore the clinical relevance of placebo responses, particularly in neuropsychiatric disorders, and their potential in personalized medicine. By integrating placebo research into healthcare, it highlights opportunities to enhance treatment efficacy, improve patient well-being, and reduce reliance on pharmacological interventions.

METHODS

A comprehensive literature search was conducted in PubMed, Scopus, and Google Scholar databases. Recent studies were reviewed to evaluate placebo effects, and the variability of the placebo response in neuropsychiatric disorders was summarized.

RESULTS

Placebo effects significantly impact treatment outcomes across various conditions, including Parkinson's disease, depression, pain syndromes, and epilepsy. The mechanisms involve neurobiological and psychological factors, with evidence suggesting that placebo interventions can modulate neurotransmitter activity and improve patient well-being.

CONCLUSION

Integrating placebo research into clinical practice may enhance treatment outcomes, reduce drug dependency, and support personalized medicine by tailoring interventions to individual placebo responsiveness. Understanding placebo and nocebo mechanisms can optimize therapeutic strategies while minimizing unnecessary pharmacological treatments.

Moderators of nocebo effects in controlled experiments: A multi-level meta-analysis

Nocebo effects are a heterogenous phenomenon in which contextual cues trigger or exacerbate symptoms independently of active interventions. Suggestion, conditioning, and social observation are widely recognised as hallmark methods for inducing nocebo effects, but the extent to which nocebo effects are differentially influenced by suggestion type (e.g., direct or indirect suggestion) and mode of administration (e.g., verbal, textual, visual, etc.) across symptom domains remains unknown. We conducted a pre-registered meta-analysis (PROSPERO registration number CRD42023402097) to quantitatively synthesize available research on the factors that moderate effects in controlled nocebo experiments. Of 8469 search results, 105 experiments comprising 5017 participants and 391 effect sizes were analyzed. A multi-level meta-analysis revealed an overall moderate effect size for nocebo effects, g=0.50, [0.39, 0.62]. The magnitude of symptom expectancy effects was a significant moderator of nocebo effects. Verbal suggestion and social observation yielded moderate and comparable nocebo effects whereas technological devices, sham stimulation, and conditioning were independently associated with the induction of large nocebo effects. Greater specificity in the reporting of nocebo induction methods is required to elucidate the efficacy of different types of suggestions in inducing nocebo effects.

Do patients with fibromyalgia syndrome and healthy people differ in their opinions on placebo effects in routine medical care?

OBJECTIVES

Placebo effects can relieve acute and chronic pain in both research and clinical treatments by learning mechanisms. However, the application of placebo-based treatment strategies in routine medical care is questioned. The current study investigated the opinions of patients with fibromyalgia and healthy controls regarding learning of placebo effects and their practical applications.

METHOD

An online survey asked 158 age- and sex-matched adult patients and controls (79 per group) to rate the perceived influence of various placebo learning mechanisms on pain relief, and the acceptability and perceived effectiveness of placebo-based strategies (open-label, closed-label, dose-extending, and treatment-enhancing strategies). Respondents' knowledge about placebo effects was obtained through a 7-item quiz.

RESULTS

The groups did not differ in the perceived influence of placebo learning mechanisms on pain relief (p = 0.217). Controls considered closed-label and treatment-enhancing strategies more acceptable than patients (p = 0.003 and p < 0.001), whereas controls perceived all strategies more effective. In both groups, closed-label strategies were significantly less acceptable than any other strategy (p-values < 0.001), and treatment-enhancing or dose-extending strategies were most acceptable. Higher acceptability was predicted by higher perceived effectiveness ratings (p < 0.001). Also, increased placebo knowledge was related to higher acceptability (p = 0.03) and perceived effectiveness (p < 0.001).

DISCUSSION

This survey suggests that both the medical history of patients and knowledge about placebo effects affect the acceptability and perceived effectiveness of placebo-based strategies. Furthermore, strategies that are transparent, assumed effective, or combined with existing medical treatments are deemed most acceptable. Keeping these factors in mind is essential for the clinical implementation of placebo-based strategies in routine medical care.

How the magnitude and precision of pain predictions shape pain experiences

BACKGROUND

In Bayesian models including predictive processing, the magnitude and precision of pain expectancies are key determinants of perception. However, relatively few studies have directly tested whether this holds for pain, and results so far have been inconclusive. Here, we investigated expectancy effects on pain experiences and associated affective responses.

METHODS

In two studies, healthy participants (n = 30 in each) received painful electrical stimuli preceded by explicit pain predictions. In study 1, the magnitude of pain predictions and administered pain intensities were varied. In study 2, the magnitude and precision of pain predictions were varied, while administered pain intensity was kept constant. Experienced pain intensity was the primary outcome in both studies.

RESULTS

Pain experiences assimilated towards both under- and overpredictions of pain. In study 1, however, effects were small, if present at all, for non-painful stimuli and effects were not necessarily larger with predictions of greater magnitude. In study 2, assimilation of pain experiences appeared regardless of precision level, while no significant effects on EMG eyeblink startle responses were observed. Moreover, under- and overpredictions caused disappointment and relief, respectively, with greater disappointment upon precise than imprecise predictions.

CONCLUSIONS

The influence of pain predictions on pain might be more complex than assumed in simple instantiations of current theoretical frameworks, with no systematically stronger assimilation of pain experiences to larger and more precise predictions. Since overpredictions are associated with relief, but underpredictions with disappointment, these findings underline the importance of providing correct predictions when preparing for upcoming painful procedures.

SIGNIFICANCE STATEMENT

Our work supports, challenges, and extends the application of Bayesian and predictive processing frameworks to the influence of pain predictions on pain. Under- and overpredictions of pain yielded assimilation of pain experiences, but assimilation was not systematically stronger with larger prediction errors or greater precision. Moreover, under- and overpredictions resulted in disappointment and relief, respectively. This research signifies the importance of establishing accurate predictions of pain in clinical practice.

Development and validation of the General attitude towards Medication Questionnaire (GAMQ)

BACKGROUND

Attitudes towards medication can affect treatment outcomes and adherence through mechanisms such as placebo and nocebo effects. Questionnaires assessing both negative and positive attitudes towards medication in general, which can be used across a variety of settings and in both patient and non-patient samples, are however lacking. To fill this gap, we developed and validated the General Attitude towards Medication Questionnaire (GAMQ).

METHODS

Items were selected and adapted from existing questionnaires by a group of experts. Validation of the original Dutch version took place in 4 samples: 2 recruited from the general population (n = 508; n = 279) and 2 patient samples (patients with rheumatoid arthritis, n = 121; patients with atopic dermatitis, n = 70). We evaluated the psychometric properties of the GAMQ by determining the factor structure and its stability across samples, internal consistency, and convergent validity.

RESULTS

The GAMQ contains 6 positive and 6 negatively worded items. A factor structure was observed with three subscales, representing 'Trust in medication', 'Concerns about medication', and 'Reluctance to use medication'. The fit of the factor structure was satisfactory across samples, considering classic cut-offs, with an adequate or close to adequate fit. The total scale showed good internal consistency, good convergent validity with related scales (i.e., Beliefs about Medicines Questionnaire - General and a single medication attitude scale), and concurrent validity as reflected in associations with expectations about pain- and itch-relieving medication. It was not consistently or strongly associated with demographic or health-related characteristics.

CONCLUSIONS

The newly developed GAMQ showed satisfactory psychometric properties in a variety of populations, although limitations should be considered. The GAMQ is the first scale to assess both positive and negative attitudes towards medication in general, providing indicators of Trust, Concerns, and Reluctance regarding medication. The scale may be an informative measure for predicting treatment outcomes and adherence, as well as placebo and nocebo effects in diverse samples.

The Effect of Placebo on Pruritus in Patients with Chronic Urticaria: A Systematic Review and Meta-Analysis of Randomized Placebo-Controlled Trials

BACKGROUND

The anti-pruritic effect of placebo in patients with chronic urticaria has gained increasing attention in clinical research. However, the extent of placebo effect and its influencing factors in the treatment of chronic urticaria are not well understood.

OBJECTIVE

The objective of this systematic review and meta-analysis was to investigate the effect of placebo on pruritus in patients with chronic urticaria and to explore relevant influencing factors.

METHODS

PubMed, Embase, Web of Science, Cochrane Library, and PsycINFO were searched from inception to 10 July, 2024. Primary outcome included pruritus scores. The secondary outcomes focused on global symptoms and quality of life. Subgroup analyses and meta-regression analyses were conducted based on drug types, sample size, participants' age, and other variables. The Grading of Recommendations Assessment, Development, and Evaluation (GRADE) system and a trial sequential analysis were employed to establish the reliability of evidence.

RESULTS

A total of 65 eligible publications (including 67 randomized controlled trials) involving 10,704 patients with chronic urticaria were included. The pruritus scores decreased following placebo treatment (moderate evidence). In addition, favorable results were observed in global symptoms (moderate evidence) and quality of life (low evidence) after placebo treatment. Subgroup analyses indicated that the type of active medication in intervention groups was an influencing factor of placebo effect of pruritus. Meta-regression analyses demonstrated that the anti-pruritic effect of placebo was inversely correlated with sample size and positively correlated with participants' age. A trial sequential analysis provided further support for the anti-pruritic effect of placebo.

CONCLUSIONS

A substantial improvement of pruritus after placebo treatment was observed in patients with chronic urticaria. The anti-pruritic effect of placebo varied with sample size, participants' age, and type of active medication used. Future research should further investigate the effect size of placebo and clarify the potential mechanism.

PROSPERO REGISTRATION

The protocol was registered in the International Prospective Register of Systematic Reviews (PROSPERO) as CRD42023482608.

Mindfulness Effects on Anxiety: Disentangling the Role of Decentering and Treatment Expectations

Mindfulness interventions have been found to lower anxiety. However, the current literature has not adequately considered the role of its individual components and of placebo effects. In an online experiment using a balanced placebo design, we aimed to disentangle effects of decentering, a key component of mindfulness, and expectations, a key component of placebo effects, on anxiety related to the COVID-19 pandemic. One hundred twenty-eight adults were randomly assigned to one of four groups: placebo/mindful decentering, placebo/sham decentering, sham/mindful decentering, and sham/sham decentering. Instructions were provided using standardized audio instructions. Current anxiety was assessed pre- and postintervention with the Short State version of the State-Trait Anxiety Inventory. Mindful decentering was found to reduce anxiety postintervention, as compared to sham decentering, regardless of induced expectations regarding its effectiveness. Participants in the mindful decentering group also mentioned more decentering-related words than those in the sham decentering group. These findings indicate that a short, standardized, and online mindful decentering intervention can effectively decrease pandemic-related anxiety independently of one's expectations. These findings provide insights into the efficacy of the individual elements of mindfulness, highlighting decentering as an effective active component for anxiety relief. Moreover, these findings suggest that, in a nonclinical sample, individuals can apply mindful decentering with minimal training.

Social Learning of Placebo Effects in Pain: A Critical Review of the Literature and a Proposed Revised Model

Relatively recently, in 2009, experimental studies were undertaken to determine the role of social observational learning in forming hypoalgesic, analgesic and hyperalgesic responses to a placebo. The research findings obtained in studies published before 2018 were integrated and formed the basis of the theoretical model of social learning of placebo effects in pain proposed by Bajcar and Bąbel. This model considered the involvement of different types of modeling (ie, behavioral modeling, symbolic modeling, and verbal modeling) in shaping placebo hypoalgesia/analgesia and nocebo hyperalgesia. The model assumed that pain expectancies might be involved in observationally induced placebo effects in pain and that the effectiveness of observational learning in shaping placebo effects could be moderated by the observer's dispositions, especially empathy. Based on the latest research data, we propose a modified and significantly extended version of this model. The revised model includes the involvement of particular types of modeling in placebo effects and their role in shaping conscious pain-related expectancies. It explains the role of dispositional empathy in shaping observationally induced placebo effects. Notably, the extended version of the model considers the contribution of the characteristics of the observed person to the magnitude of placebo effects induced by social learning. PERSPECTIVE: The paper proposes a comprehensive theoretical approach to explaining the role of observational learning in shaping placebo effects in pain. The proposed model emphasizes the potential of this form of learning in shaping placebo responses and indicates factors that can modify the effectiveness of observational learning.

Optimizing placebo and minimizing nocebo effects through communication: e-learning and virtual reality training development

BACKGROUND

The effects of many treatments in healthcare are determined by factors other than the treatment itself. Patients' expectations and the relationship with their healthcare provider can significantly affect treatment outcomes and thereby play a major role in eliciting placebo and nocebo effects. We aim to develop and evaluate an innovative communication training, consisting of an e-learning and virtual reality (VR) training, for healthcare providers across all disciplines, to optimize placebo and minimize nocebo effects through healthcare provider-patient communication. The current paper describes the development, mid-term evaluation, optimization, and final evaluation of the communication training, conducted in The Netherlands.

METHODS

The development of both the e-learning and the VR training consisted of four phases: 1) content and technical development, 2) mid-term evaluation by healthcare providers and placebo/communication researchers, 3) optimization of the training, and 4) final evaluation by healthcare providers. To ensure the success, applicability, authenticity, and user-friendliness of the communication training, there was ongoing structural collaboration with healthcare providers as future end users, experts in the field of placebo/communication research, and educational experts in all phases.

RESULTS

Placebo/communication researchers and healthcare providers evaluated the e-learning positively (overall 7.9 on 0-10 scale) and the content was perceived as useful, accessible, and interesting. The VR training was assessed with an overall 6.9 (0-10 scale) and was evaluated as user-friendly and a safe method for practicing communication skills. Although there were some concerns regarding the authenticity of the VR training (i.e. to what extent the virtual patient reacts like a real patient), placebo and communication researchers, as well as healthcare providers, recognized the significant potential of the VR training for the future.

CONCLUSIONS

We have developed an innovative and user-friendly communication training, consisting of an e-learning and VR training (2D and 3D), that can be used to teach healthcare providers how to optimize placebo effects and minimize nocebo effects through healthcare provider-patient communication. Future studies can work on improved authenticity, translate the training into other languages and cultures, expand with additional VR cases, and measure the expected effects on providers communication skills and subsequently patient outcomes.

Change in pain expectations but no open-label placebo analgesia: An experimental study using the heat pain paradigm

BACKGROUND

Open-label placebos (OLP) prescribed without deception and with a convincing rationale have been shown to evoke powerful treatment effects. Patients' treatment expectations seem to influence the magnitude of the effect.

OBJECTIVE

We examined if two different OLP rationales increased pain tolerance and reduced pain intensity and unpleasantness in a standardized heat pain experiment.

METHODS

Participants (N = 71) who self-reported reoccurring pain for at least the last 3 months were randomly assigned to one of three groups. We compared a personal-emotional and a scientific-matter-of-fact rationale with a control group (CG) that received the same placebo without any rationale. The rationale suggested a desensitizing effect on pain perception and improved pain coping of the placebo, whereas in the CG it was introduced as an ointment for measurement. The primary outcomes were pre-post changes in pain tolerance, expected and experienced pain intensity and unpleasantness.

RESULTS

Participants showed a decrease in expected pain intensity, but not expected pain unpleasantness for both rationales. There were no differences in pain tolerance and experienced pain intensity and unpleasantness.

CONCLUSIONS

Our study suggests that evoking positive treatment expectations is not sufficient to elicit an OLP response. Possibly, the magnitude of expectations change in this study was not powerful enough to evoke an OLP effect. Additionally, it is possible that OLP effects in pain are unrelated to positive treatment expectations. The failure of OLP in our study is in contrast to a number of previous studies examining the effects of OLP in experimental and clinical pain.

SIGNIFICANCE

This study provides evidence that positive treatment expectations are not sufficient to evoke an open-label placebo effect in a standardized heat pain experiment. We showed that two different rationales improved participants treatment expectations, but failed to evoke a placebo effect in comparison to a control group that received the same placebo, labelled as an ointment to improve measurement quality.

Pain Rating is Worth a Thousand Words: Nocebo Hyperalgesia Induced by Verbal Modeling Prevails Over the Effects of Symbolic Modeling and Verbal Suggestion

This study compares the effectiveness of verbal modeling, symbolic modeling, and verbal suggestion in inducing nocebo hyperalgesia. It is the first study to examine the contribution of stress to observationally induced nocebo hyperalgesia. This study's experimental groups represented various sources of social information: a group of people participating in the study (verbal modeling), a single participant (symbolic modeling), and an experimenter (verbal suggestion). During the experiment, participants received electrocutaneous stimuli at the same intensity, some of which were applied with a nocebo (sham device). Participants in the verbal modeling group were acquainted with pain ratings that had allegedly been provided by other participants. The ratings suggested that other participants experienced more pain in the nocebo trials than in the control trials. In the symbolic modeling group, participants observed a videotaped model experiencing more pain in the nocebo than in the control trials. In the verbal suggestion group, participants received a verbal suggestion of hyperalgesia in the nocebo trials and no suggestion in the control trials. No manipulations were used in the control group. To investigate whether nocebo hyperalgesia is stable over time, an additional extinction phase was conducted. Nocebo hyperalgesia was induced by verbal modeling only and was partially mediated by expectancy. Stress was a significant moderator of the induced effect. Nocebo hyperalgesia was extinguished during the extinction phase. The obtained results provide potential implications for minimizing nocebo hyperalgesia in clinical practice by, for instance, controlling patients' expectancies and stress levels. PERSPECTIVE: The study shows the role of pain-related information derived from other people in shaping negative treatment experiences in the individual. Because information from others has a particular impact on individuals experiencing stress, both this information and the stress level of patients should be monitored in the treatment process.

Learning by observing: a systematic exploration of modulatory factors and the impact of observationally induced placebo and nocebo effects on treatment outcomes

Introduction

Observational learning (OL) refers to learning through observing other people's behavior. OL has been suggested as an effective and simple tool to evoke treatment expectations and corresponding placebo and nocebo effects. However, the exact mechanisms by which OL shapes treatment outcomes, its moderating factors and possible areas of application remain unclear. We thus reviewed the existing literature with two different literature searches to answer the following questions: Which influencing factors contribute to OL-induced placebo and nocebo effects (in healthy volunteers and patients) and how large are these effects (search 1)? In which medical fields has OL been used so far to modulate treatment expectancy and treatment outcomes in patients, their caregivers, and at-risk groups (search 2)? We also aimed to explore whether and how the assessment of treatment expectations has been incorporated.

Methods

We conducted two independent and comprehensive systematic literature searches, both carried out on September 20, 2022.

Results

We identified 21 studies that investigated OL-mediated placebo and nocebo effects for pain and itch, the (placebo) efficacy of sham treatment on anxiety, and the (nocebo) induction of medication side effects (search 1). Studies showed that OL can efficiently induce placebo and nocebo effects across different presentation modes, with medium effect sizes on average: placebo effects, d = 0.79 (range: d = -0.36-1.58), nocebo effects, d = 0.61 (range: d = 0.04-1.5). Although several moderating factors have been investigated, their contribution to OL-induced effects remains unclear because of inconsistent results. Treatment expectation was assessed in only four studies. Regarding medical applications of OL (search 2), we found 12 studies. They showed that OL was effectively applied in preventive, therapeutic and rehabilitative interventions and that it was mainly used in the field of psychosomatics.

Discussion

OL effects on treatment outcomes can be both positive and negative. Future research should investigate which individuals would benefit most from OL and how OL can be implemented most effectively to induce placebo and avoid nocebo effects in clinical settings.

Systematic review registration

This work was preregistered at the Center for Open Science as open-ended registration (doi: 10.17605/OSF.IO/FVHKE). The protocol can be found here: https://archive.org/details/osf-registrations-fvhke-v1.

Placebo analgesia in physical and psychological interventions: Systematic review and meta-analysis of three-armed trials

BACKGROUND

The magnitude of placebo effects from physical and psychological 'sham' is unknown but could impact efficacy trials and treatment understanding. To quantify placebo effects, this systematic review of three-armed randomised controlled trials (RCTs) of physical and psychological interventions for pain compared outcomes in 'sham' control intervention and non-exposure arms.

METHODS

RCTs with treatment, 'sham' control intervention, and non-exposure groups were included, enrolling adults with any pain. A protocol was pre-registered (PROSPERO: CRD42023413324), and twelve databases searched from 2008 to July 2023. Trial methods and blinding were analysed descriptively and risk of bias assessed. Meta-analysis of pain measures at short-, medium- and long-term was performed with random-effects models of standardised mean differences (SMD).Studies were sub-grouped according to control intervention type.

RESULTS

Seventeen RCTs were included. The average short-term placebo effect was small (0.21 SMD, 0.1-0.33 95% CI, p = 0.0002, 1440 participants). It showed no heterogeneity (Tau2  = 0.1, I2  = 11%, p = 0.3), preventing meta-regression analyses of effect modifiers. However, sub-group analyses revealed larger placebo effects in manual control interventions compared to disabled devices and miscellaneous control interventions. Overall, placebo analgesia accounted for 39% of treatments' short-term effectiveness. No placebo effects were found at medium-term (7 RCTs, 381 participants) or long-term follow-up (3 RCTs, 173 participants).

CONCLUSIONS

The observed placebo analgesia has mechanistic and methodological implications, though its clinical importance may be limited. Control intervention design affects placebo effects, highlighting the importance of considering methodology in RCT interpretation. Review limitations include a small number of long-term studies and sample heterogeneity.

SIGNIFICANCE

This systematic review directly quantifies placebo effects from physical and psychological 'sham' control interventions and compares them to treatments' overall effectiveness. By doing so, the review enhances our understanding of placebo effects, their relative contribution in clinical trials, and their susceptibly to trial design. It poses further questions regarding the influence of blinding, participant expectations, and features of the therapeutic context. Overall, the insights provided by this review carry methodological significance and are important for the interpretation and synthesis of efficacy trials in this field.

Pain Science in Practice (Part 7): How Is Descending Modulation of Pain Measured?

SYNOPSIS: Understanding the descending pain modulatory system allows for a neuroscientific explanation of naturally occurring pain relief. Evidence from basic science and clinical studies on the effectiveness of drugs in certain patient groups led to pharmacological manipulation of the descending pain modulatory system for analgesia. Understanding mechanisms and theories helps clinicians make sense of chronic musculoskeletal pain. This editorial explains how test paradigms, including conditioned pain modulation, offset analgesia, and stress-induced analgesia work, provide an overview of a placebo analgesia circuitry, and discusses how evoking activity in the descending pain modulatory system using specific paradigms can give new insights into how specific treatments work to reduce pain. J Orthop Sports Phys Ther 2024;54(2):1-6. doi:10.2519/jospt.2024.12113.

How should uncertainty about upcoming painful procedures be communicated? An experimental study into highly uncertain pain predictions

OBJECTIVE

Uncertainty is unavoidable in the context of painful medical procedures. It is important to investigate the impact of different ways of communicating uncertainty on upcoming pain.

METHODS

In our exploratory study, healthy participants (n = 30) were repeatedly presented with three highly uncertain pain predictions communicated by a hypothetical doctor. A direct statement of high uncertainty ("I don't know") was compared to more indirect predictions (social prediction: "It varies widely among people"; range prediction: "… not painful at all to very highly painful"), followed by individually calibrated electrical stimuli of non-, moderately, or very highly painful intensity.

RESULTS

The direct expression of uncertainty led to the most intense pain sensation (for moderately painful stimuli only), lowest and most certain pain expectations, lowest trust in the hypothetical doctor, and lowest feeling of being well-informed, especially as compared to the social prediction. No differential effects on anxiety were observed.

CONCLUSIONS

Expressing high uncertainty indirectly, with reference to the common experiences of others, may be beneficial for optimizing pain experiences and enhancing patients' trust in a medical professional.

PRACTICE IMPLICATIONS

Our findings inform on how high uncertainty about upcoming pain may impact patient and health outcomes, pointing to some advantages of indirect communication.

Placebo hypoalgesia induced by operant conditioning: a comparative study on the effects of verbal, token-based, and social rewards and punishers

Operant conditioning was shown to be a mechanism of placebo hypoalgesia; however, only verbal rewards and punishers were applied in the previous study. We aimed to induce placebo hypoalgesia using more clinically relevant consequences: token-based and social. Participants were divided into three experimental groups (with verbal, social, and token-based rewards and punishers); and two control groups (with and without placebo application). During operant conditioning, participants in the experimental groups received thermal stimuli of equal intensity and were rewarded for reporting lower pain and punished for reporting higher pain compared to their pretest pain levels. The control groups did not receive any consequences. Our results revealed placebo hypoalgesia was induced by operant conditioning only in the experimental groups with social and token-based reinforcement, compared to the control groups. The hypoalgesic effect found in the group that received verbal reinforcement did not differ significantly from the control group with the placebo application. Moreover, expectations about upcoming pain intensity were found to be a mediator, and the number of reinforcers received during conditioning was a predictor of placebo hypoalgesia. These findings highlight the potential benefits of incorporating token-based and social consequences for optimizing treatment outcomes in pain management.

Learning pain from others: a systematic review and meta-analysis of studies on placebo hypoalgesia and nocebo hyperalgesia induced by observational learning

ABSTRACT

Observing someone experience pain relief or exacerbation after an intervention may induce placebo hypoalgesia or nocebo hyperalgesia. Understanding the factors that contribute to these effects could help in the development of strategies for optimizing treatment of chronic pain conditions. We systematically reviewed and meta-analyzed the literature on placebo hypoalgesia and nocebo hyperalgesia induced by observational learning (OL). A systematic literature search was conducted in the databases PubMed, PsycINFO, Web of Science, ScienceDirect, PsycARTICLES, Scopus, and Academic Search Ultimate. Twenty-one studies were included in the systematic review, 17 of which were suitable for meta-analysis (18 experiments; n = 764 healthy individuals). The primary end point was the standardized mean difference (SMD) for pain following placebo cues associated during OL with low vs high pain. Observational learning had a small-to-medium effect on pain ratings (SMD 0.44; 95% confidence interval [CI] 0.21-0.68; P < 0.01) and a large effect on pain expectancy (SMD 1.11; 95% CI 0.49-2.04; P < 0.01). The type of observation (in-person vs videotaped) modulated the magnitude of placebo hypoalgesia/nocebo hyperalgesia ( P < 0.01), whereas placebo type did not ( P = 0.23). Finally, OL was more effective when observers' empathic concern (but no other empathy-related factors) was higher ( r = 0.14; 95% CI 0.01-0.27; P = 0.03). Overall, the meta-analysis demonstrates that OL can shape placebo hypoalgesia and nocebo hyperalgesia. More research is needed to identify predictors of these effects and to study them in clinical populations. In the future, OL could be an important tool to help maximize placebo hypoalgesia in clinical settings.

Nocebo Hyperalgesia in Patients With Fibromyalgia and Healthy Controls: An Experimental Investigation of Conditioning and Extinction Processes at Baseline and 1-Month Follow-up

Nocebo effects are adverse treatment outcomes that are not ascribed to active treatment components. Potentially, their magnitude might be higher in patients with chronic pain compared to healthy controls since patients likely experience treatment failure more frequently. The current study investigated group differences in the induction and extinction of nocebo effects on pressure pain at baseline (N = 69) and 1-month follow-up (N = 56) in female patients with fibromyalgia and matched healthy controls. Nocebo effects were first experimentally induced via classical conditioning combined with instructions on the pain-increasing function of a sham transcutaneous electrical nerve stimulation device, then decreased via extinction. One month later, the same procedures were repeated to explore their stability. Results suggest that nocebo effects were induced in the healthy control group during baseline and follow-up. In the patient group, nocebo effects were only induced during follow-up, without clear group differences. Extinction was only observed during baseline in the healthy control group. Further comparisons of nocebo effects and extinction indicated no significant changes across sessions, possibly suggesting their overall magnitudes were stable over time and across groups. In conclusion, contrary to our expectations, patients with fibromyalgia did not have stronger nocebo hyperalgesia; instead, they might be less responsive to nocebo manipulations than healthy controls. PERSPECTIVE: The current study is the first to investigate group differences in experimentally manipulated nocebo hyperalgesia between chronic pain and healthy populations at baseline and 1-month follow-up. Since nocebo effects are common in clinical settings, their investigation in different populations is essential to explain and minimize their adverse effects during treatment.

Electrophysiological markers for anticipatory processing of nocebo-augmented pain

Nocebo effects on pain are widely thought to be driven by negative expectations. This suggests that anticipatory processing, or some other form of top-down cognitive activity prior to the experience of pain, takes place to form sensory-augmenting expectations. However, little is known about the neural markers of anticipatory processing for nocebo effects. In this event-related potential study on healthy participants (n = 42), we tested whether anticipatory processing for classically conditioned nocebo-augmented pain differed from pain without nocebo augmentation using stimulus preceding negativity (SPN), and Granger Causality (GC). SPN is a slow-wave ERP component thought to measure top-down processing, and GC is a multivariate time series analysis used to measure functional connectivity between brain regions. Fear of pain was assessed with the Fear of Pain Questionnaire-III and tested for correlation with SPN and GC metrics. We found evidence that both anticipatory processing measured with SPN and functional connectivity from frontal to temporoparietal brain regions measured with GC were increased for nocebo pain stimuli relative to control pain stimuli. Other GC node pairs did not yield significant effects, and a lag in the timing of nocebo pain stimuli limited interpretation of the results. No correlations with trait fear of pain measured after the conditioning procedure were detected, indicating that while differences in neural activity could be detected between the anticipation of nocebo and control pain trials, they likely were not related to fear. These results highlight the role that top-down processes play in augmenting sensory perception based on negative expectations before sensation occurs.