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The Effectiveness of Suggestive Techniques in Reducing Postoperative Side Effects: A Meta-Analysis of Randomized Controlled Trials

Kekecs, Zoltán PhD*; Nagy, Tamás MA; Varga, Katalin PhD*

doi: 10.1213/ANE.0000000000000466
Economics, Education, And Policy: Research Report

BACKGROUND: Suggestive interventions such as hypnosis and therapeutic suggestions are frequently used to alleviate surgical side effects; however, the effectiveness of therapeutic suggestion intervention has not yet been systematically evaluated. In the present study, we tested the hypotheses that (1) suggestive interventions are useful for reducing postoperative side effects; (2) therapeutic suggestions are comparable in effectiveness to hypnosis; (3) live presentation is more effective than recordings; and (4) suggestive interventions would be equally effective used in minor and major surgeries.

METHODS: We performed random effect meta-analysis with meta-regression and sensitivity analysis by moderating factors on a pool of 26 studies meeting the inclusion criteria (N = 1890). Outcome variables were postoperative anxiety, pain intensity, pain medication requirement, and nausea.

RESULTS: Suggestive interventions reduced postoperative anxiety (g = 0.40; 99% confidence interval [CI] = 0.13–0.66; P < 0.001) and pain intensity (g = 0.25; 99% CI = 0.00–0.50; P = 0.010), but did not significantly affect postoperative analgesic drug consumption (g = 0.16; 99% CI = −0.16 to 0.47; P = 0.202) and nausea (g = 0.38; 99% CI = −0.06 to 0.81; P = 0.026). No significant differences were found for intervention type, presentation method, and severity of surgery; however, sensitivity analysis only supported the effectiveness of hypnosis (g = 0.62; 99% CI = 0.31–0.92; P < 0.001) and live presentation (g = 0.55; 99% CI = 0.23–0.88; P < 0.001) for decreasing postoperative anxiety, and that of live presentation for alleviating postoperative pain (g = 0.44; 99% CI = 0.07–0.82; P = 0.002). Sensitivity analyses also suggested that suggestive interventions are only effective for decreasing pain intensity during minor surgical procedures (g = 0.39; 99% CI = 0.00–0.78; P = 0.009).

CONCLUSIONS: Suggestive techniques might be useful tools to alleviate postoperative anxiety and pain; however, strength of the evidence is weak because of possible bias in the reviewed articles. The lack of access to within-subjects data and the overlap between moderator conditions also limit the scope of the analysis. More methodologically correct studies are required with sensitivity to moderating factors and to within-subjects changes. For clinical purposes, we advise the use of hypnosis with live presentation to reduce postoperative anxiety and pain, until convincing evidence is uncovered for the effectiveness of therapeutic suggestions and recorded presentation. Pain management with adjunct suggestive interventions is mostly encouraged in minor rather than major surgeries.

Published ahead of print October 6, 2014.Supplemental Digital Content is available in the text.

From the *Department of Affective Psychology, Faculty of Psychology and Education, Eötvös Loránd University, Budapest, Hungary; and Faculty of Psychology and Education, Doctoral School of Psychology, Eötvös Loránd University, Budapest, Hungary.

Published ahead of print October 6, 2014.

Accepted for publication August 5, 2014.

Funding: The study was funded by the 2012 PhD research grant of Eötvös Loránd University, and the Hungarian Scientific Research Funds (K109187 and K100845).

Conflict of Interest: See Disclosures at the end of the article.

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s website.

Reprints will not be available from the authors.

Address correspondence to Zoltan Kekecs, PhD, Department or Affective Psychology, Faculty of Psychology and Education, Eötvös Loránd University, Budapest, Hungary, 46. Izabella u., Room 118, Budapest, Hungary H-1064. Address e-mail to kekecs.zoltan@ppk.elte.hu.

In the middle of the 19th century, suggestive techniques were frequently used as the only analgesic procedures for surgical operations until the introduction of pharmaceutical methods.1 Among these suggestive techniques, hypnosis remains the most recognized psychological intervention in modern medicine that is demonstrated to effectively alleviate postoperative side effects. Hypnosis particularly decreases postoperative distress, pain, pain medication requirement, nausea, treatment time, and improves postoperative well being and recovery.1–8 The distinctive characteristic of hypnosis is that it includes a formal “hypnosis induction” before the application of suggestions in order to increase suggestive effects. Those authors also overtly identify the applied technique as “hypnosis.”

Despite its established benefits, there is an ongoing debate whether hypnosis truly increases susceptibility to suggestions and whether it is necessary for suggestions to be effective.9 Some theories propose that patients in medical settings (e.g., being in critical condition, or waiting for an invasive operation, etc.) can experience a spontaneous trance which in itself enhances suggestibility.10 Accordingly, there is evidence that suggestions given without hypnotic induction (from here on, “therapeutic suggestions”) can influence perioperative outcome.1 The meta-analysis of Schnur et al.5 included 6 studies in which the intervention was labeled as “suggestions,” and they concluded that suggestions were less effective for reducing perioperative distress than hypnosis. However, this meta-analysis did not systematically search for suggestion studies, and they only assessed effectiveness on a single outcome variable (perioperative distress); thus, the generalizability of these results is limited. Therapeutic suggestions do not require hypnotic induction; thus, they are quicker and less expensive to use, they can be applied by more health care professionals because they do not require complex hypnotherapy training, and the common misconceptions regarding hypnosis can also be overcome by these methods. Therefore, it is important for decision makers to know whether therapeutic suggestions are a real alternative to formal hypnosis. (Appendix 1, Supplemental Digital Content 1, http://links.lww.com/AA/B9 contains more readings on hypnosis induction, whether hypnosis induction results in a meaningful increase in susceptibility to suggestions, and increased suggestibility in medical settings.)

The aim of the present study was to systematically investigate the effectiveness of therapeutic suggestions compared to hypnosis for alleviating postoperative side effects. Furthermore, we assessed how moderating factors such as the method of presentation (live or recorded) and severity of surgery (minor or major) affect the effectiveness of suggestive interventions. We hypothesized that (1) suggestive interventions significantly reduce postoperative anxiety, pain intensity, pain medication requirement, and nausea; (2) therapeutic suggestions are comparable in effectiveness to hypnosis; (3) live suggestions are more effective than recorded ones; and (4) suggestive interventions are equally effective used in minor and major surgeries.

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METHODS

Data Sources and Search Strategy

A literature search was conducted on 4 online databases (PubMed, PsycINFO, CINAHL, and ProQuest Dissertations & Theses Database) for studies published between 1980 and 2014 on hypnosis or therapeutic suggestion interventions applied in surgery with no limitations to language or publication status. Setting a minimal publication date was necessary to improve generalizability to modern surgical, anesthesia, and suggestive procedures.

The literature search was finished on February 21, 2014. We used the keywords “hypnosis,” “suggestion,” and “surgery” along with their variants and synonyms (Appendix 2, Supplemental Digital Content 2, http://links.lww.com/AA/B10 lists the exact search terms).

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Selection Criteria

Randomized controlled trials (RCTs) on the effectiveness of therapeutic suggestions or hypnosis-applied adjunct to routine surgical care were eligible for inclusion. Non-RCTs, observational studies, and case reports were excluded from analysis. Because children are more susceptible to hypnosis and respond better to suggestive interventions in clinical settings than adults, studies conducted on a pediatric population (patients younger than 17 years of age) were also excluded.5 Appendix 1 (Supplemental Digital Content 1, http://links.lww.com/AA/B9) lists reviews on the association of age and suggestibility and hypnosis applied during medical procedures with children. After data extraction, we decided to exclude studies in which suggestions were given under general anesthesia, mainly because the distribution of moderating factors were highly asymmetric in these studies. Specifically, when suggestions were presented under general anesthesia, they were always given without hypnosis induction and played from a recording. The effectiveness of suggestive techniques was compared to “regular treatment” (no psychological intervention) or “attention control” conditions.

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Data Extraction

Data extraction was performed by the first and second authors independently. Disagreements were resolved by consensus. The extracted data included number of participants by study group, presence or absence of formal hypnosis induction, type of presentation (live or recorded; if both live and recorded presentation were used as part of the intervention, it was coded as livea), timing of intervention (before, during, or after surgery), methodological quality (see Risk of Bias Assessment), and any special care not related to the suggestive intervention that could have affected postoperative outcomes (Appendix 3, Supplemental Digital Content 3, http://links.lww.com/AA/B11 is a comprehensive list). The surgical procedure used in the study was also extracted. Two physicians independently rated the procedures as being minor or major surgery according to the definitions of McGraw-Hill Concise Dictionary of Modern Medicine.11

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Outcomes

Based on previous meta-analyses3–5 and the frequency of occurrence in the reviewed studies 4 outcome measures were selected: (1) postoperative anxiety or distress, (2) postoperative pain intensity, (3) postoperative pain medication requirement, and (4) postoperative nausea. Appendix 4 (Supplemental Digital Content 4, http://links.lww.com/AA/B12) is a comprehensive list of measures used in the included studies to assess the aforementioned outcomes. Because we were interested in the short-term postoperative effects, only data measured until the ninth postsurgical day were extracted. To address ambiguities or the need for additional data, the corresponding authors of the papers were contacted via e-mail.

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Risk of Bias Assessment

Methodological quality was assessed using the Cochrane Risk of Bias Assessment Tool.12 This tool enables the evaluation of selection, performance, detection, attrition, and reporting bias with several customizable assessment categories. During the process of evaluation, studies were rated as having “low risk of bias,” “unclear risk of bias,” or “high risk of bias” on the following attributes: (1) random sequence generation, (2) allocation concealment, (3) blinding of personnel, (4) blinding of outcome assessment, (5) incomplete outcome data, and (6) selective reporting. Since hypnosis—contrary to therapeutic suggestions—requires the consent and participation of the subject, blinding of the participants is usually inappropriate.1 Thus, we did not consider lack of blinding of participants a flaw in methodological quality.

Publication bias was assessed using Begg and Mazumdar rank correlation, the random effect variant of Egger test, Duval & Tweedie trim and fill method, and the inspection of the funnel plots.

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STATISTICAL ANALYSIS

Calculating Treatment Effect

Corrected Hedges g (g) was used as a measure of effect size. If the mean and standard deviation were not reported in the original studies, effect sizes were calculated using other statistics, using the equations by Johnson and Eagly,13 and Lipsey and Wilson.14 If necessary, effect sizes were aggregated according to Rosenthal and Rubin15 and Decoster.16 For studies that did not report any test statistics or significance values for nonsignificant results, we imputed g = 0 (referred to as “imprecise inference” from here on).

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Statistical Analysis

Statistical analysis was performed using the metafor package (v1.9-3)17 in R (v3.1.1). Statistical heterogeneity (I2) yielded medium to high values that supported the application of a random-effect approach. Random effect meta-analysis was used to obtain the general effect size of suggestive methods on postoperative side effects, to assess publication bias, and to have a reference point for later sensitivity analyses. Meta-regression was used to investigate the risk of bias for all outcome variables including all categories from the Cochrane Risk of Bias Assessment Tool as binomial variables: 0 = low risk of bias; 1 = unclear or high risk of bias. A permutation-based technique was used to control for multiple hypothesis testing. Sensitivity analyses were performed to further investigate significant moderator effects by excluding studies with unclear or high-risk ratings. Moderator effects of imprecise inference and special care (see data extraction) were tested as well, accompanied by appropriate sensitivity analyses.

Subsequently 3 meta-regressions were executed for each outcome testing the moderating effect of hypnosis induction, live versus recorded presentation, and surgery type (minor versus major surgery). In addition, sensitivity analyses were also performed on datasets split by moderator conditions. One study in the anxiety and pain datasets was omitted from the analysis of the effect of surgery type because of insufficient information to determine surgery type.18 Because a relatively high risk of bias was uncovered in the study pool, threshold for statistical significance was set to P < 0.01, and 99% confidence intervals (CIs) are displayed in all analyses except for risk of bias assessments in which the traditional P < 0.05 was retained. (Appendix 1, Supplemental Digital Content 1, http://links.lww.com/AA/B9, summarizes references of text on statistical methods used for the assessment of publication bias, calculating treatment effect, and analysis of the data).

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RESULTS

Study Selection

As Figure 1 shows, 139 records were selected for full text evaluation. Sixteen of these could not be retrieved (Appendix 5, Supplemental Digital Content 5, http://links.lww.com/AA/B13), and 16 were duplicate publications. From the remaining 107 publications, 56 used hypnosis, 49 used therapeutic suggestions, and 2 used both. All non-RCTs, studies on pediatric patients, studies that did not report outcome of interest, and trials in which suggestions were given only during general anesthesia were excluded. Twenty-six studies were retained at the end of the exclusion process incorporating 1890 patients (range: n = 12–346) of which 13 applied hypnosis, 11 therapeutic suggestions, and 2 both in separate groups; 13 used live and 13 recorded presentation; and furthermore 14 were performed in major and 11 in minor surgical procedures (not enough information on surgery type in 1 study). Cholecystectomy (6 studies) and hysterectomy (4 studies) were the most commonly used surgical procedures. Four studies contained >1 relevant experimental condition. Table 1 lists the study characteristics.

Figure 1

Figure 1

Table 1

Table 1

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General Effects of Suggestive Techniques

We found a significant reduction in postoperative anxiety (g = 0.40; 99% CI = 0.13–0.66; P < 0.001) and pain intensity (g = 0.25; 99% CI = 0.00–0.50; P = 0.010), whereas no significant effect was noted for postoperative analgesic drug consumption (g = 0.16; 99% CI = −0.16 to 0.47; P = 0.202) and nausea (g = 0.38; 99% CI = −0.06 to 0.81; P = 0.026).

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Analysis of Moderators

As apparent in Figures 2–5, there is a considerable amount of heterogeneity in the total study sample. To account for this heterogeneity, moderator and sensitivity analyses were performed, the results of which can be found in Figure 6.

Figure 2

Figure 2

Figure 3

Figure 3

Figure 4

Figure 4

Figure 5

Figure 5

Figure 6

Figure 6

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Hypnosis Induction

The moderating effect of hypnosis was not statistically significant for any outcome; however, sensitivity analysis led us to different conclusions on the effects of therapeutic suggestions and hypnosis. While pooled effect size and CIs show a small nonsignificant effect for therapeutic suggestion studies on all outcomes, hypnosis had a significant medium-sized effect on postoperative anxiety and although not significant, hypnosis effect sizes were generally higher in all other outcomes as well.

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Presentation Method

Live presentation was more effective for decreasing pain ratings than recorded presentation (z = 2.18; P = 0.029); however, recordings were superior for reducing pain medication requirement (z = -2.08; P = 0.037). Although these moderator effects are statistically not significant, the sensitivity analysis also indicated differentiation in the effects of the 2 presentation methods: we found a medium-sized significant effect of live presentation on anxiety and pain intensity, while recorded presentation yielded no significant results on any outcome.

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Surgery Type

Moderator analysis did not show significant moderator effect of surgery type; nevertheless, sensitivity analysis led to somewhat differing conclusions for the effectiveness of suggestive interventions used in minor and major surgeries. Both interventions used in minor and major procedures reduced anxiety with a similar medium effect size, and neither had a significant effect on pain medication requirement or nausea. However, while studies on major surgeries showed negligible effect sizes for reducing pain and analgesic requirement, pooled effect sizes were medium sized for the same outcomes in minor procedures.

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Risk of Bias and Effects of Imprecise Inference and Special Care

A summary graph of risk of bias is displayed in Figure 7, and the results of risk of bias assessment for each study are listed in Figure 8. Meta-regression identified 2 methodological moderators as significant: random sequence generation in the anxiety dataset (z = 2.48; P = 0.018) and blinding of personnel in the nausea dataset (z = −3.84; P = 0.003; Table 2). Sensitivity analysis revealed that with the exclusion of studies having high or unknown risk of bias, the effect of suggestive techniques on postoperative anxiety is no more significant (g = 0.16; 99% CI = −0.30 to 0.60; P = 0.376). Exclusion of studies with high or unknown risk on blinding of personnel produced a slightly higher pooled effect size than the model without moderators in the nausea dataset (g = 0.49; 99% CI = −0.10 to 1.08; P = 0.032); the effect remained nonsignificant. Effect on postoperative pain and pain medication requirement was unaffected by methodological quality. There was no moderator effect of imprecise inference, and that studies with special care had higher effects compared to studies with no special care (Table 3).

Figure 7

Figure 7

Figure 8

Figure 8

Table 2

Table 2

Table 3

Table 3

There was no indication of publication bias based on funnel plots and asymmetry tests (Fig. 9). Duval & Tweedie trim and fill method does not change our interpretation for anxiety, pain intensity, and nausea. However, it predicted 4 missing studies from the right (positive) side for the pain medication dataset yielding a slightly higher but still nonsignificant effect (g = 0.31; 99% CI = −0.02 to 0.63; P = 0.015).

Figure 9

Figure 9

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DISCUSSION

We reviewed the results of 26 studies to investigate the effects of suggestive interventions in surgical settings and to explore the factors that moderate their effectiveness. Results indicate that suggestion interventions had a beneficial effect on postoperative anxiety and to a lesser extent on pain intensity, while we did not find convincing evidence to support the effectiveness of suggestive interventions as a whole in reducing postoperative analgesic use and nausea. Our findings yielded small to medium effect sizes for the effects of suggestive techniques on the studied outcome measures, which are comparable to previous reports5–7,b; however, we were more conservative in how we interpreted these results. The reason for our caution is the relatively high risk of bias in these studies mostly originating from lack of blinding of participants, study personnel and data assessors, and the lack of description of random sequence generation and allocation methods. Particularly, our analysis points out that in a large portion of the studies showing a beneficial effect on postoperative anxiety, random sequence generation is described insufficiently, and that if we omit these studies, the previously highly significant effect fades away. Furthermore, the effect on pain intensity is only at the border of statistical significance. Thus, we conclude that although results point in the right direction, we need more methodologically rigorous studies to get a clear picture of the effectiveness of suggestive interventions in surgery.

Contrary to our hypothesis but consistent with the report of Schnur et al.,5 we found that only hypnosis reduced postoperative anxiety, and we found no significant effects for therapeutic suggestions on any of the assessed outcome measures.

The effect of presentation method revealed a complex picture. Our moderator and sensitivity analyses yielded that only interventions using live presentation were effective for reducing postoperative anxiety and pain intensity. However, there was no substantial difference in the effectiveness between therapeutic suggestions and hypnosis for reducing analgesic requirement and nausea. Previous research reported mixed results about the effects of presentation method. While Schnur et al.5 supported the superiority of live presentation compared to recordings for reducing postoperative distress, 2 other meta-analyses did not find a significant difference between face-to-face and taped presentation.4,6 Although Schnur et al.5 only addressed 1 outcome, Montgomery et al.4 used a combined effect size of several outcomes during the assessment of this moderator effect. Previous reports also point out the high correspondence between moderating factors; that is, studies using live presentation also tend to use hypnosis instead of therapeutic suggestions and preoperative instead of intra- or postoperative presentation of the intervention. Therefore, reasons for differences in effectiveness by presentation method could lie in a third variable. Nevertheless, our results only support the effectiveness of live intervention.

In line with previous reports, no significant moderator effect was found for surgery type.6 Suggestive interventions had the same effectiveness for decreasing anxiety and nausea in minor and major surgeries. However, according to the sensitivity analysis, suggestions were only effective for managing pain in minor procedures. Major surgeries involve more effective analgesics compared to minor surgeries because they inflict more postoperative pain.19 Thus, it is possible that effects in major procedures are masked by the rigorous analgesic protocols. It is also possible that pain management techniques used in suggestive interventions are less effective in cases of severe pain.

Another important question is how do suggestive techniques compare in effectiveness to other adjunct nonpharmacological interventions. Early studies found small to large effect sizes for preoperative interventions such as patient education, behavioral instructions, relaxation, and cognitive interventions for reducing postoperative side effects such as pain, psychological distress, and analgesic consumption.3,20,21 In contrast, several studies are more reserved in their reporting, concluding that there is no sufficient evidence to support the beneficial effect of these interventions.22–26 Similarly, the earliest studies reported very large effects for suggestive techniques4; however, effect size estimates became increasingly tempered throughout the years,5–7 and based on our results, we now argue that the evidence is not yet strong enough to make precise estimates of effectiveness. Additional studies are needed in both fields before a meaningful comparison of efficacy can be made.

Treatment effect of the interventions could be influenced by a number of additional moderators such as the number of intervention sessions, customization of the intervention to individual needs, the experience level of the surgeon and the hypnotherapist, the level of procedure-related fear and anxiety of the patient, or the presence and amount of anxiolytic medication used. There are several possible moderators specific to suggestive techniques as well such as the number of repetitions of suggestions, positive versus negative phrasing of suggestions, specific suggestive techniques used, or the patient’s susceptibility to suggestions. Information on these factors is generally absent from previous research reports. A possible direction for future studies could be to assess the importance of these moderators, or at least to report relevant data to enable later systematic comparisons.

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LIMITATIONS

The present study has a number of limitations. A large portion of the studies did not report baseline statistics for the outcome measures; thus, only between-group comparisons were used in the analysis. Access to within-subjects data could have led to more accurate estimation of effect sizes. The meta-regressions also indicated that effects on anxiety might be biased by inappropriate random sequence generation. Because of the overlap between moderator conditions (e.g., studies with hypnosis induction were typically presented live, while therapeutic suggestions were mostly presented from recordings), the effects of live presentation and formal hypnosis are difficult to distinguish. The majority of the included studies used single-blind design (no blinding of participants) and passive control conditions (i.e., regular treatment) that might have resulted in a bias favoring the intervention because of expectancy effects. Furthermore, 16 of the 139 studies selected for detailed full text assessment could not be retrieved. We also have to keep in mind that our results only apply to the selected outcomes and cannot be generalized. Clinically relevant outcome measures differ between procedures, and there is a possibility that some of the suggestive interventions were tailored to address these specific issues (e.g., the main aim of the intervention in the study of Szeverényi et al.27 was to reduce bleeding during orthopedic surgery).

The novelty of the present study is that it included a systematic search for both therapeutic suggestion interventions and hypnosis. This way we were able to draw conclusions on suggestive interventions in general and address the difference between hypnosis and therapeutic suggestions in particular. Our results indicate that suggestive interventions might help surgical patients to cope with postoperative anxiety and pain; however, the evidence is inconclusive, mainly because of the risk of bias originating from methodological factors. For therapeutic purposes, we encourage the use of suggestions with hypnosis induction and face-to-face presentation to alleviate postoperative anxiety and pain. Further studies are needed with proper randomization, allocation, and blinding with sensitivity to within-subjects changes and incorporating rare combinations of moderator factors (e.g., recorded hypnosis, live suggestions during and after surgery and during general anesthesia, etc.). We also encourage researchers to publish full-length suggestion scripts used in their studies either as an appendix or as an online supplement so that possible suggestion-specific moderators of effectiveness can be evaluated.

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DISCLOSURES

Name: Zoltán Kekecs, PhD.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: Zoltán Kekecs has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.

Conflicts of Interest: One of the papers (Kekecs, et al., 201428) included in the review is a work of the first author’s (Zoltán Kekecs).

Name: Tamás Nagy, MA.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: Tamás Nagy has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Conflicts of Interest: This author declares no conflicts of interest.

Name: Katalin Varga, PhD.

Contribution: This author helped design the study and write the manuscript.

Attestation: Katalin Varga has seen the original study data and approved the final manuscript.

Conflicts of Interest: Two of the papers (Szeverényi, et al., 201227; Kekecs, et al., 201428) included in the review are works of the third author’s (Katalin Varga).

This manuscript was handled by: Franklin Dexter, MD, PhD.

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ACKNOWLEDGMENTS

The authors express their warmest gratitude to Klára Horváth and the library staff of Eötvös Loránd University, Budapest, for the help they provided in retrieving the papers included in the review. The authors are also grateful to Dr. Carlton A. Evans for his invaluable feedback on a previous version of the manuscript. The authors also thank the contacted authors for their cooperation in sharing details and data of their studies.

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FOOTNOTES

a We see human contact and the possibility to customize the intervention to the individual patient’s needs to be the 2 main advantages of a live (face-to-face) presentation compared to a recorded one. The rationale behind grouping live presentation and both live and recorded presentation studies together was that in both study types both human contact and possibility for individualization are present.
Cited Here...

b The markedly higher intervention effects reported by Montgomery and colleagues4 may be explained by the facts that contrary to the present meta-analysis non-RCTs were included while studies not reporting adequate statistics were excluded from their analysis, and that they used a fixed effect model.
Cited Here...

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