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).
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.
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.
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.
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.
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.
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.
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