Efficacy of therapeutic suggestions under general anesthesia: a systematic review and meta-analysis of randomized controlled trials
The present meta-analysis aimed at evaluating the efficacy of therapeutic suggestions presented during general anesthesia to patients undergoing surgery or medical procedures. Currently, the efficacy of therapeutic suggestions applied under general anesthesia has been investigated on hospitalization and patient-controlled analgesia exclusively. Our meta-analysis expands this knowledge by adding results on pain intensity, mental distress, use of medication, and recovery.
We found small, significant positive effects of therapeutic suggestions on recovery and medication use which proved to be robust and free of publication bias. When analyzing outcomes in more detail, highest effects were found for PONV and analgesic use. Comparable results of therapeutic suggestions on the amount of morphine administered via patient-controlled analgesia were also reported in the meta-analysis of Merikle and Daneman [20]. However, there was no effect of therapeutic suggestions on pain intensity or mental distress.
One reason for the small or even zero effects might be the level of awareness. Usually, therapeutic suggestions were given during general anesthesia excluding the induction of anesthesia and emergence from anesthesia that are most sensitive to intraoperative awareness [16]. Another reason could be that when suggestions are presented via tape only, rapport and therapeutic relationship are missing, which are essential components of effective hypnosis or therapeutic suggestions [4, 8]. Accordingly, higher effect sizes of suggestions to reduce postoperative side effects spoken live compared to taped suggestions were reported [5, 6].
Since study effects were quite homogeneous, we merely ran stratified analyses on PONV and antiemetic use to get an idea about potential moderators of treatment effects. In this regard, the specificity of suggestions seems to have an influence on its efficacy since studies with specific PONV related suggestions yielded significant results on PONV, while studies with unspecific suggestions only resulted in non-significant effects. Thus, our results go along with studies demonstrating an impact of suggestion specificity on its efficacy [6].
Differences in anesthesia methods did not influence the efficacy of therapeutic suggestions, although neuroleptanesthesia is known to carry a higher risk of intraoperative awareness and lower interference with memory in comparison to balanced anesthesia with inhalational or intravenous anesthetics [16]. However, intraoperative awareness and memory are not considered a pre-requisite for effects of suggestions in unconscious patients [41, 8].
When interpreting these results the exploratory nature of the respective analyses should be considered. Although research on the impact of affirmativity and specificity of therapeutic suggestions on postoperative outcomes is available [6, 29, 30, 42] this issue has not been clarified conclusively. Studies examining the most efficacious phrasing of suggestions are still pending; an optimization of therapeutic suggestions is possible and needed.
Several limitations of the present meta-analysis are noteworthy. First, we excluded studies with children and studies where pre- or postoperative suggestions were presented in addition to those given intraoperatively. Both restrictions of inclusion might have led to smaller effects of suggestions during general anesthesia. There is some evidence of a higher level of efficacy of suggestive techniques in children [5], partly due to their higher suggestibility [43]. Moreover, meta-analytic findings have shown that suggestions are more effective when delivered at least in part prior to the medical procedure rather than solely during the medical procedure [5].
Second, the reporting quality, i.e. completeness and transparency, of the included studies was rather low making it difficult to adequately evaluate potential risks of bias. Particularly, methods of randomization and allocation concealment have been reported inadequately in the majority of studies, whereas blinding of participants, personnel, and outcome assessors was reported well. From the information on the anesthesia methods provided in the included studies no conclusion can be drawn on the precise depth of anesthesia and its impact on the results, besides that standard procedures were used without techniques to control depth, if reported, the dosage of anesthetics was reasonable, and the same procedure was used for intervention and control group. Finally, the latest available randomized controlled trial dates back to 2001.
It might be argued that insufficient anesthetic depth was more common at that time, but even modern electroencephalography (EEG)-based monitoring of anesthetic depth even could only reduce but not eliminate intraoperative awareness with recall (AWR) [44]. Current recommendations for AWR prevention include earplugs or music via earphones as an essential component. Positive suggestions should be considered as well, since being proposed for prophylaxis of posttraumatic stress disorder following AWR [45]. It has been claimed that effects of intraoperative suggestions are limited to insufficient depth of anesthesia [46], but even this pre-requisite is not absent in clinical practice today.