Deviation from protocol

We decided post-protocol to exclude studies examining physically absent clowns (e.g., on videos or apps) because we did not consider physically absent and physically present clowns to be comparable. We assumed that physically present clowns are able to interact more closely with the child and according to the various situations.

We also included two studies reporting only our secondary outcome parental anxiety [11, 22], one study that only reported the secondary outcome children’s pain [23], and another study reporting our secondary outcome cooperation [24].

We could not conduct subgroup analyses on children’s age, setting, and type of clown intervention due to the low number of studies included, but performed a posteriori a subgroup analysis of the different types of anxiety-provoking procedures.

Finally, we decided a posteriori to grade quality of evidence using GRADE.

Study selection

Our searches in the electronic databases yielded a total of 137 studies; 3 additional studies were identified through reference checking [25,26,27]. Ultimately, 11 studies met our inclusion criteria. The selection process is illustrated in Fig. 1 (excluded studies are available in Additional file 3).

Fig. 1 PRISMA flow diagram of the systematic literature search Full size image

Study characteristics

The studies included 733 children aged 2 to 17 years. Study size ranged from 40 to 120 randomized patients. The studies were conducted in Italy [4, 18, 22, 27], Israel [10, 11, 19, 23, 28], Turkey [24], and the USA [17]. Data were collected from June 2003 to September 2015. Seven studies were performed in an outpatient setting, one in an emergency department [23], another study in a hospital’s burn unit [24], and in two studies, the setting was unclear [11, 17]. Potentially anxiety-provoking procedure included anesthesia [4, 18, 19, 22, 27, 28], allergy skin prick test [10], physical examination [17], insertion of an intravenous catheter [11], burn dressing change [24], and blood tests or intravenous cannulation [23].

One of the included studies reported a conflict of interest for one of the involved authors [10]. Five studies declared no conflict of interest [4, 11, 19, 24, 28], and five studies did not comment on this [17, 18, 22, 23, 27]. However, seven studies received support by organizations, such as Anna Meyer Foundation that support hospital clowns, or by clown organizations [4, 10, 18, 19, 23, 27, 28]. Two studies reported receiving no financial support at all [11, 24]; the other two were supported by individuals or institutions not associated with clowning [17, 22].

Four of the eleven studies reported data on children’s anxiety only [17,18,19, 28], whereas two studies reported data on parental anxiety only [11, 22]. Three studies reported data on both outcomes [4, 10, 18], and only one study reported data on children’s pain [23]. Additionally, we found one RCT assessing children’s cooperation [24]. We did not find RCTs assessing negative postoperative behavior. We identified three different comparison groups: parental presence or no intervention, oral midazolam (a commonly administered sedative in preoperative time), and the child life program. The child life program aims to help children develop coping skills with two major components. One component is play which is intended to help children feel more comfortable; the other component is psychological preparation [29].

Differentiation between parental presence and no intervention was difficult because parents usually accompany their children. In two studies, parental presence was not mentioned [10, 17]. Due to the fact that young children were included, we assumed that they were accompanied by their parents and combined parental presence and no intervention to one comparison group. Three studies included two comparisons; the other eight studies included only one comparison. We included eleven comparisons between clowning and parental presence or no intervention [4, 10, 11, 17,18,19, 22,23,24, 27, 28]. Two studies compared clowning and oral midazolam [18, 19], and one clowning and the child life program [17]. All studies assessing children’s anxiety used the behavioral observation scale called Modified Yale Preoperative Anxiety Scale (m-YPAS) with a score from 0 to 100 and higher value meaning higher anxiety [30]. Parental trait and state anxiety was assessed based on self-report using the State-Trait Anxiety Inventory (STAI) with a score ranging from 20 to 80 on both trait and state anxiety and higher value meaning higher anxiety in all studies reporting this outcome [31]. Additionally, one study measured parental anxiety using a verbal rating scale with a score from 0 to 45 and higher value meaning higher anxiety [22]. Rimon et al. assessed children’s pain with the Faces Pain Scale–revised (FPS-R) for children aged four to seven [32] and a visual analog scale (VAS) for children over the age of seven and combined all scores to an overall mean pain score. Yildirim et al. [24] measured cooperation using a questionnaire and a child observation form on a scale from 0 to 16 with 16 meaning worst cooperation. Detailed information on the study characteristics and outcomes are depicted in Additional files 4 and 5.

Five studies had previous performance of a comparable procedure as an exclusion criterion [4, 10, 18, 19, 28]. Three studies reported the number of patients being previously treated with a painful procedure in the comparison groups [11, 23, 24]. Three studies did not report on previous comparable procedures [17, 22, 27].

Risk of bias within studies

For risk of bias assessment of individual studies see Fig. 2. Six studies had an unclear risk of selection bias regarding the sequence generation, as their methods of sequence generation were not described in sufficient detail; it was low in the remaining five studies. Risk of selection bias regarding allocation concealment was low in three studies and unclear in eight studies. Blinding of participants and personnel was not possible, and we therefore classified all studies as having high risk of bias. The risk of detection bias was graded to be high in three studies, unclear in one, and low in three studies regarding outcome assessment of children’s anxiety. Studies with a low risk of detection bias had filmed the children and after that evaluated videos without any evidence of the clown’s presence. We graded all studies assessing parental anxiety with a high risk of detection bias, because the STAI instrument and a verbal rating scale are self-reporting instruments and clowning was visible to parents. Detection bias was also high regarding children’s pain, as self-reporting scales were used for outcome assessment. Regarding children’s cooperation, detection bias was high as blinding was not mentioned and it would have been hardly possible. We graded risk of attrition bias high in three, low in one, and unclear in seven studies. We contacted all study authors and asked to send a study protocol, but only received a protocol for one study [23]. The risk of selective reporting was judged to be at least unclear for all studies missing a protocol, but three studies even had a high risk of reporting bias. We found no other sources of bias in any study; thus, the risk of other bias was low in all the included studies.

Fig. 2 Risk of bias summary. “+”, low risk of bias; “?”,unclear risk of bias; “-”, high risk of bias. Uncoded boxes indicate that these studies did not include the corresponding outcome Full size image

Data analysis

We included ten studies in our meta-analysis. Studies synthesized in the meta-analysis used the same scales for measuring their outcomes, although this was not a prerequisite. Thus, we chose mean difference as effect estimate in all comparisons as all studies relied on the same scales. Six studies provided data concerning anxiety in children, five studies concerning parental anxiety, one concerning pain in children, and another one concerning children’s cooperation. We had to exclude one study from meta-analysis and report the available results narratively, as it reported outcome data using figures and p values without giving means and standard deviations [28].

Children’s anxiety

Clowning vs. parental presence or no intervention (Fig. 3)

During preoperative time, pooled estimated effects were significantly in favor of clowning (MD = − 7.16 [− 10.58, − 3.75], I2 = 0%). In the operation, induction, or patient room, clowning was also significantly more effective than parental presence or no intervention (MD = − 20.45 [− 35.54, − 5.37], I2 = 93%). During mask application or physical examination, however, parental presence or no intervention seemed to be more effective in reducing children’s anxiety (MD = 2.33 [− 4.82, 9.48], I2 = 52%). During the process from waiting room until skin prick test, clowning was significantly more effective than parental presence or no intervention in one study (MD = − 13.80 [− 21.28, − 6.32]). We found similar results in Kocherov et al. where children undergoing clowning demonstrated significantly lower preoperative (p = 0.032) and postoperative anxiety (p = 0.004) than children receiving only parental presence or no intervention.

Fig. 3 Children’s anxiety—clowning vs. parental presence or no intervention Full size image

We performed a sensitivity analysis to assess the impact of the inclusion of the two studies not explicitly mentioning parental presence. When excluding the two studies, clowning was even more effective in reducing children’s anxiety in operation, induction, or patient room (MD = − 25.55 [− 36.27, − 14.83], I2 = 68%).

Clowning vs. midazolam (Fig. 4)

In the preoperative period, pooled estimated effects were significantly in favor of clowning compared to midazolam (MD = − 7.60 [− 11.73, − 3.47], I2 = 0%). In the induction room, pooled estimated effects were in favor of clowning, but not statistically significantly (MD = − 9.63 [− 21.04, 1.77], I2 = 66%). During mask application, midazolam was significantly more effective in reducing children’s anxiety than clowning (MD = 12.80 [3.65, 21.95]) in one study.

Clowning vs. child life program (Fig. 5)

One study compared clowning and the child life program. In the patient room, the child life program was significantly more effective than clowning (MD = 1.40 [0.25, 2.55]). During physical examination, the child life program was more effective, but not statistically significantly (MD = 1.20 [− 0.11, 2.51]).

Fig. 5 Children’s anxiety—clowning vs. child life program Full size image

Parental anxiety

Clowning vs. parental presence or no intervention (Fig. 6)

Pooled estimated effects were significantly in favor of clowning compared to parental presence or no intervention for parental state anxiety (MD = − 4.00 [− 6.35, − 1.65], I2 = 0%) and for parental trait anxiety (MD = − 3.67 [− 6.65, − 0.69], I2 = 0%). Furthermore, clowning significantly decreased parental anxiety when measured on a verbal rating scale (MD = − 1.40 [− 2.40, − 0.40]). After excluding two studies which did not explicitly mention parental presence, clowning did not statistically significantly lower parental state anxiety (MD = − 2.83 [− 6.61, 0.36], I2 = 0%). However, clowning was still significantly decreasing parental trait anxiety (MD = − 4.45 [− 7.95, − 0.95], I2 = 0%).

Fig. 6 Parental anxiety—clowning vs. parental presence or no intervention Full size image

Clowning vs. midazolam (Fig. 7)

One study compared parental anxiety in children undergoing clowning and in children taking midazolam. Midazolam was significantly more successful in decreasing parental state anxiety than clowning (MD = 21.10 [13.95, 28.25]). Clowning seemed to be more effective in decreasing parental trait anxiety, but not statistically significantly (MD = − 4.20 [− 13.70, 5.30]).

Children’s pain (Fig. 8)

One study compared pain in children undergoing clowning and in children accompanied by at least one parent. Clowning was significantly more successful in decreasing children’s pain, which was measured 1 min after the procedure (MD = − 5.30 [− 6.77, − 3.83]).

Fig. 8 Children’s pain—clowning vs. parental presence or no intervention Full size image

Children’s cooperation (Fig. 9)

One study compared the behavior of children undergoing clowning and children accompanied by their parent. Children undergoing clowning had significant better cooperation than children with parental presence or no intervention (MD = − 6.20 [− 8.64, − 3.76]) [24].

Fig. 9 Children's cooperation-clowning vs. parental presence or no intervention Full size image

Publication bias

Since there were less than ten studies per meta-analysis, assessment of publication bias using a funnel plot was not reasonable. We found four of the included studies registered on ClinicalTrials.gov [10, 11, 19, 23], and a protocol was available for one study [23]. We identified eight ongoing RCTs on clinicaltrials.gov (IDs: NCT02199587, NCT00886314, NCT01622218, NCT02701322, NCT02668679, NCT03122015, NCT03324828, NCT03671317) (see Additional file 6). One study started in 2018; therefore, we did not request this study’s results. Four authors did not respond to our request for further information. These studies had started in 2009, 2014, 2016, and 2017. One study was completed in 2012, and the study author replied that clowning seems to have no impact. Another study started in 2016 and was ceased due to technical issues. Last but not least, one study that started in 2017 did not provide sufficient contact information. Publication bias can therefore not be ruled out.

Additional analysis

We assessed the different types of potentially anxiety-provoking procedures. Regarding anesthesia, clowning was able to reduce children’s anxiety significantly at least at one point of time [4, 18, 19, 27, 28], but did not report a statistically significant decrease of parents’ anxiety [4, 18, 22]. During mask application, children’s anxiety was significantly higher in the clowning group compared to midazolam [19]. One study that examined the effect of clowning in children undergoing allergy skin prick test found a statistically significant decreasing influence on children’s anxiety and parental state anxiety, but not for parental trait anxiety [10]. One study compared clowning with the child life program and parental presence or no intervention in children undergoing physical examination [17]. At both times, in the patient room and during examination, anxiety was higher in the clown group compared to the child life program, but only in the patient room statistical significance was reached. In comparison to parental presence or no intervention, anxiety was not significantly lower in patient room and not significantly higher during examination. Regarding intravenous access, one study showed a non-statistically significant reduction of parental state anxiety [11], while another study found that children’s pain scores were significantly lower in the medical clown group (p < 0.001) [23]. For burn dressing change, children undergoing clowning were significantly more cooperative than children with parental presence or no intervention [24].

Overall quality of evidence

GRADE assessments yielded very low quality of evidence for all outcomes. For a summary of main findings comparing clowning and parental presence or no intervention, see Table 1. For a summary of main findings regarding other comparisons, see Additional files 7 and 8.