42 experimental conditions in a number of studies (see Table 1) in total we analyzed 40 study arms (30 pain and 10 itch). Thermal pain inductions were used in 19 arms, electrical pain was used in 6, pressure pain was used in 1, and mechanical, cold pressor, hot water bath, and histamine methods were each used in 1 study arm. Only 7 studies (10 arms) induced nocebo effects on itch, one of which also included pain (this study, van Laarhoven et al., 2011, is listed under Pain in Table 1). Electrical itch was used in 3 studies, one of which (van Laarhoven et al., 2011) used additional mechanical and histamine inductions in both the pain and itch groups (see Table 1). Histamine was used in 3 more itch studies and cowhage was used in 1 study. For nocebo induction, most studies (18 pain and 4 itch studies) used a combination of classical conditioning and negative verbal suggestions, and for 3 we included additional study arms that employed verbal suggestions alone (Table 1). Verbal suggestions alone were used as the main manipulation in 10 pain studies (in total 12 arms) and 3 itch studies (in total 6 arms). Risk of bias was low within all studies, with most studies showing low risk of bias (max. 5/34) and only one study scoring in the low-moderate range with a score of 6/34 (Table 1). The funnel plots as well as a trim and fill method that suggested a small number of imputed studies (Figure 3) indicated that overall, there was a low degree of potential publication bias across all studies, with a total estimated 7 studies missing.
RkJQdWJsaXNoZXIy MTk4NDMw