Placebo and nocebo effects for itch and itch-related immune outcomes: A systematic review of animal and human studies

• Published in: Neuroscience and biobehavioral reviews 2020-06, Vol.113, p.325-337
• Main Authors: Meeuwis, Stefanie H., van Middendorp, Henriët, van Laarhoven, Antoinette I.M., van Leijenhorst, Cora, Pacheco-Lopez, Gustavo, Lavrijsen, Adriana P.M., Veldhuijzen, Dieuwke S., Evers, Andrea W.M.
• Format: Article
• Language: English
• Subjects: Conditioning; Dermatology; Expectancy; Itch; Nocebo effects; Placebo effects
• ISSN: 0149-7634; 1873-7528
• DOI: 10.1016/j.neubiorev.2020.03.025

•Social, instructional and associative learning can induce placebo effects.•These same mechanisms can underlie nocebo (or negative) effects.•Placebo and nocebo effects can influence itch and itch-related immune outcomes.•Brain areas responsible for processing of itch are associated with nocebo effects in itch.•Placebo effects may be used to improve clinical symptoms and conditions of the skin. Placebo and nocebo effects can influence somatic symptoms such as pain. For itch and other dermatological symptoms these effects have been far less investigated. This review systematically integrates evidence from both animal (mainly rodents) and human trials on placebo and nocebo effects in itch, itch-related symptoms and conditions of the skin and mucous membranes, and related immune outcomes (e.g., histamine). Thirty-one animal studies, and fifty-five human studies (k = 21 healthy participants, k = 34 patients) were included. Overall, studies consistently show that placebo and nocebo effects can be induced by various methods (e.g., suggestions, conditioning and social cues), despite high heterogeneity across studies. Effects of suggestions were found consistently across subjective and behavioral parameters (e.g., itch and scratching in humans), whereas conditioning was likely to impact physiological parameters under certain conditions (e.g., conditioning of histamine levels in stressed rodents). Brain areas responsible for itch processing were associated with nocebo effects. Future research may investigate how variations in methods impact placebo and nocebo effects, and whether all symptoms and conditions can be influenced equally.