(Van Laarhoven et al., 2011)
Aims: To study verbally induced nocebo and placebo effects on itch and pain.
Methods: 105 healthy female subjects were randomly assigned to one of 4 groups: (Itch nocebo n=36 vs Control n=20 or Pain nocebo n=33 vs Control n=16). In itch / pain nocebo groups patients were told a verbal suggestion stating that 95% of healthy people will experience Itch / pain and in control groups a neutral suggestion stating only 5% may experience Itch, 5% pain otherwise nothing. All groups tested for mechanical stimuli (Von Frey), electrical stimuli, ionophoresis of Histamine. Then all nocebo group undertook another ionophoresis stimuli. With either placebo suggestion of reduction of itch / pain or control with suggestion of same gel and that everyone feels itch or pain but not the other according to the group. All group rated pain and itch on VAS scales before and every 30 seconds during test, and after.
Results: Subjects who received verbal suggestions to induce high pain expectations reported significantly more pain than subjects who received verbal suggestions inducing low pain expectations. Subjects who received verbal suggestions inducing high itch expectations experienced significantly higher levels of itch evoked by the somatosensory stimuli than subjects who received verbal suggestions inducing low itch expectations. The nocebo effects induced by verbal suggestions appeared to be stronger for itch than for pain.
Itch levels decreased to a greater extent when suggestions of itch reduction were given than when suggestions of pain reduction were given, while the decrease in itch was not significantly different when it was suggested that itch would be reduced in comparison to neutral suggestions and same for pain.
Limitations: Only women, from one university of Netherlands, small sample. Part 2 habituation of histamine effect possible. Lab setting can be a cofounding factor. Lack control group with no expectations at all. Healthy subjects only. Small effect: MIDC not reached, low levels of itch / pain in general.
In practice: Itch and pain can be induced by only giving verbal suggestions. The perception of different ambiguous stimuli can be influenced by negative suggestions, in such a way that negative expectations can adversely influence the intensity of itch or pain experienced.
Van Laarhoven, A. I. M., Vogelaar, M. L., Wilder-Smith, O. H., Van Riel, P. L. C. M., Van De Kerkhof, P. C. M., Kraaimaat, F. W., & Evers, A. W. M. (2011). Induction of nocebo and placebo effects on itch and pain by verbal suggestions. Pain, 152(7), 1486–1494. https://doi.org/10.1016/j.pain.2011.01.043