Patients' expectations are important predictors of the outcome of analgesic treatments, as demonstrated predominantly in research on placebo effects. Three commonly investigated interventions that have been found to induce expectations (verbal suggestion, conditioning, and mental imagery) entail promising, brief, and easy-to-implement adjunctive procedures for optimizing the effectiveness of analgesic treatments. However, evidence for their efficacy stems mostly from research on experimentally evoked pain in healthy samples, and these findings might not be directly transferable to clinical populations. The current meta-analysis investigated the effects of these expectation inductions on patients' pain relief. Five bibliographic databases were systematically searched for studies that assessed the effects of brief verbal suggestion, conditioning, or imagery interventions on pain in clinical populations, with patients experiencing experimental, acute procedural, or chronic pain, compared with no treatment or control treatment. Of the 15,955 studies retrieved, 30 met the inclusion criteria, of which 27 provided sufficient data for quantitative analyses. Overall, a medium-sized effect of the interventions on patients' pain relief was observed (Hedges g = 0.61, I 2 = 73%), with varying effects of verbal suggestion (k = 18, g = 0.75), conditioning (always paired with verbal suggestion, k = 3, g = 0.65), and imagery (k = 6, g = 0.27). Subset analyses indicated medium to large effects on experimental and acute procedural pain and small effects on chronic pain. In conclusion, patients' pain can be relieved with expectation interventions; particularly, verbal suggestion for acute procedural pain was found to be effective.
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aUnit Health, Medical and Neuropsychology, Leiden University, Leiden, the Netherlands
bLeiden Institute for Brain and Cognition, Leiden University, Leiden, the Netherlands
cDepartment of Psychology and Behavioural Sciences, Aarhus University, Aarhus, Denmark
dDanish Pain Research Center, Aarhus University Hospital, Aarhus, Denmark
eRadboud Institute for Health Sciences, Radboud University Medical Center, Nijmegen, the Netherlands
fDepartment of Clinical Psychological Science, Maastricht University, Maastricht, the Netherlands
gDepartment of Medical Psychology, Radboud University Medical Center, Nijmegen, the Netherlands
Corresponding author. Address: Unit Health, Medical and Neuropsychology, Leiden University, PO Box 9555, 2300 RB Leiden, the Netherlands. Tel.: +31-71-527-3622; fax: +31-71-527-3619. E-mail address: firstname.lastname@example.org (K.J. Peerdeman).
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Received November 16, 2015
Received in revised form January 27, 2016
Accepted February 22, 2016