Predictors of perceived treatment in a double-blind placebo surgery trial

This study examined demographic variables, patient expectations, psychosocial elements, and physical functioning in order to explore the placebo effect in a double-blind fetal cell transplantation trial for the treatment of Parkinson's disease. In a previous quality of life study based on this sample, McRae et al. (2004) found a strong placebo response when comparing those who thought they received the transplant and those who thought they received the sham surgery. The purpose of this study was to evaluate whether demographic characteristics, psychosocial and physical functioning at baseline were independently, or in combination, predictive of the placebo response during the 12 month double-blind period. The first objective of this study was to determine the strongest baseline predictors of perceived treatment at 4, 8, and 12 months. The second objective was to determine if changes in Motor Functioning at 4, 8, and 12 months added to the predictive power of the baseline variables. The third objective was to determine if predictors of perceived treatment groups were consistent over time. The sample studied consisted of 29 of the 40 participants from across the United States and Canada who were randomly assigned to receive the fetal tissue transplant or sham surgery. Results indicated that Psychosocial factors were the strongest predictors of perceived treatment at all time periods. Over the double-blind period, Gender and Relationship with the Doctor consistently predicted perceived transplant or sham surgery groups. Results showed that, contrary to expectations, changing motor symptoms as objectively assessed by medical personnel did not significantly improve prediction models. It is suggested that in future studies of surgery trials, investigators obtain specific expectations about treatment benefit as well as confidence in the doctor, and use regression techniques to control for potential baseline differences in expectancies and beliefs about treatment. These are important results for elucidating the placebo response in Parkinson's disease and may be generalizable to other populations as well.