Placebo and nocebo responses in other movement disorders besides Parkinson's disease: How much do we know?

Among movement disorders and medicine in general, PD is one of the conditions for which there is a greater knowledge of the placebo and nocebo responses. In other movement disorders, the knowledge of placebo and nocebo responses is less. An advance in this field is expected to contribute to a better understanding of the nature of a therapeutic benefit in clinical research and clinical practice, and mechanisms of placebo and nocebo. We conducted a review on placebo and nocebo responses in other movement disorders besides PD by primarily examining meta-analyses of clinical trials assessing specifically the placebo and/or nocebo responses. Second, we examined both efficacy and safety results of a placebo arm in pivotal placebo-controlled trials for the different movement disorders. RLS is the movement disorder for which the most data exist, followed by tic disorders and Huntington's disease. Data available in other conditions document a placebo response in a varied phenomenology. We found different placebo responses according to clinical domains assessed, type of outcomes used for the same clinical domain, and modes of treatment administration, including rehabilitation and surgical interventions. Data on the nocebo response were very scarce. Although the data available are limited, RLS has the better documentation of placebo and nocebo responses. In contrast, atypical parkinsonisms are the group of movement disorders with the least knowledge. The clinical entities with a more robust placebo response are those that have the most beneficial available symptomatic treatments.