Placebo power.

A variable placebo response has been previously described in the multiple sclerosis (MS) literature and in sham surgical trials involving other patient populations. Conditioning and expectancy models are proposed to explain the biological mechanisms of a placebo response. Possibly, when neuroimmune mechanisms are involved in the disease process, placebo and treatment responses interact. The measurement of a placebo response related to endovascular procedures in MS may be scientifically and ethically challenging. Based on the factors associated with larger placebo responses, it is estimated that a significant placebo response may be associated with these procedures. The factors and mechanisms associated with marked placebo responses should be considered in the design of future studies related to the investigation of endovascular procedures for MS.