Domain Specific Placebo Response in the Modified Friedreich's Ataxia Rating Scale.

The placebo response in clinical trials in ataxias complicates outcome interpretation and potentially obscures genuine treatment effects. We analyzed placebo group data from past trials in Friedreich Ataxia and observed notable responses in appendicular items, in contrast to minimal changes in axial function, as measured by respective subscores of the modified Friedreich Ataxia Rating Scale (mFARS). The effect increased with the number of consecutive tests, shorter testing intervals, and older group ages. This has implications for trial design and endpoint selection, thus strengthening the utility of the Upright Stability Score (USS), a sub-score of mFARS, as an independent measure.