Minimal Clinically Important Differences for Burke‐Fahn‐Marsden Dystonia Rating Scale and 36‐Item Short‐Form Health Survey

Background Although an increasing number of trials are reported on the treatment of generalized or segmental isolated dystonia, the minimal clinically important difference thresholds for the most frequently reported outcome measures are still undetermined. Objectives To estimate the minimal clinically important difference for the Burke‐Fahn‐Marsden Dystonia Rating Scale and the 36‐Item Short‐Form Health Survey in generalized or segmental dystonia. Methods A total of 898 paired examinations of 198 consecutive patients, aged >18 years, with idiopathic and inherited (torsin family 1 member A positive) segmental and generalized isolated dystonia were analyzed. To calculate the minimal clinically important difference thresholds, both anchor‐ and distribution‐based methods were used simultaneously. Results Any improvement >16.6% or worsening larger than 21.5% on the Burke‐Fahn‐Marsden Dystonia Rating Scale indicates a minimal, yet clinically relevant, change. Threshold values for the Burke‐Fahn‐Marsden Dystonia Disability Scale were 0.5 points for both decline and improvement. Cut‐off scores for the Physical Component Summary, the Mental Component Summary, and the Global (Total or Overall) Score of the 36‐Item Short‐Form Health Survey were 5.5 and 5.5, 6.5 and 7.5, and 7.5 and 8.5 points for clinically meaningful improvement and deterioration, respectively. Conclusions The minimal clinically important difference represents the smallest change in an outcome measure that is meaningful to patients. Our estimates for the Burke‐Fahn‐Marsden Dystonia Rating Scale and the 36‐Item Short‐Form Health Survey may allow more reliable judgment of the clinical relevance of different treatments for segmental and generalized isolated dystonia. © 2020 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.