Objective: To determine the clinically important change for the movement subscale in the Fahn-Marsden Dystonia Rating Scale (FMDRS)

Background: The FMDRS is the major rating scale utilized to assess generalized dystonia, and particularly to measure efficacy of deep brain stimulation (DBS) therapy. It is composed of two clinician-rated subscales: a movement subscale (MS), based on patient examination, and a disability subscale, derived from the patient’s report of disability.  Determining the clinical important change (CIC) for the MS in clinical trials may assist in assessing and optimizing treatments for dystonia.  

Design/Methods: A consecutive series of patients with genetically determined dystonia (with either mutations in Tor1A (DYT1 GAG deletion, n=15) or THAP1(DYT6, n=1) underwent bilateral globus pallidum internum (GPi) DBS and had available video for assessment of global clinical improvement. While FMDRS was performed pre- and post- surgery by the rating clinician, video ratings were performed at later time by a different movement disorders expert.  To determine the CIC, an anchor-based method using ratings on the seven-point global clinical improvement (CGI-S) was used to compare with the FMDRS, and ROC analysis was performed.

Results: Sixteen patients (62.5% male), current mean age 42.7 years (17.9) were included. Baseline median MS score was 38.5 (IQR 32.5;53.7). A median reduction of 73.7% (60.7;89.8), 28.3 points (12 months post-DBS), and 90.0 % (83.8;96.0), 34.6 points (6 years post-DBS), of the MS, were considered the best cutoff scores to detect a substantial (very much) improvement according to the CGI-S.