Optimising Mobility Outcome Measures in Huntington's Disease

Background: Many of the performance-based mobility measures that are currently used in Huntington's disease (HD) were developed for assessment in other neurological conditions such as stroke. Objective: We aimed to assess the individual item-response of commonly used performance-based mobility measures, with a view to optimizing the scales for specific application in Huntington's Disease (HD). Method: Data from a larger multicentre, observational study were used. Seventy-five people with HD (11 pre-manifest & 64 manifest) were assessed on the Six-MinuteWalk Test, 10-MeterWalk Test, Timed "Up & Go" Test (TUG), Berg Balance Scale (BBS), Physical Performance Test (PPT), Four Square Step Test, and Tinetti Mobility Test (TMT). The Unified Huntington's Disease Rating Scale (UHDRS) Total Motor Score, Functional Assessment Scale and Total Functional Capacity scores were recorded, alongside cognitive measures. Standard regression analysis was used to assess predictive validity. Individual item responses were investigated using a sequence of approaches to allow for gradual removal of items and the subsequent creation of shortened versions. Psychometric properties (reliability and discriminant ability) of the shortened scales were assessed. Results: TUG (beta 0.46, CI 0.20-3.47), BBS (beta-0.35, CI -2.10-0.14), and TMT (beta-0.45, CI -3.14-0.64) were good diseasespecific mobility measures. PPT was the best measure of functional performance (beta 0.42, CI 0.00-0.43 for TFC & beta 0.57 CI 0.15-0.81 for FAS). Shortened versions of BBS and TMT were developed based on item analysis. The resultant BBS and TMT shortened scales were reliable for use in manifest HD. ROC analysis showed that shortened scales were able to discriminate between manifest and pre-manifest disease states. Discussion: Our data suggests that the PPT is appropriate as a general measure of function in individuals with HD, and we have identified shortened versions of the BBS and TMT that measure the unique gait and balance impairments in HD. These scales, alongside the TUG, may therefore be important measures to consider in future clinical trials.