Background Huntington’s disease can present at almost any age but traditionally, those with an onset ≤20 years are described as having juvenile onset Huntington’s disease (JOHD). They are more likely to have bradykinesia and dystonia earlier in the course of the disease. The Total Motor Score of the Unified Huntington’s Disease Rating Scale is often used as the principal outcome measure in clinical trials. Objective To identify a motor scale suitable for JOHD patients Methods A working group reviewed the motor scale and modified it by adding four further assessments. A Rasch analysis was used to study the performance of the scale in 95 patients with a mean age of 19.4 (SD 6.6) years. Results The initial analysis showed a significant overall misfit to the Rasch model and a number of individual items displayed poor measurement properties: all items relating to chorea displayed significant misfit due to under-discrimination. Additionally, a number of items displayed disordered response category thresholds, and a large amount of dependency was present within the item set (96 out of 741 pairwise differences=13%). An iterative process of scale re-structuring and evaluation was then undertaken, with a view to eliminating the largest sources of misfit and generating a set of items that would conform to Rasch model expectations. This led to a new proposed assessment instrument for JOHD. Conclusion This new instrument will now need to be evaluated further but provides the basis for developing a more useful clinical rating scale for patients with JOHD.
