Affiliations: [a] Academic Department of Rehabilitation Medicine, Leeds Institute of Rheumatic and Musculoskeletal Medicine, University of Leeds, Leeds, UK
| [b] Departments of Psychiatry, Pediatrics and Neurology, University of Iowa Carver College of Medicine, Iowa City, IA, USA
| [c] Struthers Parkinson’s Center, Minneapolis, MN, USA
| [d] Department of Neurology, University of Ulm, Ulm, Germany
| [e] Department of Clinical Neuroscience, John Van Geest Centre for Brain Repair, Forvie Site, Robinson Way, Cambridge, UK
| [f] Huntington and Rare Diseases Unit, IRCCS Casa Sollievo della Sofferenza Hospital (Rome-CSS Mendel), San Giovanni Rotondo, Italy
| [g] Neurozentrum Siloah and Department of Neurology, Swiss HD Center, University of Bern, Switzerland
| [h] Department of Clinical Genetics, Sheffield Children’s Hospital, Sheffield, UK
Correspondence to: Oliver Quarrell, Department of Clinical Genetics, Sheffield Children’s Hospital, Sheffield, UK. Tel.: +44 1142717025; Fax: +44 1142737467; E-mail: [email protected].
Abstract: 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 (UHDRS-TMS) is often used as the principal outcome measure in clinical trials. Objective:To identify a motor scale more suitable for JOHD patients. Methods:A working group reviewed the UHDRS-TMS and modified it by adding four further assessment items. Rasch analysis was used to study the performance of the modified 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. Conclusion:This post-hoc scale restructuring appears to provide a valid motor score that is psychometrically robust in a JOHD population. This scale restructuring offers a pragmatic solution to measuring motor function in a JOHD population, and it could provide the basis for the further iterative development of a more useful clinical rating scale for patients with JOHD.
Keywords: Rasch, juvenile onset Huntington’s disease, total motor score