Background The UHDRS motor section is composed of 31 items related to different aspects of the motor symptoms. These are heterogeneous in terms of functional impact, course of progression, and relation to pathological changes.

Aims The present study is aimed at investigating the correlational structure between UHDRS motor items and functional measures, cross-sectionally and longitudinally, as well as estimating the rate of progression of different UHDRS motor subdomains.

Methods Longitudinal data including UHDRS motor scores, functional scores (tfc, fa, independence scale), CAG repeat length, age, sex, on 889 HD patients, collected in the REGISTRY project over around 5 years, were provided by EHDN. Cross-sectional relationships were analysed by principal component analysis (PCA). Longitudinal analysis was performed by PCA, and PLS regression analysis, modelling the influence of change in motor scores, as well as basic demographic data, on the decline of functional scores.

Results Cross-sectional analysis shows that UHDRS motor items form four major clusters, that is chorea, dystonia, eye movements, and voluntary motor scores, the voluntary scores being most strongly, negatively correlated to the functional measures analysed (tfc, fa, is). Similarly, longitudinal analysis shows that voluntary motor impairment is most strongly correlated to the decline in functional outcomes, whereas in particular the time-course of chorea does not follow the functional decline. CAG repeat length and disease burden (CAG * age construct) were associated with a faster rate of decline in motor performance and functional outcomes.

Conclusions These preliminary results support the division of UHDRS motor items into voluntary and involuntary motor symptoms, and show that the impairment of voluntary movements is strongly linked to the decline in functional outcomes, both as viewed cross-sectionally and over time. The longitudinal analysis further indicates that the CAG repeat length, as well as the disease burden, are significant determinants of the rate of progression of clinical disability in HD.