Assessing the non-motor symptoms of Parkinson's disease: MDS-UPDRS and NMS Scale

Background and purpose: Although Parkinson's disease (PD) is characterized by typical motor manifestations, non-motor symptoms (NMS) are an outstanding part of the disease. At present, several specific instruments for assessment of NMS are available. The objective of our study was to determine the performance of the Movement Disorder Society-Unified Parkinson's Disease Rating Scale (MDS-UPDRS): Part I - Non-Motor Aspects of Experiences of Daily Living (nM-EDL) compared with the Non-Motor Symptoms Scale (NMSS).

Methods: To this purpose, 434 consecutive patients with PD were included in an international, observational, cross-sectional study. The association between scores of both scales was determined by the Spearman rank correlation coefficient. Equations for transformation of total score of a scale to the other were constructed from weighted regression models and both, transformed and observed score, contrasted by means of the Lin's Concordance Correlation Coefficient (LCCC) and Bland-Altman plot.

Results: As a whole, the prevalence of the NMS according to each scale was quite similar, and most of the correlations between their corresponding components were high (r(S) > 0.60). The total score correlation of the MDS-UPDRS Part I with the NMSS was high (r(S) = 0.81). Concerning the transformed scores, estimated scores only partially approach the observed ones (sharing about 60-64% of the variance) because residual variance increased with increasing magnitudes of the scores, i.e. the most severe patients (Bland-Altman plot; LCCC < 0.60 for severe patients).

Conclusions: (i) MDS-UPDRS Part I (nM-EDL) and NMSS showed a strong convergent validity; (ii) however, transformed scores using the equations from weighted regression models showed that for patients with the most severe NMS they are not concordant.