International study on the psychometric attributes of the Non-Motor Symptoms Scale in Parkinson disease

BACKGROUND: Nonmotor symptoms (NMS) have a great impact on patients with Parkinson disease (PD). The Non-Motor Symptoms Scale (NMSS) is an instrument specifically designed for the comprehensive assessment of NMS in patients with PD. NMSS psychometric properties have been tested in this study. METHODS: Data were collected in 12 centers across 10 countries in America, Asia, and Europe. In addition to the NMSS, the following measures were applied: Scales for Outcomes in Parkinson's Disease (SCOPA)-Motor, SCOPA-Psychiatric Complications (SCOPA-PC), SCOPA-Cognition, Hoehn and Yahr Staging (HY), Clinical Impression of Severity Index for Parkinson's Disease (CISI-PD), SCOPA-Autonomic, Parkinson's Disease Sleep Scale (PDSS), Parkinson's Disease Questionnaire-39 items (PDQ-39), and EuroQol-5 dimensions (EQ-5D). NMSS acceptability, reliability, validity, and precision were analyzed. RESULTS: Four hundred eleven patients with PD, 61.3% men, were recruited. The mean age was 64.5 ± 9.9 years, and mean disease duration was 8.1 ± 5.7 years. The NMSS score was 57.1 ± 44.0 points. The scale was free of floor or ceiling effects. For domains, the Cronbach α coefficient ranged from 0.44 to 0.85. The intraclass correlation coefficient (0.90 for the total score, 0.67-0.91 for domains) and Lin concordance coefficient (0.88) suggested satisfactory reproducibility. The NMSS total score correlated significantly with SCOPA-Autonomic, PDQ-39, and EQ-5D (rS = 0.57-0.70). Association was close between NMSS domains and the corresponding SCOPA-Autonomic domains (rS = 0.51-0.65) and also with scales measuring related constructs (PDSS, SCOPA-PC) (all p < 0.0001). The NMSS total score was higher for women (p < 0.02) and for increasing disease duration, HY, and CISI-PD severity level (p < 0.001). The SEM was 13.91 for total score and 1.71 to 4.73 for domains. CONCLUSION: The Non-Motor Symptoms Scale is an acceptable, reproducible, valid, and precise assessment instrument for nonmotor symptoms in Parkinson disease.