The metric properties of a novel non-motor symptoms scale for Parkinson's disease: Results from an international pilot study

Non-motor symptoms (NMS) in Parkinson's disease (PD) are common, significantly reduce quality of life and at present there is no validated clinical tool to assess the progress or potential response to treatment of NMS. A new 30-item scale for the assessment of NMS in PD (NMSS) was developed. NMSS contains nine dimensions: cardiovascular, sleep/fatigue, mood/cognition, perceptual problems, attention/memory, gastrointestinal, urinary, sexual function, and miscellany. The metric attributes of this instrument were analyzed. Data from 242 patients mean age 67.2 ± 11 years, duration of disease 6.4 ± 6 years, and 57.3% male across all stages of PD were collected from the centers in Europe, USA, and Japan. The mean NMSS score was 56.5 ± 40.7, (range: 0-243) and only one declared no NMS. The scale provided 99.2% complete data for the analysis with the total score being free of floor and ceiling effect. Satisfactory scaling assumptions (multitrait scaling success rate >95% for all domains except miscellany) and internal consistency were reported for most of the domains (mean α, 0.61). Factor analysis supported the a prori nine domain structure (63% of the variance) while a small test-retest study showed satisfactory reproducibility (ICC > 0.80) for all domains except cardiovascular (ICC = 0.45). In terms of validity, the scale showed modest association with indicators of motor symptom severity and disease progression but a high correlation with other measures of NMS (NMSQuest) and health-related quality of life measure (PDQ-8) (both, rS = 0.70). In conclusion, NMSS can be used to assess the frequency and severity of NMS in PD patients across all stages in conjunction with the recently validated non-motor questionnaire.