Methodological Issues on the epidemiology of non motor symptoms in Parkinson's disease.

The increasing interest in non-motor symptoms in Parkinson's disease due to the widening knowledge in the neurobiological field, brought to considering PD as a complex disease. This requires a proper characterization of the natural history of the disease using the appropriate epidemiological tools, along with the implementation in clinical practice of a standard a multi-professional approach. Available evidence on the frequency of non-motor symptoms in PD is reported in studies with very heterogeneous designs and diagnostic measures, and using a great variety of epidemiological estimates. In particular, there are no studies reporting the frequency of non-motor symptoms in a cohort of incident PD patients using point prevalence, incidence rate, and cumulative incidence. A mini-review of cross-sectional and longitudinal studies on three non-motor symptoms/conditions (dementia, depression, ICDs) in patients with PD confirms these premises and highlights the need of higher quality studies, both from a methodological and a clinical point of view.