Background: There is a considerable variation in the reported frequency of dementia in Parkinson's disease (PDD). The aim of this study was to evaluate the frequency of PDD reported in published studies and to examine the different methodological, clinical, and demographic factors that may contribute to this variation.

Methods: We conducted a systematic review, searching EMBASE and MEDLINE databases for relevant articles on PDD frequency published before May 2019. A global estimation of PDD was calculated. Different subgroup analyses were performed for methodological, clinical, and demographic characteristics. Meta-regression was also conducted to identify any significant differences within the subgroups.

Results: We included 295 studies. The global pooled dementia frequency was 26.3%. These estimations varied according to methodological (14%-55%), clinical (18%-46%), and demographic (21%-43%) variables. The declared primary objective of the studies (to study PDD), the follow-up length (≥7 years), the age of the participants (≥75 years), Parkinson's disease (PD) duration (>10 years), and the Hoehn & Yahr (H&Y) stage (>3) were important factors affecting reported dementia frequency.

Conclusions: This systematic review found that approximately one-quarter of the PD patients were diagnosed with PDD. Dementia frequency varied according to methodological, clinical and demographic variables. We cannot examine PDD frequency without considering all these variables that have an impact on it.