Conversion to MCI and dementia in Parkinson's disease: a systematic review and meta-analysis

•Cognitive decline is common and occurs relatively quickly in Parkinson's disease.•25% of patients convert from normal cognition to mild cognitive impairment within 3 years of follow-up.•20% of patients convert from mild cognitive impairment to dementia within 3 years of follow-up.•Results highlight the importance of screening and early intervention.