Conversion to MCI and dementia in Parkinson's disease: a systematic review and meta-analysis.
To systematically review and meta-analyse conversion rates from normal cognition to Mild Cognitive Impairment (MCI) and dementia in Parkinson's disease (PD) patients. Reversion rates in patients with MCI (i.e. PD-MCI) were also investigated. Electronic searches of PsycINFO, Medline and EBSCOhost were conducted in January 2018, with 1833 articles identified after duplicate removal. Articles were included if they assessed conversion/reversion in PD patients between normal cognition, PD-MCI and PD dementia (PD-D). In total, 39 articles met the inclusion criteria, representing 4011 patients (mean age range 58-75; 61% male). Within three years, in those with PD and normal cognition, 25% (95%CI 20-30%) converted to PD-MCI and 2% (95%CI 1-7%) converted to dementia. Of those with PD-MCI, 20% (95%CI 13-30%) converted to dementia while 28% (95%CI 20-37%) reverted back to a state of normal cognitive function. The conversion rates to MCI and dementia were higher, and reversion rates lower, when follow-up was ≥3 years. When International Parkinson and Movement Disorder Society (MDS) criteria were used to diagnose MCI, Level I criteria were associated with a greater reversion estimate from PD-MCI to normal cognitive function. These findings summarise the trajectory of cognitive impairment in PD and highlight that MCI is common in this patient group. Understanding cognitive trajectories in PD patients is important for patient care in terms of prognosis, as well as for identifying windows for intervention for cognitive symptoms. As the number of PD patients increases with an ageing population, this information can inform future policy and planning.