Cognition, psychiatric symptoms and conversion to dementia in Parkinson’s disease

Parkinson’s disease (PD) is a complex neurodegenerative disorder which can have a significant effect on a patient’s quality of life. Psychiatric symptoms and cognitive decline are common, with dementia (PDD) ultimately occurring in approximately 90% of patients. Mild cognitive impairment in PD (PD-MCI) is an intermediate state of cognitive decline, where patients are at a higher risk of progression to PDD than non-PD-MCI patients. The high rates of patients who develop PDD demonstrate that it is pertinent to have indicators of imminent PDD; particularly should a preventative therapy ever become available. Therefore, the aim of my PhD project was to use cognitive and neuropsychiatric measures to predict cognitive decline within a four-year period. Firstly, we examined permitted variations within Movement Disorder Society Task Force (MDS-TF) Level II diagnostic PD-MCI criteria. We aimed to identify a PD-MCI criterion which optimally captured individuals who later progressed to PDD. To evaluate this, we followed 121 non-demented PD patients for up to 4.5 years; 21% converted to PDD over this period. Three unique groups of patients were classified as PD-MCI using baseline neuropsychological assessments. Each patient was required to have two impaired cognitive test scores, with impairment defined as (1) -2SD, (2) -1.5SD or (3) -1SD below normative data. Relative risk (RR) of PDD for each criterion was calculated, with the -1.5SD criterion found to be optimal for maximizing progression to PDD over four-years. Secondly, another variation within the MDS-TF PD-MCI criteria was examined - whether the impaired tests need to occur within a single cognitive domain or only across two or more cognitive domains. The same sample of PD patients was reassessed to determine (1) RR of PDD when two impairments at -1.5SD existed within one cognitive domain, followed by (2) RR of PDD in the unique group whose two impairments at -1.5SD did not exist within one cognitive domain. The -1.5SD cut-off produced a high RR of PDD only when two impairments were identified within one cognitive domain (7.2, 95% confidence interval (CI) = 3.4–16.6, p