A Call for Change: Updating the Operational Definition for Dementia in Parkinson's Disease

In Parkinson's disease (PD), cognitive dysfunction ranges from subjective cognitive complaints to mild cognitive impairment (PD-MCI) and PD dementia (PDD). Timely identification and management of cognitive impairment are major challenges in PD, with substantial burdens on those affected and healthcare systems. Recognizing the need for criteria for different stages of cognitive impairment in PD, the Movement Disorder Society (MDS) commissioned task forces developed clinical diagnostic criteria for PDD2 (2007) and PD-MCI3 (2012) to identify cognitive impairments and ensure uniform participant criteria for therapeutic trials. The criteria, based on literature review and expert consensus, provide recommendations for diagnostic procedures that operationalize PDD4 and PD-MCI3 diagnoses and allow for Level I and II assessments (depending on available time and resources), which have both undergone formal validation. The PD-MCI criteria have not only advanced the field regarding clinical, biomarker, genetic features and the conversion to PDD, but also facilitated pathways for industry and regulatory authorities to conduct clinical trials, specifically addressing this “at risk” stage of cognitive impairment.7 At the time when the PDD criteria were established, however, there was still considerable influence from the Alzheimer's disease (AD) field and few robust biomarkers. Indeed, the only symptomatic medication approved by regulatory authorities for PDD (namely rivastigmine) used the ADAS-cog, as the primary outcome measure.8 Even recent PDD trials vary substantially in their inclusion criteria and outcome measures selected, making reliable comparisons among studies or conducting meta-analyses nearly impossible.