Understanding Cognitive Domains in Parkinson's Disease: A Scoping Review of Empirical Studies

Cognitive domains (e.g. memory, attention, or executive functions) play a crucial role in cognitive diagnostics. However, cognitive domain definitions and the assignment of cognitive tests to these domains lack both consensus and empirical justification in current guidelines for cognitive diagnostics. Focusing on this issue in Parkinson's disease (PD), this scoping review aims to provide an overview of empirical findings from dimensionality reduction analyses applied to cognitive test scores of individuals with PD. The research addressed three questions: (i) Which methods were used to empirically evaluate cognitive domain differentiations in PD? (ii) Could previously assumed cognitive domains in PD be reproduced by empirical analyses? (iii) Which cognitive domain models did the studies use as its theoretical framework? An a priori defined systematic literature search identified 22 eligible studies published between 1963 and 2022, mainly using exploratory methods such as Principal Component Analysis or Exploratory Factor Analysis to identify a cognitive domain structure in people with PD. These studies were highly heterogeneous in specific methodological configurations of statistical methods, such as rotation and extraction methods. Only few studies referred to the Movement Disorder Society (MDS) guidelines for cognitive diagnostics in PD, and none successfully reproduced previously assumed cognitive domain structures by (exploratory) empirical analysis. The inconsistent results regarding the structure of cognitive functioning might result from heterogeneous samples and methods. Overall, the results challenge the validity and reliability of cognitive diagnoses in PD, highlighting a need for more robust empirical support to improve diagnostic frameworks.