Subjective cognitive decline in idiopathic Parkinson´s disease: A systematic review

Cognitive symptoms of Parkinson's disease (PD) have been long underestimated, but are some of the most disabling non-motor features of the disease. In order to establish signs that allow for earlier detection of cognitive decline in PD, the concept of `subjective cognitive decline´ (SCD) has gained a growing interest. SCD refers to patients who report a decline in subjective cognitive capacities, while their results on neuropsychological tests are within the normal performance range, indicating adequate cognitive functions. The aim of this review was to evaluate the concept of SCD in PD and give an overview of the current research. A systematic literature search in PubMed was performed to identify articles published before December 2020. We included 18 studies with a total of n = 2,654 patients. While there is currently no consensus on research or clinical criteria for SCD in PD, this review presents the accumulated evidence for SCD in PD patients and supports the importance of early identification of cognitive deficits, due to the relatively high prevalence for SCD in PD and the added risk of future cognitive impairment it entails. The publications included in this review indicate that SCD may be part of the PD spectrum but further research is needed. Expanding research on SCD in PD will allow for earlier detection of cognitive impairment and may be considered for a preventive intervention.