Measurement of subjective cognitive decline and prediction of longitudinal clinical outcomes in older adults: a systematic review.

Abstract

Subjective cognitive decline (SCD) is a preclinical risk factor for Alzheimer's disease and may represent one of the earliest clinical indicators of underlying neurodegeneration. Prevalence estimates vary but suggest that between 20 and 30% of cognitively unimpaired older adults report some form of subjective decline (Rohr et al. 2020). Longitudinal studies indicate that individuals with SCD are at approximately twice the risk of progressing to mild cognitive impairment (MCI) or dementia compared to those without complaints. While not every older adult reporting decline will progress, early detection allows for timely intervention and risk stratification for clinical trials of dementia prevention. The current review examined longitudinal studies of adults aged ≥ 60 to assess and highlight the heterogeneity in operational definitions of SCD and risk of incident MCI or dementia. Among the 26 studies reviewed, 24 distinct definitions of SCD were used, highlighting the need for standardized measurement to improve comparability and generalizability. Of the studies reporting hazard ratios, all nine found that SCD was associated with a significantly higher risk of incident MCI or dementia compared to cognitively healthy older adults not reporting SCD; with the exception that no significant association between SCD and risk of vascular dementia was observed. The consistent association between SCD and risk of progression to MCI and dementia in these studies strengthens the evidence that suggests SCD may not be a benign complaint of aging or psychiatric conditions, but a potentially early clinical indicator of possible underlying neurodegenerative changes. These findings highlight the need for standardization in measurement tools and definitions to refine this pre-MCI construct.