A Systematic Review of Subjective Cognitive Characteristics Predictive of Longitudinal Outcomes in Older Adults

Background and objectives: Subjective cognitive decline (SCD) is a common experience of self-perceived decline without objective cognitive impairment among older adults. SCD has been conceptualized as very early Alzheimer's disease (AD), but the specific SCD features predictive of clinical or cognitive decline remain unclear. This systematic review is the first to characterize specific SCD features and their relation to longitudinal outcomes.

Research design and methods: Multiple electronic databases were searched from inception until August 2021 for longitudinal studies of adults aged ≥50 (mean ≥60) and free of dementia, with baseline SCD measurement and clinical or cognitive follow-up. Studies were screened for inclusion criteria and assessed for risk of bias using weight-of-evidence ratings.

Results: Five hundred and seventy potentially relevant studies were identified, and 52 studies were evaluated for eligibility after initial screening. Thirty-three studies with medium to high weight-of-evidence ratings were included, and results were narratively synthesized. Measurement methods varied substantially across studies: the majority (n = 27) assessed SCD symptom types and intensity, and consistently reported that a higher symptom burden increased the risk for mild cognitive impairment (MCI) and dementia. The evidence was less compelling for cognitive outcomes. A handful of studies (n = 5) suggested a predictive role for SCD symptom consistency and informant corroboration.

Discussion and implications: SCD symptom intensity emerged from our review as the most reliable predictor of future clinical outcomes. Combinations of SCD-Plus symptoms also had predictive utility. No single symptom was uniquely prognostic. Our findings support the quantitative evaluation of SCD symptoms in the assessment of risk for progression to MCI or dementia.