Cerebral Hemodynamic Patterns in People with Subjective Cognitive Decline: A Systematic Review Based on Functional Near-Infrared Spectroscopy

Background/Objectives: Subjective cognitive decline (SCD) has been proposed as an early symptomatic manifestation of dementia/Alzheimer’s disease. However, its associated brain patterns are not yet fully characterized. In recent years, functional near-infrared spectroscopy (fNIRS) has emerged as a cost-effective technique to detect hemodynamic changes associated with cognitive decline. Therefore, fNIRS may be a valuable tool to investigate SCD neural underpinnings. This systematic review synthesizes current fNIRS evidence regarding the hemodynamic correlates of SCD during both resting-state and cognitive task conditions. Methods: Following PRISMA guidelines, we searched Scopus, PubMed, and Web of Science databases for studies combining keywords (‘subjective cognitive decline’, ‘cognitive complaints’, ‘memory complaints’, ‘fNIRS’, and ‘near-infrared spectroscopy’) with Boolean operators. Results: We identified 181 records, of which 10 studies fulfilled the inclusion and exclusion criteria. All studies were cross-sectional. Furthermore, 33% lacked a healthy control group, 56% lacked a standardized SCD identification protocol, 55% used small or imbalanced samples, and 66% did not clearly differentiate prefrontal subregions. In both resting-state and task conditions, SCD individuals showed heterogeneous hemodynamic responses in prefrontal areas. The severity of SCD and the difficulty of the tasks may be useful moderators of these associations. Conclusions: The limited evidence and methodological differences prevent identifying a specific hemodynamic pattern for SCD. Adopting longitudinal, well-powered, and well-controlled designs will be essential to evaluate fNIRS-derived hemodynamic responses in SCD.