Fluid Biomarkers of Cognitive Impairments Following Traumatic Brain Injury: A Systematic Review and Meta Analysis.

Traumatic brain injury (TBI), a major cause of persistent cognitive impairment (CI), increases the long-term risk of developing dementia, including Alzheimer's disease (AD). To elucidate this association, we systematically reviewed fluid biomarkers linked to post-TBI cognitive outcomes. A comprehensive search of the PubMed, Embase, and Cochrane Library databases was performed. A total of 29 clinical studies were included, reporting on several biomarkers related to neural injury and repair, AD-like pathology, and inflammation. Among these, neurofilament light chain (NfL), ubiquitin C-terminal hydrolase L1, total tau, and glial fibrillary acidic protein (GFAP) were consistently associated with CI and brain atrophy across various TBI severities and stages. Notably, certain biomarkers assessed during the acute phase (within 7 days post-injury), such as brain-derived neurotrophic factor, neuron-specific enolase, and interleukin-1β, showed significant correlations with CI. In contrast, elevated levels of GFAP and NfL measured during the recovery phase (6 months to 8 years post-injury) were significantly associated with TBI-related CI (TBI-CI). The findings also highlighted that axonal injury, glial activation, neuroinflammation, neuronal damage, and degeneration drive TBI-CI, with tau pathology and synaptic dysfunction emerging as potential bridges from TBI to AD. This review underscores the critical temporal dynamics of fluid biomarkers in TBI-CI, revealing that stage-specific biomarker profiles mirror distinct underlying pathophysiological processes. Future longitudinal studies should focus on well-characterized patient subgroups, adopt standardized diagnostic criteria, and integrate fluid biomarkers with neuroimaging and genetic data.