Glymphatic System Function and Cognition: Converging Evidence from DTI-ALPS Imaging - A Narrative Review

The glymphatic system, a brain-wide perivascular network mediating cerebrospinal fluid-interstitial fluid exchange, has emerged as a candidate determinant of cognitive health through its role in clearing metabolic waste, neurotoxic proteins, and neuroactive metabolites. The diffusion tensor imaging analysis along the perivascular space (DTI-ALPS) index provides a non-invasive means to assess glymphatic activity in vivo, yet no synthesis has consolidated what this marker captures, how reliably it predicts cognitive outcomes, and what pathways may account for observed associations. This narrative review, informed by selected principles from the PRISMA 2020 framework but not constituting a formal systematic review, synthesizes evidence linking DTI-ALPS to domain-specific cognitive performance in healthy aging and neurological disease, encompassing the Alzheimer's disease continuum, cerebral small vessel disease, Parkinson's disease, multiple sclerosis, stroke, and HIV-associated neurocognitive disorder. Lower DTI-ALPS values were consistently associated with worse cognitive performance. The strongest associations were observed for memory and global cognition, followed by executive functions and processing speed; evidence for language and visuospatial abilities was less consistent. Longitudinal data indicate that DTI-ALPS decline may precede detectable amyloid pathology, positioning this index as a candidate early prognostic marker. Three mechanistic pathways are discussed: disruption of sleep-dependent waste clearance; compromise of gray matter integrity through neurotoxic metabolite accumulation; and facilitation of amyloid-β and tau pathology via impaired perivascular protein clearance. Limitations include the indirect nature of DTI-ALPS as a clearance measure, predominance of cross-sectional designs, small samples, and absence of sex-stratified analyses despite known hormonal modulation of glymphatic function. Longitudinal studies, randomized trials of glymphatic-targeting interventions, and acquisition protocol standardization represent priority directions for future research.