Association of glymphatic dysfunction, free water, white matter integrity and long-term memory performance in aging autistic adults

Autistic adults are at elevated risk of accelerated cognitive aging and Alzheimer’s disease and related dementias (ADRD), yet the underlying neurobiological mechanisms remain poorly understood. Dysfunction in the glymphatic system—a brain-wide network responsible for clearing waste via interstitial fluid flow—may contribute to this vulnerability by promoting extracellular free water (FW) accumulation and white matter (WM) degeneration. A total of 113 autistic and 90 age- and sex-matched neurotypical (NT) adults (aged 18–71 years) underwent multimodal MRI scanning and episodic memory assessments. Diffusion tensor image analysis along the perivascular space (DTI-ALPS) index, alongside FW maps, and fractional anisotropy (FA) maps were computed for each participant. Group comparisons, correlations, and mediation analyses were performed. Autistic adults showed significantly lower DTI-ALPS values, higher fornix FW, lower fornix FA, and poorer episodic memory scores compared to NT adults. Age-related hippocampal FW accumulation was more pronounced in autistic adults. Mediation analyses revealed that fornix FW mediated the relationship between DTI-ALPS and both fornix FA and hippocampal FW. Long-term episodic memory scores correlated with fornix FA, as well as whole-brain gray matter FW and WM FA in autistic adults. The cross-sectional design precludes causal inference regarding glymphatic function, free water accumulation, WM integrity, and cognition. In addition, our sample was not evenly balanced by sex and excluded individuals with co-occurring intellectual disability, which may limit generalizability to the broader autistic population. Our results suggest that glymphatic dysfunction and FW accumulation may contribute to aberrant WM microstructure and episodic memory challenges in autistic adults across a broad age range. These findings point to potential biomarkers for identifying and intervening in the cognitive aging process in autism.