October 30th, 2025
While the biochemistry of the brain is segregated from the biochemistry of the rest of the body by the blood-brain barrier, a lining of specialized cells wrapping blood vessels that pass through the brain, large amounts of cerebrospinal fluid flow through the brain and exit into the body, carrying away metabolic waste. The major known pathways include (a) channels in the bone of the cribriform plate that drain the olfactory bulb region of the brain, and (b) the glymphatic system that is made up of fluid filled channels that parallel blood vessels where they enter and exit the brain. Both of these pathways decline in efficiency with age: the cribriform plate channels ossify and close, while the glymphatic system loses its ability to drive fluid flow by pulsation. Researchers hypothesize that impaired drainage of cerebrospinal fluid contributes to neurodegeneration by causing a harmful buildup of metabolic waste in the brain.
The ability to measure flow of cerebrospinal fluid through the glymphatic system is a fairly recent innovation, indeed the structure and function of the glymphatic system itself is a relatively recent discovery. Now, however, researchers can use features of magnetic resonance imaging (MRI) in some portions of the glymphatic system to create a measure of the fluid flow exiting the brain. This works because MRI can measure the scatter of water molecules, and if that scatter is heavily biased in one direction, that can be taken as a flow - a technique given the unwieldy name of diffusion tensor image analysis along the perivascular space (DTI-ALPS). All that is needed is a good straight stretch of glymphatic vessel, and there is a location in human physiology that suffices for this purpose. Thus the research community can produce studies such as the one noted here, in which reduced cerebrospinal fluid drainage through the glymphatic system is correlated to dementia risk.
