Decreased Flow of Interstitial Fluid in the Brain After 50 Years of Age

A number of lines of evidence point to decreased fluid drainage from the brain into the body with aging. This drainage occurs through pathways such as the cribriform plate behind the nose and the comparatively recently discovered glymphatic system. Both of these fluid pathways are known to become dysfunctional with age. It is thought that the reduced drainage has a negative impact on the brain by allowing metabolic waste products to build up, such as the various forms of disruptive protein aggregates associated with neurodegenerative conditions.

Physiological age-related alterations in the interstitial flow in the brain, which plays an important role in waste product removal, remain unclear. Using [15O]H2O positron emission tomography (PET), water dynamics were evaluated in 63 healthy adult participants aged between 20 and 80 years. Interstitial flow was assessed by influx ratio (IR) and drain rate (DR), using time-activity concentration data.

Participants were divided into four age groups with 15-year ranges, to evaluate age-related functional alterations. At least one of the indices declined significantly with age across all groups. A significant linear negative correlation between age and both indicators was found in the scatter plots; both indicators were predominantly lower after age 50 years. These results suggest interstitial flow decreases with age, especially after 50 years. These important findings can contribute to devising therapeutic interventions for neurological diseases characterized by abnormal accumulation of waste products, and suggest the need for taking measures to maintain interstitial flow starting around the age of 50 years.