Linking brain and eye fluid dynamics: Multi-b-value diffusion derived interstitial fluid volume relates to tear fluid AQP5 levels

Motivation: A link has been suggested between ocular and cerebral waste clearance, but the relationship between eye and brain fluid dynamics in humans remains unclear. Understanding these dynamics may help clarify waste clearance mechanisms associated with Alzheimer's disease. Goal(s): To investigate the relationship between cerebral interstitial fluid characteristics and tear fluid measures in cognitively healthy elderly. Approach: With 7T multi-b-value diffusion MR imaging, a proxy of interstitial fluid volume and diffusivity were derived in thirty-four cognitively healthy elderly of which also tear fluid measures were obtained. Results: A higher aquaporin-5 tear fluid concentration significantly relates to increased interstitial fluid volume within the white matter. Impact: By identifying a relationship between tear fluid AQP5 concentration and cerebral interstitial fluid volume, this study suggests that eye and brain fluid dynamics are linked, supporting the hypothesized link between ocular and brain (glymphatic) clearance in humans.