Age-Dependent Changes in Regulation of Water Inflow Into the Vitreous Body

PURPOSE. Water inflow into the vitreous body regulated by retinal aquaporin-4 distributed within Muller cells has been observed in mice; however, the changes in this phenomenon with age remain unknown. This study aimed to evaluate whether intravenously injected H2O also flows into the vitreous body of human subjects and to investigate whether water dynamics in the human posterior eye change with age using [15O]H2O positron emission tomography (PET). METHODS. Forty-six normal adult volunteers underwent [15O]H2O PET, and the standard uptake value (SUV) in the center of the vitreous body after 1000-MBq [15O]H2O admin-istration was assessed. The SUV was fitted to an exponential curve, and y0, the steady state of the SUV, and b, the speed of increase in the SUV, were calculated. The results for patients ranging from in age from 20 to 39, 40 to 59, and 60 to 79 years were compared using analyses of variance followed by Games to Howell tests. RESULTS. For the parameter y0, statistical analysis revealed no statistically significant differ-ences among the three groups. For parameter b, statistical analysis revealed statistically significant differences between the 20 to 39 and 60 to 79 age groups (P = 0.000), the 40 to 59 and 60 to 79 age groups (P = 0.025), and the 20 to 39 and 40 to 59 age groups (P = 0.037). CONCLUSIONS. The present study revealed that H2O injected into the vein flows into the human vitreous body and that the speed of increase in water flow into the vitreous body decreases with aging. This study suggests that water dynamics in the posterior eye, or the retinal glymphatic pathway, change significantly with aging.