The impact of ocular hemodynamics and intracranial pressure on intraocular pressure during acute gravitational changes

Exposure to microgravity causes a bulk fluid shift toward the head, with concomitant changes in blood volume/pressure, intraocular pressure (IOP), and intracranial pressure (ICP). These and other factors are thought to contribute to Visual Impairment and Intracranial Pressure syndrome, a significant health concern for astronauts, characterized by degradation of visual function and ocular anatomical changes. Here we describe a lumped-parameter numerical model to simulate volume/pressure alterations in the eye during gravitational changes. The model includes the effects of blood and aqueous humor dynamics, ICP, and IOP-dependent ocular compliance. It is formulated as a series of coupled differential equations and was validated against three existing datasets on parabolic flight, body inversion, and head-down tilt (HDT). The model accurately predicted acute IOP changes in parabolic flight and HDT, and was satisfactory for the more extreme case of inversion. The short-term response to the changing gravitational field was dominated by ocular blood pressures and compliance, while longer-term responses were more dependent on aqueous humor dynamics. ICP had a surprisingly small effect on acute IOP changes. This relatively simple numerical model shows promising predictive capability. To extend the model to more chronic conditions, additional data on longer-term autoregulation of blood and aqueous humor dynamics are needed.

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