Ocular and cerebral hemo-fluid dynamics in microgravity: a mathematical model

Abstract

Purpose : Long-term exposure to microgravity is known to lead to visual impairment in many astronauts (VIIP syndrome). Upper body fluid shift, alterations in intraocular pressure (IOP), intracranial pressure (ICP), tissue biomechanics and blood flow are among the many factors hypothesized to contribute to VIIP. Due to the difficulty to single out each of these factors using in-vivo studies, we present a mathematical model to theoretically evaluate the interactions between fluid flows and pressures in the brain (blood, cerebrospinal and interstitial fluid) and eyes (blood and aqueous humor) and their possible mechanical implications in VIIP.

Methods : We propose a 0-dimensional (0d) mathematical model of fluid circulation in the eyes and brain, embedded into a 0d whole-body circulation model. The model includes retinal, choroidal and ciliary circulations in the eyes. As suggested in the literature, the effect of microgravity is accounted for by i) considering zero hydrostatic pressure, ii) imposing zero central venous pressure, iii) decreasing the blood/aqueous humor oncotic pressure difference (Δπ_p) and iv) increasing the blood-brain barrier permeability. The model simulates microgravity-induced fluid redistribution in the upper body vasculature and variations in IOP and ICP, which are critical factors for VIIP.

Results : The model predicts that, in microgravity conditions, ICP and IOP increase (Fig1a) and ocular blood flow decreases markedly in the choroid and ciliary circulations (Fig1b). Blood flow in the retina is found to experience smaller variations, owing to a purely mechanical perfusion control mechanism mainly enacted by the intraocular venous segments.

Conclusions : The model suggests that i) the venous segments play a fundamental role in controlling pressures and fluxes in the ocular circulation, owing to the possibility of their collapse, and ii) the retinal circulation is less susceptible to microgravity-induced alterations than choroid and ciliary. These findings point towards further clinical assessment of ocular venous function in microgravity as a potential determinant factor for VIIP.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.

 

Fig 1: a) IOP and ICP and b) retinal, choroidal and ciliary blood flow vs blood/aqueous humor oncotic pressure difference Δπ_p. Earth baseline values of IOP, ICP and Δπ_p are 15 mmHg, 11 mmHg and 25 mmHg, respectively. In space, Δπ_p is thought to decrease to ≈18.5 mmHg. Flows are normalized with the corresponding earth values.