Biofluid modeling of the coupled eye-brain system and insights into simulated microgravity conditions

Abstract

This work aims at investigating the interactions between the flow of fluids in the eyes and the brain and their potential implications in the development of visual impairment in astronauts, a condition also known as spaceflight associated neuro-ocular syndrome (SANS). To this end, we propose a reduced (0-dimensional) mathematical model of fluid flow in the eyes and brain, which is embedded into a simplified whole-body circulation model. In particular, the model accounts for: (i) the flows of blood and aqueous humor in the eyes; (ii) the flows of blood, cerebrospinal fluid and interstitial fluid in the brain; and (iii) their interactions. The model is used to simulate variations in intraocular pressure, intracranial pressure and blood flow due to microgravity conditions, which are thought to be critical factors in SANS. Specifically, the model predicts that both intracranial and intraocular pressures increase in microgravity, even though their respective trends may be different. In such conditions, ocular blood flow is predicted to decrease in the choroid and ciliary body circulations, whereas retinal circulation is found to be less susceptible to microgravity-induced alterations, owing to a purely mechanical component in perfusion control associated with the venous segments. These findings indicate that the particular anatomical architecture of venous drainage in the retina may be one of the reasons why most of the SANS alterations are not observed in the retina but, rather, in other vascular beds, particularly the choroid. Thus, clinical assessment of ocular venous function may be considered as a determinant SANS factor, for which astronauts could be screened on earth and in-flight. April 1, 2019 1/22 1 Microgravity conditions have been observed to induce visual function alterations in 2 many astronauts that pose serious challenges for both astronauts and their missions in 3 space [1, 2]. This syndrome, also known as spaceflight associated neuro-ocular syndrome 4 (SANS), is characterized by a large number of apparently unrelated and often not 5 concurrent symptoms. These include choroidal folds, cotton wool spots, optic nerve 6 distension and/or kinking, optic disc protrusion, posterior globe flattening, refractive 7 deficits and elevated intracranial pressure [3]. Added to the complexity of the range of 8 symptoms are the problems of susceptibility and genetic predisposition to develop visual 9problems. 10The current understanding of how weightlessness environment affects the human 11 body and may lead to SANS development is still quite rudimentary. Various studies of 12 the symptoms experienced by astronauts during long-term missions (four to six months) 13 have been performed [1], but their validity is hampered by the small size of the subjects 14 cohort. To overcome this difficulty, ground-based microgravity laboratory models have 15 been proposed, the most significant of which is the long head down tilt (LHDT) 16 experimental procedure that is used to simulate the effects of microgravity on the 17 cardiovas...

<p>Spaceflight Associated Neuro-Ocular Syndrome (SANS): A Systematic Review and Future Directions</p>

“… 7 Aside from in vivo measurements, a mathematical model of fluid flow in the eyes and brain embedded into a simplified whole-body circulation model predicted raised ICP and IOP in microgravity. 19 This model predicted decreased ocular blood flow to the choroid and ciliary body whereas retinal circulation was found to be less susceptible to microgravity induced changes.…”

“…Aside from technology that could be used inflight, there are various reports of novel approaches to increase our understanding of SANS. Salerni et al 19 reported a mathematical model of fluid flow in the eyes and brain which predicted increased ICP and IOP in microgravity, as well as decreased ocular blood flow in the choroid and ciliary body. As noted above, the lymphatic system has been proposed to play a role in contributing to SANS, and Rasmussen et al 32 described a technique that could be used to further our understanding of this.…”

Modeling the eye as a window on the body

“…To date, only preliminary mathematical models linking ocular dynamics with the cardiovascular system and the dynamics in other organs, such as the kidney and brain, are available. 9 Several mathematical models studying systems separately have been previously developed; [6][7][8] however, the coupling of the different vascular and structural components involved introduces multiple challenges. Specifically, the coupling involves the following multiscale and multiphysics components: fluid flows (e.g.…”