Spaceflight-Induced Intracranial Hypertension and Visual Impairment: Pathophysiology and Countermeasures

Abstract

Visual impairment intracranial pressure (VIIP) syndrome is considered an unexplained major risk for future long-duration spaceflight. NASA recently redefined this syndrome as Spaceflight-Associated Neuro-ocular Syndrome (SANS). Evidence thus reviewed supports that chronic, mildly elevated intracranial pressure (ICP) in space (as opposed to more variable ICP with posture and activity on Earth) is largely accounted for by loss of hydrostatic pressures and altered hemodynamics in the intracranial circulation and the cerebrospinal fluid system. In space, an elevated pressure gradient across the lamina cribrosa, caused by a chronic but mildly elevated ICP, likely elicits adaptations of multiple structures and fluid systems in the eye which manifest themselves as the VIIP syndrome. A chronic mismatch between ICP and intraocular pressure (IOP) in space may acclimate the optic nerve head, lamina cribrosa, and optic nerve subarachnoid space to a condition that is maladaptive to Earth, all contributing to the pathogenesis of space VIIP syndrome. Relevant findings help to evaluate whether artificial gravity is an appropriate countermeasure to prevent this seemingly adverse effect of long-duration spaceflight.

Spaceflight associated neuro-ocular syndrome (SANS) and the neuro-ophthalmologic effects of microgravity: a review and an update

“…60,78 CONCLUSION In summary, novel and unique neuro-ophthalmic findings have been documented in astronauts during and after LDSF and have been termed SANS. Although a single unifying and overreaching mechanism has yet to be proven, 1,2,[4][5][6][7][8][9]19,40,41,44,[55][56][57] and a multifactorial pathogenesis may be involved, it is likely that SANS may be the end result of cephalad fluid shifts to the brain and orbit brought about by extended MG exposure. Mao et al reviewed the impact of spaceflight and artificial gravity in a mouse retinal model using biochemical and proteomic analysis.…”

“…The interested reader is directed to both our own prior review articles but also the animal and human work performed by the many intramural and extramural NASA related partners working on SANS. [6][7][8][9] Despite the recognition and research related to SANS for many years, several unanswered questions remain: (1) What, if any, is the significance of potential preferential laterality (i.e., right-sided bias) seen in the anatomical changes of SANS? (unpublished data, personal communication WT); (2) Are there changes in the eye's anterior segment (in addition to the posterior segment findings) associated with SANS?…”

“…In the terrestrial environment, CSF is largely produced in the choroid plexus and drains into the lower pressure cervical venous vasculature. [1][2][3][4][5][6][7][8][9] Although vascular autoregulation stabilizes the cerebral and ONH arterial diameter, 10,11 jugular venous distention has been well documented during both head down and microgravity (MG) studies 12 suggesting that cerebral and jugular venous congestion may be present in the MG environment. Jugular venous distention alone does not necessarily prove that venous pressure is increased during spaceflight and some studies have suggested that the venous pressure may in fact be decreased.…”

Hypercapnia augments resistive exercise‐induced elevations in intraocular pressure in older individuals

“…In a recent review, Zhang and Hargens (2018) emphasized that autoregulation of IOP is also influenced by the flow of aqueous humour in the anterior part of the eye, and any factors that modify this flow will consequently also influence IOP. The effect of factors associated with spaceflight on aqueous humour flow remains unresolved.…”

Fluid shift vs. body size: changes of hematological parameters and body fluid volumes in hindlimb-unloaded mice, rats and rabbits

“…When a human body enters microgravity during a space flight, gravity ceases to pull blood to the lower part of the body, causing blood and other body fluids to redistribute. This fluid shift is manifest in the well-documented decrease of leg volume (Fortrat et al, 2017;Thornton et al, 1987), putative increase in intracranial pressure (Zhang and Hargens, 2018) and face puffing (Kirsch et al, 1993). Blood redistribution is thought to be the key initiating event for cardiovascular adaptation to weightlessness (Watenpaugh and Hargens, 1996), followed by a decrease in blood volume (Johnson, 1979), due to loss of plasma (Smith et al, 1997) and erythrocytes (Ivanova et al, 2007;Noskov et al, 1991;Poliakov et al, 1998;Tavassoli, 1982), an increase of stroke volume (Norsk et al, 2015), and alterations of heart rate (Baevsky et al, 1997;Karemaker and Berecki-Gisolf, 2009;Verheyden et al, 2009) and central blood pressure regulation (Baevsky et al, 2007;Di Rienzo et al, 2008;Fritsch et al, 1992;Morita et al, 2016;Pagani et al, 2009).…”