Intracranial Effects of Microgravity: A Prospective Longitudinal MRI Study

Abstract

A pproximately 60% of the crew members of the International Space Station have reported altered visual acuity after long-duration exposure to microgravity (1). Postflight evaluation has shown variable degrees and combinations of optic disc edema, retinal nerve fiber layer thickening, retinal hemorrhage, cotton wool spots, posterior globe flattening, and choroidal folds (1). Lacking a terrestrially based clinical correlate, this medical condition is generically referred to by the National Aeronautics and Space Administration (NASA) as the spaceflight-associated neuro-ocular syndrome. The discovery of spaceflight-associated neuro-ocular syndrome has inevitably raised concerns for the long-term health of astronauts on extended-duration interplanetary travel.MRI findings in postflight astronauts, in whom similarities to idiopathic intracranial hypertension such as posterior globe flattening are found, have implicated elevated intracranial pressure (ICP) as a hypothesized mechanism of spaceflight-associated neuro-ocular syndrome (2,3). However, contrary to expectations, ICP measured in volunteers during brief microgravity exposures in aerial parabolic flight was not elevated (ICP, 13 mm Hg), but instead remained between the supine (ICP, 15 mm Hg) and 90° upright (ICP, 4 mm Hg) baseline values (4). It has been suggested that the inherent absence of postural ICP variability in microgravity could expose astronauts to increased mean diurnal ICP because of the inability to intermittently lower ICP by upright positioning that depends on a gravitational environment (4). This is potentially problematic because chronic supine-like ICP, without intermittent decompression, simulated by 30 days of strict head-down-tilt bedrest, is associated with optic nerve edema in otherwise healthy adults (5).Although an unremitting mild increase in the mean diurnal ICP may play a role in the development of spaceflight-associated neuro-ocular syndrome, a potential

The Impact of 6 and 12 Months in Space on Human Brain Structure and Intracranial Fluid Shifts

“…Roberts and colleagues have termed these spaceflight-induced ventricular volume increases “hydrocephalus associated with long-term spaceflight” (i.e., HALS; Roberts and Petersen 2019 ). Others have suggested that spaceflight-associated ventricular volume increases are indicative of normal pressure hydrocephalus and glymphatic system dysfunction ( Kramer et al. 2020 ).…”

“…Only 2 studies to date have examined recovery of structural brain changes after return from 6-month ISS missions ( Van Ombergen et al. 2019 ; Kramer et al. 2020 ).…”

Red risks for a journey to the red planet: The highest priority human health risks for a mission to Mars

“…Based on these images, there is growing evidence that brain structural changes also occur during long-duration spaceflight. Most notably, a 10.7–14.6% ventricular enlargement (i.e., approximately a 2–3 ml increase) has been detected in astronauts and cosmonauts by multiple investigators 56 – 59 . On-orbit and post-flight cognitive testing have not revealed any systemic cognitive decrements associated with these anatomical changes.…”

A review of alterations to the brain during spaceflight and the potential relevance to crew in long-duration space exploration

“…To date, multiple MRI modalities have been used to assess the structure and function of the brains of crewmembers before and after spaceflight at both the anatomical level and the microstructural level (Supplementary Table 1) 13,14,[16][17][18][19][20][21][22] . Several studies have also used functional MRI (fMRI) to examine the connectivity within or between different brain regions during rest or activated states 15,23 .…”

“…Neuroimaging studies have detected numerous spaceflightassociated changes in the positioning and macrostructure of the brain, including alterations to the ventricular system, which contains cerebrospinal fluid (CSF) 14,[17][18][19][20][21][22] .…”