This cohort study examines the internal jugular vein flow and morphology of crew members of the International Space Station and the use of lower body negative pressure as a countermeasure to the headward fluid shift experienced during space flight.
This is the first study to examine cerebral hemodynamics using phase-contrast MRI during various angles of head-down tilt. Furthermore, the study investigated the additional effects of increased ambient carbon dioxide during head-down tilt as an analog to the environment onboard the International Space Station.
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
A small, posterior intraocular-intracranial pressure difference (IOP > ICP) is maintained during HDT, and a sustained or further decreased difference may lead to structural changes in the eye in real and simulated microgravity.Marshall-Goebel K, Mulder E, Bershad E, Laing C, Eklund A, Malm J, Stern C, Rittweger J. Intracranial and intraocular pressure during various degrees of head-down tilt. Aerosp Med Hum Perform. 2017; 88(1):10-16.
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