Space flight-associated neuro-ocular syndrome (SANS) is associated with structural and functional ocular and brain changes, including optic disc edema, and develops in approximately two-thirds of astronauts during long-duration space flight. Due to the potential of irreversible functional vision changes during exploration-class space flight missions, a countermeasure capable of preventing SANS is needed. Currently, the mechanisms underlying the development of SANS are unknown, however, exposure to a constant headward fluid shift during extended space flight is hypothesized to play a major role. Exposure to 30 days of strict 6° head-down tilt (HDT) bed rest has shown to produce optic disc edema, a hallmark sign of SANS.1 Thus, NASA plans to use this as a ground-based analog model to further understand the mechanisms underlying SANS and test potential countermeasures. NASA plans to implement two control groups in this study, a posture control group that will be upright (seated) 16 hours a day, and a strict 6° HDT bed rest control group.
This study has the following specific aims:
1. Determine if daily exposure to actual upright posture and simulated LBNP hydrostatic pressure gradients prevents ocular and cerebral structural changes during 30 days of strict 6° head-down tilt bed rest.
2. Determine if daily exposure to actual upright posture and simulated LBNP hydrostatic pressure gradients prevents ocular functional changes during 30 days of strict 6° head-down tilt bed rest.
3. Determine if daily exposure to actual upright posture and simulated LBNP hydrostatic pressure gradients prevents vascular and cerebrospinal fluid changes during 30 days of strict 6° head-down tilt bed rest.
This study will examine the responses of 72 human subjects (12 per group) among two control groups and four countermeasure treatment groups; over the course of 30 days. The two control groups will compare the effects of posture control (Group 1), in which subjects will be in an upright (seated) posture 16 hours a day and supine for 8 hours, and strict 6° HDT bed rest (Group 2), in which subjects will not be exposed to any countermeasures. Of the four treatment groups, two will include countermeasures NASA previously selected to investigate: daily exposure to exercise with veno-occlusive thigh cuffs during HDT bed rest (Group 3) and daily B-vitamin supplement intake during HDT bed rest (Group 4).
The remaining two treatment groups include strict HDT bed rest with six hours of 25 mmHg LBNP, daily (Group 5) and strict HDT bed rest for 18 hours, plus six hours of upright (seated) position exposure, daily (Group 6). The six-hour exposures (Group 5 and Group 6) will be split into two sessions of three-hour exposures, once in the morning and once in the evening. Additionally, for one day, Group 5 will undergo a continuous six-hour exposure to LBNP to test feasibility of this approach to inform future inflight countermeasure development.
This study will lay the groundwork to determine future countermeasure strategies that can prevent SANS during space flight. It is critical to determine if optic disc edema can be prevented or attenuated utilizing the most effective, currently available fluid shift countermeasures: LBNP and gravity. Overall, the quantification and comparison of physiological changes within the six groups included in this study will help to determine the necessary magnitude and duration of countermeasure needed to prevent SANS.
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