Acute exposure to negative pressure did not reverse spaceflight-induced ocular changes
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An OCT study on spaceflight crew members found ocular morphology alterations potentially related to weightlessness and chronic headward fluid shift were not reversed by short-duration lower body negative pressure countermeasures.
The NASA Fluid Shifts Study included 14 crew members. Preflight and postflight OCT scans were obtained at NASA’s Johnson Space Center, and other scans were acquired during spaceflight under normal weightless conditions and during 10- to 20-minute sessions of 25 mm Hg lower body negative pressure (LBNP) application.
The mean flight duration was 214 days. Ocular disc edema, defined as an increase of more than 19.4 µm in total retinal thickness, occurred in four of 14 participants (29%) by flight day 50 and in nine of 13 participants (69%) by flight day 150. One participant developed Frisén grade 1 edema. The greatest effects of spaceflight were seen at the optic nerve head. The minimum rim width increased by 20.6 µm on flight day 50 and by 33.8 µm on flight day 150, with a corresponding decrease in cup volume, persisting up to 30 days after return on Earth. Bruch’s membrane opening height progressively decreased and was significantly lower than baseline on flight day 150. In contrast to the increase in total retinal thickness, central macular thickness decreased by 3.7 µm on flight day 50 and 5.1 µm on flight day 150.
LBNP is a technique used to compensate for the headward fluid shift, a likely cause of spaceflight-associated neuro-ocular syndrome, by redistributing fluids from the upper body to the abdomen and legs. Brief LBNP application of 25 mm Hg for 10 to 20 minutes did not reverse ocular structural changes.
It is possible that a longer duration of exposure could better counteract the effects of chronic headward fluid shift that occur in a microgravity environment, according to the authors.
“Evidence that changes in [minimum rim width] and [total retinal thickness] occur over hours in response to pressure and posture changes suggests that it may be possible for longer-duration LBNP application to reverse spaceflight-induced ocular changes,” they wrote.
In addition, other factors potentially involved in the development of spaceflight-induced ocular changes should be considered.
Perspective
Back to TopThis group of researchers confirmed that changes occur to the optic nerve and retina when astronauts are exposed to a weightless/microgravity environment on the International Space Station (ISS) in spaceflight-associated neuro-ocular syndrome (SANS). SANS is felt to be secondary to a lack of gravity and headward fluid shift of overall total body volume, affecting the eye. This can be reversed using a potential countermeasure called lower body negative pressure (LBNP). However, the key result here was that brief LBNP was not enough to mitigate ocular changes in SANS.
This study shows how challenging it is to study SANS, both from the standpoint of understanding its pathophysiology and for the purpose of developing countermeasures. In this case, LBNP is still a highly promising and high-readiness level countermeasure, but more work will need to be done, which can include varying the level of negative pressure or time applied. However, this is tough to do on the ISS. To improve the ease of studying SANS, there is currently a ground-based human analogue in which subjects lay in a strict 6° head-down tilt position for 30 to 60 days to mimic SANS. Using this model, we are currently studying LBNP and other countermeasures on Earth (NASA grant 80NSSC20K1034).
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