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It is an exciting time in human spaceflight, with the advent of commercial spaceflight and the new Artemis Program to return astronauts to the moon. Although it has been 50 years since the Apollo 11 moon landing and 20 years since humans began continuously residing on the International Space Station (ISS), researchers continue to make novel discoveries regarding the human body in space. In this issue of JAMA Otolaryngology–Head & Neck Surgery, Inglesby et al1 report on magnetic resonance imaging opacification of the paranasal sinus and mastoid air cells after Space Shuttle and ISS missions. As with many spaceflight-related findings, the magnitude of change is greater after longer-duration ISS missions (approximately 6 months) than Space Shuttle missions (approximately 2 weeks). No differences were found in the paranasal sinuses, but long-duration spaceflight was associated with an increased odds ratio of developing mastoid effusions by 9.28 compared with short-duration spaceflight. The authors hypothesize that this may be because of eustachian tube dysfunction or venous sinus pathology. While astronauts frequently report a subjective sensation of nasal and sinus congestion, there are only a handful of documented cases of sinusitis or otitis media in space,2 making venous congestion secondary to microgravity-induced headward fluid shifts the likely culprit.3
Spaceflight leads to a well-known headward fluid shift, approaching 2 L of fluid displaced from the lower body to the upper body,4 resulting in a homeostatic volume reduction as well as a resetting of the baroreflex control system to a new spaceflight normal. This new normal is a condition in which cardiac preload is elevated,3,5 jugular vein drainage is impaired,3 and subsequent ocular6 and neural7 structural changes are documented.
There are other factors of spaceflight that likely play a role in the physiological adaptation to spaceflight, such as radiation exposure, isolation, and elevated carbon dioxide levels, but the unrelenting headward fluid volume shift likely has the biggest influence.6 In fact, strict head-down tilt bed rest, which causes a fluid shift similar to that of spaceflight, has been shown to mimic many of the ocular and vascular changes that occur in space.8 It is this long-term exposure to the supinelike headward fluid shift that is likely responsible for many of the recent physiological findings from the ISS that were not noticed during Space Shuttle missions.
The consequences of these new findings need to be better understood by space medicine community professionals to appropriately design the medical requirements for upcoming exploration missions away from Earth. The effects on both mission success and long-term astronaut health need to be considered. For example, does atrial distension increase arrhythmia risk? Would a mastoid effusion increase the risk for debilitating ear pain? On the ISS, in the event of a medical emergency, astronauts can deorbit and be treated relatively quickly (on the order of hours). On a mission to the moon or Mars, treatment would be limited to supplies and techniques available on the vehicle.
If the instigating factor for many of these physiological changes is the microgravity-induced headward fluid shift, simulating Earthlike upright gravity hydrostatic pressure may be the best countermeasure solution. Venous-occlusive thigh cuffs can trap blood in the legs, reducing some measures of the headward fluid shift; however, the magnitude of the effect is limited to the leg venous capacity. Lower body negative pressure is effective at trapping venous blood in the legs and abdomen via a vacuum chamber and can potentially shift cerebrospinal fluid into the dural sac but has no effect on the otoliths or vestibular system.3 Artificial gravity via acceleration is the most promising way to replace the vertical gravity vector (Gz), but it is also the most technically challenging, with options ranging from small human-radius centrifuges to entire rotating vehicles. The space research and medical community is aggressively working to identify the proper magnitude and duration of these countermeasures, and this work becomes increasingly important as more novel consequences of long-duration spaceflight are discovered.
Corresponding Author: Michael B. Stenger, PhD, Human Research Program, National Aeronautics and Space Administration, 2101 NASA Parkway, Houston, TX 77058 (email@example.com).
Published Online: March 26, 2020. doi:10.1001/jamaoto.2020.0274
Conflict of Interest Disclosures: None reported.
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Stenger MB, Macias BR. Sinus Space in Outer Space. JAMA Otolaryngol Head Neck Surg. Published online March 26, 2020. doi:10.1001/jamaoto.2020.0274
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