New research indicates that male astronauts are more susceptible to significant, and sometimes vision-altering, changes in the structure of their eyes during and after spaceflight. The findings, part of a growing body of evidence on the physiological effects of microgravity, highlight a notable sex difference in how the human body adapts to space, a critical consideration as space agencies plan for long-duration missions to the Moon, Mars, and beyond. While both male and female astronauts experience a range of physical adaptations in space, the ocular changes appear to be more frequent and pronounced in men.
The set of ocular and neurological symptoms observed in astronauts is known as Spaceflight Associated Neuro-ocular Syndrome, or SANS. This condition encompasses a variety of changes, including the swelling of the optic disc, folding of the choroid (a vascular layer in the eye), and alterations to the retina. The most consistent and concerning of these changes is a phenomenon known as “globe flattening,” a compression at the back of the eyeball that can affect vision. A recent study, and one of the first to specifically investigate sex-based differences in SANS, found that this globe flattening was more prevalent and severe in male astronauts. These findings underscore the need for tailored health monitoring and countermeasures to protect the vision and overall health of all crew members on extended missions.
An Emerging Health Concern in Space
SANS has become a significant focus of space medicine since it was first formally identified in 2011. Initially, astronauts on long-duration missions aboard the International Space Station (ISS) began reporting changes in their visual acuity, with some experiencing a hyperopic shift, making it more difficult to see things up close. Subsequent examinations revealed the underlying structural changes to their eyes. While many of these changes are asymptomatic, they are not insignificant. The alterations to the shape of the eyeball and the optic nerve can potentially have long-term consequences, and ensuring the continued visual health of astronauts is paramount for mission success.
The prevailing theory behind the development of SANS is the headward, or cephalad, shift of bodily fluids that occurs in the absence of gravity. On Earth, gravity pulls fluids down towards our feet, but in space, this pressure gradient is lost, leading to a redistribution of fluids towards the head. This shift is thought to increase pressure within the skull and on the back of the eye, leading to the observed anatomical changes. However, the exact mechanisms are still under investigation, and it is clear that not all astronauts are affected equally. Researchers are now exploring a range of contributing factors, including genetics, cardiovascular compliance, and even nutritional status, to better understand who is most at risk.
Sex-Based Differences in Ocular and Brain Changes
More Pronounced Eye Changes in Men
A study led by Rachael D. Seidler of the University of Florida’s Astraeus Space Institute has provided some of the clearest evidence to date of sex-based differences in the development of SANS. The research, published in the journal *npj Microgravity*, analyzed data from a cohort of astronauts and found that while both sexes can develop SANS, the structural changes to the eye were more consistently observed in men. Specifically, globe flattening was the most common and significant change, and it was more prevalent in male astronauts.
While the study’s dataset was comprised of approximately one-quarter female astronauts, a reflection of the historical gender imbalance in spaceflight, the sample size was large enough to identify these significant differences. The findings suggest that male physiology may be more susceptible to the fluid shifts that cause these ocular changes. One older study on a related condition, VIIP syndrome, noted that of 25 astronauts evaluated, 82.3% of the men and 62.5% of the women showed symptoms, with the female astronauts exhibiting milder signs. Researchers have hypothesized that factors such as higher vascular compliance in women may offer a protective effect.
Contrasting Effects on the Brain
Interestingly, the same study that identified more severe eye changes in men found a different pattern of changes in the brain. Female astronauts exhibited a greater reduction in the cerebrospinal fluid surrounding the upper part of their brains compared to their male counterparts. This suggests that the fluid shifts caused by microgravity affect the brain and eyes differently, and that the mechanisms driving the changes in each system may not be directly linked. Despite a comprehensive analysis, the researchers found no direct connection between the changes in brain structure and the incidence of globe flattening. This separation in how the brain and eyes are affected by spaceflight is a crucial finding, indicating that both systems need to be monitored independently to ensure astronaut health.
Investigating the Underlying Causes
Understanding why male astronauts appear to be more at risk for SANS is an active area of research. The answer likely lies in a combination of anatomical, hormonal, and genetic factors. For instance, differences in the structure of the skull, eyes, or optic nerve sheath between sexes could play a role in how individuals respond to increased intracranial pressure. Hormonal differences are also a potential factor, as they are known to influence vascular health and fluid regulation throughout the body.
Researchers are also delving into the genetic predispositions for SANS. It is possible that some astronauts have a genetic makeup that makes them more susceptible to the condition. One investigation identified four risk alleles related to B-vitamin status and the one-carbon metabolic pathway that were associated with a severe case of SANS in one astronaut. This suggests that a combination of genetic screening and nutritional countermeasures, such as B-vitamin supplementation, could be part of future strategies to mitigate the risks of SANS. As our understanding of these factors grows, we can move closer to personalized medicine for astronauts, tailoring preventative measures to each individual’s unique physiology.
Implications for Future Space Exploration
The findings on sex-based differences in SANS have profound implications for the future of human spaceflight. As we prepare for long-duration missions to the Moon and Mars, where astronauts will be exposed to microgravity for years at a time, protecting their vision and neurological health is a top priority. The knowledge that male astronauts may be at higher risk for severe eye changes allows for the development of targeted monitoring programs and individualized countermeasures. This could involve more frequent eye exams for male crew members during missions, or the preemptive use of devices that help draw fluids back towards the lower body.
Moreover, these findings highlight the importance of including diverse populations in space medicine research. For much of the history of spaceflight, the majority of astronauts have been male, which has limited our understanding of how female bodies respond to the space environment. As more women fly to space, researchers will be able to conduct more robust studies on sex and gender differences, leading to a more complete picture of human adaptation to microgravity. This knowledge is not only essential for ensuring the health and safety of all astronauts, but it can also provide valuable insights into terrestrial medicine, helping us better understand conditions like idiopathic intracranial hypertension, which has some similarities to SANS and primarily affects women on Earth.
