Spaceflight alters brain fluid more in women, while men face higher vision risks

A new investigation into the physiological toll of spaceflight has revealed that male and female astronauts exhibit different changes in their brains and eyes. The study, published in npj Microgravity, found that after a mission, female astronauts showed a greater reduction in the fluid surrounding the uppermost parts of the brain, while male astronauts were more likely to show signs of eye compression. These findings help illuminate how the human body adapts to microgravity and may inform health strategies for future long-duration space missions.

The human body is exquisitely adapted to Earth’s gravity. When astronauts enter the microgravity environment of space, their bodies undergo a wide range of physiological adjustments. Fluids that are normally pulled down toward the feet on Earth shift headward, altering pressure on the brain and eyes. This can lead to a condition known as Spaceflight Associated Neuro-ocular Syndrome, or SANS, which affects approximately 70 percent of astronauts and can include changes to the shape of the eyeball and swelling of the optic nerve.

A team of researchers led by Rachael D. Seidler, a professor of applied physiology and kinesiology at the University of Florida, sought to understand if demographic factors like age and sex play a role in how astronauts’ brains and eyes respond to space travel. Previous research on this topic has been limited, especially concerning sex differences, because of the historically small number of female astronauts.

“The data on sex differences in response to spaceflight are scant, given the historically low number of female astronauts,” Seidler said. “Our dataset was still only approximately one-quarter females, but since the overall dataset was fairly large we were able to examine sex differences.”

To conduct their analysis, the researchers gathered data from 37 astronauts who had completed missions aboard the International Space Station. The team analyzed magnetic resonance imaging, or MRI, scans of the astronauts’ brains taken before and after their flights.

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These scans allowed them to measure changes in brain structure, including the volume of gray matter, which contains nerve cells, and the amount of “free water,” which is the fluid that exists in the spaces around brain cells. They also obtained results from detailed ocular exams from NASA’s health repository to determine which astronauts developed signs of SANS.

Using sophisticated computational tools, the scientists compared the pre-flight and post-flight scans to map out specific changes. The team then statistically analyzed whether these brain and eye changes were associated with the astronauts’ sex, age, or mission duration. This approach allowed them to identify patterns that might not be apparent from looking at individual cases alone.

The analysis revealed no significant link between an astronaut’s age and the extent of brain changes following spaceflight. However, the researchers did identify a notable difference between the sexes. In specific regions near the top of the brain, female astronauts exhibited a greater decrease in the volume of free water compared to male astronauts after returning from space. The functional consequences of this more pronounced fluid shift in females are not yet understood.

When the team looked at eye health, they found that a flattening of the back of the eyeball was the most consistent change among crew members who developed SANS. In more than half of the cases of SANS, this globe flattening was the only symptom present. “By far the most prevalent sign of eye changes that we observed was globe flattening, suggesting that this should be the primary monitoring target for ocular health,” Seidler explained.

The study also showed that eye changes were more common in male astronauts. Although the statistical analysis was not strong enough to be conclusive, the data suggested that male crew members were more than three times as likely to develop SANS compared to their female counterparts. Another unexpected observation was the lack of a strong relationship between the structural changes in the brain and the development of SANS. This suggests that the effects of spaceflight on the brain and the eyes might be driven by separate biological mechanisms.

The researchers acknowledge certain limitations to their work, a common challenge in spaceflight studies. The sample size, while large for this field of research, is still small, particularly when looking at subgroups like female astronauts. The team also noted that some of the data was collected from different sources, which meant MRI scanning parameters were not always identical. Despite these constraints, the study provides a new perspective on how individual differences can influence adaptation to space.

Future research will need to build on these observations, especially as more women travel to space. A clearer understanding of why these sex differences occur could lead to personalized countermeasures to protect astronaut health on missions to the moon, Mars, and beyond. The finding that globe flattening is a primary indicator of SANS also has immediate implications, suggesting it should be a key focus for monitoring and diagnosing the condition in astronauts.

The study, “Crewmember demographic factors and their association with brain and ocular changes following spaceflight,” was authored by Heather R. McGregor, Kathleen E. Hupfeld, Ofer Pasternak, Nichole E. Beltran, Yiri E. De Dios, Jacob J. Bloomberg, Scott J. Wood, Roy F. Riascos, Patricia A. Reuter-Lorenz, and Rachael D. Seidler.