Male astronauts most often showed eye changes after long missions, while female astronauts showed larger shifts in brain fluid near the top of the brain.

About 70 percent of ISS astronauts experience some vision-related change in orbit, a sobering number as agencies plan months long voyages.

Effects of spaceflight on the body

The work was led by Rachael D. Seidler at the University of Florida’s Astraeus Space Institute (UF). The team pulled together brain scans and eye exams taken before and after flight.

The primary findings tie the most common eye changes to sex, yet show only a weak connection to brain structure.

Female flyers showed a larger reduction of free water, unbound water outside brain cells that reflects fluid shifts, near the crown of the head. Male flyers more often exhibited an eye change known as globe flattening after flight.

“Interestingly, eye changes were more prevalent in males than females,” said Rachael D. Seidler, director of the University of Florida’s Astraeus Space Institute and professor of applied physiology and kinesiology.

The odds of these eye changes were roughly three times higher among men in the dataset, though the statistic did not meet conventional significance. That means the pattern is notable, but still needs confirmation with larger balanced samples.

How the eyes react to spaceflight

Spaceflight Associated Neuro-ocular Syndrome (SANS), a cluster of eye and optic nerve changes linked to fluid shifts in microgravity, is now a central health concern for exploration crews.

The condition includes optic nerve head swelling, choroidal folds, changes in refraction, and the distinctive flattening of the back of the eye.

A key sign is globe flattening, a slight inward indentation of the eye’s posterior wall that can alter focusing. It can persist after landing in some cases, including up to a year.

Another thread points to venous outflow bottlenecks in the head and neck during flight, including internal jugular vein flow stasis and even a documented thrombosis case in orbit.

Countermeasures that pull fluids to the legs, such as lower body negative pressure, may help restore flow.

Links between eye findings and intracranial venous congestion have also emerged. Astronauts with postflight eye changes showed larger dural venous sinus volumes after long missions than crewmates without those signs.

The brain reacts differently

Women showed a larger postflight drop near the brain’s top, hinting at sex linked differences in how the brain adapts to weightlessness.

Even so, the team did not find a clear mapping between those brain shifts and the eye changes across individuals.

A separate ISS study saw only a weak association between retinal thickening and lateral ventricle size, and no links to white matter volume.

Taken together, the brain and eye may be reacting to the same broad stressor, but through partly independent pathways. That could explain why some astronauts show major ocular findings without large measurable brain fluid changes.

Variability in imaging protocols and timing adds noise to any pooled analysis. The number of women in astronaut cohorts also remains low, which limits how precisely sex effects can be estimated.

Limitations of the spaceflight study

This analysis mixed three overlapping astronaut samples, with dozens of crewmembers contributing MRI, ocular data, or both. That breadth helps, but the ratio of men to women is still far from even, which complicates sex specific risk estimates.

All cases classified with SANS in the study included a change in globe flattening, and in most of those cases it appeared alone. That pattern spotlights a metric that is both common and relatively straightforward to monitor in flight.

Not every suspected risk factor stood up under scrutiny here. Age did not predict the magnitude of brain changes with flight, and it did not separate those who did and did not show ocular findings.

The data also suggest that eye signs in women can look different from those in men. Females with SANS in this dataset showed globe flattening or a shift toward farsightedness, without the optic nerve head swelling seen in some male cases.

Protecting astronauts’ vision

Medical teams should track globe flattening closely during and after missions. It dominated the eye findings in this dataset, so it is a logical early warning signal for longer flights.

Screening and countermeasures will likely improve when body metrics and biological sex are factored into individualized risk plans.

The trend toward higher odds in men may steer targeted monitoring, even as larger studies test whether that pattern holds.

Noninvasive surveillance of the head and neck circulation in orbit can guide real time countermeasure use. Lower body negative pressure is one option already under evaluation for improving upper body venous return in microgravity.

More women are joining spaceflight missions, and more data are flowing from international partners and private crews.

As those records grow and standardize, researchers can finally separate causes from correlations and design fixes that keep vision sharp deep into a mission.

The study is published in the journal npj Microgravity.

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