# Brain and eye data from 89 space travelers found men had roughly three times the odds of neuro-ocular syndrome while women showed a different fluid shift

> Astronauts can return from orbit with measurable changes in their brains and eyes and a study of NASA crewmembers suggests that age and sex may influence some of those changes. Researchers also found possible links involving body size and mission duration, although...

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Byline: ARGO.net Editorial Team
Published: 2026-08-21T01:25:02+00:00
Categories: Explainer, Space

![Optician in a clinic performing an eye examination with advanced vision testing equipment. Professional setting](https://www.argo.net/wp-content/uploads/2026/08/astronaut_eye_examination.jpg)

Astronauts can return from orbit with measurable changes in their brains and eyes and a study of NASA crewmembers suggests that age and sex may influence some of those changes. Researchers also found possible links involving body size and mission duration, although the small number of people who have flown in space leaves important questions open.

The [study](https://www.nature.com/articles/s41526-025-00505-9), published in **npj Microgravity** on August 28, 2025, combined brain scans with eye examination records collected before and after spaceflight. The researchers tested whether individual traits were associated with changes in brain tissue, fluid movement and a condition called **spaceflight-associated neuro-ocular syndrome**, commonly shortened to SANS.

The results could help flight surgeons identify which crewmembers may need closer monitoring during future missions. Longer journeys beyond low Earth orbit will expose astronauts to weightlessness for months at a time, while medical support and emergency return options will become more limited.

## How spaceflight changes the brain and eyes

Gravity constantly pulls blood and other body fluids toward the lower body on Earth. In orbit, that downward pull is greatly reduced. Fluids shift toward the chest and head, producing the puffy face and thinner-looking legs often seen in astronauts during the first days of a mission.

Fluid movement around the brain may help enlarge the **brain ventricles**, which are spaces filled with cerebrospinal fluid. Earlier astronaut studies have also measured an upward shift of the brain inside the skull. Researchers are still investigating how these changes develop and how long they remain after a crewmember returns home.

The eyes can change at the same time. NASA's [SANS risk overview](https://www.nasa.gov/directorates/esdmd/hhp/risk-of-spaceflight-associated-neuro-ocular-syndrome/) explains that altered gravity can affect structures in the eye and brain. Possible signs include swelling where the optic nerve enters the eye, folds in tissue behind the retina, a change in focusing power and flattening at the back of the eyeball.

Several processes may contribute. Fluid pressure, changes in blood drainage and the space station environment are among the factors under study. NASA researchers also examine nutrition and individual biology because astronauts exposed to similar missions can have different outcomes.

## MRI scans reveal demographic differences

The research team examined several overlapping astronaut samples because every crewmember did not have every type of medical record. The largest eye-data sample included 37 people. Brain imaging analyses involved 30 crewmembers, while a group of 22 had both usable MRI data and ocular records.

All brain scans were collected with the same 3-tesla Siemens MRI scanner near Houston. Keeping the scanner consistent reduced one possible source of variation. The astronauts were scanned before flight and again after landing, allowing each person to serve as their own preflight comparison.

The team used standard structural images to measure gray matter and the size of the brain's fluid-filled spaces. They also used **diffusion MRI**, a method that tracks how water moves through tissue. Water movement can offer clues about white matter, the network of fibers that carries signals between brain regions.

Researchers performed thousands of statistical rearrangements to test whether the observed patterns could reasonably occur by chance. Their models accounted for factors including mission duration, age at launch and the time between landing and the postflight scan. Brain-size differences were also considered where appropriate.

## Age was linked to white matter changes

Age at launch was associated with differences in postflight measures of **white matter**. The researchers used several diffusion measures to examine how water traveled along and across the brain's communication fibers. They also estimated the amount of water moving freely outside cells.

The method separated a **free water** component from water movement connected more closely with tissue. Such separation is useful after spaceflight because fluid redistribution can affect an MRI signal. Without accounting for freely moving water, researchers could mistake a fluid-related signal for a change inside white matter.

Age associations do not reveal whether one group suffered greater harm. MRI measurements can identify a physical difference while leaving its medical meaning unresolved. The study did not report that the observed brain patterns produced a clear loss of thinking ability or daily function.

Age can also overlap with other factors. Older astronauts may have different flight histories, mission assignments or baseline anatomy. Statistical models can reduce some of that influence, although a few dozen participants cannot represent every combination found in a larger population.

## Male and female brains responded differently

Some brain responses differed between male and female crewmembers, pointing to possible **sex-related differences** in adaptation to spaceflight. The study examined gray matter, ventricular volume and diffusion measurements rather than relying on a single brain indicator.

Biological causes remain unresolved and the astronaut groups were uneven in size. Historically, more men than women have completed space missions, particularly during earlier periods of human spaceflight. A small female group makes it difficult to decide whether a pattern represents a broad sex difference or the histories of a few individuals.

Body size, hormones and the way fluids move through blood vessels could contribute to differing responses. Each possibility requires direct testing. The present analysis can identify associations that deserve attention, while future astronaut groups and ground-based studies can examine possible mechanisms.

Results at group level also provide limited guidance for a single astronaut. Two crewmembers of the same age and sex can respond differently because of anatomy, genetics, health history and mission conditions. Personalized monitoring may therefore become a central part of astronaut health planning.

## Who developed SANS after spaceflight

The study classified a crewmember as having **SANS** when at least one defined eye sign appeared after flight and had been absent before launch. The signs were optic disc swelling, choroidal folds, a farsighted shift greater than 0.75 diopters, or a change in globe flattening.

Among the 37 astronauts with ocular records, 21 met the study definition. The researchers then compared SANS frequency with sex, age, body mass index, mission duration and previous flight experience.

Women, younger crewmembers and astronauts in the lower body mass index groups had a lower SANS frequency in the descriptive results. SANS appeared more frequently after year-long flights, although that comparison rested on only four people in the one-year group. Novice and experienced astronauts had broadly comparable frequencies.

The demographic patterns require caution because several subgroups contained very few astronauts. Only three women were present among the 21 SANS cases shown in the demographic breakdown. A difference involving such a small group can shift sharply when one additional person is included.

## Eye flattening was the most common sign

A change in **globe flattening** appeared throughout the SANS group where the needed measurements were available. The back of the eyeball becomes less rounded, a structural change that can influence how light is focused on the retina.

For 13 of the 21 SANS cases, or 61.9 percent, globe flattening was the only recorded sign. Data on flattening were unavailable for three crewmembers who qualified as SANS cases because they had other defined eye changes.

The remaining signs were less common. Optic disc edema involves swelling where the optic nerve joins the retina. All cases of this swelling in the study were rated Frisén Grade 1, the lowest grade on a clinical scale used to describe visible optic disc swelling.

Choroidal folds form in the tissue layer containing blood vessels behind the retina. Refractive shifts change the eye's focusing power and may require a different lens prescription. NASA's [Cardiovascular and Vision Laboratory](https://www.nasa.gov/directorates/esdmd/hhp/cardiovascular-and-vision/) studies such changes while working on ways to prevent or reverse spaceflight-related effects.

## Mission length and body size may influence risk

Mission duration remains one of the strongest practical concerns in SANS research. Signs became widely recognized during long stays aboard the International Space Station, where crews commonly spend about six months in weightlessness.

"We didn't see swelling of the optic disc when we flew two-week shuttle missions," said Steve Laurie, a KBR scientist working with NASA's Human Health and Performance Directorate. His observation highlights the possibility that exposure time contributes to eye changes, although individual responses still vary.

In the 2025 study, year-long missions had the highest descriptive SANS frequency. Only four crewmembers belonged to that duration group, so the result provides a signal for further study rather than a firm risk estimate. None of the short-duration crewmembers developed optic disc swelling or refractive shifts in the reported breakdown.

Body size may influence how fluid pressure is distributed. SANS frequency was lower among astronauts in the lowest **body mass index** group and that group did not develop optic disc edema, choroidal shifts or refractive changes. Weight and BMI information was missing for three people, leaving 34 crewmembers in those statistical models.

Researchers continue to explore devices that draw fluid toward the lower body, exercise methods and other countermeasures. NASA's official [SANS research program](https://www.nasa.gov/glenn/glenn-expertise-space-exploration/human-health-performance/computational-modeling/spaceflight-associated-neuro-ocular-syndrome/) also uses computer models to study stress on the eye and the movement of fluids around the brain.

## Small astronaut samples limit the results

Human spaceflight studies face an unusual numbers problem. The astronaut population is small, medical information is sensitive and mission conditions change over time. Even a study drawing on several years of records can include only a few dozen qualified participants.

The 2025 analysis used three samples based on the records available for each question. Thirty astronauts contributed MRI data for age and sex analyses. Twenty-two had the combined information needed to compare brain changes with SANS, while 37 had ocular data for the demographic models.

**Small sample sizes** raise the chance that an apparent group pattern reflects individual variation. Unequal representation adds another difficulty. The mission-duration comparison, for example, included short flights, six-month stays and only a few one-year missions.

Scanning procedures also varied within part of the older diffusion MRI collection. The researchers documented differences in slice thickness, image resolution and repetition times, then applied careful processing and quality checks. Such technical variation remains relevant when interpreting subtle brain changes.

The study therefore provides starting points for focused research. Repeated measurements during flight, scans soon after landing and long-term follow-up could reveal when changes begin and how recovery proceeds. Pooling carefully matched data across agencies may also strengthen future estimates while protecting astronaut privacy.

## What the findings mean for longer missions

Future **deep-space missions** may keep crews away from Earth for far longer than a typical space station rotation. A Mars expedition could also prevent rapid medical evacuation. Flight doctors will need ways to detect early eye changes and judge whether they are stable or progressing.

"SANS represents a critical risk for deep space exploration," former NASA Human Research Program chief scientist Jennifer Fogarty said in a [NASA overview](https://www.nasa.gov/humans-in-space/international-space-station-research-keeps-an-eye-on-vision-changes-in-space/). The risk includes possible effects on vision during tasks that require accurate reading, navigation and equipment operation.

No astronaut has been reported by NASA to have experienced permanent vision loss from SANS, yet some eye changes can remain after landing. The health effects of longer exposure remain uncertain because very few people have spent close to a continuous year in orbit.

Demographic associations could eventually guide monitoring schedules and countermeasure testing. An astronaut with several possible risk factors might receive more frequent retinal imaging or more detailed checks of visual focus. Reliable individual predictions will require substantially more evidence.

The present findings show why a single standard response cannot describe every human body in space. Age and sex were associated with some brain measurements, while SANS frequency varied with several demographic and mission factors. Continued MRI scanning and detailed eye examinations will help mission planners protect crews as human exploration moves farther from Earth.