Astronauts who spent more than a total of a month in space experienced a variety of issues in the optic nerve and eyeball.
Astronauts who spend a month or more in space commonly experience eye problems.
This could become a serious issue during a longer mission to Mars.
Brain and eye problems have surfaced in astronauts who spent more than a month in space, marking a potential setback to plans for longer deep space missions, according to a study.
The research in the journal Radiology analyzed magnetic resonance imaging (MRI) scans of 27 astronauts with an average of 108 days in space, either on space shuttle missions or aboard the International Space Station.
Shuttle missions typically lasted a couple of weeks, while ISS stints can last more than six months. A Mars mission to bring astronauts to the red planet in the coming decades could last a year and a half.
Among the astronauts who spent more than a month in space over their lifetimes, researchers found a variety of complications that appear similar to a syndrome caused by unexplained pressure on the brain.
These symptoms included excess cerebral-spinal fluid around the optic nerve in 33 percent of the astronauts studied and flattening of the back of the eyeball in 22 percent of them.
Fifteen percent had a bulging optic nerve and 11 percent showed changes to the pituitary gland -- which is located between the optic nerves, secreting sex hormones and regulating the thyroid -- and its connection to the brain.
Similar effects, which can lead to problems with vision, have been observed in non-space travelers who suffer from unexplained pressure buildup in the brain, a condition known as intracranial hypertension.
"Microgravity-induced intracranial hypertension represents a hypothetical risk factor and a potential limitation to long-duration space travel," said lead author Larry Kramer, professor of diagnostic and interventional imaging at The University of Texas Medical School at Houston.
"The MRI findings revealed various combinations of abnormalities following both short- and long-term cumulative exposure to microgravity also seen with idiopathic intracranial hypertension."
While bone loss and temporary muscular aches and nerve abnormalities have been known to afflict astronauts in the past, the new data on eye problems has many at NASA concerned about the health of its spaceflight corps.
"NASA has placed this problem high on its list of human risks, has initiated a comprehensive program to study its mechanisms and implications, and will continue to closely monitor the situation," said William Tarver, chief of the flight medicine clinic at NASA's Johnson Space Center.
He said the findings are suspicious but not conclusive of intracranial hypertension, and said no astronauts have been rendered ineligible for future spaceflight as a result of the findings.