(HOUSTON) -- As American astronauts spend more and more time in space, they've noticed they're returning to Earth with a surprising malady: They cannot focus their eyes properly after they come home, and for some the problem seems permanent.
Astronauts with 20/20 vision found they needed glasses for the first time, says NASA. A few -- their names withheld to protect their privacy -- were told it would be unwise for them to fly in space again. At least a couple could no longer pilot private planes.
Now, a team from the University of Texas Health Science Center in Houston has done MRIs on 27 astronauts who spent more than a month in space, and reports in the journal Radiology that 60 percent have intracranial hypertension, or high fluid pressure in the skull. The more time they spent in flight, the more likely they were to have the problem.
"We've known about vision changes in orbit but in some cases we've actually found that it can be permanent," said Peggy Whitson, who has flown twice on the space station herself and is now chief of the astronaut office. She spoke with ABC News last year when the pattern among veteran space flyers first became apparent.
A fifth of the astronauts tested showed a flattening of the rear of the eyeball, affecting their ability to focus their eyes. A third showed expansion of the space surrounding the optic nerve that's normally filled with cerebral spinal fluid.
Dr. Larry Kramer, who led the team at the University of Texas that did the MRIs, said the findings could someday be useful to non-astronauts, but at the moment he's most concerned about space flyers.
"What does this mean when we want to travel beyond Earth orbit, on longer missions to Mars and elsewhere?" he said in an interview with ABC News. "There's no way to predict it, so we ought to study it now."
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