updated 9/22/2011 2:49:13 PM ET 2011-09-22T18:49:13

A new study of astronauts shows that radiation and bone loss aren't the only health risks for long-duration stays in space. About one-third of the U.S. space station crew members return with impaired vision, a condition in which at least one case was permanent.

The data has been slow in coming since astronauts can be disqualified from flying if they have serious ailments.

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"These are guys who really don't like to complain about physical issues because it may ground them. They're desire is to get back into space, so they are not complainers," neurosurgeon Bruce Ehni at Baylor College of Medicine in Houston, told Discovery News.

But in 2005, one unnamed astronaut came forward to reveal his affliction, prompting a survey of the corps. NASA discovered 35 percent of its former space station crew members, who typically spend about six months in orbit, experience visual acuity issues, agency spokesman Mike Curie told Discovery News.

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"It wasn't until a couple of years ago when we started seeing this on station that we injected true acuity scans and MRIs (magnetic resonance imaging) to produce actual medical data. That type of exact data does not exist from the shuttle astronauts," who typically spent two weeks in space, Curie added.

The condition isn't serious enough to cause blindness in the short term, but it raises concern about what would happen to astronauts during a three-year mission to Mars. The vision loss seems to be due to a swelling of the optic nerve, a condition similar to a disease on Earth called pseudotumor cerebri, which mostly afflicts heavy women.

"Nobody knows why pseudotumor cerebri occurs, but it's a condition which, if left untreated in some people who are really afflicted badly with it, can lead to complete blindness," Ehni said.

"The question is 'Is there a possibility that an astronaut on a very long mission could arrive at the end of that mission unable to see, or be so visual compromised that he'd be non-functional?' The possibility is real enough that they need to look into this," Ehni said.

With a relatively small pool of subjects to study -- around 30 U.S. astronauts have lived on the International Space Station -- doctors have not been able to determine if age, gender or previous spaceflight experience affect vision loss.

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Doctors believe the redistribution of cerebral spinal fluid in weightlessness is to blame, though that doesn't fully explain the situation.

"Right now we don't have enough data on this to see what is going to happen," said NASA astronaut Don Pettit, who is preparing for launch in December for his second stint aboard the space station. "Right now, I'm not concerned."

© 2012 Discovery Channel


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