WASHINGTON -- If NASA ever wants to send astronauts to Mars, it first must solve a problem that has nothing to do with rockets or radiation exposure.
A newly discovered eye condition -- found to erode the vision of some astronauts who've spent months aboard the International Space Station -- has doctors worried that future explorers could go blind by the end of long missions, such as a multi-year trip to Mars.
While blindness is the worst-case scenario, the threat of blurred vision is enough that NASA has asked scores of researchers to study the issue and has put special eyeglasses on the space station to help those affected see what they're doing.
"We are certainly treating this with a great deal of respect," said Dr. Rich Williams, NASA's Chief Health and Medical Officer. "This (eye condition) is comparable to the other risks like bone demineralization (loss) and radiation that we have to consider. ... It does have the potential for causing mission impact."
According to one NASA survey of about 300 astronauts, nearly 30 percent of those who have flown on space shuttle missions -- which usually lasted two weeks -- and 60 percent who've completed six-month shifts aboard the station reported a gradual blurring of eyesight.
Williams put the figure lower -- at roughly 35 percent for station crew -- but did not dispute the severity of the problem, nor the mystery surrounding it. The disorder, similar to an Earth-bound condition called papilledema, is believed to be caused by increased spinal-fluid pressure on the head and eyes due to microgravity, although the exact cause is uncertain.
Oftentimes, the problem goes away once an astronaut returns to Earth. But a recent study by the National Academies noted there had been "some lingering substantial effects on vision," and that astronauts were "not always able to re-qualify for subsequent flights" -- at least not immediately.
Williams declined to discuss specific cases, but acknowledged at least one astronaut never regained normal vision.
"We have seen visual acuity not return to baseline," he said.
Though it will be years before NASA has a rocket powerful enough to launch humans to Mars, the agency has long worried about the effects on astronauts of the nearly three-year-long round trip. But the chief worry has been exposure to cosmic radiation and, to a lesser extent, loss of bone mass due to microgravity.
For decades, though, NASA had also heard anecdotal evidence of vision problems. But the agency only began studying the issue in earnest around 2005 when an unnamed astronaut came forward.
"You didn't hear about it at all until you had one fellow come back (from space) and had problems and was very open about it. His openness led to other people reporting the same," said Garrett Reisman, a former astronaut who spent three months aboard the station in 2008.
Reisman said he noticed a slight decrease in his vision toward the end of this space station stint, but he said that was minor compared to what happened during a 2010 shuttle flight.
"Ten days into the mission, there was a carbon dioxide spike in the cabin due to problems with the carbon dioxide filter," he said.
The next day, Reisman said, he had headaches and his vision blurred so much that he had a tough time reading the cockpit gauges. The symptoms passed within 24 hours and his vision returned to mostly normal, he said, but it took two days on the ground before his sight was back to 100 percent.
"This isn't scientific, this is just what happened to me. I think it warrants further study," said Reisman, who added he was the only crewmember whose vision was affected.
Much of the research so far has centered on seven unnamed astronauts who have shown symptoms, including one astronaut whose eyesight was so affected by his third month on the station that he could "only see the Earth clearly while looking through the lower portion of his progressive reading glasses," according to a draft of a paper to appear in the medical journal Ophthalmology.
In the months after returning to Earth, the astronaut noted a "gradual but incomplete improvement in vision," the report said.
In another example cited in Ophthalmology, an astronaut was forced to use special glasses at about the three-month mark because his prescription glasses weren't strong enough.
NASA earlier this year flew to the station several new pairs of glasses, made by Superfocus, which can adjust for different vision impairments.
The seven cases were reviewed at a conference last February in Houston of about 75 scientists and doctors, from a wide range of fields.
Among the conclusions: the condition wasn't damaging enough to "cause blindness near term" but that it was "unknown" whether this could be a long-term effect, according to a summary report from the conference.
"No one has been in space long enough to know how bad this papilledema can get," said Dr. Bruce Ehni, a neurosurgeon at the Baylor College of Medicine, who has worked with NASA on this issue.
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On Earth, papilledema can lead to blindness if left unchecked, which is why Ehni said solving the issue was paramount for NASA and the space community.
"When they (NASA) start going (to) long-distance (destinations) like Mars, you can't end up having a bunch of blind astronauts."
The Houston experts recommended more research, including a better means of testing station crews while in space. It also suggested a closer examination of astronauts' backgrounds and behaviors to try to detect commonalities that determined who was affected and who wasn't.
The key is to figure out a cause so NASA can develop a countermeasure -- essential to solving an eye problem that was "unlike any clinical entity" the Houston experts had seen.
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