© NASA
NASA astronaut Michael Hopkins, performs ultrasound eye imaging in the Columbus laboratory of the International Space Station. European Space Agency astronaut Luca Parmitano assists Hopkins.
For years now, NASA has been puzzled by a mysterious effect of extended space flight: vision damage. Many, though not all, astronauts who have been in space for months at a time experienced their vision slowly degrading, and post-flight inspection revealed that the back of their eyeballs had been squished down and flattened over the course of their trip.

But new research presented this week provides a partial answer to what's causing this condition: pressurized spinal fluid. Noam Alperin, a researcher at the University of Miami's Evelyn F. McKnight Brain Institute, presented findings from research he and his peers conducted on 16 astronauts, measuring the volume of cerebrospinal fluid (CSF) in their heads before and after spaceflight. CSF floats around the brain and spine, cushioning it and protecting your brain as you move, such as when you stand up after lying down.

Alperin and his team found that astronauts who had been in space for extended trips (about six months) had much higher build up of CSF in the socket around the eye than astronauts who had only gone on short stints (about two weeks). They also designed a new imaging technique to measure exactly how "flat" the astronauts eyeballs had become after extended periods in space.

The idea is that, without the assistance of gravity, the fluid isn't pulled down and evenly distributed, allowing it to pool in the eye cavity and build up pressure, which slowly starts to warp the eye and cause the vision damage, called visual impairment intracranial pressure syndrome (VIIP). It's likely some people are more predisposed to this than others, perhaps due to the shape of their skulls, which would explain why some astronauts have not experienced VIIP. But Alperin said his findings suggest anybody could get VIIP if they're in space for a long enough period of time.

"We saw structural changes in the eye globe only in the long-duration group," Alperin told me over the phone. "And these changes were associated with increased volumes of the CSF. Our conclusion was that the CSF was playing a major role in the formation of the problem."

The results have not been published in a peer-reviewed journal, but Alperin told me the manuscript was recently accepted and will be published shortly. And these reported findings align with what scientists already suspected about the condition, according to Scott M. Smith, the manager of NASA's Nutritional Biochemistry Laboratory at the Johnson Space Center, who's been studying the vision loss issue for the last six years.

"I think this fits very well within what others seem to be thinking at the moment," Smith told me.

Many astronauts—though, importantly, not all—have experienced this unexplained reduction in eyesight after spending months on the International Space Station, some dropping from perfect 20/20 vision to 20/100 in just six months. Researchers have been gravely concerned about this effect. With plans to send humans to Mars by the 2030s, a mission that would require nine months of space flight one way, we don't really want to risk all of our astronauts going blind in the process.

"NASA ranks human health risks and the two top risks are radiation and vision issues," Smith said. "Is it number one or two? Some people would say it's number one, because we don't really know what the long-term implications are."

But the better we understand how VIIP occurs, the more likely we are to be able to create a solution. Smith's team is currently conducting a clinical trial to investigate whether polycystic ovarian syndrome—which, despite its name, may indeed occur in men—could have similar effects on vision. This research could help explain who is more likely to experience VIIP, as research like Alperin's explores the physical functions of the condition.

What a solution to the condition will look like depends what else we learn: it could be a medication, or a mechanical device to help redistribute fluid, or something else entirely. But each piece to the puzzle helps us get one step closer to sending humans to Mars, and not blinding them in the process.