The zero-G lifestyle does funny things to our bodily fluids. That’s the conclusion of one recent study, which may have found a reason for a common space travelers’ malady. The researchers presented their results [PDF] at the annual meeting of the Radiological Society of North America.

If we’re going to start sending humans to Mars and other distant destinations, we’re going to need to know if we can survive the trip. So astronauts are an incredible scientific resource, not only for what they do while in space, but also for what they experience. Living in orbit can shrink astronauts’ hearts and stretch their spines. It can also damage their ability to see: Numerous travelers who left Earth with 20/20 vision have returned to find they need glasses just to read or drive.

"People initially didn't know what to make of it, and by 2010 there was growing concern as it became apparent that some of the astronauts had severe structural changes that were not fully reversible upon return to earth,” lead author Noam Alperin of the University of Miami said in a statement.

Scientists call the phenomenon visual impairment intracranial pressure, or VIIP. The name is slightly misleading in its certainty. Researchers think the eye problems are the result of increased pressure inside astronauts’ heads, but they haven’t really been sure.

Alperin and his colleagues wondered if the problem might not be liquid—cerebrospinal fluid (CSF), to be precise. CSF is a crucial component of healthy brain and body function. It surrounds our brains and spinal cords and acts kind of like amniotic fluid in the womb, ensuring a flow of nutrients and removing waste. CSF is also somewhat adaptable and responds to changes in the position and angle of your body and head. It’s a good system, and it works.

At least where there’s gravity. The research team scanned the brains and eyeballs of seven different astronauts both before and after long stints aboard the International Space Station (ISS). They compared those scans with results from another nine astronauts who had only been on the ISS briefly.

There could be no doubt about it—longer stays in space were messing with the astronauts’ eyes. Their eyes were more flattened; their optic nerves showed more swelling; and, most interestingly, they had higher volumes of CSF in their eye sockets and in the CSF-producing part of the brain. The higher the CSF volume, the more trouble an astronaut had seeing.

"The research provides, for the first time, quantitative evidence obtained from short- and long-duration astronauts pointing to the primary and direct role of the CSF in the globe deformations seen in astronauts with visual impairment syndrome," Alperin said.

Identifying the source of the problem is the first step to correcting it. Alperin and NASA are now working to simulate the conditions that cause VIIP so they can figure out how to protect astronauts’ eyes in the future.