Scientists have finally figured out why astronauts lose their vision while in space

Radiologists have finally figured out why astronauts who spend a lot of time in space get impaired vision.

The problem, called visual impairment intracranial pressure (VIIP) syndrome, has been reported in two-thirds of astronauts who go up to the International Space Station.

And according to a new study from researchers at the University of Miami - reported Monday at the Radiological Society of North America's annual conference - those changes to the eye have everything to do with changes in the fluid that surrounds the brain and spinal cord.

There have been a few theories about why astronauts have VIIP syndrome after spending a long time in space: one of the running hypotheses had to do with vascular fluid shifting toward the brain during spaceflight, which would cause the eye to change shape.

So to examine what exactly was happening, researchers at the University of Miami took brain MRIs before and after astronauts went to space. Nine astronauts had been in space for a short duration, while seven had been up in the ISS for a long period of time.

What they found by looking at these images was that the astronauts who were in space long-term had more flattened eyes and more inflamed optic nerves. And interestingly, they also found that the astronauts who had spent more time in space experienced a larger increase in cerebral spinal fluid (CSF) - the clear fluid that surrounds and protects the brain and spine - than the astronauts that had been in space for a short amount of time.

In the end, the researchers said that CSF - not vascular fluid - "has a direct role in spaceflight induced ocular changes," the study concluded. The reason this happens has a lot to do with the difference in pressure between Earth and space.

"On Earth, the CSF system is built to accommodate these pressure changes, but in space the system is confused by the lack of the posture-related pressure changes," lead author and professor of radiology and biomedical engineering at the University of Miami Noam Alperin said in a news release.

Researchers have been looking at vision problems in astronauts for years, though for the most part researchers had considered the effects on eyesight reversible. More recently, it appears that some of those vision problems do stick around.

Now that the cause has been identified, the hope is to develop ways to prevent those eye changes from happening, which will be critical as humans plan to embark on longer space trips.