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A closer look at space travelers revealed that a buildup of brain fluids may be squishing their eyeballs from behind.

The Liquid Sloshing

Many astronauts returning to Earth after long-duration missions in space suffer from blurry vision that does not always get better. Now, after years of speculation and study, researchers believe they have finally isolated the cause: The liquid sloshing around the brain is building up in places it shouldn’t, squishing their eyeballs until they permanently flatten.

Extended Periods

The condition is called visual impairment intracranial pressure, and it afflicts almost two-thirds of the astronauts who have spent extended periods of time aboard the International Space Station. (Also see “Astronauts’ Fingernails Falling off Due to Glove Design.”)

The Mysterious Syndrome

NASA first identified the mysterious syndrome in 2005, when astronaut John Phillips’s vision went from 20/20 to 20/100 after six months in orbit. Extensive physical examinations revealed that the back of Phillips’s eyeballs had somehow gotten flatter, inflaming the optic nerve.

People initially didn’t know what to make of it, and by 2010 there was growing concern

“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,” Noam Alperin, the lead author of the study, says in a press statement.

NASA physicians knew that something was increasing pressure on the astronauts’ eyes, but they couldn’t quite pin down the cause. The leading theory was that it was somehow connected to the redistribution of vascular fluids (blood and lymph) in microgravity.

According to NASA, nearly 68 ounces of fluid—the equivalent of two large plastic bottles of soda—shifts from an astronaut’s legs toward their head while in space. Scientists suspected that this fluid buildup increased pressure on the brain, ultimately affecting the eyes.