When Scott Kelly returned from his one-year space mission last March, he admitted that he, like many of his colleagues, came back with vision troubles. In fact, nearly two-thirds of astronauts who have gone on long-duration space missions inexplicably returned with blurry vision, flattened eyeballs, and inflamed optic nerves—and now researchers have a new hypothesis as to why.

Despite years of research, the cause of the problems has remained fuzzy. However, scientists have dubbed the astronauts’ condition “visual impairment intracranial pressure syndrome” or VIIP. The name is based on the current leading hypothesis that, in space, bodily fluids normally dragged down by gravity can freely flow into the head and increase the pressure on the brain and eyeballs. A researcher at Georgia Tech is even looking into a mechanical way to draw fluid back down to the legs to spare space-goers' eyesight.

But a new study involving 16 astronauts suggests that the leading theory is a tad off-kilter; it’s not vascular fluids bubbling up to the head causing problems, but instead sloshing cerebrospinal fluid tipping toward the eyes, the new study suggests. Researchers from the University of Miami presented those findings Monday at the annual meeting of the Radiological Society of North America in Chicago.

To get to that conclusion, the researchers, led by Noam Alperin, a professor of radiology and biomedical engineering at Miami, used high-resolution magnetic resonance imaging (MRIs) of astronauts’ brains and eyes before and after space flight. They imaged seven astronauts who went on long-duration missions and nine who went on short ones. Next, the researchers analyzed changes in cerebrospinal fluid (CSF), the clear fluid that, in part, cushions the brain from pressure changes during movements, such as when a person gets out of bed and stands up.

"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," Alperin said in a press statement.

In their data, Alperin and colleagues found the signs of VIIP in the long-duration astronauts, but not in the short-duration ones. And, those symptoms were positively correlated with increased volumes of cerebrospinal fluid in the area of the eyes. The researchers noted no other differences in the brains and eyes.

Though the sample size is small and the study has not been peer-reviewed, Alperin said it points to the CSF playing a role in astronaut’s vision problems that warrants further study. Getting to the bottom of the vision problems is critical to protect crew members during lengthy space travel, he added.