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.