The mystery of why astronauts on lengthy missions suffer from vision problems has been solved for the first time in research presented today.

According to the study, which is being presented at the annual meeting of the Radiological Society of North America today, the visual impairment is a result of volume changes in the clear fluid that is found around the brain and spinal cord.

In the course of the last decade, a pattern began to emerge: astronauts who had flown long-duration space missions were reporting vision problems in significant numbers. Flight surgeons and NASA scientists were finding that their astronauts suffered blurry vision and, following further testing, several structural changes to their eyes.

The back of the astronauts’ eyeballs had flattened and the head of their optic nerves were inflamed. The syndrome is known as visual impairment intracranial pressure (VIIP) and was reported in nearly two-thirds of astronauts after long-duration missions aboard the International Space Station (ISS).

“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,” said study lead author Dr Noam Alperin, professor of radiology and biomedical engineering at the University of Miami Miller School of Medicine in Miami, Florida.

Scientists initially thought the microgravity’s effect on the body’s vascular fluid was to blame, theorising that the environment was causing it to pool near the eyes, but Alperin believed the problem instead lay with the cerebrospinal fluid (CSF). A clear fluid that keeps the brain and spinal cord cushioned while circulating nutrients and removing waste, the CSF system is well-designed to cope with the significant changes in hydrostatic pressures we face on Earth, such as standing up after lying down.

However, things are different in low-Earth orbit. The pressures that would usually be found in posture-related changes are missing in the microgravity of space, causing confusion in the CSF system.

By comparing high-resolution orbit and brain MRI scans before and shortly after spaceflights for seven long-duration mission ISS astronauts, the researchers were able to quantitatively show the role this CSF confusion was playing.

While short-duration astronauts showed little if any change, long-duration astronauts’ eyeballs had flattened significantly, with increased protrusion of their optic nerves. In addition, the CSF-producing cavities in the skull and the body cavities around the eyes of astronauts finishing long-duration flights had far greater volumes of CSF than in their short-duration counterparts.

Astronauts who reported the worst vision problems also had the greatest increases in CSF volume in both areas.

The importance of identifying the origin of the ocular changes is important, according to Alperin, because it represents the first steps towards developing methods or technologies to prevent future crews, such as those on a mission to Mars, from developing eye problems.

“If the ocular structural deformations are not identified early, astronauts could suffer irreversible damage,” he noted. “As the eye globe becomes more flattened, the astronauts become hyperopic, or far-sighted.”

Now the cause is known, a solution can be far more effectively targeted. Which is something that, according to Alperin, NASA is already working on both through simulations of VIIP and prototype countermeasures.