Cause of visual impairment in astronauts found

A visual problem affecting astronauts who serve on lengthy missions in space is related to volume changes in the clear fluid found around the brain and spinal cord, according to new research.

The research may help develop countermeasures to protect the crew from the ill effects of long-duration exposure to microgravity.

Over the last decade, flight surgeons and scientists at NASA began seeing a pattern of visual impairment in astronauts who flew long-duration space missions.

The astronauts had blurry vision, and further testing showed, among several other structural changes, flattening at the back of their eyeballs and inflammation of the head of their optic nerves.

The syndrome, known as visual impairment intracranial pressure (VIIP), 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 Noam Alperin, from University of Miami Miller School of Medicine in the US.

Scientists previously believed that the primary source of the problem was a shift of vascular fluid towards the upper body that takes place when astronauts spend time in the microgravity of space.

However, researchers led by Alperin investigated another possible source for the problem: cerebrospinal fluid (CSF), the clear fluid that helps cushion the brain and spinal cord while circulating nutrients and removing waste materials.

The CSF system is designed to accommodate significant changes in hydrostatic pressures, such as when a person rises from a lying to sitting or standing position.

"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.

Alperin and colleagues performed high-resolution orbit and brain MRI scans before and shortly after spaceflights for seven long-duration mission ISS astronauts.

They compared results with those from nine short-duration mission space shuttle astronauts. Using advanced quantitative imaging algorithms, the researchers looked for any correlation between changes in CSF volumes and the structures of the visual system.

The results showed that, compared to short-duration astronauts, long-duration astronauts had significantly increased post-flight flattening of their eyeballs and increased optic nerve protrusion.

Long-duration astronauts also had significantly greater post-flight increases in orbital CSF volume, or the CSF around the optic nerves within the bony cavity of the skull that holds the eye, and ventricular CSF volume - volume in the cavities of the brain where CSF is produced.

The large post-spaceflight ocular changes observed in ISS crew members were associated with greater increases in intraorbital and intracranial CSF volume.