Something strange has been happening to people who stay too long in space: The backs of their eyeballs start to flatten. Spider-web-like marks called choroidal folds crisscross the thin layer of blood vessels and connective tissue that surround their retinas. Their vision goes blurry, their optic nerves become inflamed. The damage can last long after the astronauts return to Earth. And scientists haven't been able to explain why.
“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,” notes Noam Alperin, a professor of radiology and biomedical engineering at the University of Miami Miller School of Medicine.
Now Alperin may have found the source of this mysterious syndrome. In a study presented this week at the annual meeting of the Radiological Society of North America, he suggests that the problem might be caused by pressure from the fluid that cushions the brain.
This is a slightly different take on visual impairment intracranial pressure syndrome (VIIP), the clunkily named malady that afflicts two-thirds of astronauts on lengthy missions to the International Space Station. Scientists had suspected that the issue was a shift in vascular fluids (blood plasma and lymph) caused by microgravity.
But Alperin pinpointed a different culprit: cerebrospinal fluid (CSF), the clear liquid that floats around the brain and spine. It helps to deliver nutrients, remove waste products and cushion the nervous system from harm. On Earth, the fluid easily redistributes itself to accommodate changes in pressure and posture: We can stand up, walk around, bend over and more without worrying about hurting our heads.
Alperin and his colleagues suspected that CSF was less able to adjust to the microgravity of space. So they performed high-resolution brain scans on seven astronauts shortly before and shortly after their long-duration missions to the space station.
Compared with astronauts who stayed aloft for a shorter time, the long-term travelers had dramatic post-flight flattening of their eyes. They experienced much greater increases in the amount of CSF surrounding the optic nerve and filling the bony part of the skull that holds the eyes. They also had more CSF in the brain cavities where the fluid is produced.
“The research provides, for the first time, quantitative evidence obtained from short- and long-duration astronauts pointing to the primary and direct role of the CSF in the globe deformations seen in astronauts with visual impairment syndrome,” Alperin said in a statement.
Though it has been more than half a century since a human first orbited the Earth, there's still a lot we don't know about the effects of extended space travel. NASA, the Russian and European space agencies, and several private space companies all say they want to send humans on a two- to three-year mission to Mars in the next several decades. No person has spent more than 438 consecutive days off Earth.
Studying VIIP and other effects of life in space is hard. Few people even make it to low Earth orbit, so there are not a lot of subjects to study. External influences are challenging to rule out, and competing demands on astronauts' time are many. It's important to note that Alperin's study examined only nine long-duration astronauts — a fairly small sample size.
But this research could be crucial to future missions. Astronauts on the ISS have seen their vision deteriorate from 20/20 to 20/100 in a matter of months. Michael Barratt, an astronaut and the former head of NASA’s human research program who experienced VIIP during a six-month ISS stint, told The Washington Post this summer that the vision problem could be the second greatest risk to human space travelers. (The top risk remains exposure to dangerous radiation.)
Barratt and a crew mate, both trained physicians, performed ophthalmoscopic exams on each other during space flight and discovered the flattening of the eyeball that's characteristic of VIIP. He thinks that confirming the fluid theory will require a more invasive procedure, like surgically implanting a monitor in astronauts' brains before they launch.
“This is one of those times I think aggressive science is extremely warranted,” Barratt said.