By Jessica Firger

Long-term exposure to microgravity during space travel causes astronauts to develop a rare neurological syndrome in which the brain shifts upward crowding the cranium.

Dr. Donna Roberts, lead author of the study published this week, said research has shown roughly 40 percent of astronauts return from space missions and report neurological issues. The top of the brain that is affected by the syndrome controls sight, and astronauts often report symptoms such as blurry vision, swelled optic disks, migraines and increased head pressure. NASA calls the constellation of symptoms visual impairment and intracranial pressure (VIIP) syndrome.

Normally, the brain floats in a bath of cerebrospinal fluid, partly a function of anatomy but, also, as the study suggests, a result of gravity.

For the study, 34 astronauts underwent MRI scans of their brain before and after space travel. Eighteen of the astronauts in the study had participated in a long journey to the International Space Station (ISS, where they spent roughly two years), while the remaining 17 went on short shuttle missions that lasted only a few weeks.

Roberts says follow-up brain scans showed virtually no changes in the brains of the short-term travel astronauts, but some “some pretty significant changes” in those who traveled to the ISS and stayed awhile. This condition is typically reversible when an astronaut returns to Earth and the body reacclimates.

VIIP is a recently recognized phenomenon. But some things about the condition stump researchers because not all astronauts return from long-term missions with changes in vision. Researchers with NASA say this suggests that there may be some sort of biological underpinning that make some astronauts more prone to VIIP than others.

While there aren't that many astronauts in the world, knowing about the impact of space travel on the brain does have some practical function. For one, if Elon Musk has his way many of us may have an up-close and personal view of the moon at some point in our lifetime.

But also, the brain state seen in astronauts does share some likeness to conditions that occur here on earth. Dr. Matthew Fink, chair of neurology at Weill Cornell Medicine and New York-Presbyterian, says changes in brain positioning are sometimes seen in patients who have sustained bed rest, as well as in avid yogis. “My wife does yoga and I tell her not to stand on her head,” he said.