Some astronauts have problems with their eyes after a long stay in space. The back of the eye can become flat, and the area where a nerve leaves the eye can swell.

Then it may be hard to see things that are close. This is called SANS. Scientists think one reason is that body fluids move up to the head in space. But they are still studying the cause.

Over the years, some astronauts have returned from space saying that they had trouble reading text up close. Doctors found that after long missions, the back of some astronauts' eyeballs had become flatter and the place where the optic nerve leaves the eye had swollen. This condition is now called SANS.

In 2011, an eye doctor named Mader and his colleagues described these changes in seven astronauts who had spent about six months on the ISS. In a survey of about 300 astronauts, around 30 percent of those on short flights and about 60 percent on long missions reported vision problems. The leading explanation is that body fluids move toward the head in weightlessness. However, other factors may also be involved.

Spaceflight-associated neuro-ocular syndrome, or SANS, refers to a set of eye changes seen in some astronauts after long missions: swelling where the optic nerve leaves the eye, flattening of the back of the eyeball, folds in the layers at the back of the eye, and a shift toward farsightedness. Because a flatter eyeball shortens the distance between lens and retina, near objects can become blurry. The first detailed report, published in 2011 by Mader and colleagues, described seven astronauts who had spent about six months on the ISS, and a survey of roughly 300 astronauts found vision complaints in about 30 percent of short-flight and 60 percent of long-duration crew members. The main suspect is the upward shift of body fluids in weightlessness, but genetics, B-vitamin metabolism and the station's high carbon dioxide levels may also contribute.

Researchers are testing countermeasures, notably lower body negative pressure, which draws fluid back toward the legs. The evidence remains limited, however. Raised pressure inside the skull cannot explain every finding, such as swelling in only one eye; ground-based bed-rest studies reproduce only some features; and reported rates vary with sample size and with when eye-scanning equipment was introduced.