Astronauts experience bone and muscle loss in the weightlessness of space, and now the first study of returning space travellers' eyes suggests that prolonged amounts of time in orbit can take a significant toll on vision, although the long-term effects aren't clear.
The new study, of more than 300 astronauts in the US space program, found that almost 50% of those who served on long missions - six months or more - developed new problems with their ability to see objects near to them while in space and for some time after returning to Earth.
Roughly 23% of astronauts who spent shorter periods in orbit reported problems with their near vision during their missions and after getting home.
The NASA-funded researchers also did physical exams on seven male astronauts who complained of vision problems after returning from six-month tours in space.
They found several signs of eye stress in all of them, including a build-up of fluid around the optic nerve, the development of folds in the bed of vessels that supply blood to the retina, flattening of the eyeball and more.
Loss of gravity causes pressure in optic nerve
"People have been flying in space for 50 years and nobody has gone blind yet," said Dr Tom Mader, an ophthalmologist at the Alaska Native Medical Center in Anchorage, who led the study. "But it's still something to be concerned about."
Dr Mader said the effects may be due to increased pressure of the fluid surrounding the brain – the result of less gravity than on Earth – that fails to drain well back into the body. But the precise mechanism is unclear.
It's possible that the loss of gravity causes pressure around the optic nerve to spike, which can damage vision, Dr Mader said. It's also possible, however, that micro gravity environments cause vision problems by lowering pressure in the eye, he added.
"It's very hard for us at this point to define exactly what is causing all of this," said Dr Mader, whose group reported its findings online in Ophthalmology.
At least one of the seven astronauts examined in the study still displayed some of the documented physical eye changes more than five years after returning from space.
A small proportion of the astronauts who completed post-flight surveys had problems with long-distance vision while in space – 6.6% of those on short missions and 12% on missions lasting more than six months.
Some 34% of the astronauts on long missions and 11% of those on short missions also reported refraction changes in their corrective lens requirements, although it's not clear whether or how long those changes lasted after returning to Earth.
Astronauts experience facial swelling in space
NASA is conducting follow-up studies, including research on the International Space Station, to pin down the mechanism, Dr Mader added. Scientists also will be using magnetic resonance imaging and other tests to carefully assess astronauts' vision and eye anatomy before and after missions.
David Robertson, who runs the Center for Space Physiology and Medicine at Vanderbilt University Medical Center in Nashville, Tennessee, said the extent of the eye changes was somewhat surprising.
However, he added, it's well-known that astronauts experience swelling in the face in space, as evidenced by in-flight pictures.
"There are more changes in the eyes that I might have expected, but I would imagine that the increased head-ward movement of fluid during travel in space, together with the puffiness of the face and facial tissues likely also affects the eye," Robertson said.
So far, studies of astronauts have found no permanent adverse effects on human health from long-term space missions, Robertson said.
Any mission to, say, Mars, likely would require at least 2.5 to 3 years, he added. However, no one has spent that long in space continuously.
The record for the most time spent in space in a single mission is held by Valeri Polyakov, who spent 438 days in 1994 and 1995 aboard the Russian Mir spacecraft while orbiting the Earth more than 7,000 times.
(Reuters Health, Adam Marcus, September 2011)