Human Spaceflight: Just two weeks in orbit causes changes in Astronauts' eyes

Animal Enclosure Modules similar to the one shown here, being inspected by Mission Specialist Tracy Caldwell, Ph.D., and Pilot Charles Hobaugh aboard Space Shuttle Endeavor (STS-118), are used to study animals in low gravity conditions. 

Scientists are reporting mice traveling aboard STS-133 showed evidence of ocular nerve damage and changes in eye gene expression. 

Credit: NASA

Just 13 days in space may be enough to cause profound changes in eye structure and gene expression, report researchers from Houston Methodist, NASA Johnson Space Center, and two other institutions in the October 2013 issue of Gravitational and Space Research.

The study, which looked at how low gravity and radiation and oxidative damage impacts mice, is the first to examine eye-related gene expression and cell behavior after spaceflight.

Patricia Chévez-Barrios

"We found many changes in the expression of genes that help cells cope with oxidative stress in the retina, possibly caused by radiation exposure," said Houston Methodist pathologist Patricia Chévez-Barrios, M.D., the study's principal investigator.

"These changes were partially reversible upon return to Earth. We also saw optic nerve changes consistent with mechanical injury, but these changes did not resolve."

"We also saw changes in the expression of DNA damage repair genes and in apoptotic pathways, which help the body destroy cells that are irreparably damaged."

Since 2001, studies have shown astronauts are at increased risk of developing eye problems, like premature age-related macular degeneration. Experts suspect the cause is low gravity, heightened exposure to solar radiation, or a combination of the two.

In Nov. 2011, a NASA-sponsored Ophthalmology study of seven astronauts showed that all seven had experienced eye problems after spending at least six months in space.

Doctors saw a flattening of the back of the eyeball, folding of the choroid (vascular tissue behind the retina), excess fluid around and presumed swelling of the optic nerve, or some combination of these.

High-energy radiation from the Sun can cause nasty, extremely damaging chemical reactions in cells, collectively called oxidative stress.

Earth's atmosphere reflects or absorbs much of this radiation and is, ironically, a much better shield than the thick metal hulls of space shuttles and the International Space Station.

Damage to eyes isn't merely a long-term health issue for some astronauts back on Earth—it could interfere with future missions.

Any loss of focus or vision will make it difficult for humans to complete long missions, such as round-trip travel to Mars (12 to 16 months) or to the moons of Jupiter (about two years).

If both radiation exposure and gravity loss are to blame, one solution to save astronauts' eyes might be a spacecraft with a more protective hull and inside, a spinning hamster wheel that simulates gravity similar to those envisioned by futurist author Arthur C. Clarke and realized in Stanley Kubrick's film, 2001: A Space Odyssey.