Scientists have long known that extended spaceflight is bad for astronauts' bones and muscles. Now evidence is building that it may cause vision problems as well.

In a new study, researchers used magnetic resonance imaging (MRI) to study the eyes and brains of 27 astronauts who spent an average of 108 days in space aboard NASA's space shuttle and/or theInternational Space Station. They found optical abnormalities similar to those that can occur in patients with intracranial hypertension, a potentially serious condition in which pressure builds up inside the skull.

"These changes that occur duringexposure to microgravity may help scientists to better understand the mechanisms responsible for intracranial hypertension in non-space traveling patients," he added.

The spaceflight-induced issues were numerous and varied. For example, nine of the 27 astronauts (33 percent) exhibited expansion of the cerebrospinal fluid space surrounding the optic nerve, and six (22 percent) showed flattening of the back of the eyeball, researchers said. [Top Milestones of Human Spaceflight]

Further, four spaceflyers (15 percent) had bulging of the optic nerve, and changes in the pituitary gland and its connection to the brain were found in three astronauts (11 percent). The pea-size pituitary secretes hormones that are important in regulating a variety of bodily functions.

These abnormalities are similar to those observed with idiopathic intracranial hypertension — cases in which no cause can be determined for the increased pressure around the brain, researchers said. This pressure causes swelling of the juncture between the optic nerve and the eyeball, sometimes leading to visual problems.

Astronauts have known for a while that their six-month stints aboard the space station can causebone mineral loss and muscle atrophy; indeed, they exercise vigorously on orbit to stave off the worst of these effects. But the new study, and others like it, suggest that spaceflyers' eyes and brains may be at risk as well.

"Microgravity-induced intracranial hypertension represents a hypothetical risk factor and a potential limitation tolong-duration space travel," Kramer said.

NASA medical officials have noted vision changes in some space station astronauts, though nothing serious enough yet to ground any spaceflyers. They're not sure if intracranial hypertension is to blame, researchers said, but they're looking into it.

"NASA has placed this problem high on its list of human risks, has initiated a comprehensive program to study its mechanisms and implications and will continue to closely monitor the situation," said William Tarver, chief of the flight medicine clinic at NASA's Johnson Space Center in Houston.

The new study appears online in the journal Radiology.

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