Loss of bone density and muscle tone are well known hazards of long-term space travel. But scientists have now discovered a new "hypothetical risk factor" for future interplanetary flights: slight eye and brain abnormalities.

MRI scans of astronauts reveal time spent in space can cause subtle deformations around the eyes, resembling a condition called idiopathic intracranial hypertension, or increased pressure around the skull with no known cause, according to a study published online this week in Radiology.

"These changes that occur during exposure to microgravity may help scientists to better understand the mechanisms responsible for intracranial hypertension in non-space traveling patients," Dr. Larry A. Kramer, the study's lead author and a professor at the University of Texas Medical School in Houston, said in a statement.

Chronic idiopathic intracranial hypertension is rare, striking about one in 100,000 people, according to the Intracranial Hypertension Research Foundation. However, it's more common among overweight women in their 20s and 30s, afflicting about one in 5,000, the group said on its website.

In the study, Kramer and his colleagues scanned 27 astronauts who spent on average 108 days aboard the International Space Station or on shuttle missions. Of these, one-third had expanded cerebral spinal fluid around the optic nerve, about one in five had flattening at the back of the eyeball, and about one in 10 had changes in the pituitary gland, a pea-sized sac that secretes sex and growth hormones. About 15 percent also had slightly bulging optic nerves, the researchers said.

Kramer said most of the results could be explained by a buildup of cerebrospinal fluid, according to the British newspaper the Guardian.

"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," Dr. William J. Tarver, chief of the flight medicine clinic at NASA/Johnson Space Center, said in a statement. However, he noted that none of the astronauts were judged unfit for space duty because of these findings.

Some of the researchers in this study were involved with a similar project to study the effects of long-term exposure to microgravity conditions on vision, which was reported on by Space Safety News last fall. In the study, the researchers tracked seven astronauts who all spent at least six months in space. Six of the seven reported near vision problems. Post-flight questionnaires given to returning astronauts also found "approximately 29 percent and 60 percent of astronauts on short and long-duration missions, respectively, experienced a degradation in distant and near visual acuity," the researchers wrote in the Journal of Ophthalmology. "Some of these vision changes remain unresolved years after flight."

According to the Guardian, Kramer said the findings raise "social, political and legal ramifications" for space travel.

"Consider the possible impact on proposed manned missions to Mars or even the concept of space tourism," he said, according to the paper. "Can risks be eventually mitigated? Can abnormalities detected be completely reversed? The next step is confirming the findings, defining causation and working towards a solution based on solid evidence."

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