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CHICAGO, Ill. -- The blurry vision that many long-duration mission astronauts experience may be linked to changes in cerebrospinal fluid (CSF) levels, researchers said here.

Based on MRI scans done before and shortly after space flights, long-duration astronauts had significantly greater post-flight increases in globe flattening indices and optic nerve protrusion indices versus short-duration astronauts, reported , of the University of Miami Miller School of Medicine, and colleagues.

Long-duration astronauts also had significantly greater post-flight increases in orbital CSF volume, the authors said in a presentation at the Radiological Society of North America (RSNA) annual meeting.

"Ocular and vision changes known as visual impairment intracranial pressure (VIIP) syndrome have been reported in nearly two-thirds of long-duration mission International Space Station (ISS) astronauts," the authors wrote. "These changes are currently attributed to cephalad vascular fluid shift induced by exposure to microgravity."

"People initially didn't know what to make of [VIIP] , and by 2010, there was growing concern as it became apparent that some of the astronauts had severe structural changes that were not fully reversible upon return to earth," Alperin said.

The study assessed ocular shape and CSF volume changes related to spaceflight to determine the underlying cause for these changes. CSF is designed to accommodate changes in hyrdrostatic pressure, such as when people rise from sitting or lying positions, Alperin explained.

"On earth, the cerebrospinal fluid system is built to accommodate these pressure changes, but in space the system is confused by the lack of the posture-related pressure changes," he said.

His group compared MRI scans from seven long-duration ISS astronauts with nine short-duration ISS astronauts. The authors did not state the duration of the flights, but according to , long missions last up to 6 months, while a short one is for 7 days.

"Post-flight increases in globe flattening and nerve protrusion were tested for association with increases in intra-orbital CSF volume, ventricular CSF volume, and brain tissue interstitial fluid volume," the authors stated.

They found that there were no significant post-flight changes in gray matter volume or white matter volume in either group. The large post-flight ocular changes observed in ISS crew members were associated with greater increases in intraorbital and intracranial CSF volume but not with interstitial brain tissue fluid volume, they noted.

Identifying the origin of the space-induced ocular changes is necessary for the development of countermeasures to protect the crew from the ill effects of long-duration exposure to microgravity, the authors concluded.

"If the ocular structural deformations are not identified early, astronauts could suffer irreversible damage," Alperin noted. "As the eye globe becomes more flattened, the astronauts become hyperopic, or farsighted."

of Stanford University in Stanford, Calif., and the co-moderator of the RSNA session, said the findings may also have relevance on earth.

"I think the findings may be relevant for a much larger group of people," Wintermark told MedPage Today. "We have patients who have a disease known as intracranial hypotension, and these patients have chronic headaches; often positional headaches ... headaches when they stand up. This is not a very well understood disease."

"We sometimes treat these patients by injecting blood in the spine," he explained. "It helps some patients, but not others. It reflects that we really don't know much about how to treat this disease."

He added that the results in astronauts could "help us understand a little bit more about intracranial hypotension. We are learning from the space studies a little more about how pressure affects the cerebrospinal fluid, which may help us understand this other condition better. While this study may help astronauts, I think it has the potential to help a larger number of patients."

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