Changes in cerebrospinal fluid volume the culprit behind astronauts' blurry vision
As missions aboard the International Space Station (ISS) have grown longer, astronauts have been repeatedly complaining of blurred vision and, fearing that this condition might not be reversible on returning from Earth, NASA has been investigating the causes. Now a team led by Noam Alperin, professor of radiology and biomedical engineering at the University of Miami Miller School of Medicine in Miami, Florida has completed a study that indicates the culprit might be a change in volume in the astronaut's cerebrospinal fluid.
For the past 10 years, NASA flight surgeons and scientists have noticed a repeated pattern of vision impairment in ISS crews. Two thirds of astronauts on long-term stays to the space station have suffered from blurred vision accompanied by flattening at the back of the eyeballs and inflammation of the head of the optic nerve. Worse, follow up studies show some of these severe structural changes don't completely reverse themselves on the astronauts' return to Earth.
The suspected cause of this blurry vision, officially called Visual Impairment Intracranial Pressure (VIIP), was a shift of vascular fluid in astronauts' bodies in zero gravity. On Earth, gravity pulls blood and other fluids downwards, but in space there's no gravity to help the process, so fluids tend to migrate upwards. The result is astronauts become puffy faced, sinuses become clogged, and the crew lose most of their sense of taste. The logical conclusion was that such fluid displacement played a part in VIIP.
But Alperin's team has concluded that the problem is much more specific and is related to cerebrospinal fluid (CSF). This is a clear fluid that insulates the brain and spinal cord against shocks and vibrations while bringing in nutrients and removing waste. According to Alperin, what is happening is that zero gravity is interfering with the body's mechanism for controlling the volume of the CSF.
In normal gravity, the CSF system can adjust to changes in hydrostatic pressures caused by simple things like moving from laying to sitting, but in space, insufficient posture-related pressure changes confuse the system, so it can't make the required adjustments.
To determine what is happening in the CSF system, Alperin's team took high-resolution MRI scans of the eyes, the bony cavity surrounding them called the orbits, and the brains of astronauts before and after seven long-duration spaceflights. These were then compared using advanced quantitative imaging algorithms to similar scans from astronauts who flew on short-duration Space Shuttle missions.
What the team found was a significant increase of post-flight flattening of the eyeballs and increased optic nerve protrusion with long-duration astronauts showing increased post-flight CSF volume around the eyeballs and the ventricles of the brain. However, there were no marked changes in the volume of the brain tissues.
Alperin says that learning about the causes of VIIP is a necessary step in finding countermeasures to combat it. NASA is currently looking at how to simulate the condition on Earth as a way of conducting detailed studies and for testing possible solutions.
"If the ocular structural deformations are not identified early, astronauts could suffer irreversible damage," says Alperin. "As the eye globe becomes more flattened, the astronauts become hyperopic, or farsighted."
The research was presented at the annual meeting of the Radiological Society of North America (RSNA).
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