Why astronauts suffer from vision problems decoded

Washington: Astronauts staying in space for extended periods of time may suffer from vision problems due the constant pressure on the back of the eye, say scientists who suggest that using a vacuum device to lower pressure may prevent the condition.

A change in vision is the top most health risk for astronauts who spend extended periods of time on the International Space Station (ISS). Researchers at University of Texas Southwestern Medical Centre in the US showed that intracranial pressure in zero-gravity conditions, such as exists in space, is higher
than when people are standing or sitting on Earth, but lower than when people are sleeping on Earth.

This finding suggests that the constancy of pressure on the back of the eye causes the vision problems astronauts experience over time. To study how zero-gravity conditions affect intracranial pressure, researchers recruited volunteer patients who had had a port (called an Ommaya reservoir) permanently placed in their head as part of treatment for cancer.

The ports provided a way for researchers to measure intracranial pressure. NASA flights then flew the eight volunteers one by one on steep up-and-down maneuvers (parabolic flights) that created 20-second intervals of weightlessness.
The researchers measured intracranial pressure during the zero-gravity intervals and compared these with intracranial pressure during standard times of sitting, lying face upward (supine) and lying with head inclined downward.

"These challenging experiments were among the most ambitious human studies ever attempted as part of the Flight Operations parabolic flight program and changed the way we think about the effect of gravity and its absence on pressure inside the brain," said Benjamin Levine, professor at UT Southwestern Medical Centre.

"The information from these studies is already leading to novel partnerships with companies to develop tools to simulate the upright posture in space while astronauts sleep, thereby normalising the circadian variability in intracranial pressure and hopefully eliminating the remodeling behind the eye," said Levine.
The researchers have continued studying whether it is possible to lower intracranial pressure by means of a vacuum device that pulls blood away from the head.

They previously showed that a negative pressure box that snuggly fits the lower body can lower intracranial pressure when applied for 20-minute periods.
They will soon be testing the effect of the lower body negative pressure device on eye remodeling when negative pressure is applied for eight-hour periods. The study appears in the Journal of Physiology.