Judging by the incredible images astronauts have taken from space, the views are spectacular. However, there are many consequences of sending a human to space. Astronauts who spend a long time in space can develop a condition called visual impairment intracranial pressure that can lead to permanent changes to eyesight. Some recent data has revealed the cause may be changes in the amount of fluid around the brain and spinal cord.
About half of long-duration mission astronauts have reported changes in their vision, usually farsightedness – their close-up vision is decreased but they retain clear distance vision. Most affected astronauts were on missions lasting 6 months, had normal preflight eye pressures and none had a disease or used medications that could increase eye pressures. Studies revealed a flattening at the back of the eyeball and inflammation of the optic nerve. For some astronauts, the changes were not fully reversible upon their return to Earth.
On Earth, gravity pulls fluids in the body down toward the feet. In space, that fluid drifts upward, including the skull. The extra fluid increases the intracranial pressure (ICP) and pushes on the brain and the eyes. Astronauts in space typically have “bird legs” and puffy faces, both signs of fluid redistribution.
NASA scientists have seen signs of increased ICP in some astronauts upon their return from space. In space, they have also observed swelling that impacts the optic nerve and flattening at the back of the eyeball.
Other factors that could be contributing to the rise in ICP may be in-flight resistance exercise, high levels of sodium in astronauts’ food and high levels of CO2 in the air they breathe on the International Space Station.
Tests for increases in ICP include spinal taps or drilling a hole in the skull for pressure sensors. These tests are too dangerous to do in space, so NASA is looking for safer ways to test the theory that ICP is responsible for changes in eyesight.
There are two relatively new devices that can be used to monitor ICP non-invasively. The Vittamed 205 ICP monitor works by applying pressure to the tissue surrounding the eye and measuring blood flow through certain areas. The second device measures tiny displacements of the eardrum. Both of these devices are safe and can be used in space.
If we can see changes in the eye to due changes in ICP, there may be other effects that we are not yet aware of that could threaten the health of astronauts. Research is underway to find out. If we are going to send people to Mars and continue having astronauts work for long stretches on the International Space Station, then a solution must be found to prevent or reverse the effects of space on the eye. As scary as increased ICP sounds, when astronauts were asked if they would give up their time in space for normal eyesight, they firmly said no. Besides, maybe the new farsightedness would make it easier to see Earth!
Medical Discovery News is hosted by professors Norbert Herzog at Quinnipiac University, and David Niesel of the University of Texas Medical Branch. Learn more at www.medicaldiscoverynews.com.