NASA reports that over 70% of astronauts are affected by SANS, which manifests as "a constellation of symptoms, including these changes in vision," according to Dr. Matt Lyon, director of the MCG Center for Telehealth. Alongside vision issues, astronauts may experience other complications as bodily fluids, such as cerebrospinal fluid, shift in space, potentially leading to structural changes in the brain.
"The changes start happening on day one," said Dr. Lyon, who also holds the J. Harold Harrison M.D. Distinguished Chair in Telehealth. "We are not entirely sure what causes these issues with vision, but we suspect it has to do with a shift in cerebrospinal fluid in the optic nerve sheath. On Earth, gravity pushes that fluid down and it drains out, but in space, it floats up and presses against the optic nerve and retina."
Using portable, handheld ultrasound machines, Dr. Lyon and his team hope to uncover the mechanism behind these changes and predict which astronauts might be most susceptible to them.
MCG has patented a method using portable ultrasound to quickly visualize damage caused by pressure and fluid changes in the optic nerve sheath-the layers of protective membranes surrounding the nerve. Dr. Lyon is investigating how this part of the brain is impacted by elevated cranial pressure and mild traumatic brain injuries (TBIs). When the brain is injured, it swells and accumulates fluid, much like other organs.
Researchers have collaborated with URSUS Medical Designs LLC, a biotech company based in Pennsylvania, to develop a 3-D ultrasound machine that enhances traditional 2-D imaging. This innovation was funded by a one-year $350,000 Small Business Innovation Research grant from the National Institutes of Health.
Dr. Lyon and the MCG research team are now utilizing these ultrasound machines to screen astronauts, determining which of them may have pre-existing damage or incompetence in their optic nerve sheaths, making them more prone to the vision changes associated with SANS.
"We discovered that when the cerebral spinal pressure goes up with mild traumatic brain injuries (TBIs), there is resulting damage to the sheath that is likely lifelong," explained Dr. Lyon. "We think that when astronauts who have experienced concussions or mild TBIs go into space and experience the low-gravity fluid shifts, the sheath dilates from the increase in volume. It is like a tire - a normal tire remains its normal shape as it is filled with air, and the shape doesn't change. When it's damaged, like bulges on the side of a tire, the fluid fills the bulges up and the sheath expands. This can cause pressure on the nerve and retina. A damaged sheath is less of a problem on Earth, but in space, the excess fluid has nowhere to go."
The team is still investigating whether the vision changes result from the volume of fluid alone, the associated pressure, or both. Polaris Dawn crew members are being trained to use these ultrasound devices to measure fluid and pressure in real time during their mission.
"If it's just volume, we suspect the cerebrospinal fluid goes up, fills this floppy bag and gets stuck. It's almost like not flushing your toilets. You're creating this toxic environment, because the cerebral spinal fluid (CSF) is what carries toxins away from your eyes and nerves, and instead the toxins sit against the optic nerve, killing it," said Dr. Lyon. "But it could be that combined with the increased pressure that comes with increased CSF, which would be like getting intermittent hypertension in your eye."
Future research may focus on developing countermeasures to reduce fluid volumes in the head during spaceflight. For instance, space agencies currently use a lower-body negative pressure device that draws fluids downward in the body, which could be further refined.
This experiment is one of many scientific endeavors that will take place during the Polaris Dawn mission. The mission is a collaborative effort involving 23 institutions, including MCG, the Translational Research Institute for Space Health (TRISH), the University of Texas, Houston, the University of Colorado, Boulder, Baylor College of Medicine, Pacific Northwest National Laboratory, and the U.S. Air Force Academy.
Related Links
Polaris Dawn
Space Medicine Technology and Systems
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