Mysterious Astronaut Eye Troubles Linked to Wonky Spinal Fluid
Long-duration spaceflight seems to mess with astronauts' vision and researchers think that cerebrospinal fluid in a microgravity environment could be to blame.
A few months into his six-month spaceflight in 2009, Canadian astronaut Bob Thirsk remembers having problems reading the stowage lists on the International Space Station, which are in eight-point font. After finding out another crew member had a problem as well, the two set about documenting their visual acuity while they were in space.
Using an ultrasound machine on board, Thirsk discovered that the back of his eyeball was swollen and his optic nerve not straight. The results were confirmed in an MRI about eight days after he landed. Since his time in orbit, other astronauts have reported changes in their vision as well.
"We still don't know what causes it," Thirsk told Seeker.
Possibilities that Thirsk (who is also a medical doctor) raised included nutrition, increased carbon dioxide on board that dilates blood vessels or perhaps even increased intercranial pressure from the Advanced Resistive Exercise Device (ARED) that is used to help astronauts stay in shape. ARED can produce up to 600 pounds of force, which is twice as much as its predecessor iRED (Interim Resistance Exercise Device), he said.
New research indicates that visual changes may be due to volume changes in the clear fluid in the brain and the spinal cord. On Earth, this cerebrospinal fluid (CSF) is needed to move around nutrients and get rid of waste materials - even if a person is moving into different positions, such as lying down, standing up and sitting down. In space, however, you remain static, pointed out lead author Noam Alperin, a radiology and biomedical engineering professor at the University of Miami Miller School of Medicine in Miami.
"On earth, the CSF system is built to accommodate these pressure changes, but in space the system is confused by the lack of the posture-related pressure changes," Alperin said in a statement this week.
While the new research is potentially groundbreaking, like many space studies it is hampered by a small sample group (since only so many people get to fly in space). Astronauts on seven ISS long-duration missions were compared to nine short-duration mission astronauts (on the space shuttle).
For all astronauts, MRI scans were performed before and shortly after spaceflight, and astronauts also did scans in orbit with the equipment available there. They found that these long-time flyers had more flattening in their eyeballs and more protrusion of their optic nerve.
They also detected changes in the CSF volume - both where it is produced in the brain, and also in the optic nerves inside the part of the skull that holds the eye. The research team says the changes in CSF are associated with changes in the crew members' eyes, and that countermeasures have to be implemented early in a mission to avoid irreversible damage.
In early October, NASA published a press release concerning intracranial pressure. When Scott Kelly and Mikhael Kornienko carried out their one-year mission in 2015-16, intracranial pressure was estimated with devices to check fluid pressure in and sound waves produced by the inner ear.
While the data from the mission is still being analyzed, NASA said there is no "pathologically large increase" in intracranial pressure, but it could still be contributing to astronauts' visual impairments. The agency is planning to look at this issue in more detail on the space station, using other techniques such as ultrasound.
"There is definitely an accumulation of cerebral spinal fluid within the space behind the eye, but we don't understand what is causing that," said Michael Stenger, lead visual impairment and intracranial pressure with technical services company KBRwyle, in a NASA statement. "Elevated venous pressure or a change in lymphatic drainage may sequester fluid behind the eye, thus causing a compartment syndrome."
For his part, Thirsk (who is now retired) said the causes must be found and resolved long before NASA embarks on a mission to Mars, which the agency says it wants to do around the 2030s. With current technology, it will take months to get there and back -- and the crew must wait at the Red Planet for the most favorable time to return, when the Earth and Mars are closest. "If six months can produce a visual impairment, what about 2.5 years?"
Image (top): Karen Nyberg images her eye with a fundoscope during Expedition 37 in 2013 (NASA)