There is a lot we don’t know about the impacts of spaceflight on the human body, but scientists investigating the physiological effects of extended periods in space are uncovering some very useful insights. A new study has revealed a significant change in the brain’s white matter in astronauts following stays on the International Space Station (ISS), and the scientists say the changes to brain volume could be permanent.
Many of the astronauts that spend time at the ISS go on to experience impaired vision. Previous research has implicated inflammation of the optic nerve, retinal hemorrhaging and structural alterations to the eyeballs in this common ailment, and also suggested that these changes linger even after the astronauts have returned to Earth.
One of the leading theories for why these changes take place is what is known as Visual Impairment Intracranial Pressure. In microgravity environments, the fluids inside the body behave differently to the way they do on Earth, where gravity typically drags them downwards. In places like the ISS, they can instead travel upwards, which scientists believe could increase the intracranial pressure of astronauts and drive changes to their vision.
“When you’re in microgravity, fluid such as your venous blood no longer pools toward your lower extremities but redistributes headward,” says study lead author Larry A. Kramer, MD, from the University of Texas Health Science Center. “That movement of fluid toward your head may be one of the mechanisms causing changes we are observing in the eye and intracranial compartment.”
The study sought to investigate these underlying mechanisms, and learn more about how they might tie in with changes to the brain’s white matter. The team conducted brain MRIs of 11 astronauts before they traveled to the ISS, and then again one day after they returned. Scans were then performed at several interval across the following year.
“What we identified that no one has really identified before is that there is a significant increase of volume in the brain’s white matter from preflight to postflight,” Kramer says. “White matter expansion in fact is responsible for the largest increase in combined brain and cerebrospinal fluid volumes postflight.”
These changes remained visible one year after spaceflight, which the researchers say indicates they could be permanent alterations. Past research has suggested that changes in the volume of cerebrospinal fluid (CSF) specifically could be a key driver of Visual Impairment Intracranial Pressure in astronauts. The authors of the new study also observed an increase in the velocity of CSF through the cerebral aqueduct, along with deformation of the pituitary gland, which they believe is related to higher intracranial pressure in microgravity.
“We found that the pituitary gland loses height and is smaller postflight than it was preflight,” Kramer says. “In addition, the dome of the pituitary gland is predominantly convex in astronauts without prior exposure to microgravity but showed evidence of flattening or concavity postflight. This type of deformation is consistent with exposure to elevated intracranial pressures.”
The same researchers are investigating how these effects could be neutralized in space. Possibilities include the use of a large centrifuge that can accommodate astronauts and spin them around to subject them to artificial gravity, and the use of negative pressure on the lower extremities to prevent the upward migration of fluids. And the research could also benefit people who haven't been into space.
“If we can better understand the mechanisms that cause ventricles to enlarge in astronauts and develop suitable countermeasures, then maybe some of these discoveries could benefit patients with normal pressure hydrocephalus and other related conditions,” says Kramer.
The research was published in the journal Radiology.
