Remember how your mother would yell at you as a kid if you were upside down too long—“Be careful. All the blood will rush to your head!” While that’s not exactly how it works, your mother’s favorite saying did have a certain amount of truthfulness to it.
Earth’s gravitation field creates a hydrostatic gradient, a pressure of fluid that progressively increases from your head down to your feet while standing or sitting. This pressure gradient, however, is only present on Earth, not space. When in space, fluid, such as your venous blood, no longer pools toward your lower extremities but redistributes headward—thus, the blood rushing to your head. Scientists hypothesize that this movement of fluid may be responsible for causing ocular structural changes in astronauts following long-duration exposure to the microgravity of space.
In a new study published in Radiology, researchers from the University of Texas Health Science Center at Houston suggest the impact of long-duration space travel is even more far-reaching than the ocular system, potentially causing brain volume changes and pituitary gland deformation.
Lead author Larry Kramer, M.D., performed brain MRIs on 11 astronauts before they traveled to the ISS. Kramer and his team then followed up with MRI studies a day after the astronauts returned home, and at several intervals throughout the ensuing year.
The MRI results showed multiple changes in and around the brain of astronauts exposed to microgravity for an extended period of time, including expansion in combined brain and cerebrospinal fluid (CSF) volumes, increase in white matter and alterations to the pituitary gland.
“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 said. “White matter expansion is responsible for the largest increase in combined brain and cerebrospinal fluid volumes postflight.”
The combined volumes remained elevated at one-year postflight, suggesting permanent alteration. Additionally, the astronauts’ pituitary gland was found to be smaller and flatter postflight than it was preflight. According to Kramer, this type of deformation is consistent with exposure to elevated intracranial pressures.
Some changes the researchers detected preflight to postflight are similar to Earth-bound conditions. For example, they observed a postflight increase in volume in the astronauts' lateral ventricles, the spaces in the brain that contain CSF. While the overall volume change would not be considered outside the range of healthy adults, the changes are similar to those seen in people who have spent long periods of bed rest with their heads tilted slightly downward.
There was also increased velocity of CSF flow through the cerebral aqueduct, a narrow channel that connects the ventricles in the brain. A similar phenomenon has been seen in normal pressure hydrocephalus, a condition in which the ventricles in the brain are abnormally enlarged. Symptoms of this condition include difficulty walking, bladder control problems and dementia. To date, these symptoms have not been reported in astronauts after space travel.
If NASA ever wants to send astronauts beyond low-Earth orbit for long durations, understanding the heath impacts of such a move is the first step. Researchers are already studying ways to counter the effects of microgravity, such as the creation of artificial gravity or the use of negative pressure on lower extremities. Ensuring astronauts can live a healthy life in space is just the first part of the equation, in addition to the ability to grow food and manufacture drugs.
Photo: Pituitary deformity examples in three crewmembers before spaceflight and after spaceflight (day 1). (a) Before flight there is normal upward convexity of the pituitary gland dome (black arrowhead) and a straight pituitary stalk (white arrowhead). In this astronaut, there is no change in the morphologic structure of the pituitary gland or stalk after spaceflight. The anterior pituitary gland (indicated by the a in a) and posterior pituitary gland (p) are indicated. (b) Before spaceflight there is normal upward convexity of the pituitary gland dome. After spaceflight, there is flattening of the pituitary gland dome. Note the CSF within the suprasellar cistern immediately above the dome of the pituitary gland. (c) Before spaceflight there is mild concavity of the pituitary gland dome. After spaceflight there is moderate concavity of the pituitary gland dome with loss of volume and new subtle posterior deviation with slight curvature of the pituitary stalk (arrows). Photo credit: Radiological Society of North America.