In recent years, astronauts have reported their vision changing as a result of long-duration spaceflight. Pre- and post-flight studies of astronauts’ eyes showed flattening along the backside of the eyeball, and scientists hypothesized that the redistribution of body fluids that occurs in microgravity could be reshaping astronauts’ eyes by increasing the intracranial pressure in their skulls.
A new study tested this hypothesis with the first-ever measurements of intracranial pressure during microgravity flights and during extended microgravity simulation (a.k.a. bedrest with one’s head pointed downward). The authors found that humans here on Earth experience substantial changes in intracranial pressure depending on our posture – while upright we experience much lower intracranial pressure than we do when we’re lying flat. In both microgravity flights and simulation, patients had intracranial pressures that were higher than earthbound upright values but lower than what is experienced when lying flat on Earth.
Since we humans on Earth spend about 2/3rds of our time upright and 1/3rd prone, our bodies are accustomed to regular variations in intracranial pressure. In space, astronauts don’t receive that regular unloading of intracranial pressure we have when we’re upright. So now researchers suggest that it is the lack of daily variation in intracranial pressure that is the culprit behind astronauts’ vision changes – not the absolute value of the pressure itself. (Image credit: NASA; N. Alperin et al.; research credit: J. Lawley et al.)