In microgravity, astronauts’ brains fill with fluid and it doesn’t drain for months after they return to Earth, which may be why their vision worsens
Source:
https://www.newscientist.com/article/2201693-astronauts-may-have-vision-problems-because-of-liquid-in-their-brains/?utm_campaign=RSS%7CNSNS&utm_source=NSNS&utm_medium=RSS&utm_content=home