Visual impairment intracranial pressure syndrome (VIIP) is named for the leading theory to explain it. On Earth, gravity pulls bodily fluids down toward the feet. That doesn’t happen in space, and it is thought that extra fluid in the skull increases pressure on the brain and the back of the eye.
[The] mysterious syndrome that affects 80 percent of astronauts on long-duration missions in space. The syndrome could interfere with plans for future crewed space missions, including any trips to Mars.