From an interesting story in the Washington Post:
During Phillips’s post-flight physical, NASA found that his vision had gone from 20/20 to 20/100 in six months.More at the link.
Rigorous testing followed. Phillips got MRIs, retinal scans, neurological tests and a spinal tap. The tests showed that not only had his vision changed, but his eyes had changed as well.
The backs of his eyes had gotten flatter, pushing his retinas forward. He had choroidal folds, which are like stretch marks. His optic nerves were inflamed.
Phillips’s case became the first widely recognized one of a 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.
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.VIIP has now been recognized as a widespread problem, and there has been a struggle to understand its cause — and even to study it.