In 2005, astronaut John Phillips took a break from his work on the International Space Station and looked out the window at Earth. He was about halfway through a mission that had begun in April and would end in October.

When he gazed down at the planet, the Earth was blurry. He couldn’t focus on it clearly. That was strange – he’d always had 20/20 vision. He wondered: was his eyesight getting worse?

“I’m not sure if I reported that to the ground,” he said. “I think I didn’t. I thought it would be something that would just go away, and fix itself when I got to Earth.”

It didn’t go away.


During Phillips’ post-flight physical, NASA found that his vision had gone from 20/20 to 20/100 in six months.

Rigorous testing followed. Phillips got MRIs, retinal scans, neurological tests, and a lumbar puncture. The tests showed that not only had his vision changed, his eyes had changed as well.

The back of his eye had gotten flatter, pushing his retina forward. He had choroidal folds, which are like stretch marks on the back of the eye. His optic nerve was inflamed.

Phillips became the first widely recognized case 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 the syndrome. On Earth, gravity pulls bodily fluids down toward the feet. That doesn’t happen in space, and it’s thought that extra fluid in the skull increases pressure on the brain and the back of the eye.

At first, NASA thought that Phillips was an isolated case. But then researchers found evidence of VIIP in other astronauts. VIIP has now been recognized as a widespread problem, and there has been a struggle not only to understand its cause, but to study it at all.


The theory that fluid builds up in the skull during space flight hasn’t actually been tested. The only proven methods of measuring intracranial pressure are invasive: a lumbar puncture or drilling a hole into the skull.

“There’s the risk for infection and just doing the procedure, quite frankly, in space is difficult,” said J.D. Polk, a senior flight surgeon at NASA. “Having to anchor somebody and do a spinal tap in space is not something we would relish.”

Here on Earth, the most similar condition is idiopathic intracranial hypertension (IIH). Patients with this condition also have increased pressure in their heads, and they experience visual changes like the astronauts.’ Another condition, papilledema, involves optic nerve swelling.

But they’re not perfect models for the astronauts’ disorder. IIH is idiopathic, which means no one knows what causes it. It comes with a deluge of other symptoms, like nausea, dizziness, and severe headache, that astronauts with VIIP don’t experience. And the medication for papilledema doesn’t work on astronauts with VIIP.

Karina Marshall-Goebel at the Institute of Aerospace Medicine in Germany is trying to study VIIP using a head-down tilt test. Participants’ entire bodies are tilted slightly to simulate the fluid shift in space, but she said it’s not ideal. The study is still affected by gravity, and they can’t keep people tilted for as long as astronauts live in space.

“It’s a unique environment, you can’t replicate it without going into space,” she said. “And you always have to keep that in the back of your head.”

Other researchers are searching for less invasive ways to assess brain health on Earth and potentially in space. Eric Bershad, an intensive care neurologist at the Baylor College of Medicine, is working on a way to measure brain pressure using ultrasound of an eye artery.

Other devices currently being developed use sound waves and radio waves to try and measure the brain through the skull, ears and eyes.

“So far, none of the non-invasive technologies are accurate enough to replace the invasive measurement, but I think within the next few years there is a good chance there will be,” Bershad said.

Ross Ethier, a biomedical engineer at Georgia Tech, is using models to simulate what happens in the body when intracranial pressure goes up. He’s looking at a potential mechanical solution: a device that could draw fluid back down to the legs in space.

Michael Barratt, the former head of NASA’s Human Research program and space medicine specialist, argues for a more radical approach.

Barratt thinks solving the puzzle of VIIP will take testing intracranial pressure in space.

VIIP could be the first sign of greater dangers to the human body from microgravity. “We’re seeing the visual and neural, ophthalmic manifestations of it,” said Barratt. “I’m fairly certain this is a bit more global than that.”