Solving a Medical Mystery Crucial to Future Space Travel

Laird Harrison

November 13, 2019

SAN FRANCISCO — Astronauts often wrestle with blurry vision during spaceflight and this problem is jeopardizing longer missions to Mars.

Half of the astronauts on the International Space Station have already experienced reduced visual acuity, most noticeable at near distances. And specialists here on Earth have observed combinations of choroidal folds, disc edema, and cotton wool spots in their eyes.

On the space station, the astronauts test themselves or one another, pulling each diagnostic instrument from a tangle of wires and equipment that covers every surface inside the station.

"It's interesting how amazing these folks are at doing tests on themselves," said Andrew Lee, MD, from the Baylor College of Medicine Center for Space Medicine in Houston. Astronauts have reported decreased near vision caused by a hyperopic shift of up to 1.5 diopters after as little as 3 weeks in microgravity.

The leading theory to explain these findings is that cephalad fluid shifts in microgravity. Without gravity pushing bodily fluids downward, the fluids redistribute toward the upper portions of the body, including the head.

Theoretically, the increase in intracranial pressure could be disproportionate to the increase in intraocular pressure, which might lead to a reduction in the head and eye pressure gradient across the lamina cribrosa at the posterior aspect of the eye.

In some ways, this condition resembles idiopathic intracranial hypertension, which is most common in obese young women, and most astronauts are otherwise healthy and fit.

It has been difficult to study what is happening to these astronauts, Lee acknowledged. First, the sample size is small; only a few people ever go to space. Second, it costs $20,000 per pound to send diagnostic instruments into space.

And third, NASA depends on the Russian space agency to ferry astronauts to and from the space station. It takes a long time for them to get from Kazakhstan, where they land, to Houston. When they arrive, they suffer from other conditions, such as muscle atrophy. "They can't stand," he explained. "If they do, they pass out; they have to be carried."

Threat to Mars Mission

Physicians began documenting abnormalities in the eyes of astronauts in 2003, when they found choroidal folds, Lee recalled here at the American Academy of Ophthalmology 2019 Annual Meeting.

Since 2016, optical coherence tomography has uncovered increases in total retinal thickness and increases in the thickness of the circumpapillary retinal nerve fiber layer, without a change in choroidal thickness.

A reduced-gravity aircraft — known as the "vomit comet" — can simulate weightlessness by flying up and down in a parabolic course. The weightlessness only lasts for half a minute at a time, but an international team of researchers took advantage of it to study the effects on intracranial pressure.

They recruited five men and three women who were previously implanted with Ommaya reservoirs, which are catheters from the lateral ventricle to cerebral spinal fluid-filled reservoirs used for drug delivery to treat hematologic malignancy. The researchers inserted fluid-filled butterfly needles into the Ommaya reservoirs and connected them to pressure transducers.

Failure is not an option.

While still on Earth, intracranial pressure in the study participants' eyes decreased when they were sitting upright and increased when they were supine. At zero gravity, intracranial pressure was between these two points taken on Earth — lower than when upright but higher than when supine.

The researchers speculate that replacing this circadian rhythm with a constant pressure between the two normal levels could be responsible for the optic remodeling.

Whatever causes spaceflight neuro-ocular syndrome, the effects on visual acuity have so far been transient. The medical officers at NASA have prepared astronauts for the problem by supplying them with corrective lenses. "We call them space anticipation lenses," Lee quipped. "You know them as reading glasses."

On Earth, the vision of the astronauts eventually returns to normal, although sometimes capillary folds remain. "No one has gone blind," he reported.

Asked about animal experiments, he said that there is no animal model for spaceflight neuro-ocular syndrome, but his group has documented some changes in the eyes of rats. He and his colleagues are currently planning to test the effects on rat eyes with gravity simulated by a centrifuge.

Audience members suggested myriad other tests, but in addition to the cost of sending instruments into space, Lee said, the astronauts have too many demands on their time to do much more testing than they already are.

"But failure is not an option. This is the mentality driving your space program," he said. "And we cannot let neuro-ocular syndrome keep us from exploring space. The answers to this puzzle rests with brain power," he added. "Human brain power. Your power."

American Academy of Ophthalmology (AAO) 2019 Annual Meeting. Presented October 14, 2019.

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