Astronauts’ Eyes Weaken During Long Space Missions, Raising Concerns About Mars Travel

Low levels of gravity (microgravity) in space cause significant changes in astronauts’ eyes and vision after six to 12 months aboard the International Space Station (ISS), according to a study published in the IEEE Open Journal of Engineering in Medicine and Biology.

Santiago Costantino, an ophthalmologist at the University of Montreal, discovered that at least 70% of astronauts aboard the ISS had spaceflight-associated neuro-ocular syndrome, or SANS.

In the biophotonics research unit that he directs at Maisonneuve-Rosemont Hospital, affiliated with UdeM, Costantino brought together a group of researchers to identify the biomechanical changes responsible for this disorder.

They analyzed data collected by the Canadian NASA team on 13 astronauts who spent between 157 and 186 days on the ISS.

The subjects were on average 48 years old and came from the American, European, Japanese and Canadian space agencies; 31% were women; eight were on their first mission.

Three parameters involved

The researchers compared three ocular parameters before and after astronauts’ space missions: ocular rigidity, intraocular pressure and ocular pulse amplitude.

They measured ocular stiffness using optical coherence tomography with a custom video module to improve the quality of choroid images. The other two parameters, intraocular pressure and ocular pulse amplitude, were measured by tonometry.

The study revealed significant changes in the biomechanical properties of the astronauts’ eyes: a 33% decrease in ocular rigidity, an 11% decrease in intraocular pressure, and a 25% reduction in ocular pulse amplitude.

These changes were accompanied by symptoms, including reduction in eye size, focal field alteration, and in some cases, optic nerve edema and retinal folds.

The researchers also found that five astronauts had choroidal thickness greater than 400 micrometers, which did not correlate with age, gender or previous space experience.

“Weightlessness changes the distribution of blood in the body, increasing blood flow to the head and slowing venous circulation in the eyes,” Costantino explained. “This is likely what causes the expansion of the choroid, the vascular layer that nourishes the retina.”

Lasting changes

According to the researchers, expanding the choroid in zero gravity could stretch the collagen in the sclera, the white outer layer of the eye, causing lasting changes in the mechanical properties of the eye.

They also believe that blood pulsations in microgravity may create a water hammer effect in which sudden changes in blood pressure cause a mechanical shock to the eye, leading to significant tissue remodeling.

Eyes return to normal

According to the researchers, these eye changes are generally not a concern when the space mission lasts six to 12 months. Although 80% of the astronauts studied developed at least one symptom, their eyes returned to normal once they returned to Earth.

In most cases, wearing corrective glasses was enough to correct the symptoms developed on board the ISS.

However, the research community and international space agencies are cautious about the consequences of longer missions, such as a flight to Mars. The effects on ocular health of prolonged exposure to microgravity remain unknown and no preventive or palliative measures currently exist.

The Maisonneuve-Rosemont research team is awaiting more data from NASA to continue their investigations.

“Observed changes in the mechanical properties of the eye could serve as biomarkers to predict the development of SANS (spaceflight-associated neuro-ocular syndrome),” Costantino said.

“This would help identify at-risk astronauts before they develop serious eye problems during long-duration missions.”