The 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).
That's what Université de Montréal opthalmology Santiago Costantino found in a study published last fall in the Journal of Engineering in Medicine and Biology.
At least 70 per cent of astronauts on the ISS have been affected by spaceflight-associated neuro-ocular syndrome, or SANS.
In the biophotonics research unit he runs at the UdeM-affiliated Maisonneuve-Rosemont Hospital, Costantino assembled a group of researchers to identify the biomechanical changes responsible for this disorder.
They analyzed data collected by the Canadian team at NASA on 13 astronauts who spent between 157 and 186 days on the ISS.
The subjects had an average age of 48 and came from the U.S., European, Japanese and Canadian space agencies; 31 per cent were women; eight were on their first mission.
Three parameters involved
The researchers compared three ocular parameters before and after the astronauts' space missions: ocular rigidity, intraocular pressure, and ocular pulse amplitude.
They measured ocular rigidity using optical coherence tomography with a customized video module to improve the quality of images of the choroid. The other two parameters, intraocular pressure and ocular pulse amplitude, were measured using tonometry.
The study found significant changes in the biomechanical properties of the astronauts' eyes: a 33-per-cent decrease in ocular rigidity, an 11-per-cent decrease in intraocular pressure, and a 25-per-cent reduction in ocular pulse amplitude.
These changes were accompanied by symptoms including reduced eye size, altered focal field and, in some cases, optic nerve edema and retinal folds.
The researchers also found that five astronauts had a choroidal thickness greater than 400 micrometres, which was not correlated with age, gender or previous space experience.
"Weightlessness alters the distribution of blood in the body, increasing blood flow to the head and slowing venous circulation in the eye," explained Costantino. "This is probably what causes the expansion of the choroid, the vascular layer that nourishes the retina."
Long-lasting changes
According to the researchers, the expansion of the choroid during weightlessness could stretch the collagen in the sclera, the white outer layer of the eye, causing long-lasting changes in the eye's mechanical properties.
They also believe that blood pulsations under microgravity can create a water-hammer effect in which sudden changes in blood-flow-pressure cause a mechanical shock to the eye, leading to significant tissue remodeling.
Eyes return to normal
According to the researchers, these ocular changes are generally not cause for concern when the space mission lasts six to 12 months. While 80 per cent of the astronauts they studied developed at least one symptom, their eyes returned to normal once back on Earth.
In most cases, wearing corrective eyeglasses was sufficient to correct the symptoms developed aboard 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 eye-health effects of prolonged exposure to microgravity remain unknown, and no preventive or palliative measures now exist.
The Maisonneuve-Rosemont research team is waiting for more data from NASA to continue its investigations.
"The observed changes in the mechanical properties of the eye could serve as biomarkers to predict the development of SANS (spaceflight-associated neuro-ocular syndrome)," said Costantino.
"This would help identify at-risk astronauts before they develop serious eye problems during long-duration missions."
About this study
"Ocular biomechanical responses to long-duration spaceflight," by Marissé Masis Solano, Remy Dumas, Mark Lesk and Santiago Costantino, was published on Sept. 5, 2024 in the Journal of Engineering in Medicine and Biology.
