According to a new study, 70% of astronauts who have spent between six and 12 months aboard the International Space Station (ISS) experienced significant changes to their vision due to a condition called “spaceflight-associated neuroocular syndrome,” or SANS.
SANS involves symptoms like swelling of the optic nerve, flattening at the back of the eye and general changes in vision. It occurs when fluids in the body shift while exposed to microgravity conditions, thereby putting pressure on the eyes.
The good news is that these changes often reverse after astronauts return to Earth, and in some cases, wearing corrective eyeglasses is enough to manage the symptoms while they’re happening. However, the long-term effects of prolonged exposure to microgravity remain uncertain, posing a significant challenge for space agencies that aim to see longer missions, such as those to Mars, become a reality.
With no proven preventive or treatment strategies currently in place, finding solutions is a top priority for ensuring astronaut health on extended space journeys.
Space agencies have been aware of SANS since the early 2000s, and researchers are actively trying to figure out the specific dynamics of the condition and cement a potential solution. One of the earliest studies concerning this topic, conducted on Russian cosmonauts who had participated in long-duration missions aboard the Mir space station, reported similar ocular changes — though the condition had not yet been identified as SANS. NASA officially recognized and named the syndrome in 2011, defining it as “distinct ocular, neurological and neuroimaging findings.” The primary suspected cause of SANS is the shift of bodily fluids toward the head in microgravity, leading to increased pressure on the brain and eyes. However, the exact mechanisms remain under investigation.
“Several theories have been suggested such as hemodynamic fluid shift, exposure to CO2 and exercise in microgravity conditions,” the science team, led by Santiago Costantino at the Université de Montréal, wrote in the new study. “Understanding the changes in the mechanical properties of ocular tissue could not only shed new light into the disease’s pathophysiology but potentially assist in both the identification of individuals at a higher risk of developing irreversible eye damage and in the development of countermeasures against SANS.”
In their study, Costantino and his team analyzed data from 13 astronauts who spent five to six months aboard the International Space Station. The group, with an average age of 48, included astronauts from the U.S., Europe, Japan, and Canada. Thirty one percent were women, and eight were on their first mission.
The researchers examined three key eye measurements before and after spaceflight: ocular rigidity, which reflects the stiffness of eye tissue, intraocular pressure, the fluid pressure inside the eye, and ocular pulse amplitude, the variation in eye pressure with each heartbeat.
They measured ocular rigidity using a special imaging technique to get clearer pictures of the choroid, which is the layer of blood vessels in the eye. For the other two measurements, they used tonometry, a common tool to check the pressure inside the eye.
The study revealed significant changes in the biomechanical properties of the astronauts’ eyes, including a 33% drop in ocular rigidity, an 11% decrease in intraocular pressure, and a 25% reduction in ocular pulse amplitude. These shifts were linked to symptoms such as a decrease in eye size, changes in focal field, and, in some cases, swelling of the optic nerve and retinal folds.
Additionally, the researchers found that five astronauts had a choroidal thickness exceeding 400 micrometers, which is higher than normal. Typically, the average choroidal thickness in healthy adults is between 200 and 300 micrometers. Interestingly, this change appeared to be unrelated to age, gender, or prior spaceflight experience.
Researchers and space agencies are working on countermeasures and treatments, including pharmaceutical interventions, nutrition and tools that apply negative pressure to the lower body to help draw fluids away from the head.
Studies like this, which improve our understanding of the syndrome’s effects on the body, will help accelerate the development of solutions.
“The observed changes in the mechanical properties of the eye could serve as biomarkers to predict the development of SANS,” Costantino said in a statement. “This would help identify at-risk astronauts before they develop serious eye problems during long-duration missions.”