70% of astronauts who spend time on the International Space Station (ISS) experience swelling at the back of their eyes, causing blurriness and impaired eyesight both in space and when they return to Earth. Sometimes, it’s permanent. Understanding the way microgravity affects the eyes, and the human body as a whole is an essential part of preparations for future long-duration spaceflights to the Moon and Mars. In an effort to understand the cause of these eye problems, researchers at the Medical University of South Carolina used MRI scans of twelve ISS astronauts to measure the intracranial venous system (veins that circulate blood to the brain) before and after flight. They’ve determined that there is a strong connection between the swelling of these veins and the onset of eye trouble.
Poor eyesight is just one of the medical challenges facing humans in space. When not having to fight against Earth’s gravity, muscles become weak and bone density decreases, while the high radiation environment of space threatens to cause long-term diseases like cancer. The ISS is largely designed as a microgravity laboratory, and many of the experiments carried out there are medical in nature, with astronauts themselves as willing test subjects – everything learned from these studies helps keep them and their peers safe. As a result, a wealth of medical data has been gathered that will reduce the risk to future spacefarers.
The effect of spaceflight on the eyes is known as spaceflight-associated neuro-ocular syndrome (SANS). SANS is so common, according to Dr. Mark Rosenberg, one of the study’s authors, that “it’s gotten to the point where astronauts actually carry extra pairs of glasses when they go into space. They know that their vision is going to be deteriorating up there, and they’ve even started calling them space anticipation glasses.”
NASA astronaut Chris Ferguson has his eyes imaged using ultrasound at the Johnson Space Center Flight Medicine Clinic, 2011, in Houston. Credit: NASA.
The physical changes in the eyes include flattening of the globes, injury to the retinas, and the swelling of nerves in the eyes. For some astronauts, the eyes recover within weeks of returning to Earth, but the healing process can sometimes take much longer.
The root cause of SANS, according to the paper Rosenberg and his colleagues produced, seems to be correlated with veins swelling behind the eyes. Weightlessness causes the distribution of blood in the body to change, with fluid moving towards the head and eyes more than is usual on Earth, where gravity reduces the flow to these areas.
These results imply “individuals with increased venous sinus compliance may be at increased risk of developing SANS,” and therefore pre-screening can help astronauts understand the risks to their eyes before they ever leave the Earth.
There is still, of course, more to learn. For one thing, the team hopes to do more research on how SANS risks might differ between men and women, using a larger sample size of astronauts (the current research is based on 2 female and ten male astronauts). They’d also like to install a mobile MRI machine on the ISS, which would allow them to do brain scans in space. The post-flight scans used in the current research leaves open the possibility that the changes they see in the intracranial venous system occur on return to Earth, rather than in orbit, and the team would like the opportunity to rule out that possibility. The team also believes their research will useful for understanding eye disorders for humans here on Earth. What is learned in space has applications in medical science right here at home.
Celia Spell, “Living in outer space: Changes in blood flow volume may be at the heart of worsening eyesight” MUSC Catalyst News.
Rosenberg MJ, Coker MA, Taylor JA, et al. “Comparison of Dural Venous Sinus Volumes Before and After Flight in Astronauts With and Without Spaceflight-Associated Neuro-Ocular Syndrome.” JAMA Netw Open. 2021.
Featured Image: Astronaut Serena Auñón-Chancellor examines her eye aboard the International Space Station with remote support from doctors on the ground (2018). Credit: NASA.