How Can Astronauts Avoid Vision Loss from Spaceflight?

Human bodies are sacks of fluids supported by skeletons. The entire human organism has evolved over billions of years on Earth in harmony with the planet’s specific gravity. But when astronauts spend too much time on the ISS in a microgravity environment, the organism responds, the fluids shift, and problems can occur.

One of those problems is with vision, and scientists are working to understand how it happens and what they can do about it.

We’re talking about Spaceflight-Associated Neuro-Ocular Syndrome (SANS). NASA says that 70% of astronauts who spend time on the International Space Station (ISS) experience at least mild SANS. Sometimes, the effect is minor and often temporary. Other times, it’s more severe and can cause long-term vision problems, including partial loss of vision.

Researchers have been dealing with microgravity and its effects on vision for a while. “Spaceflight Associated Neuro-ocular Syndrome (SANS), previously known as Visual Impairment Intracranial Pressure (VIIP), is a major risk associated with long-duration spaceflight,” wrote the authors of a 2020 paper. “During prolonged missions, optic disk edema, posterior globe flattening, decreased near vision, and hyperopic shifts are hallmarks of SANS. This risk stems from the lack of gravity, which causes a headward shift of blood and other body fluids.”

Now, a group of physicians are working with Polaris Dawn to understand the problem.

Polaris Dawn is a private spaceflight initiative operated by SpaceX. It will send four private astronauts on a highly elliptical Earth orbit that will take them 1,400 kilometres (870 mi) away from Earth. This is the furthest any human being has been from Earth since the Apollo missions.

Matt Lyon, MD, from the Medical College of Georgia (MCG) at Augusta University, is leading a team that is working with Polaris Dawn to study SANS.

“The changes start happening on day one,” said Lyon, who also is the J. Harold Harrison M.D. Distinguished Chair in Telehealth. “We are not entirely sure what causes these issues with vision, but we suspect it has to do with a shift in cerebrospinal fluid in the optic nerve sheath. On Earth, gravity pushes that fluid down and it drains out, but in space, it floats up and presses against the optic nerve and retina.”

Lyon and his colleagues are focusing on the optic nerve sheath. The optic nerve is a conduit that carries visual information from the eyes to the brain. Inside the sheath, the nerve is protected by cerebrospinal fluid. The cerebrospinal fluid (CSF) carries toxins away from the eye.

Here on Earth, MCG patented the use of ultrasound to image the optic nerve and its sheath and rapidly visualize damage associated with pressure and fluid changes in the sheath. Now, Lyon and his team are putting a portable ultrasound machine in the hands of the four Polaris Dawn astronauts and training them on how to use it.

But first they’re screening the four astronauts to try to determine which people are more susceptible to SANS. They think that people who suffered concussions or mild traumatic brain injuries (TBIs) in the past are likely more susceptible to SANS.

“We discovered that when the cerebral spinal pressure goes up with mild traumatic brain injuries (TBIs), there is resulting damage to the sheath that is likely lifelong,” he explained. “We think that when astronauts who have experienced concussions or mild TBIs go into space and experience the low-gravity fluid shifts, the sheath dilates from the increase in volume. It is like a tire — a normal tire remains its normal shape as it is filled with air, and the shape doesn’t change. When it’s damaged, like bulges on the side of a tire, the fluid fills the bulges up and the sheath expands. This can cause pressure on the nerve and retina. A damaged sheath is less of a problem on Earth, but in space, the excess fluid has nowhere to go.”

It’s critical that the private Polaris Dawn astronauts image the changes to their optical nerves and sheaths in real-time. Real-time data will help researchers understand if vision changes due to SANS are caused by the sheer volume of fluid, the increased pressure from the fluid, or interactions between the two.

The video below shows how ultrasound is used to scan the eye, including the optical nerve (0:40).

“If it’s just volume, we suspect the cerebrospinal fluid goes up, fills this floppy bag and gets stuck. It’s almost like not flushing your toilets. You’re creating this toxic environment, because the cerebral spinal fluid (CSF) is what carries toxins away from your eyes and nerves, and instead the toxins sit against the optic nerve, killing it,” Lyon said. “But it could be that combined with the increased pressure that comes with increased CSF, which would be like getting intermittent hypertension in your eye.”

The solution to SANS could be a Lower Body Negative Pressure (LBNP) device. These are large, bulky devices that counteract the headward shift of fluids in microgravity by creating ground reaction forces (GRFs). They’re typically airtight chambers that astronauts spend time in. Unfortunately, LBNPs require astronauts to be static while using them. NASA tested them during the International Microgravity Laboratory on Space Shuttle Mission STS-65.

Researchers at the University of California’s Department of Orthopaedic Surgery and Department of Bioengineering are developing a mobile version of an LBNP.

“Our new mobile gravity suit is relatively small, untethered, and flexible in order to improve mobility in space. We hypothesized that this novel mobile gravity suit generates greater ground reaction forces than a standard LBNP chamber,” wrote the authors of the 2020 paper.

Mobile Lower Body Negative Pressure suits are still under development, and scientists need more data. Polaris Dawn can help provide the needed data.

The ultrasound images of the optical nerve are part of a broader research effort that will be conducted during Polaris Dawn. The Medical College of Georgia is one of 23 institutions that the mission is working with. The data that Polaris Dawn returns should help lead to a solution for SANS.