(CNN)A study of 11 healthy astronauts onboard the International Space Station for six-month missions has revealed a new risk of long-term spaceflight.
Six of the astronauts experienced stagnant or reverse blood flow, one had a blood clot and another was found to have a potential partial blood clot.
The study, which involved nine men and two women with an average age of 46, published Wednesday in the journal JAMA Network Open. The identities of the astronauts were not included in the study.
This is the first time researchers have observed these conditions in astronauts and the implications of their discovery could impact future long-term spaceflight, such as a mission to Mars.
After more than 50 years of human spaceflight, researchers know some of the risks posed to the human body by being in zero gravity. Space motion sickness happens in the first 48 hours, creating a loss of appetite, dizziness and vomiting.
Over time, astronauts staying for six months on the station can experience the weakening and loss of bone and atrophying muscles. Astronauts also experience blood volume loss, weakened immune systems and cardiovascular deconditioning, since floating takes little effort and the heart doesn’t have to work as hard to pump blood. Scott Kelly and other astronauts in their late 40s and 50s have also complained about their vision being slightly altered. Some of them have required glasses in flight.
The weightless environment of zero gravity causes a fluid shift in the body toward the head, the opposite of what we experience standing on Earth. On Earth, humans spend about two-thirds of the day in an upright position and about a third laying down at night. This causes a daily fluid shift that varies based on our position.
But for astronauts, the fluid shift is sustained for long periods of time. It causes puffiness in the face, “bird leg” syndrome where the legs lose volume, and decreases plasma volume while increasing stroke volume — the volume of blood pumped per beat.
“A recently identified medical issue with long duration spaceflight on the International Space Station is a constellation of neuro-ocular issues that we’ve coined SANS — Spaceflight Associated Neuro-ocular Syndrome,” said Michael Stenger, study author and director of NASA’s Johnson Space Center Cardiovascular and Vision Laboratory.
“Approximately 10 years ago, we noticed that astronauts were developing optic disc edema, globe flattening, choroidal folds and permanent refractive error changes. The purpose of our experiment was to quantify the headward fluid shift in all astronauts by examining arterial and venous structure and flow characteristics in the head and neck (as well as several other parameters) and determining the relationship between these parameters and ocular structural and functional changes.”
The researchers wanted to assess how this fluid shift affected the left jugular vein. This vein carries deoxygenated blood from the head and neck to the vena cava, the largest vein in the upper body.
The researchers disclosed that one limitation of the study is that they did not image the right jugular vein, but it has been analyzed in previous spaceflight studies and there was no sign of stagnation or clotting.
The astronaut who developed a blood clot was treated with anticoagulants for the rest of the spaceflight and did not participate in the study past day 50.
The observation that blood was clotting in otherwise healthy astronauts, both male and female, due to weightlessness was a surprise to researchers, who are concerned due to the other issues blood clots can cause.