Do astronauts in microgravity have increased risk of developing blood clots?

Homo sapiens in space – an entry in the 2020 SCINEMA International Science Film Festival – is a tongue-in-cheek short film observing astronauts of the species Homo sapiens in their extraordinary new habitat: the International Space Station (ISS). 

With commentary from Frederick Atom-Burger (not to be confused with Sir David Frederick Attenborough, though somehow, they sound incredibly similar), we gain insight into the unusual daily rituals of astronauts living in microgravity in this humorous take on a nature documentary.

But while it’s fun to think about astronauts sleeping upside down in sleeping bags, there are major downsides to living in microgravity – it can have detrimental effects on the human body and health.

It’s well-known that spending prolonged periods in space damages astronauts’ eyesight, decreases muscle mass and bone density, and even alters the brain.

A new study, published this week in the journal Scientific Reports, has found that long-duration space flight alters the cerebrospinal fluid-filled spaces along veins and arteries in the brain.

The research involved imaging the brains of 15 astronauts before and after extended tours of duty on the ISS. When comparing these before and after images, they found an increase in the perivascular spaces within the brains of first-time astronauts, but no difference among astronauts who had previously served aboard the ISS.

This suggests that experienced astronauts may have reached some kind of homeostasis.

Microgravity also has profound effects on the cardiovascular system, as without the effects of gravity, blood and other body fluids are pushed up from the legs and abdomen toward the heart and head.

Does this mean astronauts are more likely to develop blood clots during space missions due to the effects of microgravity?

That’s the question NASA is currently trying to answer, after an astronaut developed a deep vein thrombosis (DVT) in their jugular vein two months into their mission on the ISS in 2020.

The blood clot was asymptomatic and was discovered completely by chance while the astronaut was taking ultrasounds of their own neck for a research study on how body fluid is redistributed in microgravity.

This was the first time a blood clot had been found in an astronaut in space, and there was no established method of treatment. So they enlisted the help of Stephan Moll – a clinical haematologist and professor in the Department of Medicine at the University of North Carolina in the US – in probably the longest-range collection of telemedicine appointments ever.

Now, the results of a long-term surveillance program spurred by this event have been reported in a new study in the journal Vascular Medicine.

Moll and fellow researchers monitored 11 astronauts over a combined total of 2,150 days in microgravity aboard the ISS.

All astronauts were evaluated before leaving Earth to get a baseline of their blood flow and blood vessel size in the veins of their neck. Once in space, astronauts performed ultrasounds on their own necks with guidance from a radiology team on Earth to monitor any changes that occurred.

“We expected some changes in flow based on the absence of gravity,” says Moll. “Gravity pulls fluid in your body down. It also creates a force on your blood vessels and this increased pressure in the veins of the legs leads to leakage of fluid from the blood vessels into the soft tissues. You can notice this when you stand for a long time and develop swelling in your ankles, feet and sometimes hands.”

But according to Moll, without gravity, fluids like blood redistribute in the body.

“When astronauts arrive in space, the lack of gravity causes the blood vessels in the neck to expand due to fluid shifting to the upper part of the body,” he explains. “Astronauts develop swelling of the neck and face from this shift. That is a normal and expected finding.”

In this study, while abnormal flow characteristics were recorded in six of the 11 astronauts, none developed blood clots.

However, the findings of slowed blood flow in the neck veins, abnormal echo findings on the examination, and even reversal of blood flow in two astronauts, raises the question whether these abnormalities could predispose these space travellers to blood clots.

But due to the small nature of the study the researchers can’t make any firm conclusions currently, and unfortunately it might take some time to do so, due to the small pool of astronauts available to participate in future studies.

In the meantime, this research will help to inform what medical supplies, like blood thinners, should be available for current and future spaceflight missions. The data will also help guide the use and development of interventions designed to minimise the potentially increased blood clot risk of spaceflight.

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