Science/Nature

How Doctors on Earth Treated a Blood Clot on the ISS

Of all the unfortunate places to be when you discover you have a blood clot, the worst has got to be 250 miles up in the sky, zooming around the world at 17,000 miles per hour aboard the International Space Station.

But in a study published last month in The New England Journal of Medicine, doctors detailed how they treated one such unlucky case. The astronaut was participating in a study on vascular flow in space and did a routine ultrasound scan on their neck. From the ground, doctors who saw the images noticed that there didn’t appear to be any blood flow in the astronaut’s left jugular vein. (The patient remains anonymous for privacy reasons.)

A followup ultrasound confirmed the problem: a venous thrombosis, or blood clot. It was the first one ever diagnosed in space. And it was quite the surprise, because the patient was showing no symptoms and had already been screened by NASA for predisposition to blood clots—and passed. Jugular clots have been associated with conditions like cancer, but an unprovoked clot in an otherwise healthy person is uncommon.

For any patient, a blood clot could be a life-threatening discovery warranting an immediate trip to the hospital. That’s because a clot can break apart and move into the lungs, leading to potentially fatal blockages there. For an astronaut on the ISS, the mid-flight discovery of a clot would take on a whole new level of complexity.

NASA couldn’t just bring the astronaut home immediately; reentry is a bit like being locked inside a washing machine, and the landing is a bit like a car crash. “We were very concerned that the decision simply to come home would take a patient who’s asymptomatic and doing fine and place them at risk of dislodging a piece of [the clot] and going to the lung or elsewhere,” says NASA flight surgeon James Pattarini, coauthor on the case study.

Luckily, the ISS carries an onboard stash of medications, including an anticoagulant, or blood thinner. A thinner prevents the clot from growing and prevents new clots from forming. The patient began injecting it immediately—but the station had only a 40-day supply. So on a resupply mission, the doctors sent up an oral anticoagulant, which the astronaut took for several months before doctors deemed it safe for them to return to Earth.

Within just 10 days of the astronaut arriving back on land, the clot had disappeared. Because this patient wasn’t at risk for blood clots in the first place, and because it went away once they returned to Earth’s atmosphere, the study’s authors think it has something to do with space itself. “Which really nails home that it’s the microgravity environment we think is playing a significant role here,” says Pattarini. “I think the biggest question is, if this has been occurring and it’s just something we’re going to routinely see in human spaceflight, what does that mean for doing these longer exploration-class missions?”

To learn more about the unprecedented medical situation aboard the ISS, check out our interview with Pattarini in the video above.


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