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Weird Medical History · Space

The Blood Clot Found in Orbit — and the Doctor Who Asked if He Could Make a House Call

In a case reported in January 2020, Stephan Moll, a blood-clot specialist in Chapel Hill, North Carolina, got a call from NASA about a new patient. The patient felt fine, had no symptoms, and was about 250 miles straight up. His first question, by his own account, was whether he could go and see them.

By the Curiosities Desk · · 5 min read · Filed under Space · Today’s round

Part of Medicine, Before It Was Sure

The Blood Clot Found in Orbit — and the Doctor Who Asked if He Could Make a House Call. Illustration.

What happened: The Astronaut's Blood Clot

The International Space Station has no hospital and no specialist on call, but it does have an ultrasound machine, which NASA was using for a research study. Long stays in weightlessness shift the body’s fluids toward the head. Researchers wanted to know what that shift does to the internal jugular vein, the large vein in the neck that drains blood from the brain back toward the heart.

Eleven astronauts on long missions took part. Each one had their neck veins scanned on the ground before launch, then scanned themselves in orbit around flight day 50 and again later in the mission, holding the probe while a team on Earth watched the images and coached them where to point it.

The results, published in JAMA Network Open in November 2019, were odd. In six of the eleven astronauts, blood in the left jugular vein had slowed to a near standstill or was flowing backward. And in one astronaut, about two months into a six-month stay, the scan showed something that should not have been there at all: a clot partly blocking the vein.

On Earth, this condition is called a deep vein thrombosis, or DVT: a blood clot inside a deep vein, usually in the leg. It is dangerous mainly because a piece can break off and travel to the lungs. Nobody had treated one in space, because nobody had ever found one there.

A second ultrasound, which the astronaut performed with real-time guidance and which two radiologists on the ground read independently, confirmed the clot. NASA reached out to Stephan Moll, a hematologist (a doctor who specializes in blood) at the University of North Carolina School of Medicine.

“My first reaction when NASA reached out to me was to ask if I could visit the International Space Station,” Moll later told the university. NASA, he said, told him it couldn’t get him to space quickly enough. So he treated his patient from Chapel Hill, by email and by phone. “It was incredible to get a call from an astronaut in space,” he said.

The standard treatment is a blood thinner, and that became a question of stock. According to the case report in the New England Journal of Medicine, the station’s pharmacy held 20 vials of an injectable blood thinner called enoxaparin, and no drug that could reverse the thinning if the astronaut started to bleed. The team started the astronaut on a full dose, then cut it after 33 days to make the supply last until a cargo ship could arrive.

On about day 43 of treatment, a resupply spacecraft delivered apixaban, a blood thinner in pill form. The clot slowly shrank, though blood still wasn’t flowing freely through that stretch of vein after 90 days.

The astronaut stopped the pills four days before heading home. Landing in a capsule is rough on the body, and an injury during re-entry could bleed badly in someone whose blood has been thinned. Back on Earth, the remaining clot had flattened against the vein wall and needed no more treatment. Ten days after landing, it was gone. NASA has never named the astronaut.

Why it was strange: The Astronaut's Blood Clot

Deep vein clots usually come from sitting still: on long flights or after surgery, blood pools in the legs and clots more easily. Astronauts are healthy, carefully screened, and exercise daily. This one had no personal or family history of clots.

And the clot wasn’t in the legs. It was in the neck, a spot where clots are unusual on Earth. Gravity normally helps drain the jugular vein toward the heart whenever we stand or sit up. Without gravity, that help disappears, and the study showed blood in the neck could slow to a crawl or even reverse.

Strangest of all, nobody would have known. The astronaut felt fine. The clot was found only because a research team happened to be pointing an ultrasound probe at that exact vein. Space sickness has been familiar since the early days of spaceflight — see The Senator Who Went to Space and Came Back as a Unit of Nausea — but this was a risk no one had been looking for.

What scientists learned: The Astronaut's Blood Clot

Doctors have long described three conditions that favor a clot: slow blood flow, a damaged vessel wall, and blood that clots too readily. The case showed that weightlessness can supply the first one, inside the neck.

It also showed that remote care can work: an astronaut performed diagnostic-quality ultrasounds on themselves, guided live from the ground, through months of treatment paced by rocket deliveries.

“These new findings demonstrate that the human body still surprises us in space,” said Serena Auñón-Chancellor, the NASA astronaut and physician who led the case report. Moll asked the obvious follow-up: “Is this something that is more common in space?”

A follow-up study he worked on, published in Vascular Medicine in 2022, tracked 11 astronauts over a combined 2,150 days in orbit. Six showed abnormal flow in the jugular vein, including reversed flow in two, but none developed a clot. The study was small, and how often space clots happen is still unknown.

How it reaches us today: The Astronaut's Blood Clot

The case changed how mission planners think about the medicine cabinet. A space station can wait 43 days for a delivery. A crew on the way to Mars could not. “The biggest question that remains is how would we deal with this on an exploration class mission to Mars?” Auñón-Chancellor said.

That means deciding which blood thinners, and which drugs to reverse them, travel with the crew, and teaching astronauts to scan their own bodies. The same approach applies on Earth: a radiology team that can guide a scan 250 miles overhead can guide one in a remote clinic. For another case where a medical problem reshaped a mission, see The Head Cold That Made Apollo 7 Refuse Its Helmets. The case belongs with the other medicine from before the answer was obvious, in Medicine, Before It Was Sure. A later tally of spacesuit wear is The Spacesuit Gloves That Loosened Astronauts’ Fingernails — Mostly in a Houston Swimming Pool.

Fun fact: The Astronaut's Blood Clot

The specialist who treated the first known blood clot in space never got anywhere near it. When he asked to examine his patient in person, NASA told him it couldn’t get him up there quickly enough.

A note on health: deep vein clots are common on Earth and very treatable. If you notice swelling, pain, or warmth in one leg, or sudden shortness of breath, especially after a long flight or surgery, it’s worth getting checked promptly.

Sources: The Astronaut's Blood Clot

#International Space Station #space medicine

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