Clots in space: How an astronaut's blocked vein brought medical insight


"Space medicine" took another small step forward after an astronaut who developed a blood clot in a neck vein was diagnosed and treated while onboard the International Space Station (ISS), physicians at NASA and elsewhere report.

The research team didn't reveal the astronaut's name, age or gender, but said the ISS crew member developed an asymptomatic thrombosis – blood clot – in the jugular vein, the major vein draining blood from the brain back to the heart.

Episode of space medicine

Back on Earth, such a case could be quickly remedied in the nearest emergency room. But the logistics of doing so in space were far more complicated, said the team that included Dr James Pattarini of Houston's National Aeronautics and Space Administration (NASA) Johnson Space Center and Dr Serena Aunon-Chancellor, of the Louisiana State University Health Science Center, in Baton Rouge.

Reporting the details of the incident in the New England Journal of Medicine, they said that in this episode of space medicine, medical decisions occurred "across multiple space agencies to overcome the numerous logistic and operational challenges".

According to the doctors, it was only by sheer luck that the astronaut's blood clot was discovered at all.

The crew member was taking part in a "vascular research study" that involved intermittent ultrasound examinations of blood vessels before, during and after the ISS space mission.

Although the astronaut showed no symptoms of vein blockage – no headache or facial redness – the jugular vein was abnormally "prominent" during a physical exam, and a follow-up ultrasound confirmed a clot.

After multiple "telemedicine" discussions with medical staff back on Earth, it was decided that the astronaut would be treated with the blood thinner enoxaparin (Lovenox), 20 vials of which had been part of the space station's medical kit.

Normal gravity

The dose was reduced, however, so that the astronaut could be treated until other blood thinners could be dispatched to the space station. Forty-two days after the crew member's clot had been diagnosed, a switch in medications was made, from enoxaparin to apixaban (Eliquis).

The clot slowly shrank over months of treatment, but blood flow through the jugular was still not fully back to normal, even three months after treatment.

However, when the astronaut finally returned to Earth – and normal gravity – blood flow in the jugular returned to normal, and treatment was discontinued. In fact, 10 days after landing the clot was completely gone.

Two experts in circulatory health who read over the report said it gives fascinating new insight into how zero-gravity conditions could compromise blood flow.

The astronaut's clot was a form of deep vein thrombosis (DVT), but these clots most often form in the legs, noted Dr Craig Greben, chief of interventional radiology at Northwell Health in New Hyde Park, New York.

It seems that "the weightlessness astronauts experience during space missions may be another unstudied cause of DVT that requires rigorous research, because it can be silent and fatal, and space travel is only increasing," Greben said.

Abnormal venous flow

Dr Maja Zaric agreed. She's an interventional cardiologist at Lenox Hill Hospital in New York City.

Zaric said the astronaut was in real danger from the clot.

"The size and proximity of documented blood clot to the heart could have easily put [the crew member] into harm's way, as it could have travelled down to the heart causing pulmonary embolism, or up extending into the head and brain veins," she explained.

And Zaric noted that ultrasounds conducted on other crew members confirmed that zero-gravity conditions can radically change the dynamics of blood flow.

"In six out of 11 studied astronauts, there was abnormal venous flow detected," she said. Instead of the steady forward movement that pushes blood through veins, the astronauts exhibited a "to and fro" or "sloshing" movement, Zaric explained. That does not "ensure effective return of head and brain blood back to the heart," she said.

In essence, gravity appears key to healthy blood flow, and without it a "stasis" appears to occur within vessels, Zaric said.

Power of telemedicine

The research team said the astronaut's survival was a tribute to coordinated medical care. But the case also highlights a new spaceflight danger.

It's now imperative that research continue into "the development of prevention and management strategies for venous thromboembolism in weightlessness, especially with future plans for prolonged space travel to the Moon and Mars," the team wrote.

Greben agreed.

"In this space case, the power of telemedicine from a room on the International Space Station to the doctor's office on Earth is sensational," he said. "The future is now, and this is what telehealth looks like – a page out of 'The Jetsons' or a 'Star Trek' script."

Image credit: iStock

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