When astronauts suddenly experience a medical situation on the International Space Station 250 miles above Earth, the terms “emergency room” or “urgent care” take on a unique meaning.

Late last year, NASA researchers suspected that one of their astronauts was suffering from a blood clot during a long duration stay on the space station.

The clot was detected during a vascular study of 11 astronauts that was intended to assess the effect of space on the internal jugular vein. In zero gravity, astronauts’ blood and tissue fluid shifts toward the head.

The study involved nine men and two women who were an average age of 46. Their identities were not included in the study.

A new assessment of the blood clot was published last Thursday in the New England Journal of Medicine.

Six of the participating astronauts experienced stagnant or reverse blood flow, another one had a blood clot, and yet another was considered to have a potential partial blood clot.

Scientists weighed the risk of the blood clot, as well as its potential to block a vessel in the absence of gravity.

Dr. Stephen Moll, from the University of North Carolina at Chapel Hill’s School of Medicine, was the only non-NASA physician who was consulted to help the affected astronaut.

He says, “My first reaction when NASA reached out to me was to ask if I could visit the International Space Station to examine the patient myself. NASA told me they couldn’t get me up to space quickly enough, so I proceeded with the evaluation and treatment process from here in Chapel Hill.”

Moll is a member of UNC’s Blood Research Center and is a blood clot expert.

“Normally the protocol for treating a patient with deep vein thrombosis would be to start them on blood thinners for at least three months to prevent the clot from getting bigger and to lessen the harm it could cause if it moved to a different part of the body such as the lungs,” Moll adds. “There is some risk when taking blood thinners that if an injury occurs, it could cause internal bleeding that is difficult to stop. In either case, emergency medical attention could be needed. Knowing there are no emergency rooms in space, we had to weigh our options very carefully.”

He spoke with the astronaut during a “phone call from space,” consulting with them as if the person were one of his other patients.

The pharmacy aboard the space station contained 20 vials with 300 milligrams each of an injectable blood thinner. Moll directed the astronaut to use them on a daily basis until an anticoagulant drug could be sent to the station during a resupply mission.

The astronaut took a higher dose of the injectable, called enoxaparin, for 33 days in order to control the risk of the blood clot. The dose was lowered after that time, as the astronaut awaited the arrival of the drug apixaban.

The researchers watched the clot shrink over time. Blood flow was then induced after 47 days through the vein, although spontaneous blood flow was not achieved, even after undergoing treatment for 90 days.

The blood clot disappeared 24 hours after landing. Six months later, the astronaut was still free of symptoms.

According to Dr. Serena Auñón-Chancellor, study author, NASA astronaut and clinical associate professor of medicine at Louisiana State University’s Health New Orleans School of Medicine, “We still haven’t learned everything about Aerospace Medicine or Space Physiology.”

She adds, “The biggest question that remains is how would we deal with this on an exploration class mission to Mars? How would we prepare ourselves medically? More research must be performed to further elucidate clot formation in this environment and possible countermeasures.”