When astronauts encounter a medical risk on the International Space Station 250 miles up from the Earth's surface, it's not exactly easy for a doctor to make a house call.

So when NASA researchers suspected a blood clot in one of their astronauts during a long-duration stay on the space station last year, they had to act quickly to treat the unexpected risk.

The blood clot was detected during a vascular study of 11 astronauts on the station 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 with an average age of 46. The identities of the astronauts were not included in the study.

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

Six of the astronauts experienced stagnant or reverse blood flow, one had a blood clot and another was found to have a potential partial blood clot. None exhibited symptoms and the findings wouldn't have been apparent without the study.

Two months into a six-month mission, an ultrasound of one of the astronaut's veins revealed a blood clot. The astronaut performed a second ultrasound, guided in real time from scientists and radiologists on the ground, to confirm the findings. The radiologists interpreted the results of the ultrasound immediately.

This risk had never been spotted in an astronaut during spaceflight before, so the scientists weighed the risk of the blood clot and its potential to block a vessel in the absence of gravity. They also assessed how treatment might impact the astronaut in this environment.

Dr. Stephan Moll at the University of North Carolina at Chapel Hill's School of Medicine was the only non-NASA physician consulted to help the astronaut in real time.

"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," Moll said. "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 said. "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."

Moll spoke with the astronaut during a "phone call from space," consulting with them as if they were one of his other patients.

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

Injections themselves can be challenging on the station. Syringes, like other supplies, are limited and zero gravity has an effect on liquids as they are drawn from containers. The astronaut took a higher dose of the injectable, enoxaparin, for 33 days to control the risk of the blood clot. That dose was lowered after 33 days as the astronaut awaited the arrival of the drug apixaban.

The researchers watched the clot shrink over time. Blood flow was induced after 47 days through the vein, but actual spontaneous blood flow was not achieved even after 90 days of treatment.

After landing, the blood clot disappeared once 24 hours had passed. Six months later, the astronaut was still asymptomatic. The astronaut also had no family history of blood clots.

"These new findings demonstrate that the human body still surprises us in space," said Dr. Serena Auñón-Chancellor in a statement, 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."

But stagnant blood flow and the blood clot made NASA aware of this potential risk, especially when considering future long duration missions to the moon or Mars.

"The biggest question that remains is how would we deal with this on an exploration class mission to Mars?" Auñón-Chancellor said. "How would we prepare ourselves medically? More research must be performed to further elucidate clot formation in this environment and possible countermeasures."