In a first, an unnamed astronaut had been treated for a blood clot while on a six-month mission on ISS sometime in the last few years.
Ultrasound examinations of the astronauts’ internal jugular veins were performed at scheduled times in different positions during the mission. Results of the ultrasound performed about two months into the mission revealed a suspected obstructive left internal jugular venous thrombosis (blood clot) in one astronaut. The astronaut, guided in real time and interpreted by two independent radiologists on earth, performed a follow-up ultrasound, which confirmed the suspicion.
Since NASA had not encountered this condition in space before, multiple specialty discussions weighed the unknown risks of the clot traveling and blocking a vessel against anticoagulation therapy in microgravity. The space station pharmacy had 20 vials containing 300 mg of injectable enoxaparin (a heparin-like blood thinner), but no anticoagulation-reversal drug. The injections posed their own challenges – syringes are a limited commodity, and drawing liquids from vials is a significant challenge because of surface-tension effects.
The astronaut began treatment with the enoxaparin, initially at a higher dose that was reduced after 33 days to make it last until an oral anticoagulant (apixaban) could arrive via a supply spacecraft. Anticoagulation-reversing agents were also sent.
Although the size of the clot progressively shrank and blood flow through the affected internal jugular segment could be induced at day 47, spontaneous blood flow was still absent after 90 days of anticoagulation treatment. The astronaut took apixaban until four days before the return to Earth.
On landing, an ultrasound showed the remaining clot flattened to the vessel walls with no need for further anticoagulation. It was present for 24 hours after landing and gone 10 days later. Six months after returning to Earth, the astronaut remained asymptomatic.
What is not known is whether weightlessness caused the clot, or whether it would have occurred regardless. The former seems very possible as the astronaut had no history of such clots, and returned to normal almost immediately upon return to Earth. As noted at the link, more research is necessary, especially in anticipation of long interplanetary flights.