Venous Thrombosis during Spaceflight

To the Editor: Approximately 2 months into an International Space Station mission, obstructive left internal jugular venous thrombosis was suspected in an astronaut during an ultrasound examination that was performed as part of a vascular research study.The astronaut reported no headache or worsening of the facial plethora that is common in conditions of weightlessness.The astronaut had no personal or family history of venous thromboembolism.The physical examination revealed a prominent ipsilateral external jugular vein.Follow-up ultrasound examination performed by the astronaut, guided in real-time and interpreted by two radiologists on Earth, confirmed the presence of venous thrombosis with subacute characteristics (Fig. S1A in the Supplementary Appendix, available with the full text of this letter at NEJM.org); the contralateral internal jugular vein and the bilateral subclavian, axillary, popliteal, and femoral veins were unremarkable.Multispecialty discussions ensued, weighing the unknown risks of thrombus embolism and retrograde extension into the sinus and cerebral veins against those of anticoagulation.Twenty vials containing 300 mg of enoxaparin each were available in the pharmacy of the space station, but no anticoagulation-reversal agent was available.Treatment with enoxaparin at a dose of 1.5 mg per kilogram of body weight once daily was started; the dose was reduced to 1 mg per kilogram once daily after 33 days to extend therapy until oral apixaban could be delivered to the space station with a supply spacecraft.Protamine and prothrombin complex concentrate were also sent to the space station.Transition to apixaban at a dose of 5 mg twice daily occurred 42 days after the diagnosis of venous thrombo-