Internal jugular vein thrombosis in astronaut

A 42-year-old astronaut is in the second month of a long mission on a space station. She feels well, trains every day and has no pain or shortness of breath. In a routine research exam, she has an ultrasound of the neck guided by specialists on the ground. The images show a dilated left internal jugular vein, with material inside and no visible flow. She uses a combined oral contraceptive to suppress menstruation during the mission. The scene is fictional and was written for teaching, inspired by a published report.

This text uses the scenario as a setting for clinical reasoning. The trap is twofold. The absence of symptoms tempts the team to think there is nothing wrong, and the imaging finding tempts the team to settle on a diagnosis of thrombus without asking whether the image can mislead. In orbit, a treatment must still be decided with limited stock and no laboratory. The text is not a space agency protocol. If you play the case of the day on NEXO, you will recognize the challenge: deciding with few clues and without the test you would order in the hospital.

Key points

• In microgravity, blood shifts toward the head and internal jugular flow can slow or even reverse.
• In a study of 11 crew members, 6 had stagnant or retrograde flow in the left internal jugular vein.
• The first case of venous thrombosis in flight was found by chance, in a person without symptoms.
• Neck pain and headache can be mistaken for space motion sickness.
• Compression ultrasound is the basis of diagnosis, but the technique is difficult in orbit.
• Anticoagulation in flight depends on the stock on board and the risk of bleeding on landing.

Why this is a reasoning scenario

The case brings together a healthy person, an environment that changes circulation and an exam performed by someone who is not a specialist. Harris and colleagues note that, because of limited resources and the lack of symptoms, tests for venous thrombosis are not done routinely in space (Harris and colleagues, 2023). When a finding appears, it comes without the usual sequence of complaint, exam and hypothesis.

Venous thrombosis in travelers is the topic of the post on thrombosis after a long trip, focused on the legs and commercial flights. Here the site is different, the neck, and so is the mechanism: it is not immobility in the seat, but the shift of fluids toward the head.

What the scene teaches

Because she has no complaints, the probability before the exam comes from the context: the time into the mission, the left side of the neck and the estrogen contraceptive. The post on pretest probability shows how these factors change the weight of an imaging finding.

Head circulation without gravity

On Earth, standing up, gravity helps blood from the head descend through the jugular veins. In orbit, that help disappears. The post on space medicine describes the shift of fluids toward the head and the puffy face of the first days. The internal jugular vein is one of the veins that feel this change the most.

A study of 11 International Space Station crew members measured the internal jugular vein with ultrasound. Around flight day 50, 6 crew members had stagnant or retrograde flow in the left internal jugular vein, and 1 developed an occlusive thrombus during the mission (Marshall-Goebel and colleagues, 2019). The authors concluded that weightlessness is associated with stasis in the internal jugular vein, which can lead to thrombosis in healthy astronauts.

What happens in the first seconds

The change begins very early. In parabolic flights, which create a few seconds of weightlessness, the area of the internal jugular vein increased on both sides when entering the zero-gravity phase, and one participant had stagnant flow in the left jugular vein that persisted in level flight (Marshall-Goebel and colleagues, 2024). The left side appears again, as on the station.

Virchow's triad in orbit

Virchow's triad organizes the reasoning: stasis, endothelial injury and a change in coagulation. Harris and colleagues describe that there are signs of change in venous flow, distension and pressure, of endothelial damage and possibly of hypercoagulability in flight, so that all sides of the triad may be altered, although the data are not conclusive (Harris and colleagues, 2023).

A systematic review of venous ultrasound in spaceflight found increased internal jugular area and pressure, associated with reduced, stagnant or retrograde flow, which on average responded to maneuvers such as lower body negative pressure, Valsalva and compression of the jugular on the other side. Compared with the standards used on Earth, in-flight ultrasound did not meet all adequacy criteria, and studies on thrombosis reported a single thrombus (Elias and colleagues, 2024).

The weight of the contraceptive

Estrogen is a known risk factor for thrombosis. In the published report, the astronaut used a combined oral contraceptive to suppress menstruation (Limper and Jordan, 2026). A review on anticoagulants in microgravity records the recommendation to switch estrogen contraceptives to progestogen-only formulations in astronauts (Watfa and colleagues, 2026). In the astronaut in this case, the contraceptive adds risk to stasis.

A little-studied risk

The suspicion that spaceflight creates a state of increased coagulation arose in the Skylab studies in the 1970s, but without documented scientific evidence at the time (Fall and Bailey, 2026). A systematic review by the European space agency, carried out after this first case, found data on dilation of head veins, increased venous pressure and lower or reversed flow in microgravity, as well as high fibrinogen, markers of thrombin generation and endothelial damage. The authors concluded that it is still unknown whether these changes actually increase the risk of thromboembolism in flight (Kim and colleagues, 2021).

A numerical model of head circulation helps explain the lower flow. Without the hydrostatic gradients, venous pressure in the body falls, the transmural pressure of the internal jugular becomes negative and the resistance of the vein rises. The vertebral plexus, with rigid walls, does not suffer this effect and starts to receive more flow (Lan and colleagues, 2021).

The report that opened the discussion

The first case of venous thrombosis associated with spaceflight was published as a case report in 2020 (Auñón-Chancellor and colleagues, 2020). A recent review summarizes the data: an astronaut in her 40s, with no personal or family history of thromboembolism, had left internal jugular thrombosis discovered on a research ultrasound. Apart from a prominent left external jugular, she had no specific symptoms, neither headache nor worsening of the usual facial swelling in space (Limper and Jordan, 2026).

The thrombus had disappeared 10 days after landing, and no specific cause was found in flight or in the thrombophilia investigation afterwards (Limper and Jordan, 2026).

Symptoms that hide

Limper and Jordan point to another problem: the most striking symptoms of neck thrombosis, neck pain and headache, can be confused with those of space motion sickness and with the facial swelling of flight (Limper and Jordan, 2026). The post about space motion sickness describes these symptoms of the first days. A new headache in the second month of a mission, when adaptation has passed, deserves another reading.

Diagnosis: what ultrasound can tell

On Earth, compression ultrasound is described as a sensitive and specific method for acute venous thrombosis in limb veins and in the jugulars. The technique relies on compressibility: a vein that collapses under the probe practically rules out a thrombus (Harris and colleagues, 2023).

In orbit, the test has two problems. Harris and colleagues describe that if the vein does not collapse, the finding may lose specificity, because the pressure in the neck veins is higher and requires more force to collapse the vein. In addition, controlling the probe pressure by hand in microgravity is difficult, and even with remote guidance from experts on the ground, the person generating the pressure is a crew member with little experience (Harris and colleagues, 2023). In the neck, there is also the risk of stimulating the carotid sinus.

Thrombus or artifact

This point is not theoretical. Limper and Jordan report that, based on the published images and the fact that the vein was not compressed, some researchers questioned the diagnosis of occlusive thrombus and suggested an artifact caused by flow disturbances in the jugular. Flow separation and reverse flow could also explain the findings (Limper and Jordan, 2026).

The post on differential diagnosis shows how to keep a competing hypothesis alive. Here the competitor is stasis with slow or reverse flow, without thrombus.

The missing tests

On Earth, a low D-dimer together with a low Wells score helps rule out thrombosis in the legs. Harris and colleagues recall that D-dimer is poorly established for the upper limbs and neck, that the normal range in flight is not known, and that neither D-dimer nor the Wells score are available or validated in spaceflight (Harris and colleagues, 2023). What remains is color Doppler ultrasound, which is available on the station (Watfa and colleagues, 2026).

Differential diagnosis in the neck

The list of hypotheses for a finding in the internal jugular includes stasis without thrombus, true thrombus, external compression of the vein and technical artifact. On Earth, catheter-related thrombosis and pharyngeal infection extending to the vein would also be included. The astronaut has no catheter, no fever and no sore throat, and these hypotheses carry less weight.

The table summarizes the clues in the case and what each one suggests.

Clue	What it suggests	Source
Second month of the mission	Window in which jugular stasis was measured	Marshall-Goebel and colleagues, 2019
Left side	Side with stagnant or retrograde flow in most crew members	Marshall-Goebel and colleagues, 2019
Estrogen contraceptive	Risk factor added to stasis	Watfa and colleagues, 2026
Vein that does not collapse	Probable thrombus, with lower specificity in orbit	Harris and colleagues, 2023
Absent flow on Doppler	Thrombus or intense stasis	Limper and Jordan, 2026
Absence of symptoms	Common in the only case described	Limper and Jordan, 2026

Repeat before deciding

When the doubt is technical, the best next clue is another well-performed test. Repeating the ultrasound with an attempt at compression, on another day and with careful remote guidance, helps separate thrombus from stasis. The post on clinical decision-making discusses when one more test changes management.

Management: anticoagulate with a limited supply

The decision to anticoagulate in orbit weighs risks that do not exist in the hospital. Watfa and colleagues describe that low molecular weight heparin, apixaban and aspirin may be available on board and are used under the guidance of the flight surgeon when there is a clinical indication (Watfa and colleagues, 2026). Supplies are small, because no one plans to treat a thrombosis in orbit.

What was done in the published report

Harris and colleagues describe the astronaut's treatment. She received enoxaparin once a day at a dose of 1.5 mg/kg, later reduced to 1 mg/kg to extend the supply (Harris and colleagues, 2023). On day 42, she switched to apixaban 5 mg twice a day, with a dose reduction at 3 months. The anticoagulant was stopped 4 days before returning to Earth, to reduce the risk of bleeding from trauma during reentry (Harris and colleagues, 2023). These doses report what was done in a single case, not a recommendation.

The original report provides the details: the enoxaparin dose was reduced from 1.5 mg/kg to 1 mg/kg once a day after 33 days, and the apixaban dose dropped from 5 mg to 2.5 mg twice a day at 3 months. Because there was no reversal agent on board, protamine and prothrombin complex concentrate were sent to the station. Ten days after landing, the ultrasound showed no thrombus (Auñón-Chancellor and colleagues, 2020).

Limper and Jordan confirm the sequence of low molecular weight heparin followed by apixaban and the reason for stopping it: the capsule's planned hard landing (Limper and Jordan, 2026).

Why apixaban

Limper and Jordan present apixaban as the anticoagulant of choice in space, because of the lower bleeding risk, because it is authorized for prophylaxis and treatment and because it has already been used in orbit. In the review's table, the treatment regimen is 10 mg twice a day for the first 7 days and then 5 mg twice a day, with no need for monitoring tests (Limper and Jordan, 2026). The authors also note that the effects of the space environment on how medicines act have barely been studied.

If there is bleeding

Reversal is another limit. In the review by Limper and Jordan, andexanet alfa, the specific antidote for factor Xa inhibitors, is not available on the station because of the complexity of storage and preparation, and the alternative listed is 4-factor prothrombin complex concentrate (Limper and Jordan, 2026). That is why the team has to think about bleeding before starting, not afterwards.

Prevention and what comes after landing

The next question is how to prevent the next case. The study by Marshall-Goebel and colleagues tested lower body negative pressure, a device that wraps around the legs and pulls blood downward. It was associated with improved jugular flow in 10 of 17 sessions during the flight, and the authors point to it as a promising countermeasure (Marshall-Goebel and colleagues, 2019). The same study later identified, on review of the images, a possible partial thrombus in another crew member.

For those already at risk, preventive medication also enters the discussion. Limper and Jordan suggest that, if prophylaxis is desired, low dose apixaban at 2.5 mg twice a day may be a rational choice, because this approach worked in preventing thrombosis in patients with cancer and after hip replacement (Limper and Jordan, 2026). The review by Watfa and colleagues, however, found no study that met the criteria to assess anticoagulants in microgravity and had to be reorganized as a narrative review (Watfa and colleagues, 2026). Direct evidence does not exist.

Selection before the flight

The same review records the switch from estrogen contraceptives to progestogen-only formulations and cites a history of unprovoked thromboembolism as a risk factor considered before the mission (Watfa and colleagues, 2026). The authors note that commercial and longer missions are likely to bring people with more varied risk profiles. The post about fitness to fly covers the assessment before boarding a commercial flight.

After the return

Back on Earth, the astronaut has gravity again, and jugular flow changes once more. In the published report, the thrombus had disappeared 10 days after landing, and the thrombophilia workup found no cause (Limper and Jordan, 2026). Follow-up includes a control ultrasound performed by a specialist, with compression, and the decision on whether to keep the anticoagulation. The post about prognostic reasoning discusses how to plan this follow-up.

Step by step reasoning in the case

For the astronaut in this case, the sequence goes like this. The finding is incidental, on a research exam. The probability before the exam is moderate: second month of the mission, left side, estrogen contraceptive. The image suggests a thrombus, but the technique has limits. The next step is to repeat the ultrasound with an attempt at compression and Doppler, with careful remote guidance.

If the thrombus is confirmed, the team on the ground discusses anticoagulation with the medicines on board, the available stock and the time until the end of the mission. Switching off the estrogen contraceptive enters the conversation. Management includes monitoring tests in orbit, the date to stop before landing and the ultrasound after the return.

Deciding without certainty

The team will never have absolute certainty. The post about diagnostic uncertainty shows how to act when the doubt will not go away. The post about confidence calibration helps to say out loud how much the team believes in the thrombus, and to revise that confidence with every new exam.

Common mistakes

The first mistake is to think that without symptoms there is no disease. The only case described was found by chance, in a person with no specific complaints (Limper and Jordan, 2026).

The second mistake is to attribute headache and neck pain to adaptation to space without thinking about the vein. The symptoms overlap (Limper and Jordan, 2026).

The third mistake is to close the diagnosis of thrombus with an image without compression. The artifact from slow or reversed flow is a real alternative (Limper and Jordan, 2026).

The fourth mistake is to apply rules from Earth that do not hold in orbit. D-dimer and the Wells score are not validated in flight (Harris and colleagues, 2023).

The fifth mistake is to forget the landing. Reentry and landing bring a risk of trauma, and anticoagulation needs a date for suspension (Harris and colleagues, 2023).

The sixth mistake is to treat evidence from Earth as if it came from space. The review by Watfa and colleagues found no study of anticoagulants in people exposed to microgravity (Watfa and colleagues, 2026), and Limper and Jordan recall that the effects of flight on the absorption and action of medicines have barely been studied (Limper and Jordan, 2026). Every decision in orbit is, in practice, an extrapolation. Saying this out loud helps the team review the approach at each follow-up exam, instead of following a fixed regimen by inertia, and to record clearly what was decided and why.

How NEXO helps train this reasoning

To use the case of the day as training, try three habits. Before the first guess, ask what the environment does to physiology. At each clue, see whether it changes the approach or only confirms what you already thought. After the result, note whether you considered the hypothesis of artifact.

Three exercises with cases

Ask about the environment. For each case, write how the place where the person is changes circulation, breathing or metabolism.

Question the exam. For each exam, write in one sentence what can make the result mislead.

Plan with limited resources. Write the approach you would take if you had only the medicines from a small kit.

The post about case-based learning in clinical practice shows how cases help fix this habit.

The limits of use of the game are described on the NEXO economy page. If you research the teaching of clinical reasoning, the page for researchers explains how to get in touch. Professionals who write or review cases find information in curation and in creators, and institutions can learn about the proposal on the partnerships page.

Frequently asked questions

Can microgravity make the flow in the internal jugular slow or reversed?

Yes. In a study with 11 crew members, 6 had stagnant or retrograde flow in the left internal jugular around day 50 of flight (Marshall-Goebel and colleagues, 2019).

Can a jugular thrombosis go without symptoms in orbit?

Yes. In the only case described, the astronaut had only a prominent external jugular (Limper and Jordan, 2026).

Can an ultrasound be done by a crew member?

Yes. The exam is done by crew members with remote guidance from specialists on the ground, with the technical limitations described by Harris and colleagues (Harris and colleagues, 2023).

Are there anticoagulants aboard the station?

Yes. Low molecular weight heparin, apixaban and aspirin may be available and are used under the guidance of the flight surgeon (Watfa and colleagues, 2026).

Did the thrombus in the published report disappear?

Yes. The thrombus had disappeared 10 days after landing (Limper and Jordan, 2026).

Final checklist

• I considered mission time and the side of the neck in the probability before the exam.
• I asked about estrogen contraceptive and other risk factors.
• I distinguished symptoms of adaptation to space from new symptoms in the middle of the mission.
• I ordered an ultrasound with an attempt at compression and Doppler.
• I kept stasis and artifact as competing hypotheses.
• I remembered that D-dimer and the Wells score are not validated in flight.
• I checked the on-board stock of anticoagulants before choosing the regimen.
• I planned what to do in case of bleeding.
• I set the suspension date before landing.
• I planned the follow-up ultrasound after the return.

Want to practice the reasoning with progressive clues? Play the case of the day.

References

1. Auñón-Chancellor SM, Pattarini JM, Moll S, et al. Venous Thrombosis during Spaceflight. N Engl J Med. 2020. PMID 31893522.
2. Elias A, Weber T, Green DA, et al. Systematic review of the use of ultrasound for venous assessment and venous thrombosis screening in spaceflight. NPJ Microgravity. 2024. PMID 38316814.
3. Fall L, Bailey DM. Houston, we have a problem: Coagulation concerns during long-term spaceflight. Exp Physiol. 2026. PMID 40448664.
4. Harris KM, Arya R, Elias A, et al. Pathophysiology, risk, diagnosis, and management of venous thrombosis in space: where are we now? NPJ Microgravity. 2023. PMID 36797288.
5. Kim DS, Vaquer S, Mazzolai L, et al. The effect of microgravity on the human venous system and blood coagulation: a systematic review. Exp Physiol. 2021. PMID 33704837.
6. Lan M, Phillips SD, Archambault-Leger V, et al. Proposed mechanism for reduced jugular vein flow in microgravity. Physiol Rep. 2021. PMID 33931957.
7. Limper U, Jordan J. Neck-vein thrombosis during spaceflight. Br J Clin Pharmacol. 2026. PMID 41035389.
8. Marshall-Goebel K, Laurie SS, Alferova IV, et al. Assessment of Jugular Venous Blood Flow Stasis and Thrombosis During Spaceflight. JAMA Netw Open. 2019. PMID 31722025.
9. Marshall-Goebel K, Lee SMC, Lytle JR, et al. Jugular venous flow dynamics during acute weightlessness. J Appl Physiol (1985). 2024. PMID 38482574.
10. Watfa A, Younis A, Winnard-Brewer A, et al. Anticoagulants for the prevention and treatment of venous thromboembolism in humans exposed to microgravity: A hybrid systematic and narrative review. Am Heart J Plus. 2026. PMID 41624399.