Space medicine: SANS, bone loss and fluids
Table of contents
In the third month of a mission on an orbital station, an astronaut tells the flight surgeon by video that her near vision has gotten worse and that she has to hold text farther away to read it. Another crew member reports facial swelling and a feeling of nasal congestion since arrival. Months later, back on Earth, a third person stands up from a chair and nearly faints. The scenes are fictional and were written for teaching. This text uses these scenarios as a setting for clinical reasoning: how microgravity changes physiology, which findings are expected adaptation and which call for investigation, and how to reason when the physician is hundreds of kilometers from the patient.
NEXO is a daily medical diagnosis game: one clinical case a day, up to six progressive clues, and ICD-10 proximity feedback across five levels.
The text avoids being a protocol. Space medicine follows agency-specific routines, with specialized teams. The interest for those who study clinical reasoning lies elsewhere: space is an extreme laboratory of context, in which the same complaint changes meaning depending on the environment. If you play the case of the day on NEXO, you will recognize the move: a contextual clue changes the weight of all the others.
Key points
- In microgravity, body fluids shift toward the head, which explains facial swelling, nasal congestion and part of the changes in the eyes.
- Spaceflight-associated neuro-ocular syndrome, known by the acronym SANS, includes optic disc edema, flattening of the globe, choroidal folds and a shift toward hyperopia.
- Bone loss on long missions is rapid and occurs mainly in the bones that bear weight on Earth, such as the spine and the hip.
- After return, orthostatic intolerance is common and calls for careful assessment before attributing fainting only to readaptation.
- Remote reasoning depends on a well-taken history, guided examinations and a willingness to reopen the differential when a finding departs from what was expected.
Why space is a setting for clinical reasoning
Hodkinson, Anderton and Posselt reviewed space medicine and describe the main physiological challenges of flight: microgravity, radiation, isolation, confinement and distance from any full medical resource (Hodkinson et al., 2017). Stepanek et al. discussed how these challenges change with the arrival of civilian passengers, with more varied ages and health conditions than those of professional astronauts (Stepanek et al., 2019).
For clinical reasoning, space has a unique feature: almost everything changes at the same time. Fluid distribution, the load on bones, sleep, diet and access to tests. A simple complaint, such as headache, may be expected adaptation, an effect of carbon dioxide in the environment, dehydration, a sign of altered intracranial pressure or a disease that would have occurred on Earth. Context raises some hypotheses and lowers others, but the reasoning stays the same: list, rank by probability and severity, and look for the finding that separates the hypotheses.
The human research program
NASA maintains a research program dedicated to health risks for humans on space missions, with a public list of the risks studied and the gaps in knowledge (NASA Human Research Program). The program's public roadmap organizes the risks by system and shows how much remains unknown (NASA Human Research Roadmap). For those who study reasoning, this list is a good example of acknowledged uncertainty: several risks have mechanisms still under study.
Civilian passengers and a new risk profile
Stepanek et al. call attention to an important change: with commercial spaceflight, older people and people with chronic diseases are now traveling, which requires pre-flight medical assessment adapted to this new profile (Stepanek et al., 2019). For clinical reasoning, this brings space medicine closer to everyday practice. The question is no longer only how microgravity affects a young, trained body and now also includes how it interacts with hypertension, diabetes, coronary disease or the use of multiple medications.
This new setting helps you think in any unusual context. A passenger with heart disease who feels unwell on a short flight has the same list of hypotheses they would have on Earth, plus the adaptations of the environment. The baseline rate of common diseases follows the person wherever they go.
Isolation, sleep and confinement
Hodkinson and colleagues include isolation and confinement among the challenges of spaceflight (Hodkinson and colleagues, 2017). Altered sleep, intense work and distance from family affect mood, attention and the way a person reports symptoms. For those providing care at a distance, this changes how you listen: a vague complaint may be the first sign of accumulated fatigue, of an emotional problem or of a physical illness that has not yet shown other signs. Asking about sleep and mood at every consultation is part of the reasoning, not just of the wellness routine.
What microgravity changes in the body
The table below is a teaching aid for organizing the main adaptations and what each one means for the clinician. It simplifies a complex field and should be read together with the sources cited in the text.
| System | What changes in space | What to think clinically |
|---|---|---|
| Fluids | Shift toward the head | Swollen face and congestion are expected, but neurological signs call for assessment |
| Eyes | Optic disc edema, choroidal folds, change in refraction | A new visual complaint deserves a fundus exam and follow-up |
| Bones | Rapid mineral loss in the spine and hip | Low back pain, kidney stones and fracture risk on return |
| Circulation | Readaptation on return, orthostatic intolerance | Fainting after return calls for assessment beyond readaptation |
| Neck veins | Slow or reversed flow in the jugular vein | An unexpected finding on a routine exam may be meaningful |
| Genes and immunity | Changes in gene expression and markers | A field under study, with data from few individuals |
The fluid shift
In microgravity, the blood and fluids that gravity would pull to the legs redistribute to the upper body. The result is the swollen face, thinner legs and nasal congestion reported by many astronauts in the first days. For clinical reasoning, it is the kind of finding that must be recognized as expected, so that attention stays free for what does not fit.
What does not fit
Nasal congestion that worsens with fever, a headache that changes pattern and a visual complaint that progresses do not belong to the expected adaptation. The reasoning here resembles that of any context: the expected serves as a baseline, and deviation from the baseline is the clue. The post on signs, symptoms and history shows how to organize this comparison.
SANS: when vision changes in orbit
The astronaut who began to see worse up close is at the center of this text. Mader and colleagues described, in seven astronauts after long missions, optic disc edema, flattening of the globe, choroidal folds and a shift toward hyperopia, plus opening pressure between 21 and 28.5 centimeters of water in the four who underwent lumbar puncture after return (Mader and colleagues, 2011). Lee and colleagues reviewed the topic and consolidated the name spaceflight-associated neuro-ocular syndrome, with the acronym SANS (Lee and colleagues, 2020).
Mechanism under study
Lee and colleagues discuss several hypotheses for the mechanism of SANS, including the fluid shift toward the head and changes in the relationship between pressure inside the eye and pressure around the optic nerve, and they acknowledge that the mechanism is not yet defined (Lee and colleagues, 2020). For clinical reasoning, this is an example of a condition in which the pattern is recognized before the cause is understood.
This situation is more common in medicine than it seems. Many syndromes were described and followed for years before the mechanism was known. What allows acting safely in that interval is the combination of three precautions: documenting the finding well, comparing it with previous exams of the same person and defining in advance what would make the team change course. In the case of SANS, this means following vision, the fundus of the eye and the images throughout the mission and after return, with attention to any sign that departs from the described pattern. The post about prognostic reasoning discusses how to think about what comes next when the cause is still under study.
The differential that remains open
The important question is what else could explain the visual complaint. On Earth, optic disc edema brings to mind intracranial hypertension, tumor, cerebral venous thrombosis, inflammation of the optic nerve or severe arterial hypertension. In space, SANS is the most likely hypothesis in an astronaut on a long mission, but the rarity of the other causes does not make them impossible. The post about differential diagnosis shows how to keep a list open without losing focus.
- Is the visual change for near, for far or both types?
- Is there headache, nausea, vomiting or double vision associated?
- Does the pattern follow the expected course of adaptation or did it worsen suddenly?
- Are there other neurological signs, such as weakness, change in speech or coordination?
- Do the fundus exam and the imaging available on board show a change compared with the previous ones?
Bones, circulation and the return to Earth
Bones and muscles without load
On Earth, body weight and movement maintain the stimulus that preserves the bones. Without that stimulus, bone loss begins early. Lang and colleagues measured bone mineral density in crew members of long missions and found significant loss in the spine and hip, with greater loss in the trabecular bone of the hip (Lang and colleagues, 2004). Hodkinson and colleagues also describe the loss of muscle mass and the increased risk of kidney stones, linked to the release of calcium from the bones (Hodkinson and colleagues, 2017).
What this means for reasoning. Low back pain in orbit may be the spine adapting to the lack of load, but it may also be a kidney stone, which calls for another reasoning and another course of action. On return, a simple fall can cause a fracture that would not have happened before the mission. The base rate of each hypothesis changes with mission duration, and the post about base rate in diagnosis explains why this weighs on interpretation.
Countermeasures as part of the history. Daily exercise with resistance equipment and a treadmill is part of the routine of long missions. For the clinician, knowing how much exercise the crew member managed to do is part of the clinical history, like knowing whether a patient on Earth is taking the medication. The question about adherence to countermeasures helps estimate the risk of bone and muscle loss.
The return to Earth: the fainting that calls for care
The crew member who nearly faints on standing up after return represents a well-known complaint. Buckey and colleagues studied astronauts after short flights and found that 9 of 14 crew members did not complete a ten-minute standing test after missions of 9 to 14 days, although all had completed it before the flight, with different pressure and vascular resistance responses between those who tolerated and those who did not tolerate the standing position (Buckey and colleagues, 1996).
Readaptation or another cause? Orthostatic intolerance on return is expected, but fainting can also have other causes: dehydration, arrhythmia, bleeding, medication effect or an acute cardiovascular event. The clinical question is whether the picture follows the expected pattern of readaptation or whether there is a sign that departs from it: chest pain, palpitation, shortness of breath, fainting without prodrome or worsening over the days. The post about clinical decision-making shows how to choose exams when the most likely picture is benign, but there are serious hypotheses to rule out.
The neck as an unexpected clue. Marshall-Goebel and colleagues examined the internal jugular vein of crew members by ultrasound during flight and found slow or reversed flow in some of them, and an occlusive thrombus in one case identified in the routine exam (Marshall-Goebel and colleagues, 2019). The finding is an example of how a research exam can reveal something clinically relevant and of how the context creates risks that were not on the initial list.
Diagnosis at a distance
In space, the physician is rarely on board. Foale and colleagues described how crew members without medical training performed ultrasound exams on the space station with quick training and real-time guidance from specialists on Earth, obtaining images useful for diagnosis (Foale and colleagues, 2005). This model of guided exam shows how clinical reasoning adapts when the person examining and the person interpreting are in different places.
What distance demands
Distance demands a more structured history, because every question takes up communication time. It also demands clarity about what one wants to see in the exam, because the person examining needs precise instruction. And it demands a willingness to reopen the case when the answer departs from what was expected. These skills are the same as those of teleconsultation on Earth. The post about telemedicine shows how to keep clinical reasoning without an in-person exam.
An example of a guided consultation
Imagine the astronaut from the opening scene. The flight physician starts with the history: when the vision changed, whether it was for near or for far, whether there is headache, nausea or double vision. Then the physician asks for a targeted exam, with precise instructions about what to record and how. Then the physician compares the images with those from the start of the mission. Only then does the physician decide whether the picture follows the expected pattern or whether more investigation is needed. Each step chooses the information that best separates the hypotheses, because communication time is limited. The post about simulation of care shows how to train this kind of structured assessment.
When evacuation changes the decision
On Earth, there is almost always a next level of care. In space, emergency return is complex and slow. This changes the weight of each decision: the choice between observing and acting depends on what is possible to do on board and on the time until a hospital. The post about diagnostic uncertainty brings strategies for deciding well when the picture is not yet clear and resources are limited.
The biases of the space setting
Space is an environment of high technology, rigid routine and great attention to known risks. Pat Croskerry describes the most frequent cognitive errors in diagnosis and proposes metacognition, stopping and examining one's own reasoning, as the main way to reduce them (Croskerry, 2003). Graber, Franklin and Gordon showed that premature closure was the most frequent cognitive error in cases of diagnostic error in internal medicine (Graber and colleagues, 2005).
Attribution to the environment
The first bias is attributing to space what belongs to the person. The headache becomes fluid shift, the visual complaint becomes SANS, the fainting becomes readaptation. The correction is to write next to each expected hypothesis the serious hypothesis that must be ruled out and the finding that would rule it out.
Overconfidence in routine
The second bias is trusting routine exams too much. A well-designed routine detects what it was planned to detect. The jugular thrombus found by Marshall-Goebel and colleagues is a reminder that important findings can appear outside the expected list. The post about examples of heuristics shows when shortcuts help and when they get in the way.
Generalizing from few cases
The third bias is treating as a rule what was observed in a few people. The NASA twins study compared an astronaut on a mission of almost a year with his twin brother on Earth and found changes in gene expression, telomeres, microbiome and cognition, most of them reversed after return (Garrett-Bakelman and colleagues, 2019). The study is valuable precisely because it shows what can happen, but with a single pair of individuals it generates hypotheses, and the conclusions call for confirmation in larger groups.
How NEXO helps train this reasoning
In NEXO, the case of the day is fictional, with a real clinical presentation and up to six progressive clues. You propose a diagnosis by looking for the code in ICD-10 and receive feedback on proximity at five levels, which shows whether your guess landed on the exact code, the same category, the same group, the same chapter or a different chapter. This format connects with the theme of this text: when the context clue seems to explain everything, the proximity feedback shows whether you stayed in the right chapter.
To use the case of the day as training, try three habits. Before each guess, separate what is expected for the context from what departs from it. When a clue seems to explain everything, ask what it leaves unexplained. After the result, note down which clue you overvalued.
Three exercises with cases
Separate the expected. In each case, write down what would be expected for the context and mark the finding that departs from the baseline.
Examine from a distance. Imagine you can ask only three questions and order one test. Which would you choose to separate the hypotheses?
Be suspicious of routine. After each case, ask which finding would have gone unnoticed if you had only looked at the expected list.
The post about learning based on clinical cases shows why training with cases helps consolidate this type of decision.
The limits on using 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 under curation and under creators, and institutions can learn about the proposal on the for partnerships page.
Frequently asked questions
What is SANS?
It is spaceflight-associated neuro-ocular syndrome, a set of findings that includes optic disc edema, flattening of the eyeball, choroidal folds and a shift toward hyperopia, observed in astronauts after long missions.
Why does the face swell in space?
Because, without gravity, body fluids redistribute toward the head. Facial swelling and nasal congestion are expected adaptations in the first days, and new or progressive signs call for assessment.
How does bone loss enter clinical reasoning?
It changes the baseline rate of some hypotheses. Low back pain may be a kidney stone, and a simple fall on return may cause a fracture. Asking about exercise and countermeasures helps estimate the risk.
Is fainting on return to Earth always readaptation?
Readaptation is the most common explanation, but dehydration, arrhythmia, bleeding and medication effects also enter the differential. Chest pain, palpitations or fainting without warning call for a broader assessment.
How do you make a diagnosis with no doctor on board?
With a well-structured history, tests performed by the crew with real-time guidance from specialists on Earth and decisions that take into account the time to a hospital. The skills are similar to those of a teleconsultation.
What does space teach medicine on Earth?
That context changes the weight of each clue, that the expected serves as a baseline and that attention to what departs from the pattern is the central skill of clinical reasoning in any setting.
Final checklist
- I separated the findings expected for the context from the findings that depart from the baseline.
- Next to each expected hypothesis I wrote the serious hypothesis to rule out.
- I considered common causes from Earth even in an unusual context.
- I used the baseline rate adjusted for time of exposure and countermeasures.
- I planned questions and tests thinking about who will examine and who will interpret.
- I reopened the case when a finding departed from the expected.
Want to practice reasoning with progressive clues? Play the case of the day.
References
- Buckey JC Jr, Lane LD, Levine BD, et al. Orthostatic intolerance after spaceflight. J Appl Physiol. 1996. PMID 8828642.
- Croskerry P. The importance of cognitive errors in diagnosis and strategies to minimize them. Acad Med. 2003. PMID 12915363.
- Foale CM, Kaleri AY, Sargsyan AE, et al. Diagnostic instrumentation aboard ISS: just-in-time training for non-physician crewmembers. Aviat Space Environ Med. 2005. PMID 15945407.
- Garrett-Bakelman FE, Darshi M, Green SJ, et al. The NASA Twins Study: A multidimensional analysis of a year-long human spaceflight. Science. 2019. PMID 30975860.
- Graber ML, Franklin N, Gordon R. Diagnostic error in internal medicine. Arch Intern Med. 2005. PMID 16009864.
- Hodkinson PD, Anderton RA, Posselt BN, et al. An overview of space medicine. Br J Anaesth. 2017. PMID 29161391.
- Lang T, LeBlanc A, Evans H, et al. Cortical and trabecular bone mineral loss from the spine and hip in long-duration spaceflight. J Bone Miner Res. 2004. PMID 15125798.
- Lee AG, Mader TH, Gibson CR, et al. Spaceflight associated neuro-ocular syndrome (SANS) and the neuro-ophthalmologic effects of microgravity: a review and an update. NPJ Microgravity. 2020. PMID 32047839.
- Mader TH, Gibson CR, Pass AF, et al. Optic disc edema, globe flattening, choroidal folds, and hyperopic shifts observed in astronauts after long-duration space flight. Ophthalmology. 2011. PMID 21849212.
- 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.
- NASA. Human Research Program. nasa.gov.
- NASA. Human Research Roadmap. humanresearchroadmap.nasa.gov.
- Stepanek J, Blue RS, Parazynski S. Space Medicine in the Era of Civilian Spaceflight. N Engl J Med. 2019. PMID 30865799.