Renal colic in spaceflight: the astronaut's kidney

A 46-year-old astronaut is in the fourth month of a mission on a space station. In the middle of the afternoon, he feels a severe pain in the left flank, in waves, that moves down to the groin. He cannot stay still, he sweats and vomits once. The urine strip shows blood. He has no fever. The flight surgeon on the ground follows by videoconference and requests a kidney ultrasound, which another crew member performs with remote guidance. The scene is fictional and was written for teaching.

This text uses the scene as a setting for clinical reasoning. The trap is thinking of the astronaut as a healthy 46-year-old man with back pain. In orbit, bone loss, more concentrated urine and changes in the kidney itself make a stone more likely than on Earth. At the same time, the nearest hospital is hours or days away, and the decision to return changes the mission for everyone. The text is not a space agency protocol. If you play the case of the day on NEXO, you will recognize the challenge: context changes the probability before any test.

Key points

• Astronauts have a higher risk of kidney stones than before flight, and the risk in orbit is estimated to be even higher.
• Bone loss increases calcium in the urine, and urine becomes more concentrated during flight.
• Part of the risk seems to come from a change in the kidney itself, not only from bone.
• Waves of pain from the flank to the groin, blood in the urine, nausea and vomiting form the classic picture.
• Ultrasound is the test available on board and, on Earth, it is already enough in many scenarios.
• Drinking more fluids is the simplest way to reduce the risk.

Why this is a reasoning scenario

The case combines a known clinical picture with a place where almost everything changes: physiology, probability, tests and treatment options. A review on calcium in spaceflight notes that the occurrence of a stone in orbit would probably lead to medical evacuation of the crew member and, given the nature of the spacecraft, to the end of the mission for everyone (Smith et al., 2012). A common diagnosis on Earth becomes a decision of great weight.

The post on space medicine covers bone loss and fluid shift. Here the focus is the kidney: what happens to urine and to the organ itself during months without gravity.

What the scene teaches

The astronaut's pain has all the features of colic. What changes is the weight of the alternatives. The post on baseline rate in diagnosis shows why the frequency of a disease in a specific group must enter the reasoning before any test. In the astronaut, that frequency is not that of an ordinary 46-year-old man: it is that of someone who has spent four months with bone losing calcium and urine more concentrated than before flight.

The risk of stones in space

The numbers are striking. A broad study on the kidney in spaceflight summarizes that astronauts have an unusually high rate of kidney stones: in the year after flight, the incidence is 2 to 7 times above the estimate before flight, and the risk in orbit is estimated at about twice that (Siew et al., 2024). The authors note that an episode in orbit on a Soviet mission nearly ended the mission because of the intensity of symptoms and only resolved when the stone passed on its own.

A review on the priority risks of a Mars mission records that clinical nutritional assessment of astronauts identified, among other findings, a higher risk of kidney stones (Patel et al., 2020).

Before and after landing

A numerical model from the American space agency, fed with the urine chemistry of astronauts, estimated that in the first year after flight the incidence of calcium oxalate stones is between 2 and 7 times that before flight. In the group assessed in flight, a little more than one fifth of the astronauts had a relative risk above the high-risk threshold (Goodenow-Messman et al., 2022). The risk does not end when the person returns to Earth.

Long missions

A 2025 review of endocrine, metabolic and renal function in space recalls that the hypercalciuria of flight has always been attributed to bone loss, but that recent data suggest a transient and self-limited increase, a point still controversial, while bone loss continues over time. The same review describes stone formation as a direct consequence of the space environment, and only partly of bone loss (Magni et al., 2025). The astronaut in this case is in the fourth month of the mission.

Pathophysiology: from bone to kidney

The best-known explanation starts in the bone. Without the load of weight, bone is reabsorbed, and the released calcium leaves through the urine. Smith et al. describe that, in human studies, in flight and in prolonged bed rest, what increases is bone resorption, with bone formation equal or slightly lower (Smith et al., 2012). That is not the whole story: a commentary on the study by Siew et al. recalls that it challenged the old idea that stones in flight are only a consequence of bone resorption (Hoorn and Gameiro, 2024).

The post about from basic cycle to clinic shows how to connect the calcium metabolism studied in the early years to the picture that appears on the ward.

More concentrated and less protected urine

A review of the kidney in the space environment lists among the mechanisms of stone formation hypercalciuria, hypocitraturia, altered antidiuretic hormone and sodium handling (Pavlakou et al., 2018). Citrate inhibits crystal formation, and less citrate leaves the urine less protected.

The same review describes fluid regulation in flight as a two-phase process. At first, the lack of gravity shifts fluid to the chest and increases central blood volume, with the puffy face and thin legs typical of the first days. Later, plasma volume falls, and part of the fluid moves from the vessels into the cells and the space between them, without much change in total weight (Pavlakou et al., 2018). The authors also describe lower renal sodium excretion. The practical result, for the kidney, is a smaller volume of urine in a body that has lost its usual reference for thirst and fluid balance.

The kidney itself changes

The study by Siew et al. adds a piece. The fractional excretion of calcium increased significantly in flight and returned to normal on return; urine volume fell; oxalate, phosphate and uric acid rose less. In this data set, urinary citrate did not change significantly. The authors conclude that the metabolic changes favor stone formation, but do not explain all the risk, and propose that part of it comes from a change in the kidney itself (Siew et al., 2024).

How the picture presents

The classic presentation does not change in orbit. A review of the treatment of renal colic in the emergency department describes flank pain that radiates to the groin, accompanied by microscopic hematuria, nausea and vomiting, as well as pain on percussion of the costovertebral angle (Golzari et al., 2014). The same review recalls that the pain of colic is often described as the worst the patient has ever felt.

The astronaut in this case has wave-like pain from the flank to the groin, restlessness, sweating, vomiting and blood in the urine. He has no fever. The set is typical. The post about signs, symptoms and history shows how the shape of the pain, and not only the location, guides the diagnosis.

Questions that help

It is worth asking whether he has had a stone before, how he is drinking water on the mission, whether the urine has been getting darker and whether there is pain on urination or fever. A history of previous stones and concentrated urine increase the probability. Fever or chills change the severity of the picture.

Differential diagnosis

Even with a typical picture, the list must stay open. Pyelonephritis, with fever and lower back pain, calls for another urgency. Lumbar muscle pain, common with intense exercise and with changes in the spine in flight, worsens with movement and does not usually come with hematuria. In a 46-year-old man, dissection or rupture of an aortic aneurysm is rare, but serious, and can mimic colic. Other abdominal causes also enter: appendicitis, when the pain is on the right, and intestinal problems.

The post about differential diagnosis shows how to order the hypotheses from the most likely to the most dangerous.

Hypothesis	Supporting clues	Clues against in this case	Source of context
Kidney stone	Waves of pain toward the groin, hematuria, nausea	Typical picture, no clue against	Golzari and colleagues, 2014
Increased risk in orbit	Long mission, concentrated urine	Favorable context, no clue against	Siew and colleagues, 2024
Pyelonephritis	Lower back pain	Absence of fever	Clinical reasoning
Muscle pain	Daily exercise	Hematuria and waves of pain	Clinical reasoning
Aortic aneurysm	Age, intense lower back pain	Waves of pain, hematuria	Clinical reasoning
Infection with obstruction	Probable stone	Absence of fever so far	Clinical reasoning

The most dangerous combination

A stone obstructing the ureter together with an infection is the most serious situation. On Earth, this calls for urgent drainage. In orbit, there is no way to drain, and fever in an astronaut with a probable stone changes the conversation about return.

What can be examined on board

Ultrasound is the imaging exam available on the station. On Earth, a systematic review with consensus from several specialties concluded that ultrasound or no additional imaging is enough in specific clinical scenarios, and that reduced-radiation-dose CT should be used when CT is needed (Moore and colleagues, 2019). The authors cite a trial in which initial ultrasound reduced radiation exposure without worsening important patient outcomes.

In orbit, there is no CT. The ultrasound performed by a crew member with remote guidance looks for dilation of the collecting system, a sign of obstruction, and sometimes the stone itself. An ultrasound without dilation does not rule out the stone, but it reduces concern about significant obstruction.

The exam in orbit has limits worth stating. The person holding the probe is not a specialist, the image is interpreted remotely, and the patient must be restrained so as not to float. So the question put to ultrasound should be simple: is there or is there not dilation of the kidney on the side of the pain, and is it similar to or larger than on the other side? Repeating the exam after a few hours, comparing it with the first image, usually says more than trying to see the stone directly.

The urine

The urine strip confirms the blood and can show signs of infection.

Management: relieve the pain and decide about return

The first goal is to control the pain. The review by Golzari and colleagues describes nonsteroidal anti-inflammatory drugs and opioids as the basis of treatment for renal colic, alone or combined (Golzari and colleagues, 2014). Anti-inflammatory drugs act on prostaglandins, which are at the origin of the pain. Opioids work well, but bring nausea, vomiting, sedation, dizziness and respiratory depression among the adverse effects, which weighs heavily in an environment where the crew member needs to keep functioning. This text does not give doses, because they depend on the kit on board and on the flight surgeon's decision.

Hydration

Drinking more fluid is the simplest measure. Smith and colleagues describe increased fluid intake as the easiest and proven effective way to reduce the risk of a stone (Smith and colleagues, 2012). During the attack, this helps maintain urine flow; after it, it becomes prevention.

When to think about returning

The decision to end the mission depends on how things evolve. Controlled pain, no fever and improvement within hours or days suggest the stone may pass on its own, as in the Soviet episode described by Siew and colleagues (Siew and colleagues, 2024). Pain that does not yield to what is available on board, fever, signs of infection or persistent obstruction on ultrasound weigh in favor of return. The post about clinical decision-making discusses how to weigh the risk of acting against the risk of waiting.

Prevention: what has been studied in orbit

Two measures appear in the studies. The first is hydration, already mentioned. Smith and colleagues describe increased fluid intake as the easiest and best proven way to reduce the risk of stones (Smith and colleagues, 2012). On Earth, a review on hydration notes that current guidelines recommend fluid intake of at least 2.5 liters per day to prevent stones, and that increasing urine volume is the basis of prevention for all types of stones (Courbebaisse and colleagues, 2023).

The second is potassium citrate. Smith and colleagues explain that it alkalinizes the urine and reduces the potential to form stones (Smith and colleagues, 2012). This text does not give a dose.

Missions of the future

After landing

The risk continues on return. Smith and colleagues describe that increased fluid intake works as a measure after the flight, a period in which the risk of stones remains above the level from before the mission, and that in some astronauts changes in urine chemistry indicate the need for additional measures while still in orbit (Smith and colleagues, 2012). For the astronaut in this case, follow-up after landing includes analysis of the stone, if it is recovered, a 24-hour urine collection and bone assessment.

Step-by-step reasoning in the case

For the astronaut in this case, the sequence is as follows. The probability before the examination is already high: fourth month of the mission, concentrated urine and expected bone loss. The picture is typical of colic. The first hypothesis is a stone, and the most dangerous one to rule out is infection with obstruction. The pain is treated with what is in the kit, fluid intake increases, and the ultrasound looks for dilation. Temperature is monitored closely. The team records the time the pain started, the response to each analgesic and the volume of fluid ingested, to compare the evolution in the following hours with concrete data, not with impressions.

If the pain improves and there is no fever, the team monitors and discusses prevention for the rest of the mission. If fever, pain that does not subside or significant dilation appear, the conversation shifts to return. The post on diagnostic uncertainty helps define in advance which signs will change the decision.

A pause before closing

The typical picture tempts the team to close the diagnosis early. A diagnostic pause forces them to ask what else it could be and what would make the picture more serious.

A final point in the reasoning is the review after the outcome. If the stone passes on its own, the team compares what it expected with what happened: the time until the pain subsided, the evolution of dilation on ultrasound and the response to analgesics. If the picture worsens and return is necessary, the same review helps understand which signs appeared first. This comparison between prediction and result leaves a useful lesson for the next mission, and it applies both to space medicine and to the shift on Earth, where the doctor also decides with incomplete information and needs to know afterward whether they were right for the right reason.

Common errors

The first error is using the probability from Earth. In orbit, the risk of stones is estimated to be much higher (Siew and colleagues, 2024).

The second error is thinking only about bone. Part of the risk seems to come from changes in the kidney itself (Siew and colleagues, 2024).

The third error is treating the pain without looking at temperature. Infection with obstruction is the most serious combination. A new fever in an astronaut with colic changes the conversation about return, because there is no way to drain the kidney on board.

The fourth error is forgetting hydration. Supersaturation of the urine, a necessary condition for stones, always comes from reduced urine volume or excess solutes (Courbebaisse and colleagues, 2023).

The fifth error is thinking the risk ends at landing. The estimated incidence in the first year after return is between 2 and 7 times that from before the flight (Goodenow-Messman and colleagues, 2022). The post on prognostic reasoning covers what to monitor after the diagnosis.

How NEXO helps train this reasoning

To use the case of the day as training, try three habits. Before the first guess, ask how the setting changes the probability. With each clue, see whether it changes management or only confirms what you already thought. After the result, note which serious hypothesis you ruled out and with which clue.

Three exercises with cases

Adjust the base rate. For each case, write whether the person's group has a disease that is more or less frequent than average.

Link mechanism and management. Write the mechanism of the main hypothesis and the prevention that follows from it.

Plan far from the hospital. Write what you would do if you had only an ultrasound and a small kit of medicines.

The post about learning based on clinical cases shows how cases help fix this habit.

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 creators, and institutions can learn about the proposal on the partnerships page.

Frequently asked questions

Do astronauts have more kidney stones?

Yes. In the year after the flight, the incidence is 2 to 7 times above the estimate before it, and the risk in orbit is estimated at about double that (Siew et al., 2024).

Does bone loss raise calcium in the urine?

For the most part, yes. During flight, bone resorption increases (Smith et al., 2012), but the space environment also seems to act directly on the kidney (Magni et al., 2025).

Can ultrasound be enough to assess renal colic?

Yes. On Earth, ultrasound or no additional imaging is enough in several clinical scenarios (Moore et al., 2019).

Does drinking more fluid help prevent it?

Yes. Increasing fluid intake is described as the simplest and best proven way to reduce the risk (Smith et al., 2012).

What is potassium citrate for?

It alkalinizes the urine and reduces the potential to form stones (Smith et al., 2012).

Final checklist

• I adjusted the probability of stones to the mission duration.
• I asked about previous stones and fluid intake.
• I confirmed the pattern of the pain and the presence of blood in the urine.
• I measured the temperature and looked for signs of infection.
• I kept pyelonephritis, muscle pain and aneurysm in the differential.
• I requested an ultrasound with remote guidance, looking for dilation.
• I treated the pain with what is in the kit, following the flight surgeon.
• I increased fluid intake.
• I defined in advance which signs would lead to return.
• I planned prevention and follow-up after landing.

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

References

1. Courbebaisse M, Travers S, Bouderlique E, et al. Hydration for Adult Patients with Nephrolithiasis: Specificities and Current Recommendations. Nutrients. 2023. PMID 38068743.
2. Golzari SE, Soleimanpour H, Rahmani F, et al. Therapeutic approaches for renal colic in the emergency department: a review article. Anesth Pain Med. 2014. PMID 24701420.
3. Goodenow-Messman DA, Gokoglu SA, Kassemi M, et al. Numerical characterization of astronaut CaOx renal stone incidence rates to quantify in-flight and post-flight relative risk. NPJ Microgravity. 2022. PMID 35091560.
4. Hoorn EJ, Gameiro J. Cosmic kidney disease: a spaceflight-induced tubulopathy. Clin Kidney J. 2024. PMID 39664989.
5. Magni P, Ricci G, Narici M, et al. Impact of spaceflight on endocrine, metabolic and kidney function: current evidence, open issues, and potential countermeasures. BMC Biol. 2025. PMID 41462245.
6. Moore CL, Carpenter CR, Heilbrun ME, et al. Imaging in Suspected Renal Colic: Systematic Review of the Literature and Multispecialty Consensus. Ann Emerg Med. 2019. PMID 31402153.
7. Patel ZS, Brunstetter TJ, Tarver WJ, et al. Red risks for a journey to the red planet: The highest priority human health risks for a mission to Mars. NPJ Microgravity. 2020. PMID 33298950.
8. Pavlakou P, Dounousi E, Roumeliotis S, et al. Oxidative Stress and the Kidney in the Space Environment. Int J Mol Sci. 2018. PMID 30326648.
9. Siew K, Nestler KA, Nelson C, et al. Cosmic kidney disease: an integrated pan-omic, physiological and morphological study into spaceflight-induced renal dysfunction. Nat Commun. 2024. PMID 38862484.
10. Smith SM, McCoy T, Gazda D, et al. Space flight calcium: implications for astronaut health, spacecraft operations, and Earth. Nutrients. 2012. PMID 23250146.