Altitude sickness: Lake Louise, descent and acetazolamide

On the second day of a trek at high altitude, a traveler in her early thirties reaches the hut with a headache, nausea and little appetite. She says it is tiredness from the climb and that she will feel better after sleeping. At night, a colleague notices that she stumbles on the way to the bathroom and answers questions slowly. The guide asks a doctor in the group whether she can continue the next day. The scene is fictional and was written for teaching. This text uses that picture as a setting for clinical reasoning: how to recognize acute mountain sickness, when it progresses to something serious, why descent is the central treatment and what other causes look like it.

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. Mountain medicine guidelines are detailed, and management in each case depends on the place, the group and the resources. The interest here is reasoning: how to separate the expected tiredness from an illness that can kill within a few hours, with little more than the history and the bedside exam. If you play the case of the day on NEXO, you will recognize the move: one contextual clue changes the weight of all the others.

Key points

• Acute mountain sickness is common in people who climb fast, and its frequency rises with the altitude reached.
• The Lake Louise score organizes the symptoms, with headache as the central piece, and was revised to remove the sleep item.
• Changes in gait, confusion or drowsiness suggest high-altitude cerebral edema, and shortness of breath at rest suggests high-altitude pulmonary edema, conditions that call for immediate descent.
• Descent is the central treatment for the severe forms, and oxygen, a portable hyperbaric chamber and medications help buy time when descent is delayed.
• Dehydration, hypothermia, hypoglycemia, poisoning and infection enter the differential, and the "it is just tiredness" bias delays the diagnosis.

Why altitude is a setting for clinical reasoning

Maggiorini and colleagues examined 466 mountaineers in four huts in the Swiss Alps and found acute mountain sickness in about 9 out of every 100 people at 2850 meters and in about 53 out of every 100 at 4559 meters (Maggiorini and colleagues, 1990). The authors also described cases of high-altitude pulmonary and cerebral edema in the highest hut (Maggiorini and colleagues, 1990). These numbers show two things at once: the condition is frequent, and the frequency changes with the context.

For clinical reasoning, altitude has a useful and dangerous feature. Useful because the context points to a small group of illnesses. Dangerous because that same context tends to explain everything. Headache, nausea, tiredness and poor sleep are expected in people who climb. The same list of symptoms appears in dehydration, hangover, gastroenteritis and carbon monoxide poisoning in a poorly ventilated hut. The challenge is to use the context without letting it close the case too early.

Guidelines that acknowledge uncertainty

The Wilderness Medical Society publishes guidelines for the prevention and treatment of acute altitude illnesses, with recommendations graded according to the quality of the evidence and the balance between benefits and risks (Luks and colleagues, 2019). The most recent update expands the text to diagnosis and includes guidance on travel to altitude after COVID-19 infection (Luks and colleagues, 2024). For anyone studying reasoning, this grading is a good example of explicit uncertainty: some recommendations have strong evidence, others depend on the experience of specialists.

Time and speed of ascent

The altitude reached matters, but so does the speed at which one gets there. People who drive or fly to a high town, with no time to acclimatize, may have symptoms even without physical effort. People who climb on foot in short stages tend to adapt better. Asking how the ascent went, how many days have passed and whether the person slept higher than the night before is part of the history, as much as asking about the pain.

Who needs more attention

Some people deserve closer attention before and during the ascent. This group includes those who have had altitude illness on previous trips, those who will ascend quickly because of the itinerary, those with heart or lung disease, and those who will sleep much higher than they slept the night before. This is not about forbidding the trip, but about adjusting vigilance. For the person assessing, this changes the pretest probability: the same headache weighs more in someone with a previous history of high-altitude pulmonary edema than in someone who has always ascended well.

It is also worth remembering that the whole group is exposed to the same environment. When one person falls ill, the others may also be at the start of a similar picture. A good field practice is to use one patient's assessment to quickly ask how the others are doing.

Acute mountain sickness and the Lake Louise score

The Lake Louise score has been used for decades to standardize the diagnosis of acute mountain sickness. Roach and colleagues presented the 2018 revision, produced by international consensus, which removed poor sleep from the questionnaire because studies showed it relates more to hypoxia itself than to the illness (Roach and colleagues, 2018).

The current score rates headache, gastrointestinal symptoms, fatigue or weakness, and dizziness. Headache plays a central role: the diagnosis assumes recent ascent, presence of headache, and a total score of 3 or more across the four symptoms (Roach and colleagues, 2018). For clinical reasoning, the score is an organizing tool, not a ruler that replaces the examination. A person with a low score and altered gait is sicker than another with a high score and a normal examination.

What the score leaves out

The score measures reported symptoms. It depends on the honesty and the ability of the person answering. In groups with summit goals, people tend to minimize what they feel. In someone already confused, the answer loses value. That is why a simple neurological examination, observation of gait, and conversation with those accompanying the person complete the assessment. The post on signs, symptoms and history shows how to combine the report with what is observed.

The symptoms that call for more attention

Maggiorini and colleagues identified that severe headache, vomiting, dizziness, rapid breathing, and marked pulmonary crackles characterize severe acute mountain sickness and indicate immediate descent (Maggiorini and colleagues, 1990). This sentence sums up much of the field reasoning: there is a point where observing stops being prudent and becomes risky.

When the picture becomes severe: cerebral and pulmonary edema

Acute mountain sickness is usually uncomfortable and transient. The severe forms, high-altitude cerebral edema and high-altitude pulmonary edema, can kill if they are not recognized and treated in time. The Wilderness Medical Society guidelines address the three conditions together, with specific guidance for each (Luks and colleagues, 2024).

High-altitude cerebral edema

The traveler in the scene stumbled and began responding slowly. These two findings change the case. Ataxia, confusion, drowsiness, and altered behavior in someone at altitude suggest cerebral edema, which is usually seen as the extreme evolution of acute mountain sickness. Hackett and colleagues described, in patients with severe high-altitude cerebral edema, reversible white matter edema on MRI and microhemorrhages that persisted after clinical recovery (Hackett and colleagues, 2019).

In the field, there is no MRI. The practical test is simple: ask the person to walk in a straight line, placing the heel of one foot against the toe of the other. Anyone who cannot do this, with no other explanation, must be treated as a possible case of cerebral edema. The decision to descend does not wait for confirmation.

High-altitude pulmonary edema

High-altitude pulmonary edema can appear with or without acute mountain sickness. Signs include shortness of breath out of proportion to exertion, which progresses to shortness of breath at rest, cough, decline in performance and, sometimes, crackles on auscultation. Maggiorini and colleagues included rapid breathing and marked pulmonary crackles among the signs of severity (Maggiorini and colleagues, 1990). Pulse oximetry helps, but it must be interpreted with care, because oxygen saturation in healthy people also falls at altitude.

Warning signs in the field

At altitude, some signs call for immediate action:

• difficulty walking in a straight line, stumbling or falling;
• confusion, drowsiness or a change in behavior;
• shortness of breath at rest or worsening day by day;
• persistent cough, sometimes with pink-tinged sputum;
• headache that does not improve with a simple analgesic and rest;
• repeated vomiting.

Descent as treatment

In severe forms, descent is the central treatment. Maggiorini and colleagues concluded that people with signs of severe acute mountain sickness should descend immediately to lower altitudes (Maggiorini and colleagues, 1990). The Wilderness Medical Society guidelines propose management approaches for each form of acute altitude illness (Luks and colleagues, 2024).

When descent is not possible

Weather, night, terrain and lack of transport can delay descent. In these cases, other measures help buy time. Bärtsch and colleagues showed, in a randomized trial at 4559 meters, that one hour in a portable hyperbaric chamber, equivalent to descending about 2250 meters, relieved the symptoms of acute mountain sickness in the short term, without lasting benefit after twelve hours (Bärtsch and colleagues, 1993). Keller and colleagues compared the chamber with dexamethasone and concluded that the chamber brings faster relief and the medication brings longer-lasting improvement, suggesting combining the two when descent or evacuation are not possible (Keller and colleagues, 1995).

How to organize the descent

The descent does not need to be long to help. Symptoms usually ease after descending 300 to 1000 meters, but the drop needed varies from person to person, and the guidance is that no one should descend alone (Luks and colleagues, 2024). What matters is starting early, preferably in daylight and with companions. Anyone with signs of cerebral edema should not descend alone, because confusion and lack of coordination increase the risk of falling. Anyone with pulmonary edema should make as little effort as possible, which may require carrying the person's backpack or using transport.

During the descent, reassessment continues. Is the gait improving? Is the shortness of breath decreasing? Is the person responding better? These answers help decide whether stopping at an intermediate altitude is enough or whether it is necessary to continue to a health service. The post on prognostic reasoning discusses how to think about what comes after the diagnosis.

The reasoning behind the chamber

The chamber studies teach something that applies beyond altitude. A rapid improvement can be temporary. Someone who leaves the chamber feeling well remains at the same altitude and can worsen again. For clinical reasoning, this means that the response to treatment is a clue, not a conclusion. Management must include what happens next: descent as soon as possible, periodic reassessment and a clear criterion for worsening.

Acetazolamide and other medications

Acetazolamide is the most studied medication for preventing acute mountain sickness. Wang and colleagues gathered 28 randomized trials in a network meta-analysis and concluded that moderate doses of acetazolamide prevent acute mountain sickness, with dexamethasone and ibuprofen also reducing the incidence (Wang and colleagues, 2025). The same authors highlighted that no medication reduced the incidence of severe forms in the analysis (Wang and colleagues, 2025).

What this result teaches

This finding has a practical consequence for reasoning. Someone who used preventive medication may have fewer mild symptoms, but remains subject to severe forms. Hearing "I already took the medicine" should not lower vigilance for alarm signs. The dose, the contraindications and the choice among the options rest with the professional who follows the traveler, according to the guidelines.

Medications as a bridge

In treatment, medications and oxygen work as a bridge until descent. Keller and colleagues showed that dexamethasone brought more lasting improvement of acute mountain sickness than the hyperbaric chamber alone (Keller and colleagues, 1995). The Wilderness Medical Society guidelines grade prevention and treatment recommendations according to the quality of the evidence (Luks and colleagues, 2024). In high-altitude cerebral edema, dexamethasone is the main drug treatment, and acetazolamide may come in as an adjuvant (Luks and colleagues, 2024). The post about clinical decision-making helps to think about approaches such as choices with risks and benefits.

The differential: not everything at altitude is mountain sickness

Being at altitude makes mountain sickness likely, but it remains one hypothesis among several. The table below organizes the differential in a didactic way. It simplifies a broad field and should be read together with the sources cited in the text.

Hypothesis	What brings it closer	What helps to separate it
Acute mountain sickness	Recent ascent, headache with nausea or fatigue	Improvement with rest, descent or oxygen
High-altitude cerebral edema	Ataxia, confusion, drowsiness	Altered walking test, progressive worsening
High-altitude pulmonary edema	Shortness of breath at rest, cough	Crackles, low saturation for the altitude
Dehydration	Low water intake, long exertion	Dry mouth, dark urine, improvement with fluids
Hypothermia	Cold, wind, wet clothing	Low temperature, shivering, confusion that improves with warming
Hypoglycemia	Little food, diabetes, exertion	Low blood glucose, rapid improvement with sugar
Poisoning	Alcohol, sedatives, stove in a closed place	History of exposure, other people with symptoms
Infection	Fever, contact with sick people	High fever, localized signs

Hypothermia

Brown and colleagues published a review on accidental hypothermia, useful for those who treat patients in cold environments (Brown and colleagues, 2012). Confusion and loss of coordination are also part of the picture in someone who gets cold. In the mountains, cold and altitude go together. A confused and unsteady person may have cerebral edema, hypothermia or both. Measuring the temperature, when possible, and warming the person while organizing the descent addresses both hypotheses.

Carbon monoxide poisoning

In shelters and tents, stoves and heaters used in closed places can generate carbon monoxide. Headache, nausea and confusion appear in both pictures. A useful clue is the pattern in the group: when several people who slept in the same place wake up with similar symptoms, environmental exposure moves up the list. The post about pretest probability helps to think about how context changes the weight of each hypothesis.

Dehydration and hypoglycemia

Long exertion, dry air, little appetite and nausea easily lead to dehydration and hypoglycemia. Both pictures cause fatigue, headache and dizziness. Asking how much the person drank and ate, measuring blood glucose when a device is available and observing the response to fluids and sugar are simple steps that change the differential.

The "it's just fatigue" bias

Croskerry described how cognitive errors lead to diagnostic errors and proposed strategies to reduce them (Croskerry, 2003). Graber and colleagues showed that cognitive factors, such as premature closure, are present in a good part of diagnostic errors in internal medicine (Graber and colleagues, 2005). In the mountains, these biases gain strength.

The explanation that arrives first

Climbing is tiring. This simple truth becomes a trap when fatigue starts to explain any symptom. The traveler in the scene called the headache and nausea fatigue, and the group accepted it. The antidote is to ask what fatigue leaves unexplained: the pain that does not go away with rest, the stumble, the slow response. The post about heuristics in clinical reasoning shows how useful shortcuts can mislead.

The pressure of the goal

Groups with an arrival date, expensive trips and personal expectations push to keep going. The sick person tends to minimize the symptoms. Whoever is assessing may feel that recommending descent is an overreaction. This scenario favors the bias of following the plan even in the face of signs of worsening. Agreeing before the trip that certain signs lead to descent, without discussion, protects the group from this pressure.

Excessive confidence

Anyone who has climbed without problems may conclude they are protected. Susceptibility varies, and a faster or higher climb can change the outcome. The post on calibrating confidence discusses how to adjust certainty to what the evidence allows.

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 searching for the ICD-10 code and receive proximity feedback at five levels, which shows whether your guess stayed on the exact code, in the same category, in the same group, in the same chapter or in a different chapter. This format speaks to the theme of this text: when the context 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 falls outside it. When a clue seems to explain everything, ask what it leaves unexplained. After the result, note which clue you underestimated.

Three exercises with cases

Look for the alarm sign. In each case, write which finding would make you change from observing to acting.

Test the easy explanation. When the answer seems obvious, list two hypotheses that would produce the same symptoms.

Think about the group. Ask whether other people in the same environment would have similar symptoms, and what that would change.

The post on differential diagnosis with clues shows how to train this type of reasoning with progressive cases.

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

Frequently asked questions

What is acute mountain sickness?

It is a set of symptoms that appears in people who climb to high altitudes, with headache accompanied by nausea, fatigue or dizziness. It usually improves with rest, acclimatization or descent.

What is the Lake Louise score for?

To standardize the assessment of acute mountain sickness symptoms. The 2018 revised version scores headache, gastrointestinal symptoms, fatigue and dizziness, and removed the sleep item.

Which signs indicate a severe condition?

Difficulty walking in a straight line, confusion, drowsiness, shortness of breath at rest, persistent cough and repeated vomiting. These signs call for immediate descent and assessment.

Why is descent so important?

Because it treats the cause, which is the lack of oxygen at altitude. Oxygen, a portable hyperbaric chamber and medications help buy time while the descent is organized.

Does acetazolamide protect against severe forms?

It reduces the frequency of acute mountain sickness, according to meta-analyses, and vigilance for alarm signs remains necessary even in those who use the medication.

What other conditions look like mountain sickness?

Dehydration, hypothermia, hypoglycemia, carbon monoxide poisoning, the effect of alcohol or sedatives, and infections. The history of exposure and the response to simple measures help tell them apart.

Final checklist

• I asked about the speed of the climb and the altitude at which the person slept.
• I used the Lake Louise score as an organizing tool, completing it with examination and observation.
• I tested gait and looked for signs of cerebral and pulmonary edema.
• I defined the alarm sign that leads to immediate descent.
• I considered dehydration, hypothermia, hypoglycemia and poisoning in the differential.
• I asked what the "it's just tiredness" leaves unexplained.

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

References

1. Bärtsch P, Merki B, Hofstetter D, et al. Treatment of acute mountain sickness by simulated descent: a randomised controlled trial. BMJ. 1993. PMID 8495155.
2. Brown DJ, Brugger H, Boyd J, Paal P. Accidental hypothermia. N Engl J Med. 2012. PMID 23150960.
3. Croskerry P. The importance of cognitive errors in diagnosis and strategies to minimize them. Acad Med. 2003. PMID 12915363.
4. Graber ML, Franklin N, Gordon R. Diagnostic error in internal medicine. Arch Intern Med. 2005. PMID 16009864.
5. Hackett PH, Yarnell PR, Weiland DA, Reynard KB. Acute and Evolving MRI of High-Altitude Cerebral Edema: Microbleeds, Edema, and Pathophysiology. AJNR Am J Neuroradiol. 2019. PMID 30679208.
6. Keller HR, Maggiorini M, Bärtsch P, Oelz O. Simulated descent v dexamethasone in treatment of acute mountain sickness: a randomised trial. BMJ. 1995. PMID 7767194.
7. Luks AM, Auerbach PS, Freer L, et al. Wilderness Medical Society Clinical Practice Guidelines for the Prevention and Treatment of Acute Altitude Illness: 2019 Update. Wilderness Environ Med. 2019. PMID 31248818.
8. Luks AM, Beidleman BA, Freer L, et al. Wilderness Medical Society Clinical Practice Guidelines for the Prevention, Diagnosis, and Treatment of Acute Altitude Illness: 2024 Update. Wilderness Environ Med. 2024. PMID 37833187.
9. Maggiorini M, Bühler B, Walter M, Oelz O. Prevalence of acute mountain sickness in the Swiss Alps. BMJ. 1990. PMID 2282425.
10. Roach RC, Hackett PH, Oelz O, et al. The 2018 Lake Louise Acute Mountain Sickness Score. High Alt Med Biol. 2018. PMID 29583031.
11. Wang J, Lian N, Tang K, et al. Comparative effects of pharmacological interventions for the prevention of acute mountain sickness: A systematic review and Bayesian network meta-analysis. Travel Med Infect Dis. 2025. PMID 40383249.