Altitude Sickness: Symptoms, Prevention & Emergency Treatment
📊 Quick facts about altitude sickness
💡 The most important things you need to know
- Gradual ascent is the best prevention: Above 3,000m, increase sleeping altitude by no more than 300-500m per day
- Physical fitness does NOT protect you: Elite athletes can be affected just as easily as anyone else
- Never ascend with symptoms: If you develop headache or other symptoms, stop and rest or descend
- Descent is the definitive treatment: Even 500-1,000m descent can provide significant relief
- Watch for danger signs: Confusion, loss of coordination (ataxia), or severe breathlessness require immediate descent
- Acetazolamide can help: Starting 1-2 days before ascent can speed acclimatization
What Is Altitude Sickness and Why Does It Happen?
Altitude sickness (acute mountain sickness) is a condition caused by the body's inability to adapt quickly enough to the reduced atmospheric pressure and lower oxygen levels at high elevations. The air at high altitude contains the same percentage of oxygen (21%) but the lower pressure means fewer oxygen molecules per breath, leading to hypoxia (low blood oxygen).
When you ascend to high altitude, your body undergoes a complex series of physiological changes in an attempt to maintain adequate oxygen delivery to vital organs. This process, called acclimatization, normally takes several days to weeks to complete. Altitude sickness occurs when the rate of ascent exceeds the body's ability to adapt, resulting in a cascade of symptoms that can range from mild discomfort to life-threatening conditions.
The fundamental problem is reduced partial pressure of oxygen. At sea level, atmospheric pressure is approximately 760 mmHg, and the partial pressure of oxygen in inspired air is about 159 mmHg. At 3,000 meters (10,000 feet), atmospheric pressure drops to approximately 526 mmHg, reducing the partial pressure of inspired oxygen to only 110 mmHg – a 31% reduction. At the summit of Mount Everest (8,849m), atmospheric pressure is only about 253 mmHg, with an inspired oxygen partial pressure of just 53 mmHg – one-third of sea level values.
This reduction in available oxygen triggers immediate compensatory responses: increased breathing rate and depth (hyperventilation), increased heart rate, and redistribution of blood flow to vital organs. Over time, additional adaptations occur including increased red blood cell production, changes in cellular metabolism, and alterations in how the body handles fluids. However, these adaptations take time, and ascending too quickly overwhelms the body's compensatory mechanisms.
The pathophysiology of altitude sickness
The exact mechanisms causing altitude sickness symptoms are not fully understood, but current research suggests several contributing factors. Hypoxia leads to increased cerebral blood flow as the brain attempts to maintain oxygen delivery, which can cause brain swelling (cerebral edema). Additionally, hypoxia affects the blood-brain barrier, allowing fluid to leak into brain tissue. The resulting increased intracranial pressure is thought to be responsible for the characteristic headache of altitude sickness.
Fluid retention also plays a significant role. At high altitude, the kidneys initially retain sodium and water, leading to peripheral edema and contributing to pulmonary symptoms. Hyperventilation causes respiratory alkalosis (increased blood pH), which triggers the kidneys to excrete bicarbonate – this is actually beneficial as it allows further hyperventilation but takes 2-3 days to fully develop. This is why the first few days at altitude are the most dangerous period for developing altitude sickness.
Who is at risk for altitude sickness?
Anyone who ascends to high altitude can develop altitude sickness, regardless of age, sex, or physical fitness level. In fact, young, fit individuals may be at increased risk because they tend to ascend more quickly and push themselves harder. Previous altitude sickness is the strongest predictor of future episodes – if you've had it before, you're likely to get it again under similar circumstances.
Other risk factors include rapid rate of ascent, high sleeping altitude (the altitude at which you sleep matters more than maximum altitude reached during the day), individual susceptibility (some people are simply more prone to altitude sickness than others), and certain medical conditions including respiratory diseases, heart conditions, and anemia. Living at low altitude (especially at sea level) without recent high-altitude exposure also increases risk.
What Are the Symptoms of Altitude Sickness?
The cardinal symptoms of altitude sickness include headache (present in nearly all cases), nausea or vomiting, fatigue and weakness, dizziness or lightheadedness, and difficulty sleeping. Symptoms typically begin 6-12 hours after arriving at high altitude and are usually worst on the second or third day before improving with acclimatization.
Altitude sickness exists on a spectrum from mild acute mountain sickness (AMS) to life-threatening high altitude cerebral edema (HACE) and high altitude pulmonary edema (HAPE). Understanding these different manifestations is crucial for appropriate management.
Mild to moderate acute mountain sickness (AMS)
The most common presentation of altitude sickness is mild to moderate AMS. Headache is the hallmark symptom, present in approximately 95% of cases. The headache is typically described as throbbing or pulsating, often worse in the morning or when lying down, and frequently located in the frontal region. It's usually bilateral and may be aggravated by straining, coughing, or bending over.
Accompanying the headache, individuals typically experience one or more of the following symptoms: nausea with or without vomiting, loss of appetite (anorexia), fatigue and weakness disproportionate to the level of exertion, dizziness or lightheadedness, and difficulty sleeping (insomnia). Sleep disturbances at high altitude often include periodic breathing (Cheyne-Stokes respiration), where breathing alternates between periods of deep breaths and periods of apnea, causing frequent awakenings.
These symptoms are often compared to a hangover or mild flu. Most people with mild AMS can continue their activities with appropriate rest and symptom management, though ascending further should be avoided until symptoms resolve.
| Severity | Symptoms | Lake Louise Score | Required Action |
|---|---|---|---|
| Mild AMS | Headache plus mild nausea, fatigue, or dizziness | 3-5 | Stop ascent, rest, symptomatic treatment |
| Moderate AMS | Severe headache, significant nausea/vomiting, marked fatigue | 6-9 | Consider descent, medical treatment, monitor closely |
| Severe AMS/HACE | Ataxia, confusion, altered consciousness | 10+ | Immediate descent, emergency treatment |
| HAPE | Severe breathlessness at rest, cough, pink frothy sputum | N/A | Immediate descent, oxygen, nifedipine |
High Altitude Cerebral Edema (HACE)
HACE represents the severe end of the AMS spectrum and is a life-threatening medical emergency. It occurs when brain swelling becomes severe enough to cause neurological dysfunction. HACE typically develops from progressive AMS, though it can occasionally appear rapidly.
The defining feature of HACE is ataxia – loss of coordination that makes it difficult to walk in a straight line. This is tested by having the person walk heel-to-toe in a straight line (tandem gait); inability to do this without stumbling suggests HACE. Other neurological signs include confusion, disorientation, irrational behavior, hallucinations, and eventually progression to coma if untreated.
HACE is relatively rare, occurring in less than 1% of those ascending above 4,000 meters, but it has a significant mortality rate if not treated promptly. The treatment is immediate descent – every minute counts. Dexamethasone can be given as an adjunctive treatment but should never delay descent.
High Altitude Pulmonary Edema (HAPE)
HAPE is another life-threatening condition that can occur independently of AMS or HACE. It is characterized by fluid accumulation in the lungs due to increased pulmonary artery pressure in response to hypoxia. HAPE is the most common cause of death from altitude sickness.
Early symptoms include decreased exercise tolerance and a dry cough, which may initially be attributed to the cold, dry air at high altitude. As HAPE progresses, symptoms include shortness of breath at rest (not just during exertion), productive cough with pink, frothy sputum, gurgling or crackling sounds when breathing (rales), and cyanosis (bluish discoloration of lips and fingernails). Fever and rapid heart rate are common.
HAPE can progress rapidly, sometimes within hours. Like HACE, the definitive treatment is descent. Supplemental oxygen and nifedipine can help stabilize the patient for evacuation.
- Confusion, disorientation, or altered mental status
- Loss of coordination (ataxia) – unable to walk heel-to-toe
- Severe breathlessness at rest
- Persistent cough with pink or frothy sputum
- Extreme fatigue – unable to care for yourself
- Symptoms that worsen despite rest at the same altitude
These symptoms indicate HACE or HAPE and require immediate descent to a lower altitude. Call emergency services if evacuation is needed.
How Can I Prevent Altitude Sickness?
The most effective prevention for altitude sickness is gradual ascent, allowing your body time to acclimatize. Above 3,000 meters, increase your sleeping altitude by no more than 300-500 meters per day, with rest days every 3-4 days. The principle of "climb high, sleep low" – ascending during the day but returning to a lower altitude to sleep – is highly effective.
Prevention of altitude sickness is based on three main strategies: gradual ascent, pharmacological prophylaxis, and avoiding factors that worsen symptoms. Understanding and implementing these strategies can dramatically reduce your risk of developing altitude illness.
Gradual ascent and acclimatization
The body requires time to adapt to high altitude, and the most important prevention strategy is allowing adequate time for this acclimatization to occur. Current guidelines from the Wilderness Medical Society and International Society for Mountain Medicine recommend the following ascent profile:
Below 3,000 meters (10,000 feet), ascent rate is less critical, though some individuals may experience symptoms at these moderate elevations. Above 3,000 meters, the sleeping altitude should increase by no more than 300-500 meters (1,000-1,500 feet) per day. A rest day (no altitude gain) should be scheduled every 3-4 days or every 1,000 meters of altitude gain.
The "climb high, sleep low" principle is highly effective. This means you can climb to higher altitudes during the day for acclimatization but should descend to sleep at a lower altitude. For example, on a Himalayan trek, you might climb from camp at 4,000m to a pass at 4,500m, then descend to camp at 4,200m for the night.
Pre-acclimatization – spending time at moderate altitude before ascending to high altitude – can also be beneficial. Even 4-5 days at 2,500-3,000 meters can significantly reduce the risk of AMS at higher elevations.
Pharmacological prevention
Acetazolamide (Diamox) is the most well-studied medication for altitude sickness prevention and is recommended by major medical organizations for those at moderate to high risk. It works by causing a mild metabolic acidosis, which stimulates breathing and speeds acclimatization. The typical prophylactic dose is 125-250mg twice daily, starting 1-2 days before ascent and continuing for 2-3 days after reaching maximum altitude.
Common side effects of acetazolamide include tingling in the extremities (paresthesias), altered taste (particularly of carbonated beverages), and increased urination. These side effects are generally mild and well-tolerated. Acetazolamide is a sulfonamide and should not be used by those with sulfa allergies.
Dexamethasone is an alternative for those who cannot take acetazolamide. It works by reducing brain swelling and inflammation. The typical dose is 2mg every 6 hours or 4mg every 12 hours. Unlike acetazolamide, dexamethasone does not aid true acclimatization – it only masks symptoms – so it should be used cautiously, and users should be aware they may experience rebound symptoms when stopping the medication.
- Ascend gradually – no more than 300-500m sleeping altitude gain per day above 3,000m
- Include rest days every 3-4 days or every 1,000m gain
- Climb high, sleep low
- Stay well hydrated (3-4 liters daily)
- Avoid alcohol, sedatives, and heavy meals
- Consider acetazolamide 125-250mg twice daily for rapid ascents or high-risk individuals
- Get adequate sleep
Lifestyle factors
Staying well hydrated is important at high altitude. The combination of cold, dry air and increased breathing rate leads to significant fluid loss. Aim for 3-4 liters of water daily, though be aware that overhydration can also cause problems. Use urine color as a guide – it should be light yellow, not clear or dark.
Avoid alcohol during the first few days at altitude. Alcohol is a respiratory depressant that can worsen hypoxia, particularly during sleep. It also contributes to dehydration. Similarly, sedative medications (including sleeping pills) should be avoided as they can suppress breathing during sleep when periodic breathing is already common.
Eating light, high-carbohydrate meals may be helpful, as carbohydrates require less oxygen to metabolize than fats. Heavy, fatty meals should be avoided. Many people experience decreased appetite at high altitude anyway.
How Is Altitude Sickness Treated?
The primary treatment for altitude sickness is descent to a lower altitude – even descending 500-1,000 meters can provide significant relief. For mild symptoms, stopping ascent and resting at the current altitude while taking pain relievers and anti-nausea medication may be sufficient. Acetazolamide can speed recovery, and dexamethasone is used for moderate to severe cases.
Treatment of altitude sickness depends on symptom severity. The fundamental principle is that descent is always definitive treatment – when in doubt, go down. Other treatments are supportive or adjunctive and should never delay descent when it is indicated.
Treatment of mild acute mountain sickness
For mild AMS (headache with one or two other mild symptoms), the most important intervention is to stop ascending. Continue to rest at the current altitude until symptoms resolve, which typically takes 24-48 hours. Most cases of mild AMS will resolve spontaneously with rest and time.
Symptomatic treatment includes acetaminophen (paracetamol) or ibuprofen for headache, anti-emetics such as ondansetron or metoclopramide for nausea, and ensuring adequate hydration. Avoid strenuous activity until symptoms resolve.
Acetazolamide 250mg twice daily can speed recovery from mild AMS by accelerating acclimatization. It is particularly useful if symptoms are preventing sleep or if there is pressure to continue ascending (though ascending with symptoms is never recommended).
Treatment of moderate to severe AMS
Moderate AMS (Lake Louise Score 6-9) warrants consideration of descent, especially if symptoms are worsening or not improving with rest. Treatment includes acetazolamide 250mg twice daily plus dexamethasone 4mg every 6 hours. Supplemental oxygen (2-4 L/min) can provide significant relief if available.
For severe AMS approaching HACE (ataxia, confusion), immediate descent is mandatory. Dexamethasone 8mg initially, then 4mg every 6 hours, should be given. Supplemental oxygen should be used if available. A portable hyperbaric chamber (Gamow bag) can simulate descent and buy time when actual descent is delayed or impossible, but it should not replace descent when possible.
Treatment of HACE
HACE is a medical emergency requiring immediate descent. The patient should be accompanied and may need to be carried if unable to walk safely. Dexamethasone 8mg immediately, then 4mg every 6 hours, should be administered. Supplemental oxygen at the highest available flow rate should be used. If descent is impossible due to weather, darkness, or terrain, a portable hyperbaric chamber should be used.
Even with appropriate treatment, HACE can be fatal. The priority is getting the patient to lower altitude as quickly and safely as possible. Symptoms typically improve rapidly with descent, though full recovery may take several days.
Treatment of HAPE
Like HACE, HAPE requires immediate descent. However, patients with HAPE are often too weak to walk, making evacuation challenging. The patient should be kept warm, as cold stress worsens pulmonary hypertension. Minimize exertion – the patient should not help with their own evacuation if possible.
Supplemental oxygen is highly effective in HAPE and should be used at the highest available flow rate (4-6 L/min or higher). Nifedipine 30mg extended-release every 12 hours can help reduce pulmonary artery pressure. Phosphodiesterase inhibitors (sildenafil, tadalafil) are alternatives if nifedipine is unavailable.
A portable hyperbaric chamber can be life-saving when descent is delayed. HAPE can progress rapidly, so treatment should not be delayed while awaiting evacuation.
Acetazolamide (Diamox): 125-250mg twice daily for prevention; 250mg twice daily for treatment. Speeds acclimatization.
Dexamethasone: 4mg every 6-12 hours for prevention; 8mg then 4mg every 6 hours for treatment. Reduces brain swelling.
Nifedipine: 30mg extended-release every 12 hours for HAPE. Reduces pulmonary artery pressure.
Ibuprofen: 600mg three times daily has been shown to help prevent AMS in some studies.
When Should I Seek Medical Care?
Seek medical care immediately for any signs of HACE (confusion, ataxia) or HAPE (severe breathlessness at rest, productive cough). Also seek care if mild symptoms don't improve within 24-48 hours of rest, if symptoms worsen despite staying at the same altitude, or if you're unsure whether descent is needed. In remote areas, err on the side of descending rather than waiting.
In high-altitude environments, access to medical care is often limited. Understanding when symptoms require professional evaluation versus self-management is crucial. The general rule is: when in doubt, descend.
Seek emergency medical care immediately if:
- You or a companion develop signs of HACE: confusion, disorientation, ataxia (unable to walk heel-to-toe), unusual behavior, or decreasing level of consciousness
- Severe breathlessness at rest develops (not just with exertion)
- A productive cough develops, especially with pink or frothy sputum
- Symptoms rapidly worsen despite stopping ascent
- The person becomes unable to care for themselves
In these situations, begin descent immediately while arranging for medical evaluation and potential evacuation. Find your local emergency number and arrange helicopter evacuation if available and conditions permit.
Contact medical services or descend if:
- Mild symptoms don't improve after 24-48 hours of rest at the same altitude
- Symptoms prevent sleep for more than one night
- Moderate symptoms develop (significant nausea/vomiting, severe headache unresponsive to medication, marked fatigue)
- You're unsure about the severity of symptoms
Who Needs Special Consideration at High Altitude?
Certain groups require special consideration when traveling to high altitude, including people with heart or lung disease, those with sickle cell disease, pregnant women, and children. While many of these individuals can safely travel to moderate altitudes with appropriate precautions, medical consultation before travel is essential.
People with cardiovascular disease
Those with stable coronary artery disease, well-controlled heart failure, or properly managed hypertension can generally travel to moderate altitudes (up to 3,000m) safely. However, high altitude increases cardiac workload due to increased heart rate and blood viscosity. Anyone with heart disease should consult their cardiologist before high-altitude travel and ensure their condition is optimally controlled.
Those with unstable angina, uncontrolled arrhythmias, or severe heart failure should avoid high altitude. People with pacemakers or implantable defibrillators can generally travel safely as these devices are not affected by altitude.
People with respiratory disease
Individuals with COPD, asthma, or other respiratory conditions may have difficulty at high altitude due to baseline compromised oxygen exchange. Those with mild to moderate, well-controlled respiratory disease can often travel to moderate altitudes but may need supplemental oxygen. A pre-travel assessment including spirometry and possibly a hypoxic challenge test can help predict high-altitude tolerance.
Those with pulmonary hypertension are at high risk for HAPE and should generally avoid high-altitude travel.
Children at high altitude
Children can develop altitude sickness just like adults. Young children (under 3 years) cannot communicate their symptoms, making diagnosis more challenging. Signs of altitude sickness in young children include irritability, loss of appetite, decreased playfulness, and disrupted sleep patterns.
The same ascent guidelines apply to children as adults. Acetazolamide can be used in children at a dose of 2.5mg/kg twice daily (maximum 125mg per dose). Parents should be especially vigilant and have a low threshold for descending if symptoms develop.
Pregnancy and high altitude
Limited data exist on pregnancy and high altitude. Brief visits to moderate altitudes (up to 2,500m) are generally considered safe during uncomplicated pregnancies. However, higher altitudes and longer exposures may pose risks due to reduced fetal oxygen delivery. Pregnant women should avoid altitudes above 3,500m and should not take acetazolamide (pregnancy category C) unless the benefits clearly outweigh the risks.
What About Flying to High-Altitude Destinations?
Many popular destinations are at high altitude, including Cusco (3,400m), La Paz (3,640m), Lhasa (3,650m), and ski resorts in Colorado. Flying directly to these destinations doesn't allow gradual acclimatization, so altitude sickness is common. Consider arriving at a lower elevation first if possible, taking acetazolamide prophylaxis, and planning light activity for the first 24-48 hours.
Modern air travel allows rapid ascent to high-altitude destinations that would have taken weeks to reach historically. While this is convenient, it eliminates the opportunity for gradual acclimatization and increases altitude sickness risk.
Popular high-altitude destinations
Many popular tourist destinations are at elevations where altitude sickness is common. Cusco, Peru (3,400m/11,150ft), the gateway to Machu Picchu, has AMS rates of 30-50% among visitors. La Paz, Bolivia (3,640m/11,940ft) is one of the highest capital cities in the world. Lhasa, Tibet (3,650m/11,975ft) requires a permit and many visitors experience symptoms. Ski resorts in Colorado, including Breckenridge (2,926m/9,600ft) and Leadville (3,094m/10,152ft), see significant rates of altitude sickness.
For these destinations, consider the following strategies:
- Staged ascent: If possible, spend 1-2 nights at intermediate altitude before proceeding higher. For Cusco, this might mean spending nights in the Sacred Valley (lower elevation) before ascending to the city.
- Pharmacological prophylaxis: Start acetazolamide 125-250mg twice daily 1-2 days before arrival
- Minimize activity: Plan light activities for the first 24-48 hours to allow partial acclimatization
- Stay hydrated: Increase fluid intake to counter the effects of dry air and increased breathing
- Avoid alcohol: For at least the first 24-48 hours after arrival
Frequently Asked Questions About Altitude Sickness
Medical References and Sources
This article is based on current medical research and international guidelines. All claims are supported by scientific evidence from peer-reviewed sources.
- Wilderness Medical Society (2024). "Clinical Practice Guidelines for the Prevention and Treatment of Acute Altitude Illness." WMS Guidelines Evidence-based clinical guidelines. Evidence level: 1A
- Luks AM, Auerbach PS, Freer L, et al. (2024). "Wilderness Medical Society Clinical Practice Guidelines for the Prevention and Treatment of Acute Altitude Illness: 2024 Update." Wilderness & Environmental Medicine. Updated clinical practice guidelines from leading wilderness medicine organization.
- International Society for Mountain Medicine (ISMM) (2024). "Consensus Statement on the Prevention and Treatment of Altitude Illness." International expert consensus on altitude illness management.
- Roach RC, Hackett PH, Oelz O, et al. (2018). "The 2018 Lake Louise Acute Mountain Sickness Score." High Altitude Medicine & Biology. 19(1):4-6. Standardized diagnostic scoring system for AMS.
- Luks AM, Swenson ER (2023). "High Altitude Medicine." New England Journal of Medicine. Comprehensive review of high altitude physiology and pathophysiology.
- Bartsch P, Swenson ER (2013). "Acute High-Altitude Illnesses." New England Journal of Medicine. 368(24):2294-302. DOI: 10.1056/NEJMcp1214870 Landmark review article on altitude illness.
- World Health Organization (2023). "International Travel and Health: High Altitude." WHO Travel Advice WHO recommendations for travelers to high altitude.
Evidence grading: This article uses the GRADE framework (Grading of Recommendations Assessment, Development and Evaluation) for evidence-based medicine. Evidence level 1A represents the highest quality of evidence, based on systematic reviews of randomized controlled trials.
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