Altitude Sickness

What causes altitude sickness

Air pressure falls as you go higher. At 8,000 ft (2,500 m) each breath delivers about 25 percent less oxygen than at sea level; at 18,000 ft (5,500 m), about half. The body adapts (acclimatizes) over days by breathing faster and deeper, increasing heart rate and later making more red blood cells. Altitude sickness happens when you climb faster than this process can keep up.

Risk factors:

• Rate of ascent: the biggest factor, especially sleeping more than 500 m (1,600 ft) higher each night above 3,000 m
• Altitude reached, particularly the sleeping altitude
• Previous altitude sickness
• Living at low altitude
• Flying or driving directly to high destinations, such as Cusco, La Paz, Lhasa or high ski resorts

Fitness does not protect against altitude sickness, and young, fit people often climb too fast. Age is not a strong risk factor.

Symptoms

Acute mountain sickness (AMS)

AMS usually starts 6 to 24 hours after arrival. The main symptom is headache, with one or more of:

• nausea, vomiting or poor appetite
• tiredness or weakness
• dizziness or lightheadedness
• difficulty sleeping, with frequent waking

It feels much like a hangover. Mild AMS usually improves within 1 to 3 days if you stop climbing.

High-altitude cerebral edema (HACE)

HACE is swelling of the brain (cerebral edema) and a progression of severe AMS. Signs:

• confusion, irritability or unusual behavior
• loss of coordination (ataxia): stumbling, unable to walk a straight line
• extreme drowsiness, leading to coma

It can be fatal within 24 hours.

High-altitude pulmonary edema (HAPE)

HAPE is fluid leaking into the lungs, a form of pulmonary edema. It can occur with or without AMS, usually on the second to fourth night at altitude:

• breathlessness on exertion that becomes breathlessness at rest
• a dry cough that becomes wet, sometimes with pink, frothy sputum
• chest tightness, a fast heart rate and fatigue
• crackling sounds in the chest and blue lips

HAPE is the leading cause of death from altitude illness, but it reverses quickly with descent and oxygen.

Comparing the forms of altitude illness

Diagnosis

Altitude sickness is diagnosed from symptoms in the setting of a recent ascent. The Lake Louise score rates headache, gastrointestinal symptoms, fatigue and dizziness. Pulse oximetry can show low oxygen levels, especially in HAPE, but low readings are normal at altitude, so symptoms matter more than the number. Always consider other causes such as dehydration, exhaustion, hypothermia, infection or carbon monoxide from stoves in tents.

Treatment

Acute mountain sickness

• Do not go higher until symptoms resolve
• Rest and avoid strenuous activity
• Drink fluids, but not excessively
• Pain relief: ibuprofen or acetaminophen (paracetamol) at the label dose for headache
• Anti-nausea medicine if needed
• Acetazolamide (prescription) speeds acclimatization and treats moderate AMS
• Descend 300 to 1,000 m (1,000 to 3,300 ft) if symptoms worsen or do not improve after 24 hours

HACE and HAPE

• Descend immediately, even at night, if it is safe to do so. Even 300 to 1,000 m can be life-saving
• Oxygen if available
• Dexamethasone (a steroid) for HACE
• Nifedipine for HAPE
• Portable hyperbaric chamber (a pressurized bag) when descent is not possible
• Evacuation to medical care

Preventing altitude sickness

• Ascend gradually: above 3,000 m (10,000 ft), increase your sleeping altitude by no more than 500 m (1,600 ft) a night
• Take a rest day every 3 to 4 days, or for every 1,000 m (3,300 ft) gained
• "Climb high, sleep low": day hikes to higher points, returning to sleep lower
• Spend a night at an intermediate altitude before flying or driving to very high places
• Avoid alcohol and sleeping pills in the first 48 hours
• Stay hydrated and eat enough
• Acetazolamide taken from the day before ascent for people at moderate or high risk, after checking for sulfa allergy and other contraindications with a doctor
• Know the signs and agree with companions to watch each other

Who should be cautious

People with heart failure, severe lung disease such as COPD, pulmonary hypertension, sickle cell disease, or pregnancy should talk to a doctor before high-altitude travel. Children can get altitude sickness and may show it as fussiness, poor feeding, vomiting or disturbed sleep.

Frequently asked questions

What are the first symptoms of altitude sickness?

A headache is usually the first symptom, starting 6 to 24 hours after arriving at altitude. It often comes with nausea, loss of appetite, tiredness, dizziness and poor sleep. It feels similar to a hangover.

How long does altitude sickness last?

Mild acute mountain sickness usually improves within 1 to 3 days if you stop climbing and rest. Symptoms clear quickly after descending. HACE and HAPE need immediate descent and treatment and can take days to recover from fully.

How do you prevent altitude sickness?

Ascend slowly, increasing sleeping altitude by no more than 500 m (1,600 ft) a night above 3,000 m, take regular rest days, avoid alcohol early on, and stay hydrated. Acetazolamide can be prescribed for people at higher risk or when a fast ascent cannot be avoided.

At what altitude does altitude sickness start?

It typically starts above about 8,000 feet (2,500 meters), although sensitive people can have symptoms from around 6,500 feet (2,000 meters). The higher and faster you go, the more likely it is.

Can you die from altitude sickness?

Yes, from its severe forms. High-altitude cerebral edema and high-altitude pulmonary edema can be fatal within hours without descent and treatment. Recognizing danger signs early and descending promptly prevents almost all deaths.