What Are The 3 Stages Of Altitude Sickness?

What Are The 3 Stages Of Altitude Sickness?

The three stages of altitude sickness are mild, moderate, and severe. Medical professionals often refer to these as mild acute mountain sickness, moderate acute mountain sickness, and severe acute mountain sickness. The severe stage includes two distinct emergency conditions: high altitude cerebral edema (HACE) and high altitude pulmonary edema (HAPE).

The stages are not always a straight line. A person can move from mild to severe within hours. The progression depends on how high you go, how fast you ascend, and your individual physiology. The only reliable treatment for any stage is descending to a lower altitude.

What Causes Altitude Sickness?

Altitude sickness occurs because the air pressure drops as you gain elevation. Lower air pressure means fewer oxygen molecules enter your lungs with each breath. Your blood still carries the same percentage of oxygen, but the total amount is lower. Your brain and body sense this deficit.

At sea level, blood oxygen saturation is typically 95% to 100%. At 8,000 feet, it drops to around 90% to 92%. At 14,000 feet, it can fall to 80% or lower. The body tries to compensate by breathing faster and increasing heart rate. For most people, these changes cause mild symptoms. For others, the response triggers fluid leakage in the brain or lungs.

The key trigger is how fast you climb. Flying directly to a high-altitude city like La Paz, Bolivia, or Cusco, Peru, is far more likely to cause sickness than hiking up gradually. Your body needs time to produce more red blood cells and adjust its chemistry. This process is called acclimatization.

Stage 1: Mild Acute Mountain Sickness

Mild AMS is the most common form of altitude sickness. It typically begins 6 to 12 hours after arriving at elevations above 8,000 feet. The hallmark symptom is a headache. The headache is often throbbing and worsens with movement.

Other symptoms include fatigue, dizziness, loss of appetite, and trouble sleeping. Some people feel short of breath with mild exertion. The symptoms feel similar to a hangover or a viral illness. Many people mistake it for dehydration or exhaustion.

The Lake Louise Scoring System is the standard tool doctors use to diagnose AMS. A person must have a headache plus at least one other symptom to qualify. Those symptoms are gastrointestinal upset, fatigue, dizziness, or difficulty sleeping. The system is not used for the severe stages.

Mild AMS does not require evacuation. Rest and hydration often resolve it within 24 to 48 hours. Over-the-counter pain relievers like ibuprofen or acetaminophen can reduce the headache. The critical rule is to stop ascending until symptoms improve. Continuing to climb with mild symptoms is dangerous.

Stage 2: Moderate Acute Mountain Sickness

Moderate AMS is significantly worse than the mild form. The headache becomes severe and does not respond well to pain medication. Nausea and vomiting are common. A person may feel extremely weak and unable to eat or drink.

The defining feature of moderate AMS is reduced coordination. This is called ataxia in medical terms. A person may stagger when walking heel-to-toe or struggle to button a shirt. This symptom signals that the brain is beginning to swell.

Shortness of breath at rest is another warning sign. At moderate altitude, breathing should return to normal when you stop moving. If it does not, that is a red flag. Some people develop a dry cough or feel like they have fluid in their chest.

Moderate AMS requires immediate descent. Descending 1,000 to 3,000 feet is usually enough to relieve symptoms. Do not leave the person alone. Do not let them sleep without checking their breathing regularly. If descent is impossible, supplemental oxygen or a portable hyperbaric chamber may help temporarily.

Stage 3: Severe Acute Mountain Sickness (HACE and HAPE)

Severe AMS is a medical emergency. It involves one or both of two conditions: high altitude cerebral edema and high altitude pulmonary edema. Both involve fluid leaking into organs where it does not belong.

HACE is swelling of the brain. It is the progression of moderate AMS to a dangerous level. Symptoms include confusion, irrational behavior, severe lethargy, and loss of consciousness. A person with HACE may not recognize their own family members. They may be unable to walk a straight line even with support.

HAPE is fluid in the lungs. It can occur alone or with HACE. Symptoms include extreme breathlessness at rest, a persistent cough producing pink or frothy sputum, and a gurgling sound in the chest. The person may feel like they are drowning. HAPE can develop in people who showed no signs of mild AMS.

Both conditions are fatal without rapid treatment. The only definitive treatment is immediate descent. Do not wait to see if symptoms improve. Call for rescue if possible. Administer supplemental oxygen if available. A portable hyperbaric chamber can simulate descent temporarily but is not a substitute for actually going down.

How Fast Can Altitude Sickness Progress?

The timeline varies widely between individuals. Mild AMS can appear within hours of reaching altitude. Moderate symptoms can follow within a day. Severe symptoms can develop within 12 to 24 hours after moderate symptoms begin.

In rare cases, HAPE develops suddenly without warning. A climber may feel fine and then collapse from fluid-filled lungs within hours. This unpredictability is why the rule is simple: if symptoms worsen at the same altitude, go down immediately.

The most dangerous situation is sleeping at altitude. Breathing slows during sleep, which lowers oxygen levels further. Many climbers feel acceptable during the day and deteriorate overnight. This is why altitude medicine experts advise checking on anyone with symptoms during the night.

How To Prevent Altitude Sickness

Prevention is far more effective than treatment. The most reliable strategy is gradual ascent. The general clinical guidance is to avoid increasing your sleeping elevation by more than 1,600 feet per day once you are above 9,800 feet. This is a widely accepted rule in altitude medicine.

Acclimatization days help. If you are climbing above 10,000 feet, plan an extra day at intermediate elevations. The classic mountaineering phrase is “climb high, sleep low.” You can hike to a higher point during the day and return to a lower camp to sleep.

Hydration matters but does not prevent AMS. Drinking extra water will not stop altitude sickness. It can prevent dehydration, which worsens symptoms. Avoid alcohol and sleeping pills at altitude. Both suppress breathing and reduce oxygen levels during sleep.

Acetazolamide, sold as Diamox, is a prescription medication that reduces the incidence of AMS. It works by making your blood slightly more acidic, which stimulates breathing. Some clinicians recommend it for people with a history of altitude sickness or those making rapid ascents. It can cause tingling in the fingers and a metallic taste in the mouth. Discuss it with your doctor before traveling.

When To Descend Immediately

Certain symptoms require immediate descent regardless of how you feel overall. Confusion or altered mental status is one. Inability to walk a straight line is another. Severe shortness of breath at rest is a third. Vomiting that prevents fluid intake is also a reason to descend.

Do not wait for a headache to become unbearable. Do not try to “tough it out.” Altitude sickness does not resolve by staying at the same elevation. It only gets worse until you descend or receive oxygen.

The rule of thumb in mountain medicine is simple: if you have any symptoms of AMS, do not ascend further. If you have moderate symptoms, descend. If you have severe symptoms, descend immediately and get emergency help. This is not a guideline that varies between experts. It is the standard of care.