Altitude Sickness: AMS, HACE, and HAPE Recognition and Treatment

The Three Altitude Syndromes

Altitude illness occurs because oxygen pressure decreases with altitude. At 5,500m, each breath contains roughly half the oxygen available at sea level. The body responds by breathing faster and making blood chemistry adjustments, but this acclimatization takes time. Push the altitude gain faster than the body adapts and you get illness.

The three syndromes exist on a spectrum, not as entirely separate conditions:

Acute Mountain Sickness (AMS) — The mildest form. Most people will experience some AMS symptoms above 3,000m without proper acclimatization. Uncomfortable, but not immediately dangerous if recognized and managed.

High Altitude Cerebral Edema (HACE) — AMS progressing to brain swelling. This is a medical emergency. A person who was confused at high altitude and did not descend has died from this. Descent is non-negotiable.

High Altitude Pulmonary Edema (HAPE) — Fluid accumulation in the lungs at altitude. The most common cause of death from altitude illness. Can develop in otherwise fit individuals, sometimes without preceding AMS.

Recognizing AMS

The Lake Louise Criteria define AMS as: recent gain in altitude plus headache plus at least one of:

• GI symptoms (nausea, vomiting, anorexia)
• Fatigue or weakness
• Dizziness or lightheadedness
• Difficulty sleeping

A headache alone at altitude is common and may not represent AMS, but it should be taken seriously. Rate the headache — a throbbing, positional headache that worsens when lying flat is more concerning than a mild tension-type headache.

What AMS feels like: Many patients describe it as similar to a hangover. Headache, nausea, fatigue, poor sleep. It typically develops 6-12 hours after arriving at a new altitude.

Recognizing HACE

HACE = AMS + altered mental status or ataxia (loss of balance and coordination)

The tandem gait test: ask the patient to walk heel-to-toe in a straight line for 10 steps. Inability to do this at altitude without another explanation = positive test for ataxia = treat as HACE.

Mental status changes: confusion, irrational behavior, unusual drowsiness, or inability to perform simple tasks are all warning signs.

HACE can progress from confusion to coma in hours. Do not wait to see if it improves on its own.

Recognizing HAPE

HAPE = exercise intolerance + at least two of the following:

• Cough (often dry early, becomes productive with pink-tinged frothy sputum late)
• Breathlessness at rest
• Chest tightness
• Weakness and fatigue disproportionate to exertion level

Plus two of these signs:

• Crackling sounds in the lungs (bilateral)
• Central cyanosis (blue lips, fingernails)
• Rapid respiratory rate at rest (above 25/min)
• Rapid heart rate at rest (above 110/min)

The key red flag: breathlessness at rest. Anyone who cannot breathe comfortably while sitting still at altitude has a problem requiring immediate action.

HAPE can develop in 6-12 hours and can kill overnight. It often worsens during sleep when respiratory drive decreases.

Treatment

AMS Treatment

Do not ascend until fully symptom-free. This is the primary intervention.

Descend 300-500m if symptoms are moderate to severe and do not improve with rest. Descent of even 300m produces meaningful improvement in many cases.

Ibuprofen 600mg every 8 hours — Effective for altitude headache. Multiple RCTs confirm better results than acetaminophen for altitude-specific headache.

Acetaminophen 1000mg every 6 hours — Alternative if ibuprofen is contraindicated.

Rest and hydration — Do not exert yourself. Mild dehydration worsens symptoms.

Acetazolamide (Diamox) 125-250mg twice daily — If available, acetazolamide both treats and prevents AMS by stimulating breathing and accelerating acclimatization. Take at least 24 hours before ascent for prevention, or immediately upon symptom onset for treatment.

Low-flow supplemental oxygen if available — provides symptomatic relief but does not treat the underlying adaptation problem.

HACE Treatment

Descend immediately. This is not optional.

Target minimum descent of 1,000m, or until symptoms improve. Descend at any hour, in any weather, if physically possible.

Dexamethasone 8mg immediately (any route: oral, IM, IV), followed by 4mg every 6 hours. This reduces brain swelling and buys time during descent. It is a bridge, not a cure. A patient who improves on dexamethasone still needs to descend.

Supplemental oxygen at 2-4 L/min if available. Portable hyperbaric chambers (Gamow bags) are effective when descent is impossible.

Do not leave the HACE patient alone. Mental status changes mean the patient cannot self-monitor. Descend with them.

HAPE Treatment

Descend immediately. HAPE is the most immediately dangerous of the altitude syndromes.

Nifedipine 30mg extended release (or 10mg immediate release, repeated in 30 minutes) reduces pulmonary artery pressure and is the standard pharmacological treatment. If available and if blood pressure allows (do not give if hypotensive or systolic below 90).

Supplemental oxygen at 4-6 L/min — often dramatically improves symptoms within 15-30 minutes. The response to oxygen is itself diagnostic.

Minimize exertion — The patient should not exert themselves during descent. Being carried is better than walking if possible.

Portable hyperbaric chamber if descent is delayed or impossible.

Tadalafil or sildenafil (PDE5 inhibitors) are used for HAPE prevention in high-risk individuals but are less established than nifedipine for acute treatment.

Descent Protocol

Descent is the definitive treatment for all altitude illness. Medications are tools to make descent possible and safer, not alternatives to descent.

How far to descend:

• AMS: 300-500m minimum, continue until symptoms resolve
• HACE: Minimum 1,000m, or until mental status fully normalizes
• HAPE: Minimum 1,000m, or until resting respiratory distress resolves

Reascent: After HACE or HAPE, do not attempt to reascend until completely symptom-free for at least 24 hours. Even then, reascend slowly and with medical evaluation first if possible.

Acclimatization Protocol

Prevention is vastly preferable to treatment.

Above 3,000m (10,000 feet):

• Limit sleeping altitude gain to 300-500m per night
• Take a rest day every 3rd day (sleep at the same altitude two nights in a row)
• Ascend slowly, descend to sleep lower if possible
• Avoid alcohol and sedatives the first 48 hours at a new altitude (suppress breathing drive)
• Eat lightly, hydrate well

High-risk individuals — Previous HACE or HAPE is the strongest predictor of future episodes. If you've had HAPE, you're likely to get it again. Take prophylactic nifedipine beginning 24 hours before ascent and during ascent.