Altitude Sickness

AMS, HACE & HAPE — Prevention, Recognition & Medication Guide for High-Altitude Travel

Why Altitude Matters for Travelers

High-altitude travel is among the most medically underestimated risks in international tourism. Every year, trekkers in Nepal, tourists visiting Cusco and Machu Picchu, skiers in Colorado, and pilgrims traveling to high-altitude holy sites experience altitude-related illness, ranging from the uncomfortable to the life-threatening.

The good news: altitude sickness is predictable, preventable, and treatable when travelers understand the risks, acclimatise correctly, and know when to descend. This guide covers everything you need to know before traveling above 2,500 metres (8,200 feet).

2,500 m (8,200 ft) Altitude at which significant risk of AMS begins for most travelers

25–85% of Trekkers Develop AMS symptoms above 3,500 m depending on ascent rate and destination

HACE & HAPE Fatal Both can kill within hours if descent and treatment are delayed

Highly Preventable Correct ascent profile and medication reduce risk dramatically

  
  Who is at risk? Previous altitude illness is the strongest predictor of recurrence. Physical fitness does not protect against altitude sickness — elite athletes are equally susceptible. Age, sex, and underlying health play a smaller role than ascent rate and individual physiology.

Understanding Altitude & Acclimatisation

As altitude increases, atmospheric pressure falls and the partial pressure of oxygen decreases. At 3,500 m (11,500 ft), the available oxygen is approximately 65% of sea-level values. At the summit of Everest (8,849 m), it is just 33%.

The body responds to hypoxia by increasing breathing rate, raising heart rate, and — over days to weeks — producing more red blood cells. This process is called acclimatisation. When ascent is faster than the body can adapt, altitude illness results.

The golden rule: "Climb high, sleep low." Ascend during the day to a higher altitude for activity, but return to a lower sleeping altitude at night to allow the body to adapt gradually.

Altitude Zones

High Altitude: 2,500–3,500 m (8,200–11,500 ft)

AMS possible. Acclimatisation recommended. Destinations: Cusco, Lhasa, most of the Alps.

Very High Altitude: 3,500–5,500 m (11,500–18,000 ft)

AMS common. HACE and HAPE risk increases significantly. Destinations: Everest Base Camp, Annapurna Circuit, high Andes.

Extreme Altitude: Above 5,500 m (18,000 ft)

Significant physiological deterioration inevitable. Only experienced mountaineers with acclimatisation and medical support.

Death Zone: Above 8,000 m (26,200 ft)

Acclimatisation impossible. Supplemental oxygen essential. Prolonged exposure leads to rapid deterioration and death.

The Three Altitude Illness Syndromes

AMS

Acute Mountain Sickness (AMS)

Most Common

AMS is the mildest and most common form of altitude illness. It typically begins 6–12 hours after arrival at a new high altitude and resembles a hangover. It is defined by the Lake Louise Score (LLS) — a headache at altitude plus one or more additional symptoms.

Primary Symptom Headache at altitude — typically bifrontal, worsening with exertion, bending over, or lying flat.

Additional Symptoms Nausea or vomiting, fatigue, dizziness, difficulty sleeping, loss of appetite.

Onset & Duration Typically 6–12 hours after arriving at altitude. Resolves in 12–48 hours with acclimatisation — if ascent is halted.

Incidence 25% of visitors to 2,500 m. Up to 75–85% of trekkers arriving rapidly at 3,500–4,500 m (e.g. flying to Lhasa or Cusco).

Lake Louise Score (LLS)

Score each symptom 0–3. AMS is diagnosed with headache + LLS ≥3.

Symptom	Score
Headache	0 (none) – 3 (severe incapacitating)
GI symptoms	0 (none) – 3 (severe, vomiting)
Fatigue/weakness	0 (none) – 3 (severe)
Dizziness/lightheadedness	0 (none) – 3 (severe)

Score ≥3 with headache = AMS. Do not ascend further until symptoms fully resolve.

Management of mild AMS: Stop ascent. Rest at current altitude. Ibuprofen or paracetamol for headache. Hydrate well. Descend if symptoms worsen or do not improve within 24 hours. Acetazolamide may be used to speed acclimatisation.

HACE

High-Altitude Cerebral Edema (HACE)

Life-Threatening

HACE is a life-threatening medical emergency. It represents end-stage AMS — brain swelling caused by vasogenic edema. It can progress from confusion to coma and death within hours if descent and treatment are delayed. Anyone with HACE must descend immediately — no exceptions.

Defining Signs Altered mental status (confusion, disorientation, unusual behaviour) AND/OR ataxia (inability to walk heel-to-toe in a straight line).

Symptoms Severe progressive headache, vomiting, extreme fatigue, confusion, hallucinations, loss of coordination, drowsiness, coma.

Onset Usually develops after 1–3 days at altitude, typically following untreated AMS. Can develop rapidly — progression from AMS to coma can occur in <12 hours.

Incidence Rare — approximately 1% of trekkers above 4,000 m. Most cases are preventable with correct ascent protocols and early AMS management.

Ataxia test: Ask the person to walk heel-to-toe in a straight line for 3 metres. Inability to do so without staggering = positive test = descend immediately. Do not wait for morning. Do not wait for weather. Descent is the cure.

Emergency treatment (while arranging descent): Dexamethasone 8 mg immediately, then 4 mg every 6 hours. Portable hyperbaric chamber (Gamow bag) if available. Supplemental oxygen. Descend a minimum of 300–1,000 m immediately.

HAPE

High-Altitude Pulmonary Edema (HAPE)

Most Deadly

HAPE is the most common cause of death from altitude illness. It is caused by fluid accumulation in the lungs — non-cardiogenic pulmonary edema — resulting in progressive respiratory failure. Critically, HAPE can occur without preceding AMS. Mortality without treatment exceeds 50%; with prompt descent it is less than 3%.

Early Signs Decreased exercise tolerance, dry cough, breathlessness on exertion greater than expected for altitude and fitness level, feeling of chest tightness.

Late Signs Breathlessness at rest, pink or frothy sputum, cyanosis (blue lips/fingertips), gurgling sound when breathing, inability to lie flat.

Onset Typically on the 2nd or 3rd night at a new altitude. Often worse at night and in early morning when breathing rate is lowest.

Incidence 0.1–4% of trekkers above 4,000 m. Higher in those with prior HAPE history (up to 60% recurrence risk). Men affected more than women.

HAPE can occur in apparently healthy, fit individuals with NO previous AMS. A fit trekker who becomes unusually breathless or develops a cough at altitude should be assessed for HAPE immediately.

Emergency treatment (while arranging descent): Nifedipine 30 mg extended-release immediately (or 10 mg immediate-release then 30 mg ER). Supplemental oxygen to maintain SpO2 >90%. Portable hyperbaric chamber if available. Descend immediately — at least 300–1,000 m. Tadalafil or sildenafil as alternatives if nifedipine unavailable.

Condition	Key Symptoms	Onset	Severity	Emergency Treatment
AMS	Headache + nausea, fatigue, dizziness, poor sleep	6–12 hrs after arrival	Uncomfortable — rarely dangerous alone	Stop ascent. Rest. Ibuprofen. Acetazolamide. Descend if worsening.
HACE	Confusion, ataxia, altered consciousness, severe headache	1–3 days; can progress rapidly from AMS	Life-threatening — death possible within hours	Descend immediately. Dexamethasone 8 mg. O₂. Gamow bag.
HAPE	Breathlessness at rest, dry cough → frothy sputum, cyanosis	2nd–3rd night at new altitude	Most deadly — >50% mortality untreated	Descend immediately. Nifedipine. O₂. Gamow bag.

Check Your AMS Risk

Not sure if your symptoms may indicate Acute Mountain Sickness (AMS)? Use our interactive Lake Louise Score (LLS) Calculator to score your symptoms and understand when you should stop ascending.

Use the LLS Calculator

Prevention — The Ascent Profile

Gradual ascent is the single most important preventive measure for all forms of altitude illness. No medication fully substitutes for adequate acclimatisation time. Follow these evidence-based ascent guidelines from the Wilderness Medical Society (WMS) and the International Society for Mountain Medicine (ISMM).

Limit Sleeping Altitude Gain

Above 3,000 m, limit sleeping altitude gain to 300–500 m per day. Do not rush — most AMS, HACE, and HAPE cases occur in travelers who ascended too quickly.

For every 1,000 m gained above 3,000 m, take a rest day — sleep two nights at the same altitude before ascending further.

Fly-In Destinations Need Extra Care

Flying directly to high-altitude cities (Cusco 3,400 m, Lhasa 3,650 m, La Paz 3,640 m, Addis Ababa 2,355 m) bypasses gradual ascent entirely. Risk is significantly higher than trekking at the same altitude.

Plan a 1–2 day acclimatisation stop at an intermediate altitude before flying to very high-altitude destinations. Consider prophylactic acetazolamide.

Recognise & Respect AMS Early

Any AMS symptoms = do not ascend further. Attempting to "push through" AMS dramatically increases risk of progression to HACE or HAPE. Rest at current altitude until completely symptom-free.

The most dangerous phrase in high-altitude travel: "I'll feel better at the top." You won't — ascending with AMS symptoms is how people die.

Hydration & Diet

Increased breathing at altitude causes significant fluid loss. Maintain adequate hydration — 3–4 litres of water per day during active acclimatisation. Avoid alcohol in the first 48 hours at a new altitude.

Eat a high-carbohydrate diet during acclimatisation — carbohydrate metabolism requires less oxygen than fat metabolism. Avoid heavy meals that impair breathing.

Avoid Sedatives & Respiratory Depressants

Sleeping pills, opioids, antihistamines, and alcohol suppress the breathing drive — worsening nocturnal hypoxia at altitude. Avoid all sedating medications during the acclimatisation period.

Sleep disturbance and periodic breathing (Cheyne-Stokes) are normal at high altitude and do not require treatment with sedatives.

Pre-Acclimatisation Where Possible

Spending 1–2 nights at an intermediate altitude (2,000–3,000 m) before the main ascent significantly reduces AMS incidence. For Everest Base Camp trekkers, spending 2 nights in Namche Bazaar (3,440 m) before ascending is standard practice.

Prior exposure to altitude within the preceding 2 months offers some protective acclimatisation carry-over effect.

Example Safe Ascent Schedules

Destination	Arrival Altitude	Recommended Approach	Acetazolamide?
Cusco, Peru	3,400 m (11,200 ft)	Fly to Lima (154 m) first. Transit via Arequipa (2,335 m) for 1–2 nights before flying to Cusco. Rest 1–2 days in Cusco before Machu Picchu.	Recommended if flying direct
Lhasa, Tibet	3,650 m (11,975 ft)	Fly via Chengdu or Kathmandu. Rest 2–3 days in Lhasa before any excursions. Avoid exertion on arrival day.	Strongly recommended
Everest Base Camp Trek	5,364 m (17,598 ft)	Fly to Lukla (2,860 m). Spend 2 nights Namche (3,440 m). 1 acclimatisation day per 1,000 m gained. Total trek 12–14 days minimum.	Optional — prophylactic or rescue
Kilimanjaro	5,895 m (19,341 ft)	Marangu or Machame route (6–8 days) preferred over faster routes. Pole pole (slowly slowly) is the guiding principle.	Consider for fast routes
Colorado Ski Resorts	2,800–3,500 m (9,200–11,500 ft)	Spend 1–2 nights in Denver (1,609 m) before heading to ski resorts. Avoid heavy exertion and alcohol on day of arrival.	For highly susceptible individuals

Medications for Altitude Illness

Prescription required in the US: Acetazolamide, dexamethasone, and nifedipine all require a prescription. Discuss your itinerary with a travel medicine physician before departure and obtain prescriptions in advance.

Acetazolamide (Diamox)

First Choice — AMS Prevention

Acetazolamide is a carbonic anhydrase inhibitor that speeds acclimatisation by stimulating breathing — increasing the rate of acclimatisation approximately 2-fold. It is the only medication with strong evidence for AMS prevention and is the first-choice drug recommended by the WMS and ISMM.

Prophylactic Use125–250 mg twice daily, starting 1–2 days before ascent above 2,500 m. Continue for 2 days after reaching highest altitude or until acclimatised.

Treatment of AMS250 mg twice daily to speed acclimatisation in established mild-moderate AMS. Combined with rest at current altitude — do not ascend.

ContraindicationsSulfonamide allergy (cross-reactivity). Pregnancy (relative). Severe renal or hepatic disease. Addison's disease. Hyponatraemia.

Side EffectsTingling in hands and feet (paraesthesia) — common and harmless. Increased urination. Altered taste of carbonated drinks. Mild diuresis. Photosensitivity.

Pregnancy & ChildrenAvoid in pregnancy if possible. Paediatric dosing available (2.5 mg/kg twice daily). Not for breastfeeding.

Important NoteAcetazolamide treats the cause of AMS by accelerating acclimatisation — it does not simply mask symptoms. This is why it is preferred over symptomatic treatments alone.

Who should consider prophylactic acetazolamide: Anyone with a prior history of AMS, those flying directly to high altitude, trekkers on fast ascent schedules (e.g. Kilimanjaro 5-day routes), and those who cannot afford any altitude illness disruption.

Dexamethasone

HACE Emergency Treatment

Dexamethasone is a potent corticosteroid that reduces brain edema. It is the emergency treatment for HACE and a second-line option for AMS prevention when acetazolamide is contraindicated. Unlike acetazolamide, it does not accelerate acclimatisation — it suppresses symptoms. If used prophylactically, AMS may rebound when the drug is stopped.

HACE Emergency Dose8 mg immediately (oral, IM, or IV), then 4 mg every 6 hours until descent is complete. This buys time — descent remains mandatory.

AMS Prevention (2nd line)2 mg every 6 hours or 4 mg every 12 hours starting day of ascent. Use only when acetazolamide is contraindicated.

ContraindicationsSystemic infection, diabetes (raises blood glucose significantly), peptic ulcer disease. Avoid long-term use — adrenal suppression.

Key LimitationDoes not accelerate acclimatisation. Treats the symptom, not the cause. Rebound AMS can occur if ascent continues after stopping the drug.

Dexamethasone for HACE is a bridge to descent — not a substitute for it. Even if symptoms improve dramatically with dexamethasone, the patient must still descend.

Nifedipine

HAPE Treatment & Prevention

Nifedipine is a calcium channel blocker that reduces hypoxic pulmonary vasoconstriction — the primary mechanism causing HAPE. It is the first-line pharmacological treatment for HAPE and is also used prophylactically in individuals with a prior HAPE history.

HAPE Emergency Treatment10 mg immediate-release stat, then 30 mg extended-release every 12–24 hours. Continue until descent and clinical improvement.

HAPE ProphylaxisFor individuals with prior HAPE history: 30 mg extended-release twice daily, starting the day before ascent above risk altitude.

ContraindicationsHypotension (systolic BP <90 mmHg). Not for routine prevention in those without prior HAPE. Interaction with PDE5 inhibitors (sildenafil/tadalafil).

AlternativesTadalafil (10 mg twice daily) or sildenafil (50 mg three times daily) as HAPE prophylaxis alternatives with similar efficacy. Do not combine with nifedipine.

OTC

Ibuprofen & Symptomatic Relief

Over the Counter

Ibuprofen for AMS Headache600 mg every 8 hours significantly reduces AMS headache. A 2012 RCT demonstrated ibuprofen reduced AMS incidence in trekkers on rapid ascent schedules. Take with food.

Paracetamol (Acetaminophen)500–1000 mg for headache relief. Widely used and safe at altitude. Does not prevent AMS but reduces headache discomfort during acclimatisation rest.

What NOT to UseDo not use antiemetics or sedatives to manage AMS symptoms. Do not use aspirin in children. Avoid paracetamol overdose — liver metabolism is impaired at very high altitude.

Important ReminderSymptomatic treatment of AMS headache does not accelerate acclimatisation. Do not ascend while symptomatic even if pain relievers are controlling the headache.

Medication	Primary Use	Dose	Rx Required?	Key Caution
Acetazolamide (Diamox)	AMS prevention & treatment	125–250 mg twice daily	Yes	Sulfa allergy contraindication
Dexamethasone	HACE emergency; AMS 2nd-line prevention	8 mg stat then 4 mg/6h (HACE)	Yes	Does not acclimatise; bridge to descent only
Nifedipine	HAPE treatment & prevention	10 mg IR stat, 30 mg ER bd	Yes	Avoid if hypotensive; don't combine with PDE5i
Tadalafil / Sildenafil	HAPE prophylaxis (prior history)	Tadalafil 10 mg bd; sildenafil 50 mg tds	Yes	Do not combine with nifedipine or nitrates
Ibuprofen	AMS headache relief	600 mg every 8 hours	No (OTC)	Take with food; does not treat AMS cause

Emergency Equipment & When to Descend

Portable Hyperbaric Chamber (Gamow Bag)

A Gamow bag simulates descent by 1,500–2,500 m when inflated. Used as a bridge to actual descent for HACE and HAPE when immediate physical descent is impossible due to terrain or weather.

Simulates 1,500–2,500 m descent Bridge only — not a substitute for descent

Supplemental Oxygen

Portable oxygen canisters and cylinders are used as emergency treatment for HACE and HAPE while arranging descent. Aim to maintain SpO₂ above 90%. Pulse oximeters are essential kit for high-altitude trekkers.

Maintain SpO₂ >90% Pulse oximeter essential

Descent — The Definitive Treatment

For HACE and HAPE, descent of at least 300–1,000 m is the single most effective treatment. Begin descent immediately — do not wait for improvement with medication or supplemental oxygen before descending.

Never leave a HACE or HAPE patient alone. Confusion and ataxia mean the patient cannot safely assist in their own descent.

Situation	Action	Urgency
Mild AMS (headache + 1 symptom, LLS 3–4)	Stop ascent. Rest. Ibuprofen/paracetamol. Acetazolamide. Monitor closely.	Pause ascent
Moderate AMS (LLS 5–6, not improving in 24 hrs)	Descend 300–500 m. Acetazolamide. Do not re-ascend until fully symptom-free.	Descend
Severe AMS (LLS ≥7, significant impairment)	Descend immediately. Dexamethasone. Supplemental O₂ if available.	Descend immediately
HACE (confusion / ataxia)	Descend minimum 1,000 m NOW. Dexamethasone 8 mg. O₂. Gamow bag if descent delayed.	Emergency — immediate descent
HAPE (breathlessness at rest / cyanosis)	Descend minimum 1,000 m NOW. Nifedipine. O₂. Gamow bag if descent delayed.	Emergency — immediate descent

Special Considerations

Cardiac Conditions

Altitude increases cardiac workload. Well-controlled mild-moderate hypertension and stable coronary artery disease do not preclude high-altitude travel, but unstable angina, recent MI, heart failure, or pulmonary hypertension are relative to absolute contraindications. Seek cardiology clearance before trekking above 3,500 m.

Respiratory Conditions

Mild-moderate stable asthma is usually well-tolerated at altitude — cold dry air may trigger bronchospasm, so carry rescue inhalers. Moderate-severe COPD, pulmonary hypertension, and significant sleep apnoea carry significant risk. Medical review essential before travel above 2,500 m.

Pregnancy

Altitude travel during pregnancy reduces oxygen delivery to the fetus. Avoid new ascents above 3,500 m during pregnancy. Brief stays at moderate altitude (2,500–3,000 m) are generally tolerated in low-risk pregnancies. Acetazolamide is contraindicated in pregnancy.

Children

Children develop AMS as readily as adults but may not reliably report symptoms. Watch for irritability, loss of appetite, and decreased activity as AMS signs in young children. Acetazolamide can be used in children (2.5 mg/kg twice daily). Dexamethasone is appropriate in emergencies.

A Note from Traveler Health MD

Altitude illness does not respect fitness, experience, or determination. The mountaineers who suffer the most are often those who push through early warning signs out of pride or schedule pressure. The most important altitude medicine principle is simple: if you feel unwell at altitude, stop. If you don't improve, go down. No summit or itinerary is worth a life.

Before any high-altitude trip, obtain prescriptions for acetazolamide and emergency medications, carry a pulse oximeter, brief your group on HACE and HAPE warning signs, and have a clear descent plan before you need it.

World Elevation Finder — Interactive Map

Sources & further reading: CDC Yellow Book — Altitude Illness · Wilderness Medical Society Guidelines · ISMM — International Society for Mountain Medicine · ISTM Guidelines