High-altitude illness

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Barometric pressure falls with elevation while the oxygen fraction remains about 21%, reducing inspired oxygen partial pressure. Acclimatisation increases ventilation and produces other physiological adaptations. Rapid ascent can outpace this response and cause a spectrum from AMS to cerebral or pulmonary oedema.

AMS and HACE sit on a cerebral spectrum, whereas HAPE is a non-cardiogenic pulmonary oedema driven by uneven hypoxic pulmonary vasoconstriction and high pulmonary artery pressure. HAPE can occur without preceding AMS, and HACE and HAPE can coexist.

The central decision is whether symptoms are mild enough to rest at the same altitude or severe enough to descend now. In a remote environment, clinical pattern and trajectory govern that decision; pulse oximetry, a scoring system or medication response must not overrule ataxia, altered consciousness or dyspnoea at rest.

Key points

Altitude illness usually follows rapid ascent above about 2,500 metres, although susceptible people can become ill lower; sleeping altitude and ascent rate matter more than fitness.
Acute mountain sickness requires recent altitude gain with headache and symptoms such as nausea, dizziness, fatigue or disturbed sleep after excluding a better explanation.
Never ascend to sleep higher while symptomatic. Mild AMS should improve with rest at the same altitude; worsening or persistent illness calls for descent.
High-altitude cerebral oedema presents with ataxia, confusion, behavioural change, drowsiness or coma and is an immediate descent and oxygen emergency.
High-altitude pulmonary oedema causes disproportionate exercise intolerance progressing to breathlessness at rest, cough, chest tightness, tachycardia, hypoxaemia and crackles; fever may be mild and does not prove pneumonia.
Descent is the definitive treatment for HACE and HAPE. Medication must never create permission to continue upward or delay evacuation.
Acetazolamide accelerates acclimatisation and is used for selected prevention or AMS treatment; it is not an instant oxygen substitute and can cause paraesthesia, diuresis and taste disturbance.
Dexamethasone can improve severe AMS or HACE while descent proceeds but does not acclimatise the traveller, so rebound is possible after stopping at altitude.
Nifedipine is a specialist rescue adjunct for HAPE when oxygen and descent are delayed or insufficient; hypotension can make a sick casualty worse.
Pre-travel assessment is important for previous severe altitude illness, pregnancy, pulmonary hypertension, unstable cardiac disease, significant anaemia, chronic lung disease or an itinerary with unavoidable rapid ascent.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Rapid ascent

Ascending faster than acclimatisation permits is the main modifiable risk for acute mountain sickness and its cerebral or pulmonary complications.

Individual susceptibility

Previous altitude illness strongly predicts recurrence, while fitness does not reliably protect against hypobaric hypoxia, whose relevance depends on the complete exposure and clinical history.

High sleeping altitude

Greater attained altitude and large daily sleeping-height gains increase hypoxic stress, particularly during the first nights.

Exertion and intercurrent illness

Heavy early exertion, respiratory infection and pre-existing cardiopulmonary disease may reduce reserve and worsen symptoms during acclimatisation.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Hypobaric hypoxaemia
Falling barometric pressure lowers inspired oxygen tension, reducing alveolar and arterial oxygen despite unchanged oxygen concentration.
2
Cerebral response
Hypoxia increases cerebral blood flow and vascular permeability; maladaptation produces headache and, when severe, vasogenic oedema and raised intracranial pressure.
3
Uneven pulmonary vasoconstriction
Regional hypoxic vasoconstriction raises pulmonary arterial pressure and directs excess flow into less constricted capillaries, thereby altering ventilation, gas transfer or respiratory mechanics.
4
Capillary leak
High capillary stress causes non-cardiogenic alveolar oedema, worsening hypoxaemia and creating a self-amplifying cycle in high-altitude pulmonary oedema.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Acute mountain sickness

Headache after recent ascent with nausea, anorexia, dizziness, fatigue or poor sleep supports AMS. Symptoms often begin within hours; dehydration alone does not usually explain the full cluster.

Severe AMSRed flag

Incapacitating headache, repeated vomiting, inability to perform normal activity or worsening symptoms despite rest means the traveller should descend and be observed closely for evolving HACE.

High-altitude cerebral oedemaRed flag

Truncal or gait ataxia, confusion, unusual behaviour, hallucination, marked drowsiness or reduced consciousness at altitude is HACE until proved otherwise. Headache may not be emphasised by an obtunded casualty.

Early HAPERed flag

An unexpected drop in pace, breathlessness disproportionate to companions, dry cough, chest tightness and tachycardia may precede resting symptoms. Relative exercise intolerance is often the first practical clue.

Established HAPERed flag

Dyspnoea at rest, hypoxaemia out of proportion to peers, crackles, cyanosis and wet or blood-stained cough indicate advanced pulmonary oedema. Descent must not wait for chest imaging.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Altitude and ascent profileFirst step
Why
Establish exposure intensity and pre-test probability.
Interpretation and limitations
Record sleeping elevations, daily gain, rest days, time since arrival, previous episodes and prophylaxis. A rapid gain or previous illness increases risk, but a careful itinerary does not exclude it.
02
Focused neurological examination
Why
Detect HACE before consciousness is lost.
Interpretation and limitations
Assess behaviour, orientation, speech and gait or truncal ataxia when safe. Inability to walk a straight line in context is concerning; intoxication and hypoglycaemia remain differentials.
03
Pulse oximetry trend
Why
Support respiratory assessment and response to oxygen.
Interpretation and limitations
Compare with altitude, device quality and companions where useful. Normal sea-level values are not expected, and an individual reading neither diagnoses AMS nor safely excludes HAPE.
04
Respiratory and cardiovascular examination
Why
Identify HAPE and alternative cardiopulmonary emergencies.
Interpretation and limitations
Record rest and exertional breathlessness, respiratory rate, heart rate, crackles, wheeze, chest pain and perfusion. Crackles may be absent early and are not required before descent.
05
Capillary glucose and basic field observations
Why
Find immediately reversible mimics during evacuation.
Interpretation and limitations
Correct hypoglycaemia and assess temperature, hydration and urine output, but do not let normal glucose or a label of dehydration postpone treatment of neurological signs.
06
Chest imaging and lung ultrasound
Why
Confirm pulmonary oedema and evaluate mimics once definitive care is reached.
Interpretation and limitations
Patchy opacities or diffuse B-lines can support HAPE; focal consolidation, pneumothorax and embolic disease require consideration. Imaging is not required before emergency descent.
07
ECG, blood gas and laboratory profile
Why
Assess severe illness and competing diagnoses in hospital.
Interpretation and limitations
Hypoxaemia and respiratory alkalosis are expected at altitude; acidosis, marked electrolyte disorder, cardiac injury or infection signals complication or another cause. Interpret relative to elevation and treatment.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Dehydration and exhaustion

Headache and fatigue may reflect fluid loss or exertion, but new symptoms after ascent warrant presumptive altitude-illness precautions while dangerous progression is excluded.

Infection

Fever, focal respiratory signs or diarrhoea suggests infection, although high-altitude pulmonary oedema can also cause cough and low-grade systemic symptoms.

Carbon monoxide exposure

Stove or heater exposure in enclosed accommodation causes headache, nausea and confusion across several people, requiring immediate removal and oxygen.

Pulmonary embolism

Pleuritic pain, focal thrombosis risk and disproportionate unexplained hypoxaemia may indicate embolism rather than altitude oedema.

Primary neurological emergency

Focal deficit, seizure, trauma or symptoms unrelated to ascent may indicate stroke or haemorrhage, while ataxia and confusion remain high-altitude cerebral warning signs.

Additional chapter-specific clues

Alternative diagnosisRed flag

Hypoglycaemia, hyponatraemia, dehydration, sepsis, carbon monoxide from a stove, pulmonary embolism, asthma, pneumothorax and exhaustion can mimic or coexist. Treat immediate threats without delaying descent from severe altitude illness.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01Mild AMSStop ascent and reassessFirst stepHeadache with compatible symptoms after ascent, without ataxia, rest dyspnoea or altered consciousness.

1Do not ascend farther or sleep higher; rest, avoid alcohol and sedatives, maintain sensible oral intake and tell a companion or leader.
2Use simple analgesia and antiemetic support when safe, and consider acetazolamide under the expedition or travel-medicine plan.
3Reassess symptoms, gait, breathing and function repeatedly; descend if symptoms worsen, fail to improve over the agreed interval or evacuation access may close.
4Resume ascent only after complete recovery, more slowly and with a revised itinerary; medication must not conceal symptoms to preserve a schedule.

02HACEDescend with cerebral-oedema treatmentAtaxia, confusion, marked drowsiness, abnormal behaviour or coma at altitude.

1Begin descent immediately with competent assistance, protect the casualty from falls and cold, give supplemental oxygen and never allow them to walk alone.
2Give dexamethasone according to the current expedition emergency protocol while evacuation proceeds, documenting dose and response.
3Use a portable hyperbaric chamber only when descent is temporarily impossible, continuing evacuation preparation because improvement may reverse after chamber exit.
4DefinitiveAt definitive care, assess airway, brain, glucose, trauma and coexisting HAPE, and continue specialist monitoring until stable at lower altitude.

03HAPERestore oxygen and reduce altitudeBreathlessness at rest, severe exercise intolerance, hypoxaemia, crackles or wet cough after ascent.

1Stop exertion, give oxygen, keep the casualty warm and upright if comfortable, and organise immediate assisted descent to a substantially lower elevation.
2Use a portable hyperbaric chamber as a bridge when weather or terrain prevents descent, with trained monitoring and plans for rapid extraction.
3Consider nifedipine only under the current specialist protocol when oxygen or descent is unavailable or insufficient; monitor blood pressure and do not give routine diuretics.
4Treat coexisting HACE, and in hospital distinguish pneumonia, pulmonary embolism, asthma and cardiogenic oedema while continuing oxygen and supportive care.

04PreventionBuild acclimatisation into the itineraryTravel planning for sleeping altitude around or above 2,500 metres.

1Avoid a single rapid gain from below 1,200 metres to above 3,500 metres where possible and include acclimatisation before strenuous activity.
2Above 3,000 metres, plan sleeping-height gains generally no greater than about 500 metres daily with a rest day every three or four days, following current TravelHealthPro advice.
3Consider acetazolamide for moderate- or high-risk itineraries or prior susceptibility after checking contraindications, interactions and the current prescribing reference.
4Agree turnaround rules, communication, evacuation and insurance before departure, and obtain specialist advice for pregnancy or important cardiopulmonary disease.

Key medicines and prescribing safety4 treatments · regimens, roles and cautions

Accelerate ventilatory acclimatisation and improve AMS prevention or recovery in selected travellers.

Acetazolamide

Prevention in an adult at moderate or high risk: 125 mg orally twice daily, started 1 day before ascent and continued for at least 2 days after reaching the highest altitude; stop on descent if asymptomatic. This is unlicensed and supplements, never replaces, gradual ascent.

Paraesthesia, polyuria, taste change and metabolic acidosis occur. Check renal disease, electrolytes, pregnancy, interactions and previous severe sulfonamide reaction; it never replaces descent for deterioration.

Reduce cerebral oedema and temporise severe neurological altitude illness during evacuation.

Dexamethasone

For severe AMS or HACE, give the protocol emergency loading and repeated regimen while descent proceeds; verify current adult and paediatric instructions.

Does not promote acclimatisation and symptoms can recur when stopped at altitude. Hyperglycaemia, mood change, infection and gastrointestinal effects matter; do not use improvement to justify reascent.

Reduce hypoxic pulmonary vascular pressure as an adjunct in selected HAPE emergencies.

Modified-release nifedipine

Use only the current HAPE rescue regimen with blood-pressure monitoring when oxygen or prompt descent is unavailable or inadequate.

Hypotension can impair evacuation and perfusion. It is not routine prophylaxis, not a substitute for oxygen or descent, and should not be combined casually with other vasodilators.

Reverse hypobaric hypoxaemia in AMS complications, HACE and HAPE.

Supplemental oxygen

Titrate to clinical improvement and safer saturation using expedition equipment, preserving supply for evacuation and following specialist advice in severe illness.

Equipment can fail or empty in cold remote settings. Oxygen provides time but does not lower altitude; descent remains definitive when HACE or HAPE is suspected.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

High-altitude cerebral oedema

Progressive brain swelling causes ataxia, confusion, reduced consciousness, coma and death without urgent descent and oxygenation.

High-altitude pulmonary oedema

Rapid alveolar flooding produces cough, breathlessness at rest, severe hypoxaemia and potentially fatal respiratory failure, and increasing the burden of otherwise local respiratory disease.

Accidental injury

Impaired judgement, balance and physical performance increase falls, navigation errors and exposure to cold or terrain hazards.

Hypothermia

Illness, immobility and emergency descent in harsh conditions accelerate heat loss and further impair cardiorespiratory and neurological function.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Record headache, nausea, activity limitation, gait, behaviour, respiratory rate, resting breathlessness and cough at frequent intervals rather than relying on one symptom score.
Trend pulse oximetry with altitude, temperature, perfusion and device quality documented; look for change within the individual rather than imposing a sea-level threshold.
During descent, monitor consciousness, walking safety, cold exposure, oxygen supply and evacuation progress because transient improvement can reverse if altitude is regained.
After dexamethasone or nifedipine, check neurological function or blood pressure respectively and continue rescue actions regardless of early symptomatic response.
At definitive care, monitor gas exchange, imaging, fluid balance and cardiac or infective differentials until HAPE resolves and oxygen is no longer required.
Before any future ascent, review the episode, itinerary, medication plan, comorbidity and rescue limits with an experienced travel or altitude-medicine clinician.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Fitness does not confer immunity

Elite aerobic capacity does not guarantee faster acclimatisation. Fit travellers may ascend rapidly or dismiss symptoms, increasing exposure before the illness is recognised.

Sleeping height predicts risk

A high daytime excursion followed by sleep lower is generally better tolerated than the same gain in sleeping elevation. Itineraries should be judged by where nights occur.

Ataxia changes the diagnosis

A mildly nauseated trekker may have AMS; inability to walk heel-to-toe with confusion marks probable HACE and converts rest-at-altitude advice into immediate assisted descent.

HAPE is not fluid overload

The mechanism is uneven hypoxic pulmonary vasoconstriction, not simple excess total-body water. Routine furosemide can worsen volume depletion and is not standard treatment.

Children need behavioural observation

Young children may express headache or nausea only as irritability, reduced play, feeding difficulty or sleep change. Parents and guides should compare behaviour with the child's baseline.

11Common pitfallsFrequent interpretation and management errors.

01
Allowing a symptomatic traveller to ascend because a fixed itinerary, summit target or flight booking seems difficult to change.
02
Calling HACE intoxication or exhaustion without checking gait, behaviour, glucose and recent ascent.
03
Waiting for crackles, a chest radiograph or a particular saturation before descending a person with rest dyspnoea.
04
Using acetazolamide, dexamethasone or nifedipine as permission to continue upward.
05
Giving diuretics routinely for HAPE and worsening intravascular depletion.
06
Placing a casualty in a portable chamber without continuing descent and evacuation planning.
07
Assuming pregnancy, childhood or chronic cardiopulmonary disease can use an adult generic itinerary without specialist review.
08
Failing to consider carbon monoxide from cooking or heating equipment inside a tent as an alternative cluster diagnosis.