High Altitude Illness
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Direct answer
High altitude illness covers acute mountain sickness, high altitude pulmonary oedema and high altitude cerebral oedema, all arising when rapid ascent above roughly 2,500 metres outpaces acclimatisation. Hypoxic pulmonary vasoconstriction drives pulmonary oedema, while cerebral oedema is a vasogenic process in severe cases. Descent is the definitive treatment for all three, with oxygen, nifedipine and dexamethasone as adjuncts, and acetazolamide used for prophylaxis and mild disease.
What you must remember
- Acute mountain sickness: headache plus at least one of gastrointestinal upset, fatigue, dizziness or sleep disturbance, scored by the Lake Louise questionnaire where a total of 3 or more with headache is diagnostic (the 2018 revision dropped the sleep item).
- High altitude pulmonary oedema: the most serious altitude threat for most climbers, typically presenting within two to four days of arrival with breathlessness at rest, dry cough, crackles and hypoxaemia.
- High altitude cerebral oedema: ataxia and altered consciousness are the defining features and demand immediate descent with dexamethasone.
- Acetazolamide: a carbonic anhydrase inhibitor that promotes ventilation during ascent; paraesthesia, polyuria and altered taste are common side effects.
- Ascent rule: above about 3,000 metres, sleeping height should rise by no more than roughly 300 to 500 metres a day, with a rest day every three to four days.
- Treatment priorities: descent first, then oxygen where available, nifedipine or a phosphodiesterase inhibitor for pulmonary oedema, and dexamethasone for cerebral oedema.
- Hyperbaric therapy: a portable hyperbaric chamber is used for cerebral or pulmonary oedema when descent is impossible.
A three-day ascent in the Himalayas
A 24-year-old traveller reaches Leh by road, climbing to about 3,500 metres within two days. Three members of the group then develop three different problems, and the table sets out each one.
| Climber | Key features | Diagnosis and response |
|---|---|---|
| The 24-year-old traveller | Headache, nausea and poor sleep on the second evening; Lake Louise score meets the threshold | Acute mountain sickness: stop ascending, rest, take analgesia and start acetazolamide, with no further gain in altitude until symptoms settle |
| Second member of the group | Breathless at rest, dry cough, crackles and low oxygen saturation on the fourth morning | High altitude pulmonary oedema: immediate descent, oxygen where available and nifedipine, not fluid restriction or diuresis |
| Third member of the group | Ataxic and drowsy | High altitude cerebral oedema: immediate descent, with dexamethasone given while the descent proceeds |
The lesson is that ataxia, not headache, marks the move from mountain sickness to a life-threatening syndrome.
Where the exam tries to catch you
Examiners often present the three conditions in one stem and expect candidates to rank them by danger.
- Trap: treating headache with ataxia as simple mountain sickness Correct: ataxia is the discriminating sign for cerebral oedema, while headache alone is acute mountain sickness, so a stem with both belongs to the more severe category.
- Trap: offering acetazolamide for established cerebral oedema Correct: it is prophylaxis and a mild-disease drug, and dexamethasone with descent is the correct pairing.
- Trap: choosing drugs while the patient stays at altitude Correct: descent is the mainstay of every form of high altitude illness, so an option proposing drugs without descent is wrong.
- Trap: prescribing acetazolamide without considering sulfa allergy Correct: acetazolamide is a sulfonamide, so caution is needed in sulfa allergy. This becomes a viva point when a patient asks about prophylaxis for a pilgrimage trip.
Frequently asked questions
What scoring system grades acute mountain sickness?
The Lake Louise Acute Mountain Sickness score grades acute mountain sickness, scoring headache together with gastrointestinal symptoms, fatigue, dizziness and sleep disturbance. A total of 3 or more with headache confirms the diagnosis, and it guides whether a climber should stop or descend.
What is the first-line treatment for high altitude pulmonary oedema?
Immediate descent is the first-line treatment for high altitude pulmonary oedema, supplemented by oxygen where available. Nifedipine is the usual adjunct to lower pulmonary artery pressure, and most patients improve within hours of descending.
Which drug treats high altitude cerebral oedema?
Dexamethasone is used for high altitude cerebral oedema, given alongside immediate descent, which remains the definitive treatment. Ataxia or reduced consciousness at altitude should prompt urgent descent rather than waiting for symptoms to settle.
Why does acetazolamide help prevent acute mountain sickness?
Acetazolamide causes renal bicarbonate loss and mild metabolic acidosis, which stimulates ventilation and improves oxygenation during ascent. It is used prophylactically in climbers who ascend rapidly, and tingling, polyuria and altered taste are its common side effects.