Altitude Sickness: Pre-travel Assessment and Prevention Factsheet

Altitude Sickness: Pre-travel Assessment and Prevention Factsheet

Definition & Altitudes

• altitude illness: collective term for syndromes occurring in unacclimatised individuals travelling above 2500m due to hypobaric hypoxia
• high altitude: 2500m–3500m (e.g. European Alps, Rockies, Bogota, Cusco)
• very high altitude: 3500m–5500m (e.g. Everest Base Camp, Annapurna Circuit)
• extremely high altitude: >5500m (e.g. Mt Kilimanjaro, Mt Aconcagua)

Key Concepts, Rate of Ascent & Sleeping Height Guidance

• sleeping altitude: the altitude above sea level at which an individual spends the night; guidelines consider sleeping altitude more critical than daytime peak height for determining illness risk
• sleeping altitude gain: the net increase in sleeping elevation between two consecutive nights
• rate of ascent: the single most critical modifiable risk factor for altitude illness
  • first night target: aim to sleep at <2800m on Day 1 of high-altitude travel; flying directly into cities >2800m significantly increases initial risk
  • safe ascent rate: limit sleeping altitude gain to 300m–500m per 24-hour period above 3000m
  • rapid ascent: any sleeping altitude gain >500m between consecutive nights above 3000m; markedly increases altitude sickness risk
• acclimatisation & rest days: physiological adaptation to reduced barometric pressure/oxygen levels
  • build in 1 extra rest day (sleeping at the same altitude) after every 1000m gain in sleeping altitude (or every 2–3 days of trekking)
  • "climb high, sleep low" strategy: daytime excursions to higher elevations encourage acclimatisation while overnighting at a lower altitude reduces hypoxic stress
• misconceptions: high physical fitness and younger age do not protect against altitude illness; heavy physical exertion increases risk regardless of baseline fitness
• pulse oximetry: not recommended for diagnosis; oxygen saturation readings do not reliably correlate with clinical symptoms

Clinical Features & Spectrum

• acute mountain sickness (AMS): onset typically appears during or after the first night
  • symptoms: headache plus fatigue, dizziness, anorexia, nausea, or insomnia
  • incidence: up to 25% with rapid ascent >2500m
• high altitude cerebral oedema (HACE): life-threatening cerebral pathophysiology / end-stage AMS
  • symptoms: ataxia, confusion, altered mental status, severe lethargy
  • incidence: up to 1% at 4000m–5000m
• high altitude pulmonary oedema (HAPE): non-cardiogenic pulmonary oedema
  • symptoms: dyspnoea at rest, cough, cyanosis, tachycardia, rales/crackles, reduced exercise performance
  • incidence: up to 6% at 4500m

Pre-Travel Risk Stratification

• low risk: ascent to <2800m, or >2800m with gradual ascent (<500m/day sleeping altitude gain) and built-in rest days
• moderate/high risk: history of AMS/HACE/HAPE, rapid ascent (>500m/day sleeping altitude gain), ascending >2800m on Day 1 without pre-acclimatisation

Contraindications to High Altitude Travel

• advanced restrictive or obstructive lung disease
• decompensated heart failure or myocardial infarction / stroke within past 6 months
• unstable angina or poorly controlled seizures
• pulmonary hypertension or sickle cell disease
• high-risk pregnancy
• untreated high-risk cerebrovascular abnormalities (aneurysm / AV malformation)

Pharmacological Prevention (note: prophylactic agents are off-label in the UK; usually issued via private prescription)

• high altitude headache:
  • ibuprofen: 600mg TDS orally during ascent
• AMS / HACE prophylaxis (moderate to high risk):
  • 1st line: acetazolamide 125mg BD (moderate risk) or 250mg BD (high risk); start night before ascending above 2800m; continue for 2–4 days after reaching maximum altitude, or until starting descent
  • 2nd line (if sulphonamide allergy/SJS history): dexamethasone 2mg QDS or 4mg BD
• HAPE prophylaxis (only for individuals with a history of HAPE):
  • 1st line: nifedipine ER 30mg BD; start 24 hours prior to ascent; continue 4–7 days after reaching max altitude or until descent
  • 2nd line: tadalafil 10mg BD or sildenafil 50mg TDS
  • 3rd line: dexamethasone 8mg BD

General Clinical Advice

• lifestyle: stay well hydrated (dehydration mimics altitude illness); avoid alcohol during high-altitude travel
• action on symptoms:
  • mild/moderate AMS: cease further ascent; rest until symptoms resolve completely
  • severe AMS / HACE / HAPE: descend immediately by at least 500m–1000m; administer oxygen if available

Post-Travel Differential Diagnoses (If Symptoms Persist at Sea Level)

• AMS differentials: dehydration, hangover, hyponatraemia, carbon monoxide poisoning, exhaustion, migraine
• HACE differentials: stroke, intracranial lesions, hypoglycaemia, drug toxicity, severe hyponatraemia
• HAPE differentials: pneumonia, pulmonary embolism, MI, heart failure, pneumothorax, asthma/bronchospasm

Check the respective Summary of Product Characteristics (SPC) before prescribing any of the medication mentioned above.

Reference:

1. Adhikari S et al. Prevention of altitude illness in adults: preparing people for travel to high altitude.BMJ 2026; 394 :e100532.