Altitude Sickness

No locations in Australia reach an altitude sufficient to cause altitude illness, but many Australians travel to international destinations where altitude illness is a real risk. Preventive measures and awareness of subtle early symptoms are essential for anyone travelling above 2500 m.

Cause

Increasing altitude reduces atmospheric pressure and available oxygen. This causes variable (organ-dependent) vasoconstriction/vasodilation and vascular fluid leakage that produce symptoms — the precise mechanism is incompletely understood.

Relative risk factors: rate of ascent, maximum height achieved, sleeping elevation, and strenuous exertion at altitude.

The three syndromes

Syndrome	Diagnostic features
Acute mountain sickness (AMS)	Headache with a recent gain in altitude, plus at least one of: GI symptoms (anorexia, nausea, vomiting), fatigue/weakness, dizziness/lightheadedness, or difficulty sleeping
High altitude cerebral oedema (HACE)	“End-stage”/severe AMS — a recent gain in altitude with either (a) mental status change and/or ataxia in a person with AMS, or (b) both mental status change and ataxia without prior AMS
High altitude pulmonary oedema (HAPE)	At least 2 of: dyspnoea at rest, cough, weakness/decreased exercise performance, chest tightness — plus at least 2 of: crackles/wheeze in ≥1 lung field, central cyanosis, tachypnoea, tachycardia

Severity of AMS can be estimated with the Lake Louise Score (www.high-altitude-medicine.com/AMS-worksheet.html) — serial measurements matter more than a single absolute score for tracking response to treatment.

Prevention

Gradual ascent is the best strategy. Ideally take 2+ days to reach an initial 3000 m, then increase sleeping elevation by no more than 500 m/day.

Above 3000 m:

• Moderate risk: increasing sleeping elevation by >500 m/day; ascending >2800 m in a day; or a history of AMS with ascent >2500 m in a day.
• High risk: ascending >3500 m in a day; a history of AMS with ascent >2800 m in a day; increasing sleeping elevation by >1000 m/day; very rapid ascents (e.g. Mt Kilimanjaro); or a history of HAPE/HACE.

Prophylaxis

A sensible ascent profile has no substitute, but where unavoidable:

• Acetazolamide 125 mg (child: 2.5 mg/kg up to 125 mg) PO 12-hourly, starting the day before ascent, continued until acclimatised (usually 2–3 days at target altitude) or until descent begins. It’s a carbonic anhydrase inhibitor acting mainly as a respiratory stimulant; cross-reacts with sulfonamide allergy. Most troublesome adverse effect: perioral/digital paraesthesia. Also: anorexia, polyuria, polydipsia, tinnitus, headache.
• Dexamethasone — reserved for those allergic to or intolerant of acetazolamide (greater adverse-effect burden with continuous use, and not recommended as prophylaxis in children): 2 mg PO 6-hourly or 4 mg PO 12-hourly, from the day of ascent, for no longer than 10 days.
• Nifedipine, tadalafil, sildenafil or inhaled salmeterol may help prevent HAPE in those with a prior HAPE history — specialist advice only.

Treatment

• Descent is the primary treatment. For HACE/HAPE, descent is the highest priority — symptoms improve with descent of 300–1000 m — and further ascent is not advised.
• A portable hyperbaric chamber can temporise pending evacuation.
• Oxygen 2–3 L/min to keep saturations above 90%.
• Severe AMS or HACE: dexamethasone 8 mg PO/IV/IM first dose, then 4 mg 6-hourly (children: 0.15 mg/kg up to 4 mg 6-hourly). Acetazolamide 250 mg (child: 2.5 mg/kg up to 125 mg) PO 12-hourly can also be added for HACE.
• HAPE: nifedipine controlled-release 20 mg 8-hourly or 30 mg 12-hourly.
• If both HAPE and HACE are present, treat both concurrently.

Other considerations

• Sleep disturbance at altitude: acetazolamide (prophylactic dose) helps; other pharmacological options if it persists.
• Comorbidities: medical or psychiatric clearance/specialist discussion before high-altitude travel.
• Pregnancy: travel above 2500 m in the first trimester carries a high miscarriage risk and is strongly discouraged; 2nd/3rd trimester risk is less well studied but obstetrician approval should be sought — most altitude medications are contraindicated in pregnancy.

From https://tgldcdp.tg.org.au/viewTopic?topicfile=altitude-illness&guidelineName=Toxicology%20and%20Wilderness (eTG Toxicology and Wilderness). Cross-checked against TravelHealthPro’s altitude illness factsheet (>3500 m in a day, or >500 m/day sleeping elevation above 3000 m, as the core risk thresholds) — consistent.