Altitude sickness - the range of medical conditions caused by the body's inadequate adaptation to reduced oxygen availability at high elevation - is the primary safety concern for trekkers and climbers in Nepal, and the most important piece of medical knowledge for anyone planning a trip above 3,000 metres. Understanding altitude sickness is not optional preparation for a Nepal trek - it is a fundamental part of mountain safety that can mean the difference between a manageable inconvenience and a life-threatening emergency. This guide covers the three main conditions (AMS, HACE, HAPE), their recognition, their prevention, and their treatment in the Nepal high-altitude context.
Why Altitude Makes You Sick
At sea level, the air contains approximately 21% oxygen at a pressure of 101 kPa (1 atm). As altitude increases, atmospheric pressure drops: at 3,500 m (approximately the elevation of Namche Bazaar), the available oxygen per breath is roughly 65% of sea-level. At 5,364 m (Everest Base Camp), it is approximately 53%. At 8,849 m (Everest summit), it is approximately 33%. The human body compensates for reduced oxygen through increased breathing rate, elevated heart rate, and over days to weeks, through production of additional red blood cells and physiological changes that improve oxygen uptake efficiency. These adaptations take time - which is why ascent rate is the most important factor in altitude illness prevention. Ascend too fast, and the body cannot adapt before the physiological stress becomes acute.
Acute Mountain Sickness (AMS)
AMS is the mildest and most common altitude-related illness - essentially a severe headache combined with one or more of the following symptoms: nausea, vomiting, fatigue, dizziness, poor sleep, and loss of appetite. The Lake Louise Score (a validated clinical assessment tool used by mountain medicine practitioners worldwide) defines AMS as a headache plus at least one of the above symptoms in the context of recent ascent. AMS typically appears within 6-12 hours of arrival at a new elevation and usually resolves within 24-48 hours if the ascent is stopped and time is given for acclimatisation.
What to do if you have AMS: Stop ascending. Rest at the current altitude. Drink 3-4 litres of fluid per day. Take ibuprofen or paracetamol for the headache. Do not ascend further until symptoms have fully resolved (typically 1-2 days). If symptoms do not improve within 24 hours of rest, descend 300-500 m. AMS that does not improve with rest and does not resolve after 24-48 hours should be treated as potential progression to HACE - see below.
High Altitude Cerebral Oedema (HACE)
HACE is a medical emergency - the severe end of the AMS spectrum, caused by swelling of the brain due to fluid accumulation at high altitude. HACE is fatal if not treated urgently. Symptoms: severe, persistent headache that does not respond to pain relief, loss of coordination (ataxia - the patient cannot walk a straight line, stumbles, or cannot stand without support), confusion, altered consciousness, and eventual coma. The progression from AMS to HACE can be rapid (hours), which is why the ataxia test (can the patient walk heel-to-toe in a straight line?) is the single most important HACE screening tool at altitude.
Treatment: DESCEND IMMEDIATELY. HACE requires immediate descent of at least 500-1,000 m - the single most effective treatment for any severe altitude illness. While preparing for descent: administer dexamethasone 8 mg immediately, then 4 mg every 6 hours (all Adventure Peaks Nepal expedition groups carry dexamethasone in the group first-aid kit). If descent is impossible due to terrain or weather, use a Gamow bag (portable hyperbaric chamber) - Adventure Peaks Nepal carries Gamow bags on all Khumbu and high-altitude expedition operations. Arrange helicopter evacuation as soon as weather permits. Do not leave a HACE patient alone or allow them to sleep without a guardian present.
High Altitude Pulmonary Oedema (HAPE)
HAPE - fluid accumulation in the lungs at altitude - is the most common cause of altitude-related death. It can develop without significant prior AMS symptoms and can progress from mild to fatal within hours. Early symptoms: reduced exercise tolerance (becoming breathless on terrain you managed easily the day before), dry cough that progressively becomes productive, crackling sound in the lungs when breathing deeply. Advanced symptoms: extreme breathlessness even at rest, pink or frothy sputum, cyanosis (blue lips and fingernails), inability to lie flat without breathlessness.
Treatment: DESCEND IMMEDIATELY. Like HACE, HAPE requires immediate descent - do not wait for the patient to deteriorate further. Administer nifedipine 30 mg slow-release immediately (carried in all Adventure Peaks Nepal group first-aid kits), supplemental oxygen if available, and use a Gamow bag if descent is impossible. Arrange helicopter evacuation urgently. HAPE can be reversed completely by descent if treated promptly - the lungs clear within hours to days once the patient reaches a lower altitude.
The Golden Rule of Altitude - Acclimatise, Don't Rush
The single most effective prevention for all altitude illness is controlled ascent. The widely cited rule - "climb high, sleep low" - summarises the most important principle: above 3,000 m, do not increase your sleeping altitude by more than 300-500 m per day, and build in rest days every 2-3 days of ascent. On the Everest Base Camp Trek, the standard itinerary includes acclimatisation days at Namche Bazaar (3,440 m) and Dingboche or Pheriche (4,200-4,400 m) for precisely this reason - the days are not wasted time but physiologically necessary pauses that allow the body's red blood cell production and breathing pattern adaptation to catch up with the altitude gained.
Practical acclimatisation advice:
- Never increase sleeping altitude by more than 500 m per day above 3,000 m.
- On each acclimatisation day, do a "climb high, sleep low" day hike: ascend 400-600 m above your sleeping altitude, then return and sleep lower.
- Drink 3-4 litres of fluid per day at altitude (more than you feel you need).
- Avoid alcohol and sleeping pills at altitude - both suppress the respiratory drive that assists altitude adaptation.
- Eat even if you have no appetite - adequate caloric intake supports the physiological demands of acclimatisation.
- If you feel worse, do not ascend - rest for 24 hours before making a descent decision.
Diamox (Acetazolamide) - Does It Work?
Diamox (acetazolamide) is a carbonic anhydrase inhibitor that accelerates acclimatisation by stimulating increased breathing rate and improving oxygen uptake at altitude. It is clinically proven to reduce AMS incidence and severity. Standard preventive dose: 125 mg twice daily, starting 1-2 days before ascent. Side effects: tingling in hands and feet (common, benign), increased urination, occasional metallic taste in carbonated drinks. Diamox is sulfa-based - people with sulfa drug allergies should not take it. It requires a prescription in most countries but is available over the counter in Nepal. Diamox is a supplement to - not a substitute for - proper acclimatisation. It does not make you immune to altitude illness and should not be used to enable faster ascent than your acclimatisation schedule would otherwise allow.
Our Emergency Protocol
Every Adventure Peaks Nepal expedition and trekking group above 3,500 m is covered by a documented altitude emergency protocol: a group first-aid kit including dexamethasone, nifedipine, and supplemental oxygen; a Gamow bag on all Khumbu and high-altitude expeditions; a pulse oximeter for daily blood oxygen monitoring; and a satellite communicator for emergency contact. All our guides hold wilderness first responder certification with specific altitude medicine training. We monitor our clients' blood oxygen saturation (SpO2) every morning on all high-altitude trips - a SpO2 reading below 80% at a given altitude is an early warning indicator that additional acclimatisation time or descent may be required. Every client is briefed on AMS, HACE, and HAPE recognition before the trek begins - with the explicit understanding that they should report symptoms to the guide immediately and honestly, without concern that doing so will end their trip.
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