Acute mountain sickness is the main reason people abandon treks in Nepal. It does not depend on age, fitness or experience. A marathon runner can be prone to it, while a 70-year-old walker may feel nothing at all. The good news: it can almost always be prevented, as long as you know the rules and stick to them.
What is acute mountain sickness?
The higher you go, the less oxygen there is in the air. At 3,500 m, each breath brings in about a third less than at sea level; at 5,400 m, half as much. The body takes several days to adapt. If you climb faster than it can adapt, it suffers. This is called acute mountain sickness, or AMS. It usually appears above 2,500 m, and especially above 3,000 m.
The symptoms to know
AMS feels like a hangover. It starts 6 to 12 hours after arriving at a new altitude:
- headache, the most common sign;
- loss of appetite, nausea;
- unusual tiredness, dizziness;
- poor sleep, waking up often.
These signs are harmless if they stay mild and go away with rest. They become serious if they get worse despite rest, or if you develop a cough with breathlessness at rest, confusion, an unsteady walk or repeated vomiting. These signs point to fluid on the lungs or the brain. The only answer then is to go down at once, even at night.
The five rules that prevent almost all problems
- Climb slowly. Above 3,000 m, do not raise your sleeping altitude by more than 300 to 500 m a day. What counts is the altitude where you sleep, not the highest point of the day.
- Plan acclimatisation days. One rest day for every 1,000 m gained. On the Everest trek, these are the days in Namche Bazaar and Dingboche. On the Annapurna Circuit, the day in Manang. These days are not wasted time: they are what get you over the pass.
- Climb high, sleep low. On rest days, take a short walk up to a higher point, then come back down to sleep. The body adapts faster.
- Drink plenty. 3 to 4 litres a day. The dry air and the effort dehydrate you without you noticing. Avoid alcohol and limit coffee.
- Never go up with symptoms. A mild headache: stay at the same altitude for one more night. Symptoms getting worse: go down. This rule is simple and it saves lives.
Medication
Acetazolamide (Diamox) speeds up acclimatisation. Many trekkers take it as a preventive from 3,000 m, at a low dose. It has side effects (tingling, needing to urinate often, fizzy drinks tasting different). It does not replace a gradual ascent and does not hide an oedema. Talk to your doctor before you leave: they will tell you whether you can take it and at what dose. An ordinary painkiller such as paracetamol relieves the headache.
How we build our itineraries
All our high-altitude treks include acclimatisation days in the right places. Mohan walks with you every day and checks in every evening: how you slept, how you ate, whether you have a headache. If someone shows signs of AMS, the next day’s stage is adjusted: one more night, or a descent. The stages are flexible: it says so on every trek page, and it is true. A successful trek is one you come back from.
For the highest treks, such as Everest via Gokyo and the Cho La or the Annapurna Circuit, we advise choosing the longer version if you have never slept above 4,000 m.
Insurance: essential
In a case of severe AMS, evacuation is by helicopter. It costs several thousand euros and must be paid before take-off if you are not insured. Check that your travel insurance covers trekking up to the altitude of your trek (often 5,000 or 6,000 m) and helicopter repatriation. Bank card insurance does not usually cover you above 3,000 m. We ask you for a certificate of insurance before departure.
Frequently asked questions
At what altitude do you need to be careful?
From 2,500 m for sensitive people, and for everyone from 3,000 m. Treks that stay below 3,500 m, such as the Annapurna Balcony or Mohare Danda, rarely cause problems.
Can you train for altitude before you go?
Not really: acclimatisation wears off within a few weeks. On the other hand, good fitness makes the trek more enjoyable and gives you more margin on the day of the pass. Walk with a pack, up and down hills, several times a week in the two months before you leave.
Are children more sensitive?
They are not more sensitive, but they are less good at expressing their symptoms. With children, stay below 3,000 m or plan an even more gradual ascent, and keep an eye on their appetite and mood.
This article gives general information. It does not replace advice from your doctor, whom you should see before any high-altitude trek.
