Acute mountain sickness (AMS) is the mild form of altitude illness caused by ascending faster than the body adapts to reduced oxygen, and its prevention rests on a sleeping-altitude rule of no more than 500 m per day above 3,000 m. The Himalayan Rescue Association's Pheriche clinic recommends exactly this limit, and it is the rule our itineraries are built around. Mingma Tendi Sherpa, our safety lead and a graduate of the Himalayan Rescue Association's high-altitude medicine seminar, wrote this guide, reviewed by lead guide Passang Dawa Sherpa.
The sections below cover how altitude illness develops, the Lake Louise scoring used to grade it, the severe forms to recognize, the role of acetazolamide, and the guide protocols that keep our departures safe. This is safety-critical information; it supports but does not replace a briefing from your own guide and doctor.
In this guide 5
How does altitude sickness develop on a trek?
Altitude sickness develops when a trekker ascends faster than the body can adapt to the falling oxygen pressure, and it can affect anyone above 3,000 m regardless of fitness. The first form is acute mountain sickness, which presents as headache, nausea, fatigue, dizziness, and poor sleep, usually within 6 to 12 hours of arriving at a new altitude. Fitness offers no protection: a marathon runner who ascends too fast is at more risk than a moderate walker who follows a measured schedule, because the limiting factor is physiological adaptation, not cardiovascular capacity.
The prevention that works is controlled ascent. The Himalayan Rescue Association's Pheriche clinic recommends ascending no more than 500 m of sleeping altitude per day above 3,000 m, with a rest day every 1,000 m of gain, and our itineraries build in exactly these acclimatization days. The Everest Base Camp trek fixes rest days at Namche (3,440 m) and Dingboche (4,410 m) for this reason.
What is the Lake Louise score?
The Lake Louise score is a standard self-assessment that grades AMS severity by rating symptoms on a point scale, and guides worldwide use it to decide whether a trekker can continue. It scores headache, gastrointestinal symptoms, fatigue, and dizziness from 0 to 3 each, in the presence of a recent altitude gain. A total of 3 to 5 with a headache indicates mild AMS; 6 or more indicates severe AMS that requires descent. Our guides apply this scoring at the twice-daily checks above 4,000 m so a rising score is caught before it becomes dangerous.
The score's value is that it turns vague "feeling off" into a tracked measure, so a trekker cannot talk themselves past worsening symptoms. A climbing score over consecutive checks is a clear instruction to stop ascending, and a high score is a clear instruction to descend.
What are HAPE and HACE?
The severe forms of altitude illness are HAPE and HACE, and both are medical emergencies requiring immediate descent. High-altitude pulmonary edema (HAPE) is fluid in the lungs, presenting as breathlessness at rest, a wet cough, and extreme fatigue, and it can develop from untreated AMS or on its own. High-altitude cerebral edema (HACE) is fluid on the brain, presenting as confusion, loss of coordination (a stumbling, drunk-like walk), and altered consciousness, and it is the most dangerous form. Both can progress from first symptoms to life-threatening within hours.
The treatment for both is the same and non-negotiable: immediate descent, oxygen if available, and evacuation. Our guides carry a pulse oximeter and know the descent routes and helicopter pickup points on every itinerary. The insurance that funds an evacuation is covered in the helicopter evacuation insurance coverage guide, and no trekker should go above 4,000 m without it.
Does acetazolamide prevent altitude sickness?
Acetazolamide (Diamox) speeds acclimatization and reduces the incidence of AMS, and it is the standard prophylaxis our guides discuss at the pre-altitude briefing. Wilderness medicine guidance and the Himalayan Rescue Association describe a preventive dose of 125 mg twice daily started the day before ascending above 3,000 m, though the exact dose is a decision for you and your doctor before the trek. It is a prevention aid, not a cure: it does not mask worsening symptoms, and a trekker on acetazolamide who develops HAPE or HACE still descends immediately.
Trekkers should discuss acetazolamide with a travel doctor 6 to 8 weeks before departure, because it is a sulfonamide and carries contraindications for some people. It also causes tingling in the fingers and a changed taste in fizzy drinks, harmless side effects worth knowing about in advance. Hydration, a measured ascent, and honesty about symptoms remain the foundation; the drug is an aid to those, not a substitute.
What are our guide protocols for altitude?
Our altitude protocols are fixed on every high departure and centre on measurement and authority. Above 4,000 m, guides log each client's oxygen saturation twice daily, at breakfast and dinner, so a falling reading is caught early. Above 3,000 m, sleeping-altitude gains stay under 500 m per day with rest days every 1,000 m, matching the Pheriche clinic guideline. Crucially, the guide holds the authority to turn a trekker around or order a descent, and that decision is not open to negotiation regardless of how much the client wants to continue.
These protocols are why our clients summit rather than turning back at the last camp, and why our evacuations, four coordinated above 5,000 m, all ended in full recovery. The rules apply identically in every season, including the winter departures where cold compounds the altitude challenge.
FAQ
Common questions
How do I prevent altitude sickness on a Nepal trek?+
Ascend slowly: no more than 500 m of sleeping-altitude gain per day above 3,000 m, with a rest day every 1,000 m, the Himalayan Rescue Association's Pheriche clinic guideline. Stay hydrated, avoid alcohol at altitude, and be honest about symptoms. Acetazolamide 125 mg twice daily can aid acclimatization if your doctor approves it, but a measured ascent is the foundation.
What are the warning signs I should descend?+
Descend immediately for any sign of the severe forms: breathlessness at rest with a wet cough (HAPE), or confusion and a stumbling, uncoordinated walk (HACE). For milder AMS, a Lake Louise score of 6 or more, or symptoms that worsen rather than settle with a rest day, also mean descent. Descent is the only reliable cure, and our guides hold the authority to order it.
Does fitness protect against altitude sickness?+
No, altitude sickness affects anyone above 3,000 m regardless of fitness, because the limiting factor is physiological adaptation, not cardiovascular capacity. A very fit trekker who ascends too fast is at more risk than a moderate walker who follows a measured schedule. This is why acclimatization days and controlled ascent matter more than training for the altitude itself.
Should I take Diamox for a Nepal trek?+
Acetazolamide (Diamox) speeds acclimatization and reduces AMS incidence, and many trekkers use it as prophylaxis at 125 mg twice daily. It is a prescription decision for you and a travel doctor 6 to 8 weeks before departure, because it carries contraindications for some people. It aids prevention but does not cure severe illness, so descent still applies if HAPE or HACE develops.
