AMS, HAPE and HACE
Acute Mountain Sickness (AMS) is common above about 2,500 m and usually mild. It means your body hasn't yet adjusted to the lower oxygen. HAPE (high-altitude pulmonary oedema) is fluid on the lungs, and HACE (high-altitude cerebral oedema) is swelling of the brain. Both are rarer, and both can kill within hours if the person doesn't descend.
| Condition | Symptoms | What to do |
|---|---|---|
| Mild AMS | Headache plus one of: nausea, loss of appetite, tiredness, dizziness, poor sleep | Don't go higher. Rest, drink water, take paracetamol or ibuprofen. Continue only when symptoms are gone |
| Moderate to severe AMS | Headache that painkillers don't fix, vomiting, marked weakness | Descend 300–1,000 m. Diamox may help, under medical guidance |
| HAPE | Breathless at rest, dry cough then frothy or pink sputum, blue lips, gurgling chest, very low energy | Descend immediately; oxygen if available; evacuate |
| HACE | Confusion, loss of coordination (can't walk heel-to-toe in a line), drowsiness, behaviour change | Descend immediately, with someone else; evacuate. Life-threatening |
The ataxia test
Ask the person to walk heel-to-toe along a straight line. If they can't do it without stumbling, and they aren't drunk, treat it as HACE and start descending straight away, even at night.
The acclimatisation rules
- Above 3,000 m, increase your sleeping altitude by no more than 300–500 m per night.
- Take a rest day for every 1,000 m gained, or every 3–4 days above 3,000 m.
- Climb high, sleep low: on rest days, walk 300–500 m higher and come back down to sleep.
- Drink 3–4 litres a day. Dehydration doesn't cause AMS, but it causes headaches that are easy to mistake for it.
- Avoid alcohol and sleeping pills above 3,000 m, because they suppress breathing at night.
- Never go up with symptoms. Wait or descend.
Standard itineraries build this in. On the Everest route you rest in Namche (3,440 m) and in Dingboche or Pheriche. On the Annapurna Circuit you rest in Manang (3,540 m). On ABC the gradual ascent through Chhomrong, Dovan and Deurali usually does the job. The risky itineraries are the ones that skip these days to save time.
Diamox (acetazolamide)
Diamox speeds up acclimatisation and is widely used to prevent and treat AMS. Wilderness-medicine guidelines commonly give 125 mg twice a day for prevention, started the day before going above 3,000 m, but the right dose for you is a decision for your doctor. It is sold without a prescription in Pokhara and Kathmandu pharmacies.
- Common side effects: tingling fingers and toes, needing to pass urine more often, and fizzy drinks tasting flat.
- It is a sulfonamide-type drug. Tell your doctor if you're allergic to sulfa drugs.
- It does not hide HAPE or HACE, and it doesn't replace a slow ascent.
- Try a dose at home first to check for side effects.
Consult a doctor
Discuss Diamox, and any existing heart, lung or blood-pressure conditions, with a travel doctor before your trek.
When to descend
- Mild symptoms: stay at the same altitude. If they are gone the next morning, you can continue.
- Symptoms getting worse despite resting a day: descend.
- Any sign of HAPE or HACE: descend at once, even at night, with a companion.
- Can't walk: arrange a horse, porter or helicopter through your guide, lodge or insurer.
Descending 300–500 m often brings quick relief. Never leave someone with altitude symptoms alone in a lodge while the group goes on.
How high do popular treks go?
| Trek | Highest point | Highest sleep | Altitude risk |
|---|---|---|---|
| Ghorepani Poon Hill | 3,210 m | 2,860 m (Ghorepani) | Low |
| Khopra Danda | 3,660 m | 3,660 m | Low–moderate |
| Mardi Himal | 4,500 m (upper viewpoint) | 3,580 m (High Camp) | Moderate |
| Annapurna Base Camp | 4,130 m | 4,130 m | Moderate |
| Langtang Valley | 4,984 m (Tserko Ri, optional) | 3,870 m (Kyanjin Gompa) | Moderate |
| Tilicho Lake | 4,919 m | 4,150 m (Tilicho Base Camp) | High |
| Everest Base Camp | 5,545 m (Kala Patthar) | 5,164 m (Gorak Shep) | High |
| Manaslu Circuit | 5,106 m (Larke La) | 4,460 m (Dharamsala) | High |
| Annapurna Circuit | 5,416 m (Thorong La) | 4,540 m (High Camp) or 4,450 m (Phedi) | High |
Frequently asked questions
At what altitude does altitude sickness start?
Mild AMS can start from about 2,500 m, and it becomes common above 3,000 m, especially if you ascend quickly. Treks below 3,000 m, such as Ghandruk, Australian Camp and Panchase, carry very little risk.
Can you get altitude sickness on Poon Hill?
It is possible but uncommon. Poon Hill is 3,210 m and you sleep at Ghorepani (2,860 m). A mild headache is the usual symptom. If one doesn't go away with rest and water, don't go higher.
Should I take Diamox for Everest Base Camp?
Many trekkers do, and wilderness-medicine guidelines support it for people going above 3,000 m quickly or with a history of AMS. Discuss the dose and any allergies with a doctor before you go. Diamox doesn't replace the standard rest days in Namche and Dingboche.
How many acclimatisation days do I need for Everest Base Camp?
Standard itineraries include two: one in Namche Bazaar (3,440 m) and one in Dingboche or Pheriche (around 4,300 m). This is why most EBC itineraries are 12–14 days from Lukla and back.
Does fitness prevent altitude sickness?
No. Fitness makes the walking easier but doesn't change how your body adapts to low oxygen. Fit people sometimes get sick precisely because they ascend too quickly.
What are the first signs of altitude sickness?
A headache is the usual first sign, often with loss of appetite, nausea, tiredness or poor sleep. Breathlessness at rest, confusion or stumbling are signs of HAPE or HACE and mean you must descend immediately.
Treks to read next
Everest
Everest Base Camp Trek
11–16 days · Challenging · max 5,545 m
Annapurna · from Pokhara
Annapurna Base Camp Trek
6–12 days · Moderate · max 4,130 m
Annapurna · from Pokhara
Mardi Himal Trek
4–7 days · Moderate · max 4,500 m
Annapurna · from Pokhara
Annapurna Circuit Trek
10–18 days · Challenging · max 5,416 m
Manaslu · from Pokhara
Manaslu Circuit Trek
12–18 days · Challenging · max 5,106 m
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