Altitude sickness on Annapurna treks

Most Annapurna treks begin near Pokhara at about 800 m, where the air holds plenty of oxygen. As you climb, the air pressure drops, and each breath carries less oxygen into your lungs. At 3,000 m you breathe in roughly a third less oxygen than at sea level; at 5,400 m, on Thorong La, about half. The body adapts by breathing faster, raising the heart rate and, over days, making more red blood cells. That adaptation is acclimatisation, and it takes time that no amount of fitness can replace.

On the low treks around Pokhara altitude is not a concern. Poon Hill tops out at 3,210 m and the highest night is Ghorepani at 2,860 m; some people notice a mild headache, but serious illness is rare. Panchase, the Royal Trek and Dhampus stay well below that. The picture changes on the three higher routes: Annapurna Base Camp, Mardi Himal and the Annapurna Circuit. Fit, young trekkers are not protected; in fact they often climb too fast because they feel strong.

What counts most is where you sleep, not how high you walk during the day. These are approximate overnight heights on the main routes, taken from the standard trekking figures.

Altitude illness comes in three forms. Acute mountain sickness, AMS, is common and usually mild: a headache plus one or more of nausea, loss of appetite, tiredness, dizziness and poor sleep. It often appears six to twelve hours after arriving at a new height and usually improves with rest at the same height. The two serious forms are rare on these routes but can kill, and they need immediate descent.

Cold, dehydration and plain tiredness can look like altitude sickness, and so can a stomach bug from the trail. Treat any headache above 3,000 m as altitude-related until it is clearly something else. It is far safer to lose a day resting at Deurali or Manang than to push on and find out the hard way.

• Mild AMS: headache, poor appetite, nausea, tiredness, broken sleep. Rest, drink, do not go higher until it clears.
• Worsening AMS: headache that painkillers do not touch, vomiting, unusual exhaustion. Go down.
• HACE, high-altitude cerebral oedema: confusion, unsteady walking, drowsiness, odd behaviour. Descend at once, even at night.
• HAPE, high-altitude pulmonary oedema: breathlessness at rest, a dry cough that becomes wet, blue lips, a gurgling chest. Descend at once.
• A simple test: ask the person to walk heel to toe in a straight line. If they cannot, treat it as HACE.

Mountain doctors give the same basic advice everywhere, and it works. The rules below are the backbone of every itinerary we plan in the Annapurnas, and we build the days around them rather than around how fast the trail can be walked.

The hardest rule to follow is the last one: telling your guide early. Many trekkers hide a headache because they do not want to slow the group. Mild symptoms caught on the evening they appear can usually be fixed with a rest day; the same symptoms ignored for two days of climbing can end a trek by helicopter.

• Above 3,000 m, sleep no more than 300 to 500 m higher than the night before.
• Take a rest day for every 1,000 m or so gained, spent walking higher and coming back down to sleep.
• Drink 3 to 4 litres of water a day; urine should be pale.
• Avoid alcohol and sleeping pills above 3,000 m.
• Eat well, especially carbohydrates, even if appetite drops.
• Walk slowly enough to talk; the trekkers who rush the first days suffer most.
• Tell your guide about any symptom the day it starts.

Annapurna Base Camp is the route where people are most often caught out, because the trail climbs quickly at the end. Many itineraries go from Bamboo at about 2,310 m to Deurali at 3,230 m and then to ABC at 4,130 m in two days. Most people manage with a headache, but if you feel unwell at Deurali, sleep at MBC rather than going on, or add a night. Starting via Poon Hill gives extra nights near 2,800 m, which helps.

On Mardi Himal the climb from Forest Camp to High Camp is steep in height, and the dawn walk to the viewpoint goes to about 4,200 m. We put a night at Low Camp in the plan for this reason. On the Annapurna Circuit the key is Manang: take at least two nights there, walk high on the rest day toward Ice Lake or Praken Gompa, and go from Manang to Yak Kharka and Thorong Phedi in short days. In the trekking season the Himalayan Rescue Association runs a health post in Manang with daily talks on altitude illness; go to one.

Side trips need the same care. Tilicho Lake, at about 4,919 m, is a popular detour from Manang on the Circuit, and it should only be attempted after proper acclimatisation in Manang, with a night at the base camp lodges below the lake. Ice Lake above Braga is a good acclimatisation walk precisely because you come back down to sleep.

Acetazolamide, sold as Diamox, is widely used to help prevent and treat AMS. It is a prescription medicine with side effects, including tingling fingers and more frequent urination, and it is not suitable for everyone. Talk to your own doctor or a travel clinic before your trip about whether to carry it and how to use it. It is not a substitute for a sensible ascent.

Our guides carry a first-aid kit and a pulse oximeter, which measures blood oxygen from a fingertip. A reading alone does not diagnose altitude sickness, because healthy trekkers often show low numbers at height, but a falling trend together with symptoms helps the decision to go down. Painkillers for headache and rehydration salts are useful in every daypack.

The best treatment for altitude illness is going down. Descending 300 to 500 m usually brings relief within hours. On ABC that means walking from base camp to MBC or Deurali; on the Circuit, from Phedi to Yak Kharka or Manang. If someone cannot walk, porters can carry them, and in serious cases a helicopter can fly from Pokhara or Kathmandu in good weather.

Your travel insurance must cover trekking to the highest point of your route, including Thorong La at 5,416 m if you are doing the Circuit, and helicopter evacuation. Read the small print: many policies stop at 3,000 or 4,000 m unless you buy an extension. Carry a copy of the policy and the emergency number, and give a copy to your guide before the trek starts.

Finally, keep perspective. Thousands of trekkers reach ABC, Mardi Himal and Thorong La every season without serious trouble. Those who get into difficulty have usually climbed too fast, ignored early symptoms or walked on alone. A slow plan, a guide who knows the signs and the willingness to turn back for a day are what keep a trek safe.