What altitude sickness feels like, and the three rules that matter

What it feels like

Mild AMS starts as a headache, usually in the evening after you arrive somewhere higher. Along with it comes some mix of nausea, poor appetite, tiredness out of proportion to the day, dizziness and a night when you hardly sleep.

Trekkers routinely explain all of that away. The headache is dehydration, the tiredness is the long day, the bad sleep is the plywood walls. Sometimes that's right. Above 3,000 m, though, the safe assumption is that a new headache is altitude until you can show it isn't. Our guides work to a simple rule of thumb. If you feel unwell at altitude, it's the altitude, and you prove it's something else before you act as if it is.

The three rules

One: if you have symptoms, you don't go any higher. Not 'a bit higher', and not 'just to the next lodge'. You stay where you are until they've gone.

Two: if your symptoms get worse while you're resting, you go down, and you go then, not tomorrow morning. Dropping even three or four hundred metres reverses mild AMS remarkably reliably, and going down is the only treatment that always works.

Three: never leave someone with symptoms on their own, and never let them go down alone. Judgement is one of the first things altitude takes, and a person who only seems tired can get worse quickly.

When it gets serious

High altitude cerebral oedema is AMS that has gone on to affect the brain. The giveaway is loss of coordination. The person can't walk a straight line heel to toe, and they may also be confused, behave oddly or be drowsy. This is a medical emergency, and the response is to go down straight away, at night if necessary.

High altitude pulmonary oedema affects the lungs. Look for breathlessness at rest that doesn't settle, a cough that won't go away, a crackling sound when breathing, extreme fatigue and sometimes pink or frothy sputum. It's an emergency too, and it's also treated by going down. Both are uncommon on the standard teahouse routes, and people survive both when they're recognised early. What kills people is delay, such as waiting until morning or carrying on up because a flight is booked.

How to climb so it doesn't happen

Above about 3,000 m, sleep no more than roughly 300 to 500 m higher than the night before, and add a rest day for every 1,000 m or so you gain. We build our itineraries around this, and it explains most of why a trek takes as many days as it does.

Climb high, sleep low. On an acclimatisation day you don't lie in. You walk several hundred metres up and come back down to sleep. That's what the Manang day on the Circuit and the Namche day on Everest are for, and skipping them is the most common cause of trouble further up.

Drink a lot more than you feel like. Three to four litres a day is normal. Eat even when you aren't hungry, because your body needs the calories to adapt. Leave alcohol and sleeping tablets alone at altitude.

Walk slowly, absurdly slowly, at a pace that feels wrong for the first twenty minutes. Our guides call it bistarai, Nepali for slowly, and every experienced guide in Nepal starts a climb slower than their clients would like.

Diamox, and what it won't do

Acetazolamide, sold as Diamox, helps the body acclimatise and is widely used. It's a prescription drug, not a vitamin, and whether you take it is a decision for a doctor who knows your history, especially if you have a sulfa allergy or kidney problems.

Diamox is a diuretic, so it makes you pee more and you have to drink more to make up for it. Tingling in the fingers and toes is common and harmless. Some people say fizzy drinks taste strange on it, which is an odd side effect.

It doesn't make it safe to climb faster. On a sensible ascent profile it lowers the chance of AMS, but it doesn't excuse a bad one, and it can hide early symptoms. That's why the three rules still apply while you're taking it.

What we do about it

Our guides carry a pulse oximeter and check everyone in the group each evening from around 3,000 m. It takes thirty seconds and turns a vague worry into a number you can watch over several days.

We plan the ascent profile first and the day count second, which is why our ABC trek stops a night at Deurali. When someone asks us to cut days, the profile is what we protect, and it's why we sometimes say no.

We also turn people around, several times a season, and we never treat it as failure. A trek you came down from is a trek you can do again.

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