Altitude Sickness Symptoms: Early Signs to Watch For

Acute mountain sickness is defined by a headache appearing within roughly six to twelve hours of arriving above about 2,500 metres, together with at least one of nausea or loss of appetite, unusual fatigue, dizziness, or difficulty sleeping. It is common — depending on the ascent rate, somewhere between a quarter and half of people arriving at 3,500 metres will get it. It is also usually self-limiting. The signs that matter most are the ones that suggest it is progressing to high altitude cerebral or pulmonary oedema: confusion, an unsteady walk, or breathlessness at rest. Those mean descend, immediately.

Knowing the difference between an uncomfortable first night and a genuine emergency is the whole point of learning the symptom list.

Why altitude does this

At 2,500 metres the atmospheric pressure is around 750 hectopascals compared with roughly 1,013 at sea level. At 4,000 metres it is about 616, and at 5,500 metres roughly half of sea level. The proportion of oxygen in the air is unchanged, but with less total pressure there is less oxygen pressure driving gas into your blood.

The body responds within minutes — breathing rate rises, heart rate rises, and over days the kidneys adjust blood chemistry and red cell production increases. Acute mountain sickness occupies the gap between arriving and adapting. The current understanding is that mild swelling of brain tissue, combined with changes in cerebral blood flow, produces the headache and the associated symptoms.

Individual susceptibility varies enormously and does not track fitness. Very fit people get altitude sickness routinely, sometimes more often, because fitness lets them ascend faster.

The early symptoms, in the order they usually appear

Headache. Almost always first, and required for the diagnosis. Typically both sides, often frontal or all-over, dull and throbbing rather than sharp, worse when bending forward, worse at night and on waking, and usually only partially responsive to ordinary painkillers.

Loss of appetite and nausea. Food becomes unappealing, and some people vomit. This is one of the most common accompaniments and one of the most useful, because it distinguishes altitude sickness from a simple tension headache.

Fatigue and weakness out of proportion to the effort. Not ordinary tiredness — a heaviness where routine tasks feel disproportionately hard.

Dizziness or light-headedness. Usually mild, often noticed on standing.

Disturbed sleep. Frequent waking, vivid dreams, and periodic breathing — a cycle where breathing speeds up, then pauses, then resumes. Bed partners often notice the pauses before the sufferer does. This is nearly universal above 3,000 metres and disproportionately unpleasant.

Symptoms typically start six to twelve hours after arrival, often overnight, which is why so many people describe waking up feeling terrible on their first morning at altitude.

Scoring it: the Lake Louise system

The standard tool is the Lake Louise Acute Mountain Sickness Score. You need a headache to qualify, then score four items from 0 to 3:

• Headache: none, mild, moderate, severe or incapacitating
• Gastrointestinal symptoms: none, poor appetite or nausea, moderate nausea or vomiting, severe and incapacitating
• Fatigue or weakness: none, mild, moderate, severe
• Dizziness or light-headedness: none, mild, moderate, severe

A total of 3 to 5 with a headache present indicates mild acute mountain sickness; 6 to 9 moderate; 10 to 12 severe. It is a blunt instrument, but the discipline of scoring the same four things each morning is genuinely useful on a trip, because it turns a vague sense of feeling rough into a trend you can act on.

The red flags

These are not more severe altitude sickness. They are different, more dangerous conditions, and they require descent rather than patience.

High altitude cerebral oedema. The distinguishing signs are neurological: confusion, altered behaviour, drowsiness that is hard to rouse, and above all ataxia — an unsteady, drunken walk. The standard field test is the tandem gait test, walking heel-to-toe along a straight line. Someone who cannot do it who could yesterday needs to go down now. This condition can be fatal within hours.

High altitude pulmonary oedema. Breathlessness at rest, a persistent dry cough that may later produce frothy or pink sputum, extreme fatigue, a marked drop in exercise capacity, and often a raised resting heart rate. It can develop without much of a headache at all, which is why people miss it. Also potentially fatal within hours.

The two can occur together. In both cases, descent is the treatment. Everything else — oxygen, dexamethasone, nifedipine, a portable hyperbaric bag — buys time to descend, not permission to stay.

What actually prevents it

Ascend slowly. The single most effective measure. Above 3,000 metres, the standard guidance is to increase sleeping altitude by no more than 300 to 500 metres per night, with a rest day every 3 to 4 days or every 1,000 metres. Climbing high during the day and sleeping lower is fine and helpful — it is sleeping altitude that matters.

Do not fly straight to a high destination and start walking. Arriving by air at 3,500 metres and immediately trekking higher is the classic setup for trouble. Spend a night or two before going higher.

Hydrate sensibly, but do not overdo it. Dehydration is common at altitude because of dry air and increased breathing. Overhydration is also possible and can be dangerous. Drink to thirst plus a little.

Avoid alcohol and sedatives, especially on the first nights. Both suppress breathing, which is the last thing you want when hypoxia is the underlying problem.

Consider acetazolamide. It speeds acclimatisation by prompting the kidneys to correct blood chemistry more quickly. Commonly started a day before ascent and continued for the first days at altitude. Side effects include tingling in the fingers, frequent urination and, memorably, fizzy drinks tasting strange. It is a prescription decision, not a casual one.

Skip the day-one exertion. Take the first day at a new altitude easy.

If you already get pressure headaches at sea level

There is a reasonable overlap between people who are sensitive to weather-driven pressure changes and people who report bad first nights at altitude, though the two are not the same mechanism. Weather pressure changes are tiny — perhaps 20 to 40 hectopascals across a major storm — while a trip to 3,500 metres is a sustained drop of around 350.

If you already track your symptoms against the barometer, an altitude trip is a useful data point at the extreme end of the scale. Comparing your response there with your response to a storm somewhere with a genuinely active barometer — Fox Crossing, Montana, for instance, where chinooks and Arctic outbreaks produce fast, large swings — can help you work out whether the size of the change or the speed of it is what your body reacts to.

FAQ

At what altitude does it start?

Meaningfully above about 2,500 metres, though sensitive people report symptoms from around 2,000. Below that it is rare.

How long does it last?

Mild cases usually resolve in 24 to 48 hours as acclimatisation proceeds, provided you do not go higher. Symptoms that worsen rather than settle are a signal to descend.

Does being fit protect me?

No. Fitness has no reliable protective effect, and fit people sometimes fare worse because they ascend faster. Prior history at altitude is a much better predictor.

Can I treat the headache with ordinary painkillers?

Paracetamol or ibuprofen can take the edge off, and ibuprofen has some evidence for prevention. But masking a headache while continuing to ascend is dangerous — treat the symptom and still respect what it is telling you.

Is altitude sickness the same as a migraine?

No, though they can be confused and altitude can trigger a genuine migraine in someone who gets them. The distinguishing features are covered in more detail in the comparison of acute mountain sickness and migraine.

What if symptoms appear after several days at altitude?

New symptoms after a stable period are less typical of acute mountain sickness and more concerning for pulmonary oedema or an unrelated illness. Get it assessed.

The short version

A headache within twelve hours of arriving above 2,500 metres, plus nausea, fatigue, dizziness or broken sleep, is acute mountain sickness. Do not ascend further until it settles. Confusion, an unsteady walk or breathlessness at rest are different and far more serious — descend immediately. Slow ascent is the best prevention there is.

Curious how much smaller everyday pressure changes compare? Track them with Pressure Pal.