Reading the Signs: Recognising and Responding to Altitude Sickness
A practical field guide to the three faces of altitude illness — from the common, mild headache to the rare emergencies — with the symptoms that distinguish them and the clear, unglamorous rule that keeps every traveller safe.
Altitude illness sounds frightening and is, in truth, mostly mild and manageable — provided it is recognised honestly and not ignored. The single rule that underpins everything below is short enough to memorise: never ascend to a higher sleeping altitude while you have symptoms, and if symptoms are severe or worsening, descend. Almost all serious altitude trouble begins with that rule being broken.
This guide sets out the three forms altitude illness takes, the symptoms that tell them apart, and what to do about each. It is written so a traveller — or a travelling companion — can read the signs early, while a problem is still small and the response is still easy.
Acute mountain sickness: common and usually mild
Acute mountain sickness, or AMS, is the everyday form, and on a journey to Cusco’s 3,400 metres or the altiplano a good proportion of travellers will feel a touch of it. Its defining symptom is a headache, usually joined by some mix of fatigue, poor appetite, nausea, light-headedness and broken sleep. It typically appears six to twelve hours after arriving at a new altitude and eases over a day or two as the body adjusts.
Think of mild AMS as a hangover that the high country hands you — uncomfortable, not dangerous, and a clear instruction to pause. The correct response is to stop ascending, rest, hydrate, eat lightly, and treat the headache with a simple painkiller. Stay at your current altitude until you feel well again. Mild AMS is a reason to wait, never a reason to push on.
The warning shift: when mild becomes serious
The rare dangerous forms of altitude illness almost never strike out of a clear sky — they are nearly always preceded by AMS that was ignored or pushed through. That is what makes honest reporting so important: the danger is not the first headache, it is the decision to climb higher despite it.
Watch for a change in character rather than just intensity. AMS that fails to improve with rest, or steadily worsens, is a warning. So is the arrival of new symptoms — confusion, unsteadiness, breathlessness at rest — that do not belong to ordinary AMS at all. A worsening picture is the cue to act, and to act early.
HACE: fluid on the brain
High-altitude cerebral oedema (HACE) is swelling of the brain, and it is a medical emergency. Its hallmark is a change in how the mind and body work: confusion, drowsiness, difficulty walking a straight line — the classic test is asking someone to walk heel-to-toe along a line, which they cannot — and behaviour that is simply not like the person. A severe, unrelenting headache and vomiting often accompany it.
HACE is uncommon, particularly on well-paced itineraries, but it is unforgiving of delay. The response is immediate descent — do not wait for morning — alongside emergency oxygen and medication, and a HACE patient must never be left alone or allowed to descend unaccompanied. On our high journeys, guides are trained to spot the early loss of coordination precisely because catching it early changes everything.
HAPE: fluid in the lungs
High-altitude pulmonary oedema (HAPE) is fluid accumulating in the lungs, and it is the other altitude emergency. Its signature is the breathing: breathlessness at rest, not merely on exertion, a marked drop in exercise tolerance, a persistent cough that may bring up frothy or pink-tinged sputum, unusual chest tightness and a fast heartbeat. It can develop without a bad headache, which is part of why it catches people out.
HAPE, like HACE, demands immediate descent, kept oxygen and urgent medical help. A simple distinction is worth carrying: if you are breathless and coughing badly while completely at rest, that is not ordinary acclimatisation and not the dry mountain cough — it is a reason to descend and get help now. Both emergencies are rare; both are survivable when answered quickly and fatal when ignored.
The response that always works: descent
Every form of altitude illness has the same ultimate cure, and it is not a drug or a cylinder of oxygen — it is going down. A descent of even 500 to 1,000 metres often produces a dramatic improvement, and for the serious forms it is the treatment, with oxygen and medication serving only to buy time while the descent happens.
This is why our high journeys are built the way they are. Guides on Andes to Antarctica, The Silk Road Reborn and The Long Way East carry pulse oximeters and oxygen, check travellers daily, and crucially have the experience and the authority to call a descent early. The itineraries themselves keep a lower altitude within reach. Recognising altitude sickness is half the skill; the other half is the humility to respond — and descent, never stubbornness, is always the answer.
Quick answers
How do I tell ordinary altitude sickness from something dangerous?
Mild acute mountain sickness is essentially a headache with some fatigue, poor appetite, nausea or broken sleep, and it eases with rest at the same altitude. The dangerous forms announce themselves differently: confusion, drowsiness or an inability to walk a straight line point to brain swelling (HACE); breathlessness and a cough while completely at rest point to fluid in the lungs (HAPE). Either of those is an emergency.
What should I do if I get a headache at altitude?
Treat a mild headache as ordinary acute mountain sickness: stop ascending, rest, drink water, eat lightly and take a simple painkiller, and do not move to a higher sleeping altitude until it clears. Tell your guide so it can be monitored. If the headache is severe, ignores rest and painkillers, or comes with confusion, unsteadiness or breathlessness, treat it as serious and descend.
Is descent always necessary, or can oxygen and medication be enough?
For mild altitude sickness, simply pausing at your current altitude until you recover is usually enough — no descent needed. For severe or worsening illness, and for the emergencies HACE and HAPE, descent is the definitive treatment; oxygen and medication only buy time. The guiding principle is simple: when in doubt, go down, and go down promptly.
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