Altitude Sickness in Nepal: What It Feels Like Before You Realise What It Is
The problem with altitude sickness is that the early stages feel like tiredness or a bad night's sleep. By the time it is clearly something else, you are already in difficulty.
Altitude sickness in Nepal trekking is not a dramatic event that announces itself. It is a gradient. It starts as a cold you think you are fighting off, a headache you blame on the wind, an appetite loss you blame on the food. By the time you are willing to call it altitude, you are at least a day behind where you should have turned around. The guides know this. The trekker rarely does.
Above 3,000 metres, the body's accommodation to oxygen changes from a question to an ongoing negotiation. Most people manage the negotiation. A substantial minority do not. What separates the two groups is almost never fitness, training, or prior experience. It is the willingness to notice symptoms early and to descend when descent is still an option, and that willingness is the hardest thing to hold onto when the trek is going well, the group is moving, and the summit is a day away.
The masquerade phase of altitude sickness on Nepal trekking routes
The first phase of acute mountain sickness does not look like altitude sickness. It looks like everything else. A guide who has watched this happen dozens of times described the pattern: Most of the times, I have found trekkers to be anxious because they have sudden onset headaches. More likely it is the combination of cold wind and dehydration. He is not wrong about the headache, and the cold wind and dehydration are real contributors. They are also the disguise. The same guide described a case that began the same way: it started as general symptoms of Cold and Flu. My client presented herself early in the morning with stuffy nose and some coughing with a mild headache. The client did not have the flu. She had altitude sickness, and the first twelve hours of it looked exactly like a head cold.
What makes the masquerade dangerous is that the rationalisations are all plausible. Dehydration is real at altitude; the air is dry, the water sources are spread out, and most trekkers under-drink. Cold is real. Bad food happens. Tiredness is expected. The overlap between these and early AMS is what buys the sickness its second day before the trekker accepts what it is.
The fitness fallacy
A belief that marathon runners circulate quietly, and that some trekking companies do not correct, is that cardiovascular fitness protects against altitude sickness. It does not. The best a strong baseline does is buy a margin of comfort on the days the body is still acclimatising. It does not prevent onset. A trekker in a reasonably fit group of fifteen reported the outcome without flinching: It doesn't really matter how fit you are either, the fittest people aren't exempt from altitude sickness, and there were 2/15 people who did not make it to Base camp. Two out of fifteen is not an outlier rate. It is close to the median for commercial EBC groups in any given season.
The mechanism is straightforward. Fitness affects how efficiently the cardiovascular system transports oxygen. Altitude sickness is about how much oxygen is available to transport. The first is a quantity that improves with training. The second is a function of altitude, which does not care.
AMS, HACE, and HAPE in plain language
The three tiers are worth knowing. Acute Mountain Sickness is what most trekkers experience. Mild headache, mild nausea, poor sleep, appetite loss. A guide's summary is accurate: Mild Headaches, mild shortness of breath, disturbed sleep patterns, lack of appetite, etc are all mild symptoms... Consider them the unholy combination of altitude sickness. AMS is manageable. You pause the ascent. You drink water. You sleep one or two nights at the same altitude. If it does not clear, you descend 300 to 500 metres and usually it does.
High Altitude Cerebral Edema, HACE, is AMS that has progressed into fluid on the brain. The symptom that distinguishes it is the inability to walk heel-to-toe in a straight line. The headache is severe. Judgment is impaired. The trekker may not realise they are unwell. A companion may. HACE is an emergency. Descend immediately, regardless of time of day.
High Altitude Pulmonary Edema, HAPE, is fluid in the lungs. The diagnostic sign is breathlessness at rest that does not ease with a few minutes of stillness. A dry cough develops. The chest sounds wet when you breathe. A medical source summarised the progression honestly: Most people will feel short of breath as they acclimatize, however headaches, vomiting, difficulty sleeping, and the onset of pulmonary oedema... are serious symptoms. HAPE is also an emergency. Descent is the only effective treatment. Oxygen and medication buy time.
Diamox and what it actually does
Acetazolamide, sold as Diamox, is the prophylactic commercial itineraries most often discuss. It does not cure altitude sickness. It speeds acclimatisation by making the blood slightly more acidic, which drives faster breathing, which delivers more oxygen. A prophylactic dose is 125mg twice daily starting the day before ascent. A treatment dose is higher. Diamox is not a substitute for acclimatisation, and it does not make unsafe ascents safe.
The side effects are tolerable for most people and intolerable for some. Tingling in the fingers and face is near-universal. Frequent urination is near-universal. A metallic taste in the mouth, particularly with carbonated drinks, is common. Some people have allergic reactions, particularly those with sulfa allergies. A trial run at sea level before the trek is prudent. What prevents altitude sickness is ascending slowly. Diamox helps people who are already doing that. For anyone considering it as a way to push an itinerary, it is not that.
Climb high, sleep low
The acclimatisation rule is three words. Climb high, sleep low. Most commercial itineraries claim to follow it. Many do not. An acclimatisation day in Namche at 3,440 metres that involves a day-hike to 3,800 metres and a return to 3,440 is within the rule. An itinerary that sleeps one night at 4,400 after sleeping the previous night at 3,900, and which does not include a higher day-hike between them, is outside it. The difference is whether the body has seen the next altitude before it has to sleep there.
Sleep matters because breathing rate drops during sleep and the body is unsupervised by the conscious mind. Trekkers who feel fine at the teahouse at dusk can wake at 2am with a headache that did not register during the day. A trekker who had ignored that signal described the result: 3 days later i felt altitude sickness...very strong. I told my friend, we should go down, no, i don't want to go down... Stupid as i was, i hiked again until my power was completely gone. The social momentum of the group carried the decision past where the body was willing to go. This is the most common pattern in trekking evacuations.
CIWEC, rescue, and what descent looks like
Kathmandu's CIWEC Clinic is the reference point for altitude medicine in Nepal. It publishes annual data, has English-speaking physicians, and can be reached by phone from most trekking routes. For any trekker running a serious symptom, CIWEC is the call, not the lodge owner.
Helicopter rescue from the main regions is routinely available in daylight and in clear weather. Insurance is non-negotiable. Without it, a rescue bill reaches five figures in USD. With it, the helicopter can be on the ground within an hour of a confirmed evacuation call on a popular trail.
Descent is the only reliable treatment. A trekker on a different route described the moment the decision was made: at about 15,000 ft I lost my appetite, had no energy and developed a pounding headache. Our guide watched me closely and gave me the option to head down. That the guide watched closely is the variable. On a trek with a competent guide, symptoms are caught early. On a solo trek, or in a group whose guide is under social pressure not to interrupt the itinerary, they may not be.
A parallel hazard is gastrointestinal illness. One trekker documented how it compounded the altitude experience: I apparently picked up some intestinal bug just prior to the beginning of the trek... I had diarrhea and vomited once. A gut bug at low altitude is unpleasant. The same bug above 4,000 metres impairs hydration and calorie intake, and altitude reserves collapse faster than they would otherwise. If you are sick at the trailhead, delay the trek.
What the mountain asks
Altitude sickness does not negotiate. It does not respect training, plans, or the money already spent on the permit. Its only demand is that you descend when it begins, and its only trick is that it disguises its beginning as something else. The trekkers who come back from Nepal in good health are not the fittest, and they are not the luckiest. They are the ones who went down 300 metres the night the headache started, and who slept a second night at the same altitude when the first night was not enough. The trek will still be there in the morning. The mountain does not move.
