You can train for six months, prepare every piece of gear, and arrive in Kathmandu in excellent physical condition. Altitude sickness on the Everest Three Passes trek can still find you. The Khumbu does not grade trekkers on their preparation. At 5,000m, atmospheric pressure is roughly half what it is at sea level, and that gap between what the body is used to and where you are standing is what altitude sickness is built from.

What does protect you is knowing what altitude sickness looks like on this specific circuit, how to score it accurately each day, when to stay and when to descend, and how to manage the particular problem that makes Three Passes medically different from every other major Nepal trek: on day 16 of this circuit, cumulative fatigue from 15 trekking days produces symptoms that are nearly identical to altitude illness, at the highest and most remote point of the entire route, with no teahouse in either direction for 10 to 12 hours.

This guide gives you the complete picture. Read it before the day-by-day breakdown here before you head out for the trek.

What Altitude Sickness is and Why It Happens on Everest Three Passes Trek?

Altitude sickness is not caused by thin air in the way most people imagine. The percentage of oxygen in the atmosphere is the same at Lukla as it is at sea level: 21%. What changes with elevation is atmospheric pressure. As pressure drops, each breath delivers fewer oxygen molecules to the lungs and bloodstream. At 5,000m, atmospheric pressure is roughly half what it is at sea level.

The body's response is to adapt: breathing rate increases, deeper ventilation is driven, and over days the body begins producing more red blood cells to carry available oxygen more efficiently. Altitude sickness happens when the ascent rate outpaces that adaptation process. You are climbing faster than your body can keep up.

On the Everest Three Passes trek, this matters in a specific way that shorter or lower treks do not create. You sleep above 4,000m for 8 to 10 consecutive nights. You cross 4 separate altitude events above 5,300m. From Lobuche onward, the body stops fully recovering overnight. Each day begins slightly more depleted than the last. That compounding effect is what separates the altitude medicine reality of this circuit from the Annapurna Base Camp trek, which reaches 4,130m and descends the same day, or the Manaslu Circuit's single crossing of Larkya La at 5,160m.

If you want to understand how altitude sickness presents on those routes, our Annapurna Base Camp altitude guide and Manaslu Circuit altitude guide cover the specific conditions of each.

Fitness does not protect against altitude sickness. A highly trained athlete and a less fit trekker on the same ascent profile develop AMS at similar rates. The belief that fitness provides protection is one of the most dangerous misconceptions on this circuit, because it leads fit, confident trekkers to dismiss symptoms that a less confident trekker would take seriously.

Why Everest Three Passes Trek is Medically Different From EBC?

The additional acclimatization stop at Thame (3,800m) is the most underappreciated safety advantage of this circuit. Two nights at Thame, 360m above Namche, before pushing to Lungden (4,380m) and Renjo La (5,360m), gives the body a meaningful extra step of altitude preparation. EBC trekkers going directly from Namche toward Tengboche have one acclimatization stop. Three Passes trekkers going clockwise have two. The difference in physiological preparation for the first pass crossing is real.

AMS, HACE, and HAPE: Three Different Conditions

These are not the same condition at different severity levels. They are 3 distinct medical situations affecting different parts of the body with different timelines and different levels of danger.

AMS: Acute Mountain Sickness

AMS is the warning. Your body signaling that acclimatization has fallen behind. Uncomfortable, sometimes significantly, but manageable with correct action.

HACE: High Altitude Cerebral Oedema

HACE is AMS that has progressed to the point where fluid accumulates around the brain. It is distinguished from severe AMS not by headache intensity but by neurological signs: loss of coordination and altered mental state.

HAPE: High Altitude Pulmonary Oedema

HAPE is the most dangerous of the three. Fluid builds in the lungs. Without treatment, the mortality rate reaches 50 percent. Unlike AMS or HACE, HAPE can develop without any preceding headache. A trekker with no AMS symptoms at all can develop HAPE overnight.

The Lake Louise Score: How to Diagnose AMS on the Three Passes Trek

The Lake Louise Acute Mountain Sickness Score is the standard diagnostic tool used by guides and altitude medicine physicians on this route. It was first established in 1991 and last revised in 2018. The 2018 revision made one change most trekkers do not know about: sleep disturbance was removed from the scoring criteria.

Research showed that disturbed sleep above 4,000m is a normal physiological response to altitude hypoxia and correlates poorly with true AMS. Waking repeatedly at Lobuche or Gorak Shep is expected. It is not, by itself, an AMS signal. Use the four symptoms below only.

Score yourself every evening above Namche, before dinner, after you have stopped walking and had water. Be honest. A mild headache (1) plus poor appetite (1) plus mild fatigue (1) is a total of 3 with a headache present. That is AMS, even though each symptom feels manageable individually. The score is what matters.

Why You Cannot Fully Trust Your Own Judgment Above 5,000m on This Trek?

Hypoxia at altitude affects cognitive function measurably. Reaction time slows, working memory contracts, complex reasoning becomes effortful, and judgment is impaired in ways that are not always obvious to the person experiencing them. At 5,364m at Everest Base Camp, cognitive effects are present and documented. At 5,535m on Kongma La, on day sixteen of the circuit, after fourteen trekking days of accumulated fatigue, those effects are compounded.

This creates a specific problem for self-assessment. A trekker with early HACE may genuinely believe they are fine while showing ataxia on the heel-to-toe test. A trekker with worsening AMS may feel that pushing through to Chukhung is a reasonable decision because their impaired judgment cannot access the full picture of how they are doing. The confidence a trekker feels in their own assessment above 5,000m is itself less reliable than the confidence they would feel at sea level, and they cannot perceive that difference.

Your guide is observing you from outside the impairment. They may notice changes in how you are moving, the quality of your answers, slowing in your responses, before you notice any of these things yourself. A guide's observation that something is off should be weighted at least equally with a trekker's insistence that they feel fine.

Tell your guide the Lake Louise Score honestly each evening. Not the number that makes the next morning's plan work, but the actual number.

AMS Risk Day by Day on the Everest Three Passes Trek

The Day 16 Problem: Fatigue Versus Altitude Illness on Kongma La

Day 16 of the Everest Three Passes trek is the most medically complex single day of the circuit and the one that most requires preparation in advance rather than judgment in the moment.

After fifteen trekking days that include Renjo La, Cho La, Everest Base Camp, and Kala Patthar, cumulative fatigue produces: headache, reduced appetite, disrupted sleep, and generalized weakness. The Lake Louise Score of a completely healthy trekker on day sixteen, experiencing only fatigue and no altitude illness whatsoever, might register 2 or 3. An early-AMS trekker on the same morning might register the same number. The difference is not obvious from the inside.

Two tools separate them.

The ataxia test:

Ask every member of the group to walk a straight-line heel-to-toe for 3 to 4 metres before leaving Lobuche. A trekker who is fatigued can do this. A trekker with HACE cannot, even at an early stage when they feel and sound fine. This test takes 20 seconds per person and is the definitive field distinction between the two states. Freedom Adventures guides perform this check as standard practice on this specific morning.

The Lake Louise Score in trend:

Compare the morning score to the scores from the previous two evenings. A stable or declining trend (3, 2, 2) suggests fatigue rather than progressing AMS. A rising trend (2, 3, 4) indicates AMS irrespective of how each individual day's number feels.

The temptation to attribute a score of 3 or 4 to fifteen days of trekking rather than to altitude illness is the specific psychological pressure that makes this day dangerous. A score is a score. Treat it accordingly.

The Remote Section Problem: Where Help is and Where It is Not

The Himalayan Rescue Association clinic at Pheriche (4,371m) has operated since 1973. It is staffed by qualified physicians during the main trekking seasons. The free daily altitude talks at the clinic are strongly recommended for all trekkers passing through the EBC corridor. The problem is that the remote sections of the Three Passes circuit, Lungden, Thangnak, and Gokyo, are on the western side of the Khumbu. Pheriche is on the eastern side. If you develop serious AMS at Lungden and cannot continue, the foot route to medical help runs through Thame and Namche, a two-day walk, not directly to Pheriche.

How to Prevent AMS on the Everest Three Passes Trek?

What to Do If You Have AMS on Everest Three Passes Trek?

The Rule That Covers All Three Conditions

Never ascend to sleep at a higher elevation while experiencing any AMS symptoms. If symptoms worsen at the same altitude, descend. There is no version of this trek where pushing through worsening symptoms to reach a planned stop is the right decision. A person with HACE may insist they feel fine while failing the heel-to-toe test. Act on the test, not the verbal report.

Medications for Altitude Illness

All medication decisions must be discussed with your doctor before departure. The information below is educational only. Freedom Adventures does not prescribe or recommend medications.

Hydration and Its Relationship to AMS on This Route

Dehydration and AMS share several symptoms: headache, fatigue, nausea, and reduced appetite. A trekker who is dehydrated and mildly altitude-sick may try to address what is actually altitude illness with water alone, while the AMS progresses. Staying consistently hydrated removes that overlap and makes the Lake Louise Score more reliable when you use it.

Drink 3 to 4 litres daily above 3,000m. Cold dry air at altitude drives significant fluid loss through breathing at an elevated rate, less visibly than sweat at sea level. Thirst signals are suppressed. Pale yellow urine is the field indicator of adequate hydration. Dark or amber means drink more immediately, regardless of thirst level. On acclimatization days, the target is the same as on walking days. The altitude dehydration mechanism does not pause because you are resting.

Sleep at Altitude: What is Normal and What is Not

Above 4,000m, most trekkers experience periodic breathing during sleep: a cycle of gradually deeper breaths, then shallower ones, sometimes a brief pause, then a sharper breath. This pattern is called Cheyne-Stokes respiration and it is normal physiology at altitude. It is driven by the CO2 balance disruption that underlies acclimatization. Waking with a sudden breath in the night at Lobuche or Gorak Shep is startling but not dangerous in an otherwise healthy person acclimatising normally.

The 2018 Lake Louise revision removed sleep disturbance from the AMS diagnostic criteria for exactly this reason. Disturbed sleep above 4,000m correlates poorly with true AMS. Waking repeatedly is expected. It is not, by itself, a signal.

What does matter is the quality of that waking. Waking with a gasp and settling quickly back to sleep is Cheyne-Stokes. Waking with breathlessness that does not ease when you sit up, a cough that is worsening, or tightness in the chest is different. Assess for HAPE before going back to sleep.

Your Complete AMS Framework for the Everest Three Passes Trek

Continue Reading All the Everest Three Passes Guides

Everest Three Passes Day-by-Day Breakdown

Every stage of the clockwise itinerary from Lukla to Lukla: terrain, conditions, and what each day demands.

Difficulty and Fitness Guide

Pass-by-pass physical demands, the hardest 72 hours on any teahouse trek, and the 14-week

Food and Accommodation Guide

What teahouses on this circuit actually provide, what to eat at altitude, and the Kongma La day logistics.

Best Time to Visit

Season-by-season conditions, pass-specific timing, and the honest verdict.

Packing List

What goes in the bag for glaciated terrain and multiple nights above 4,800m.

Altitude Guides for Other Nepal Treks

Compare the Altitude Profiles

• Altitude Sickness on the Everest Base Camp Trek : The EBC altitude profile, day-by-day risk, and the full AMS framework for that route.

• Altitude Sickness on the Annapurna Circuit Trek : Thorong La at 5,416m and how a single high-pass crossing differs from the Three Passes medical picture.

• Altitude Sickness on the Manaslu Circuit Trek: Larkya La at 5,160m in a remote restricted permit zone with limited medical access en route.