Altitude Skills · Part 3 Updated

How to tell if acclimatisation is not keeping up.

Acute mountain sickness is not a low oxygen number by itself. It is a symptom pattern after ascent. Use the 2018 Lake Louise self-check, then watch the dangerous brain and lung branches.

~8 min
Onset 2-12 h after arrival or further ascent, often during or after the first night.
Assess 6 h+ after reaching a new sleeping altitude, per Lake Louise 2018.
Self-check 3+ total points plus at least 1 point for headache matches AMS by this score.
01

AMS is the common, recoverable form.

The cardinal symptom is headache, plus at least one of appetite loss, nausea, fatigue, weakness, dizziness, or light-headedness. If symptoms start more than 3 days after arrival without further ascent, the CDC warns against simply blaming AMS.

Arrival 0 h

Do not score the moment you step off the plane or roadhead.

First check 6 h+

Lake Louise 2018 says to assess at least 6 hours after arrival.

Typical onset 2-12 h

Symptoms often show during or after the first night.

Hold and improve 12-48 h

AMS usually eases if you stop ascending further.

02

The Lake Louise tap-scorer.

Score each item from 0 to 3. This page says "matches AMS" because a field score is not a medical diagnosis.

Headache
Appetite / nausea
Fatigue / weakness
Dizziness
0/12 No AMS by this score

Tap a number in each row. AMS by this score needs headache at least 1 and total at least 3, in the setting of recent ascent.

The 2018 update removed the old disturbed-sleep item. Poor sleep at altitude tracks hypoxia, but it is not part of the current AMS score.

03

The two red branches.

AMS is common and usually recoverable. HACE and HAPE are different: one is the brain, one is the lungs, and both change the field decision.

AMS

The common trunk

Headache after ascent plus appetite loss, nausea, fatigue, weakness, or dizziness. Hold altitude and reassess; do not keep climbing through it.

HACE · brain

Confusion or ataxia

CDC describes HACE as end-stage AMS: altered mental status, ataxia, confusion, drowsiness, and behaviour that can look like alcohol intoxication.

Companion test: heel-to-toe walking. Stumbling or stepping off is the clearest field sign.

HAPE · lungs

Breathless at rest

Early HAPE can look like cough, chest congestion, exertional breathlessness, and falling performance. Later: breathless at rest, wet cough, crackles, and sometimes pink sputum.

The mechanism

Uneven hypoxic pulmonary vasoconstriction raises pressure in parts of the lung circulation. Capillary stress failure lets fluid leak into the air spaces.

HAPE can occur without AMS. Do not wait for a headache before taking lung symptoms seriously.

04

Some signs mean stop now.

A Lake Louise score can tell you whether the common symptom pattern matches AMS. It does not replace a companion's judgement when someone is confused, cannot walk straight, or is breathless at rest. Part 4 is the treatment and descent line.

Sources

  1. Roach RC, Hackett PH, Oelz O, et al. The 2018 Lake Louise Acute Mountain Sickness Score. High Altitude Medicine & Biology, 2018. PMC6191821.
  2. CDC Yellow Book. High-Altitude Travel and Altitude Illness. 2026 edition, updated April 23, 2025.
  3. Luks AM, et al. Wilderness Medical Society Clinical Practice Guidelines for the Prevention, Diagnosis, and Treatment of Acute Altitude Illness: 2024 Update. Wilderness & Environmental Medicine, 2024. PMID 37833187.

Educational only, not medical advice. If symptoms worsen at altitude, the safe default is to stop ascending and descend.