Acute Mountain Sickness | HACE, HAPE & Surviving at Altitude

Acute Mountain Sickness (AMS) is common amongst climbers, hikers and hill walkers at 2500m asl. Although AMS in itself is not life threatening, it has the potential to progress to High Altitude Cerebral Edema (HACE). Depending on the individual, this can happen rapidly over a matter of hours or progress more slowly over days. High Altitude Pulmonary Edema (HAPE), another form of mountain sickness, is twice as common as HACE, yet comparatively rare to AMS which effects 20-80% of people at high altitude.


As you ascend in altitude, your body has to adapt to reduced oxygen availability. The PaO2 of air drops from 143mmHg at sea level to just 43mmHg at the summit of Everest. In order to do this the body undergoes some physiological changes:

  • Increase in tidal volume – you begin to breathe deeper, using some of your reserve lung volume in order to increase the amount of oxygen available
  • Increased respiratory rate – the normal physiological respiratory rate at sea level (8-12 respirations per minute) is increased with altitude to roughly 20 breaths per minute at 6000m asl.
  • Polyuria – the hypobaric hypoxic stimulus of altitude decreases circulating anti-diuretic hormone, renin and aldosterone. This, in combination with a respiratory alkalosis, causes hypoxia-induced diuresis leading to hypovolaemia.
  • Polycythaemia – after one week at altitude, there is up-regulation of erythropoiesis, leading to increased red blood cells available to carry oxygen.


The physiological response to altitude progresses over time to improve tissue oxygenation. This includes increasing ventilation, decreasing cardiac demand and enhancing the ability of the blood to deliver oxygen to tissues. The Initial increase in heart rate and cardiac output slowly decrease over time, thus requiring less oxygen.

flow diagram to demonstrate the cardiovascular adaptations to altitude
Cardiovascular adaptations to altitude (Nephrol Dial Transplant. 2014 Mar; 29(3): 497–506.)

Different people acclimatise at different speeds. ‘Slow’ acclimatisers will need to ascend more slowly in order to avoid developing acute mountain sickness. This can often be difficult in organised tour groups with a fixed schedule. Therefore, it is really important to be aware of the signs and symptoms and to alert your guide early if you notice either yourself or a fellow climber developing symptoms.

Often it is the fit, young members of the group who are reluctant to admit they are suffering. Consequently, it is often these people that develop life-threatening consequences rather than the unfit, older members of the group who tend to be more cautious.


If the acclimatisation mechanisms do not have time to work because an ascent is too fast then acute mountain sickness develops. The signs and symptoms are due to an accumulation of fluid around the brain. The mechanism of this is not well understood, although it is thought that there is increased permeability in the blood-brain barrier and increased capillary leakage.

Symptoms appear between 12-24 hours after a quick ascent. For a diagnosis to be made there must be a headache, characteristic of increased intracranial pressure. The headache will often be worse lying down or bending forward. Additionally, one of the following must be present:

  • Gastrointestinal symptoms – nausea, vomiting, decreased appetite.
  • Sleep disturbance – frequent wakening or poor quality sleep.
  • Dizziness, fatigue and weakness.

The Lake Louise score is a self-assessment questionnaire to assess if AMS is present and to monitor progression.Chart showing the Lake Louise scoring system for Acute mountain sickness

Mild AMS – A Lake Louise score of 5 or less

Moderate – Severe AMS – A Lake Louise Score of 6 or more.

Typically, acute mountain sickness will resolve given time to rest at the same altitude. This can vary from a few hours to a few days, depending on your acclimatisation speed. However, if there is further altitude gained, it can reappear at the new higher altitude until given time to acclimatise at that height.


HACE is a more severe form of acute mountain sickness. If AMS fails to resolve or progresses it becomes life threatening and descent is the only treatment. It is important to recognise the signs and symptoms of HACE as it may come on rapidly and sufferers are often confused, consequently acting dangerously.

  • Severe headache that will not go away.
  • Decreased coordination/ Ataxia – You may frequently fall over or have difficulty unscrewing a bottle top.
  • Behaviour change – aggression & agitation may be present.
  • Reduced consciousness – You may become drowsy and confused ultimately progressing to unconsciousness.

Testing for HACE

There are several simple tests that can be performed to detect if HACE is present.

  1. Finger nose test – you may have difficulty continuously alternating between touching your partner’s finger with your outstretched index finger then touching your nose.
    A diagram illustrating the finger nose test
  2. Heel-toe walking – you may be unable to walk in a straight line placing one heel in front of the other toes.
    A diagram illustrating heel-toe walking.
  3. Rombergs test – You may be unable to stand with your feet together and eyes closed without falling over.
  4. Mental tests – You may be unable to spell your name backwards or remember simple details such as your address or date of birth. You can also diagnose HACE by an inability to subtract 3 from 40 and repeatedly subtract 3 from the answer.


By a similar process to HACE, HAPE develops due to accumulation of fluids on the lungs. It may develop without any prior signs of acute mountain sickness. Hypoxia leads to pulmonary vasoconstriction, increasing pulmonary artery pressure. Consequently, the increased pressure disrupts the pulmonary capillary junctions leading to alveolar flooding.

HAPE often presents on the second day after an ascent and may come on rapidly, over hours, or more progressively, over days.

Signs & Symptoms

  • Tiredness
  • Initial dry cough then a cough productive of white/ blood stained frothy sputum.
  • Breathlessness – initially on exertion and when laying flat, progressing to breathlessness at rest.
  • Increased respiratory rate – normal at 6000m asl is 20 breaths per minute
  • Crackles – either heard with a stethoscope or by lacing an ear against the posterior base of the lungs
  • Cyanosis – if there is not enough oxygen being delivered the patient will develop blue lips and extremities.
  • Fever & chest pain in extreme circumstances
  • Reduced consciousness is a sign of severity


If you are suffering from acute mountain sickness, HACE or HAPE, you are at risk of being confused and making poor decisions. It is vital that the team leader, team medic or even another team member takes control of the situation. Without proper guidance, climbers suffering from any form of mountain sickness can quickly develop hypothermia, hypoglycaemia and dehydration.

Acute mountain sickness is treated with a period of acclimatisation at the same altitude, rarely does it require descent. Ensure you maintain well hydrated and eat appropriately – 70% of calories should come from carbohydrates at altitude.

Other medical conditions: Ensure you have excluded alternative diagnoses such as a pulmonary embolus, in HAPE, or meningitis, in HACE. If you are unable to exclude these alternatives then treat for both the altitude illness and alternative diagnosis.

Descent – If you suspect HACE or HAPE the most important form of treatment is immediate descent. As you reduce your altitude the symptoms should begin to reverse. You should aim for >1000m of descent.

Oxygen – The mechanisms of developing altitude sickness are precipitated by hypoxia. Giving supplemental oxygen is imperative to perfuse tissues, prevent permanent damage and alleviate symptoms.

Position – Keeping a sufferer sat up helps reduce symptoms of breathlessness and headache.

Medication – Take analgesia if chest pain or headache are present. If you suspect HACE give 8mg dexamethasone, followed by 4mg every 6 hours until you have medical support. If you suspect HAPE, give 30mg Nifedipine (modified release) every 12 hours for 3 days. Anyone who has developed any degree of acute mountain sickness should receive acetazolamide. The dose varies from 125mg 12 hourly to 250mg 8 hourly depending on severity. If you are suffering from disrupted sleep at altitude, try taking 125mg acetazolamide one hour before going to sleep.


Typically, routine use of acetazolamide is not recommended. It is useful if you have developed recurrent acute mountain sickness or when a rapid ascent is necessary (>1000m / day). In this case take acetazolamide at the following doses:

Previous AMS/ rapid ascent: 125mg every 12 hours from the start of an ascent until returning below 2500m.

Flying directly to altitude: 125mg commencing 24 hours before flying and continuing for three days after arrival.

Take necessary precautions when taking acetazolamide with suncream, hats and eye protection as it can increase photosensitivity.

Photo of Cerro Torre - argentina with lake in foreground

If you are planning a trip to altitude, take some inspiration from our tough mountain challenge guide. 

If you want to read more about Altitude sickness, check out these articles on Human response to altitude and the kidney response to high altitude


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Acute Mountain Sickness (AMS) is common amongst climbers, hikers and hill walkers at 2500m asl. Although AMS in itself is not life threatening, it...
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