Core Skills, News & Features — 6 September 2013 at 11:17 am

Introduction to Altitude Illness

Kitty Duncan / Consultant Anaesthetist, Borders General Hospital, Scotland

If you are interested in this article, you may be interested in these others relating to altitude:

Altitude Elective in Nepal

Drugs in the Mountains

Medication Use Among Mount Everest Climbers

Altitude illness is a core topic for expedition medics. In this article, Dr Kitty Duncan introduces us to some of the key points, including definitions, the Lake Louise Score and the treatment of AMS, HACE and HAPE. Kitty Duncan is a consultant anaesthetist and mountaineer who has worked at altitude in Nepal, holds the Diploma in Mountain Medicine and a Postgraduate Certificate in Aeromedical Retrieval and writes the ‘Mountain Doc’ column at

Millions of people are exposed to high altitude every year, not only the increasing numbers of commercial trekkers and mountaineers but also those living and working at altitude. Most people can function normally despite increasing environmental hypoxia until an oxygen pressure of around 67kPa is reached. This level is 66% of sea level atmospheric pressure or roughly equivalent to 3500m/8000ft, which is why commercial aircraft are pressurised to just below this level.

The term altitude illness encompasses Acute Mountain Sickness (AMS), High Altitude Cerebral Oedema (HACE) and High Altitude Pulmonary Oedema (HAPE) which are all caused by acute exposure to high altitude hypoxia. AMS typically develops at altitudes greater than 2500m, HAPE greater than 3000m and HACE greater than 4000 to 5000m. Approximately 50% of un-acclimatised people who ascend from sea level to 4500m will have some symptoms of AMS. HACE and HAPE are much rarer but serious conditions with a very approximate incidence of 0.1-4%.

Acute Mountain Sickness and HACE

AMS is common, benign and should be self limiting if recognised and treated promptly. The risk of suffering from AMS depends on speed of ascent, altitude reached, degree of prior acclimatization, age and individual inborn susceptibility

For diagnosis a person must have recently ascended to altitude above 3000m, have a headache and some or all of the following symptoms: nausea/vomiting; fatigue; dizziness; and difficulty sleeping. Symptoms usually present at 8-24 hours after ascent to a new altitude.

A Lake Louise Score (LLS) can be calculated from these symptoms and AMS graded as mild, moderate or severe. The LLS was originally developed as a research tool but is helpful for making decisions about treatment. Symptoms are non specific and can be related to other conditions that are common at altitude, especially dehydration and hypothermia. When treating a patient these should always be addressed, and dehydration alone increases the risk of developing AMS.

HACE is defined as all the above symptoms, usually in their severest form, with the addition of ataxia and confusion. At lower altitudes it usually only occurs if a patient with AMS ascends higher without treatment, but can present rapidly at very high altitude without obvious prodromal AMS.

It is likely, although not proven, that AMS and HACE share a common pathophysiological pathway, with HACE being at the severe end of the spectrum. Despite much research the exact mechanism is unknown, but we know that brain oedema occurs in HACE.

Prophylaxis and Treatment of AMS

The best way to avoid getting AMS is to ascend slowly. Recommended guidelines for ascent were published by the Wilderness Medical Society in 2010. Once above 3000m, un-acclimatised people should ascend no more than 300-500m per day sleeping at altitude, with an additional rest day every 1000m or 3 days. The majority of people should remain symptom free with this ascent profile; however some people may still have symptoms.

The only recommended pharmacological treatment for AMS is Acetazolamide (Diamox) which can be used for prophylaxis and treatment. Acetazolamide works by altering the biochemistry of the carbon dioxide pathway, speeding up respiratory rate and therefore aiding a person’s acclimatisation to a new altitude. It does not mask worsening symptoms and is a safe drug. The only common side effects are tingling in the hands/feet and increased urination. Anaphylaxis has also been described.

Prophylactic Acetazolamide is only recommended in certain circumstances:

  1. It can be used to help insomnia. Poor sleep at altitude can be caused by a number of things. One is a type of Cheyne-Stokes breathing caused by hypoxia. The sufferer is roused multiple times during the night gasping after a period of apnoea. Acetazolamide stimulates the breathing centres, promoting regular breathing and uninterrupted sleep.
  2. Diamox can be used by those who have had problems with AMS before despite slow ascent (I myself am one of these people). A prophylactic dose of 125mg BD can be taken from the altitude at which they have previously had symptoms. This should then be continued until they are acclimatised at a final altitude (around 4 days) or until they are descending again.
  3. Diamox can also be used as prophylaxis if a person has no choice but to ascend quickly, for example if they are on a rescue team or in a military situation.
  4. Lastly, Diamox can also be used if someone is suffering from very mild AMS symptoms, i.e. those which are generally improving after 24hours at a given altitude but return on further ascent (i.e. they are acclimatising but slowly). Examples include mild loss of appetite and or a headache that appears in the evening but is gone by the next morning.

Diamox is also used for treatment of AMS, at a dose of 250mg BD. Treatment depends on the severity of the symptoms.

  • Mild: (LLS <3) In general those with mild symptoms should remain at their current altitude for at least 24 hours, commence Diamox at treatment dose, rest, drink plenty of fluid, keep warm and eat well. If they improve they can then ascend. If not they should descend.
  • Moderate: (LLS3-6) These people should be commenced on Diamox. Oxygen can be given if available and usually helps symptoms rapidly. They should not ascend further and should descend as soon as possible. Treatment should not delay descent, but aid it.
  • Severe: (LLS>6): These patients need immediate treatment and descent as soon as possible, oxygen if available, Acetazolamide 250mg BD rest and hydration. A portable pressure chamber (for example a Gamow bag) can be used instead of oxygen. If severe, and descent is not possible immediately, it is reasonable to give a dose of Dexamethasone (8mg stat then 4mg 6hourly).

Dexamethasone has also been shown to work for prophylaxis of AMS (2mg 6 hourly), but because of its side effects it is only recommended for emergency treatment and for prophylaxis if a person is allergic to Diamox (or other sulpha drugs).

There is no contraindication for taking simple analgesia for altitude headache and it does not mask severe symptoms.

Treatment of HACE

If a patient develops HACE they need to descend as soon as possible, at least 500-1000m. This means day, night, storm or snow! Treatment should not delay descent but Dexamethasone (8mg stat then 4mg 6hourly) should be given. Oxygen or a Gamow bag can be used whilst waiting for evacuation. Care should be taken with the airway if they become unconscious.


High altitude pulmonary oedema (HAPE) is a life-threatening non-cardiogenic pulmonary oedema.
It usually starts two to five days after ascent to altitude. The incidence of HAPE depends primarily on rate of ascent, intensity of exercise and absolute altitude attained however some people are genetically susceptible and will have symptoms lower and earlier than others.

Early symptoms are excessive shortness of breath with a dry cough on exercise. This progresses to dyspnoea at rest, high heart rate, low grade fever and variable hypoxaemia. Pink, frothy sputum is a late sign.

The pathophysiology is entirely separate from that of AMS/HACE and is complex. The current leading theory is that an excessive and uneven hypoxic pulmonary vasoconstrictor response causes patchy regional over-perfusion leading to stress failure and leakage from the microvasculature. Other important factors are nitric oxide productions abnormalities and reduced transepithelial water/sodium clearance from the lungs.


Pharmacological prophylaxis of HAPE remains a controversial area. As per the Wilderness Medical Society recommendations, a gradual ascent profile should be the primary method of preventing HAPE . Nifedipine, 60mg SR in divided doses, commencing from 24 hours before ascent is recommended only for those who are known to be HAPE-susceptible.


The best treatment is descent. This cannot be overemphasised. Oxygen or hyperbaric treatment should only be used as a temporising measure to allow descent. Keeping the patient warm and minimising exertion will reduce any additional sympathetic contribution to the condition. The only current medication with good evidence base is Nifedipine, 60mg SR in divided doses. Its main side effect is systemic hypotension, which is a risk as these people tend to be dehydrated.

Phosphodiesterase-5 (PDE-5) inhibitors are pulmonary vasodilators and Sildenafil and Tadalafil have both been studied for HAPE prophylaxis in susceptible individuals. As yet there are no clinical trials of PDE-5 inhibitors for the treatment of HAPE; therefore these drugs cannot yet be recommended for first line use in the field.

It should be remembered that up to 50% of patients suffering from HAPE may also have symptoms of AMS, and such people should also be treated with Acetazolamide as well.


Acute altitude illness remains the bane of climbers across the continents, and the increasing ease of global traffic and altitude tourism prompts the suspicion that its prevalence will continue to rise. It covers a spectrum of disorders ranging from the irritating to the life-threatening, but the elusive pathophysiology means that knowledge is limited, and further research into the cause, personal susceptibility and management is warranted. Despite this, with the sensible adherence to ascent guidelines where possible, a savvy approach to symptom recognition and the prompt initiation of prophylaxis and treatment (including descent), altitude morbidity and mortality can hopefully be reduced and kept to a minimum.



  1. Hackett and Roach. High Altitude Illness. New England Journal of Medicine Vol 345:107-114. 2001
  2. Wilderness Medical Society Consensus Guidelines for the Prevention and Treatment of Acute Altitude Illness. Andrew M. Luks, MD; Scott E. McIntosh, MD, MPH; Colin K. Grissom, MD; Paul S. Auerbach, MD, MS; George W. Rodway, PhD, APRN; Robert B. Schoene, MD; Ken Zafren, MD; Peter H. Hackett, MD. WILDERNESS & ENVIRONMENTAL MEDICINE, 21, 146–155 (2010).
  3. High Altitude Pulmonary Oedema. David P. Hall, Kitty Duncan, J Kenneth Baillie. Journal of the Royal Army Medical Corps. Volume 157;No 1; March 11 – Medicine and Physiology at High altitude