Pathology of the Week – High-altitude illness

During the past few years I had the opportunity to learn mountaineering and ice/rock climbing from a few storied guides. I even got to climb one of Canada’s sixteen 10,000′-plus peaks. Although neither of us developed any form of high-altitude illness the trip prompted me to research the disease. High-altitude illness encompasses a group of acute pathologies which affect the bodies of individuals not accustomed to altitudes over 2,500m (8,202 feet) above sea level. These are acute mountain sickness (AMS), high-altitude cerebral oedema (HACE) and high-altitude pulmonary oedema (HAPE). AMS is far more common than HAPE and HACE with the latter two occurring in less than 0.1 to 4.0% of ascent cases. AMS equally affects both men & women as well as paediatric & adult patients. Strangely individuals over 50 seem to have a lower risk of developing AMS.  I will reference Basnyat & Murdoch, 2003 (free public access) as well as the Merck page on Altitude Sickness. See also BaseCamp MD.

Signs and Symptoms

Climbing Mt. Baldwin, summit: 10,682 ft

We in Canada actually contributed to the diagnosis of AMS!  The Lake Louise Consensus Group defines AMS as a new onset headache, in an unacclimatized person, who has recently travelled above 2500m as well as at least one of the following: fatigue, nausea, vomiting, loss of appetite, dizziness, and/or sleep disturbances. The symptoms typically begin 6 to 10 h after ascent and spontaneously subside after one to two days.

Differential Diagnosis

A differential diagnosis list for AMS comprises the numerous potential causes of acute headache with accompanying GI symptoms. Amongst the many are carbon monoxide poisoning, CNS infection, diabetic conditions, hypothermia, migraine as well as toxins, drugs and/or alcohol. AMS is commonly mistaken for hangovers, especially the morning after an apres-ski/chalet experience. Consider AMS in a patient fitting the Lake Louise criteria who has recently ascended to more than 2,500m and who does not have a history of toxin ingestion.


Not all individuals undergoing rapid ascension will develop AMS. The reasons for this as well as the aetiology of AMS in general are not entirely understood. One model suggested by Hackett and Roach (2001) is the “tight brain” theory. As a person ascends into higher altitudes the partial pressure of oxygen is reduced and their body may become hypoxic.  As part of the body’s compensatory response to the resultant hypoxaemia cerebral bloodflow and capillary pressure increases. The tight brain theory suggest some individuals do not have the capacity to withstand the extra intracranial and intraspinal pressure and thus the mainly neurologic effects of AMS set in. This model would also account for the lower occurence rates amongst individuals over 50 as their brain matter to CSF ratio is relatively lower (Hackett & Roach, 2001).

Management and Treatment

Individuals who normally live below 900m (2,952 feet) may benefit from an acclimitization night at an intermediate altitude. During a mountaineering excursion a climb high/sleep low regimen is commonly recommended. Most guidelines suggest limiting the daily increase in sleep altitute to less than 600m (1,968 feet)/day. The treatment of minor AMS symptoms is usually to halt the ascent and rest for a few days.

In some cases prophylaxis may be reccomended and trials have shown mixed results with three drugs. Acetazolamide (Diamox), by way of the kidneys, raises the blood’s acidity level. This in turn raises the ventilatory rate which may help to reduce hypoxaemia and thus speed acclimatization. Ginkgo Biloba has also been studied as a prophylactic measure with some clinical success. Dexamethasone (Decadron, Dexasone) on the other hand is used for symptom relief once AMS has already set in.

Implications for Prehospital Care

During any high-altitude travelling adventure monitor your party for signs of AMS. GI symptoms and/or headaches can be common in travel and so individuals may shrug off the beginning stages. It is important to halt the ascent at the first signs of AMS to prevent deterioration to HACE or HAPE. The symptoms will usually diminish after one or two days of rest. Supplemental oxygen for a few hours may help to attenuate the effects of hypoxaemia and speed acclimatization. If the patient does not improve after a few days descend to a lower altitude and consider portable hyperbaric treatment.

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