Diseases of high terrestrial altitudes

Ascent to altitudes above 2500 m leads to exposure to hypobaric hypoxia. This affects performance on first arrival at high altitude and disturbs sleep, but physiological changes occur over time to defend arterial and tissue oxygenation and allow the individual to adjust. This process of acclimatization includes (1) an increase in the rate and depth of breathing; and (2) an increase in red cell mass, and in red cell 2,3-diphosphoglycerate. Acclimatization is no longer possible at extreme altitude (>5800 m) and the exposed individual will gradually deteriorate.

Altitude illness results from a failure to adjust to hypobaric hypoxia at altitude. Risk is increased by ascent to higher altitudes, by more rapid gain in altitude, and (in some people) genetic predisposition; the condition may be avoided in most cases by slow, graded ascent. Clinical presentation occurs soon after arriving at a new altitude, most often manifest as one of three conditions:

Acute mountain sickness (AMS)

A common condition that presents with non-specific symptoms, including headache and anorexia. The victim is likely to be apathetic, but clinical examination is generally unremarkable. Mild cases usually resolve with rest and avoidance of further ascent. Those whose symptoms fail to resolve (or worsen) should descend immediately. Treatment with acetazolamide (which can also be used as prophylaxis) or dexamethasone is often given in severe cases.

High-altitude cerebral oedema

An uncommon condition that typically presents with worsening symptoms of AMS and ataxia, with progressive neurological symptoms including behavioural changes, confusion, and impairment of consciousness. Papilloedema and focal neurological signs may be present. Treatment is urgent, with the most important measure being descent. Oxygen or simulated descent using a portable hyperbaric chamber can be helpful. Dexamethasone is widely recommended (and can be used as prophylaxis).

High-altitude pulmonary oedema

A relatively uncommon condition with significant mortality that typically presents with dyspnoea and cough. Signs include low-grade fever, tachycardia, tachypnoea, basal crepitations, and (in late disease) cyanosis. Treatment is urgent, with the most important measure being descent. Oxygen should be given if available. Simulated descent using a portable hyperbaric chamber can be helpful. Nifedipine reduces pulmonary artery pressure, relieves symptoms, and is usually given (and can be used as prophylaxis).

Chronic mountain sickness (Monge’s disease) is a disease of adults who reside for prolonged periods at high altitude and develop polycythaemia and eventually cor pulmonale. Symptoms appear to resolve with descent, but treatment with venesection has been attempted in those who remain at altitude. High-altitude pulmonary hypertension has been described in both infants and adults, predominantly native lowlanders who ascend to and reside at high altitude: this also appears to resolve on descent.

Pre-existing medical conditions are mostly little affected by ascent to altitude, but people particularly likely to be affected by hypoxia/ altitude include those with (1) coronary ischaemia and a strongly positive exercise treadmill test; (2) sickle cell disease or trait; (3) chronic pulmonary disease, especially pre-existing pulmonary hypertension from any cause.