The cause of altitude illness is a matter of oxygen physiology. At sea level the concentration of oxygen is about 21% and the barometric pressure averages 760 mmHg. As altitude increases, the concentration remains the same but the number of oxygen molecules per breath is reduced. At 12,000 feet (3,658 meters) the barometric pressure is only 483 mmHg, so there are roughly 40% fewer oxygen molecules per breath. In order to oxygenate the body effectively, your breathing rate (even while at rest) has to increase. This extra ventilation increases the oxygen content in the blood, but not to sea level concentrations. Since the amount of oxygen required for activity is the same, the body must adjust to having less oxygen. In addition, high altitude and lower air pressure cause fluid to leak from the capillaries which can cause fluid build-up in both the lungs and the brain. Continuing to higher altitudes without proper acclimatization can lead to potentially serious, even life-threatening illnesses.

Acute mountain sickness
The commonest illness at altitude is acute mountain sickness. This is sometimes colloquially referred to as altitude sickness or mountain sickness and in South America it is called soroche. Most people experience acute mountain sickness as a relatively mild, self-limiting illness.

What are the symptoms?
The most prominent symptom is usually headache, and most people also experience nausea and even vomiting, lethargy, dizziness and poor sleep. Symptoms are very similar to a really bad hangover. Acute mountain sickness can be diagnosed using a self-assessment score sheet. If you have recently ascended to over 2500m, have a headache and your total score is 3 points or more on the score sheet, then you have acute mountain sickness.

Who gets acute mountain sickness?
Anyone who travels to altitudes of over 2500m is at risk of acute mountain sickness. Normally it doesn’t become noticeable until you have been at that altitude for a few hours. Part of the mystery of acute mountain sickness is that it is difficult to predict who will be affected. There are many stories of fit and healthy people being badly limited by symptoms of acute mountain sickness, while their older companions have felt fine.
There are a number of factors that are linked to a higher risk of developing the condition. The higher the altitude you reach and the faster your rate of ascent, the more likely you are to get acute mountain sickness. On the Apex high altitude research expeditions, flying from sea level to the Bolivian capital, La Paz (3600m), caused over half of the expedition members to have acute mountain sickness on the day after they arrived. If you have a previous history of suffering from acute mountain sickness, then you are probably more likely to get it again. Older people tend to get less acute mountain sickness – but this could be because they have more common sense and ascend less quickly.

What causes acute mountain sickness?
How a shortage of oxygen leads to acute mountain sickness is not known. Some scientists believe that it is due to swelling of the brain but the evidence for this hypothesis is not conclusive. The theory is that In susceptible individuals, swelling could cause a small increase in the pressure inside the skull and lead to symptoms of acute mountain sickness. The swelling may be due to increased blood flow to the brain or leakiness of blood vessels in the brain.

What to do?
It is better to prevent acute mountain sickness than to try to treat it. Ascending at a sensible rate (300m gain in altitude per day), should mean that your body can acclimatise as you ascend and so you will be less likely to develop acute mountain sickness. However, if you need to go up more quickly, you could consider taking a drug called acetazolamide (also known as Diamox). There is now good evidence [BMJ. 2004;328:797] that acetazolamide reduces symptoms of acute mountain sickness in trekkers, although it does have some unusual side-effects: it makes your hands and feet tingle, and it makes fizzy drinks taste funny.

If you do have acute mountain sickness, the best treatment is descent. Painkillers may ease the headache, but they don’t treat the condition. Acetazolamide may be helpful, especially if you need to stay at the same altitude, and resting for a day or two might give your body time to recover. It is essential that you should NEVER go up higher if you have acute mountain sickness.

If a travelling companion has symptoms of acute mountain sickness and becomes confused or unsteady, or develops an extremely severe headache or vomiting, they may have a life-threatening condition called high altitude cerebral oedema (HACE).