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New Study Helps Quantify the Risk of Developing Acute Mountain Sickness

A new study by Johannes Burtscher and colleagues at the University of Lausanne has gone some way to helping us estimate the chances of developing acute mountain sickness (AMS) when we ascend quickly to high altitude. Jeremy Windsor explains.

The human body has an incredible capacity to adapt to its surroundings. Nowhere is this more obvious than following an ascent to high altitude. But if you try to rush the process you’re likely to be left exhausted or in some cases, faced with a bout of acute mountain sickness (AMS). But it's not always the case. There are plenty of people who drive out to Chamonix, spend their first night at the Torino Hut (3375m) and climb the Dent du Géant (4013m) the next day. But at the same time there’s quite a few others who spend the night awake and descend exhausted and disappointed the next day. So is there any way to estimate the risk of developing AMS? A new study has attempted to do just that.

Last year, a group of high altitude experts scoured the medical literature and identified 12 studies that quantified the risk of developing AMS at a given height. In seven studies the volunteers ascended slowly over 2 or 3 days, whilst those in the remaining five studies ascended in just a couple of hours.

In those who ascended over 2 to 3 days, the chances of developing AMS increased steadily with height. Whilst noone developed AMS at just over 2000m, 52% of those who ascended to 4559m developed the condition. Meanwhile, those who flew directly to altitudes of between 3350m and 3740m saw their chances of developing AMS range from 39 to 84%. Put simply, the study found that there is a 4.5 times greater risk of AMS if you fly to your high altitude destination compared to if you take a little time and walk to it. 

The 12 studies are divided into 2 groups - those who ascended in hours (orange) or 2 to 3 days (blue). There are often several data points for each study.
The numbers beside the data points are references to the studies identified by Burtscher and his colleagues

For me, this is all much clearer if you take a look at the graph. There are two lines of “best fit” that make it possible to estimate the chances of developing AMS at any given altitude. Using the trip to the Torino Hut (3375m) as an example. If you drive a minibus with ten passengers to Chamonix and jump straight on the Pointe Helbronner cable car, there’s a good chance that almost half of you will develop AMS by the next morning. If instead, you spend a couple of long days cragging on the Brevent, it’s likely that only 1 or 2 will encounter symptoms at the Torino Hut.

The difference is even starker the higher you go. Rapid ascents to 4,000m or more seem to all but guarantee a bout of AMS. However, even short periods of time spent acclimatising at lower altitudes can reduce the risk dramatically. 

All of this seems to confirm what mountaineers have known for a long time, that ascending more slowly reduces the risk of Acute Mountain Sickness. However, having clear probabilities for different altitudes and rates of ascent is of great help with decision-making, particularly for groups who have not previously been to higher altitudes and do not know how they personally cope. It also confirms that even a day or two of acclimatising at lower altitudes is not wasted time, but a way of making a real difference to your risk of developing AMS.

Johannes Burtscher's paper can be downloaded here.



Jeremy Windsor is the director of the Centre for Mountain Medicine at the University of Central Lancashire (UCLan)