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Prior mountain sickness doesn't predict future illness

By C. E. Huggins

NEW YORK (Reuters Health) - A history of acute mountain sickness may not predict future bouts of the condition, according to a team of Canadian researchers.

Likewise, those who have never experienced such altitude sickness may not necessarily be protected against it in the future.

The Wilderness Medical Society's current consensus guidelines to prevent acute mountain sickness (AMS) are partly based on an individual's prior history of the illness. But the new study's results suggest that strategy may not be useful.

"A previous history of AMS (or the absence of AMS) doesn't seem to be as informative as we used to think," lead study author Martin J. MacInnis, a doctoral candidate at the University of British Columbia, told Reuters Health via e-mail.

"Individuals who have developed AMS on a previous ascent to altitude may not develop AMS next time," he said. "Similarly, individuals who did not develop AMS on their last ascent aren't guaranteed to be safe on their next ascent."

Each year, more than 35 million people ascend to heights of 9,842 feet or greater, according to estimates from the World Health Organization. These include not only mountain climbers, but also hikers and skiers, as well as miners, emergency workers and those making religious pilgrimages.

Acute mountain sickness usually occurs at 8,202 feet, according to MacInnis, but it can also happen at lower altitudes as well.

Previous studies have suggested that a past bout of acute mountain sickness raises the likelihood of experiencing it again. Yet the increased risk was perhaps too small to be considered useful in planning preventive treatment.

MacInnis and his colleagues reviewed 17 prior studies to clarify whether a history of acute mountain sickness can be used to predict future responses. The studies included 7,921 participants.

Overall, the evidence indicated that a prior history of AMS cannot reliably predict future susceptibility, the researchers report in the British Journal of Sports Medicine.

A history of high-altitude climbs without experiencing AMS appeared to be somewhat useful in predicting who was not likely to have the problem. Still, neither a positive nor a negative history "was sufficient… to plan ascents to high altitudes," MacInnis and his co-authors write.

The varying quality of the included studies may have affected the results, the researchers speculate. For example, some studies included participants who used medications to prevent AMS, such as acetazolamide - whose side effects of nausea and lethargy mirror the symptoms of altitude sickness - and dexamethasone.

Some studies also included individuals who had recently spent time at altitudes above 9,842 feet to prevent AMS during higher climbs.

Despite the current study's implications, an individual's prior history of AMS should not simply be ignored, the report indicates. It should be taken into consideration along with plans for a future ascent, particularly if the conditions will greatly differ from what was encountered in the past.

Although predicting AMS remains somewhat of a mystery, there are steps that can decrease a person's risk.

Ascending to higher altitudes slowly is key, according to the U.S. Centers for Disease Control and Prevention (CDC). It takes about three to five days for the human body to acclimate to high altitudes, the CDC indicates, so individuals should get used to spending time at 8,000 to 9,000 ft for a few days before ascending to higher altitudes.

"Those who ascend to altitude at a slow rate, taking sufficient time to acclimatize en route, will be less likely to develop AMS," MacInnis said. "Those interested in going to altitude should also speak with a knowledgeable physician about the use of medications to prevent AMS."

"Everyone should be cautious when ascending to altitude, and they should not rely too heavily on their previous history to predict how they will feel," he added.

SOURCE: http://bit.ly/1h3QW4F British Journal of Sports Medicine, online December 2, 2013.

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