What are the effects of interventions to prevent acute mountain sickness?

BENEFICIAL

Slow Ascent (or Acclimatization). We found no systematic review or randomized controlled trial (RCT) evaluating the effect of different rates of ascent or acclimatization in preventing acute mountain sickness. A non-randomized controlled trial, observational studies, and a consensus opinion suggest that slower ascent reduces the risk of acute mountain sickness compared with faster ascent.

Acetazolamide. One systematic review and two RCTs showed that acetazolamide reduced the incidence of acute mountain sickness compared with placebo. The RCTs showed that acetazolamide caused polyuria, paraesthesia, or both in a large proportion of persons. We found no RCT of sufficient quality comparing acetazolamide with dexamethasone.

Dexamethasone. One systematic review and two RCTs showed that dexamethasone was more effective than placebo for preventing acute mountain sickness. One RCT showed no significant difference between dexamethasone and placebo in the incidence of acute mountain sickness, but it may have lacked power to detect a clinically significant difference. However, the review showed that adverse effects (including depression) occurred in one fourth of persons on withdrawal of dexamethasone. We found no RCT of sufficient quality comparing dexamethasone with acetazolamide.

What are the effects of treatments for acute mountain sickness?

LIKELY TO BE BENEFICIAL

Dexamethasone. One small RCT that included climbers with symptoms and signs of acute mountain sickness showed that dexamethasone reduced mean acute mountain sickness scores compared with placebo.

Descent Compared with Resting at the Same Altitude. We found no systematic review or RCT on the effects of descent compared with resting at the same altitude in persons with acute mountain sickness. Consensus opinion suggests that persons with acute mountain sickness should descend if possible. However, we found no RCT examining the effects of different distances of descent, or the balance of risks and benefits in persons who might find it difficult to descend.

UNKNOWN EFFECTIVENESS

Acetazolamide. We found no systematic review or RCT of sufficient quality on the effects of acetazolamide compared with placebo for treating persons with acute mountain sickness.

Definition

Altitude sickness (or high altitude illness) includes acute mountain sickness, high-altitude pulmonary edema, and high-altitude cerebral edema. Acute mountain sickness typically occurs at altitudes greater than 2,500 m (8,202 ft) and is characterized by the development of some or all of the symptoms of headache, weakness, fatigue, listlessness, nausea, insomnia, and suppressed appetite. Symptoms may take days to develop or may occur within hours, depending on the rate of ascent and the altitude attained. More severe forms of altitude sickness have been identified. High-altitude pulmonary edema is characterized by symptoms and signs typical of pulmonary edema (e.g., shortness of breath, coughing, production of frothy or bloodstained sputum). High-altitude cerebral edema is characterized by confusion, ataxia, and decreasing level of consciousness.

Incidence

The incidence of acute mountain sickness increases with absolute height attained and with the rate of ascent. A survey in Taiwan of 93 persons ascending above 3,000 m (9,843 ft) showed that 27 percent experienced acute mountain sickness.¹ A survey in the Himalayas of 278 unacclimated hikers at 4,243 m (13,921 ft) showed that 53 percent developed acute mountain sickness.² A survey in the Swiss Alps of 466 climbers at four altitudes between 2,850 and 4,559 m (9,350 and 14,957 ft) showed the prevalence of two or more symptoms of acute mountain sickness to be 9 percent of persons at 2,850 m; 13 percent at 3,050 m (10,007 ft); 34 percent at 3,650 m (11,975 ft); and 53 percent at 4,559 m.³

Etiology

A survey in the Himalayas identified the rate of ascent and absolute height attained as the only risk factors for acute mountain sickness.² It showed no evidence of a difference in risk between men and women, or that previous episodes of altitude sickness experience, load carried, or recent respiratory infections affected risk. However, the study was too small to exclude these as risk factors or to quantify risks reliably. A systematic review (search date 1999) comparing prophylactic agents with placebo found that, among persons receiving placebo, the incidence of acute mountain sickness was higher with a faster rate of ascent (54 percent of persons at a mean ascent rate of 91 m [299 ft] per hour; 73 percent at a mean ascent rate of 1,268 m [4,160 ft] per hour; and 89 percent at a simulated ascent rate in a hypobaric chamber of 1,647 m [5,404 ft] per hour).⁴ A survey in Switzerland of 827 mountaineers ascending to 4,559 m examined the effects of susceptibility, preexposure, and ascent rate on acute mountain sickness.⁵ In this study, preexposure was defined as having spent more than four days above 3,000 m in the preceding two months, and slow ascent was defined as ascending in more than three days. The survey showed that, in susceptible persons (who had previously had acute mountain sickness at high altitude), the prevalence of acute mountain sickness was 58 percent with rapid ascent and no preexposure, 29 percent with preexposure only, 33 percent with slow ascent only, and 7 percent with both preexposure and slow ascent.⁵ In nonsusceptible persons, the corresponding values were 31, 16, 11, and 4 percent. The overall odds ratio for developing acute mountain sickness in susceptible persons compared with non-susceptible persons was 2.9 (95% confidence interval, 2.1 to 4.1).⁵

Prognosis

We found no reliable data on prognosis. It is widely held that if no further ascent is attempted, the symptoms of acute mountain sickness tend to resolve over a few days. We found no reliable data about long-term sequelae in persons whose symptoms have completely resolved.