Acute Mountain Sickness (AMS)

Original Editor - Kapil Narale

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Introduction

Acute Mountain Sickness is a self-limiting disease, which is individually variable, and is considered similar to having a hang-over or a migraine. It can be elicited by initial exposure of non-acclimatised individuals to moderate altitude. Moderate altitude would be above 2000-3000m, and is when acute mountain sickness would start to occur, and will get worse with increasing altitude, thus acclimatisation would be very important for performance.

It is usually experienced 6 hours after ascending to an altitude above 2500m, or rapidly ascending to such an altitude. It can display its maximum effect between 12 and 96 hours.

Prevalence and Risk Factors

There is a prevalence of 0-25% at moderate altitudes, which depends on individual physical characteristics. Severe obesity and pulmonary disease are some risk factors to consider. At a moderate altitude, as in Columbia, the prevalence of AMS can be 25%. Mountaineers in the Himalayas, or in the Alps, ascending >600m/day, there is a prevalence of AMS of 30-40% of between 4000m and 5600m.

The main cause of AMS is through hypoxia, subsequent to quick ascent to above 2500m.

Living or sleeping at moderate altitude would not cause discomfort from hypoxia. Exposures of a few minutes to a few hours at greater altitudes (5000-6000m) are too short of a duration to cause AMS, hence would be tolerated by healthy athletes at rest and during exercise. In addition, life-threatening high-altitude pulmonary oedema (HAPE) and cerebral oedema hardly occur below 3000-4000m. However, they can occur after many days of exposure at a higher altitude.

Important determinants of AMS, similar to HAPE is altitude, rate of ascent, and individual risk factors.

Symptoms

Symptoms can include a headache, and one of:

• Fatigue,
• Dizziness,
• Nausea,
• Dyspnoea
• Anorexia,
• Insomnia, or
• Sleep disturbance.

These symptoms can heighten after spending the first night at altitude, however they can spontaneously resolve after 1-2 days without further ascent.

It is seen that AMS can occur subsequent to High-Altitude Pulmonary Oedema (HAPE). This is not always the case, but it is very likely that severe AMS is a risk factor for HAPE. This can be explained by studies which show that individuals with severe AMS have a low PaO₂, and/or a low hypoxic ventilatory drive which may increase the risk of developing HAPE.

Outcome Measure

The severity of the symptoms of AMS progressively increase with higher altitudes. These can be assessed with the Lake Louise Questionnaire for the symptoms of Acute Mountain Sickness.

Please see the respective page for information on the Questionnaire.

Treatment and Prevention

AMS can be treated by rest and can be prevented by slow or gradual ascent.

As mentioned on the High-Altitude Pulmonary Oedema page, nifedipine helps with the treatment of HAPE, however it is not effective with the treatment of AMS. Acetazolamide, is a prophylactic drug, which helps promote acclimatisation by making the blood acidic, which leads to an increased minute ventilation (V_E). Acetazolamide, which is used for AMS prevention, is seen to blunt hypoxic vasoconstriction in animals, but it is uncertain if that is the same case in humans. AMS can also be prevented and treated with the use of a prophylaxis and dexamethasone. Dexamethasone greatly dampens the increase in pulmonary pressure at high altitude.

AMS and Age

A meta-analysis by Wu et al (2018), examined whether age was a predominant factor of experiencing AMS. After examining 17 studies with 1810 subjects with AMS, and 3014 subjects without AMS, ranging from 10-76 years old, the meta-analysis showed that there was no statistical significance and association between AMS and age, with a MD = 0.10; 95% CI between -0.38-0.58, and P = 0.69.

Moraga et al & Hackett et al showed that children may be more at risk of AMS compared to adolescents and adults. However Honigman et al showed that children are less at risk of AMS, and that the risk factor increases with age. Tang et al showed that the elderly were more prone to AMS.

It is estimated that the elderly have a lower likelihood of developing AMS. This would be due to the mechanism of brain swelling. It is seen that brain size decreases with age, which can lead to a reduction in AMS due to an increase in cranial compliance.

It can be noticed that younger individuals can have a higher activity level than older individuals, even upon arriving at altitude. For this reason, it was concluded that younger individuals have a higher risk of AMS from the increased oxygen consumption in the brain, caused by movement, and decreased oxygen saturation in brain tissue. Because of the hypoxia, there is an increase in the release of free oxygen radicals and calcium. This would lead to vascular permeability, thus brain swelling.

Tang et al also noted that the elderly were more prone to experiencing AMS because of poor sleep quality. As it was already mentioned, sleep quality is reduced with age, and low sleep quality can increase the risk of AMS.

Generally, it is seen that studies with different views were conducted with inconsistent factors, or with inconsistent diagnostic criteria .