What is Altitude Sickness?

Altitude sickness brings together a series of symptoms that appear at altitudes above 2,500 meters. Learn all about it in today's article.
What is Altitude Sickness?

Last update: 31 December, 2022

Altitude sickness, also known as acute high altitude illness (AHAI) or acute mountain sickness, brings together a series of symptoms that appear when a person is exposed to hypoxia at high altitudes.

It’s very common, especially when ascending to altitudes above 2,500 meters above sea level (MASL). Some people are prone to developing it, primarily those with lung or heart disease.

It’s estimated that up to 75% of non-acclimatized travelers who exceed 3,000 masl develop acute mountain sickness. The non-acclimatized criterion is very important, as the adaptation process can prevent complications upon ascent. We’ll review the characteristics, symptoms, causes, and what things you can do about altitude sickness in the following article.

The causes of altitude sickness

The characteristic symptoms of altitude sickness are a reaction of the body to the reduced level of oxygen in the air. As the Centers for Disease Control and Prevention (CDC) remind us, at 3,000 MASL the PaO2 (partial pressure of oxygen in arterial blood) is a little more than two-thirds of what it is at sea level.

As a consequence of the low level of oxygen in the blood, people develop tissue hypoxia. There are many variables that affect its appearance, but the most important are the actual level of elevation, the rate of ascent, and the duration of exposure. Non-acclimatized people are at higher risk of developing altitude sickness in contrast to acclimatized people.

Although it can appear at lower elevations, the symptoms of acute mountain sickness manifest around 2,500 MASL. The human body adapts very well to changes in the oxygen level, although it takes time to do so. This process is known as acclimatization, and depending on the characteristics of the person affected, it can take from a couple of hours to 5 days.

Most people who develop acute high altitude illness do so from failure to acclimatize. This is, by exposing the body to a rapid rise in altitude without it previously getting used to the characteristics of the environment.

Humidity, air pressure, and increased ultraviolet (UV) radiation further encourage the effect of hypoxia. In addition to them, experts point out the following risk factors:

  • People who practice mountaineering or hiking who live above sea level or in low-altitude areas (less than 900 MASL)
  • People who have had altitude sickness in the past
  • Subjects with heart or lung conditions
  • Those who maintain a sedentary lifestyle (practicing sports conditions the lungs and the body to make better use of oxygen)
  • A genetic predisposition to tolerate changes in altitude better or worse

Types of altitude sickness

Altitude sickness has several causes
The symptoms of altitude sickness are usually derived from problems with oxygen supply to the brain tissue.

The term altitude sickness is used to describe three manifestations that occur from rapid ascent to high altitude terrain. Researchers distinguish acute mountain sickness, high-altitude cerebral edema, and high-altitude pulmonary edema. Let’s see what each of them is about:

  • Acute Mountain Sickness: Also known as AMS. It usually occurs 4-24 hours after ascent, often resolving within 2-3 days at a constant altitude. It can be mild, moderate, or severe. These latter cases are known as chronic mountain sickness or Monge’s disease.
  • High altitude cerebral edema: This is also known as HACE. It usually occurs 24 hours after the symptoms of AMS. From a clinical point of view, it’s considered the final stage of AMS. It can progress into a coma and death as a result of a brain herniation within 24 hours, so specialists warn about the importance of rapid and efficient diagnosis and control.
  • High altitude pulmonary edema: This is also known as HAPE and it’s a form of non-cardiogenic pulmonary edema. According to experts, if left untreated, it can cause death in up to 50% of people.

Fortunately, both high-altitude pulmonary edema and high-altitude cerebral edema are rare. Most people only develop acute mountain sickness. This is because the presence of symptoms causes them to change their ascent itinerary. For example, they choose to acclimatize, leave the area, or seek medical assistance.

The symptoms of altitude sickness

The symptoms of altitude sickness are very varied. Not only do they depend on the specific type, but the rate of ascent and the altitude mediate also its intensity. With this caveat in mind, we leave you with a typical picture:

  • Fatigue and loss of energy
  • Difficulty breathing
  • Dizziness (which can lead to fainting)
  • Difficulty focusing
  • Loss of appetite
  • Coordination problems
  • Nausea and vomiting
  • Chest tightness
  • Confusion

When a person develops high-altitude pulmonary edema, they manifest symptoms such as dyspnea at rest, rales, dry cough, and cyanosis. On the other hand, high-altitude cerebral edema is characterized by ataxia, fatigue, and altered mental status. Symptoms are often made worse by sleeping at high altitude terrain.

In the same way, if rapid elevation persists without the respective acclimatization, the signs tend to worsen. In these cases, the consequences can be life-threatening. They’re especially so in the absence of timely medical treatment.

How to prevent altitude sickness

A backpacker standing on a cliff looing out at a valley.
Making a progressive ascent allows you to reduce the risk of suffering from mountain sickness.

Acute high-altitude illness can be prevented. The most important variables that affect its development are the level of elevation, the speed of the ascent, and the duration of the ascent. By controlling them, you can prevent symptoms, and you do this through acclimatization.

Acclimatization consists of ascending gradually. Therefore, experts recommend not exceeding ascents of more than 2,500 MASL in 24 hours. From then on, you should advance only 500 MASL per day. As it’s not always possible to plan ascents in this way, experts recommend taking acetazolamide to speed up acclimatization.

Other ways to prevent symptoms are to avoid alcohol intake 48 hours before the ascent and to avoid exercise or strenuous activities at such altitudes. Similarly, people are encouraged to plan ascents based on their experience. That is, avoid sudden ascents when you’ve never been at a certain altitude above sea level before.

Fortunately, most episodes of altitude sickness are mild. These resolve completely when descending in altitude, and they do so within 24 to 72 hours. If the signs are very intense, don’t hesitate to seek medical assistance immediately.


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