The 4 Types of Altitude Sickness: AMS, CMS, HACE, and HAPE

Altitude sickness is your body’s natural response to low-oxygen environments like those at high altitudes. However, it isn’t a single disease, it actually includes four distinct types of altitude sickness. These include acute mountain sickness (AMS), chronic mountain sickness (CMS), High-Altitude Cerebral Edema (HACE), and High-Altitude Pulmonary Edema (HAPE). Here’s a complete introduction to the four types of altitude sickness, their symptoms, and treatment methods.

What Causes The Four Different Types of Altitude Sickness?

Regardless of the specific type of altitude sickness, the cause is always the same. Air pressure drops as you gain altitude because there is less air above you pushing down. This means that it is harder for your body to absorb oxygen, as there is less air pressure forcing it across the cell membranes into your body. Anyone who rapidly ascends above 8,500 feet is at risk of developing one of the following four types of altitude sickness. 

1. Acute Mountain Sickness (AMS)

AMS is the most common and mild form of altitude sickness. It presents with a range of symptoms that include:

  • Fatigue and exhaustion
  • Confusion
  • Dizziness
  • Nausea and vomiting
  • Headache

Most people who visit high altitudes experience mild AMS, defined as experiencing mild symptoms that do not interfere with regular activity. You can treat the symptoms of mild AMS with ibuprofen.

If your headache is not relieved despite taking over-the-counter medication, if you begin to vomit, or if your symptoms interfere with regular activities, you have moderate AMS. It would be best if you descended immediately, as ascending further will lead to deterioration until you cannot move on your own anymore.

Severe AMS occurs when your symptoms inhibit your ability to keep moving. This is a major medical emergency, as you may be stranded at a high altitude where your condition will continue to deteriorate and become life-threatening. Never let AMS progress to this point if you can help it.

2. Chronic Mountain Sickness (CMS)

While AMS symptoms develop quickly after ascending to a higher altitude, CMS takes months or years to develop among people who live permanently at a high altitude. The symptoms are similar to AMS, with the main difference between their longevity.

Typical AMS will clear up after 2-3 days once your body acclimatizes to the elevation. CMS does not improve over time, and will usually require medical treatment or a permanent change to a lower altitude before the symptoms improve. If you live in the mountains long-term and experience the symptoms of AMS for more than 1-2 weeks, consider talking to your doctor about CMS.

3. High-Altitude Cerebral Edema (HACE)

HACE is a life-threatening form of altitude sickness characterized by a swelling of the brain caused by fluid. While the exact mechanism is still uncertain, it tends to affect those already struggling with moderate-to-severe AMS. It rarely develops without any warning. Symptoms include typical AMS signs, but it is especially associated with confusion, dizziness, and loss of coordination. While HACE itself is life-threatening, it also can lead you to make poor decision that lead to injury or death from exposure or a fall.

If someone is experiencing moderate or severe AMS symptoms with significant neurological presentations, descend immediately. If that is not possible, call search and rescue right away before things get worse.

4. High-Altitude Pulmonary Edema (HAPE)

HAPE is the most deadly of the four types of altitude sickness. Fluid fills your lungs making it difficult to breath, quickly causing unconsciousness, coma, and death. HAPE typically develops among those suffering from moderate and severe AMS, but it can sometimes develop out of nowhere. It’s associated with shortness of breath, coughing, and other respiratory symptoms and requires immediate descent and medical attention.

Preventing All Types of Altitude Sickness

All four types of altitude sickness can be prevented through two tactics. The first is acclimatization: giving your body time to adjust and acclimate to higher altitude before ascending further. This takes 2-3 days to do, though the general rule of thumb is to ascend 1,000 feet each day, with a rest day every three days.

The second method is preventative medication, usually Acetazolamide/or Dexamethasone. Most people start taking the medication 1-2 days before they begin climbing to give it time to take effect. Talk to your doctor to get more information or to get a prescription for either type of medication.

The Four Types of Altitude Sickness: Now You Know!

As you can see, altitude sickness is not one condition. It is actually an entire family of different diseases, with four types of altitude sickness that each affects you differently. The key is to take the time to acclimate and prepare before you reach the climb, rather than waiting for it to strike high up on the mountain. I hope you found this guide helpful, safe travels on the trail!

Additional Altitude Sickness Resources

Here are a few additional articles and resources about the four different types of altitude sickness, including AMS, CMS, HAPE, and HACE.

Alex Derr, Creator of The Next Summit

Alex is an Eagle Scout and mountaineer living in Denver, Colorado. He created The Next Summit to help others stay safe exploring the mountains and advocate to preserve the peaks for the future. You can subscribe to his Next Summit Newsletter here.

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