What is Acute Mountain Sickness? Understanding AMS, HAPE and HACE

Today I’m talking about a term we’re all familiar with — altitude sickness.

Before you read this, I hope you’ve read my previous article about how altitude affects your body. If you haven’t, then read it now. It is important that you know some background first. You’ll find it here if you missed it.

Now let’s get started.

First, what is altitude sickness?

Interestingly “altitude sickness” itself doesn’t mean much.

It is a blanket term for different kinds of illnesses — it could be something as simple as a slight swelling in your fingers to something as severe as fluid accumulation in your brain.

For the most rudimentary understanding of the term, “altitude sickness” means that your body is not adjusting well to altitude.

With that in mind, today, I’ll talk about the most common forms of altitude sickness we see on our treks — AMS, HAPE and HACE.

Let’s start with the mildest form and progress to the more nasty forms.

1. What is Acute Mountain Sickness (AMS)?

AMS is very common when you’re trekking in the Himalayas.

How common is it?

Well, in a team of 20, we see at least 3-4 people showing symptoms of AMS. Most of them recover within a few hours.

We’ve also noticed that all of them don’t show symptoms at the same time. Some of them might show symptoms at a lower camp, some at a middle camp and some at a higher one.

Some show symptoms even while descending!

Now that might surprise you.

But it’s something you should keep in mind. AMS doesn’t always affect you while ascending. It might have hit you at your previous campsite or at the highest point on your trek, and the symptoms might show up the next day when you’re descending.

So you have to be vigilant at all times. There’s no way out of it.

Getting to the symptoms, I’ll tell you what we see most often.

Most trekkers affected by AMS display gastrointestinal symptoms. They either go to the loo very often, or they feel nauseous. They sometimes vomit. Especially late in the evening or after bedtime.

In our experience, GI symptoms (gastrointestinal) are a lot more frequent than the notorious headache that all manuals will tell you about.

Having said that, we have seen trekkers suffering from persistent headaches too. It builds up while trekking. It persists through the afternoon and evening. Most confuse it with dehydration. Or they assume it’s the heat. Most often they are right, but they could be very wrong too.

Moving on…

They find it hard to sleep at night.

You’ll often hear them saying “I’m feeling uneasy.” They won’t be able to explain that uneasiness.

These are all classic symptoms of AMS.

There are more symptoms — fatigue, loss of appetite, dizziness.

Of the three, look out for trekkers who are supercharged on day one and suddenly lagging behind on day two. (They’ll most likely tell you that they’re slow because they are taking in the scenery. Most of them don’t like to admit to any symptoms.)

Watch, especially, for not wanting to eat.

What I’ve learnt is that when you’re at high altitude, any symptom you have must first be attributed to AMS and then to anything else.

If these symptoms are treated immediately, AMS can be cured and the trekker can proceed further. Most of our trekkers who have mild AMS go on to complete the trek after medication.

A video debunking 5 Common Myths About AMS | Altitude Sickness

If left untreated and ignored, mild AMS could transform into its more sinister cousins – HAPE or HACE.

2. What is High Altitude Pulmonary Edema (HAPE)?

Pulmonary –> Relating to lungs

Edema An abnormal accumulation of fluid in parts of the body

HAPE is an advanced form of AMS. And a more dangerous form.

I really would not want to see you or anyone getting HAPE. This is when things have started to turn nasty. It requires a very experienced hand to get you out of this mess.

I’m saying this because HAPE can and does turn fatal very quickly. I am not trying to scare you. These are things that happen at high altitudes.

| What actually happens?

When a trekker has HAPE, fluid from his blood leaks into air sacs in his lungs (because of high pressure in the arteries of his lungs).

To put it simply, HAPE is like death by drowning. The trekker’s lungs are drowning in his own body fluids.

And when his lungs are incapacitated, you can imagine what could happen. The rest of his body will soon stop functioning.

The symptoms of HAPE start off much like AMS symptoms, with fatigue, loss of appetite and difficulty in breathing.

But HAPE could appear suddenly without AMS preceding it too. It’s very normal for HAPE to climb onto a trekker within hours — we notice that it worsens at night when the trekker sleeps and his breathing becomes shallow. So sleeping at high altitudes with mild HAPE is a big NO.

For this, identifying the symptoms is very important. And the symptoms of HAPE are pretty evident.

| Here’s what we see on treks

The HAPE-affected trekker is absolutely exhausted. He/she finds it difficult even to go up to the toilet tent. His breathing is heavy, even during mild walks. An experienced trekker will hear him breathing hard even while sitting.

He coughs quite a bit. A dry kind of cough. Not often, but obvious enough.

And here’s the final, sure-shot evidence: When you put your head to his chest, you’ll hear a rattling, gurgling noise. You don’t need a stethoscope. You just need to keep your head near his chest.

Scary as it might sound, there’s light at the end of the tunnel — HAPE does not occur too regularly. And it can be treated. At Indiahikes, in the last two years, we have seen just 15 cases of HAPE on our treks. All of them were evacuated to safety.

The key lies in identifying symptoms as early as possible and acting quickly.

It’s a similar case with HACE. Let’s talk about that next.

3. What is High Altitude Cerebral Edema (HACE)?

Cerebral –> Related to the brain

Edema

HACE is as serious as HAPE because the altitude is now playing with your nerve centre.

When a trekker is affected by HACE, his brain starts to swell. This happens because blood vessels in the brain dilate, filling the brain with fluids.

Since HACE affects the brain, the symptoms are clearly identifiable.

A headache is almost universal amongst trekkers who have HACE — a headache that won’t go with any medication.

You’ll find affected trekkers acting a little strange — they generally talk excitedly or show irritation. Their behaviour is sometimes quite irrational. One trekker we know asked for a helicopter to evacuate her. This sounds rational, but when you know the trekker’s normal behaviour, she would never say something like that.

They sometimes walk woozy, as if sloshed after a night of drinking.

This is medically termed ataxia, where the brain tells the body something, but the body does something else. It’s a basic loss of muscular coordination.

These are very typical symptoms of HACE.

You may also see them struggling to talk, hallucinating, unable to see clearly.

Don’t ask a trekker affected by HACE to do simple maths, like 5+3. They have been affected by HACE, but they are not idiots. They will get it right but may still be affected by HACE.

A trekker who’s been hit by HACE deteriorates within hours and could go into a coma.

So it’s important to act before it gets severe.

Thankfully, HACE is very very rare. From January 2017 until September 2024, we have seen just two cases of HACE. 

What you need to know about altitude sickness?

Well, altitude sickness depends a lot on how fast you gain altitude, your physiology and your body’s capability at the time of your trek

Altitude sickness does NOT depend on your prior high altitude experience, your fitness preparation or the food you eat.

Having said that, we have noticed that those who are better prepared for the trek get affected less. Their lungs are prepared to work harder despite having less oxygen.

When Sandhya UC suffered from altitude sickness

Our co-founder, Sandhya — a fit, experienced and healthy eater, suffered from a bad case of altitude sickness in the form of AMS at a relatively low altitude on the Buran Ghati trek. Thankfully, she knew of the symptoms and medications and could rescue herself from the clutches of AMS. Read about her AMS experience here. There is a lot to learn from her.

What I’m trying to say is that anyone can be affected. And anyone can save themselves or others. All it requires is knowledge.

Conclusion

Hope you found this post useful. I suggest you read about how to treat Acute Mountain Sickness, HAPE and HACE. After that, it is advisable to read How To Prevent Altitude Sickness (AMS), HAPE and HACE.

I recommend sharing this content with your fellow trekkers. This will help anyone stepping into high altitude.

Still, got questions that need answering? Just scroll down and put in your comments below, we’ll help you out!