High-Altitude Cerebral Edema (HACE): Causes, Symptoms, and How to Prevent It?

Trekking is all fun and games until you face realities like acute mountain sickness, which needs practical preparations. These medical situations are not the bogeyman of the mountains, but a reality that shows up in 50% of people as AMS, .5-1% of people as HACE, and 6% as HAPE (mostly in altitudes over 4,000 m).

Each of these conditions comes with its own set of problems and solutions. In hindsight, you might find a patient who develops one after the other, but it’s rare and their sharing an etiology is a question with more confusion than answers. Therefore, every condition deserves an article of its own. 

This article especially deals with a not-so-common, but fatal condition called High Altitude Cerebral Edema (HACE). Knowing its symptoms, risks, prevention, and treatment is essential for early detection and swift rescue. At, Himalayan Dream Treks, you always get accompanied by trained professionals, but a little awareness goes a long way. Who knows, your team member might be one of the 1% of people susceptible to HACE and need your help! 

Note: HACE is rarely recognized by the person experiencing it. Often, their only hope is you. 

What is High-Altitude Cerebral Edema (HACE)?

First, let’s clear out a common myth about HACE, here’s what it is not: A chronic case of AMS (Acute Mountain Sickness). Although AMS can facilitate a 50% increased chance of edema, it isn’t a guaranteed precursor to HACE (Kallenberg et al., 2007). 

In layman’s terms, HACE is defined as the following:

“High Altitude Cerebral Edema (HACE) is brain swelling caused by fluid buildup, triggered by lack of oxygen at high altitudes.”

During HACE, your brain experiences extracellular brain edema within the genu and splenium of the corpus callosum and often within the subcortical white matter. HACE survivors also show microbleeds throughout their cerebral matter, which are not observed in AMS and isolated HAPE patients. I say “isolated” because HAPE and HACE can show in a patient at the same time too, studies show that 15% of HAPE patients also have HACE (Hochstrasser et al., 1986). 

Edema has three types and HACE is edema of vasogenic origin. This extracellular edema causes a subtle or complete breakdown of the blood-brain barrier (BBB). 

Glossary
Edema	Abnormal fluid buildup in the brain that causes swelling and pressure.
Extracellular Edema	Swelling outside the cells, in the spaces between them.
Genu and Splenium of the Corpus Callosum	Parts of the thick band of nerve fibers that connect the brain’s two hemispheres.
Blood-Brain Barrier (BBB)	A protective barrier that controls what substances can enter the brain from the bloodstream.
Vasogenic	Related to blood vessels; in this context, swelling is caused by fluid leaking from blood vessels into the brain due to a compromised blood-brain barrier.

How is HACE Different from Other Altitude-Related Illnesses?

HACE is different than other Altitude-Related Illnesses because:

1. Affected organ: Brain 
2. Effects: Swelling in the brain, confusion, loss of coordination (ataxia), and coma.
3. Who’s Affected: 1% of the climbers, trekkers, and travelers at high altitudes.
4. Diagnosis: No self-diagnosis.
5. Worst Case Scenario: Brain herniation and death.
6. Solution: Immediate descent, oxygen therapy, and dexamethasone.
7. Reaches chronic stage: Within 24 hrs of reaching the unsuitable altitude where HACE develops (Hackett & Shlim, 2024).

These are some ways in which HACE differs greatly from HAPE, AMS, and HAH (High Altitude Headache). HAH, at times, is an indicator of acute AMS, but the same cannot be said for HAPE and HACE. 

What Causes HACE?

The following are the causes considered for HACE:

• Hypoxia: HACE is primarily triggered by low oxygen levels at high altitudes. This leads to cerebral vasodilation and increased blood flow, which raises intracranial pressure.
• The rapid speed of ascending above 2,500 m: Rapid gains in altitude without allowing the body to adjust can overwhelm the brain’s ability to manage pressure and fluid balance.
• Individual predisposition: Some people are genetically or physiologically more prone to altitude illnesses. Other contributing factors like poor cerebral autoregulation also play a role.
• Lack of acclimatization: Failing to give the body time to adapt to thinner air increases the risk of HACE. Proper acclimatization involves gradual ascent, hydration, and rest days.
• Physical Exertion at High Altitude: Strenuous activity while ascending can exacerbate oxygen deprivation. This accelerates symptoms of altitude sickness.
• Dehydration: At high altitudes, the body loses more water through respiration, and dehydration can thicken blood, increasing the risk of cerebral edema.
• Sleep Apnea at Altitude: Altitude-induced periodic breathing during sleep can cause intermittent hypoxia, which may contribute to HACE development.

Who is at Risk of Developing HACE?

There’s no textbook answer to this question, but there are times when people are at a greater risk of developing HACE. Studies prove that any person’s aerobic capacity falls by 1% as they gain every 100 m of altitude after an altitude of 1500 m (Schommer & Bärtsch, 2011). So, with that math,

Aerobic capacity falls by 10% at 2,500 m.

Aerobic capacity falls by 25% at 4,000 m. 

Aerobic capacity falls by 65% at 8,000 m. 

Therefore, the risk of increased heart rate due to lack of oxygen content increases. This makes people with a prior history of heart and lung diseases more susceptible to HACE. Trekkers with pre-existing cardiopulmonary diseases should be assessed by medical professionals first-hand. Only when they get a green flag, they’re okay to trek to altitudes higher than 4,000 m. 

Lastly, trekkers who are likely to develop HACE are the ones who avoid proper acclimatization. Your body needs time to adjust, give it that. 

Note: Do not take 4,000 m being a safe line as something written on stone. A case report from 2024 says, “A woman had HACE after rapidly ascending to 2,400 m” (Walsh & Agarwal, 2024).

What Are the Symptoms of HACE?

HACE is a complicated condition, with symptoms that if not caught early become fatal. These symptoms don’t show themselves easily, you have to look for them. So, here’s the checklist for you:

• Headache 
• Lethargy 
• Worsening Malaise 
• Loss of coordination (ataxia)
• Lack of ambulation 
• Markedly altered consciousness and agitated
• Mumbling incomprehensible sounds (chronic symptoms)
• Only opening eyes to voice (chronic symptoms) 
• Withdrawing to pain (chronic symptoms) 
• Not answering questions 
• Not following commands
• Intermittently combative 
• Fighting to get out of the place 
• Various levels of confusion 
• Behavioral abnormalities 
• Getting angry very easily
• Papilledema (Turner et al., 2021)
• There are also findings consistent with organic brain syndrome, i.e., delirium too. 

Symptoms after treatment: No memory of descent, EMS transport, time in hospice care. Usually seen in patients who develop severe HACE to the point of unconsciousness (Walsh & Agarwal, 2024).  

How Can You Prevent HACE?

HACE is curable with early detection, but there are also ways to avoid developing HACE completely. The ways to prevent HACE are explained in detail by the following points: 

1. Acclimatize Properly

If you experienced HACE at a certain altitude once, it isn’t necessary to experience it again. This is possible when you give your body enough time to acclimatize by taking rest days and ascending slowly. Rapid ascent from lower to higher altitudes does not provide your body enough time to adjust. Take time to acclimatize and your body starts to perform the same tasks with

• Slower heart rate,
• Less shortness of breath, and
• Lesser feelings of exertion.

2. Stay Hydrated and Eat Well

Dehydration contributes to HACE by exacerbating altitude sickness by thickening the blood and reducing oxygen circulation. Proper hydration, therefore, is one of the many preventative measures to avoid cerebral edema. Drink plenty of fluids, but avoid excessive caffeine intake and maintain zero alcohol consumption; they’re both top contributors to dehydration. 

Eating well is equally important because quick energy is required at high altitudes to maintain optimal oxygen utilization in your body. Usually, the food attributed to these qualities is a high-carbohydrate diet. Foods rich in complex carbohydrates, such as whole grains, fruits, and vegetables, can support better acclimatization. A balanced diet also maintains sodium and electrolyte balance to prevent unnecessary stress on the body. 

3. Use Medications if Needed

Effects of altitude sickness, including HACE, are mitigated using certain medications, and can even be prevented from further development in the case of AMS. The common use case medication to speed up acclimatization is Diamox (acetazolamide), it stimulates breathing and prevents fluid build-up in the brain. It is proven effective in the case of AMS but doesn’t work wonders for HACE, if not in combination with Dexamethasone (a corticosteroid used to reduce brain swelling and inflammation). 

However, medication is not a substitute for proper acclimatization and other preventative measures. It is mostly the last resolve before descent. More about administering medicine in the following section!

4. Avoid Overexertion

Nothing is a race and trekking certainly is not. Whenever you think you need rest and you might be left behind on your schedule or goal, don’t hesitate to put your health and fitness before it, and rest. 

The atrial oxygen saturation at high altitudes gets very low; as low as 20% less than the specific normal value. This indicates that overexertion will do you no good, and only harm normal bodily functions, increasing the risk of developing HACE.

5. Be Mindful of Early Symptoms

Trekkers, at times, are keen on completing their trek, and with the fear of being judged as weak, they do not admit to symptoms. The early onset of HACE can easily be confused with AMS symptoms, and you might decide to rest through the day. This soon becomes the last mistake you make as sooner than later ataxia (a symptom that is seen in 60% of HACE cases) catches up to you. Hereon, you are a risk to yourself due to all the confusion and altered mental status. 

Therefore, before the condition reaches ataxia and eventually unconsciousness, admit to any minor or major inconveniences you are feeling. Early signs can range anywhere from social withdrawal, and drowsiness to mild headaches and nausea. If you doubt one of your team members might have it, here are a few symptoms to diagnose and treat:

• They are behaving out of character, saying things that aren’t like themselves.
• When asked to walk in a straight line, they fail. Their walk is more in line with a drunk than a normal person. 

Note: AMS, if left untreated, can be a cause of concern too and lead to dire consequences. So, in any case, reporting to your trek leader should be a priority, be it HACE or AMS. 

How is HACE Treated?

Descend. Descend. Descend. These are the three magic words to treat HACE. Nothing will cure you better than the decreasing altitude, but that said, below are the points with detailed explanations to treat HACE:

1. Descend Immediately

The sole reason for developing HACE is “High Altitude.” A person ascends and the body fails to acclimatize, and cerebral edema develops. Therefore, altitude is THE FACTOR, and losing some (going at around 2,500 m or below) is the safest bet to prevent the edema from increasing and leading to brain herniation. 

Thus, descend some, descend some more, and then descend some more on top of it. You are thereby out of any dangers of HACE. The symptoms will leave you in a few hours to 1 or 2 days, but you’re no longer at risk.

2. Provide Oxygen Therapy

In all cases, immediate descent may not be possible, here, supplemental oxygen is a lifesaver. It stabilizes the patient by increasing oxygen levels in the blood, reducing brain swelling, and slowing the progression of HACE. This is possible because the whole condition starts due to a lack of normal partial pressure of oxygen in the blood.  

While using supplemental oxygen, keep these in mind:

• Use high-flow oxygen (4–6 L/min via mask) to maintain SpO₂ above 90%.
• If oxygen is unavailable, a portable hyperbaric chamber (Gamow bag) works too. It simulates descent by increasing air pressure around the patient.

Note: While oxygen therapy helps, it is not a cure, it only buys time. Descent remains the only definitive treatment for HACE. Another thing to note is that supplemental oxygen and a portable hyperbaric chamber are rare during treks. But, you need not panic, there are several ways to rescue and this is one among them. 

3. Administer Medications

HACE falls under the umbrella of acute mountain illnesses, but the commonly suggested treatment for AMS, Diamox (acetazolamide), is not always safe for HACE patients. If the patient is concomitantly dehydrated, Diamox may cause hypotension—worsening dyspnea. Therefore, hasty decisions shouldn’t be taken with any suspicion of HACE. 

Instead of relying solely on Diamox, a combination of acetazolamide and dexamethasone should be used. These medications can be administered in one of three ways:

• Oral (preferred) – As long as the patient is conscious and can swallow.
• Intramuscular (IM) or Intravenous (IV) – If the patient is unconscious or unable to take oral medication.

Medication Dosage for HACE Evacuation

Stage	Diamox (Acetazolamide)	Dexamethasone (Dex)	Notes
First Dose	250 mg (1 tablet in India)	8 mg (16 tablets in India as each tablet is .5 mg)	Take them all together to reduce swelling and boost energy for descent. Provides ~6-hour window.
Ongoing Treatment (if still at a high altitude i.e., above 2,500 m)	250 mg	4 mg (Luks et al., 2019)	Maintain this dose if not at a safe altitude.
Ongoing Treatment (at a safe altitude i.e., below 2,500 m)	250 mg	4 mg	Reduce Dex dosage gradually.
Weaning Off Dex (after every 6 hours)	Not Required	3 mg → 2 mg → 1 mg → 0 mg	Gradually winding down to prevent withdrawal effects.

Key Takeaways:

1. Immediate action is critical: Take 8 mg Dex + 250 mg Diamox as soon as symptoms appear, then descend immediately.
2. Dexamethasone must be tapered off: Stopping it abruptly can cause complications. Reduce dosage every 6 hours once at a safe altitude.
3. Provides a temporary survival window: The first dose gives around 6 hours to descend before another dose is required.

Caution: Are you suspecting HACE? Avoid hasty decisions. A wrong approach can worsen the condition rather than improve it. 

4. Seek Medical Evacuation

At times, HACE looks as bad as a swollen face and head. This shows its severity and an immediate threat to life, therefore, professional medical help for an emergency should be contacted immediately. The window for medications known to laymen to cure the patient is closed and medical professionals need to take over them immediately. At this point, time wasted is a life wasted.

Final Thoughts 

My final thoughts on HACE are, “It’s not as fatal with experienced people, but it’s also not as benign as AMS. If I trek to altitudes higher than 4,000 m or above, I want someone experienced, not only my enthusiasm.” High Altitude Cerebral Edema (HACE) is not scary, with the right people and knowledge. 

The right knowledge part is covered in the article above, you’ll know once you read it. For the part about the right people, I got two suggestions: An experienced trekker who knows HACE first-hand or trained trek leaders at Himalayan Dream Treks. So, which one is it? Let me know in the comments!

FAQs

Can HACE cause permanent damage? 

Yes, in cases where immediate actions and cures are not administered while descending, the person can suffer permanent damages. These damages include:

• Memory loss: Trouble remembering things long-term.
• Balance issues: Walking feels unsteady or shaky.
• Constant headaches: Pain lingers even after recovery.
• Blurry vision: Seeing becomes difficult.
• Seizures or coma: This can lead to unconsciousness or worse.

What’s with HACE vs HAPE?

There’s no HACE vs HAPE. Where HACE affects the brain, HAPE affects the lungs with swelling of alveolar tissues due to fluid accumulation. Studies show that ~85-100% of patients diagnosed with HACE also had HAPE (Turner et al., 2021), this means they can overlap or occur together, but there is no definite proof of their etiology.  

Can someone who has had HACE trek again? 

Yes, with proper training, and acclimatization, they can. HACE occurs when the body fails to adapt—but if you train it to adjust, you can trek again. Start below 2,500 m and gradually increase altitude to build natural adaptation. 

References:

Walsh, B., & Agrawal, S. (2024). High-altitude Cerebral Edema and High-Altitude Pulmonary Edema Diagnosed in the Desert: A Case Report. Clinical practice and cases in emergency medicine, 8(3), 202–205. https://doi.org/10.5811/cpcem.3851 

Turner, R. E. F., Gatterer, H., Falla, M., & Lawley, J. S. (2021). High-altitude cerebral edema: its entity or end-stage acute mountain sickness? Journal of Applied Physiology, 131(1), 313–325. https://doi.org/10.1152/japplphysiol.00861.2019

Kallenberg, K., Bailey, D. M., Christ, S., Mohr, A., Roukens, R., Menold, E., … & Knauth, M. (2007). Magnetic resonance imaging evidence of cytotoxic cerebral edema in acute mountain sickness. Journal of Cerebral Blood Flow & Metabolism, 27(5), 1064-1071. 

HACE (High-Altitude Cerebral Edema). (2025, February 17). Cleveland Clinic. https://my.clevelandclinic.org/health/diseases/high-altitude-cerebral-edema-hace

https://pmc.ncbi.nlm.nih.gov/articles/PMC3254048/: Schommer, K., & Bärtsch, P. (2011). Basic medical advice for travelers to high altitudes. Deutsches Arzteblatt international, 108(49), 839–848. https://doi.org/10.3238/arztebl.2011.0839

Hackett, P., & Shlim, D. (2024). High Elevation Travel & Altitude Illness | CDC Yellow Book 2024. CDC.gov. https://wwwnc.cdc.gov/travel/yellowbook/2024/environmental-hazards-risks/high-elevation-travel-and-altitude-illness 

Luks, A. M., Auerbach, P. S., Freer, L., Grissom, C. K., Keyes, L. E., McIntosh, S. E., Rodway, G. W., Schoene, R. B., Zafren, K., & Hackett, P. H. (2019). Wilderness Medical Society Clinical Practice Guidelines for the Prevention and Treatment of Acute Altitude Illness: 2019 update. Wilderness & Environmental Medicine, 30(4_suppl). https://doi.org/10.1016/j.wem.2019.04.006 

Hochstrasser, J., Nanzer, A., & Oelz, O. (1986). Altitude edema in the Swiss Alps. Observations on the incidence and clinical course in 50 patients 1980-1984. Schweizerische Medizinische Wochenschrift, 116(26), 866-873.