Conrad Anker
Pro climbers like Conrad Anker know the secrets of adjusting to thin air. But new science shows that ill-prepared amateurs—or anyone who ascends too fast—may be putting their brains at risk

Are the Mountains Killing Your Brain?

Alarming new science shows that thin air can wreck brain cells—at lower altitudes than you'd think. Here's how to protect yourself.

Conrad Anker
Douglas Fields

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“YOU HAVE TO BE poco loco to be a climber,” says Dr. Nicholás Fayed. A neuroradiologist at the Clinica Quirón de Zaragoza, in northern Spain, Dr. Fayed leads me into his office and pulls out a collection of MRI images. They’re brain scans, taken from amateur and professional mountain climbers after they came back from major expeditions, and the results aren’t pretty.

Conrad Anker

Conrad Anker Pro climbers like Conrad Anker know the secrets of adjusting to thin air. But new science shows that ill-prepared amateurs—or anyone who ascends too fast—may be putting their brains at risk

“Atrophy of the frontal lobes,” Fayed says, pointing to a black-and-white slice of brain on one MRI. The frontal cortex—the region just behind the forehead that handles higher-level mental functions—looks like a piece of dried fruit. This kind of damage can leave patients with an impaired ability to plan, focus, and make complex decisions. And it’s permanent.

“Cortical atrophy, subcortical lesion…” Fayed continues, pointing to the scans of eight amateur climbers whose MRIs were taken in 1998, just after a trip up Argentina’s 22,834-foot Aconcagua. “This guy suffered the most serious damage,” he says. He hands me a picture of a robust young climber standing on the mountain’s snowy slopes, looking fit and determined. “When he came back, he couldn’t remember his own phone number. His wife would send him to the store for a loaf of bread and he would forget why he was there and come home without it.”

Fayed is an internationally recognized scientist who studies abnormalities and damage in the brain caused by various health disorders. Since 1992, he and his colleagues, neurologist Dr. Pedro Modrego and neuroradiologist Dr. Humberto Morales, have been collecting these scans of 35 climbers returning from peaks like Aconcagua, Everest, Kilimanjaro, and Mont Blanc. The scans are giving us the clearest picture yet of what happens to the brain at altitude, and it’s fair to say the results won’t make you want to scamper off to Everest. But the good news for most climbers is that Fayed’s studies also suggest that proper acclimatization can reduce the risk of brain damage a great deal.

SCIENTISTS HAVE long known that the brain can be harmed by extreme conditions such as high-altitude cerebral edema (HACE), in which blood vessels leak fluid into surrounding tissue, causing the brain to swell and press against the skull wall. But Fayed’s scans are the first to indicate that brain damage can show up even in people who displayed no symptoms of altitude sickness during their climbs, or had just the usual nausea and lethargy familiar to any hiker in the mountains. And, disturbingly, it seemed to happen to climbers going not much higher than 15,000 feet.

On 15,771-foot Mont Blanc, for example, seven trekkers reached the summit in 1998 without experiencing any symptoms of mountain sickness. When scanned a few days later, three showed major abnormalities. Two displayed enlargement of their brains’ Virchow-Robin (VR) spaces, gaps in the brain matter surrounding blood vessels that look like white birdshot on MRIs. (Enlarged VR spaces are found in the elderly and in people with Alzheimer’s disease, but they don’t normally show up in people in their twenties and thirties, the age of these climbers.) One trekker had cortical atrophy—a permanent loss of gray matter that can cause “spaciness” and other problems—and one had a subcortical lesion, damage to the network of neural pathways in the white matter, which can cause any number of serious issues.

Similar effects were seen on Aconcagua. Most of the eight amateur climbers never got above 21,000 feet, and one reached only 18,000 feet. Still, none of the scans came back normal. Four of the climbers suffered multiple subcortical lesions, seven had widespread enlargement of their VR spaces, and all showed signs of cortical atrophy—even though half of the team displayed either no symptoms of mountain sickness or mild ones during the climb.

The risks at extreme altitudes are clear. A few studies have shown that climbers who spend years in the Himalayas without supplemental oxygen frequently have problems that can be seen on an MRI. But the effects found at moderate altitudes are much more disconcerting.

“It’s easy to understand illness at 23,000 feet,” says Dr. Gianni Losano, director of the Angelo Mosso Institute, one of the world’s leading high-altitude-research laboratories, located in the Alps near Turin, Italy. “But on Mont Blanc?”

I READ ABOUT FAYED’S work in a 2006 issue of The American Journal of Medicine, and, being a climber and a neuroscientist, it disturbed me. I’ve never scaled anything like Aconcagua, but, at 56, I’ve spent 37 years tackling peaks around the U.S. and the Alps, all below 14,500 feet. Like most normal climbers, I’ve had my share of altitude sickness, so Fayed’s work made me wonder: Had all that time in the mountains damaged my brain? To find out, I decided to use myself as a test subject, doing one popular climb—on Washington’s Mount Rainier—and then flying to Spain to meet Fayed and have my brain scanned.

Rainier is a good place to court altitude sickness. The glacier-draped volcano rises steeply to 14,410 feet, and most of the hundreds of amateurs who attempt it each year start their trips by flying in to nearby Seattle, at sea level. Like Mont Blanc, Rainier is often attempted in a weekend push that happens too quickly for the body to acclimatize.

My 26-year-old son, Dylan, came with me, and on our first night out we slept at 3,180 feet, at a roadside campsite near the base of the mountain. The next day, we drove to a trailhead at 5,420 feet and started climbing in a whiteout. A storm forced us to bivy at 8,440 feet, where we waited out 50-mile-per-hour winds overnight.

The next morning, we followed our compass needle through the white blur to the public shelter at Camp Muir, where we found a few teams and four guides from International Mountain Guides, one of the commercial outfits on Rainier.

One guide, Karl Rigrish, estimated that about 40 percent of the company’s clients suffer at least mild altitude illness on Rainier. The key to avoiding that, he and other guides said, is taking it slow. Gary Talcott, a guide with Rainier Mountaineering Inc., recommends doing no more than 2,000 feet per day and staying hydrated, which helps thin the blood by replacing lost fluids.

Medical texts are even more conservative, calling for an ascent rate of 1,000 feet per day above 5,000 feet to avoid acute mountain sickness, or AMS. But who has time for that? On Rainier, that would mean taking five days just to get to the mid-mountain camp, which most climbers do in a day and we did in two. Climbing services will gladly lead trips at a slower pace, but most clients push for cheaper, more rapid ascents, relying on the better-acclimatized guides to take care of them if they get into trouble.

Notably, the results of Fayed’s study hinted that the brain damage from high-altitude climbing might be reduced or even eliminated through proper acclimatization, the kind that amateurs often don’t perform very diligently. The amateur team on Mont Blanc took just two days to climb roughly 8,000 feet to the summit, and half the team showed clear signs of damage when scanned a few days later. The amateurs on Aconcagua gave themselves six days of acclimatizing for that 9,000-vertical-foot climb (as opposed to the two to three weeks taken by commercial teams), and every brain scan showed problems. (A second scan three years later showed no improvement.) Overall, five of the 23 amateurs the Spaniards studied had irreversible subcortical lesions—the most serious brain injury the team found. None of the 12 professionals had them.

The pros weren’t biochemically gifted—their blood showed similar levels of oxygen-carrying red blood cells—but it appears that they acclimatized better through proper technique, while avoiding the kinds of amateur mistakes that stress the body.

“Amateurs have something to prove, so they kill themselves to get to base camp and they’re predisposed to getting sick,” says climber and photographer Jimmy Chin. “When we’re hiking to base camp, a pro like Conrad Anker is in the back of the line, taking his time, smelling the flowers. I think that helps.”

On our climb, the storm never let up, so we descended from Camp Muir, at 10,080 feet. During the trip down, we got a reminder that the mountains hold risks beyond thin air. About an hour after leaving camp, Dylan broke through a weak snow bridge over a crevasse. He slithered to safety as the crumbling block fell into the chasm below.

A few days later, I flew to Spain for my brain scan. The ceiling slipped away as my head slid into the chamber of an MRI machine in a Zaragoza hospital. When it was over, Fayed and Modrego scrolled through the slices of my brain, magically peeling away layers of undulating cortex. “A small VR space,” Fayed said. Flipping through a few more, he said, “Another one.”

“Perfectly normal,” Modrego assured me. “For your age.”

Fayed burned the 3-D images of my brain onto a CD and handed it to me with a smile. I was in the clear but somehow didn’t feel at ease.

THIS IS SCIENCE that’s still in its infancy. There haven’t been many studies of brain changes in climbers at moderate altitudes, partly because it’s difficult to get the required approvals for research on humans in a situation that puts them at risk for injury and partly because most climbers who return from such trips appear to be healthy, not in need of a $3,000 MRI. But the few high-altitude studies that have been done seem to bear out the Spanish team’s findings.

What is still unclear is how high you have to go, or how fast, before your neurons start dying en masse. The greatest risk lies above 15,000 feet, but there’s no reason to assume it can’t happen lower. My normal scan is by no means the kind of data one can base a sweeping conclusion on, but to me it suggests that someone climbing mountains for years around the U.S. (outside of Alaska) should be fine, if he or she is careful to acclimatize well.

Still, it’s also clear that one high-altitude climb can really hurt. Before the Kilimanjaro trip, all seven of the trekkers in Fayed’s study had a brain scan to confirm they had no preexisting damage; afterwards, one hiker’s scan revealed the white-birdshot look of enlarged VR spaces in his brain. And since the damage can occur without signs of altitude sickness, we can only assume that the worse you feel, the more at risk you are.

We’re also learning that the older you are, the more susceptible you become to the effects of high-altitude hypoxia. According to Fayed’s latest study, published online last May in the journal Neurological Research, the risks of altitude sickness and potential brain damage grow with age—climbers in their late thirties and early forties are more likely to have either AMS or brain damage than climbers in their late twenties and early thirties.

You’d think all of this might give pause to people whose passions take them into the highest mountains. But I asked Fayed if any of the climbers in his studies quit the sport after seeing the damage to their brains.

“They are all still climbing,” he said. “Our purpose is not to convince anyone to stop climbing. It is to make people aware of the dangers and the need to acclimatize properly.”

Most of the climbers I talked to seemed unfazed by the risks. Mountaineers are already undeterred by exposure to much more immediate and lethal hazards. And many have already suspected for years that high-altitude climbing has an effect on the brain.

“High-altitude mountaineering kills brain cells—no doubt,” says RMI guide Melissa Arnot. “But this is what I do. It’s my profession.” One internationally known climber confided to me that he isn’t sure whether his cognitive function recovers completely after big climbs, or if he just gets accustomed to the diminished capacity. Another, RMI guide Alex Van Steen, once told me, “Sometimes you’re never quite right afterwards.”

I, for one, will not stop climbing. But Fayed’s science is sound, and it’s changed the way I’ll go about it. I’m not going to be as tempted to push through the pain of altitude illness to try to reach the summit. Climbers are always looking for external warning signs that they should turn around: approaching weather, weakening teammates, unstable snow. It’s clear now that mountain sickness is an internal warning that they should treat with just as much respect.

The Damage Dictionary


Climbing The key to avoiding altitude-related brain injury: Sleep low, climb slow, stay hydrated, and turn back if you begin to feel sick


Widening of spaces surrounding blood vessels in the brain. They are caused by brain swelling or atrophy and are associated with age-related cognitive decline, dementia, and various brain diseases.


Loss of neurons in the cerebral cortex—the surface layer of the brain, which carries out conscious thought, physical perception, and higher-level control of body movements.


Damage to the white matter beneath the cerebral cortex. In a climber’s brain, the damage is often caused by small strokes—clots that form in the thickened blood, starving the surrounding tissue of oxygen. White matter is the network that transfers signals between parts of the brain, so damage causes widespread and irreversible problems.

Protect Your Brain

Follow these steps to prevent high-altitude trouble in your head.

Enlarged VR spaces

Enlarged VR spaces Enlarged VR spaces—more common in the elderly—are clearly visible on this young amateur climber's brain scan after Aconcagua

1. Coming from sea level? Spend night one at about 5,000 feet.

2. Ascend as slowly as possible. Medically speaking, the safest rate is 1,000 feet per day above 9,000 feet.

3. Minimize time above 19,500 feet.

4. Climb high, sleep low. The higher elevation will kick-start the acclimatization process, while descending at night allows the body to adapt at a safer elevation. Or build in a rest day every 2-3 days.

5. Listen to your body. Never ascend with obvious symptoms of altitude sickness; descend if symptoms worsen.

6. Stay hydrated, avoid excess salt, and eat foods rich in carbohydrates.

7. Don’t drink alcohol—it’s dehydrating and depresses breathing.

I’m Not Feeling So Good…

Jimmy Chin

Jimmy Chin On Jimmy Chin's expeditions, he takes it easy at base camp. “Your body gets weakened with stress. Resting really well and getting acclimatized at a lower elevation is more important than getting fatigued from constantly trying to climb high and sleep low”

Altitude sickness can strike as low as 6,000 feet but more commonly occurs above 8,000 feet, usually in people who’ve reached that elevation rapidly. The first phase is Acute Mountain Sickness (AMS), which always involves mild brain swelling that, according to the Spanish study, can cause lasting damage. Symptoms include headache, nausea, and malaise; if these start, climbers should descend until they disappear. If the swelling continues to worsen, it can become High-Altitude Cerebral Edema (HACE), a very serious condition. Delusions, confusion, and emotional instability are early symptoms; it can progress to cause lack of coordination, unconsciousness, and death.

Here’s what happens: At altitude, the lack of oxygen causes your heart and respiration rates to increase. This causes you to exhale too much carbon dioxide, which upsets the water and electrolyte balance in the blood. That, in turn, damages the walls of brain (and lung) capillaries, causing them to leak fluid into surrounding tissue and make the brain swell. The blood also becomes thicker as more red blood cells are produced to transport oxygen and as water is pulled out by dehydration. In the most serious cases, clots develop in the thickened blood, causing minor strokes.

The Results Are In (and Kinda Scary)

The Spanish report is not the only one to examine brain damage in otherwise healthy high-altitude climbers.

By comparing scans of nine climbers’ brains before and after trips to K2 or Everest, Dr. Margherita Di Paola and her colleagues at the University of Rome found that the climbers lost both gray and white matter. The study also showed greater damage to the brain regions controlling the dominant side of the body—presumably from the greater oxygen demands in the parts controlling movement.

A 1996 study in the British journal Clinical Science compared brain scans of 21 elite climbers who had climbed above 26,000 feet and a control group of 21 people who had never been to high altitude. Sixty percent of the elite climbers showed signs of mild cortical atrophy or damage in the white matter deep in the brain. Notably, seven elite Sherpas who climbed that high but lived at high altitude were also studied. Only one showed similar effects. The authors recommended slower acclimatization.