Acute Mountain Sickness

Contents

1. The illness and what causes it
2. Symptoms and their timing
3. How the diagnosis is scored
4. Conditions that resemble it
5. When it becomes more serious
6. Prevention and treatment in the guidelines
7. What remains unsettled

Candace Cooley practiced in Breckenridge, Colorado, from 2011 to 2014, in a ski town whose lodgings sit near 9,600 feet according to the CDC Yellow Book’s list of popular high altitude destinations.⁴ At that height two things are easily confused: the ordinary fatigue of a long trip, and a true altitude illness with a predictable course. A 1997 Spokesman Review report on Colorado ski visitors found that most people who fell ill assumed they had caught the flu or were suffering from jet lag, and so never sought care, although the condition affects up to a quarter of those who come to the state to ski or sightsee.⁷ That condition is acute mountain sickness, usually shortened to AMS. It is the mildest and most common of three altitude illnesses; the other two, high altitude cerebral edema and high altitude pulmonary edema, are rarer and can kill.²

The illness and what causes it

Risk begins at elevations above about 8,200 feet. A Medical Dialogues summary of the Wilderness Medical Society guidelines notes that travel above that height carries a risk of one or more forms of altitude illness, and that the recommendations apply to anyone who goes high, whether for work, hiking, skiing, trekking or mountaineering.¹ The underlying stress is low oxygen. The CDC chapter notes that at roughly 10,000 feet the pressure of inspired oxygen is only 69 percent of its sea level value, enough to drop arterial oxygen saturation to between 88 and 91 percent on first arrival.⁴

Figure

Sleeping heights at popular high altitude destinations

• Vail lodgings8,150
• Breckenridge lodgings9,600
• Cusco, Peru11,000
• La Paz, Bolivia12,000
• Lhasa, Tibet12,100

feet

Why that shortfall produces headache is less certain. A high altitude emergency physician writing for the University of Colorado Anschutz Medical Campus explains that the symptoms are thought to come from increased pressure around the brain when the body fails to acclimatize, and that the illness is rare below 8,200 feet but common above it.⁶ Researchers at Stanford, in a trial reported by Medscape, started from a related idea: that thinner air sets off inflammation in the brain, which an anti inflammatory drug might blunt.⁹

Symptoms and their timing

Headache is the cardinal symptom. The CDC defines AMS by a recent ascent plus headache with at least one of poor appetite, dizziness, fatigue, nausea or, occasionally, vomiting. Onset usually comes 2 to 12 hours after arrival, often during or after the first night, and the illness generally clears within 12 to 48 hours if the traveler climbs no higher. Very young children who cannot yet describe a headache may show only loss of appetite, irritability and pallor. Symptoms that begin more than three days after arrival, without further ascent, should not be blamed on AMS.⁴

The British travel health service TravelHealthPro lists sleep disturbance and weakness alongside the core complaints.⁵ Severity varies widely. A physician interviewed by the Vail Daily described visitors whose only sign was a minor headache that faded within hours, and others whose headache lasted for days with nausea, vomiting and an inability to sleep.⁸

How the diagnosis is scored

AMS has no laboratory test. Because the early illness has no definitive physical signs, a review of high altitude illnesses in the NIH archive explains, clinicians and researchers rely on what the person reports, and several self scoring systems were built to decide whether AMS is present and how bad it is. The most familiar is the Lake Louise questionnaire, in which each symptom is graded from 0 to 3 and a headache plus a total of 3 or more is usually counted as AMS.¹⁰ The same review cautions that other cut off points and other scales are in common use, that the systems do not give equivalent results, and that study findings often depend on which one was chosen.¹⁰

Measurements add little. The CDC notes that AMS has no characteristic physical findings and that pulse oximetry is usually normal for the altitude or slightly low, although a high saturation for the altitude seems to protect against it.⁴ The Wilderness Medical Society, as summarized in American Family Physician, does not use fixed altitude thresholds for the diagnosis, since AMS can appear as low as 6,500 feet.² What can be scored in advance is risk, which the CDC sorts by past history, first night sleeping altitude and pace of ascent.⁴

Figure

Risk categories for acute mountain sickness

1. 01
   LowNo history of altitude illness or only mild AMS, a first night below about 9,000 feet, and a slow gain in sleeping altitude above 9,800 feet.
2. 02
   MediumA past episode of moderate to severe AMS, or a first night between about 9,000 and 11,150 feet, or a faster gain offset by extra acclimatization days.
3. 03
   HighA past episode of cerebral or pulmonary edema, a first night above about 11,150 feet, or a fast gain with no extra days to acclimatize.

Conditions that resemble it

With no test to settle the question, ruling out look alikes matters. The guideline panel, in OutdoorEd’s summary of the Wilderness Medical Society recommendations, names carbon monoxide poisoning, dehydration, exhaustion, low blood sugar, hypothermia and low blood sodium as disorders whose signs can mimic AMS.³ The CDC adds migraine to the list and draws one useful line: unlike a viral illness, AMS brings no runny nose, fever, chills or muscle aches. Rapid improvement after a descent of 1,000 feet or more also points toward AMS.⁴ Habitual caffeine users who go without it can develop a withdrawal headache that is easily mistaken for the altitude kind, and some clinicians treat an isolated altitude headache as a separate entity from AMS.⁴

Dehydration deserves special mention, since dry mountain air makes it common. Its symptoms overlap with AMS, so adequate fluid matters, but the guidelines summarized in American Family Physician found that forced overhydration does not prevent altitude illness and raises the risk of low blood sodium.² TravelHealthPro puts the working rule plainly: symptoms at altitude are altitude illness until proven otherwise.⁵

When it becomes more serious

TravelHealthPro describes the cerebral form by confusion, altered consciousness and poor coordination, and the pulmonary form by mounting breathlessness, breathlessness when lying flat, cough, chest tightness and blood tinged sputum.⁵ The CDC calls high altitude cerebral edema the end stage of AMS, an encephalopathy that is rare below about 14,000 feet. High altitude pulmonary edema can arise with or without AMS, runs about 1 case per 10,000 skiers in Colorado, and can be more rapidly fatal than the cerebral form; it often begins with chest congestion, cough and unusual breathlessness on exertion.⁴

Figure

Acute mountain sickness and its severe form

	Acute mountain sickness	High altitude cerebral edema
Hallmark	Headache with loss of appetite, dizziness, fatigue or nausea	Altered mental status, loss of coordination, confusion and drowsiness
Physical findings	None characteristic	Neurological findings on examination
How common	The most common form of altitude illness	Rare, especially below about 14,000 feet
Course without further ascent	Usually resolves within 12 to 48 hours	Coma can follow within 24 hours of onset

At resort elevations these severe forms are uncommon. The Vail physician said pulmonary edema occurs only once in a while there, and cerebral edema rarely if ever.⁸ The 1997 report noted three deaths from altitude illness in Colorado since 1990 in state health records, while physicians suspected more went unrecognized.⁷

Prevention and treatment in the guidelines

The Wilderness Medical Society treats sleeping altitude as more important than the highest point reached while awake, and a gradual rise in sleeping altitude as the best prevention. Acetazolamide and dexamethasone can both prevent AMS, but only acetazolamide speeds acclimatization; the most important treatment is descent of 1,000 to 3,300 feet, with oxygen if it is available.² The Colorado physician reports that one night at a moderate elevation near 5,280 feet sharply lowers the odds of symptoms, and that people who skip that stop are up to four times more likely to fall ill.⁶ The pace of that gradual rise, and the body changes behind it, are the subject of altitude acclimatization, and the broken nights that often come with the first days high are covered in sleep at altitude.

Once AMS appears, the panel says ascent should stop; climbing may resume when symptoms resolve, but never while they persist, and no one with the cerebral form should descend alone. Portable hyperbaric chambers can treat severe illness when descent is blocked, though they need constant tending and should never delay a needed descent.³ The CDC reduces the safety message to three rules: know the early symptoms, never go higher to sleep while they are present, and descend if they worsen despite rest.⁴

What remains unsettled

Several questions are still open. Ibuprofen is one. In the 2012 Medscape report, 43 percent of hikers given ibuprofen developed AMS after a night at about 12,570 feet, against 69 percent given placebo, but the authors warned that the result might not extend to higher climbs.⁹ The guideline review notes that two studies favored ibuprofen, a smaller one found no benefit, and comparisons with acetazolamide were mixed.² Evidence on preacclimatization is similarly split, with some studies showing fewer cases and others none.²

Figure

Ibuprofen against placebo in a 2010 field trial

• Ibuprofen group43
• Placebo group69

percent

Who falls ill is also poorly understood. The CDC calls susceptibility partly inherited but notes there is no simple screening test, and that training and fitness do not change risk.⁴ The Vail physician recalled that the sickest altitude patient he had treated was an elite ultramarathon runner, and that people who have had the illness nearly always get it again, for reasons no one has explained.⁸ For athletes the point carries over to the entry on sport and the female body. How any of this applies to one woman’s trip, given her own health, is a question her own clinician is best placed to answer.