The incidence of acute mountain sickness (AMS) varies with location, depending on both absolute altitude reached and rate of ascent to altitude. It has been estimated that 15 to 40% of Colorado resort skiers (depending on the altitude of the resort) develop AMS, and studies have shown an incidence of 40% in Mt. McKinley climbers and 70% in Mt. Rainier climbers. Given the huge numbers of Colorado tourists (10 million a year), this is not a trivial problem. High altitude cerebral edema (HACE), or brain swelling, is defined as the progression of cerebral symptoms and findings of ataxia (difficulty with balance, walking, and muscular coordination) and change in consciousness.
The sleeping altitude is the critical factor, with 9,000 feet being a significant threshold for illness (>20% incidence), and 8,000 to 9,000 feet less of a problem (perhaps 10 to 15% incidence), while below 8,000 feet, AMS is uncommon (but still possible). Susceptibility to AMS is not related to physical fitness or gender, although women less frequently suffer from pulmonary edema (fluid in the lungs). Older adults may be less susceptible, while limited data suggest that children probably have the same incidence as does the general adult population.
Individual susceptibility and reproducibility are well documented. Contributing factors include low lung capacity, a less vigorous breathing response to conditions of low oxygen content in the blood, and exaggerated pulmonary hypertension (high pressures in the circulation of the lungs) in response to hypoxia (for high altitude pulmonary edema, or HAPE). Brain circulatory responses and dynamics play an important role, but are difficult to test at sea-level.
Currently, past history of AMS is the most significant risk factor and best predictor. Early diagnosis is the key to successful management and a high index of suspicion is critical. The setting is rapid ascent to a higher altitude in unacclimatized persons. The symptoms include headache, poor or no appetite, dizziness, nausea, insomnia, feeling tired, fatigue, and shortness of breath. Difficulty with (erratic) breathing is common during sleep, but not a sign of AMS. Early AMS feels exactly like a hangover. In the early stages, physical findings may be lacking. When advanced, the findings are those of fluid in the lungs and brain swelling. Ataxia, change in mental status and bluish skin discoloration (particularly noted in the fingers and toes and around the lips, also known as cyanosis) are the most useful indicators of serious illness.
The differential diagnosis of AMS includes dehydration, exhaustion, carbon monoxide poisoning (this is very important indoors, or in a tent or igloo), infections of lung or brain, viral syndromes, migraine events, transient ischemic attack (TIA, of the brain), hypothermia, drugs, and psychiatric problems.
The pathophysiology of moderate to severe AMS and HACE is clearly related to brain swelling. Whether early AMS, especially the headache, is due to brain swelling is not yet established. Factors contributing to brain swelling include, but are not limited to, the degree and rate of onset of hypoxemia (low oxygen content in the blood), inadequate breathing (known as hypoventilation, which can be due to low innate breathing response to hypoxemia, respiratory depressant drugs, or ascent too rapid for adequate acclimatization), poor gas exchange (oxygen for carbon dioxide) in the lungs, fluid retention, individual anatomy (such as ability to accommodate increased brain volume).
As brain volume increases, the pressure within the brain (intracranial pressure, or ICP) rises, although very little (perhaps only 20 to 30 milliliters) until a critical threshold is reached. A dehydrated brain is much more compliant than a “wet” brain. Dilation of cerebral blood vessels causes increased cerebral blood flow and increased cerebral blood volume, engorging the brain and making it stiffer and less compliant. As brain swelling continues, ICP rises beyond the ability of blood to flow into brain tissue. Eventually (and sometimes quite rapidly), cerebral blood flow stops, causing death.
Treatment is directed toward reducing brain volume and stopping any leak of fluid from the blood vessels into brain tissue:
1. Increase oxygenation and thereby reduce low oxygen concentration in the blood and tissues:
a. Descent – 1,000 feet may be adequate to effect improvement, but one should descend as far as is necessary until there is visible clinical improvement.
b. Administer supplemental oxygen if it is available. This is especially good for headaches and altered mental status.
c. Initiate hyperbaric oxygen therapy (e.g., within a portable pressure bag) if such is available
2. Speed the process of acclimatization:
a. Administer acetazolamide (Diamox) 125 to 250 mg by mouth every 12 hours. For children, the dose is 5 mg/kg of body weight/day. This drug promotes increased urination, stimulates ventilation, and decreases cerebrospinal fluid formation. Because acetazolamide carries some cross-reactivity with “sulfa” drugs, it should be used with extreme caution in persons suspected or known to be allergic to sulfa drugs.
b. Acclimatization at the same altitude is okay for mild AMS, but a sick person should never be left alone.
3. Treat symptoms:
a. For the headache, use analgesics.
b. For nausea and vomiting, use antiemetics, such as ondansetron (Zofran) dissolving wafer tablets 4 mg by mouth every 4 hours as needed
4. Reduce the fluid leak from the brain capillaries:
a. Administer dexamethasone 4 mg by mouth (or if a health care professional, by injection) every 6 hours. This may need to be continued until the victim is evacuated to a lower altitude, since rebound brain swelling may occur with cessation of this medication, and because the drug per se does not improve or hasten acclimatization.
Prevention of altitude illness:
1. As best possible, ascend slowly. “Climb high and sleep low.” The ideal rate of ascent is difficult to establish because of marked individual variation in the ability to acclimatize. A reasonable recommendation is to not sleep at an altitude 2,000 feet higher than the previous night’s sleeping altitude once above 8,000 feet. Take an extra day for acclimatization with every 3,000 to 4,000 feet of elevation gain.
2. A high (>70%) carbohydrate diet reduced AMS by 30% in some studies, but had little effect in other reports. It is not likely to be harmful and might help.
3. Avoid respiratory depressants (especially sleeping pills), and only ingest alcohol in small amounts.
a. Indications are forced rapid ascent or history of recurrent illness.
b. Take acetazolamide by mouth up to 5 mg/kg body weight/day divided into 2 or 3 doses, beginning one day prior and until one day after ascent. 125 mg twice a day may be sufficient for most persons. Recall that there may be a cross-reactivity (allergic reaction) in persons allergic or sensitive to “sulfa drugs.”
c. Take dexamethasone 4 mg by mouth every 6 to 12 hours, or 2 mg every 6 hrs. This is useful for persons intolerant of acetazolamide, or if the travel will be to extreme altitude. The drug may need to be continued for three or four days, since it does not speed acclimatization. It is commonly used by climbers on “summit day.”
d. Use of acetazolamide and dexamethasone simultaneously is promoted by some – acetazolamide to speed acclimatization and dexamethasone to prevent brain swelling, but only in first few days of ascent.
e. Ginkgo biloba was used in three studies, and shown to reduce AMS from 35 to 100%. It may be more effective during a moderate rate of ascent. The dose is 100 mg by mouth twice a day starting 2 to 3 days before and while at altitude. It is safe and inexpensive, but it has not been proven effective in all studies. Furthermore, preparations of the compound vary.