Acute mountain sickness
The most common form of altitude illness is a constellation of symptoms known as Acute Mountain Sickness (AMS). Although there are no laboratory tests to identify AMS, useful clinical criteria are the presence of at least three of the following seven possible symptoms: headache, nausea or vomiting, sleep disturbance, dizziness, shortness of breath, anorexia, and fatigue.
Headache leads the list of complaints, with about 70% of visitors above 8000 feet suffer from at least mild headache. Seven percent of visitors describe severe headache, throbbing and bitemporal in nature, usually not responsive to over-the-counter medications.
The second most common complaint is sleep disturbance. Over thirty percent of people going above 8,000 feet note difficulty sleeping. Possibly secondary to periodic breathing (Cheyne-Stokes respiration). This symptom is very disconcerting to the traveler spending the night in a strange hotel room who may awaken with the sensation of not breathing!
Fatigue is the third most common complaint voiced by AMS sufferers. The fatigue is more severe than can be explained by the previously mentioned insomnia, and occurs in nearly 3 of 10 visitors.
Shortness of breath and dizziness ensue in about equal numbers of patients, both occurring about twenty percent of the time. Most will notice increased dyspnea with exertion, but dyspnea at rest may suggest increased severity of illness.
Oxygen saturation (SaO2) measured by pulse oximetry is a very useful tool in evaluating altitude illnesses. Even though the patient may complain of significant shortness of breath, the SaO2 is within the normal range or at most minimally reduced. At 9000 feet SaO2 measurements between 85-95% are common. Measurements below 85% imply more serious impairment of oxygen exchange than is seen with uncomplicated AMS and should prompt further investigation.
Nausea and anorexia are less common presenting complaints, occurring in less than five percent of cases. This symptom can be merely a loss of appetite or may be severe enough to impair adequate fluid intake. Vomiting indicates more severe illness and also can lead to significant dehydration.
Acute Mountain Sickness has been reported to occur in 17 to 24% of those who travel from sea level to above 8,000 feet. Higher altitudes increase the incidence of symptoms with 67% becoming ill at 14,000 feet. The illness usually begins within 4 hours of arrival in 60 percent of those affected. On occasion the onset of symptoms may be delayed for two to three days. Onset of symptoms after more than a week should call the diagnosis into question. AMS is generally a self limited condition spontaneously resolving within three to four days. Still, the patient is often miserable until resolution occurs. The presence of ataxia indicates progression to more serious disease.
The rate of ascent to altitude is probably an important determining factor in who gets sick. Some may have a predisposition to altitude illness, suffering with each visit, though many will not have a recurrence. Predicting who will get sick is not possible. AMS occurs with equal frequency in males and females, and children often suffer from the malady. Older persons may actually be less likely to develop AMS, perhaps because of reduced activity levels. Excellent physical conditioning does not seem protective.
Various treatments for AMS have been employed in the past, usually based on anecdotal evidence. In studies at high altitude both acetazolamide (Diamox) and dexamethasone have been reported to be helpful. Aceazolamide is carbonic anhydrase inhibitor whose mode of action is thought to be increased urinary excretion of bicarbonate resulting in a metabolic acidosis which stimulates respiration. Unpleasant side effects of acetazolamide such as tingling of the fingers and around the mouth and alteration of the taste of carbonated beverages may limit patient acceptance. The incidence of these symptoms may be decreased by prescribing smaller doses than previously recommended. One half of a 250 milligram tablet taken twice a day beginning the day prior to ascent and continued for the first 2-3 days while at altitude is usually adequate. Acetazolamide should not be used by people with a sulfa allergy.
Dexamethasone may act by inducing a non-specific state of euphoria thereby allowing the patient to tolerate uncomfortable altitude symptoms. It does not promote acclimatization and should be reserved for severe cases until descent is possible. The initial dose is 8 mg. followed by 4 mg. every six hours.