Incidence of Acute Mountain Sickness
A Study of Winter Visitors to Six Colorado Resorts
Charles S. Houston, M.D.
MANY THINK OF altitude illness as an uncommon affliction of foolhardy mountaineers but the steadily increasing interest in skiing, hiking and trekking has been paralleled by growing awareness that altitude illness is common even at modest altitudes and has important medical and economic consequences. We wanted to determine just how frequently Acute Mountain Sickness (AMS) affected winter visitors to six Colorado mountain resorts between 8000 and 9500 feet high. Previous studies have been concerned only with much higher altitudes.
Altitude illnesses form a continuum of several components: an unpleasant, but generally benign syndrome called Acute Mountain Sickness (AMS) which usually subsides in a day or two; a more serious and occasionally fatal form High-Altitude Pulmonary Edema (HAPE), and the equally serious problem called High-Altitude Cerebral Edema (HACE). Sometimes AMS may progress to HAPE and/or HACE, but any of the three may appear alone. Whether or not one gets altitude illness depends on the altitude reached, rate of ascent, length of stay, and individual susceptibility. In this study we investigated only AMS, defined as the presence of any three of the following: headache, nausea and/or vomiting, weakness, shortness of breath and sleep disturbance. We designated visitors those who had come from a lower altitude within the five days preceding the interview. Though we tried to do a study which would be epidemiologically impeccable, little money and few helpers made this difficult.
The interviewer handed a detailed questionnaire to persons encountered in restaurants, ski area buses, and lounges in the six resorts during the winter of 1982. She introduced herself and stated that our purpose was to determine what if any effect the altitude had on visitors; she avoided further discussion until the questionnaire was completed, which took about five minutes. The forms were coded and the data analyzed by computer. We did not try to interview persons under medical care or sick at home, nor did we evaluate the presence of HAPE or HACE or try to select out other conditions which might cause similar symptoms.
655 or 17% of the 3906 persons questioned said they were having symptoms at the time, which they attributed to the altitude. Two-thirds of these (12% of the entire group) reported three or more symptoms and thus were considered to have AMS. 1077 (28%) said they had had AMS in the past, half of them or 14% of the total at or below 9500 feet. Of the entire group 87% had been at the altitude for five days or less and thus were presumably not acclimatized to high altitude. Of the 655 persons with symptoms, 93% had been at altitude five days or less.
About half (52.5%) were women; less than 1.0% were under ten years of age, 17.5% between 11 and 20, 65.2% between 21 and 40, and 16% over 40. 70% lived below 3000 feet, and 25% between 3000 and 6000 feet. In order of frequency, the symptoms complained of (by those who had symptoms) were headache, shortness of breath, weakness, sleep disturbance, and nausea and/or vomiting.
To determine incidence one must have a denominator (persons at risk) as well as a numerator (persons affected). We were unable to find an accurate source for the number of persons at risk, but by interviewing at six different resorts we felt the sample would be representative of the population visiting. Furthermore when the data for each month were compared, the percentages were almost identical. Although one can challenge this approach we believe the data represent the true incidence of AMS.
Singh, describing 1905 cases of “Acute Mountain Sickness” in Indian troops taken rapidly from the plains of India to 12-14,000 feet in 1962, reported an incidence ranging from 0.8 to 8.0% in different companies, but he included all forms of altitude illness, and a wide range of altitudes, rates of ascent, and length of stay, and did not explain the tenfold difference between different groups. Hackett and Rennie obtained 278 completed questionnaires from 330 trekkers passing through a clinic at 14,000 feet near Mount Everest, finding that 52.5% had one or another form of altitude illness, and noted that some of the more serious cases may not have been able to get that high. They included cases of HAPE and HACE. Larson and Roach comparing placebo with several drugs in climbers on Mount Rainier (14,410 feet) found that 25% of the placebo and untreated subjects had AMS. Brendel reported that 35% of 260 trekkers in the Peruvian Andes had “high altitude discomfort.” Milledge pointed out the difficulty of obtaining accurate data from climbers and trekkers because of the many variables. We have not found any study which defines incidence or level of risk among people vacationing between 8000 and 10,000 feet.
Our study has several defects. By interviewing persons who were active, the incidence may be understated because those who felt too badly to be up and about are not included. Since the interviewees were on buses going to or from a ski area, or in restaurants or lounges at the area, we presumed they were typical of the ski population, but this supposition may be inaccurate and could bias the study in either direction. We did not try to separate out other causes of these symptoms, such as hangover or flu, but accepted the individual’s belief that the symptoms were due to altitude; this is imprecise and may have produced an incidence which is too high, but our findings at the six resorts and for each of the four months are similar. Because this study was made in the winter months, it concerned primarily a skiing population which might differ somewhat from a summer population. We did not determine to what degree, if any, activities were limited by symptoms among those who felt ill although our data do show that those who had been most active were the most affected.
Despite these deficiencies, some of which might have been avoided had more resources been available, our data are compatible with smaller studies at higher elevations and we believe give a reliable estimate of the incidence of AMS at modest altitude.
The data are important not only because of the miserable and occasionally serious symptoms experienced, but because of the economic impact on resort business. Estimates of visitation to the Rocky Mountains are only approximate but suggest that more than half of the 25-30 million individuals who visit the mountain states each year go above 8000 feet. Our findings indicate that 12% to 17% of these will have symptoms due to altitude.
If one assumes, conservatively, that each of these two to three million persons will refrain from spending $25 for food or drink or recreation because of feeling poorly, we estimate that resort businesses may be losing 50 to 75 million dollars a year because of altitude illness among visitors. These estimates do not include those who are seriously ill and leave earlier than planned, or those who feel poorly enough not to return, and we believe them low rather than high. An epidemic of infectious disease which cost this much would surely have high priority for prevention, early diagnosis and treatment.
Of 3906 men, women and children interviewed during three winter months in six Colorado resorts from 8000 to 9500 feet in altitude, 12% were experiencing three or more symptoms attributed to altitude at the time of interview and thus were considered to have Acute Mountain Sickness (AMS). 27% reported having had altitude illness in the past, half of them at or below 9500 feet. Half of the group interviewed were male, and 65% were between 21 and 40 years of age. This is the first survey of altitude illness in a general population at altitudes between 8000 and 10,000 feet. An attempt is made to estimate the loss of revenue to resort operators due to mild altitude illness.
The study was supported by a grant from the Snake River Health Services, Inc. of Keystone, Colorado. Interviews were done by Patricia Zurenda, RN, and data analysed by Dr. Irving Weiss.
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