Backpackers need not climb the Sierra’s highest peaks to experience altitude sickness, and although most hikers do just fine at moderate altitude, they should know what to do if an illness occurs at elevation. In a June 2010 article in “Wilderness and Environmental Medicine,” a panel of experts assembled by the Wilderness Medical Society provides a review of the research on altitude illness and evidence-based guidelines for its prevention and treatment.
A spectrum of disorders
Altitude sickness is a spectrum of syndromes that ranges from acute mountain sickness, a mild, self-limiting illness, to high-altitude pulmonary and cerebral edema (often abbreviated HAPE and HACE), medical emergencies characterized by the collection of fluid in the lungs and brain, respectively. Altitude illness occurs in response to the decreasing partial pressure of oxygen and barometric pressure as altitude increases. These decreasing pressures induce a state of oxygen deprivation in the body, resulting in symptoms of illness in some individuals. The incidence of acute mountain sickness at moderate elevation (6,600 to 11,500 feet) in the United States is 10 to 40 percent.
According to the Wilderness Medical Society panel, high-altitude cerebral edema is the end stage of acute mountain sickness. The relationship between high-altitude pulmonary edema and acute mountain sickness is less clear, but both kinds of edema are most often associated with a rapid change in elevation—flying to a high-altitude destination, for example—or ascent to very high or extreme high altitude (11,000 to 18,000 feet and above).
Generally self-limiting, acute mountain sickness varies in its presentation depending on individual susceptibility, the rate of ascent and the altitude. Backpackers with acute mountain sickness experience nausea, headache, dizziness, vomiting and fatigue. The onset of breathlessness even at rest, cough, chest tightness or congestion, confusion, clumsiness, altered mental status and drowsiness signal illness that may be progressing to pulmonary or cerebral edema. The signs and symptoms of dehydration, exhaustion, hypoglycemia, hypothermia and hyponatremia (water intoxication) can mimic those of acute mountain sickness and high-altitude cerebral edema.
From hundreds to thousands
Backpackers from Fresno (elevation ~300 feet) who hike to moderate elevations in the Sierra do have a slight risk of developing altitude sickness. Acute mountain sickness generally occurs at elevations above 8,000 feet—not a difficult elevation to reach popular backpacking destinations in the Sierra. Tuolumne Meadows, for example, a common starting point for backpackers in Yosemite National Park, lies at 8,600 feet, and several of the High Sierra camps in nearby national parks are situated that high as well. Most backpacking trips in the Sierra will traverse elevations above 8,000 feet, and it’s even possible to car camp above this elevation in the wilderness areas near Fresno.
Not far from Fresno in Kings Canyon and Sequoia National Parks, the Sierra Nevada rises to its highest points: 12 peaks lie more than 14,000 feet above sea level in the crest of the Sierra at the eastern border of the parks. In Kings Canyon National Park, the Goddard and Monarch Divides, populated with peaks above 13,000 feet, extend westward from the crest. The Great Western Divide, a subrange of the Sierra in Sequoia National Park, runs parallel to the crest of the Sierra and stretches higher than 12,000 feet.
The highest summit in the continental United States—popular Mt. Whitney, at 14,491 feet—is situated on the east side of the Great Western Divide in Sequoia National Park. Many of the trails in this region of the central Sierra cross through mountain passes at high elevation (10,000 feet and higher).
Prevention and treatment
According to the article, backpackers can lessen the risk of acute mountain sickness by making their ascents gradually, taking into account the altitude at which they’ll sleep rather than the altitude they’ll attain during a day’s hike. For example,
~ Hiking to 8,000 to 9,800 feet over two days and increasing sleep elevation less than 1,600 feet per day presents a low risk for acute mountain sickness.
~ Ascending to 9,200 feet in a single day or increasing sleeping elevation by 1,600 feet per day above 9,800 feet poses a moderate risk of illness.
Individuals with a history of altitude sickness are at a greater risk. The highest risks accompany rapid ascents to elevations 11,500 feet and above, hikes for which most recreational backpackers in the Sierra are likely to plan ahead and acclimatize.
Some medications are effective in preventing altitude sickness and may be recommended in high-risk scenarios. These medications are useful in treatment as well, but the best treatment is to hike to progressively lower elevation (accompanied, if possible) until symptoms clear. After a descent of 1,000 to 3,200 feet, most symptoms subside. Backpackers should always descend—reducing exertion as much as possible, including having others carry their pack—if they are showing signs of high-altitude pulmonary edema.
Dr. Eric A. Weiss offers these Golden Rules of Altitude Illness:
(1) Above 8,000 feet, headache, nausea, shortness of breath and vomiting should be considered altitude sickness until proven otherwise.
(2) No one with mild symptoms of altitude illness should ascend any higher until symptoms have resolved.
(3) Anyone with worsening symptoms or severe symptoms of altitude illness should descent immediately to a lower altitude.
The bottom line for high elevation
Although altitude sickness can be serious, the large majority of backpackers who travel in the high country do so without incident. Backpackers who have concerns should read the Wilderness Medical Society panel’s article closely and consult with their doctors.
While it’s smart to be prepared and knowledgeable about the health risks of backcountry travel, what takes your breath away in the high country probably won’t be high-altitude pulmonary edema—it will be the incredible views of the Sierra’s majestic peaks and the breathtaking vistas from their summits.
Harris, N. S. and Nelson, S. W. Altitude Illness—Pulmonary Syndromes.
Luks, A. M., et al. Wilderness Medical Society Consensus Guidelines for the Prevention and Treatment of Acute Altitude Sickness. Wilderness and Environmental Medicine, 21, 146–155 (2010). (PDF)
Weiss, Eric A. A Comprehensive Guide to Wilderness and Travel Medicine. Oakland, CA: Adventure Medical Kits; 2005. (PDF)
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