As the number of international, adventure, and wilderness travelers increase, physicians in all locations and types of practices may be asked to counsel and provide prophylaxis or self-treatment for a variety of travel-related illnesses. At higher altitudes, the decreased partial pressure of oxygen can cause several pathological presentations, including High Altitude Pulmonary Edema, High Altitude Cerebral Edema, and the milder, but much more common, Acute Mountain Sickness (also referred to as Altitude Illness or Altitude Sickness). High Altitude Pulmonary Edema and High Altitude Cerebral Edema are both life-threatening emergencies requiring immediate treatment, with a descent to lower altitude (or higher pressure artificial environment) as quickly as can be safely arranged and executed. In contrast, Acute Mountain Sickness symptoms can be prevented or managed with oral medication, and Acute Mountain Sickness does not typically require descent or oxygen supplementation. High altitude environments exist at elevations over 1,500 meters. The higher the altitude, the less the oxygen saturation and risk of hypoxemia.
Acute Mountain Sickness is caused by the body’s reaction to the reduced oxygen level in respired air and resultant tissue hypoxia. At baseline metabolic levels, the brain is the most sensitive organ regarding hypoxia and oxygen stress. Thus, the symptoms of Acute Mountain Sickness (discussed below) are mediated by the central nervous system (CNS). In many travelers at altitude, respirations during sleep develop a periodic pattern that may contribute to the development of symptoms.
The incidence of Acute Mountain Sickness increases with increasing altitude. While Acute Mountain Sickness is very uncommon under 2500 m, the percentage of non-acclimated travelers affected at 3000 m approaches 75%. Any travelers with prior episodes of Acute Mountain Sickness are at greater risk than those who have tolerated similar trips in the past.
Pre-existing diseases can increase Acute Mountain Sickness risk by magnifying the effects of the hypoxia. The most common conditions in this category include anemia, with a reduced oxygen-carrying capacity of the blood, and chronic obstructive pulmonary disease, due to the reduced degree of oxygenation occurring in the lungs.
Given the suspected severity of the underlying process, careful pre-trip measures should include: screening to characterize the severity of the disease, additional treatment of the underlying condition, a lower threshold altitude to begin prophylaxis to reduce the risk of hypoxia, enhanced preparation for treatment during the trip, and recommendations to modify the itinerary.
As the brain has the highest baseline need for oxygen supply, the symptoms of Acute Mountain Sickness are neurologic (central nervous system CNS). As the generalized hypoxia can affect multiple functions, CNS symptoms are not specific or localizing. Lower respiratory rate, typical during sleep, increases the risk for Acute Mountain Sickness, and symptoms are often first noticed upon awakening. For the majority of people, acclimatization at altitudes over 5600 meters is not possible and prolonged stay at these heights can be associated with respiratory distress.
The hallmark of Acute Mountain Sickness is a headache, with other symptoms including nausea, vomiting, loss of appetite, fatigue/malaise (particularly at rest), sleep disturbance, and dizziness/lightheadedness. Acute Mountain Sickness symptoms can begin after only a few hours and typically present the first day at a given altitude, resolving after one to three days, even without treatment, as the body adjusts physiologically (acclimates) to the lower oxygen levels.
The presence of facial or extremity edema can be present with or without Acute Mountain Sickness symptoms and is felt to be a marker for not yet being acclimated to the altitude. Rarely, retinal hemorrhages can occur and affect visual fields.
Acute mountains sickness is a clinical diagnosis based on the presence of a headache, with or without the other typical symptoms, in the appropriate setting. There is no specific lab or other testing recommended. Since the symptoms of Acute Mountain Sickness are nonspecific, the presence of atypical symptoms, such as diarrhea, should trigger an evaluation for other causes. Also, the patient may have increased their alcohol intake while traveling, and this should be considered when evaluating. Finally, the presence of additional or more severe neurologic symptoms is indicative of possible High Altitude Cerebral Edema and should be treated as a medical emergency. 
For a traveler going to an at-risk altitude, the most significant modifiable risk factor is the rate of ascent. More gradual ascent allows the body’s physiological processes to adjust to the reduced partial pressure of oxygen at the new altitude. Planning the trip to allow for such acclimation is the most natural method of prevention and, when possible within the parameters of the patient’s overall travel plans, is preferred to eliminate the risk of side-effects from pharmacologic management. This method will also reduce the risk of worsening or triggering any oxygen-sensitive underlying conditions which are present (discussed above). Because of the change in respiratory pattern during sleep, the altitude at which the traveler sleeps is more important for AMS risk than the maximal altitude reached during the day. The optimal rate of ascent (sleep altitude) should be no more than 500 m per day at levels greater than 2500 m. Also, allowing at least one day to acclimate around 2500 m before further ascent, and then again for every additional 1000 m ascent, will reduce risk. Avoiding exercise and alcohol for the first 48 hours until acclimated may also minimize the risk of symptoms. If Acute Mountain Sickness does occur, further ascent is not advisable until acclimated. Having the patient descend is also curative when symptoms are persistent and should be considered when possible in the rare cases when symptoms are progressive despite treatment. Regarding pharmacologic measures, acetazolamide is currently used most commonly for both prophylaxis and treatment of Acute Mountain Sickness. Prophylaxis is used if going to greater than 9000 feet without acclimatization and can be considered at somewhat lower levels if the traveler has had Acute Mountain Sickness previously or has other risk factors. Acetazolamide decreases the pH level of the blood, which then causes an increased respiratory rate. It is well tolerated in general, but patients should be advised that use can cause tingling of the digits and even lips.
Dexamethasone use is less frequent for Acute Mountain Sickness, and it is used primarily as a treatment, though occasionally may be considered for prophylaxis in certain settings. Ibuprofen may also be effective as a non-prescription option, and some travelers may be more comfortable using ibuprofen since they are familiar with it. All three treatments may cause gastrointestinal (GI) symptoms, which may overlap with those seen in some instances of Acute Mountain Sickness. Assuming the medications are tolerated, use should continue until the traveler has at least one to two days at the highest altitude reached to acclimate.
Also, while accessing the potential risk of altitude to the traveler, it is important to note that the stress of reduced oxygen levels may worsen other medical conditions such as angina, congestive heart failure, and sickle cell (trait or disease), all independent of Acute Mountain Sickness symptoms.
The management of acute mountain sickness is an interprofessional. While the physician may treat the acute symptoms, the key is to prevent the symptoms in the first place. The sports nurse, respiratory therapist, pulmonologist and physical therapist should educate the patient on the importance of staged ascents. If ever the individual develops symptoms at a certain height, the key is to go back down immediately. Anyone with symptoms following a mountain climb should seek immediate medical assistance, because the cerebral symptoms can quickly take a negative downturn. Finally, the pharmacist should educate the patient on routine prophylaxis with acetazolamide and discontinue smoking when climbing mountains. Today, there are portable oxygen sensors that one can wear which will sense the oxygen concentration in the body.  (Level V)
For most patients the outcome after suffering from acute mountain sickness is good; however, complete recovery may take time. Many climbers will develop symptoms of acute mountain sickness on an ascent. However, sometimes the symptoms may be delayed for 24-48 hours. One can reduce the intensity of symptoms by descending slightly and then resuming climbing. Complete resolution of symptoms can take 2-5 days, but if one continues to climb in the presence of symptoms, one can develop confusion, disorientation and lapse into a coma. Brain herniation has been reported and often the course is fatal. Those who survive may have memory and gait deficits that persist for months. (Level V)
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