Acute Mountain Sickness

Acute Mountain Sickness (AMS) is a common condition occurring in individuals ascending to altitudes usually above 8,000 feet (2,500 m). It is usually a benign, self-limited condition, but it can progress to more severe manifestations. The greatest concern is its progression to either High Altitude Pulmonary Edema (HAPE) or High Altitude Cerebral Edema (HACE).

Because of this concern, it is important to know how to prevent this condition and how to identify those that become more serious. Presently, AMS is detected by having an awareness of symptoms and complaints. It is often preceded by a headache, which may be due to infection, overexertion, dehydration, and central venous pressure variations.

Atmospheric barometric pressure decreases predictably with increasing altitude, leading to reduced PaO2, which causes various physiological changes in the body. The initial response to hypoxia is an increase in minute ventilation to raise blood O2 content, decrease PaCO2, and shift the Hb-O2 curve to the left. Acclimatization to altitude involves marked increases in pulmonary ventilation and hypoxic ventilatory drive, probably mediated by tissue hypoxia and the effects of carotid body O2 sensors.

Despite these changes, Hb saturation and arterial O2 content fall slightly at altitudes above sea level and decrease over time at higher altitudes. Mild AMS is a frequent complaint and occurs in both experienced climbers going to altitude rapidly and in individuals climbing slowly or driving by motor vehicle to a lofty high-altitude site. The frequency of AMS correlates with the rate of ascent and altitude achieved. Preventive strategies and curative measures will be addressed separately.

Definition and Symptoms

Definition and Symptoms of Acute Mountain Sickness Acute Mountain Sickness (AMS) is a common issue for individuals who ascend to a high elevation too quickly. Symptoms are nonspecific and include headache, lightheadedness, nausea, vomiting, and fatigue. These symptoms can be classified as neurologic (i.e., headache, lightheadedness) or systemic (i.e., nausea, loss of appetite, fatigue). Hiking at high elevations, one may conduct observations of neurological symptoms, such as some individuals applying pressure to their forehead with their hands, one of the first self-report symptoms of AMS.

Systemic symptoms may include a loss in appetite, moaning, and food that appears regurgitated. In addition, the difficulty in performing activities (i.e., hiking) is one of the main reasons people experience high-altitude discomfort. The most common self-paced activity included resting, increased breathing rate, and increased talking. Symptomology in individuals is highly variable. Despite temporary fog-related disorientation, some individuals ran a 100-mile trail race at an average elevation of 10,000 feet. In contrast, other athletes terminated races taking place at 6,000 feet. However, in some cases, consulting a physician may be advisable when AMS symptoms worsen.

Although equally susceptible, a male twin can generally exhibit a more severe headache and desire to discontinue exercise when AMS symptoms occur. Moreover, a male twin indicated a twofold increase in symptoms, which generated psychological disturbances. Understanding when to consult a physician for prevention or intervention is important to minimize complications. Often (but not always), symptoms develop six to eight hours after arrival at elevations of 8,000 feet or more. Prompt recognition results in preventing more severe illness.

Causes and Risk Factors

The primary and most widely accepted cause of AMS relates to the reduction in oxygen as barometric pressure decreases with higher altitudes. For example, ambient oxygen levels at 8,000 feet are approximately 25% lower compared to sea level. The ambient oxygen fraction is clinically indistinguishable between 10,000 and 20,000 feet due to the sigmoidal shape of the oxyhemoglobin dissociation curve. T

he fraction of inspired oxygen is also slightly less than double that of the ambient oxygen fraction, as the humidity and body temperature increase the absolute amount of water vapour, thereby edging down the percent composition of the remaining gases. Thus, perhaps not unexpectedly, trekkers, adventurers, mountaineers, and pilots begin to complain of AMS shortly after reaching an elevation of 2,500 m. At this point, barometric pressure drops by approximately 20% compared to sea level, and a mirror image of this drop in inspired oxygen is seen.

Proposed mechanisms behind the physiologic dysregulation in AMS occur irrespective of any other obvious risk factors. The condition is compounded by sudden exposure to altitude mere hours after departure from low elevation, previous history of AMS, and the individual’s genetic predilection, energy status, sleep, and exercise capacity. Other factors that could cause a hypovolemic state and dehydration include fever, vomiting, and diarrheal illness, and these should also be addressed.

Only pack animals, such as hikers, can compensate for their high-altitude exposure by drinking water to prevent dehydration. An unusual case of resistant AMS has been described in an ultramarathoner with glycogen storage disease type 1a, leading to severe exertional hypoglycemia. Plasma concentrations of glucose and uric acid can help exclude altered energy balance as the cause of AMS in this age group. Our collective knowledge helps to stratify the basics of the physiologic stress on the body at high altitudes, as well as an individual's susceptibility to becoming unwell due to hypoxia. Unfortunately, a rapid ascent to a high altitude does not allow the body the time it requires to make necessary physiologic adaptations. To combat AMS, and even more severe forms of high-altitude pulmonary oedema and high-altitude cerebral oedema, the trekkers' love of the outdoors makes learning about who is at risk and how to prevent AMS a top priority.

The causes of reduced oxygen availability at high altitudes must be clearly outlined before we can define those at particular risk. Reduced oxygen combined with dehydration has an additive effect on the extent of AMS. In the post-flight and/or uplift scenario, the cause of dehydration at high altitudes seems to be associated with the diuretic effects of hypoxia, rather than excess urination alone. Individual susceptibility does vary, both at a genetic and environmental level, with high-altitude residency essential in conferring some degree of natural altitude tolerance.

Other proposed risk factors include a history of AMS, speed of ascent, individual variability, preexisting medical conditions, sleep disturbances, dietary habits, enhanced adrenergic responses, and stress. So the susceptibility to AMS is multifactorial. It is important to recognize these basic principles behind such a cascade of risk factors so that trekkers can take steps to reduce these problems. Heightened risks of morbidity are primarily based on rapid ascent and individual predisposition, and good self-awareness of these factors is a prerequisite for making an informed decision about any rapid ascent pre-ascent.

Preventive Measures

One of the most important ways to prevent acute mountain sickness is to undergo acclimatization before and during the acclimatization. Acclimatization involves allowing the body enough time to adjust to the decreased partial pressure of oxygen it will encounter at high altitudes.

Because it takes several days for the body to produce a higher amount of red blood cells and, therefore, maximize the concentration of haemoglobin before it can acclimatize, one of the typical acclimatization techniques is to "climb high, sleep low." This idea is consistent with a general rule of 300 to 500 meters per day to acclimatize. The rate of ascent differs, of course, because the initial elevations reached will likely differ among readers. The importance of acclimatization cannot be overemphasized.

Staying well-hydrated is one of the easiest and most effective ways to avoid acute mountain sickness. Not only does staying hydrated make recovery from altitude sickness easier, but it can also keep any mild symptoms of mountain sickness from worsening. Another preventive measure for staying well-hydrated is to drink two to four quarts of water daily, depending upon your body size, the weather, your activity level, and your particular body type. While it is true that it is easier to become dehydrated at higher altitudes, mild dehydration is not, in and of itself, thought to cause hypoxia.

However, dehydration can make the symptoms of acute mountain sickness worse and can certainly compromise physical and mental performance at higher elevations. Proper nutrition can also prevent acute mountain sickness. Not consuming enough calories or unnutritious foods could also make you tire more easily and impair your performance, mental function, and decision-making. Protein-rich snacks—such as cheese, beef jerky, eggs, sardines, nuts, and soy, among others—are good for increasing energy at higher altitudes. Therefore, it is important to combine protein-rich snacks with other foods.

Acclimatization Techniques

Mountain climbers should use acclimatization techniques to reduce the incidence of AMS. Many of the methods in this category monitor the effects of altitude, because of the influence of genetic factors and variability of individual responses. In general, acclimatization comes naturally with time ascending, allowing the body to adapt to the reduced availability of oxygen.

At higher altitudes, the body has increased red blood cell production and increased ventilation capacity. However, it is advisable to use methods that help this process. - Session plan: Ascents should be gradual, with a slight increase in the area of 900 to 1000 meters a day. Depending on the maximum point and the overall time, rest days depending on the intensity of effort must be included in the sessions. - Gradual elevation gain: This involves sleeping at a lower point than the maximum reached every night.

Days are built into the base plan and the main climbs to maintain acclimatization. It is necessary to record the hours spent resting as currently accepted to maintain acclimatization. - Altitude training: Altitude training developed before the ascent is known to have a positive effect on people who have a rapid acclimatization. By learning about the symptoms and the physiological effects of acute exposure to altitude stress, the risks can be minimized. Furthermore, these methods are used for the general safety of the climbers.

The most important fact is that symptoms appear early when the victims are still conscious, as this may encourage people to be cautious in taking any further risks. Supervised adherence to the recommended plan for ascending helps not only the climber's safety but also in developing their previous accurate mountain experience.

Proper Hydration and Nutrition

Proper hydration can help prevent the onset of Acute Mountain Sickness by reducing the dehydrating effects of high altitude. Before you travel, increase your fluid consumption to 3-4 quarts of liquid per day, especially if you are an athlete. This is in addition to any other fluids you might normally drink, 8 to 12 quarts of water, juice, and other fluids daily while at high altitude. As your altitude increases and your fluid needs also increase, this means that you'll have to drink more than your urine output. Drink regularly and take advantage of every chance to drink more fluids. Carb drinks give your body energy. You should be hydrated enough to maintain a clear-to-light yellow urine volume throughout the day.

Food intake is another focus for maintaining energy levels and overall physical health throughout work and high-altitude play. 10-15% of consumed calories at altitude should come from protein, and 55-65% should come from carbohydrates, which are the most reasonable source of energy for the efforts of both altitude and work. Excessive protein intake increases the body's water needs and may decrease energy levels. Increase your carbohydrate energy level by 500-1,000 kilocalories per day while at altitude, depending on exertion levels. More than 60-25% of your total diet should be from nonfat sources, so remember to celebrate the fats of the altitude dining.

Electrolyte balance can also be an issue at altitude. Not many people like low-sodium, low-sugar food. While not an issue to obsess over, it is a problem that can emerge, particularly if you start a new high-altitude, intense activity plan for those who sweat a lot. You can also avoid the main meal of the evening before the climb but do not leave the area without a hearty breakfast and the right carb energy to help you sustain strong performance from the first step. If you think you're at risk for this or are doing work or playing in very high elevations, consider talking it over with someone who knows the disorders that could occur in the mountains and make sure you're rounding up the carbs high enough.

Medical Interventions

Although the definitive treatment is to retreat to a lower altitude, there are several alternate medical interventions for treating AMS. Supplemental oxygen therapy is primary among those, and it should ameliorate most if not all, symptoms associated with hypobaric hypoxia. Mild carbon dioxide retention is futile during conditions of hypobaric hypoxia and will not have detrimental effects on altitude residents. The provision of supplemental oxygen should be viewed as the definitive treatment of the condition. The sooner reliable oxygen is given, the sooner results are noted. In situations of extreme hypoxia, improvement in cognition and overall feeling of well-being is noted within minutes.

Pharmacological treatments are also part of the medical interventions for AMS. Not all affected persons require treatment. Many may simply need more time for acclimatization. Medications can assist in reducing the time required for acclimatization, or in the case of helium, not requiring acclimatization, immediate movement downward. The most widely used drug to either prevent or treat AMS is a carbonic anhydrase inhibitor, acetazolamide.

A 250 mg dose once or twice per day before ascent is a good idea because of its efficacy, low cost, and many years of experience with the drug for preventing AMS in high-altitude travellers. There is no hard and fast rule for the timing of when to give any of these medications. This can often be determined by the acuity of onset of symptoms, comorbid medical conditions, and prior treatment with these or any other altitude medications. The side effects of medications can greatly mimic the symptoms of AMS, and that must be considered.

For acetazolamide, the side effects are fatigue and paresthesias, which generally require a lower dose or cessation of the medication. Those who are allergic to sulfa-based medications should not receive acetazolamide. The timing of when to administer descent is the same as when to administer high-flow oxygen. It should be done as soon as possible to prevent further ill effects of hypobaric hypoxia. Knowledge of these preventative and first-line treatment methodologies is the most important thing that travellers and the treating doctor must be alike about.

Supplemental Oxygen Therapy

Oxygen therapy is the most important medical treatment for individuals suffering from mild exertional dyspnea to severe high-altitude cerebral oedema. Delivery of supplemental oxygen restores normal respiratory physiology quickly and helps eliminate the symptoms of acute mountain sickness. Breathlessness during mild exercise or at rest (dyspnea) can be relieved in a matter of minutes when supplemental oxygen is breathed by an individual suffering from acute mountain sickness.

Headache, lassitude, dizziness, gastric discomfort, and sleep disturbances improve within a few days if higher percentages of supplemental oxygen are breathed continuously. There are several ways to administer supplemental oxygen to individuals developing symptoms of acute mountain sickness. The most common and practical methods are:

• Portable oxygen tanks that deliver a mixture of oxygen and nitrogen or pure oxygen. These can be rented or purchased from manufacturers in the town closest to the mountains.

• Electrically powered oxygen generators can deliver between 1-6 litres/minute with oxygen concentrations greater than 90%. The devices weigh in at about 20 pounds and are frequently used in various locations, as well as by manufacturers and clinics.

• Everyone, but especially those with confined exercise capacity, should sleep with oxygen.

Supplemental oxygen elevates the amount of oxygen reaching the brain on a per unit red blood cell and per unit haemoglobin molecule basis. Hyperventilating air also elevates the amount of oxygen that reaches the brain, but it must be maintained continuously and frequently to impact headaches, sleep disturbances, and central nervous system function. Supplemental oxygen elevates the amount of oxygen that reaches the brain for the duration of time that it is breathed.

It can be given to an individual by facemask or via nasal cannula. The device used to deliver the oxygen flow rate is set to deliver between 2-4 litres/minute to ensure that the reservoir of air with a greater percentage of oxygen is kept constant and the individual will breathe the desired percentage of oxygen. In various studies at sea level, individuals with altitude headache and dyspnea were given one of three treatment conditions and asked if they felt better: (1) room air, (2) humidified oxygen via a nasal cannula at a flow rate of 2-6 litres/minute, (3) non-humidified oxygen via mask at a flow rate of 6-8 litres/minute.

The only group to feel better was those who breathed humidified oxygen. Oxy-hood and non-humidified mask oxygen should be avoided since it often causes nasal drying and discomfort. Deep breaths of oxygen upon arrival may lead to loss of consciousness and should also be avoided. Individuals who have used supplemental oxygen to decrease acute mountain sickness symptoms have not reported altitude-related problems.

It is not known if oxygen use on a routine basis by healthy individuals to improve acute mountain sickness symptoms would increase morbid occurrences at altitude. In practice, the clinician should consider oxygen as a backup for either the individual who cannot be evacuated or the medications that have not worked.

Pharmacological Treatments

The main pharmacological strategy for preventing and treating acute mountain sickness (AMS) and high-altitude pulmonary oedema (HAPE) is to facilitate acclimatization. Some agents facilitate acclimatization by reversing hypoxic vasodilatation, while others acidify the blood, which stimulates the breathing drive and thus the chemoreceptor drive.

One of the most commonly used agents discussed in the context of high-altitude illness is acetazolamide, a carbonic anhydrase inhibitor that facilitates acclimatization to hypobaric hypoxia by inducing mild metabolic acidosis and increased ventilation. Acetazolamide is the most commonly used prophylactic agent against AMS. The prophylactic oral dosage is 125 to 250 mg every 12 hours in divided doses.

The only definitive treatment for both AMS and high-altitude cerebral oedema (HACE) remains to descend to a lower altitude. Opiates and dexamethasone can be used temporarily until the descent, but they should not be used as an alternative to descending. The time to be used is enough to reduce the symptoms so that descent can be safely performed.

Side effects of acetazolamide treatment include polyuria, paresthesia, decreased libido, depression, and vomiting. These signals indicate that people with chronic obstructive pulmonary disease should not take it, especially those with hypercapnia. Acetazolamide should be introduced before hypoxic exposure at an elevation or continued for 24 hours before significant exposure to high altitudes with rapidly increased sleeping elevation. Initial studies of acetazolamide at an altitude of 3,459 m are mixed, but a recent study found that 2–3 mg of acetazolamide in response to well-tolerated early oxygen triggers helped provide modest benefit.

Emergency Management

Emergency Treatment. Mild acute mountain sickness usually requires nothing more than rest and perhaps some mild medication for headaches. Relief usually follows a good night's sleep. If symptoms persist or become worse, the person should descend to a lower altitude as soon as possible. In the meantime, oxygen administered by a mask or nasal cannula at a flow rate of 2-4 Lt/min will usually relieve the symptoms.

Should a mask or cannula not be available, oxygen may be made available by use of an "oxygen tent." At the caprine level, which frequently peaks at 36-48 hours after arrival, the person with mild acute mountain sickness must be watched closely in case of deterioration. Nutritious food and plenty of water are to be encouraged. Sleep is soundest in the reclining position, which may have to be enforced so that adequate hours of sleep may be had.

Severe Acute Mountain Sickness with Cerebral and/or Pulmonary Edema. The immediate emergency in the case of severe acute mountain sickness is employing the one sure cure: lower altitude. At this juncture, oxygen may provide good comfort and sometimes just enough of an advantage so that rapid descent is possible. There is no time for oxygen to provide a safe margin, and questionable cases must descend regardless of the presence of oxygen.

The sooner the descent, the more sure and the safer will be the recovery regardless of the symptoms, be they severe headache, vomiting, loss of coordination, drowsiness, or unconsciousness. If at all possible, rapid descent should be continued down to at least 4,000 feet in the case of cerebral oedema, and even 2,500 feet in the case of pulmonary oedema. If descent is impossible, an attempt should be made to decrease the cerebrospinal fluid pressure by lumbar puncture.

Doses of prescribed medication can provide relief to the suffering patient and aid in descent. Before any of these alternatives are chosen, medical advice should be obtained if at all possible. During the descent, the expertise of a qualified physician is to be sought.

Recognizing Severe Symptoms

You must recognize severe symptoms of AMS, which may lead to one of the more serious high-altitude illnesses, such as high-altitude cerebral oedema or high-altitude pulmonary oedema. If anyone is below you and climbing, get them out of danger as quickly as possible. If you have your pulse oximeter, use it to monitor for dangerous levels of hypoxemia.

Cheyne-Stokes respiration is not as dangerous at altitude as it is at sea level, but it is a sign of hypoxemia and needs to be watched carefully. Severe symptoms of acute mountain sickness include shortness of breath at rest (whether AMS is suspected or not), an altered mental status, and problems with gait or balance. There are other signs of impending HACE or HAPE, which are known mostly to the literate mountaineering community, and your friends there can help to keep an eye out for them.

During your climb, eat, drink, urinate, and move as close to normal as possible. If rapid descent is not an option and a Gamow bag is unavailable, call for a rescue and position the victim for as much physical rest and comfort as possible. Urgent: Even if people in your party seem to feel their symptoms of AMS are minor or can be dealt with, keep pushing them for more information.

High altitude illnesses can creep up on victims over time, and other complications of altitude can masquerade as AMS. Upon arrival at your final camp, make sure that everyone is drinking, eating, and urinating, and refer anyone with severe symptoms of AMS to higher medical care for evaluation before further ascent.

Evacuation Protocols

Evacuation should always be considered in the event of marked or unrelenting symptoms or in the event of symptoms of high-altitude cerebral oedema or high-altitude pulmonary oedema. Additionally, there are very few contraindications to evacuating someone with acute mountain sickness due to the life-threatening potential of high-altitude cerebral oedema or high-altitude pulmonary oedema; if left untreated, these conditions will be exacerbated by longer exposure to hypoxia.

The only true contraindications to evacuating a person with acute mountain sickness are those that preclude safe evacuation, such as severe weather or daytime temperatures too cold for safe overland travel.

Ideally, he or she should be evacuated to an area of lower altitude to receive more definitive care. In the event of severe or unrelenting symptoms, high-altitude cerebral oedema, high-altitude pulmonary oedema, or one of the very few true contraindications to safe evacuation, immediate descent should be initiated anyway.

Since descending 500 meters achieves the goal of initial and definitive treatment, it is beneficial if the person can be evacuated to an altitude at least this low. Symptoms will generally begin to improve within hours of descent. If significantly lower altitudes cannot be reached, interim measures must be taken to treat symptoms as much as possible until lower altitudes can be used for definitive treatment.

Conclusion

Acute mountain sickness (AMS) is a well-established problem that affects many unacclimatized individuals ascending to high altitude. However, little is known about when people who have AMS seek treatment; who provides treatment for people experiencing AMS; and whether these treatments are sufficient to prevent the progression of the disease without significant side effects.

To learn more on these topics, download our e-book  Summit Survival: Medical Essentials for High Altitude Adventures. With the knowledge and understanding of the potential health risks at altitude, we can control them by taking preventive measures. Find the download link HERE.