Management of Altitude Illnesses

Ken Zafren

INTRODUCTION

Acute high altitude illnesses range from acute mountain sickness (AMS), which is merely unpleasant, to high altitude cerebral edema (HACE) and high altitude pulmonary edema (HAPE), which can be fatal. Acute high altitude illnesses can be prevented, primarily by gradual ascent and can be treated, usually by rapid descent. Patients who survive acute altitude illness generally make a complete clinical recovery. Wilderness EMS (WEMS) providers who are involved in operations at high altitude need to know how to prevent and treat high altitude illness for their own safety as well as for the benefit of their patients.

People living in areas at or near sea level, who travel to areas above 2,400 m (7,874 ft), are at risk of acute high altitude illness. Tens of millions of people visit high altitude areas worldwide every year. There are an estimated 40 million visitors annually to areas above 2,400 m (7,874 ft) in the western United States alone.¹ Many subjects of high altitude illness are visitors to wilderness areas or other remote areas with little or no medical infrastructure. There are millions of cases of AMS and an unknown number of fatal cases of HACE and HAPE every year.

Definitions

Acclimatization

Acclimatization is the process of physiologic changes by which people adapt to high altitude. A person who lives at sea level taken suddenly to the altitude of the summit of Mt. Everest (8,848 m or 29,029 ft) would become unconscious in a few minutes. The same person, acclimatizing for a few weeks on an expedition to Mt. Everest, might be able to climb to the summit breathing ambient air. Altitude illness is caused by decreased availability of oxygen due to low atmospheric pressure (hypobaric hypoxia). The percentage of oxygen in air is 21% at all altitudes, but atmospheric pressure decreases as altitude increases, causing hypoxia. Acclimatization enables people to live and work at high altitude without suffering from high altitude illness.

High Altitude—1,500 to 3,500 m (4,921 to 11,483 ft)

The physiologic effects of low atmospheric partial pressure of oxygen become evident above about 1,500 m (4,921 ft). These effects include decreased exercise performance and increased resting ventilation. Oxygen saturation (SaO₂) at rest remains above 90% until about 3,500 m (11,483 ft), although the arterial partial pressure of oxygen (PaO₂) is markedly lower than at sea level. The abbreviation SaO₂ refers to oxygen saturation measured by co-oximetry (blood gas analysis), while SpO₂ refers to oxygen saturation measured by pulse oximetry. SpO₂ readings are usually lower than measured SaO₂ when SaO₂ is below 85% to 90%. “High Altitude” is the most common range for acute altitude illness, because so many people ascend rapidly to altitudes between 2,500 m (8,202 ft) and 3,500 m (11,483 ft).

Very High Altitude—3,500 to 5,500 m (11,483 to 18,045 ft)

Above 3,500 m (11,483 ft), resting SaO₂ is less than 90% and resting PaO₂ is below 50 mm Hg. Severe hypoxemia may occur during exercise, during sleep, or with HAPE. The risk and severity of altitude illness tend to increase with increasing altitude. “Very high altitude” is the most common range for severe altitude illness, because relatively few people ascend to altitudes above 5,500 m (18,045 ft).

Extreme Altitude—above 5,500 m (18,045 ft)

“Extreme altitude” is associated with severe hypocapnemia, hypocapnia, and alkalemia at rest as well as with exercise. Rapid ascent without a period of acclimatization or use of supplemental
oxygen carries a very high risk of severe altitude illness. At extreme altitude, progressive deterioration of physiologic function eventually overwhelms acclimatization,²^,³ causing weakness, lethargy, weight loss, sleep disturbances, and severe polycythemia. For this reason, there is no permanent human habitation above 5,500 m (18,045 ft). The highest permanent settlement in the world is at Aconquilcha, Chile, at 5,340 m (17,520 ft).

Scope of Discussion

The emphasis in this chapter is prevention, recognition, and treatment of acute high altitude illnesses, primarily AMS, HAPE, and HACE in wilderness and in other remote settings where there are limited medical resources. Other high altitude conditions include high altitude headaches, high altitude syncope, neurologic conditions, retinopathies, visual problems, high altitude pharyngitis and bronchitis, high altitude peripheral edema, high altitude flatus expulsion (HAFE), disturbed sleep, and periodic breathing.

WEMS responders who are based at altitudes lower than that of the rescue scene are themselves at risk of high altitude illnesses, especially if they are able to reach the scene but are unable to descend immediately. WEMS responders also run risks due to limitations of human and machine performance at high altitudes.

EPIDEMIOLOGY

AMS and HACE affect the brain, while HAPE affects the lungs. AMS or HACE may occur alone or in combination with HAPE. HAPE may also occur alone or in combination with AMS or HACE. HACE and HAPE frequently occur together.

AMS is rare below 2,400 m (7,874 ft). HACE is rare below 3,000 m (9,843 ft) and, in most settings, is uncommon, even above 3,000 m. HACE most frequently occurs in very high mountains, such as the Himalayas. HAPE is more common than HACE in most settings, especially in areas below 4,000 m (13,123 ft). The incidence of HAPE is increased by colder conditions.⁴ HAPE is common above 3,000 m (9,843 ft), but may be seen well below 2,400 m (7,874 ft) in occasional patients with congenital absence of a pulmonary artery.⁵^,⁶

In most high altitude areas, the incidence of high altitude illness can only be roughly estimated. This is especially true in wilderness and remote areas where the number of people at risk is often unknown. Many travelers who suffer from altitude illness, especially AMS, do not seek medical attention. Medical care is often unavailable in wilderness and remote areas. Visitors to high altitude may blame the symptoms of acute altitude illness on other causes.

Risk is increased primarily with rapid ascent. The higher the altitude, the higher are the risks of both HACE and HAPE. At a given location, AMS and HACE are more common during periods of low barometric pressure, which causes a higher effective altitude.⁴ HAPE is more common during periods of cold weather.⁴ HAPE also seems to be more common in travelers such as trekkers and climbers who are exerting themselves and seems to be less common in those who reach high altitude by passive means and who do not participate in strenuous activities once they arrive.

In the western United States, where tens of millions of visitors from lowland locations sleep above 2,400 m (7,874 ft), the estimated incidence of AMS is about 22% for those sleeping at 2,500 m (8,202 ft) and 17% to 42% for those sleeping above 3,000 m (9,843 ft).⁷ The overall incidence of HAPE or HACE is about 0.01%. Trekkers in the Khumbu (Mt. Everest) region of Nepal typically sleep at altitudes from 3,400 m (11,155 ft) to 5,100 m (16,732 ft) and reach altitudes of 5,400 to 5,700 m (17,717 to 18,701 ft). In a classic study, almost half of the trekkers who flew to 2,800 m (9,186 ft) and ascended in several days to 4,300 m (14,108 ft) developed AMS, compared to about a quarter of those who spent an extra week or so walking from the lowlands.⁸ The incidence of HACE or HAPE was 1% to 2% of those who flew to 2,800 m (9,186 ft) and about 0.05% of those who walked from the lowlands.

Climbers and hikers attempting summits of high mountains are at risk of altitude illness. On Mt. Rainier (4,392 m or 14,410 ft), in the Cascade Mountains of Washington State, most climbers live near sea level and sleep one night at about 3,048 m (10,000 ft) on the way to the summit. The incidence of AMS is estimated to be about 67%, with a negligible incidence of HACE and HAPE.⁹ On Mt. Kilimanjaro (5,895 m or 19,341 ft), hikers typically sleep at altitudes ranging from 2,700 to 4,700 m (8,858 to 15,420 ft), taking 2 to 6 days to reach the summit. The incidence of AMS is estimated to be 50% to 83%.¹⁰^,¹¹ Although the incidence of HACE and HAPE is not known, deaths from altitude illness are common. On Denali in Alaska, climbers typically take 3 days to a week or more to reach the summit (6,190 m or 20,310 ft) after reaching 3,000 m (9,843 ft), sleeping at 3,000 to 5,300 m (17,388 ft). The incidence of AMS is about 30%.¹² The incidence of HACE and HAPE is about 2% to 3%.

The highest incidences of AMS are found in situations with very rapid ascents, especially in travelers who fly to places like Lhasa, Tibet (3,656 m or 11,995 ft), and La Paz, Bolivia, where the airport is at about 4,000 m (13,123 ft) and most travelers sleep at about 3,500 m (11,483 ft). These are neither wilderness nor remote locations and are low enough that HACE and HAPE are rare. The combined incidence of HACE and HAPE is below 1% in most settings, but the incidence of HACE has been reported in an amazing 31% of pilgrims ascending to Gosainkund Lake (4,380 m or 14,370 ft) in Nepal.¹³ Most of the pilgrims
live at about 1,340 m (4,400 ft). They ascend to the lake in 1 to 2 days. The incidence of HAPE was estimated to be about 5%.

CLINICAL MANAGEMENT

In contrast to the management of many types of illnesses and injuries, management of high altitude illnesses by WEMS providers is relatively straightforward and seldom requires sophisticated equipment for diagnosis or treatment.

Identification

Acute Mountain Sickness

CLINICAL FEATURES

AMS is at the mild end of the spectrum of high altitude illness that affects the brain. The severe end of the spectrum is HACE. AMS is a syndrome of cerebral symptoms that can occur after recent ascent to high altitude. For practical purposes, as well as for research, AMS is defined by the Lake Louise Score (LLS) (Table 15.1) as the presence of a headache and at least one other symptom on a specific list of brain-mediated symptoms. The LLS symptoms are gastrointestinal disturbance (anorexia, nausea, and vomiting), “fatigue and/or weakness,” dizziness/lightheadedness, and poor sleep.¹⁴ Poor sleep has long been recognized as being normal at high altitude rather than a symptom of AMS.¹⁵^,¹⁶ The symptom of “poor sleep” is in the process of being eliminated from the LLS. This will likely occur in 2017.

AMS usually starts 6 to 12 hours after arrival at a new, higher altitude,¹⁷ but may begin as early as 2 hours and as late as 4 days.¹⁸ Symptoms may range from mild to severe. The headache is classically described as throbbing, bitemporal, or occipital. It tends to be worse at night and even worse on awakening in the morning. Bending over or Valsalva maneuvers worsen the headache. Gastrointestinal symptoms can include poor appetite alone, nausea, or vomiting. “Fatigue and/or weakness” is a subjective term that, if present, may be reported as mild, moderate, or severe. “Dizziness/lightheadedness” is another term that has purposely been left vague to allow for individual and cultural variation. The term dizziness can include vertigo.

Respiratory symptoms are common at high altitude, even in the absence of AMS and are not related to AMS. Dyspnea on exertion is normal after arrival at altitude. Dyspnea at rest may be a symptom of HAPE.

The presence of abnormal physical findings is not a feature of AMS. Resting heart rate generally rises with ascent to altitude. In AMS, the resting heart rate is usually normal or slightly high, but may also be low. Blood pressure usually rises with acute ascent to altitude even in the absence of AMS. Low blood pressure would be concerning for conditions other than AMS. Fever suggests HAPE or an infectious process. Peripheral edema is a common finding after ascent to altitude. Although AMS is associated with decreased urine output, peripheral edema is usually not a sign of AMS. Resolution of AMS is often associated with markedly increased diuresis.

Table 15.1 The Lake Louise Acute Mountain Sickness Scoring System (LLS)

AMS SELF-REPORT SCORE
Headache
No headache	0
Mild headache	1
Moderate headache	2
Severe headache, incapacitating	3
Gastrointestinal Symptoms
None	0
Poor appetite or nausea	1
Moderate nausea or vomiting	2
Severe nausea and vomiting	3
Fatigue and/or Weakness
Not tired or weak	0
Mild fatigue/weakness	1
Moderate fatigue/weakness	2
Severe fatigue/weakness	3
Dizziness/Lightheadedness
Not dizzy	0
Mild dizziness	1
Moderate dizziness	2
Severe dizziness, incapacitating	3
Difficulty Sleeping
Slept as well as usual	0
Did not sleep as well as usual	1
Woke many times, poor sleep	2
Could not sleep at all	3
A diagnosis of acute mountain sickness (AMS) is based on a recent gain in altitude, at least several hours at the new altitude and the presence of a headache and at least one of the following symptoms: gastrointestinal upset (anorexia, nausea, or vomiting), fatigue or weakness, dizziness or lightheadedness, and difficulty sleeping. Add together the individual scores for each symptom to get the total score. A score of 3 points or greater on the AMS self-report questionnaire alone, or in combination with the Clinical Assessment score, constitutes AMS. Note: Symptoms of “Difficulty Sleeping” will be eliminated from the LLS in the future. Adapted from Roach R, P B, Oelz O, et al. The Lake Louise acute mountain sickness scoring system. In: Wood S, Roach R, editors. Hypoxia and Molecular Medicine. Burlington, VT: Queen City Printers; 1993. p. 272-4.

SpO₂^* decreases with altitude. In AMS, SpO₂ is usually normal for the altitude although it may be slightly low. SpO₂ cannot be used to diagnose AMS or predict whether AMS will occur with further ascent, in spite of several studies that assert its utility for these purposes.¹⁹

DIFFERENTIAL DIAGNOSIS

AMS is a nonspecific syndrome. Many other conditions can mimic AMS, but the diagnosis is usually clear in the setting of recent ascent to an altitude above 2,500 m (8,202 ft). Fever and
muscle aches suggest a viral syndrome. A hangover can cause symptoms of AMS and may be found in association with AMS. The diagnosis of hangover in the morning is based on a history of drinking alcohol the night before. Exhaustion may mimic AMS. Volume depletion can also cause symptoms consistent with AMS, but, unlike AMS, dehydration is rapidly relieved by drinking fluids. Carbon monoxide (CO) poisoning can closely mimic AMS. CO poisoning should be considered if multiple patients are affected in the presence of combustion in a closed space such as a tent or snow cave. Cooking stoves are the most likely source of CO in the wilderness.

High Altitude Cerebral Edema

CLINICAL FEATURES

HACE is the severe end of the spectrum of high altitude illness that affects the brain. It is an encephalopathy characterized by ataxia or altered mental status.²⁰^,²¹ Although it is the severe form of AMS, HACE may occur without antecedent symptoms of AMS. When HACE occurs without preceding AMS, it is often rapidly progressive. Ataxia is tested by having a patient walk heel-to-toe (tandem gait). Inability to walk heeltotoe in a straight line on level ground or any unsteadiness is abnormal. The initial symptom of HACE may be inability to keep up with a group. Another early symptom may be extreme lassitude. Focal neurologic signs are said to be possible, but they are rare.²⁰^,²² The presence of focal neurologic signs suggests a diagnosis other than HACE. HACE often presents with extreme lassitude and drowsiness that may progress to stupor and coma. When HACE is associated with HAPE, the diagnosis of HACE may be missed. Untreated HACE is fatal. Coma usually develops in 1 to 3 days, but may occur in less than 12 hours to as long as 9 days.²⁰^,²² Comatose patients with HACE have 60% mortality.²³

DIFFERENTIAL DIAGNOSIS

Usually HACE occurs as a progression of AMS. In such cases, the diagnosis is usually clear. When HACE occurs without a preceding headache or other symptoms of AMS, it can be difficult to make the diagnosis. Encephalopathy due to any cause can mimic HACE. Fever is usually absent in HACE, unless HAPE is also present. Heat stroke or hypothermia can cause altered mental status or ataxia, but touching the skin should be sufficient to make a diagnosis of heat illness or hypothermia. The effects of sedative/hypnotics, including alcohol, can mimic HACE. A clinical history is usually sufficient to make the distinction. Neurologic causes of altered mental status that can occur at high altitude include subarachnoid hemorrhage,²⁴ cerebral venous thrombosis, mass lesion,²⁵ migraine, or delirium.²⁶ A patient with altered mental status with or without headache, in the absence of other causes, should be considered to have HACE until proven otherwise.

If an apparent case of HACE does not resolve with descent, further investigation is necessary. In severe cases of HACE, ataxia may persist for months after descent. In such cases the ataxia may be so pronounced that the patient seems to have muscle weakness. A case of sagittal sinus thrombosis causing a headache that persisted after descent has been reported.²⁷ The patient was misdiagnosed as having HACE.

High Altitude Pulmonary Edema

CLINICAL FEATURES

HAPE is a noncardiogenic form of pulmonary edema. It is the most common severe high altitude illness and the most common cause of death from high altitude illness.²⁸ HAPE begins with fatigue, weakness, shortness of breath on exertion, and a nonproductive cough. HAPE can occur in the absence of AMS, but AMS or HACE are commonly found in association with HAPE. Coexisting severe HACE with altered mental status or coma may complicate the diagnosis. The onset of HAPE is usually described as sudden, after the second night at altitude, but HAPE may occur earlier or later.²⁹^,³⁰ It is not clear if the onset is truly sudden or if the apparent abruptness is due to worsening of HAPE while the patient is sleeping followed by a sudden awareness of dyspnea on awakening.

HAPE causes shortness of breath at rest and also when lying flat (orthopnea). Vital signs are abnormal, with tachycardia, tachypnea and a low-grade fever.³¹^,³² The earliest physical exam finding is usually crackles in the right middle lobe. These are best heard in the right axilla.¹ As HAPE progresses, crackles may occur throughout both lungs. HAPE may also occur with clear lungs. A cough with frothy pink sputum is a late and ominous sign. SpO₂ in HAPE is usually much lower than expected at a given altitude. HAPE can be diagnosed solely on clinical presentation, but pulse oximetry can be helpful to confirm the diagnosis. A pulse oximeter will markedly underestimate the SpO₂ at lower saturations. SpO₂ readings, even in the 50s and 60s, should not unduly alarm a WEMS provider, because the true value is much higher. WEMS providers should use the clinical appearance of a patient, rather than a number, to guide treatment.

Risk factors for HAPE include cold exposure and physical exertion.³³^,³⁴^,³⁵^,³⁶ These factors are thought to increase the risk of HAPE by increasing pulmonary artery pressure. Males are more likely to develop HAPE than females.³⁷ This may be due to increased exertion after ascent to high altitude. Children who live at high altitude are at risk of developing HAPE when they return home after a stay of at least 10 to 14 days at low altitude.³⁰^,³⁸^,³⁹ This has been called “reascent” pulmonary edema. The etiology is unclear and may differ from that of HAPE in travelers to high altitude.

Pulmonary hypertension is a risk factor for HAPE. Patients have been found to develop HAPE below 2,000 m (6,562 ft)
due to congenital absence of one pulmonary artery.⁵^,⁶ These patients were asymptomatic near sea level.

A person who has had HAPE is at risk of recurrence with further ascents to altitude. Most of the research into the prevention and treatment of HAPE has been carried out on subjects who have had HAPE multiple times after rapid ascent.⁴⁰^,⁴¹ These individuals are termed HAPE-susceptible (HAPE-S). Research involving HAPE-S subjects may not be applicable to the majority of presumably non-HAPE-S individuals.

DIFFERENTIAL DIAGNOSIS

Before HAPE was described, most cases were diagnosed as pneumonia.⁴² Although fever and cough are present in both HAPE and pneumonia, the fever in HAPE is usually low grade and the cough does not produce purulent sputum. Like HAPE, pulmonary embolism (PE) can cause shortness of breath, tachycardia, tachypnea, and low-grade fever. Both HAPE and PE benefit from descent and oxygen, but only HAPE resolves quickly with treatment. Congestive heart failure (CHF) can cause cardiogenic pulmonary edema, which may be mistaken for HAPE. In CHF, unlike in HAPE, there may be jugular venous distension and crackles confined to the lower lung fields.

Conditions at High Altitude Other Than AMS, HACE, and HAPE

HIGH ALTITUDE HEADACHE

Headache is usually the first symptom of AMS, but a headache may occur at high altitude without the later development of AMS. Any type of headache, including migraine, can occur at high altitude. Treatment of a headache at high altitude does not differ from treatment at low altitude except that supplemental low-flow oxygen (typically 2 L/min) often provides complete relief in 10 to 15 minutes.⁴³ There is no accepted definition of high altitude headache. For practical purposes, high altitude headache could be defined as a headache occurring without other symptoms of AMS, or with symptoms of AMS that do not reach a total of 3 points on the LLS, that is relieved by supplemental oxygen.

NEUROLOGIC CONDITIONS OTHER THAN AMS OR HACE

Any neurologic condition, including stroke, can occur at high altitude.⁴⁴^,⁴⁵ Thrombotic strokes have been reported in climbers⁴⁶ and soldiers⁴⁷ at extreme altitude. Other neurologic syndromes that have been described at high altitude include isolated cranial nerve palsies,⁴⁸ transient monocular blindness,⁴⁹ and cortical blindness.⁵⁰ Cerebral sinus thrombosis can cause neurologic signs such as ataxia and speech difficulties.⁵¹ Transient global amnesia has also been reported at high altitude,⁵² although it may be a psychiatric rather than a neurologic condition.

Many apparent transient ischemic attacks (TIAs) have been reported at altitudes above 5,500 m (18,045 ft), usually in young healthy climbers. Several participants in a simulated “climb of Mt. Everest” in a high-altitude chamber also experienced apparent TIAs.⁵³ These episodes have resolved completely without descent. It is possible that some solo climbers have fallen to their deaths on difficult terrain due to hemiplegia. The etiology of apparent TIAs at altitude is unclear, but may represent vascular spasm or atypical migraines. Subsequent magnetic resonance images of the brain have been normal.

HIGH ALTITUDE SYNCOPE

Syncope can occur at high altitude. Near syncope due to acute hypoxemia is frequent after rapid transport to altitudes above 4,000 m (13,123 ft). A benign condition, “high altitude syncope,” is common in the first 2 to 3 days at moderate altitude,⁵⁴ especially at night and after large meals and alcohol consumption. Syncope at high altitude may be related to vasomotor instability and cerebral hypoperfusion due to hypocapnemia.⁵⁵ It is not clear whether “high altitude syncope” differs from syncope at low altitude.

HIGH ALTITUDE RETINOPATHY

The most common form of retinal pathology (retinopathy) at high altitude is high altitude retinal hemorrhage (HARH).⁵⁶^,⁵⁷ HARH is common above 5,000 m (16,404 ft). Above 5,000 m HARH is not related to AMS,⁵⁸ although retinal hemorrhage may be correlated with AMS at 4,300 m (14,108 ft). The location of the hemorrhage seems to be random, but blindness may result if the hemorrhage occurs over the macula. Otherwise, HARH does not cause any symptoms. Additional hemorrhages may occur with further ascent,⁵⁹ but resolve with descent. In the case of HARH over the macula, blindness usually resolves with descent, but a blind spot may persist for years or may be permanent.⁶⁰

HIGH ALTITUDE VISUAL PROBLEMS

Drying of the eye is the most common cause of blurred vision at high altitude.⁶¹ Drying is caused by dry air exacerbated by wind. Blurred vision is often unilateral, especially in windy conditions. Blurring resolves when the affected person gets out of the wind. Saline eye drops may also help. Blurred vision in one eye may also be the result of a person touching a transdermal scopolamine patch and then touching the eye. A unilateral dilated pupil without neurologic signs is the usual presentation. Transient monocular blindness can also occur at high altitude.⁴⁴

Corneal surgery to improve refraction can cause visual problems at high altitude.⁶¹ Persons who have had radial keratotomy can undergo severe changes in refraction at high altitude that may be disabling. Photorefractive keratotomy does not lead to refractive changes at altitude. Laser in situ keratomileusis (LASIK) can cause refractive changes above 5,500 m (18,045 ft), but some
climbers have ascended above 8,000 m (26,247 ft) without problems after LASIK.

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