Acute Mountain Sickness and High-Altitude Cerebral Edema
Definitions
- Acute mountain sickness (AMS): neurologic syndrome with no physical findings occurring > 6 hours after ascent to altitudes > 2,500 m (rarely, 1,500 m)
- High-altitude cerebral edema (HACE): diffuse encephalopathy without focal neurological deficits, always preceded by AMS
Epidemiology
- Because AMS becomes HACE, the epidemiological factors are the same.
- Prevalence: 40%–90% based on altitude
- Risk factors:
- Prior history of high-altitude sickness
- Rate of ascent
- Exertion (lack of physical fitness is not a risk factor)
- Neck irradiation or surgery damaging carotid bodies
- Age > 50 years reduces risk.
Etiology
All symptomatology of AMS and HACE is related to hypoxia. The central nervous system (CNS) is the most sensitive organ to hypoxia.
Clinical presentation
- AMS:
- Symptoms:
- Headache (most common)
- Nausea
- Fatigue
- Dizziness
- Insomnia
- Physical exam is usually unremarkable.
- Symptoms:
- HACE:
- Symptoms and signs:
- Ataxia
- Altered level of consciousness
- Physical exam findings:
- Papilledema
- ± retinal hemorrhage
- No focal neurologic findings
- Symptoms and signs:
Pathophysiology
- Exact mechanism is not well understood.
- AMS:
- High altitude → decreased partial pressure of oxygen in inspired gas (hypobaric hypoxia)
- Hypobaric hypoxia → low arterial oxygen saturation (SPA02 is > 90%)
- Low SPA02 → increased cerebral blood flow:
- Raised intracranial pressure
- Increased sympathetic activity
- Hypoventilation
- Fluid retention
- Further impaired gas exchange
- HACE:
- As above, but swelling progresses, leading to vasogenic cerebral edema
- Swelling puts pressure on the brain, resulting in:
- Papilledema
- Diffuse neurological symptoms
Significant morphometric edge flow indicating brain volume swelling during high-altitude exposure (Test 2A) and 2 months after return to sea level (Test 3B) compared with baseline before ascent to high altitude (Test 1)
Image: “Reversible Brain Abnormalities in People Without Signs of Mountain Sickness During High-Altitude Exposure” by Cunxiu Fan et al. License: CC BY 4.0Diagnosis
Both AMS and HACE are diagnosed clinically by noting the characteristic symptoms in a patient who ascends to high altitude.
- Pulse oximetry, vital signs, and laboratory exams are not useful markers as they are often normal.
- Clinical trials often use the 2018 Lake Louise AMS scoring system for diagnosis:
- Score of ≥ 3 including headache is considered diagnostic of AMS.
- Not routinely used for diagnosis in clinics, but scoring systems such as the Lake Louise AMS may be useful for screening purposes
Headache |
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Gastrointestinal (GI) symptoms |
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Fatigue/weakness |
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Dizziness/lightheadedness |
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AMS clinical functional score: Overall, if you had AMS symptoms, how did they affect your activities? |
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Prevention
- Gradual ascent is best.
- Spending 1 night at intermediate altitude before ascending to higher ones
- Proper hydration, but not overhydration
- Acetazolamide for patients with prior history of AMS or when rapid descent necessary (e.g., rescue missions)
Management
- AMS:
- Mild:
- Discontinue ascent.
- Rest.
- Carbonic anhydrase inhibitors (acetazolamide)
- Ultimate treatment is descent.
- Moderate:
- Immediate descent
- Supplemental oxygen
- Acetazolamide and/or dexamethasone
- Hyperbaric therapy
- Mild:
- HACE:
- Immediate descent
- Portable hyperbaric chamber (when descent not possible)
- Supplemental oxygen
- Dexamethasone
High-Altitude Pulmonary Edema
Definition
High-altitude pulmonary edema (HAPE) is a non-cardiogenic pulmonary edema (normal capillary wedge pressure) occurring 2–4 days after arrival at high altitude.
Epidemiology
- Incidence ranges from 0.01%–15.5% based on altitude.
- Risk factors:
- Prior history of HAPE
- Rapid ascent
- Respiratory tract infection
- Cold temperatures
- Exercise
- Pre-existing cardiopulmonary abnormalities leading to pulmonary hypertension
Pathophysiology
- Hypobaric hypoxia from high altitude → decreased nitric oxide and increased endothelin 1 in pulmonary vasculature causing vasoconstriction
- Patchy pulmonary vasoconstriction → overperfusion and increased capillary pressure
- Increased pulmonary capillary pressure → interstitial fluid leak → HAPE
Clinical presentation
- Symptoms:
- Cough (most common, but nonspecific)
- Hemoptysis
- Fatigue
- Dyspnea at rest
- Signs:
- Tachypnea and tachycardia, especially at rest (important markers)
- Crackles on lung auscultation
- Signs of HACE
Diagnosis
- Diagnosis is primarily clinical; requires:
- 2 or more of the following symptoms:
- Chest tightness or pain
- Cough
- Dyspnea at rest
- Decreased exercise tolerance
- 2 or more of the following exam findings:
- Central cyanosis
- Rales/wheezes
- Tachycardia
- Tachypnea
- 2 or more of the following symptoms:
- Imaging and lab testing support the diagnosis:
- Chest X-ray: patchy/localized opacities and/or streaky interstitial edema
- Electrocardiogram (ECG): right ventricular strain and/or hypertrophy
- Laboratory tests: hypoxia, respiratory alkalosis (unless on acetazolamide)
- Ultrasound: findings consistent with pulmonary edema
Diagram of pulmonary edema: In HAPE, the alveoli fill up with fluid, interrupting proper gas exchange.
Image by Lecturio.High altitude pulmonary edema (HAPE) in a Himalayan trekker: Initial Chest X-ray showing pulmonary infiltrates in the right lung especially in the right mid and lower lung zones indicative of pulmonary edema
Image: “Initial Chest x-ray” by Nepal International Clinic, Travel and Mountain Medicine, Kathmandu, Nepal. License: CC BY 2.0
Prevention
- Gradual ascent is best.
- Patients with prior history of HAPE or when rapid descent necessary (e.g., rescue missions):
- Sustained-release nifedipine
- Salmeterol
- Tadalafil
- Dexamethasone
Management
- Immediate descent
- Portable hyperbaric chamber (when descent not possible)
- Rest and warm temperatures
- Oxygen therapy to > 90% saturation
- Extended-release nifedipine
References
- Hackett, P. H., & Davis, C. B. (2016). High-altitude disorders. In J. E. Tintinalli, J. S. Stapczynski, O. J. Ma, D. M. Yealy, G. D. Meckler, & D. M. Cline (Eds.), Tintinalli’s emergency medicine: A comprehensive study guide, (8e). New York, NY: McGraw-Hill Education. accessmedicine.mhmedical.com/content.aspx?aid=1121514038
- Roach, R. C. et al. (2018). The 2018 Lake Louise acute mountain sickness score. High Altitude Medicine & Biology, 19(1), 4–6. https://doi.org/10.1089/ham.2017.0164
- Luks, A. M., Swenson, E. R., & Bärtsch, P. (2017). Acute high-altitude sickness. European Respiratory Review, 26(143). https://doi.org/10.1183/16000617.0096-2016