Acute Mountain Sickness

The headache felt like my skull was splitting in half.

It was summit day on Kilimanjaro, and as I slowly climbed toward the peak, I found myself wondering why I was even there. Kilimanjaro, the highest mountain in Africa at a whopping 5,895 meters (19,341 ft), is also the world's largest freestanding mountain. Here, the guides and climbers are keenly aware of the risks associated with high altitude, constantly monitoring for early signs of altitude sickness. I was fortunate—my headache resolved as we started our descent. But it was a powerful reminder of the importance of recognizing altitude-related conditions early before they escalate into serious life-threatening emergencies. 

Bringing us back to the real world: “You can't diagnose it if you don’t think about it.”  As emergency medicine providers, we’re trained to consider various causes of life-threatening, severe conditions. But one group of conditions that often slip through the cracks is high-altitude illnesses—conditions we might associate more with mountaineering expeditions than with daily practice. We must ensure we are familiar with the many medical conditions that may need to be higher on our differential diagnosis. 

Some conditions that are not commonly thought of as a significant cause of symptoms are high altitude-related illnesses, as these conditions are only thought of as occurring during mountaineering expeditions that we read about or see in films. 

However, in the United States alone, there are many popular ski destinations and hiking regions known for their beautiful landscapes and outdoor activities, which are all situated at elevations where the risk of developing acute mountain sickness (AMS) is significant.  This is especially true for individuals who ascend rapidly or are not adequately acclimatized. This blog will dive into the pathophysiology, risk factors, and detailed diagnostic criteria for AMS and its more severe forms—High-Altitude Cerebral Edema (HACE) and High-Altitude Pulmonary Edema (HAPE)—providing the tools necessary for prompt recognition and intervention.

Pathophysiology and Acclimatization

High-altitude illnesses primarily result from hypobaric hypoxia, where low atmospheric pressure at high elevations leads to decreased oxygen availability. This hypoxia triggers a cascade of physiological responses aimed at improving oxygen delivery but can also result in pathologic states if the body’s compensatory mechanisms are overwhelmed.

Partial Pressure and Oxygen Availability

Although the oxygen percentage in the air remains constant (21%) at higher altitudes, the atmospheric pressure decreases, reducing the partial pressure of inspired oxygen (PiO2). For instance, at the summit of Mount Everest (8,848 meters/29,029 feet), PiO2 is only about 28% of what it is at sea level. This drop in barometric pressure (PB) is the fundamental cause of high-altitude illnesses.

Acclimatization

Human physiology adapts remarkably to hypoxia given sufficient time. Acclimatization involves immediate (within minutes) and long-term (over weeks to months) physiological changes, including increased minute ventilation, enhanced renal bicarbonate excretion, and hematopoietic responses like increased hemoglobin concentration and red blood cell production.

However, rapid ascent without allowing for acclimatization overwhelms these mechanisms, leading to AMS, HACE, or HAPE.

Risk Factors for Acute Mountain Sickness:

• Rapid Ascent

The primary risk factor for AMS is rapid elevation gain, especially above 2,400 meters (7,874 feet). Overnight stays at high altitudes heighten the risk.

• Pre-existing Medical Conditions

Conditions like coronary artery disease (CAD), pulmonary hypertension, chronic obstructive pulmonary disease (COPD), and sickle cell disease can make individuals more vulnerable to altitude-related illnesses. Pregnant women may also face increased risks at higher elevations.

• Altitude and Sleeping Altitude

The altitude reached, particularly the sleeping altitude, significantly influences the likelihood of developing AMS. The risk increases with higher sleeping altitudes due to prolonged exposure to hypobaric hypoxia.

Diagnostic Criteria and Spectrum of High-Altitude Illnesses

High-altitude illnesses range from mild AMS to life-threatening conditions like HACE and HAPE. The severity of symptoms and neurological or pulmonary involvement primarily differentiate these conditions.

AMS:

• Symptoms: Characterized by headache and at least one of the following: gastrointestinal disturbance, dizziness, fatigue, or sleep disturbance. These symptoms typically appear 1-6 hours after ascent but may be delayed.
• Diagnostic Tools: 
  • The Lake Louise Questionnaire Score (LLQS) is commonly used. A score of 3-5 indicates mild AMS, while a score of 6 or higher suggests moderate AMS. Symptoms often mimic an alcohol hangover, with bifrontal headaches that worsen with exertion and Valsalva maneuvers.

• The clinical functional score (CFS) has a similar diagnosing profile as the LLQS. ≥2 indicates AMS
  • “Overall, if you had any symptoms, how did they affect your daily activity”?
    • No reduction of daily activity = 0
    • Mild reduction = 1
    • Moderate reduction = 2 
    • Severe reduction = 3

HACE:

• Symptoms:  progressive neurological deterioration, such as severe headache, ataxia, confusion, and, in extreme cases, coma or seizures. It’s a severe form of AMS that demands immediate intervention.
• Pathophysiology: The exact mechanism is unclear but may involve disruption of the blood-brain barrier, leading to vasogenic edema. 

HAPE:

• Symptoms: Non-cardiogenic pulmonary edema presenting with dyspnea at rest, dry cough, and marked exercise intolerance. The cough may become productive as HAPE progresses, and hypoxemia, cyanosis, and generalized rales develop.
• Pathophysiology: HAPE is primarily driven by high pulmonary vascular pressures due to hypoxia-induced vasoconstriction. Resting SaO2 is often significantly reduced; symptoms manifest 2-4 days after ascent.

Conclusion

Understanding the intricacies of high-altitude illnesses is crucial for emergency medicine practitioners. With the growing number of people living in and visiting high-altitude regions, being well-versed in the risk factors, pathophysiology, and diagnostic criteria for AMS, HACE, and HAPE can save lives. Early recognition and appropriate intervention remain the cornerstones of managing these potentially fatal conditions. By educating patients and preparing for altitude exposure, we can help mitigate the public health impact of acute mountain sickness and its severe forms.