Acute Mountain Sickness – Symptoms, Prevention, and Practical Insights for Mountain Trekkers

Acute Mountain Sickness (AMS) is a frequently observed condition that tends to be mild and self-limiting, affecting those exploring mountainous regions. However, when disregarded or underestimated, it can escalate into severe issues like High-Altitude Cerebral Edema (HACE) and High-Altitude Pulmonary Edema (HAPE). The primary cause of AMS is well understood—rising hypoxemia levels due to decreased partial pressure of inhaled oxygen as altitude increases. Proper acclimatization and gradual ascent, following established acclimatization guidelines, serve as effective safeguards against AMS development.

The factors triggering the onset of AMS remain shrouded in mystery, with limited scientific research in this realm. Contributing elements likely include pre-existing health conditions, genetic predisposition, and dehydration. Individuals previously afflicted by AMS are at a heightened risk of recurrence during subsequent expeditions.

Some individuals may start experiencing initial symptoms of AMS even at altitudes as low as 2000 meters above measured sea level (mamsl). Generally, those accustomed to lower altitudes can achieve full acclimatization for continuous stays at around 5000 mamsl. In my experiences as a medic on expeditions, AMS symptoms typically surfaced at altitudes of 4000-5000 mamsl. Beyond 6000 mamsl, processes of bodily deterioration begin to outweigh regenerative mechanisms, making extended stays at such heights incredibly arduous and draining. Mountaineers can acclimatize to brief altitudinal sojourns, even exceeding 7000 mamsl. Nonetheless, acclimatization within the “death zone” (above 8000 mamsl) appears unattainable for humans, as remaining without oxygen there for a longer period of time leads to swift fatality.

Principles of healthy acclimatization

Gradual elevation gain stands as the fundamental strategy for averting AMS. Generally embraced guidelines (e.g., UIAA) advise ascending by 300-500 meters each day, with an additional night’s halt at the same altitude after every 1000-meter climb. While daytime acclimatization treks are beneficial for “testing” new altitudes, the rest of the day should prioritize rest and spending the night at the attained elevation. Terrain constraints can occasionally impede strict adherence to acclimatization rules, necessitating consultation with an expedition medical professional. Regrettably, financial constraints often lead expedition companies to compromise healthy acclimatization principles. A striking instance is Kilimanjaro expeditions, which, despite being a non-technical climb, are often restricted to 5-6 days instead of the recommended 9. This curtailed timeframe results in elevated AMS instances and a lower summit success rate (merely 65%), despite the absence of technical challenges.

Symptoms of AMS and Daily Evaluation of High-Altitude Expedition Participants

The hallmark and initial symptom of AMS is headache, affecting up to 70% of trekkers at specific altitudes. Additional indications encompass reduced appetite, nausea, vomiting, fatigue, and dizziness.

Daily assessment involves utilizing a finger pulse oximeter to measure pulse oximetry and applying the Lake Louise Score to evaluate participants’ conditions during expeditions.

Expedition medical kits should invariably contain a pulse oximeter and spare batteries for high-altitude journeys. A healthy individual’s sea-level SpO2 level ranges from 95-100%. As elevation increases, SpO2 levels decrease, though they should not dip below 75%. Cold-induced vasoconstriction can influence readings, prompting measures like warming hands, trying alternate fingers, or using the earlobe for measurement.

The next evaluation point is participants’ well-being appraisal using the Lake Louise Scale.

Carrying a laminated copy of the scale is advisable, ensuring accessibility even when electronic devices are unavailable.

Interpretation of Lake Louise Scale

A score of 3-5 signifies mild AMS, warranting pain relief (e.g., 500-1000 mg Paracetamol / 200-400 mg Ibuprofen / 500-1000 mg Metamizole) every 6 hours, rest, and adequate hydration (2-4 liters daily during mountain treks) with electrolyte supplementation. Initial signs of the condition may include a participant’s deceleration (from leading to trailing) and reluctance to vacate their tent or sleeping bag. This conduct should raise concerns among fellow trekkers, prompting checks on those remaining in tents. Encouraging affected individuals to socialize and delaying sleep until evening is advisable.

A score of 6-9 denotes moderate AMS. While such individuals can maintain their current altitude, ascending further is discouraged until symptoms abate. Protocol mirrors that of mild AMS. If symptoms persist despite altitude stability, Acetazolamide administration (e.g., Diuramid, Zolamide, Diamox) (125-250 mg 2x/day) is prudent, ideally initiated 24 hours before ascending and continued until expedition completion. Potential side effects, like increased urination, necessitate heightened hydration. In instances of concurrent moderate AMS symptoms, low saturation, and non-resolving/aggravating symptoms, expeditions should be halted, requiring participants to descend at least 500-1000 meters. Subsiding symptoms permit expedition continuation.

A score of 10-12 flags severe AMS, posing risks of developing high-altitude cerebral (HACE) or pulmonary edema (HAPE). Acetazolamide, at 250 mg 2x/day, should be administered, with a descent of at least 500-1000 meters or helicopter evacuation contingent on the individual’s condition.

Moderate to severe AMS permits low-flow nighttime oxygen therapy (approx. 1 l/min) for regeneration, under rigorous pulse oximetry control and an on-site medic. Trips lacking medical personnel may render this procedure hazardous and impede accurate patient assessment.

Debate surrounds the routine Acetazolamide prophylactic usage at expedition onset

Natural acclimatization, sans prophylactic intervention, is favored for optimal acclimatization. However, individuals previously afflicted by severe AMS episodes are advised to commence prophylactic Acetazolamide (125 mg 2x/day) from the outset.

Mastery of Discretion

At a certain altitude, oxygen deficiency becomes a less-than-ideal setting for human functioning. Consequently, many will encounter symptoms like mild headaches and general discomfort. Prioritizing self-care—especially sleep, warmth, hydration—and keenly observing bodily signals becomes paramount. A crucial skill for every high-altitude enthusiast is the ability to objectively assess situations and abort expeditions when health becomes jeopardized.

Note: This article is intended for informational purposes only and should not be considered a substitute for professional medical advice.