What are the recommendations for preventing acute mountain sickness (AMS) at high altitudes?

Prevention of Acute Mountain Sickness (AMS) at High Altitudes

The most effective prevention strategy for acute mountain sickness is slow ascent (<400 m/day above 2,500m), combined with prophylactic acetazolamide (125mg twice daily) starting 24 hours before ascent for those at higher risk. 1

Primary Prevention Strategies

Gradual Ascent

• Slow ascent (<400 m/day above 2,500m) is the most effective non-pharmacological prevention method 1
• Include a rest day for every 600-1200m gained in altitude 1
• For those with access to altitude/hypoxic facilities, long pre-acclimatization (about 2 weeks with >8 h/day at progressively increasing altitudes) is preferable to shorter exposures 2

Pharmacological Prevention

• Acetazolamide (125mg twice daily) starting 24 hours before ascent is the primary pharmacological preventive measure 1
• Dexamethasone (4mg every 12 hours) can be effective for AMS prevention when acetazolamide is contraindicated 3
• Lower doses of dexamethasone (1mg or 0.25mg every 12 hours) are relatively ineffective 3

Risk Assessment and Monitoring

Screening for AMS Risk

• Systematic screening using the Lake Louise Scoring system is recommended, especially for women who appear to have a statistically higher AMS risk 2
• The Lake Louise Scoring system rates severity (0-3) of: headache, nausea, dizziness, and fatigue 2
• For individuals without previous mountain experience, a chemosensitivity test (assessing relationship between pulmonary ventilation and SpO₂) prior to ascent can indicate physiological responses to hypoxia 2

Special Considerations for Women

• Women may have a higher risk of AMS than men 1
• The mid-luteal phase of the menstrual cycle may be more appropriate for acute exposure to high altitude due to higher hypoxic ventilatory response during this phase 2, 1
• Women should check iron profiles 6 weeks prior to altitude exposure, as they are at higher risk of iron deficiency which can affect haematological adaptations 2

Management of AMS

Early Recognition and Treatment

• If AMS develops, immediate management includes:
  • Descent of at least 300m as soon as possible 1
  • Supplemental oxygen to maintain SpO₂ >90% 1
  • Dexamethasone for cerebral symptoms 1
  • Nifedipine specifically for high-altitude pulmonary edema (HAPE) 1

Hydration and Nutrition

• Maintain proper fluid intake to ensure adequate hydration 1
• Altitude exposure increases carbohydrate oxidation during exercise, potentially requiring greater dietary carbohydrate intake 2
• Women may require a smaller increase in carbohydrate intake at altitude compared to men due to less sensitivity to substrate shift 2

Common Pitfalls and Caveats

• Rapid ascent risk: Most altitude-related illnesses occur when ascent is too rapid without sufficient acclimatization time 4
• Individual susceptibility: Susceptibility to high-altitude syndromes varies between individuals but is generally reproducible in the same person 4
• Exercise caution: Strict control of exercise intensities, particularly during the first days (acclimatization phase), is especially important for women 2
• Medication interactions: Combined use of acetazolamide with other diuretics may increase dehydration risk 1
• Serious complications: Be vigilant for progression to more serious conditions like high-altitude cerebral edema (HACE) or high-altitude pulmonary edema (HAPE), which occur in approximately 0.5-1.0% of visitors to elevations above 10,000 feet 5

By following these evidence-based recommendations, the risk of developing acute mountain sickness can be significantly reduced, allowing for safer experiences at high altitudes.