Acute Mountain Sickness (AMS) 

Acute Mountain Sickness (AMS) is a hypoxia-induced syndrome occurring in non-acclimatized individuals after rapid ascent to altitude > 2500 meters.

It represents the mildest form of high-altitude illness and may progress to:

• High-Altitude Cerebral Edema (HACE)
• High-Altitude Pulmonary Edema (HAPE)

1. Altitude Physiology – The Foundation

 Barometric Changes

—> FiO₂ remains 21%, but partial pressure of oxygen drops → arterial hypoxemia.

 Immediate Physiological Responses

1. Hyperventilation

• Triggered by carotid body hypoxia
• ↓ PaCO₂ → Respiratory alkalosis

2. Renal Compensation

• Kidneys excrete bicarbonate
• Takes 24–48 hours

3. Sympathetic Activation

• Tachycardia
• Increased cardiac output

4. Hypoxic Pulmonary Vasoconstriction (HPV)

• Uneven pulmonary vasoconstriction
• Predisposes to HAPE

2. Pathophysiology of AMS

AMS is primarily vasogenic cerebral edema due to hypoxia.

Mechanisms:

 1. Cerebral Vasodilation

• Hypoxia → cerebral blood flow ↑
• Increased capillary hydrostatic pressure

 2. BBB Disruption

• Endothelial dysfunction
• VEGF-mediated permeability

 3. Mild Brain Edema

• Increased intracranial pressure (ICP)
• Stretch of pain-sensitive structures → headache

 4. Impaired CSF Absorption

• Mild CSF pressure rise

 AMS = early stage of HACE spectrum

3. Risk Factors

Not strongly related to:

• Sex
• Physical fitness

4. Clinical Features

Onset: 6–12 hours after ascent
Peak: 24–48 hours

Core Symptom → Headache (Mandatory for diagnosis)

Lake Louise Criteri

Diagnosis requires:

• Recent altitude gain
• Headache PLUS ≥1 of:

5. Differential Diagnosis 

 Ataxia = HACE until proven otherwise

6. Investigations

Usually clinical diagnosis.

In ICU setting:

• ABG → hypoxemia, respiratory alkalosis
• MRI (if HACE suspected) → splenium changes
• CXR → if HAPE suspected
• CBC → hemoconcentration possible

7. Management of AMS 

 GOLDEN RULE: STOP ASCENT

 1. Mild AMS

• Rest at same altitude
• Oral fluids
• Analgesics (paracetamol, ibuprofen)
• Observe 24 hrs

 2. Moderate–Severe AMS

 Oxygen

• Target SpO₂ > 90%

 Acetazolamide (First-line drug)

Mechanism:

• Carbonic anhydrase inhibitor
• Induces metabolic acidosis
• Stimulates ventilation

Side effects:

• Paresthesia
• Polyuria
• Sulfa allergy caution

 Dexamethasone (If severe or progressing)

Mechanism:

• Reduces vasogenic edema
• Stabilizes BBB

 Does NOT help acclimatization

Descent (Definitive Treatment)

Descend:

• 500–1000 meters minimum
• Immediate descent if worsening

Portable Hyperbaric Chamber

Used in remote areas.
Simulates descent by increasing ambient pressure.

8. Prevention 

 Ascent Strategy

• <300–500 m sleeping altitude per day above 3000 m
• Rest day every 1000 m gain

Pharmacologic Prophylaxis

Acetazolamide

Dexamethasone

• Alternative if acetazolamide intolerant
• Not preferred for prophylaxis

9. Comparison: AMS vs HACE vs HAPE