Acute Mountain Sickness

Acute mountain sickness is the commonest altitude illness, a self-limiting syndrome of headache with other symptoms that develops 6 to 12 hours after arrival at a new altitude over 2500 m (sometimes at 2000 m). Incidence depends on ascent rate and altitude: about 25% at 2500 m, 50 to 75% at 4500 m after rapid ascent, and over 80% at 5500 m in unacclimatized people.

Symptoms peak at 24 to 48 hours and settle within 2 to 4 days of staying at the same altitude, but it can progress to cerebral or pulmonary edema.

Mechanism: hypobaric hypoxia triggers hypoxemia, cerebral vasodilation, increased capillary pressure, fluid shifts, mild brain swelling and altered intracranial compliance. Individuals with less intracranial reserve, a blunted hypoxic ventilatory response and rapid ascent are more susceptible.

Risk factors: previous episode, rapid ascent, higher sleeping altitude, exertion, dehydration, obesity, age under 50, and migraine. Cardiopulmonary fitness is not protective.

Diagnosis uses a self-reported symptom score. Complications: progression to high altitude cerebral edema or pulmonary edema, dehydration from vomiting, and rarely retinal hemorrhage and thrombosis.

• Headache (required for diagnosis), usually throbbing, bilateral, worse at night and on waking, with straining or when lying down
• One or more of: nausea and vomiting, anorexia, fatigue and weakness, dizziness or light-headedness, and insomnia with frequent waking
• Peripheral edema and reduced urine output
• Mild disease: score 3 to 5. Moderate: 6 to 9. Severe: 10 to 15 (each of the five symptoms scored 0 to 3)
• Red flags: ataxia, confusion, severe persisting vomiting, headache unresponsive to analgesia, dyspnea at rest and drowsiness (these indicate cerebral or pulmonary edema)

1. Clinical diagnosis: headache after arrival at altitude plus at least one other symptom, with a total self-reported score of at least 3.
2. Examination: tandem gait test and mental state at each review, pulse oximetry (it is not diagnostic, since values vary), chest auscultation for crackles and fundoscopy.
3. Capillary glucose to exclude hypoglycemia in anyone with confusion.
4. Exclusion of other causes: dehydration, exhaustion, hypothermia, hyponatremia, carbon monoxide poisoning, infection, migraine and intoxication.
5. Imaging only when features suggest another diagnosis.

Differentials: dehydration, exhaustion, migraine, viral illness, hyponatremia, carbon monoxide poisoning, hypothermia and alcohol hangover.

1. Mild disease:
   • Stop ascent, rest and stay at the same altitude until symptoms resolve (usually 1 to 2 days), then resume.
   • Paracetamol 1 g orally every 6 hours or ibuprofen 600 mg orally every 8 hours with food for headache.
   • Ondansetron 4 mg orally or sublingually every 8 hours for nausea.
   • Acetazolamide 250 mg orally every 12 hours (children 2.5 mg/kg every 12 hours, maximum 125 mg a dose) speeds acclimatization.
   • Oral fluids, avoiding alcohol and sedative hypnotics.
2. Moderate disease: stop ascent and descend 300 to 1000 m if symptoms do not improve in 24 hours, or worsen. Acetazolamide, with dexamethasone 4 mg orally, IV or IM every 6 hours (children 0.15 mg/kg a dose) until symptoms resolve and for no longer than needed. Low-flow oxygen 1 to 2 L/min.
3. Severe disease or any ataxia or confusion: immediate descent and oxygen 2 to 4 L/min, with dexamethasone 8 mg once then 4 mg every 6 hours (managed as cerebral edema).
4. Sleep: acetazolamide 125 mg orally at night improves periodic breathing. Avoid benzodiazepines and opioids.
5. Acetazolamide effects: paresthesia, increased urine output, taste change and a mild metabolic acidosis. Contraindicated in sulfonamide allergy, severe renal or hepatic impairment, and avoided in the first trimester of pregnancy.

Prevention

• Gradual ascent: over 3000 m, an increase in sleeping altitude of at most 300 to 500 m a day with a rest day every 3 to 4 days.
• Acetazolamide 125 mg orally every 12 hours starting the day before ascent and for 2 days after reaching the highest altitude (250 mg every 12 hours for forced rapid ascent).
• Alternative: dexamethasone 2 mg orally every 6 hours, or 4 mg every 12 hours (for those with a sulfonamide allergy, or as adjunct for forced ascent), taken during ascent and for no more than 5 days.
• Ibuprofen 600 mg orally every 8 hours reduces symptoms in some trials.
• Avoid alcohol and heavy exertion in the first days, and treat early.

Counsel travelers that symptoms are a warning that must be respected: never ascend with symptoms, descend if they worsen, and never leave a sick person alone.