Acute mountain sickness is the most common form of altitude illness, affecting 25–43% of travelers ascending above 2,500 m (8,200 ft), characterized by headache plus at least one of: nausea, fatigue, dizziness, or anorexia, developing within hours to 3 days of ascent. [1-2] It is typically self-limited (resolving in 12–48 hours) but can progress to life-threatening high-altitude cerebral edema (HACE) in <1% of cases. [3]
1. History
- Key HPI questions: Altitude reached, rate of ascent, sleeping elevation, time of symptom onset relative to arrival at altitude
- Symptom characterization: Headache is the cardinal symptom — typically dull, bifrontal/temporal, worsened by movement or Valsalva [1-2]
- Timing: Onset usually 2–12 hours after arrival at altitude; symptoms starting >3 days after arrival without further ascent should not be attributed to AMS [2]
- Associated symptoms: Anorexia, nausea/vomiting, fatigue, lightheadedness, insomnia [4-5]
- Important negatives: No fever, coryza, chills, or myalgia (distinguishes from viral illness); no focal neurologic deficits; no altered mental status or ataxia (distinguishes from HACE) [2]
- Ask about prior episodes of AMS (recurrence risk up to 60% with similar ascent profiles) [3]
2. Alarm Features
- Ataxia (earliest sign of HACE — test with heel-to-toe walking) [4]
- Altered mental status, confusion, drowsiness, inability to provide self-care → HACE [2][4]
- Dyspnea at rest, dry cough progressing to pink frothy sputum, cyanosis → HAPE [1][6]
- Worsening symptoms despite rest and treatment at current altitude
- Coma can ensue within 12–24 hours of ataxia onset in untreated HACE [2]
- Focal neurologic deficits or seizures → consider intracranial lesion, hyponatremia, or stroke rather than HACE [2]
3. Medications
Prevention (moderate-to-high-risk ascent profiles)
- Acetazolamide 125 mg PO q12h — start day before ascent, continue 2–4 days at target altitude; aids acclimatization [2][4]
- Dexamethasone 2 mg PO q6h or 4 mg q12h — alternative if acetazolamide intolerant; does NOT aid acclimatization [4][6]
- Ibuprofen 600 mg PO q8h — alternative if both above are contraindicated [2][6]
Treatment
- Mild AMS: NSAIDs (ibuprofen 600 mg q8h) or acetaminophen 500 mg q8h for headache; antiemetics (ondansetron 4 mg ODT) as needed [1-2]
- Mild-moderate: Consider acetazolamide 250 mg PO q12h [1][4]
- Moderate-severe: Dexamethasone 4 mg PO/IV/IM q6h — more reliable than acetazolamide for treatment; consider adding acetazolamide as adjunct [4][6]
- HACE: Dexamethasone 8 mg loading dose, then 4 mg q6h [1][4]
Contraindications/Cautions
- Acetazolamide: contraindicated with history of anaphylaxis or Stevens-Johnson syndrome to sulfonamides (cross-reactivity with sulfa antibiotics is extremely unlikely) [2][6]
- Avoid opioid analgesics (respiratory depression risk at altitude) [2]
- Avoid sedative-hypnotics unless combined with acetazolamide [7]
- Dexamethasone: do not use >10 days without taper; does not facilitate acclimatization — rebound symptoms possible if stopped at altitude before acclimatization [2][6]
4. Diet
- Maintain adequate hydration — dehydration is common at altitude due to increased respiratory water loss and decreased thirst drive
- Avoid alcohol — mimics and worsens AMS symptoms, impairs acclimatization, and worsens sleep quality
- Light, high-carbohydrate meals may be better tolerated during acute symptoms
- Avoid heavy, high-fat meals which can worsen nausea
5. Review of Systems
- Neuro: Headache quality/severity, dizziness, balance problems, confusion, visual changes
- GI: Nausea, vomiting, anorexia
- Pulmonary: Dyspnea (exertional vs. rest), cough (dry vs. productive), exercise tolerance
- Sleep: Insomnia, frequent awakenings, sensation of suffocation (central sleep apnea is very common at altitude) [1]
- Constitutional: Fatigue, malaise, functional impairment
- Cardiac: Palpitations (common at altitude, usually benign)
6. Collateral History and Family History
- Travel companions can provide critical observations about gait instability, confusion, or behavioral changes that the patient may not recognize [3]
- Prior altitude exposure history and outcomes in similar conditions
- Family history is generally not a major factor, though animal studies suggest genetic differences in respiratory drive may contribute to individual susceptibility [3]
- Social context: recreational trekking vs. occupational/military rapid deployment affects risk profile
7. Risk Factors
- Rapid ascent rate (>400 m/day gain in sleeping elevation) — OR 4.69 for AMS [3]
- Altitude attained — prevalence increases ~13% per 1,000 m above 2,500 m [3]
- Prior history of AMS — strongest individual risk factor (recurrence OR up to 12) [3]
- Lack of preacclimatization (no altitude exposure in prior 1–2 months) [3]
- Younger age (<50 years) — risk is ~2-fold higher [3]
- Migraine history, obesity, anxiety may increase risk [3]
- Physical fitness does NOT reduce risk [3]
- Smoking and alcohol consumption do not appear to increase risk [3]
- Exertion at altitude increases risk [8]
8. Differential Diagnosis
- Carbon monoxide poisoning — consider in enclosed shelters/tents with stoves at altitude [4]
- Dehydration/exhaustion — common at altitude, overlapping symptoms
- Hyponatremia — from excessive water intake during exertion
- Hypoglycemia — especially in diabetics or with prolonged exertion
- Hypothermia — altered mentation, overlapping setting
- Migraine — can be triggered by altitude; distinguish by prior history and aura
- Alcohol hangover — symptoms closely mimic AMS [2]
- Viral illness — presence of fever, coryza, myalgia argues against AMS [2]
- Drug intoxication — consider in appropriate context
- Intracranial pathology (stroke, mass lesion) — suspect if focal neurologic deficits present [2]
9. Past Medical History
- Prior episodes of AMS/HACE/HAPE — most important predictor of recurrence [3]
- Migraine disorder — may lower threshold for altitude headache
- Chronic lung disease, pulmonary hypertension — may increase susceptibility to HAPE
- Cardiac conditions — relevant for high-altitude travel counseling [1]
- Sickle cell trait — altitude is a risk factor for exertional sickling, splenic infarction [9]
- Medication history: current use of diuretics, antihypertensives, or respiratory depressants
10. Physical Exam
- Vital signs: Tachycardia and mild tachypnea are common at altitude; SpO2 will be lower than sea level (expected range varies by altitude). A notably low SpO2 for the altitude may suggest more severe illness [2]
- Neuro exam: Must be normal in AMS — intact mental status, no ataxia. Perform heel-to-toe walking to screen for HACE [4]
- Lung exam: Should be clear in AMS; crackles/rales suggest HAPE [3]
- General: Assess hydration status, pallor, cyanosis
- No characteristic physical findings distinguish AMS from other causes of similar symptoms [2]
- Papilledema, if present, confirms cerebral edema [4]
- POCUS: Optic nerve sheath diameter is NOT reliable for AMS/HACE diagnosis [4]
11. Lab Studies
- No laboratory test establishes the diagnosis of AMS — diagnosis is entirely clinical [3-4]
- Labs are used to rule out mimics:
- BMP: Hyponatremia, hypoglycemia, renal function
- CBC: If infection suspected
- Blood glucose: Point-of-care if altered mentation
- Carboxyhemoglobin: If CO exposure suspected (enclosed shelter with combustion)
- SpO2 monitoring: Useful for gauging acclimatization; a high SpO2 for the altitude appears protective [2]
12. Imaging
- Imaging is generally unnecessary for straightforward AMS [4]
- CT/MRI brain: Only if focal neurologic deficits, seizures, or concern for intracranial pathology other than HACE
- Chest X-ray: If concern for HAPE (patchy infiltrates, typically right middle lobe predominant)
- Lung ultrasound: May detect B-lines suggesting subclinical pulmonary edema at altitude, but clinical significance is debated
13. Special Tests
Diagnostic Scoring Systems
- Lake Louise Acute Mountain Sickness Score (2018 revision): Scores headache, GI symptoms, fatigue, and dizziness on 0–3 scale; score ≥3 with headache = AMS. Mild AMS: 3–5; Moderate-severe AMS: 6–12 [4][10]
- Clinical Functional Score (CFS): Single question — "How did symptoms affect your daily activity?" (0–3 scale). Score ≥2 has positive LR 3.2 for severe AMS; simplest bedside tool [3]
- AMS-Cerebral (AMS-C) score: ≥0.70 indicates AMS; used primarily in research [3]
Point-of-Care Tests
- Pulse oximetry: Inexpensive, useful for monitoring acclimatization and detecting HAPE [2]
- Heel-to-toe walk test: Screens for ataxia/HACE [4]
14. ECG
- ECG is not routinely indicated for AMS
- Sinus tachycardia is expected at altitude
- Consider ECG if chest pain, palpitations, or syncope to rule out arrhythmia or acute coronary syndrome
- Right heart strain pattern may be seen with severe HAPE (right axis deviation, P-pulmonale, RV strain)
15. Assessment
Severity Stratification (per Wilderness Medical Society 2024 Guidelines): [4]
- AMS is typically self-limited, resolving in 12–48 hours without further ascent [2-3]
- Progression to HACE occurs in <1% of AMS cases [3]
- Atypical presentations: symptoms without headache (uncommon); preverbal children may present with irritability, pallor, and poor feeding [2]
The following figure illustrates the proposed pathophysiology linking high-altitude hypoxemia to both cerebral and pulmonary forms of altitude illness:
16. Treatment Plan
Initial Stabilization
- Stop ascent; rest at current elevation [1][4]
- Assess for HACE/HAPE alarm features
Mild AMS
- Symptomatic treatment: ibuprofen 600 mg q8h or acetaminophen 500 mg q8h [1-2]
- Antiemetics PRN: ondansetron 4 mg ODT [2]
- May consider acetazolamide 250 mg q12h to speed acclimatization [1]
- Remain at current altitude; do not ascend until symptoms resolve [4]
Moderate-Severe AMS
- Dexamethasone 4 mg PO/IV/IM q6h for 24 hours [1][4][6]
- Consider adding acetazolamide 250 mg q12h as adjunct [6]
- Descent 300–1,000 m if symptoms persist or worsen [4][6]
- Supplemental O₂ at 1–2 L/min if available [2]
HACE
- Immediate descent — this is the definitive treatment [1][4]
- Dexamethasone 8 mg loading, then 4 mg q6h [1][4]
- Supplemental O₂ to SpO₂ >90% [4]
- Portable hyperbaric chamber (Gamow bag) if descent not feasible — simulates ~2,000 m descent at 2 psi [7]
HAPE
- Descent + supplemental O₂ [1][11]
- Nifedipine SR 30 mg q12h if O₂ unavailable and descent not feasible [1][4]
- No role for diuretics [11]
17. Disposition
May remain at altitude (with close observation)
Descent indicated
- Moderate-severe AMS not improving after 1–3 days of treatment [4]
- Any signs of HACE (ataxia, altered mental status) [4][12]
- Any signs of HAPE (rest dyspnea, cyanosis, productive cough) [1]
- Worsening symptoms despite appropriate treatment [2]
Admission/higher-level care
- HACE or HAPE — require monitoring, supplemental O₂, and pharmacotherapy [4][11]
- Altered mental status, hemodynamic instability
Resuming ascent
- Permitted only after complete symptom resolution [4][12]
- Use prophylactic acetazolamide 125 mg q12h during reascent [4]
- Further ascent is contraindicated with any ongoing symptoms [4]
18. Follow Up / Return Precautions
Symptoms requiring immediate reassessment
- New or worsening ataxia, confusion, or drowsiness (HACE) [2][4]
- Dyspnea at rest, cough with pink sputum, cyanosis (HAPE) [1]
- Persistent vomiting preventing oral intake or medication
- Symptoms worsening despite rest and treatment at current altitude [2]
Patient counseling
- AMS is common and usually self-limited; most cases resolve in 12–48 hours [2]
- The "golden rule": never ascend to a higher sleeping elevation with symptoms of AMS [4][7]
- For future trips: ascend gradually (≤500 m/day increase in sleeping elevation above 3,000 m, with rest days every 3–4 days) [1]
- Consider prophylactic acetazolamide for future ascents if history of AMS [4][6]
- Prior AMS does not preclude future high-altitude travel — it requires better planning and slower ascent [3]
Expected recovery: Symptoms typically resolve within 1–3 days if no further ascent occurs. Rapid improvement with descent of even 300 m is both therapeutic and diagnostically confirmatory. [2]
References
1. Medical Conditions and High-Altitude Travel. — Luks AM, Hackett PH. The New England Journal of Medicine. 2022.
2. High-Altitude Travel and Altitude Illness. — Peter H. Hackett and David R. Shlim CDC Yellow Book. 2025.
3. Does This Patient Have Acute Mountain Sickness?The Rational Clinical Examination Systematic Review. — Meier D, Collet TH, Locatelli I, et al. The Journal of the American Medical Association. 2017.
4. Wilderness Medical Society Clinical Practice Guidelines for the Prevention, Diagnosis, and Treatment of Acute Altitude Illness: 2024 Update. — Luks AM, Beidleman BA, Freer L, et al. Wilderness & Environmental Medicine. 2024.
5. Altitude Illnesses. — Gatterer H, Villafuerte FC, Ulrich S, et al. Nature Reviews. Disease Primers. 2024.
6. Acute Altitude Illness: Updated Prevention and Treatment Guidelines from the Wilderness Medical Society. — American Academy of Family Physicians (2020). 2020.
7. High-Altitude Illness. — Hackett PH, Roach RC. The New England Journal of Medicine. 2001.
8. The Cerebral Effects of Ascent to High Altitudes. — Wilson MH, Newman S, Imray CH. The Lancet. Neurology. 2009.
9. Mass Participation and Tournament Event Management for the Team Physician: A Consensus Statement (2022 Update). — Herring SA, Kibler WB, Putukian M, et al. Medicine and Science in Sports and Exercise. 2024.
10. The 2018 Lake Louise Acute Mountain Sickness Score. — Roach RC, Hackett PH, Oelz O, et al. High Altitude Medicine & Biology. 2018.
11. Acute High-Altitude Illnesses. — Bärtsch P, Swenson ER. The New England Journal of Medicine. 2013.
12. Wilderness Medical Society Clinical Practice Guidelines for the Prevention and Treatment of Acute Altitude Illness: 2019 Update. — Luks AM, Auerbach PS, Freer L, et al. Wilderness & Environmental Medicine. 2019.