Mountain sickness
Reading File
Finding Sources
Searching PubMed
"acute mountain sickness" OR "high altitude illness"
Searching the Web
mountain sickness altitude illness pathophysiology treatment 2024 2025
Reading File
Reading File
Reading File
Reading File
Reading File
Reading File
Loading Image
I now have comprehensive information from multiple authoritative sources. Let me compile the full answer.
Mountain Sickness (Altitude Illness)
Mountain sickness encompasses a spectrum of acute altitude illnesses triggered by hypobaric hypoxia at elevation. There are three main syndromes that exist on a continuum: Acute Mountain Sickness (AMS), High-Altitude Cerebral Edema (HACE), and High-Altitude Pulmonary Edema (HAPE).
Epidemiology
- AMS affects 22–50% of travelers ascending to 1,850–4,240 m; rates up to 70% are reported with rapid ascent on Mt. Kilimanjaro
- Incidence rises with altitude: rare below 2,400 m (8,000 ft), most common above 3,000 m
- Occurs across all ages and fitness levels; children and adults are equally susceptible; people >50 years may be somewhat less affected
- No significant gender difference in AMS incidence
— Murray & Nadel's Textbook of Respiratory Medicine, p. 2431; Harrison's Principles of Internal Medicine 22E
The Three Syndromes
1. Acute Mountain Sickness (AMS)
The most common form. A neurologic syndrome with nonspecific symptoms developing 6–12 hours after ascent.
Lake Louise Diagnostic Criteria (requires headache + at least one of):
- Gastrointestinal upset (anorexia, nausea, vomiting)
- Fatigue or general weakness
- Dizziness or lightheadedness
Headache is typically bitemporal, throbbing, worse at night and on awakening. Symptoms peak at 24–48 hours, resolve in most individuals by day 3–4. Sleep disturbance (Cheyne-Stokes breathing) is nearly universal at high altitude but is no longer part of the diagnostic criteria.
2. High-Altitude Cerebral Edema (HACE)
AMS and HACE represent a continuum. HACE is an encephalopathy — the critical distinguishing features are:
- Ataxia (truncal, cerebellar)
- Altered consciousness (confusion, disorientation, stupor, coma)
- Diffuse cerebral involvement without focal neurologic deficits (usually)
- Papilledema and retinal hemorrhages (occur frequently at ≥5,000 m even without AMS symptoms)
MRI in HACE shows increased T2 signal in white matter, particularly the splenium of the corpus callosum — considered the hallmark finding. Hemosiderin deposits in the corpus callosum may persist long-term as footprints of HACE.
3. High-Altitude Pulmonary Edema (HAPE)
The most lethal form. Presents with:
- Dyspnea at rest (dyspnea on exertion is universal and expected at altitude)
- Dry cough progressing to pink frothy sputum
- Crackles, cyanosis, marked hypoxemia
- Mechanism: hypoxic pulmonary vasoconstriction → non-cardiogenic pulmonary edema from elevated pulmonary artery pressure and capillary stress failure
Pathophysiology
The key inciting event is hypobaric hypoxia. From there, the causal chain remains incompletely understood, but the major mechanisms include:
| Mechanism | Details |
|---|---|
| Cerebral vasodilation | Hypoxia → cerebral blood flow ↑ (can be >normal); impaired autoregulation |
| Blood-brain barrier disruption | VEGF, histamine, arachidonic acid — promote vasogenic (interstitial) edema |
| Intracranial pressure | Raised ICP documented in HACE; debated in AMS ("tight fit hypothesis") |
| Trigeminovascular activation | Primary mechanism for altitude headache; arachidonic acid pathway, inflammation |
| Nitric oxide & adenosine | Promote cerebral vasodilation |
| Venous outflow obstruction | Contributes to raised brain capillary pressure and edema |
| Pulmonary vasoconstriction | Uneven → high-pressure capillary leak → HAPE |
| Fluid retention | Aldosterone-mediated; reduced urine output common |
Vasogenic edema may become cytotoxic in severe HACE.
— Harrison's 22E; Murray & Nadel's
Risk Factors
- Rate of ascent — the single most important factor
- Prior history of altitude illness
- High exertion at altitude (lack of fitness is NOT a risk factor)
- Sleep desaturation
- Low ventilatory response to hypoxia
- Carotid body damage (neck irradiation/surgery)
- Respiratory tract infections
- Dehydration
Differential Diagnosis
The nonspecific nature of AMS mandates consideration of:
Tension/migraine headache • Dehydration • Hypothermia • Hyponatremia • Alcohol hangover • Carbon monoxide poisoning (critical in poorly ventilated high-altitude shelters with stoves/generators) • Caffeine withdrawal • Meningitis/encephalitis • Intracranial hemorrhage or mass • Cerebral venous sinus thrombosis • Acute angle-closure glaucoma
Any ataxia or altered mental status = HACE until proven otherwise → immediate descent
— Rosen's Emergency Medicine; Tintinalli's Emergency Medicine
Diagnosis
- AMS is a clinical diagnosis — no confirmatory lab tests exist
- Optic nerve sheath diameter (ONSD) ultrasonography: serial measurements showing enlargement support worsening AMS/HACE; single measurements have limited utility due to individual variability
- MRI (if available): corpus callosum splenium signal on T2 = pathognomonic of HACE
- Pulse oximetry: lower SpO₂ is associated with AMS but should not alone be used to predict AMS on a trek
Management
Graded by Severity (Harrison's 22E)
| Condition | Management |
|---|---|
| AMS, mild | Stop ascent; rest; acetazolamide 250 mg q12h; descend if not improving |
| AMS, moderate | Immediate descent for worsening; low-flow O₂ if available; acetazolamide 250 mg q12h ± dexamethasone 4 mg q6h; hyperbaric therapy |
| HACE | Immediate descent/evacuation; O₂ 2–4 L/min; dexamethasone 8 mg then 4 mg q6h PO/IM/IV; portable hyperbaric chamber if descent impossible |
| HAPE | Immediate descent/evacuation; minimize exertion; O₂ 4–6 L/min (target SpO₂ >90%); nifedipine 30 mg ER q12h (adjunct); hyperbaric therapy if descent impossible |
Key Drug Summary
| Drug | Role | Dose |
|---|---|---|
| Acetazolamide | Treatment & prevention (AMS); carbonic anhydrase inhibitor → stimulates ventilation, reduces CSF production | 125–250 mg PO q12h; prophylaxis 125 mg q12h starting 24h before ascent |
| Dexamethasone | Treatment of AMS/HACE; HAPE adjunct; blocks hypoxic VEGF upregulation | 4 mg q6h (AMS); 8 mg loading + 4 mg q6h (HACE) |
| Nifedipine | HAPE — reduces pulmonary artery pressure | 30 mg ER q12h |
| Tadalafil / Sildenafil | HAPE prevention (PDE-5 inhibitors → pulmonary vasodilation) | Tadalafil 10 mg BID; sildenafil 50 mg TID |
| Ibuprofen / aspirin / acetaminophen | High-altitude headache in AMS | Standard doses |
| Prochlorperazine | Nausea in AMS; also augments hypoxic ventilatory response | Standard doses |
⚠️ Avoid narcotics — they suppress the hypoxic ventilatory response.
⚠️ Dexamethasone masks symptoms but does not facilitate acclimatization — rebound AMS can occur on discontinuation.
Prevention
- Gradual ascent — the most effective strategy ("Climb high, sleep low")
- Above 3,000 m: increase sleeping altitude ≤300 m/day
- Take an extra acclimatization day for every 3,000–5,000 ft gained above 10,000 ft
- First night ideally ≤9,200 ft (2,800 m)
- Acetazolamide prophylaxis: 125 mg PO BID starting 24h before ascent, for moderate/high-risk individuals or rapid ascent scenarios. Children: 2.5 mg/kg/dose BID (max 125 mg/dose)
- Dexamethasone: 2 mg q6h or 4 mg q12h — for military/rescue personnel or acetazolamide contraindication (sulfa allergy)
- Ibuprofen (600 mg TID) is equally efficacious to acetazolamide for headache prevention
- Adequate hydration (clear urine); avoid alcohol and sedatives; moderate exertion
- Hypoxic pre-exposure tents: limited benefit if <8–12 hours duration
Special Considerations
- Children: AMS may present as fussiness, decreased appetite, sleep disturbance — differential must remain broad; if serious illness is suspected, descend for evaluation
- Carbon monoxide at altitude: hypobaric hypoxia + CO poisoning is synergistic and more dangerous than at sea level
- Retinal hemorrhages: common at >5,000 m even without symptoms; do not by themselves mandate descent unless they affect the macula and impair vision
- Globus pallidus lesions (hypoxia-sensitive): HACE can cause Parkinson's-like sequelae
Recent Evidence
Recent systematic reviews (2024–2025) are broadly consistent with established guidelines:
- PMID 39316567 — Systematic review/meta-analysis on mountain sickness prevalence in Latin American altitude inhabitants (2024)
- PMID 39323005 — Pulse oximetry for AMS prediction: systematic review confirms SpO₂ alone has limited predictive value (2024)
- The 2024 Wilderness Medical Society guidelines updated recommendations for prevention, diagnosis, and treatment of AMS, HACE, and HAPE — the current reference standard for field management
Sources: Harrison's Principles of Internal Medicine 22E • Rosen's Emergency Medicine • Tintinalli's Emergency Medicine • Murray & Nadel's Textbook of Respiratory Medicine • Guyton & Hall Medical Physiology
This is a shared conversation. Sign in to Orris to start your own chat.