Acute mountain sickness physiology

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Acute Mountain Sickness (AMS) - Physiology

Definition & Epidemiology

AMS is a neurologic syndrome that develops 6-12 hours after ascent to high altitude (typically >2500-3000 m). It is characterized by headache, fatigue, dizziness, nausea, vomiting, and sleep disturbance. Symptoms typically peak within the first day and last 3-5 days. It can occur in some people at altitudes as low as 2100 m and occurs in most people above 3500 m.

AMS and high-altitude cerebral edema (HACE) are thought to represent opposite ends of a continuum of altitude-related neurologic disorders.

Harrison's Principles of Internal Medicine, 22E, p. 3798

The Primary Trigger: Hypobaric Hypoxia

At altitude, barometric pressure falls, reducing the partial pressure of inspired oxygen (PiO2). This produces hypobaric hypoxia - the fundamental driver of all altitude illnesses. The body's initial compensatory response is hyperventilation, driven by peripheral chemoreceptors (carotid bodies). However, this is often inadequate in the short term, leading to insufficient O2 delivery to the brain.

Medical Physiology (Boron & Boulpaep), p. 689

Pathophysiology - Central Mechanisms

1. Cerebral Vasodilation and Edema (Core Mechanism)

Hypoxia causes cerebral arteriolar dilation, increasing cerebral blood flow (CBF). This raises capillary hydrostatic pressure, promoting fluid transudation into brain tissue.

Vasogenic (interstitial) edema is now recognized as the dominant form in AMS/HACE, supported by MRI evidence showing increased T2 signal in white matter - especially the splenium of the corpus callosum:

HACE MRI - hyperintense splenium of corpus callosum

T2 MRI showing marked swelling and hyperintense splenium of corpus callosum in HACE. (Harrison's, Fig. 475-1)

In severe HACE, vasogenic edema can progress to cytotoxic (intracellular) edema.

Harrison's, p. 3799; Guyton & Hall, p. 554

2. Blood-Brain Barrier Disruption

Hypoxia induces several chemical mediators that increase BBB permeability:

VEGF (Vascular Endothelial Growth Factor) - first proposed in 1995, shown to promote capillary leakage; dexamethasone (an effective AMS treatment) blocks hypoxic VEGF upregulation
Histamine and arachidonic acid - inflammatory mediators
Nitric oxide - calcium-mediated; promotes cerebral vasodilation
Adenosine - neuronally mediated; contributes to cerebral vasodilation
Harrison's, p. 3799

3. Impaired Cerebral Autoregulation

Normally, cerebral blood flow is kept constant across a range of perfusion pressures. In AMS, hypoxia impairs this autoregulation, so hypoxic vasodilation leads to uncontrolled increases in capillary pressure and edema formation.

4. Venous Outflow Obstruction

Increased brain capillary pressure from venous outflow obstruction is also thought to contribute to HACE pathophysiology, compounding the vasogenic edema.

5. High-Altitude Headache - Trigeminovascular Activation

The headache of AMS - the cardinal symptom - arises via the trigeminovascular system. Since brain parenchyma is insensate, only the meninges (which have trigeminal sensory fibers) generate pain. Cerebral swelling stretches the meninges, activating this pathway. Both mechanical and chemical factors (arachidonic acid/prostaglandins, inflammation) converge on this final common pathway - which is why NSAIDs and glucocorticoids are effective treatments.

Harrison's, p. 3799

Pathophysiology - Pulmonary Mechanisms (HAPE)

High-Altitude Pulmonary Edema (HAPE)

HAPE is the most lethal form of altitude illness. Its mechanism involves:

Hypoxic Pulmonary Vasoconstriction (HPV) - hypoxia causes pulmonary arteriolar constriction, raising pulmonary vascular resistance
Uneven vasoconstriction - HPV is non-uniform; some vessels constrict more than others, forcing blood through fewer remaining open capillaries
Focal capillary hypertension - in the still-perfused segments, capillary pressure rises dramatically
Capillary stress failure and transudation - the high pressure causes fluid leakage into alveolar spaces, producing pulmonary edema
VEGF and inflammatory cytokines may further increase pulmonary capillary permeability

Individuals with an exaggerated HPV response are particularly susceptible to HAPE. O2 supplementation reverses the process by relieving HPV.

Guyton & Hall, p. 554; Medical Physiology (Boron), p. 689

Risk Factors

Factor	Details
Rate of ascent	Most important modifiable factor
Prior altitude illness	Strong predictor of recurrence
Exertion at altitude	Increases risk; physical fitness does NOT protect
Blunted hypoxic ventilatory response (HVR)	Less hyperventilation → worse hypoxemia → greater vasodilation
Low oxygen saturation on exercise	Independent predictor of severe illness
Patent foramen ovale (PFO)	Allows right-to-left shunting, worsening hypoxemia
Carotid body damage	Neck irradiation/surgery impairs chemoreception
Dehydration, respiratory infection	Additional risk factors
Age >50	May be less susceptible than younger adults

People least likely to develop AMS tend to ventilate more robustly in response to hypoxia, maintaining higher PaO2 and lower PaCO2. The lower PaCO2 blunts the degree of cerebral vasodilation, and the higher PaO2 minimizes pulmonary vasoconstriction.

Medical Physiology (Boron), p. 689; Harrison's, p. 3798

The "Tight Fit" Hypothesis

An attractive (though still speculative) hypothesis is that AMS develops in people who have inadequate cerebrospinal fluid (CSF) buffering capacity to accommodate the brain swelling that occurs at altitude. In people with smaller intracranial CSF compartments, even modest cerebral edema produces symptoms because there is less room to buffer volume expansion.

Harrison's, p. 3798

Acclimatization vs. AMS (Why Most People Adapt)

Normal high-altitude acclimatization involves:

Immediate hyperventilation (raises PaO2, lowers PaCO2 → respiratory alkalosis)
Renal bicarbonate excretion compensates for alkalosis over 2-3 days
Gradual increase in erythropoietin → polycythemia over weeks
Increased 2,3-DPG → rightward shift of O2-Hb curve

AMS develops when acclimatization fails or is overwhelmed by too-rapid ascent. People who acclimatize well have a stronger hypoxic ventilatory response, producing less cerebral vasodilation and less pulmonary hypertension.

Treatment - Physiologic Rationale

Condition	Treatment	Mechanism
All AMS/HACE	Descent	Removes hypobaric hypoxia - the root cause
All AMS/HACE	Supplemental O2	Directly corrects hypoxemia
Mild/Moderate AMS	Acetazolamide (250 mg q12h)	Carbonic anhydrase inhibitor; forces renal bicarbonate excretion, mimicking acclimatization; stimulates ventilation
Moderate AMS / HACE	Dexamethasone (4-8 mg)	Reduces cerebral edema; blocks VEGF upregulation
HAPE	Nifedipine (30 mg ER q12h)	Pulmonary vasodilator; counteracts HPV
If descent impossible	Portable hyperbaric chamber	Simulates descent by increasing ambient pressure

Harrison's, Table 475-1, p. 3799

Summary of Pathophysiologic Cascade

High Altitude
     ↓
Hypobaric Hypoxia (↓ PiO2)
     ↓
↓ PaO2 → ↑ Hypoxic Ventilatory Response (HVR)
     ↓                    ↓
Inadequate HVR        Good HVR → Acclimatization
     ↓
Persistent Hypoxemia
     ↙                  ↘
BRAIN                  LUNG
↑ CBF, vasodilation    HPV → uneven vasoconstriction
↑ Capillary pressure   ↑ Focal capillary pressure
↑ BBB permeability     ↑ Pulmonary permeability
(VEGF, histamine,      (VEGF, cytokines)
nitric oxide)
     ↓                        ↓
Vasogenic cerebral edema   Pulmonary edema
(AMS → HACE)               (HAPE)
     ↓
Trigeminovascular activation → Headache
Raised ICP → Nausea, ataxia, altered consciousness

Recent literature note: A 2024 Nature Reviews Disease Primers review (PMID 38902312) and a 2026 review in Signal Transduction and Targeted Therapy (PMID 41571626) on altitude hypoxia and hypoxemia further explore molecular mechanisms - including HIF-1α upregulation, mitochondrial adaptations, and potential therapeutic targets - consistent with the mechanisms above.

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A detailed medical flowchart showing the pathophysiology of Acute Mountain Sickness (AMS). The flowchart starts at the top with "High Altitude" flowing down through: Hypobaric Hypoxia → Reduced PaO2. Then it splits into two parallel branches. Left branch (Brain pathway): Cerebral Vasodilation → Increased Cerebral Blood Flow → Raised Capillary Pressure + BBB Disruption (VEGF, Histamine, Nitric Oxide) → Vasogenic Cerebral Edema → AMS symptoms (Headache, Nausea, Fatigue) → HACE (Ataxia, Altered Consciousness). Right branch (Lung pathway): Hypoxic Pulmonary Vasoconstriction → Uneven Vasoconstriction → Focal Capillary Hypertension → Pulmonary Edema → HAPE. At the bottom, both branches converge into a Treatment box showing: Descent, Oxygen, Acetazolamide, Dexamethasone, Nifedipine. Use clean medical diagram style with blue and red color coding for brain and lung pathways respectively, white background, clear arrows, bold headings, and professional typography.

Here is the Acute Mountain Sickness Pathophysiology Flowchart covering:

Trigger - High altitude → Hypobaric Hypoxia → ↓ PaO2
Brain pathway (blue) - Cerebral vasodilation → ↑ CBF → BBB disruption (VEGF, Histamine, NO) → Vasogenic cerebral edema → AMS → HACE
Lung pathway (red) - Hypoxic pulmonary vasoconstriction → Uneven constriction → Focal capillary hypertension → HAPE
Treatment box - Descent, O2, Acetazolamide, Dexamethasone, Nifedipine

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