Env: Altitude Medicine: AMS, HAPE, HACE for MRCP Part 1
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TL;DR
Env: Altitude Medicine: AMS, HAPE, HACE is a frequently tested environmental medicine topic in MRCP Part 1. Candidates should be able to distinguish acute mountain sickness (AMS), high-altitude pulmonary oedema (HAPE), and high-altitude cerebral oedema (HACE), understand the underlying physiology, and know the emergency management principles. Questions commonly focus on acclimatisation, hypobaric hypoxia, acetazolamide, and the recognition of life-threatening complications.
Why altitude medicine matters in MRCP Part 1
Altitude medicine is well suited to MRCP-style questions because symptoms overlap and management differs significantly between conditions.
Candidates are expected to:
Recognise AMS, HAPE, and HACE clinically
Understand acclimatisation physiology
Identify high-risk ascent patterns
Know prevention strategies
Select appropriate emergency management
Distinguish HAPE from pneumonia or heart failure
A classic MRCP trap is focusing excessively on oxygen saturation values rather than the overall clinical picture.
Core physiology: what happens at altitude?
At high altitude, atmospheric pressure decreases. Although oxygen concentration remains approximately 21%, the partial pressure of inspired oxygen falls, producing hypobaric hypoxia.
Physiological adaptations
The body responds through several mechanisms:
Hyperventilation
Increased sympathetic activity
Tachycardia
Increased cardiac output
Pulmonary vasoconstriction
Increased erythropoietin production over time
Hyperventilation initially causes respiratory alkalosis, which is another common examination point.
Pulmonary vasoconstriction plays a key role in the development of HAPE.
Acute Mountain Sickness (AMS)
AMS is the mildest and most common altitude illness.
Typical onset
Usually occurs within 6–12 hours after ascent
More common above 2500 metres
Often follows rapid ascent without acclimatisation
Symptoms
The hallmark symptom is headache.
Other features include:
Nausea
Dizziness
Fatigue
Poor sleep
Anorexia
Malaise
Examination findings
Examination may be largely normal apart from:
Mild tachycardia
Mild tachypnoea
Diagnosis
AMS is a clinical diagnosis based on:
Recent ascent
Headache
Compatible symptoms
Management
Mild AMS
Stop ascent
Rest
Oral hydration
Analgesia
Moderate symptoms
Acetazolamide
Observation
Severe or worsening symptoms
Descent
Supplemental oxygen
High-Altitude Pulmonary Oedema (HAPE)
HAPE is a potentially fatal non-cardiogenic pulmonary oedema caused by exaggerated hypoxic pulmonary vasoconstriction.
It is one of the highest-yield altitude emergencies in MRCP Part 1.
Clinical presentation
Typical symptoms include:
Dyspnoea on exertion progressing to dyspnoea at rest
Dry cough
Reduced exercise tolerance
Chest tightness
Pink frothy sputum in severe disease
Examination findings
Tachycardia
Tachypnoea
Cyanosis
Bibasal crackles
Hypoxia
Low-grade fever may occur, which can mislead candidates into diagnosing pneumonia.
AMS vs HAPE vs HACE
Feature | AMS | HAPE | HACE |
Main symptom | Headache | Breathlessness | Ataxia/confusion |
Organ system | General systemic | Pulmonary | Neurological |
Fever | Rare | Mild possible | Rare |
Crackles | No | Yes | No |
Mental state changes | No | Sometimes late | Prominent |
Immediate danger | Moderate | Severe | Severe |
Investigations in HAPE
Chest X-ray
Patchy bilateral infiltrates
Non-cardiogenic oedema pattern
ECG
Sinus tachycardia
Oxygen saturation
Markedly reduced
Management of HAPE
Immediate priorities
Descent
High-flow oxygen
Rest
Pharmacological therapy
Nifedipine may reduce pulmonary artery pressure
Phosphodiesterase inhibitors can be used in prevention
Important MRCP point
Diuretics are not standard treatment because HAPE is not caused by fluid overload or left ventricular failure.
High-Altitude Cerebral Oedema (HACE)
HACE represents severe altitude illness with cerebral oedema and neurological dysfunction.
It often develops as progression from severe AMS.
Pathophysiology
Hypoxia causes:
Cerebral vasodilatation
Capillary leakage
Raised intracranial pressure
Symptoms
Severe headache
Confusion
Hallucinations
Drowsiness
Reduced consciousness
Most important sign
Ataxia is the classic hallmark feature tested in MRCP examinations.
Management of HACE
Immediate descent
Oxygen therapy
Dexamethasone
Portable hyperbaric chamber if descent impossible
Untreated HACE can rapidly become fatal.
Acetazolamide: a favourite MRCP drug
Acetazolamide is commonly tested in altitude medicine.
Mechanism of action
Acetazolamide inhibits carbonic anhydrase, causing bicarbonate diuresis and mild metabolic acidosis. This stimulates ventilation and improves acclimatisation.
Uses
Prevention of AMS
Reduction of symptoms during ascent
Side effects
Paraesthesia
Polyuria
Taste disturbance
Metabolic acidosis
Examination pearl
Acetazolamide assists acclimatisation but does not replace gradual ascent.
The 5 most tested altitude medicine subtopics
1. Differentiating AMS, HAPE and HACE
Clinical recognition remains central to examination questions.
2. Respiratory alkalosis at altitude
Hyperventilation causes low carbon dioxide levels and alkalosis.
3. Emergency management
Descent and oxygen remain definitive therapies.
4. Drug therapy
Know the indications for:
Acetazolamide
Dexamethasone
Nifedipine
5. Non-cardiogenic pulmonary oedema
HAPE is not caused by heart failure.
Practical mini-case
A 28-year-old climber ascends rapidly to 3400 metres. Twelve hours later, she develops headache, nausea, fatigue, and dizziness. Examination is otherwise normal.
Most likely diagnosis
Acute mountain sickness (AMS)
Why?
The combination of recent ascent and headache with constitutional symptoms strongly suggests AMS. There are no pulmonary findings suggestive of HAPE and no neurological deficits suggesting HACE.
SBA-style MRCP question
A 34-year-old trekker at 4300 metres develops dyspnoea at rest, tachycardia, bilateral crackles, and hypoxia. Temperature is 37.4°C.
What is the most appropriate immediate management?
A. Intravenous furosemideB. Oral amoxicillinC. Immediate descent and oxygenD. Fluid restrictionE. Aspirin
Correct answer
C. Immediate descent and oxygen
Explanation
This patient has classic HAPE. The definitive treatment is descent and oxygen therapy. Furosemide is not routinely indicated because HAPE is non-cardiogenic.
Common pitfalls in MRCP Part 1
Confusing HAPE with cardiogenic pulmonary oedema
Missing ataxia as a hallmark sign of HACE
Forgetting that oxygen concentration remains 21% at altitude
Assuming AMS always requires emergency descent
Treating HAPE as simple pneumonia without considering altitude exposure
Practical study checklist
Use this checklist before your MRCP Part 1 examination:
Revise acclimatisation physiology
Memorise hallmark symptoms:
AMS = headache
HAPE = dyspnoea/crackles
HACE = ataxia/confusion
Learn altitude drug indications
Review respiratory alkalosis compensation
Practise emergency medicine SBAs
Compare pneumonia versus HAPE presentations
Understand non-cardiogenic pulmonary oedema mechanisms
You can consolidate revision using:MRCP Part 1 lectures
For timed practice sessions, try:MRCP mock tests
Suggested related reading:
British Thoracic Society guidance on altitude illness
FAQs
What altitude commonly causes acute mountain sickness?
AMS usually develops above 2500 metres, particularly after rapid ascent without adequate acclimatisation.
What is the hallmark sign of HACE?
Ataxia is one of the most important and highly tested signs of HACE in MRCP examinations.
Why is acetazolamide used in altitude illness?
Acetazolamide promotes metabolic acidosis, which stimulates ventilation and improves acclimatisation.
Is HAPE cardiogenic?
No. HAPE is a non-cardiogenic pulmonary oedema caused by hypoxic pulmonary vasoconstriction and capillary leakage.
What is the most important treatment for severe altitude illness?
Immediate descent and oxygen therapy are the most important interventions for both HAPE and HACE.
Ready to start?
Altitude medicine is a compact but extremely testable area within MRCP Part 1. Candidates who understand acclimatisation physiology and can distinguish AMS, HAPE, and HACE are well positioned to answer integrated respiratory and emergency medicine questions.
For structured revision and exam-focused practice, explore:
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