High-Altitude Disorders

1. THE 2-MINUTE PHYSIOLOGY (Rapid Pathophysiology)

• Hypobaria-Induced Hypoxia: High-altitude illness (HAI) encompasses a spectrum of diseases triggered by exposure to low atmospheric pressure (hypobaria) and the resulting low oxygen states during a rapid ascent, typically to elevations above 8,000 feet (2440 m).
• Cerebral Pathology (AMS and HACE): Acute Mountain Sickness (AMS) and its lethal progression, High-Altitude Cerebral Edema (HACE), result from hypoxia-induced cerebral vasodilation. This vasodilation alters the permeability of the blood-brain barrier, leading to capillary leak and progressive cerebral edema.
• Pulmonary Pathology (HAPE): High-Altitude Pulmonary Edema (HAPE) is the leading cause of death from altitude illness. It is driven by uneven hypoxic pulmonary vasoconstriction, which causes severe focal pulmonary hypertension, endothelial capillary leak, and the rapid accumulation of protein-rich, non-cardiogenic pulmonary edema.

2. THE BEDSIDE ACTION PLAN (Rapid ER Management)

• Immediate Stabilization: The universal, definitive treatment for all forms of high-altitude illness is immediate descent and supplemental oxygen. Apply oxygen via nasal cannula or face mask, titrated to maintain an SpO2 >90%.
• Simulated Descent: In remote environments where immediate physical descent is impossible, utilize a portable hyperbaric chamber to simulate descent for HACE and HAPE.
• Targeted Pharmacotherapy & Dosages:
• For HACE: Administer Dexamethasone 8 mg PO, IM, or IV immediately, followed by 4 mg every 6 hours.
• For HAPE: Administer Nifedipine 30 mg extended-release PO every 12 hours to reduce pulmonary artery pressure. If unavailable, Tadalafil 10 mg PO every 12 hours is an alternative. Consider CPAP to stent open fluid-filled alveoli.
• For AMS: Treat symptomatically with analgesics and antiemetics, and consider Acetazolamide (which actively accelerates acclimatization) or Dexamethasone.
• Fluid Resuscitation Caution: While IV hydration combats altitude-related diuresis and improves mild AMS, extreme caution must be used if administering IV fluids to patients with HACE or HAPE, as aggressive volume expansion will dangerously worsen cerebral or pulmonary edema.

3. THE DIAGNOSTIC GRID (Differential Diagnosis & Workup)

• Critical "Can't-Miss" Mimics:
• Carbon Monoxide (CO) Poisoning: Indistinguishable clinically from AMS. Frequently caused by combustion stoves used in poorly ventilated high-altitude tents/cabins.
• Pulmonary Embolism (PE): Hyperviscosity (from dehydration and elevated hematocrit) combined with venous stasis (from cramped sleeping bags/tents or preceding long flights) creates a high risk for PE, which is notoriously misdiagnosed as HAPE.
• Stroke (CVA) / Intracranial Hemorrhage: Must be ruled out in cases of HACE.
• Pneumonia / Asthma Exacerbation: Major mimics of HAPE.
• Diagnostic Workup Strategy:
• Co-Oximetry / Carboxyhemoglobin: Mandatory to rule out CO poisoning in patients presenting with headache and nausea after sleeping in enclosed high-altitude shelters.
• Arterial Blood Gas (ABG): Will typically demonstrate severe hypoxemia and a compensatory respiratory alkalosis in HAPE.

4. THE VISUAL BOARD (ECG / POCUS / Imaging)

• Chest Radiography (CXR): In patients with suspected HAPE (typically presenting 2-4 days after ascent), the CXR will reveal patchy, diffuse bilateral infiltrates characteristic of non-cardiogenic pulmonary edema.
• Electrocardiogram (ECG): Explicitly look for right heart strain (e.g., right axis deviation, S1Q3T3, or T wave inversions in V1-V4) which can be secondary to the severe hypoxic pulmonary vasoconstriction of HAPE or a concomitant PE.
• Neuroimaging (CT/MRI): In HACE, MRI classically demonstrates increased T2 signal intensity indicative of edema localized specifically in the splenium of the corpus callosum.
• Fundoscopy: Look for high-altitude retinal hemorrhages (HARHs), which serve as a visual clue to the microvascular shear stress occurring in the CNS.

5. THE SCORING MATRIX (Risk Stratification & Guidelines)

• The Lake Louise Criteria: This is the validated scoring tool used to clinically diagnose Acute Mountain Sickness.
• Criteria Components: To be diagnosed with AMS, the patient must have:

1. A recent gain in altitude (typically >8,000 feet).
2. The presence of a headache.
3. At least one of the following: Gastrointestinal upset (anorexia, nausea, or vomiting), general weakness/fatigue, or dizziness/lightheadedness.

• Disposition Cutoff: Patients with AMS must not ascend any further until all of their symptoms have completely resolved.

6. THE DANGER ZONE (Pitfalls & Critical Actions)

• Pitfall (Exertional vs. Resting Dyspnea): A deadly cognitive trap is brushing off shortness of breath as a normal physiologic response to altitude. While dyspnea on exertion is nearly universal at high altitudes, dyspnea at rest is highly abnormal and is the critical early warning sign of High-Altitude Pulmonary Edema.
• Pitfall (Ignoring Ataxia): Assuming a climber is just clumsy or fatigued. Ataxia is a hallmark sign of HACE and mandates immediate evacuation.
• CRITICAL ACTION: You must initiate immediate descent for any "hard" neurologic finding (ataxia, slurred speech, altered mentation, focal deficits). Unlike mild AMS, HACE cannot be managed by simply resting in place; failure to descend and administer dexamethasone can result in permanent disability or death.
• CRITICAL ACTION: If CO poisoning is suspected as the cause of the headache/nausea, you must test the patient's enclosed space and assess all other tent-mates who may have been exposed.

7. MCQ MASTERCLASS (Written Exam Tips)

• Buzzword: "Ataxia" + "high altitude" $\rightarrow$ High-Altitude Cerebral Edema (HACE).
• Buzzword: "Pink frothy sputum" + "dyspnea at rest" + "2-4 days after ascent" $\rightarrow$ High-Altitude Pulmonary Edema (HAPE).
• Common Distractor: A question stem will ask which medication actually accelerates acclimatization in a climber with AMS. Distractors will include ibuprofen, oxygen, and dexamethasone. The correct answer is Acetazolamide; the others only treat the symptoms without aiding the physiologic acclimatization process.
• Common Distractor: A patient presents with hypoxia and rales midway down a mountain. Options will offer "slow descent" or "IV fluids." The correct answer is to apply supplemental oxygen and immediate descent.

8. THE BOARDROOM SCRIPT (OSCE & Oral Board Tips)

• High-Stress Articulation: "Given the patient's recent ascent to 10,000 feet, severe headache, and new-onset truncal ataxia, my primary diagnosis is High-Altitude Cerebral Edema. This is a life-threatening neurologic emergency. I am immediately halting any further ascent and preparing for an emergency descent."
• Mandatory Physical Exam Maneuver: "I will perform a focused cardiopulmonary exam to rule out concomitant pulmonary edema, specifically auscultating for rales. I will then perform a complete neurologic exam, including a strict assessment of their gait for ataxia, and a fundoscopic exam to check for high-altitude retinal hemorrhages."
• Communication Pearl: "Nurse, please place the patient on 4 Liters of oxygen via nasal cannula and titrate to an SpO2 greater than 90%. I need to administer Dexamethasone 8 mg IV push right now. Please have respiratory therapy bring the portable hyperbaric chamber to the bedside to simulate descent while we arrange for emergent physical evacuation."