Chronic Mountain Sickness (CMS) / Monge Disease

Definition

Chronic Mountain Sickness (CMS), also called Monge disease, is a maladaptive disorder occurring in long-term residents (>1–2 years) at high altitude (usually >2500–3000 m), characterized by:

• Excessive erythrocytosis
• Severe chronic hypoxemia
• Pulmonary hypertension
• Right heart failure (late)

First described by Carlos Monge Medrano in 1925 in the Peruvian Andes.

Epidemiology

• Common in Andean highlanders
• Less frequent in Tibetans (genetic adaptation differences)
• Seen in:
• Andes (Peru, Bolivia)
• Tibetan plateau
• Himalayas (Ladakh region — relevant to India)

Prevalence:

• 5–30% depending on altitude and ethnicity
• More common in males
• Incidence rises with age

Pathophysiology 

Chronic Hypobaric Hypoxia

At altitude:

• ↓ Barometric pressure → ↓ Inspired PO₂ → ↓ PaO₂
• Chronic hypoxia triggers:

(A) Excessive Erythropoietin (EPO) Production

• Renal hypoxia → ↑ EPO → ↑ RBC mass
• Hb often >21 g/dL (men)
• Hematocrit >65% common

(B) Ventilatory Maladaptation

• Blunted hypoxic ventilatory response
• Relative hypoventilation
• CO₂ retention in advanced disease

(C) Pulmonary Vasoconstriction

• Hypoxic pulmonary vasoconstriction
• Medial hypertrophy of pulmonary arteries
• Progressive pulmonary hypertension

(D) Hyperviscosity

• ↑ Hematocrit → ↑ blood viscosity
• Microvascular flow impairment
• Cerebral symptoms

Molecular Mechanism 

Hypoxia-inducible factor (HIF) pathway:

• HIF-1α stabilization
• ↑ EPO transcription
• Genetic variations:
• Tibetans: EPAS1 mutation → protection
• Andeans: no protective mutation → excessive erythrocytosis

 Diagnostic Criteria (Qinghai Criteria)

Diagnosis requires:

1. Excessive Erythrocytosis:

• Hb ≥21 g/dL (men)
• Hb ≥19 g/dL (women)

2. Plus ≥1 of:

• Dyspnea
• Cyanosis
• Sleep disturbance
• Headache
• Tinnitus
• Dilated veins
• Paresthesia

3. Living at high altitude

 Clinical Features

 Neurological (Hyperviscosity)

• Headache
• Dizziness
• Cognitive impairment
• Sleep apnea
• Tinnitus

 Cardiopulmonary

• Dyspnea on exertion
• Cyanosis
• Loud P2
• RV heave
• Signs of cor pulmonale

 Hematological

• Plethora
• Ruddy complexion

Pulmonary Hypertension & Cor Pulmonale

Progression:
Chronic hypoxia → Pulmonary vasoconstriction → PH → RV hypertrophy → RV failure

Complications:

• Peripheral edema
• Ascites
• Hepatomegaly

Investigations

 Blood Tests

• Hb markedly elevated
• Hct >65%
• ABG: chronic hypoxemia
• Secondary polycythemia pattern

 Pulmonary

• Echo: pulmonary hypertension
• PFT: may show mild restriction

 Imaging (If Neuro Symptoms)

• MRI to rule out stroke (hyperviscosity risk)

 Differential Diagnosis 

Critical Care Complications

• Stroke (Hyperviscosity)
• Acute Decompensated RV Failure
• Venous Thromboembolism
• High-Altitude Pulmonary Edema superimposed

Management 

 Definitive Treatment: DESCENT

• Most effective therapy
• Move to lower altitude (<1500 m)
• Reverses erythrocytosis

 Phlebotomy

Indication:

• Symptomatic hyperviscosity
• Hct >65–70%

Remove:

• 250–500 mL periodically

 Risk: Iron deficiency

 Acetazolamide

Mechanism:

• Carbonic anhydrase inhibitor
• Induces metabolic acidosis
• ↑ Ventilation
• ↓ Hematocrit over time

 Used in both acute mountain sickness & CMS.

 Long-Term Oxygen Therapy (LTOT)

Indicated if:

• Severe hypoxemia
• Cor pulmonale

 Sildenafil / Pulmonary Vasodilators

For:

• Severe pulmonary hypertension

 Emerging Therapies

• HIF pathway modulators (research)
• Rho-kinase inhibitors (experimental)

CMS vs Acute Mountain Sickness vs HAPE