Morphogenesis, Malformations, and Natural Course of CCAM

I. Steps in Morphogenesis of Respiratory System (3 Marks)

Lung development occurs in 5 overlapping stages.

Embryonic Stage (Weeks 4–7):
- Lung bud (respiratory diverticulum) arises from the ventral foregut.
- Tracheoesophageal septum separates trachea from esophagus.
- Branching into lobar and segmental bronchi.
Pseudoglandular Stage (Weeks 5–17):
- Conducting airways form up to terminal bronchioles.
- Appearance resembles a gland.
- Key: No gas exchange is possible; respiration is impossible if born now.
Canalicular Stage (Weeks 16–26):
- Formation of respiratory bronchioles and alveolar ducts.
- Vascularization: Capillaries invade the mesenchyme.
- Viability: Survival becomes possible towards the end (24-26 weeks) as the blood-gas barrier forms.
Saccular Stage (Weeks 24–36):
- Formation of Terminal Sacs (primitive alveoli).
- Surfactant: Type II pneumocytes begin secretion (significant by 28-32 weeks).
Alveolar Stage (Week 36 – 8 Years):
- Development of true alveoli.
- Septation: Secondary septation increases surface area.
- Note: 85-90% of alveoli form postnatally.

These are often grouped as Congenital Thoracic Malformations (CTMs).

A. Bronchopulmonary Anomalies

Congenital Pulmonary Airway Malformation (CPAM/CCAM): Hamartomatous cystic lesions.
Congenital Lobar Emphysema (CLE): Overdistension of a lobe (usually LUL/RML).
Bronchogenic Cyst: Foregut duplication cyst.
Pulmonary Agenesis / Aplasia / Hypoplasia: Failure of development.
Bronchial Atresia: Proximal obliteration with distal mucous impaction.

B. Vascular/Combined Anomalies

Pulmonary Sequestration: Non-functioning lung tissue with anomalous systemic blood supply.
- Intralobar: Within normal pleural cover.
- Extralobar: Separate pleural cover.
Scimitar Syndrome: Hypoplastic right lung + Partial anomalous pulmonary venous drainage (to IVC).
Pulmonary AV Malformation.

The natural history varies significantly from fetal life to childhood.

Growth Phase: Lesions typically grow rapidly between 20–25 weeks of gestation.
Plateau/Regression: Growth usually plateaus after 26 weeks, and many lesions appear to shrink relative to the growing fetus.
Complications:
- Mediastinal Shift: Large lesions compress the heart and contralateral lung.
- Hydrops Fetalis: Esophageal compression (polyhydramnios) and cardiac compression (venous obstruction) can lead to hydrops. CPAM Volume Ratio (CVR) > 1.6 predicts high risk of hydrops and fetal demise.
Spontaneous Resolution: Some lesions become “isoechoic” or invisible on prenatal USG, but true histological disappearance is rare (persisting as small cysts/abnormalities on postnatal CT).

Neonatal Respiratory Distress: ~25% of neonates present at birth with distress due to air trapping/mass effect (requires emergency surgery).
Recurrent Infection: If untreated, the cyst acts as a reservoir for infection, leading to recurrent pneumonia or lung abscess in infancy/childhood.

There is a small but definite risk of malignant transformation within the CPAM, typically in Type 1 (large cyst) or Type 4 lesions.

Pleuropulmonary Blastoma (PPB): Strongly associated with Type 4 CPAM (often indistinguishable).
Bronchioloalveolar Carcinoma (BAC): Late complication (adulthood) in Type 1 CPAM.
Management implication: This risk drives the recommendation for elective resection of asymptomatic CPAMs.

Hydrops: Poor prognosis (survival <50% without intervention).
Non-Hydrops: Excellent survival (>95%).
Post-Resection: Excellent long-term lung function due to compensatory growth of remaining lung.