- Most common foetal lung mass. Usually incidental, no genetic cause.
- Solid-appearing (microcystic) to multilocular cystic (macrocystic) lesion
- Supplied by pulmonary artery. 95% unilateral, affecting 1 lobe.
- Lesion growth stabilises or regresses by 29 weeks. Hydrops confers poor prognosis (95% mortality).
Description
Congenital Pulmonary Airway Malformation (CPAM), previously known as Congenital Cystic Adenomatoid Malformation (CCAM), is a developmental hamartomatous abnormality of lung with adenomatoid proliferation of cysts resembling bronchioles. This developmental anomaly of the lower respiratory tract predominantly affects one lobe of the lung, leading to cystic masses or overgrowth of the normal lung tissue.
Pathogenesis
CPAM is a result of an early embryologic insult at 6-7 weeks of gestation, leading to an abnormal proliferation of bronchial structures. It results from abnormal branching morphogenesis with a hamartomatous proliferation of bronchial structures at the expense of the alveoli. The aetiology is unknown, but the theory of airway obstruction during a critical period of development is widely accepted.
Subtype
- Type I (Macrocystic): Characterised by large cysts (>2 cm in size). Most common subtype.
- Type II (Microcystic): Characterised by small cysts (<2 cm in size).
- Type III (Solid): Rare, characterised by non-cystic (or microcystic, < 5 mm), solid-appearing mass of lung tissue.
- Type IV (Peripheral cystic): Very rare, characterised by peripheral, large cysts (typically > 10 cm).
Epidemiology, Risk Factors & Associations
- Occurs in 1 in 25,000 to 35,000 pregnancies.
- No known gender preference or hereditary pattern.
- No genetic causes.
- Associated with other congenital anomalies in up to 50% of cases.
Clinical Features
- Asymptomatic in many neonates.
- May present with respiratory distress in the neonatal period if large.
- In older children and adults, can present with recurrent respiratory infections, pneumothorax or haemoptysis.
Complications
- In utero, compression of foetal heart or great vessels may causes foetal hydrops.
- Compression of normal foetal lung may rarely cause pulmonary hypoplasia.
- Potential for respiratory distress in neonates.
- Increased risk of malignancy in adulthood, particularly bronchoalveolar carcinoma.
- Recurrent pneumothorax, haemopneumothorax, pyopneumothorax.
Pathological Features
Histopathology
- Macroscopic: Can range from macrocystic to microcystic or solid appearances.
- Microscopic: Composed of bronchiolar or bronchial-type epithelium with various degrees of cystic changes.
Serology
Not applicable.
Biochemistry
Not applicable.
Radiological Features
General Features
- Cystic, solid or mixed cystic-solid lung lesions, usually unilateral and unilobar.
- May displace the heart
- CPAM volume ratio (CVR) is used to predict foetal prognosis.
- CPAM volume = length x width x height x 0.52
- CPAM volume is then divided by head circumference
- CVR > 1.6 signifies increased risk of hydrops and foetal demise
- Associated polyhydramnios may be seen (amniotic fluid index > 25 cm, maximal vertical pocket > 8cm, overall amniotic fluid volume >1500 – 2000 mL
- No lobar predilection (unlike pulmonary sequestration and congenital lobar overinflation)
- Communicates with airway (unlike pulmonary lobar sequestration)
XR
- Type I and II CPAM may demonstrate an air-filled multicystic lesion
- Type III lesions appear solid
- Lesions may change in size on interval imaging with expansion from collateral ventilation via pores of Kohn
- Day 1 CXR: Dense lungs, increased volume on the affected side.
- Day 2+ CXR: Fluid resorption, replaced by air-containing spaces.
US
- B-mode:
- Microcystic CPAM: Uniformly echogenic, well-defined mass with cysts < 5 mm
- Macrocystic CPAM: Poorly defined mass with ≥ 1 cyst of varying sizes > 5 mm
- Antenatal: Hydrops foetalis and polyhydramnios may develop
- Colour Doppler: Vascular supply from pulmonary artery. Venous drainage to pulmonary vein.
MRI
- T2WI:
- Microcystic CPAM: Uniformly T2-hyperintense mass
- Macrocystic CPAM: T2 hyperintense with discrete cysts
Differential Diagnosis
Imaging-based
Differentials for foetal echogenic lung:
- Pulmonary sequestration: Characterised by the presence of systemic arterial supply from aorta, which is not seen in CPAM, detectable on ultrasound. 90% left-sided, lower lobe predominant. May be intralobar or extralobar. Grayscale appearance indistinguishable from microcystic CPAM. Ipsilateral pleural effusion highly suggestive. Does not communicate with airway (unlike CPAM).
- Bronchial atresia
- Usually unilateral or uni-lobar echogenic lung
- If bilateral, consider Congenital High Airway Obstruction Syndrome (CHAOS)
- Congenital Diaphragmatic Hernia: Abnormally located fluid-filled stomach with evidence of peristalsis. Abdominal circumference often small.
- Bronchogenic, oesophageal duplication or neurenteric cyst
- Teratoma: Solid and cystic components of mass within mediastinum or pericardial space. Calcifications are highly suggestive.
Management
Management varies based on symptoms and risk of malignancy.
- Asymptomatic neonates might be monitored with serial imaging, with increased frequency if there are concerns for developing hydrops.
- Symptomatic neonates and those with large lesions or hydrops foetalis often need prenatal or immediate postnatal surgery. Microcystic CPAMs may be treated with betamethasone intramuscular injections.
- In older children and adults, surgical resection is recommended due to the risk of recurrent infections and potential malignant transformation. CPAM volume ratio is used to predict foetal prognosis.