What Is It? Understanding Congenital Pulmonary Airway Malformation (CPAM)

Terminology and Definition

Congenital Pulmonary Airway Malformation (CPAM) is a developmental abnormality of the fetal lung, historically referred to as Congenital Cystic Adenomatoid Malformation (CCAM). Although both terms describe the same condition, CPAM is now the preferred term in current clinical literature and guidelines⁷. CPAM is characterized by the abnormal overgrowth of terminal bronchioles, which results in the formation of cystic or solid masses within the lung. These lesions can include fluid-filled cysts, gland-like tissue structures, or dense solid tissue depending on their classification.

CPAM Lesion (Type II) adjacent to heart

Occurrence and Epidemiology

CPAM is relatively rare, affecting approximately 1 in 25,000 to 35,000 pregnancies³. It is most commonly detected during routine prenatal ultrasound between 18 and 22 weeks of gestation. Although CPAM usually affects only one lobe of the lung, it can develop in any lobe and is typically unilateral. Awareness of its prevalence and detection timing is essential for early intervention and ongoing monitoring.

Location and Types of Lesions

CPAM lesions are categorized based on their sonographic appearance and underlying histology. They are typically classified into macrocystic and microcystic types⁴:

Macrocystic lesions contain large, fluid-filled cysts that are often clearly visible on ultrasound.
Microcystic lesions appear more solid on ultrasound due to the presence of many small cysts that are not individually distinguishable.
Mixed lesions contain a combination of large cysts (macrocysts) and smaller cysts or more solid-appearing areas (microcysts).

The lesion’s size and composition influence its clinical significance and potential for complications.

Mixed CPAM Lesion (Rt)

Prenatal Ultrasound and Imaging Features

Ultrasound is the primary tool for detecting CPAM. The lesions may present as a bright echogenic mass, or as one or multiple fluid-filled cysts. When the lesion is large, it can cause a mediastinal shift, where the heart and surrounding structures are displaced. This displacement may impair fetal swallowing and lead to polyhydramnios.

In complex cases, fetal MRI may be used to further assess the lesion and differentiate it from other thoracic anomalies, such as congenital diaphragmatic hernia. Color Doppler ultrasound can help rule out systemic arterial supply, which would suggest bronchopulmonary sequestration instead of CPAM. It also enhances lesion characterization by visualizing internal flow and vascular architecture.

CPAM vs. CCAM: What’s in a Name?

Though the terms CPAM and CCAM are often used interchangeably, CPAM has become the standardized term in modern diagnostic practice. It reflects a more accurate understanding of the lesion's structure and origin. Clinicians and educators should be familiar with both terms, especially when reviewing older literature or collaborating across institutions that may use varied terminology⁷.

Key Point: CPAM and CCAM refer to the same condition, but CPAM is the preferred term in current clinical guidelines.

Stocker’s Histological Classification & Imaging Correlates⁶

Type	Histology	US Appearance	Cyst Size	Notes
Type 0	Bronchial	Not usually detected antenatally	N/A	Rare, incompatible with life
Type I	Large cysts	Multiple large cysts (3–10 cm)	Macrocystic	Most common
Type II	Small cysts	Multiple <2 cm cysts ± solid areas	Mixed	Often associated with other anomalies
Type III	Tiny cysts	Solid homogenous echogenic mass	Microcystic	May displace the heart
Type IV	Large cysts (alveolar origin)	May appear microcystic antenatally	Microcystic	Associated with malignancy risk

Why It Matters: Clinical Implications

Most CPAM lesions are benign and asymptomatic, but some may enlarge significantly during gestation, leading to complications such as:

Compression of the fetal heart
Displacement of intrathoracic organs
Development of hydrops fetalis, a severe condition involving fluid buildup in multiple fetal compartments

These complications can have serious implications for fetal health and survival, requiring early recognition and coordinated management.

Postnatal Management and Prognosis

Infants with CPAM are often asymptomatic at birth, particularly if the lesion regresses or remains stable during pregnancy. However, surgical resection is commonly recommended between 6 and 9 months of age, especially if the lesion persists or the child develops symptoms such as recurrent respiratory infections or respiratory distress.

The long-term prognosis is excellent for most children following surgery, with normal lung development and minimal complications reported in the majority of cases.

Timing Matters – The 5 Stages of Lung Development

Lung development is a precisely timed and highly coordinated process that spans from early embryonic life well into childhood. Disruptions in this timeline, particularly in the early stages, can result in congenital abnormalities such as Congenital Pulmonary Airway Malformations (CPAMs). Understanding the timing of normal lung development is critical for interpreting prenatal imaging, predicting lesion behavior, and planning appropriate clinical management.

1. The Five Stages of Lung Development⁴

The lung develops in five histologically distinct stages:

Embryonic Stage (Weeks 4–7)

This foundational stage begins with the formation of the lung bud from the ventral foregut endoderm. The trachea and primary bronchi develop, setting the structural basis for all future airway branching. Disruptions at this stage can result in major defects such as pulmonary agenesis (complete absence) or pulmonary aplasia (bronchial stump without parenchyma).

Pseudoglandular Stage (Weeks 5–17)

During this critical period, extensive branching morphogenesis occurs, forming bronchial and terminal bronchiolar structures. This is the stage where most CPAMs originate, especially between weeks 5 and 6. Abnormal or excessive branching during this phase can result in cystic lesions composed of non-functioning airway tissue.

Canalicular Stage (Weeks 16–25)

In this phase, respiratory bronchioles and primitive alveolar ducts begin to form. There is also early vascularization, an important step for enabling future gas exchange. Malformations here may affect both airway and vascular architecture.

Saccular Stage (Weeks 24–38)

Development of terminal sacs (primitive alveoli) begins, and type II pneumocytes start to produce surfactant, essential for postnatal lung function. Lesions detected late in pregnancy are often evaluated to assess their impact on surfactant production and potential need for postnatal support.

Alveolar Stage (Late fetal period – childhood)

This stage continues after birth, with the formation and maturation of alveoli extending into early childhood. While most CPAMs are established earlier, postnatal lung growth can influence compensatory development after lesion resection.

2. Abnormal Lung Bud Formation

Lung development starts with the emergence of the lung bud from the foregut around week 4. Failures at this stage can result in severe abnormalities:

Pulmonary agenesis – no lung formation due to absent bud
Pulmonary aplasia – bud forms but lacks parenchymal development
CPAMs – excessive or disorganized branching resulting in cystic lesions

These abnormalities underscore the importance of early embryologic events in shaping both structure and function of the lungs.

3. CPAMs: When and How They Form

CPAMs are non-functioning lung masses that typically form during the pseudoglandular stage, when airway branching is at its peak. These lesions are usually unilateral, localized to a single lobe, and result from disrupted airway development between weeks 5 and 6. They are histologically classified into Types 0–IV under the Stocker system, each reflecting different developmental origins and tissue patterns.

Clinical Insight:
Type I CPAM, the most common form³, typically arises due to disrupted branching between the primary and secondary bronchi during the pseudoglandular phase. It often presents as a single large cyst or multiple dominant cysts.

4. Molecular and Genetic Mechanisms⁵

Recent studies reveal that CPAMs are not merely structural anomalies but may involve molecular misregulation of key genes responsible for lung development:

Gene / Protein	Role in Lung Development	CPAM Association
SOX-2	Regulates branching morphogenesis and airway identity	Overexpression promotes cyst formation
TTF-1 (NKX2-1)	Controls epithelial cell differentiation	Misexpression found in CPAM tissues
RALDH1	Involved in retinoic acid synthesis	Suggests altered signaling in lesion development

These molecular disruptions may explain the histological diversity and unpredictable behavior of some CPAM lesions.

5. Why Embryology Matters in Clinical Practice

An understanding of lung development stages is not just academic—it directly informs prenatal imaging interpretation, diagnosis, and clinical decision-making. By linking the timing of lesion development with ultrasound findings, clinicians can:

Anticipate the growth trajectory of fetal lung lesions
Differentiate CPAMs from other anomalies like bronchopulmonary sequestration or congenital diaphragmatic hernia
Predict risk of complications like hydrops or respiratory distress
Plan postnatal management including timing of surgical intervention

Chart: Stages of Lung Development⁴

Stage	Timeline	Key Events
Embryonic	Weeks 4–7	Lung bud forms from foregut; trachea and bronchi begin to develop
Pseudoglandular	Weeks 5–17	Branching of airways down to terminal bronchioles; CPAMs often arise here
Canalicular	Weeks 16–25	Formation of respiratory bronchioles and early pulmonary vasculature
Saccular	Weeks 24–38	Terminal sac development; surfactant production begins
Alveolar	Late fetal–childhood	Formation and maturation of alveoli; continues postnatally

Summary Takeaways

CPAMs arise early, typically during the pseudoglandular stage when airways are rapidly branching.
Early disruption in lung bud formation or airway branching can lead to malformations such as CPAM.
Molecular markers like SOX-2 and TTF-1 provide insight into the developmental biology of these lesions.
A solid grasp of embryology allows clinicians to interpret imaging more accurately and guide appropriate management.

General Differential Diagnoses⁷

When a cystic or echogenic mass is identified within the fetal chest, CPAM is a common consideration—but not the only one. Several congenital thoracic anomalies share overlapping imaging features. Below are the most relevant differentials and how they can be distinguished:

1. Bronchogenic Cyst⁸

A bronchogenic cyst is a congenital anomaly that results from abnormal budding of the tracheobronchial tree during early embryonic development.

Typical Imaging Findings:
- Unilocular, well-circumscribed cyst
- Most often located in the mediastinum or lungs
- Appears fluid-filled on ultrasound or MRI
- Does not communicate with the airway, so no air is present
- No systemic arterial supply

How to Differentiate from CPAM:
CPAM often presents with multiple cysts or solid components and may be connected to airways postnatally, whereas bronchogenic cysts are solitary and isolated without systemic vascularity.

2. Pulmonary Sequestration⁴

Pulmonary sequestration refers to non-functioning lung tissue that lacks normal communication with the tracheobronchial tree and receives its blood supply from the systemic circulation, usually the aorta.

Typical Imaging Findings:
- Appears as an echogenic solid mass on ultrasound
- Color Doppler reveals a feeding vessel arising from the aorta
- Can be intralobar or extralobar
- May coexist with CPAM in what’s termed a hybrid lesion

How to Differentiate from CPAM:
Doppler is key—systemic arterial supply confirms sequestration. CPAM typically lacks this vascular pattern unless part of a hybrid lesion.

3. Congenital Diaphragmatic Hernia (CDH)¹⁰

CDH results from a defect in the diaphragm, allowing abdominal organs (stomach, bowel, liver) to herniate into the thoracic cavity.

Typical Imaging Findings:
- Stomach and/or bowel loops seen in the chest
- Mediastinal shift toward the opposite side
- Stomach absent in abdominal cavity
- Often associated with polyhydramnios
- Discontinuity of the diaphragm may be visible

How to Differentiate from CPAM:
CDH involves actual abdominal contents in the thorax, not cystic lung tissue. MRI and careful evaluation of stomach position are essential for differentiation.

4. Congenital Lobar Emphysema (CLE)⁹

CLE is characterized by overinflation of one pulmonary lobe, often due to defective bronchial cartilage. Unlike CPAM, it is not cystic.

Typical Imaging Findings:
- Appears as a hyperlucent (radiolucent), overinflated lobe on imaging
- No discrete cysts or mass
- Most commonly affects the left upper lobe
- Normal pulmonary blood supply
- Can produce a mass effect but remains aerated

How to Differentiate from CPAM:
CPAM may contain cysts or solid components; CLE is typically air-filled and not present until after birth, making it less frequently diagnosed prenatally.

5. Localized Congenital Cystic Bronchiectasis⁹

This rare anomaly involves chronic dilation of bronchi, leading to cystic spaces that may contain air-fluid levels.

Typical Imaging Findings:
- Cystic spaces with air-fluid levels
- More often diagnosed postnatally or in older children
- Seen on CT, particularly when Type I CPAM is suspected
- Usually results from scarring or traction bronchiectasis

🔍 How to Differentiate from CPAM:
The diagnosis of bronchiectasis is rarely made prenatally and typically involves acquired or post-inflammatory changes, whereas CPAM is congenital and identified earlier in gestation.

Comparison Chart: CPAM vs. Common Differentials⁷

Condition	Cystic?	Solid?	Systemic Blood Supply?	Key Imaging Clues	Timing of Diagnosis
CPAM	Yes (macro/micro)	Sometimes	No	Cystic or echogenic mass; may cause mediastinal shift	Mid-trimester (18–22 weeks)
Bronchogenic Cyst	Yes	No	No	Single unilocular cyst, no airway connection	Often 2nd trimester or postnatal
Pulmonary Sequestration	No	Yes	Yes (from aorta)	Echogenic solid mass with Doppler-visible feeding vessel	Second trimester
Congenital Diaphragmatic Hernia	Variable	No	No	Bowel/stomach in chest, stomach absent in abdomen	Often detected mid-trimester
Congenital Lobar Emphysema	No	No	No	Overinflated lobe, appears radiolucent postnatally	Rarely prenatal
Cystic Bronchiectasis	Yes	No	No	Air-fluid levels, typically postnatal on CT	Rare prenatal; usually postnatal

Conclusion: Why Differentiating CPAM Matters

Correctly distinguishing CPAM from other thoracic anomalies is essential for accurate counseling, prognosis, and perinatal planning. While CPAM is the most common congenital lung lesion, others—such as sequestration, CDH, or bronchogenic cysts—require different management approaches.

Utilizing key imaging tools like color Doppler and fetal MRI, along with an understanding of typical timing and lesion characteristics, allows for greater diagnostic clarity. Recognizing hybrid lesions (e.g., CPAM + sequestration) and staying alert to mimics like CDH can prevent misdiagnosis and optimize outcomes.

Summary Snapshot

Feature	CPAM / CCAM
Condition Type	Congenital lung malformation
Terminology	CPAM = modern term, CCAM = older term
Frequency	1 in 25,000–35,000 pregnancies
Detection	Prenatal ultrasound, confirmed by MRI
Risk Factors	Rare complications (hydrops, cardiac compression)
Postnatal Treatment	Surgical removal in infancy
Prognosis	Excellent with appropriate management

Prenatal Diagnostic Imaging: A Focused Approach to Congenital Pulmonary Airway Malformation

Chapter 1 Introduction to Congenital Pulmonary Airway Malformation: Foundations for Prenatal Diagnos