# The Pediatric Chest: Normal Development and Common Pathology

## Introduction

Imaging the pediatric chest requires an understanding of **age-related anatomic variations** that can mimic pathology. The thymus, airways, and lungs undergo significant changes from birth through adolescence. This lecture reviews normal developmental anatomy, common congenital anomalies, and acquired pediatric chest pathology.

## Normal Developmental Anatomy

### The Thymus

The normal thymus is prominent in **neonates and infants**, occupying the anterior mediastinum. On a frontal radiograph, the right thymic lobe produces the classic **"sail sign."** The thymus may appear wavy along its lateral margin due to indentation by the anterior ribs, producing the **thymic wave sign**. Importantly, the thymus **does not displace or compress** adjacent structures but rather molds around them. Stress, illness, or steroid therapy causes **thymic involution**, and rebound thymic hyperplasia may occur during recovery.

### Airways

The **pediatric trachea** is more pliable than in adults and may buckle to the right on expiration, a phenomenon known as pseudobuckling. Subglottic tracheal narrowing is normal in children and should not be confused with croup. The **carina** is located at approximately T4 in infants, compared to T5-T6 in adults.

### Lungs and Vasculature

Neonatal lungs may retain fetal fluid for 24-48 hours, producing **transient tachypnea of the newborn**. Pulmonary vascularity assessment helps distinguish cardiac from pulmonary causes of respiratory distress. **Increased pulmonary vascularity** suggests a left-to-right shunt or fluid overload.

## Congenital Anomalies

### Congenital Diaphragmatic Hernia (CDH)

**Bochdalek hernia** is a posterolateral defect that occurs on the left side in 80-90% of cases and is the most common type of CDH. The chest radiograph shows bowel loops in the thorax with contralateral mediastinal shift, and the condition is associated with **pulmonary hypoplasia** on the affected side. **Morgagni hernia** is an anterior, right-sided defect that is less common and often presents later in childhood.

### Congenital Pulmonary Airway Malformation (CPAM)

Previously termed congenital cystic adenomatoid malformation (CCAM), CPAM consists of abnormal lung tissue that appears as a cystic or solid mass replacing normal parenchyma. **Type 1** is the most common, featuring large cysts greater than 2 cm with a generally good prognosis. **Type 2** features smaller cysts (0.5-2 cm) and is more commonly associated with other anomalies. Air-fluid levels may be present after birth when the lesion communicates with the airways.

### Bronchopulmonary Sequestration

A bronchopulmonary sequestration is non-functioning lung tissue with a **systemic arterial supply**, typically from the aorta. The **intralobar** type accounts for 75% of cases and lies within the visceral pleura, with venous drainage via the pulmonary veins. The **extralobar** type (25%) has its own pleural covering and drains via systemic veins such as the azygos or IVC. CT angiography or MR angiography identifies the **aberrant feeding artery**, which is diagnostic.

## Acquired Pathology

### Neonatal Respiratory Distress

| Condition | Radiographic Findings | Clinical Context |
|-----------|----------------------|-----------------|
| RDS (surfactant deficiency) | Diffuse granular opacities, air bronchograms | Premature infant |
| TTN (retained fetal fluid) | Perihilar streaky opacities, fissural fluid; resolves 24-72 hrs | Term/near-term, C-section |
| Meconium aspiration | Coarse asymmetric opacities, hyperinflation | Post-dates, meconium-stained |
| Neonatal pneumonia | Indistinguishable from RDS | GBS; may have pleural effusion |

**Respiratory distress syndrome (RDS)** results from surfactant deficiency in premature infants and presents with diffuse granular opacities and air bronchograms. **Transient tachypnea of the newborn (TTN)** is caused by retained fetal lung fluid, producing perihilar streaky opacities and fluid in the fissures that resolve within 24-72 hours. **Meconium aspiration syndrome** produces coarse, asymmetric opacities with hyperinflation and may cause pneumothorax. **Neonatal pneumonia** is radiographically indistinguishable from RDS, with Group B Streptococcus being the classic organism.

### Pediatric Airway Disease

**Croup (laryngotracheobronchitis)** produces the **steeple sign** on the AP radiograph, representing subglottic narrowing. **Epiglottitis** produces the **thumb sign**, representing a swollen epiglottis on the lateral radiograph, though it is now rare due to Hib vaccination. **Foreign body aspiration** is most common in children ages 1-3, with the right main bronchus more commonly affected. Inspiratory and expiratory radiographs or **decubitus views** demonstrate air trapping. Most aspirated foreign bodies in children are radiolucent, such as peanuts.

### Round Pneumonia

Round pneumonia is a phenomenon seen in children under **8 years of age** due to underdeveloped pores of Kohn and channels of Lambert, which normally allow collateral ventilation. It appears as a **well-defined round opacity**, often in the posterior lower lobes, and can mimic a mass. Clinical history of fever and cough is critical for the diagnosis, and follow-up imaging after antibiotic therapy should show resolution.

## Clinical Pearls

A prominent thymus in an infant is **normal** and should not be mistaken for mediastinal pathology; it does not cause mass effect. The **sail sign** and **thymic wave sign** are characteristic normal findings on pediatric chest radiographs. Round pneumonia is essentially unique to children under 8 and should resolve with antibiotics; if the opacity persists, further workup is warranted. Always obtain **inspiratory and expiratory films** (or bilateral decubitus views in uncooperative children) when foreign body aspiration is suspected.

## References

1. ACR Appropriateness Criteria: Respiratory Illness in the Neonate. *J Am Coll Radiol*. 2019;16(5S):S318-S326.
2. Lee EY, et al. Pediatric Chest Imaging: Current Concepts. *Radiol Clin North Am*. 2017;55(4):683-706.
3. Newman B. Congenital Bronchopulmonary Foregut Malformations: Concepts and Controversies. *Pediatr Radiol*. 2006;36(8):773-791.
4. Imaging of Pediatric Airway Emergencies. *Radiographics*. 2020;40(5):1257-1275.