Residency · Residency · Diagnostic Radiology

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

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.

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