Fetal Lung Lesions

Congenital Malformations of the Respiratory Tract

Embryologic development of the respiratory tract requires the appropriate growth of the upper airway and the six ‘trees’ that make up the lower respiratory tract – bronchial, arterial (systemic and pulmonary), venous (systemic and pulmonary) and lymphatic – together with a normal thoracic volume, a normal thoracic skeletal structure and normal neuromuscular function.

Defects in any of these elements (except the systemic venous system because there are none known) can affect anatomical organisation resulting in a variety of congenital abnormalities. Whereas some lesions can be detected by direct prenatal sonographic visualisation, other lesions are suspected because of the presence of nonspecific findings (e.g., mediastinal shift) or form part of a more generalised genetic syndrome, for example, the pulmonary hypoplasia seen in skeletal dysplasias such asphyxiating thoracic dystrophy. Other congenital malformations of the lungs only become apparent in the postnatal period when they cause symptoms.

Not all congenital malformations have a detrimental impact on a fetus or postnatal respiratory function. Advances in sonographic technology allow prenatal detection of subtle lesions which may have no immediate clinical impact. A lack of evidence surrounding the natural history of asymptomatic cystic lung lesions has resulted in divergent postnatal management strategies and difficult prenatal counseling.

In this chapter, we will identify thoracic malformations, excluding congenital diaphragmatic hernia (covered in Chapter 31 ), that can be detected directly on prenatal sonography, describe a system to classify these lesions sonographically, define their salient pathological features and discuss the merits of prenatal and postnatal management options.

Thoracic Malformations Detected on Prenatal Ultrasound

Thoracic malformations detectable on prenatal sonography are detailed in Table 30.1 ; however, a definitive diagnosis of these lesions (excluding congenital diaphragmatic hernia (CDH)) requires histologic confirmation. A prenatal or postnatal diagnosis is not possible by radiologic means alone. The imaging appearance of different types of lesions can be identical, rendering specific pathological diagnoses redundant and risking confusion in communication between medical professionals and families. Moreover, it is now apparent that there can be considerable overlap in histologic features within lesions, further highlighting the increasing complexity of diagnosis and the limitations of diagnosis based on imaging modalities alone.

In light of these difficulties, a system whereby malformations detected by prenatal sonography are described meticulously in simple language, based on their appearance, and without the presumption of a single pathological diagnosis has been recommended. Within this system, all thoracic malformations are described under the umbrella term congenital thoracic malformation (CTM). Malformations are then defined further using descriptive terms, including the presence and size of cysts, the presence of a feeding vessel, the degree of echogenicity, the presence or absence of mediastinal shift, polyhydramnios and the presence of anomalies in other systems. In practical terms, lesions have been most usefully classified sonographically as either macrocystic or microcystic (see Table 30.1 ).

Detection of CTMs, often at the routine 20-week anomaly scan, allows detailed planning of further prenatal management, including serial sonographic monitoring, delineation of the lesion and intrauterine therapeutic interventions if required. Planning for appropriate neonatal support on delivery can be prepared in advance. The exception to this pattern are pleural effusions, which often present later when scanning is undertaken because of a suspicion of increased amniotic fluid or as an incidental finding on ultrasound (USS) undertaken during the third trimester.

The appeal of enhanced prenatal radiologic definition of lung CTMs has led to the exploration of magnetic resonance imaging (MRI) as an additional modality. MRI has been used in the delineation of fetal lung lesions and the identification of feeding vessels, although this is readily done using Doppler ultrasound. Whether this enhanced imaging provides any additional information of practical value over ultrasound alone has yet to be determined, and in most centres at present, MRI is not part of routine practice.

Macrocystic Lung Lesions

Macrocytic lung lesions include congenital pulmonary airway malformations (CPAMs), bronchogenic cysts, enteric cysts, bronchial atresia and congenital lobar emphysema (see Table 30.1 ). The sonographic appearance includes a cystic lesion(s) of varying size in the thorax with or without mediastinal shift ( Fig. 30.1 ). An essential differential diagnosis to consider on identification of a macrocystic lung lesion is a left-sided diaphragmatic hernia, in which the stomach or bowel herniated into the thorax can be mistaken for a cystic structure. The crucial importance of this differentiation is the subsequent management strategy for CDH, particularly the place of delivery, associations with chromosomal abnormalities and genetic syndromes and general prognosis. Additional sonographic features that assist in the correct identification of a CDH include the absence of a stomach within the abdomen, the visualisation of peristalsis within the thorax, the paradoxical movement of abdominal viscera within the thorax during fetal breathing movements and absence of a diaphragm. However, differentiation can remain difficult on occasion. In a case series of 110 fetuses diagnosed with a CTM, two were later identified as having a diaphragmatic hernia, both after serial prenatal scanning.