Ultrasound Evaluation of the Fetal Thorax

Summary of Key Points

The most common chest masses are congenital pulmonary airway malformations (CPAMs) and congenital diaphragmatic hernias (CDHs). For both, the displacement of the heart from its normal position is the most commonly recognized ultrasound finding.
CPAMs are unilateral 98% of the time and appear as echogenic areas of the lung, with or without identified cysts. The CPAM volume ratio (CVR) can be used to subdivide these malformations into high- and low-risk categories for development of fetal hydrops.
Fluid-filled dilated airways should raise suspicion for bronchial or laryngeal atresia. Bilateral hyperechogenic lungs with fluid-filled airways are characteristic of congenital high airway obstruction syndrome (CHAOS), whereas a unilateral hyperechogenic lung with fluid-filled airways suggests bronchial atresia or congenital lobar emphysema (CLE).
Hydrothorax may be primary or secondary. Primary hydrothorax is usually chylous in origin and may progress to hydrops, usually associated with significant skin edema. A large unilateral hydrothorax, even if associated with significant hydrops, may respond to prenatal drainage and shunting.
CDHs are left-sided and posterior in 85% of cases. They are associated with other anomalies (structural and chromosomal) in 40% to 50% of cases.
Prognosis for CDHs depends on the presence of associated abnormalities, the gestational age at delivery, and the degree of pulmonary hypoplasia.
Estimation of lung volume by two-dimensional ultrasound lung-head ratios or volumes obtained through magnetic resonance imaging (MRI) or three-dimensional ultrasound may help guide counseling.

The neonatal identification of chest lesions frequently relies on decompensation of the newborn. Prenatal identification allows for appropriate triaging for site of delivery as well as early and appropriate treatment. Better definition and prenatal identification of chest lesions along with enhanced understanding of the natural history of these lesions has resulted in improvements in counseling. In addition, prenatal therapies are now available for many different types of chest abnormalities. The appropriate use of these therapies depends on accurate identification and assessment of the lesion and the associated pathologic changes. Identifying prognostic factors facilitates continued improvement in the understanding of the natural course of the disease.

This chapter will explore the range of chest lesions that can be identified prenatally and will discuss factors associated with a thorough sonographic diagnosis to facilitate complete and accurate counseling for patients.

Embryology

Although an intact pulmonary system is not required for intrauterine viability, the prenatal development of the respiratory system is integral to ex utero life and survival. To allow successful transition to neonatal life, fetal lungs must proceed through both structural and functional maturation processes, which occur from the early embryonic period and continue after birth. During structural development, there is branching of the airways and development of alveolar spaces, allowing gas exchange to occur once the first breath is taken at delivery. Functional development results in the creation of a surfactant system. This system is composed of phospholipids that decrease alveolar surface tension, inhibiting alveolar collapse during exhalation. The surfactant system develops in the third trimester and is typically mature at 36 weeks' gestation. Birth prior to the development of the surfactant system or completed pulmonary development results in neonatal respiratory compromise.

The respiratory diverticulum (lung bud) appears as an outgrowth of the ventral wall of the foregut. The epithelium of the internal lining of the larynx, trachea, bronchi, and lungs is entirely of endodermal origin. The cartilaginous, smooth muscle, and connective tissues of the trachea and lungs are derived from splanchnic mesoderm that surrounds the tubular framework. The lung bud is in open communication with the foregut. The diverticulum develops in a craniocaudal direction, leading to development of the upper respiratory structures (nose and pharynx) prior to the lower structures. Two tracheoesophageal ridges separate the diverticulum from the foregut. The dorsal portion of the foregut divides into the esophagus and the ventral portion into the trachea and lung buds.

The pulmonary circulation is a highly specialized vascular network, connecting the interdependent heart and lungs ( Fig. 12-1 ). The vascular bed parallels the airways and links the arterial and venous poles of the heart. The pulmonary vasculature has been described as appearing de novo within the mesoderm ventral and lateral to the foregut endoderm, suggesting that this mesoderm pool gives rise to the future pulmonary vasculature as lung develops from the foregut. The lungs have two sets of lymphatic vessels, a superficial set located beneath the pleura and a deep set which follows the blood vessels and extends along the bronchi. Both sets end within the bronchial glands. Efferent lymphatic vessels travel up the trachea, ending at the left-sided thoracic duct or the right-sided lymphatic duct. The fetal lungs play a key role in amniotic fluid volume maintenance: 15 mL/kg of body weight in fluid is produced by the lungs and flows out via the trachea and mouth, circulating and contributing to the amniotic fluid that is swallowed.

FIG 12-1, The pulmonary circulation. The right and left pulmonary veins and arteries and the branching capillaries are illustrated.

The basic structure of the diaphragm is also established early in gestation. Initially, the septum transversum develops, lying caudal to the heart and rostral to the umbilicus, in a position that eventually divides the intraembryonic cavity into the pleuropericardial cavity and the peritoneal cavity. The diaphragm subsequently undergoes programmed muscularization; this represents the final stage in the formation of the basic foundation of the diaphragm and is completed by 14 weeks' gestation.

Fetal lung development occurs in five stages: embryonic, pseudoglandular, canalicular, saccular, and alveolar ( Fig. 12-2 ). The timing of these phases is approximate. Transitions between stages are gradual, with overlap from one stage to the next.

FIG 12-2, A, The five stages of normal fetal lung development. B, Histologic sections illustrating progressive stages of lung development. 1. Pseudoglandular period (about 8 weeks). 2. Late canalicular period (about 24 weeks). 3. Early terminal sac period (about 26 weeks). 4. Early alveolar period (newborn infant). Note that the alveolocapillary membrane is thin and that some of the capillaries have begun to bulge into the primordial alveoli.

Embryonic Phase

The embryonic phase of lung development takes place during the first 4 to 5 weeks of gestation. During this phase, the larynx, trachea, and lung bud form from the foregut. The embryonic phase of lung development begins at about day 28 with the formation of a groove in the ventral lower pharynx, the sulcus laryngotrachealis. A few days later, at about day 30, a bud forms from the lower part, creating the true lung primordium. Development progresses in the 8-week-old embryo as the lobar buds subdivide and form the bronchopulmonary segments. Early on, the asymmetry of the main bronchi is established, with the smaller bud on the left directed more laterally than the larger one on the right, which is parallel to the esophagus and directed more caudally. The embryonic phase proceeds with the unequal dividing of the endodermal branches followed by further division. At the end of the embryonic period, the five lobes of the lungs (three right and two left) are present.

Pseudoglandular Phase

The pseudoglandular stage takes place between the 7th and 16th weeks of embryonic development. Conducting airways are formed by progressive branching of the original lung buds into smaller and more numerous areas. Each bud eventually becomes an independent respiratory unit, served by a bronchiole surrounded by capillary vessels that will bring blood to the lungs for oxygen. During this stage, the first differentiation of lung epithelium occurs. By 13 weeks' gestation, cilia appear in the proximal airways.

Canalicular Phase

Lasting until approximately 25 weeks' gestational age, the canalicular phase is crucial for the development of the gas-exchanging portion of the lung. By 20 weeks, there is differentiation into type I pneumocytes, the primary structural cell of the alveolus. Capillaries grow in close proximity to the distal surface of the alveolar cells. Lamellar bodies develop in type II alveolar cells and are the site of surfactant storage prior to release into the alveoli. A sufficient differentiation of the type II pneumocytes into the type I pneumocytes and the proliferation of the capillaries into the mesenchyme marks an important step toward the fetus being able to survive outside the uterus. By the end of this stage, structural development has progressed sufficiently such that gas exchange is possible and neonatal survival can occur. Amniotic fluid is required for the canalicular stage to occur. Fetal lung growth is stimulated in part by the distending force of lung fluid in the airways, which occurs during fetal breathing and is inhibited by anhydramnios.

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