The Fetal Chest



The Fetal Chest





INTRODUCTION

The examination of the fetal chest in the first trimester includes the assessment of the right and left lung, the bony and cartilaginous thoracic cage, the diaphragm, and the heart with surrounding vasculature. Because of the importance and prevalence of cardiac anomalies, normal and abnormal anatomy of the heart and surrounding vasculature are presented in Chapter 11. Normal and abnormal appearance of lungs, diaphragm, and rib cage in the first trimester are discussed in this chapter. Pentalogy of Cantrell, involving a sternal defect, and ectopia cordis are discussed in Chapter 12.


EMBRYOLOGY

The respiratory diverticulum or lung bud is first seen around day 22 from fertilization as a ventral outgrowth of the primitive foregut. As the lung bud grows, it is surrounded by mesoderm, which gives rise to the lung vasculature, connective tissue, and muscle within the bronchial tree. The lengthening lung bud bifurcates on day 28 into the right and left lung buds, which gives rise to the right and left lung, respectively. Growth and bifurcation of the lung buds along with the surrounding mesenchyme continues throughout pregnancy. The terminal bronchioles are seen by the 28th week of gestation (menstrual) and the terminal sacs are formed by the 36th week of gestation. Maturation of the alveoli occurs between the 36th week of gestation and term. Alveolar growth continues into early childhood.






Figure 10.1: Schematic drawing of the developing diaphragm. Note that the diaphragm is formed by the fusion of the septum transversum, the pleuroperitoneal membranes, the mesoderm of body wall and the esophageal mesoderm, with the central tendon being primarily formed from the septum transversum.

The four embryonic structures—septum transversum, pleuroperitoneal membranes, mesoderm of body wall, and the esophageal mesoderm—coalesce to form the diaphragm (Fig. 10.1). The central tendon of the diaphragm is primarily formed from the septum transversum. The diaphragm is completely formed by the end of the 10th to 11th week of gestation.


In the sixth week of embryogenesis, the sternum arises from the somatic mesoderm as paired longitudinal sternal bars. These bars fuse in the midline to form a cartilaginous sternum at around the 10th week. Sternal ossification starts at day 60 in a segmental arrangement. The xiphoid process does not ossify until after birth. The vertebral bodies and ribs are also derived from the paraxial mesoderm.






Figure 10.2: A: Axial plane of the upper abdomen in a fetus at 13 weeks of gestation showing the right (RL) and left (LL) lungs and the diaphragm (white arrows), separating the lungs from the liver. Note that the lungs are slightly more echogenic than the liver. B: Axial view of the thorax at the level of the four-chamber view in the same fetus. Note the position of the heart in the left hemithorax surrounded by the RL and LL. The ribs (yellow arrows) are seen laterally in both A and B.






Figure 10.3: Axial planes (A and B) of the fetal chest at the level of the four-chamber view in a fetus at 12 weeks of gestation. The four-chamber view plane is optimal for the visualization of the right (RL) and left (LL) lungs. The lungs are highlighted in B.


NORMAL SONOGRAPHIC ANATOMY

The systematic visualization of the fetal chest in the first trimester is generally achieved from multiple sonographic planes. Axial views, at the level of the upper abdomen (Fig. 10.2A), the chest (Figs. 10.2B, 10.3, and 10.4A), and mediastinum
(Fig. 10.4B), allow for the evaluation of the diaphragm, right and left lungs, midline structures such as the esophagus, trachea/bronchi, and the thymus gland. In the normal fetus, the lungs appear slightly more echogenic than the liver and cardiac muscle (Fig. 10.2). At the four-chamber view plane, the right and left lungs are seen and the rib cage assessed (Figs. 10.2B, 10.3 and 10.4A). Comprehensive evaluation of the lungs in axial views requires the assessment at the level of the four-chamber view (Figs. 10.2B, 10.3 and 10.4A) and superiorly into the upper mediastinum at the three-vessel-trachea view (Fig. 10.4B). Assessment of cardiac position and axis in the chest (Fig. 10.4A) is helpful in the identification of lung abnormalities.






Figure 10.4: Axial views in color Doppler at the four-chamber view (A) and the three-vessel-trachea view (B) in a fetus at 13 weeks of gestation. Note that the right (RL) and left (LL) lungs are seen in both planes (A and B). Comprehensive evaluation of the fetal lungs in axial views requires an evaluation at the level of the four-chamber view (A) and the three-vessel-trachea view (B). The cardiac axis and position as evaluated in the four-chamber plane (A) is not only important for detecting cardiac abnormalities, but also for suspecting lung anomalies.






Figure 10.5: Right (A) and left (B) parasagittal planes of the fetal chest at 13 weeks of gestation. Note in the right thorax (A) the slightly hyperechoic lung as compared to the liver and the diaphragm in between. The bowel has the same echogenicity as the lung. The parasagittal view on the left (B) shows the lung, portion of the heart, the diaphragm, and the stomach (asterisk).

The right (Fig. 10.5A) and left (Fig. 10.5B) parasagittal views of the fetal chest are important for assessment of individual lung lobes, the diaphragm, and the rib cage (Fig. 10.6A). The
ribs can also be assessed from an axial plane of the chest at the level of the four-chamber view (Fig. 10.6B). In our opinion, the evaluation of the diaphragm is best achieved in coronal views (Fig. 10.7), starting from the posterior coronal view of the spine and moving more anteriorly toward the sternum. In these planes the diaphragm muscle and tendon on the right and left chest can be well visualized. The transvaginal approach improves visualization of all chest structures due to higher resolution (Fig. 10.7B). Three-dimensional ultrasound in a surface (Fig. 10.8) or tomographic display (Fig. 10.9)

can help in demonstrating various chest structures, especially in transvaginal scanning where transducer manipulation is limited. Clear visualization of the lungs can be achieved from about the 12th week of gestation onward.






Figure 10.6: A: A parasagittal plane of the thorax at the lateral chest wall in a fetus at 12 weeks of gestation showing the rib cage laterally with the normal arrangement of the ribs. B: An axial plane of the chest at the level of the four-chamber view in the same fetus demonstrating the ribs laterally.






Figure 10.7: Coronal views of the fetal abdomen and chest at 13 weeks of gestation obtained by the transabdominal (A) and transvaginal (B) approach in the same fetus. Note in A, the right (RL) and left (LL) lung seen in a coronal view with the diaphragm (arrows) separating the chest from the abdomen. The kidneys (K) are seen in the abdomen in A. In the transvaginal approach (B), the borders of the RL and LL are better seen and the diaphragm is clearly delineated (arrows). The stomach is also seen in B as an anechoic structure in the abdomen (asterisk).






Figure 10.8: Surface display of a three-dimensional transvaginal ultrasound volume of the chest in a fetus at 13 weeks of gestation showing the posterior coronal plane (A) and right parasagittal plane (B). Note the full display of the right (RL) and left (LL) lungs in A and the RL in B.






Figure 10.9: Tomographic display of a three-dimensional transvaginal ultrasound volume of the chest and abdomen in a fetus at 13 weeks of gestation. The volume displays the coronal planes of the fetus showing in the chest the thoracic cage with ribs (yellow arrows), lungs, heart, diaphragm, and in the abdomen the stomach (asterisk), liver, and bowel.


CHEST ABNORMALITIES


Hydrothorax/Pleural Effusion



Ultrasound Findings

Accumulation of fluid around the lungs is relatively easy to detect on ultrasound on axial (Figs. 10.10, 10.11, and 10.12A), coronal, or sagittal views (Fig. 10.12B). A typical sign for hydrothorax involves the presence of fluid between the lateral borders of the lungs and the ribs (Figs. 10.10, 10.11 and 10.12). This sign allows for differentiating hydrothorax from pericardial effusion, which can be difficult in some cases. In pericardial effusion, the fluid surrounds the heart and is on the medial aspects of the lungs (Fig. 10.13). The presence of severe hydrothorax results in lung compression with the typical “butterfly” appearance of the lungs. The association of hydrothorax with fetal hydrops is easily seen and is commonly noted with increased nuchal translucency (NT) and genetic abnormalities (Figs. 10.10 and 10.11). Diagnostic or therapeutic thoracocentesis is typically reserved for the second or third trimester of pregnancy. Figure 10.12 shows a fetus with an isolated unilateral hydrothorax that resolved by the second trimester of pregnancy.

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Oct 14, 2019 | Posted by in ULTRASONOGRAPHY | Comments Off on The Fetal Chest
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