Prenatal abnormal features of fetal mainstem bronchi: back to the basics

The segmental approach, which is widely used in the imaging work-up of congenital heart disease, consists of a three-step evaluation of the cardiac anatomy after determining fetal laterality. The important step 1 consists of determining atrial arrangement or situs. Theoretically, this should be done by describing the position of each atrium in relation to each other, irrespective of where the heart is within the chest. The shape of the atrial appendages is the most constant feature of the atrium, but it has not been systematically imaged in child and even more specially in fetus. In current paediatric cardiology practice, atrial situs is most commonly recognized from the arrangement of the abdominal great vessels, aorta and inferior vena cava, with respect to the spine, at the level of the diaphragm. Indeed, prenatally, appendages may be identified by ultrasound at best by an axial view a little bit higher that the level of the 4-chamber view or in a parasagittal view. The visceroatrial situs is then determined. Three different anatomic configurations may be observed: situs solitus (normal), situs inversus (inverted) or situs ambiguus (ambiguous). In step 2, the atrioventricular junction is described. Then, the ventricles are identified on the basis of their internal morphologic features as well as their position in the chest and by the morphologically appearance of the atrioventricular (mitral and tricuspid) valves. In step 3, the ventriculo-arterial junction is determined by determining which great artery connects to which ventricle. We reviewed two cases of situs ambiguous or heterotaxy diagnosed prenatally for which prenatal evaluation of fetal main bronchus characteristics helped clarify the left and right types of isomerism respectively. The characteristics of the two fetuses are detailed in Table 1. Ethical approval was not required according to the French law in this specific case of descriptional study. Heterotaxy or situs ambiguous is a unique clinical entity in which there are anomalies in the normal lateral asymmetrical arrangement of organs; the lateralisation of the thoracoabdominal organs is abnormal, typically with isomerism of the bronchial tree and atrial appendages. It is now recognized that heterotaxy is better segregated on the basis of isomerism of the atrial appendages rather than other features. Situs solitus reflects the normal anatomical arrangement. The situs inversus is the arrangement mirroring the normal anatomy. The left and right isomerisms (also called heterotaxies) are characterized by the presence of atriarelated appendages. However, the morphological assessment of those appendages is challenging and not always feasible, especially prenatally. Whether in the postnatal or prenatal period, these anomalies often exhibit, as for our two cases, abnormalities in the position of the stomach, in the orientation of the tip of the heart and in cardiac architecture. The identification of the subtype of isomerism is often related to indirect extracardiac defects. Indeed, most of the time, the appendages are not reliably identifiable at radiologic imaging, and the localization of noncardiac organs is more helpful for determining the situs. Thus, the absence of inferior vena cava with azygos or hemi azygos continuation is consistent with a left isomerism, and polysplenia is often associated, whereas the presence of an inferior vena cava associated with abnormal pulmonary venous return and asplenia is consistent with a right isomerism. It has long been known that the characteristics of the main bronchi almost always reflect the atrial situs (Figure S1), are easier to identify postnatally (Figure S2) and more tricky prenatally. Prenatal assessment of the trachea, the carina and the main bronchial tree can be performed prenatally by using cross-sectional axial views (Figure S3) of the fetal chest. Those entities are identified as being anechoic round structures behind the convergence of the ductal and aortic arches visible from either side of the level of the three-vessel and trachea