Visceral Situs, Heart Position, 7 and Aortic Arch Position The second step in interpretation of a chest radiograph and (6) the azygos vein. The basic concepts regarding vis- obtained in a new patient is to ascertain visceral situs, the ceral situs is discussed in detail in Chapter 2. heart position, and the position of the aortic arch relative In situs solitus, the gastric air bubble is on the left side, to the trachea. For example, the radiographic report may and the larger lobe of the liver is on the right side (Fig. 7.2). start with this sentence: “The radiograph shows situs solitus, The splenic tip can often be identified when the stomach levocardia and a left aortic arch.” and adjacent bowel loops are filled with air. When the bronchial air column can be traced, an asymmetric bronchial branching pattern with a short right and a long left main bronchus can be appreciated on the frontal radi- ■ Visceral Situs ograph. Normally the left main bronchus is 1.5 to 2 times longer than the right main bronchus. At the pulmonary Visceral situs refers to the pattern of arrangement of the hilum, the left pulmonary artery is seen slightly higher body organs relative to the midline. There are four types than the right pulmonary artery. The left pulmonary of visceral situs: situs solitus, situs inversus, heterotaxy artery is seen above the left upper lobe bronchus, with thoracic right isomerism, and heterotaxy with whereas the right pulmonary artery (in fact, its descend- thoracic left isomerism (Fig. 7.1).1–4 Visceral heterotaxy ing branch) is seen below the right upper lobe bronchus. has also been described as situs ambiguus, which means An asymmetric branching pattern of the bronchi and pul- uncertain situs. This term should be abandoned because monary arteries is also evident on the lateral radiograph. the situs is not uncertain but rather needs more words to As the upper lobe bronchi normally have horizontal describe it precisely. The structures that are helpful in de- courses, they are seen as two round lucencies arranged termination of the situs include (1) the gastric air bubble, superoinferiorly in the middle mediastinum. The right (2) the larger lobe of the liver, (3) the tip of the spleen, (4) upper lobe bronchus is located approximately one verte- the bronchi and pulmonary arteries, (5) the minor fissure, bral height above the left upper lobe bronchus. The right Fig. 7.1 Four types of visceral situs. GB, gallbladder; IVC, inferior vena cava; PA, pulmonary artery; SVC, superior vena cava. 68 7 Visceral Situs, Heart Position, and Aortic Arch Position 69 Fig. 7.2 Situs solitus. Magnified views of the hilar anatomy (lower panels). Asterisks on the magnified frontal view mark the upper lobe bronchi. The left main bronchus is twice as long as the right main bronchus in this individual (solid lines). pulmonary artery lies anterior and slightly inferior to the the lateral view with a right-sided gastric bubble on the right upper lobe bronchus. The left pulmonary artery lies frontal view is highly suggestive of situs inversus even if posterior and superior to the left upper lobe bronchus. In the splenic shadow is not identified. short, the right upper lobe bronchus lies higher than the In right isomerism, the hepatic shadow usually extends left, and the left pulmonary artery lies higher than and across the upper abdomen (Fig. 7.4). The gastric bubble posterior to the right pulmonary artery. The minor fissure can be seen on either side but tends to be closer to the may cast a horizontal linear shadow over the midzone of midline. Interestingly, about 15% of patients with right a trilobed right lung. isomerism have a hiatal hernia (Fig. 7.5).5 Hiatal hernia Situs inversus is characterized by a mirror-image can be regarded as a manifestation of visceral heterotaxy. arrangement of the visceral organs (Fig. 7.3). The gastric Bilaterally short bronchi can be appreciated in a well- bubble is on the right and the larger lobe of the liver on taken frontal radiograph (Fig. 7.6). However, this feature the left. However, it is important to understand that the is often unclear in infants. On the lateral radiograph, situs should not be called “situs inversus” solely because the end-on shadows of the upper lobe bronchi are seen at of the inverted positions of the gastric bubble and hepatic the same or similar horizontal level, and the pulmonary shadow. Plain film diagnosis of situs inversus can only be arterial shadow is seen mostly in front of the bronchi made when an inverted bronchial and pulmonary arterial (Fig. 7.4). The presence of bilateral minor fissures indicates branching pattern is clear. A lateral radiograph is helpful right isomerism (Fig. 7.7). In fact the diagnosis of right in differentiating the lateralized situs (i.e., situs solitus isomerism can be entertained when the minor fissure is and inversus) from the symmetric situs (i.e., right iso- present on the same side as the stomach even when a minor merism or left isomerism). Normal hilar arrangement of fissure is identified only on one side (Fig. 7.4, left panel). the upper lobe bronchi and branch pulmonary arteries on This is because the minor fissure and stomach cannot be 70 II Systematic Approach to Chest Radiographs Fig. 7.3 Situs inversus. The frontal view shows mirror-image arrange- situs solitus (Fig. 7.2) but note the difference in labeling. This patient ment of the bronchial and pulmonary arterial trees. Asterisks mark the had Kartagener’s syndrome with subtle pulmonary infiltrates in the upper lobe bronchi.The lateral view appears identical to that seen in right lower lung. on the same side in either situs solitus or situs inversus, congenital heart disease in almost all cases. We have not and no minor fissure is present in either lung in left iso- seen any single case of right isomerism without complex merism. However, a potential pitfall is the presence of an congenital heart disease. As pulmonary atresia or stenosis accessory fissure, which can simulate a horizontal fissure is present in the majority of cases, the pulmonary vascu- (Fig. 7.8).6,7 Right isomerism is associated with complex larity is usually reduced (Figs. 7.4, 7.5, 7.6, and 7.7). Fig. 7.4 Heterotaxy with thoracic right isomerism. The hepatic silhou- evidence of right isomerism. On the lateral view, the similar length of ette extends symmetrically across the upper abdomen. The stomach the right and left main bronchi results in the upper lobe bronchi being with a nasogastric tube in place is on the right side, lying somewhat projected at a similar horizontal level. The right and left pulmonary close to the midline. A minor fissure is visible in the right lung. The arteries are projected mostly in front of the upper lobe bronchi. presence of a minor fissure on the same side of the stomach is conclusive 7 Visceral Situs, Heart Position, and Aortic Arch Position 71 Fig. 7.5 Ectopic location of a part of the stomach in the thorax, which is often described as hiatal hernia, in a neonate with abdominal heterotaxy and thoracic right isomerism. Left isomerism is also characterized by a symmetric solitus or inversus (Fig. 7.9). The gastric bubble can be visceral arrangement. However, the hepatic shadow in on either side. The splenic shadow can be identified left isomerism is usually less symmetric than in right along the greater curvature of the stomach. On the isomerism, and it is not uncommon for the hepatic frontal radiograph, bilaterally long bronchi can often be shadow to be indistinguishable from that seen in situs appreciated (Fig. 7.9, right panels). On the lateral radi- ograph, the upper lobe bronchi cast end-on shadows at the same or similar horizontal level, and the pulmonary arterial shadow is mostly seen behind the bronchi. The presence of a minor fissure in either lung excludes the Fig. 7.6 Symmetrically short main bronchi are shown in this frontal radiograph in a child with right isomerism. This feature is not usually as apparent in neonates and small children. Asterisks indicate the origins Fig. 7.7 Bilateral minor fissures (arrows) in a neonate with right of the upper lobar bronchi. isomerism. 72 II Systematic Approach to Chest Radiographs diagnosis of left isomerism. In approximately 80% of cases, left isomerism is associated with interruption of the inferior vena cava with azygos or hemiazygos venous continuation.8 In this situation, the dilated azygos vein can be identified in the area of the tracheobronchial angle (Fig. 7.9, left panels). Absence of the inferior vena caval shadow on the lateral radiograph has also been described in patients with an interrupted inferior vena cava. We find this feature unreliable. As interruption involves the postrenal prehepatic segment of the infe- rior vena cava, the posthepatic segment of the inferior vena cava is present and collects the hepatic veins. Therefore, a web-like shadow, albeit somewhat smaller than normal, can be identified in the lateral radiograph. In addition, the inferior vena caval shadow can be hid- den by the posteriorly enlarged heart. In contrast to right isomerism, left isomerism is not always associated with congenital heart disease. Left isomerism may be seen as an incidental finding in otherwise normal indi- Fig. 7.8 Accessory left anomalous fissure simulating the appearance viduals or in patients with an arrhythmia or biliary of bilateral minor fissures in an infant with situs solitus and hypoplas- atresia.9 tic left heart syndrome. Right isomerism was suspected, but there was a situs solitus arrangement of the other organs.