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. An accessory left It should be remembered that there are exceptions, minor fissure is not an uncommon normal variant.6,7 although rare, to these rules, as discussed in Chapter 2.

Fig. 7.9 Heterotaxy with left iso- merism. Magnified views of the hilar anatomy (lower panels). The hepatic silhouette is not as asymmetric as it is in situs solitus or situs inversus, but is not as symmetric as it is in right iso- merism. The splenic silhouette is barely visible lateral to the stomach (arrow). Symmetric bronchial branch- ing can be appreciated on the frontal radiograph. The dilated azygos vein is seen above the right tracheobronchial angle. The trachea is mildly bent to the right by the left aortic arch. The lateral view shows the upper lobe bronchi at a similar horizontal level because of a similar length of the right and left main bronchi. The right and left pulmonary arteries cast a shadow mostly behind and above the upper lobe bronchi. The inferior vena caval shadow is not obvious on the lateral view. 7 Visceral Situs, Heart Position, and Aortic Arch Position 73

Fig. 7.10 Three cardiac positions defined by the location of the main part of the heart relative to the midline.

When defining the position of the heart on a chest ■ Heart Position radiograph, it is important to check whether there is any obliquity of radiographic projection. A subtle obliquity The cardiac position is described by using the terms levo- may bring the cardiac silhouette to the other side of the cardia, dextrocardia, and mesocardia (Fig. 7.10), which thorax (see Fig. 6.2). In general, the cardiac silhouette is describe where the main part of the heart is located brought to the right when the radiograph is obtained in a within the thorax. These terms are not used when the left anterior or right posterior oblique projection, whereas heart is displaced to one or the other side due to extracar- it is brought to the left when the radiograph is obtained in diac pathology, such as hypoplasia of a lung or deformity a right anterior or left posterior oblique projection. of the thoracic cage (Fig. 7.11). When the heart is partially Although there are many exceptions, levocardia tends or completely outside the thorax, it is called ectopia to have the right-sided right ventricle and the left-sided left cordis. Usually the cardiac apex points toward the side ventricle (so-called D-loop ventricles), whereas dextro- where the main part of the heart is positioned. However, cardia tends to have the left ventricle on the right and the there are exceptions where the cardiac position and the right ventricle on the left (so-called L-loop ventricles). This base-apex orientation are not matched. tendency explains why congenitally corrected transposition

Fig. 7.11 (A) Rightward displacement of the heart due to right lung The displaced cardiac position in scimitar syndrome is often called hypoplasia in a patient with absence of the right pulmonary artery in dextroposition, which is a confusing term. the mediastinum and (B) in another patient with scimitar syndrome. 74 II Systematic Approach to Chest Radiographs

Fig. 7.12 Discordant visceral situs and heart position. Situs solitus and Ao, ascending aorta; LA, left atrium; LV, left ventricle; RA, right atrium; dextrocardia (left panel). Situs inversus and levocardia (right panel). RV, right ventricle. Both cases had congenitally corrected transposition of the great arteries.

of the great arteries is the most common diagnosis when whereas the right aortic arch courses backward on the right there is discordance between the heart position and vis- side of the trachea and above the right main bronchus. ceral situs, such as dextrocardia with situs solitus (Fig. 7.12, Chest radiographic determination of the aortic arch left panel) and situs inversus with levocardia (Fig. 7.12, position depends largely on the indentation of the tracheal right panel). air column. A left aortic arch indents and bends the tra- chea to the right side, whereas a right arch indents and bends the trachea to the left (Fig. 7.13). In almost all cases, the aortic arch continues with the proximal descending ■ Position of the Aortic Arch aorta on the same side, which is usually identifiable as a vertical stripe along the spine. The rare exceptions are the The reference structures used for the determination of the so-called circumflex retroesophageal aortic arches in left- or right-sidedness of the aortic arch are the trachea and which the distal segment of the aortic arch crosses the main bronchi.10 The left aortic arch courses backward on the midline behind the esophagus to connect to the descend- left side of the trachea and above the left main bronchus, ing aorta on the opposite side10,11 (Fig. 7.14). When there

Fig. 7.13 Aortic arch positions relative to the trachea. The trachea nor- side of the aortic arch. Bilateral indentation is typically seen in double mally shows indentation on the side of the aortic arch. The most proxi- aortic arch but other forms of vascular ring cannot be excluded. mal part of the descending aorta (arrows) can be traced on the same Fig. 7.14 Circumflex retroesophageal right aortic arch (right aortic nect to the descending aorta on the left side of the spine. The tracheal arch with a left descending aorta). The proximal aortic arch is on the indentation is on the right side with the descending aorta seen on the right side, whereas the distal arch has a retroesophageal course to con- left (arrows).

A

Fig. 7.15 Cervical left aortic arch in a 14-year-old patient. (A) Chest radiograph and coronal magnetic resonance (MR) image show the aortic arch (arrows) reaching the upper thorax above the level of the clavicle (C). (B) Contrast-enhanced MR angiogram seen from the left side shows a tortuous aortic arch, aneurysmal dilatation (asterisks) in the distal aortic arch and left subclavian artery (LSA) and severe long segment narrowing of the proximal descending aorta. Dilated intercostal arteries are seen as collateral channels. As an incidental finding the left innominate vein (LIV) takes a retroaortic course. Ao, B ascending aorta; LV, left ventricle. 76 II Systematic Approach to Chest Radiographs

is double aortic arch, the trachea is bent to neither side or unusually high, its apex reaching above the level of the bent only slightly to the side of the smaller aortic arch. clavicle (Fig. 7.15). This condition is called a cervical aor- The trachea may show bilateral indentations with con- tic arch.10,12 It occurs more commonly with a right aortic centric narrowing of its lumen (Fig. 7.14). The descending arch, often taking a retroesophageal course. It is com- aorta can be seen on either side, although a left-sided monly associated with tortuosity, aneurysmal dilatation, descending aorta is more frequent. The aortic arch can be and narrowing.

Pearls

■ The situs can be accurately defined when the ■ Cardiac position has nothing to do with the type bronchial branching pattern is clearly shown. of visceral and atrial situs. ■ A right-sided gastric air bubble is not a specific sign ■ Levocardia tends to occur with D-loop ventricles for situs inversus, as it can also be seen with right (right ventricle on the right), and dextrocardia and left isomerism. A right-sided gastric air bubble with L-loop ventricles (right ventricle on the simply means that the situs is not normal. left), but there are many exceptions. ■ The presence of a minor fissure in both lungs is a ■ A left aortic arch indents the left side of the definitive sign for right isomerism. However, its trachea, and a right aortic arch indents the right absence means nothing. side of the trachea. ■ The presence of a minor fissure on the same side of ■ The stripe of the descending aorta is seen on the the stomach is a definitive sign of right isomerism. same side as the aortic arch in most cases. ■ Symmetry of the right and left lobes of the liver is ■ In double aortic arch, the trachea bends to highly suggestive of right isomerism, especially neither side if the arches are symmetric in size. when the heart is not enlarged and the pulmonary When the arches are asymmetric in size, the vascularity is diminished. trachea may bend to the side of the smaller ■ A dilated azygos vein on either side is highly arch. The descending aorta can be seen on suggestive of left isomerism, but can be seen in either side. other conditions such as superior or inferior vena caval obstruction.

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