Download Visceral Situs, Heart Position, and Aortic Arch Position

7
Visceral Situs, Heart Position,
and Aortic Arch Position
The second step in interpretation of a chest radiograph
obtained in a new patient is to ascertain visceral situs, the
heart position, and the position of the aortic arch relative
to the trachea. For example, the radiographic report may
start with this sentence: “The radiograph shows situs solitus,
levocardia and a left aortic arch.”
■ Visceral Situs
Visceral situs refers to the pattern of arrangement of the
body organs relative to the midline. There are four types
of visceral situs: situs solitus, situs inversus, heterotaxy
with thoracic right isomerism, and heterotaxy with
thoracic left isomerism (Fig. 7.1).1–4 Visceral heterotaxy
has also been described as situs ambiguus, which means
uncertain situs. This term should be abandoned because
the situs is not uncertain but rather needs more words to
describe it precisely. The structures that are helpful in determination of the situs include (1) the gastric air bubble,
(2) the larger lobe of the liver, (3) the tip of the spleen, (4)
the bronchi and pulmonary arteries, (5) the minor fissure,
and (6) the azygos vein. The basic concepts regarding visceral situs is discussed in detail in Chapter 2.
In situs solitus, the gastric air bubble is on the left side,
and the larger lobe of the liver is on the right side (Fig. 7.2).
The splenic tip can often be identified when the stomach
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 radiograph. Normally the left main bronchus is 1.5 to 2 times
longer than the right main bronchus. At the pulmonary
hilum, the left pulmonary artery is seen slightly higher
than the right pulmonary artery. The left pulmonary
artery is seen above the left upper lobe bronchus,
whereas the right pulmonary artery (in fact, its descending branch) is seen below the right upper lobe bronchus.
An asymmetric branching pattern of the bronchi and pulmonary arteries is also evident on the lateral radiograph.
As the upper lobe bronchi normally have horizontal
courses, they are seen as two round lucencies arranged
superoinferiorly in the middle mediastinum. The right
upper lobe bronchus is located approximately one vertebral 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.
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7 Visceral Situs, Heart Position, and Aortic Arch Position
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
right upper lobe bronchus. The left pulmonary artery lies
posterior and superior to the left upper lobe bronchus. In
short, the right upper lobe bronchus lies higher than the
left, and the left pulmonary artery lies higher than and
posterior to the right pulmonary artery. The minor fissure
may cast a horizontal linear shadow over the midzone of
a trilobed right lung.
Situs inversus is characterized by a mirror-image
arrangement of the visceral organs (Fig. 7.3). The gastric
bubble is on the right and the larger lobe of the liver on
the left. However, it is important to understand that the
situs should not be called “situs inversus” solely because
of the inverted positions of the gastric bubble and hepatic
shadow. Plain film diagnosis of situs inversus can only be
made when an inverted bronchial and pulmonary arterial
branching pattern is clear. A lateral radiograph is helpful
in differentiating the lateralized situs (i.e., situs solitus
and inversus) from the symmetric situs (i.e., right isomerism or left isomerism). Normal hilar arrangement of
the upper lobe bronchi and branch pulmonary arteries on
the lateral view with a right-sided gastric bubble on the
frontal view is highly suggestive of situs inversus even if
the splenic shadow is not identified.
In right isomerism, the hepatic shadow usually extends
across the upper abdomen (Fig. 7.4). The gastric bubble
can be seen on either side but tends to be closer to the
midline. Interestingly, about 15% of patients with right
isomerism have a hiatal hernia (Fig. 7.5).5 Hiatal hernia
can be regarded as a manifestation of visceral heterotaxy.
Bilaterally short bronchi can be appreciated in a welltaken frontal radiograph (Fig. 7.6). However, this feature
is often unclear in infants. On the lateral radiograph,
the end-on shadows of the upper lobe bronchi are seen at
the same or similar horizontal level, and the pulmonary
arterial shadow is seen mostly in front of the bronchi
(Fig. 7.4). The presence of bilateral minor fissures indicates
right isomerism (Fig. 7.7). In fact the diagnosis of right
isomerism can be entertained when the minor fissure is
present on the same side as the stomach even when a minor
fissure is identified only on one side (Fig. 7.4, left panel).
This is because the minor fissure and stomach cannot be
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II Systematic Approach to Chest Radiographs
Fig. 7.3 Situs inversus. The frontal view shows mirror-image arrangement of the bronchial and pulmonary arterial trees. Asterisks mark the
upper lobe bronchi.The lateral view appears identical to that seen in
on the same side in either situs solitus or situs inversus,
and no minor fissure is present in either lung in left isomerism. However, a potential pitfall is the presence of an
accessory fissure, which can simulate a horizontal fissure
(Fig. 7.8).6,7 Right isomerism is associated with complex
Fig. 7.4 Heterotaxy with thoracic right isomerism. The hepatic silhouette extends symmetrically across the upper abdomen. The stomach
with a nasogastric tube in place is on the right side, lying somewhat
close to the midline. A minor fissure is visible in the right lung. The
presence of a minor fissure on the same side of the stomach is conclusive
situs solitus (Fig. 7.2) but note the difference in labeling. This patient
had Kartagener’s syndrome with subtle pulmonary infiltrates in the
right lower lung.
congenital heart disease in almost all cases. We have not
seen any single case of right isomerism without complex
congenital heart disease. As pulmonary atresia or stenosis
is present in the majority of cases, the pulmonary vascularity is usually reduced (Figs. 7.4, 7.5, 7.6, and 7.7).
evidence of right isomerism. On the lateral view, the similar length of
the right and left main bronchi results in the upper lobe bronchi being
projected at a similar horizontal level. The right and left pulmonary
arteries are projected mostly in front of the upper lobe bronchi.
7 Visceral Situs, Heart Position, and Aortic Arch Position
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
visceral arrangement. However, the hepatic shadow in
left isomerism is usually less symmetric than in right
isomerism, and it is not uncommon for the hepatic
shadow to be indistinguishable from that seen in situs
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
of the upper lobar bronchi.
solitus or inversus (Fig. 7.9). The gastric bubble can be
on either side. The splenic shadow can be identified
along the greater curvature of the stomach. On the
frontal radiograph, bilaterally long bronchi can often be
appreciated (Fig. 7.9, right panels). On the lateral radiograph, 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. 7.7 Bilateral minor fissures (arrows) in a neonate with right
isomerism.
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II Systematic Approach to Chest Radiographs
Fig. 7.8 Accessory left anomalous fissure simulating the appearance
of bilateral minor fissures in an infant with situs solitus and hypoplastic left heart syndrome. Right isomerism was suspected, but there
was a situs solitus arrangement of the other organs. An accessory left
minor fissure is not an uncommon normal variant.6,7
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 inferior 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 hidden 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 individuals or in patients with an arrhythmia or biliary
atresia.9
It should be remembered that there are exceptions,
although rare, to these rules, as discussed in Chapter 2.
Fig. 7.9 Heterotaxy with left isomerism. 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 isomerism. The splenic silhouette is
barely visible lateral to the stomach
(arrow). Symmetric bronchial branching 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
Fig. 7.10 Three cardiac positions defined by the location of the main part of the heart relative to the midline.
■ Heart Position
The cardiac position is described by using the terms levocardia, dextrocardia, and mesocardia (Fig. 7.10), which
describe where the main part of the heart is located
within the thorax. These terms are not used when the
heart is displaced to one or the other side due to extracardiac pathology, such as hypoplasia of a lung or deformity
of the thoracic cage (Fig. 7.11). When the heart is partially
or completely outside the thorax, it is called ectopia
cordis. Usually the cardiac apex points toward the side
where the main part of the heart is positioned. However,
there are exceptions where the cardiac position and the
base-apex orientation are not matched.
Fig. 7.11 (A) Rightward displacement of the heart due to right lung
hypoplasia in a patient with absence of the right pulmonary artery in
the mediastinum and (B) in another patient with scimitar syndrome.
When defining the position of the heart on a chest
radiograph, it is important to check whether there is any
obliquity of radiographic projection. A subtle obliquity
may bring the cardiac silhouette to the other side of the
thorax (see Fig. 6.2). In general, the cardiac silhouette is
brought to the right when the radiograph is obtained in a
left anterior or right posterior oblique projection, whereas
it is brought to the left when the radiograph is obtained in
a right anterior or left posterior oblique projection.
Although there are many exceptions, levocardia tends
to have the right-sided right ventricle and the left-sided left
ventricle (so-called D-loop ventricles), whereas dextrocardia tends to have the left ventricle on the right and the
right ventricle on the left (so-called L-loop ventricles). This
tendency explains why congenitally corrected transposition
The displaced cardiac position in scimitar syndrome is often called
dextroposition, which is a confusing term.
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Fig. 7.12 Discordant visceral situs and heart position. Situs solitus and
dextrocardia (left panel). Situs inversus and levocardia (right panel).
Both cases had congenitally corrected transposition of the great arteries.
of the great arteries is the most common diagnosis when
there is discordance between the heart position and visceral situs, such as dextrocardia with situs solitus (Fig. 7.12,
left panel) and situs inversus with levocardia (Fig. 7.12,
right panel).
■ Position of the Aortic Arch
The reference structures used for the determination of the
left- or right-sidedness of the aortic arch are the trachea and
main bronchi.10 The left aortic arch courses backward on the
left side of the trachea and above the left main bronchus,
Fig. 7.13 Aortic arch positions relative to the trachea. The trachea normally shows indentation on the side of the aortic arch. The most proximal part of the descending aorta (arrows) can be traced on the same
Ao, ascending aorta; LA, left atrium; LV, left ventricle; RA, right atrium;
RV, right ventricle.
whereas the right aortic arch courses backward on the right
side of the trachea and above the right main bronchus.
Chest radiographic determination of the aortic arch
position depends largely on the indentation of the tracheal
air column. A left aortic arch indents and bends the trachea 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
aorta on the same side, which is usually identifiable as a
vertical stripe along the spine. The rare exceptions are the
so-called circumflex retroesophageal aortic arches in
which the distal segment of the aortic arch crosses the
midline behind the esophagus to connect to the descending aorta on the opposite side10,11 (Fig. 7.14). When there
side of the aortic arch. Bilateral indentation is typically seen in double
aortic arch but other forms of vascular ring cannot be excluded.
Fig. 7.14 Circumflex retroesophageal right aortic arch (right aortic
arch with a left descending aorta). The proximal aortic arch is on the
right side, whereas the distal arch has a retroesophageal course to con-
nect to the descending aorta on the left side of the spine. The tracheal
indentation is on the right side with the descending aorta seen on the
left (arrows).
A
B
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,
ascending aorta; LV, left ventricle.
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II Systematic Approach to Chest Radiographs
is double aortic arch, the trachea is bent to neither side or
bent only slightly to the side of the smaller aortic arch.
The trachea may show bilateral indentations with concentric narrowing of its lumen (Fig. 7.14). The descending
aorta can be seen on either side, although a left-sided
descending aorta is more frequent. The aortic arch can be
unusually high, its apex reaching above the level of the
clavicle (Fig. 7.15). This condition is called a cervical aortic arch.10,12 It occurs more commonly with a right aortic
arch, often taking a retroesophageal course. It is commonly associated with tortuosity, aneurysmal dilatation,
and narrowing.
Pearls
■
The situs can be accurately defined when the
bronchial branching pattern is clearly shown.
■
Cardiac position has nothing to do with the type
of visceral and atrial situs.
■
A right-sided gastric air bubble is not a specific sign
for situs inversus, as it can also be seen with right
and left isomerism. A right-sided gastric air bubble
simply means that the situs is not normal.
■
Levocardia tends to occur with D-loop ventricles
(right ventricle on the right), and dextrocardia
with L-loop ventricles (right ventricle on the
left), but there are many exceptions.
■
The presence of a minor fissure in both lungs is a
definitive sign for right isomerism. However, its
absence means nothing.
■
A left aortic arch indents the left side of the
trachea, and a right aortic arch indents the right
side of the trachea.
■
The presence of a minor fissure on the same side of
the stomach is a definitive sign of right isomerism.
■
The stripe of the descending aorta is seen on the
same side as the aortic arch in most cases.
■
Symmetry of the right and left lobes of the liver is
highly suggestive of right isomerism, especially
when the heart is not enlarged and the pulmonary
vascularity is diminished.
■
■
A dilated azygos vein on either side is highly
suggestive of left isomerism, but can be seen in
other conditions such as superior or inferior vena
caval obstruction.
In double aortic arch, the trachea bends to
neither side if the arches are symmetric in size.
When the arches are asymmetric in size, the
trachea may bend to the side of the smaller
arch. The descending aorta can be seen on
either side.
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