Visceroatrial situs refers to the position and configuration of the cardiac atria,
the tracheobronchial tree, and the thoracoabdominal viscera [1].
Situs solitus with a trilobed right lung and a bilobed left lung,
normal position of atria,
levocardia with levoversion,
liver on the right side of upper abdomen and stomach and spleen on the left side is the normal anatomic position of the thoracic and abdominal organs [2].
In situs solitus,
the right upper lobe bronchus is superior and posterior to the descending branch of the right pulmonary artery (epiarterial bronchus),
while the left upper pulmonary artery courses posterior to the left upper bronchus and over the left upper lobe bronchus (hyparterial bronchus) [1-4].
Any disturbance in this normal arrangement of thoracoabdominal organs is called situs abnormality [2].
Situs inversus is a mirrored image of the situs solitus.
It is the most common form and characterized by complete reversed transposition of al thoracoabdominal organs and often associated with dextrocardia.
Heterotaxy syndrome or so-called situs ambiguous (right isomerism,
left isomerism) is when there are lack of lateralization and symmetry of organs in the abdominal and thoracic cavities.
Heterotaxy can be grouped into two major classes of asplenia and polysplenia syndrome or heterotaxy with right isomerism and heterotaxy with left isomerism.
Left isomerism is a rare type of heterotaxy syndrome [5].
In left isomerism or polysplenia syndrome,
patients have isomeric left atrial appendages,
bilaterally bilobed lungs,
and multiple spleens [6].
Patients with right isomerism or Ivemark syndrome have bilateral trilobed lungs with eparterial bronchi,
double right atria and often other heart malformations,
a transverse liver,
and a stomach in determinate position,
and asplenia [6-8].
There are also minor forms of situs ambiguous like M-anisosplenia,
F-anisosplenia.
They are characterized by a bifurcated spleen and bronchial symmetry [6].
M-anisosplenia is seen in male population with relatively normal visceral status,
with bilateral trilobed (right) lungs and anisosplenia.
F-anisosplenia is a variant of heterotaxy syndrome that seen in females and characterized by bilateral left-sidedness and bifurcated spleen,
nearly always associated with severe cyanotic congenital heart disease [2,
6].
It is important to distinguish different types of situs abnormalities.
Situs solitus with dextroversion,
situs inversus with levoversion or situs abnormalities with mesoversion and heterotaxy syndromes are associated with high incidence of severe congenital heart diseases (CHD) [2,
9].
Some of them,
like right isomerism syndrome,
are associated with complex cyanotic CHD (85%) and asplenia,
which lead to higher mortality risk due to infections [7].
Management of patients with visceral situs abnormalities associated with CHD is complex and depends on specific anatomy of cardiac and noncardiac thoracoabdominal structures.
Imaging of these abnormalities can be done with conventional radiography,
ultrasonography,
CT,
MRI,
and cardiac catheterization.
Conventional chest X-ray and/or echocardiography are the first methods of choice in evaluation of such anomalies in all cases.
Poor acoustic window and right isomerism may decrease informativeness of echocardiography.
Catheterization being an invasive tool and associated by higher radiation dose than CT is nowadays deputizing for non-invasive methods like MRI and CTA.
MRI allows detailed assessment of complex cardiac and extracardiac anomalies with lack of ionizing radiation [9].
By its wide availability and short acquisition time,
CTA is validated and reliable diagnostic tool for characterization of thoracoabdominal organs.
However,
dose modulation techniques should be applied to reduce radiation exposure on pediatric population [10].
A resource-poor setting can lead to delay in diagnosis of critical congenital diseases which are associated with higher mortality and morbidity risks [11].
Thus,
the use of CTA versus MRI or other method is based on hospital equipment,
the patient’s characteristics and possible procedure contraindications.