2. Situs classification
1. SITUS SOLITUS: the normal configuration of thoracic and abdominal organs
2. SITUS INVERSUS: mirror image of the normal configuration
3. SITUS AMBIGUUS (HETEROTAXY): an intermediate configuration with duplication
(isomerism)
.
• When situs solitus and situs inversus occur, the atrial situs always corresponds to the
visceral situs. In situs ambiguus, the bronchial anatomy always corresponds to the atrial
situs.
3. Heterotaxy, derived from Greek (hetero, meaning "different," and taxy, meaning "arrangement")
defined as an abnormal arrangement of the internal thoracic-abdominal organs across the left-
right axis of the body
Patients with heterotaxy have been historically stratified into either the subsets of asplenia
syndrome or polysplenia syndrome
Occurs in 1 per 10,000 – 40,000
It is caused by disruption of left-right axis orientation during early embryonic development.
Introduction:
4. Left isomerism
Known as polysplenia syndrome:
• multiple splenules without a parent spleen
• azygos or hemiazygos continuation of the inferior vena
cava
• bilateral hyparterial bronchi
• bilateral bilobed lungs
• bilateral pulmonary/left atria
• midline/transverse liver
• intestinal malrotation
6. Right isomerism
Known as asplenia syndrome:
• severe cyanotic congenital heart diseases
• absence of spleen
• bilateral eparterial bronchi
• bilateral trilobed lungs
• bilateral right atria
• midline/transverse liver
• intestinal malrotation
7. •severe/complex congenital heart
disease (50%), especially cyanotic congenital cardiac
• total anomalous pulmonary venous
• endocardial cushion defect (85%)
• transposition of the great arteries (72%)
• single ventricle (51%)
• double outlet right ventricle (DORV)
Associations
Gastrointestinal
• gallbladder agenesis
• intestinal malrotation (up to 100% in
small series)
• microgastria
• imperforate anus
genitourinary
• horseshoe kidney
• fused / horseshoe adrenal gland or absent left
adrenal gland
• bicornuate uterus
• bilobed urinary bladder
vascular
• duplication of the superior vena cava
• absent coronary sinus
• juxtaposition of the IVC in front (usually) of the
abdominal aorta (piggyback configuration)
8. Determination of atrial situs
Basic determination of situs relies on the ability to discriminate right and left atria, which depends
on the morphologic characteristics of the atria and other features including the:
internal and external anatomy of the atrial appendages
connections of systemic and pulmonary vascular systems to the heart
anatomy of the interatrial septum
11. 14 yr old male with intermittent cramy abdominal pain of years and with recent
exacerbation of abdominal Pain and billous vomiting
12.
13. Right atrium
connections: superior and inferior vena cavae
crista terminalis: it is a smooth thickened muscular
structure which represents junction between sinus
venosus and developing heart
right atrial appendages: triangular or pyramidal
shaped and broad-based; it contains pectinate
muscles extending towards the atrioventricular
valve
see: normal anatomy of the left atrium
Left atrium
connections: pulmonary veins
crista terminalis: absent in left atrium
left atrial appendages: long slender
finger-like; contains pectinate muscles
coronary sinus always runs
inferoposteriorly to the left atrium
see: normal anatomy of the right
atrium
14. Bronchial anatomy
Bronchial anatomy accurately reflects atrial situs. The bronchial anatomy on the left and
right can be recognized on a well-penetrated radiograph and consists of two main
bronchi that are anatomically different:
hyparterial bronchus (below artery): supplies the bilobed left lung
eparterial bronchus (alongside the artery): supplies the trilobed right lung
In situs ambiguus, there is duplication of either the hyparterial or eparterial bronchus.
The associated atria are also duplicated and there are specific changes below the
diaphragm (although not left-sided or right-sided duplication)
Editor's Notes
Situs is best thought of as a spectrum with situs solitus and situs inversus at either end of the spectrum. Situs ambiguus, as the name suggests, falls somewhere between the two ends of the spectrum.
Atrial situs refers to the relative position of the cardiac atria in relation to abdominal viscera and the midline
Out of these features, the most reliable method is the morphologic characteristic differences between the right and left atria.
