Situs ambiguus
Encyclopedia
Situs ambiguus or situs ambiguous (the former spelling is more correct etymologically, but the latter spelling is very common), also known as heterotaxy, is a rare congenital defect in which the major visceral organs
are distributed abnormally within the chest
and abdomen
.
The normal position of the organs is known as situs solitus
; situs inversus
is a condition in which the usual positions of the organs are reversed from left to right as a mirror image
of the normal condition. If these are the two extreme positions on a continuum of asymmetric thoracic and abdominal organ formation, situs ambiguus covers everything in between.
More rarely, vascular abnormalities are found, including interrupted inferior vena cava, bilateral superior or inferior venae cavae, intrahepatic interruption of the inferior vena cava with connection to the azygos
or hemiazygos vein
s, and aberrant portal veins.
is poorly understood, it has been found to be linked to maternal diabetes mellitus
, family history of malformations, and maternal cocaine
use, suggesting both genetic and environmental factors play a role.
Genetic associations of heterotaxy include:
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...
are distributed abnormally within the chest
Chest
The chest is a part of the anatomy of humans and various other animals. It is sometimes referred to as the thorax or the bosom.-Chest anatomy - Humans and other hominids:...
and abdomen
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...
.
The normal position of the organs is known as situs solitus
Situs solitus
Situs solitus refers to the normal position of thoracic and abdominal organs. Anatomically, this means that the heart is on the left with the pulmonary atrium on the right and the systemic atrium on the left along with the cardiac apex...
; situs inversus
Situs inversus
Situs inversus is a congenital condition in which the major visceral organs are reversed or mirrored from their normal positions. The normal arrangement is known as situs solitus...
is a condition in which the usual positions of the organs are reversed from left to right as a mirror image
Mirror image
A mirror image is a reflected duplication of an object that appears identical but reversed. As an optical effect it results from reflection off of substances such as a mirror or water. It is also a concept in geometry and can be used as a conceptualization process for 3-D structures...
of the normal condition. If these are the two extreme positions on a continuum of asymmetric thoracic and abdominal organ formation, situs ambiguus covers everything in between.
Presentation
Classically, it comprises:- CardiacHeartThe heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
looping malformations – commonly Fallot’s tetralogyTetralogy of FallotTetralogy of Fallot is a congenital heart defect which is classically understood to involve four anatomical abnormalities...
, transposition of the great vesselsTransposition of the great vesselsTransposition of the great vessels is a group of congenital heart defects involving an abnormal spatial arrangement of any of the primary blood vessels: superior and/or inferior vena cavae , pulmonary artery, pulmonary veins, and aorta...
, pulmonary valve stenosisPulmonary valve stenosisPulmonary valve stenosis is a heart valve disorder in which outflow of blood from the right ventricle of the heart is obstructed at the level of the pulmonic valve. This results in the reduction of flow of blood to the lungs. Valvular pulmonic stenosis accounts for 80% of right ventricular outflow...
, and ventricularVentricular septal defectA ventricular septal defect is a defect in the ventricular septum, the wall dividing the left and right ventricles of the heart.The ventricular septum consists of an inferior muscular and superior membranous portion and is extensively innervated with conducting cardiomyocytes.The membranous...
and atrial septal defectAtrial septal defectAtrial septal defect is a form of congenital heart defect that enables blood flow between the left and right atria via the interatrial septum. The interatrial septum is the tissue that divides the right and left atria...
s. - Deranged abdominal organ asymmetry - the stomachStomachThe stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...
and spleenSpleenThe spleen is an organ found in virtually all vertebrate animals with important roles in regard to red blood cells and the immune system. In humans, it is located in the left upper quadrant of the abdomen. It removes old red blood cells and holds a reserve of blood in case of hemorrhagic shock...
are particularly prone to isolated reversal, and the stomach, liverLiverThe liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
, and a single adrenal glandAdrenal glandIn mammals, the adrenal glands are endocrine glands that sit atop the kidneys; in humans, the right suprarenal gland is triangular shaped, while the left suprarenal gland is semilunar shaped...
are occasionally found in the midline. - Organ malformations - chiefly aspleniaAspleniaAsplenia refers to the absence of normal spleen function and is associated with some serious infection risks. Hyposplenism is used to describe reduced splenic functioning, but not as severely affected as with asplenism.-Congenital:...
-polyspleniaPolysplenia-Associated conditions:There are frequent associated congenital anomalies all related to deviations in the development of anatomical asymmetries in early embryonic stages...
, and more rarely a failure of the head of the pancreasPancreasThe pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as a digestive organ, secreting pancreatic juice containing digestive enzymes that assist...
to form, and horseshoe adrenals and kidneysHorseshoe kidneyHorseshoe kidney, also known as renal fusion or super kidney, is a congenital disorder affecting about 1 in 400 people. In this disorder, the patient's kidneys fuse together to form a horseshoe-shape during development in the womb...
. - Rotation errors, causing volvulusVolvulusA volvulus is a bowel obstruction with a loop of bowel whose nose has abnormally twisted on itself.-Types:* Volvulus Neonatorum.* Volvulus Small Intestine.* Volvulus Caecum.* Volvulus Sigmoid Colon .* Gastric volvulus....
and/or faulty peritonealPeritoneumThe peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom — it covers most of the intra-abdominal organs — in amniotes and some invertebrates...
attachments.
More rarely, vascular abnormalities are found, including interrupted inferior vena cava, bilateral superior or inferior venae cavae, intrahepatic interruption of the inferior vena cava with connection to the azygos
Azygos vein
The azygos vein is a vein running up the right side of the thoracic vertebral column. It can also provide an alternate path for blood to the right atrium by allowing the blood to flow between the venae cavae when one vena cava is blocked.-Structure:...
or hemiazygos vein
Hemiazygos vein
The hemiazygos vein is a vein running superiorly in the lower thoracic region, just to the left side of the vertebral column.-Structure:...
s, and aberrant portal veins.
Causes
Although its etiologyEtiology
Etiology is the study of causation, or origination. The word is derived from the Greek , aitiologia, "giving a reason for" ....
is poorly understood, it has been found to be linked to maternal diabetes mellitus
Diabetes mellitus
Diabetes mellitus, often simply referred to as diabetes, is a group of metabolic diseases in which a person has high blood sugar, either because the body does not produce enough insulin, or because cells do not respond to the insulin that is produced...
, family history of malformations, and maternal cocaine
Cocaine
Cocaine is a crystalline tropane alkaloid that is obtained from the leaves of the coca plant. The name comes from "coca" in addition to the alkaloid suffix -ine, forming cocaine. It is a stimulant of the central nervous system, an appetite suppressant, and a topical anesthetic...
use, suggesting both genetic and environmental factors play a role.
Genetic associations of heterotaxy include:
| Type | OMIM | Gene | Locus |
|---|---|---|---|
| HTX1 | ZIC3 ZIC3 Zinc finger protein ZIC 3 is a protein that in humans is encoded by the ZIC3 gene.-Further reading:... |
Xq26.2 | |
| HTX2 | CFC1 | 2q21.1 | |
| HTX3 | PA26 | 6q21 | |
| HTX4 | ACVR2B ACVR2B Activin receptor type-2B is a protein that in humans is encoded by the ACVR2B gene. ACVR2B is an activin type 2 receptor.-Interactions:ACVR2B has been shown to interact with SYNJ2BP and ACVR1B.-Further reading:... |
992 | |
| HTX5 | NODAL NODAL Nodal is a protein that in humans is encoded by the NODAL gene. It is a member of the TGF beta superfamily. Like many other members of this superfamily, it is involved in cell differentiation... |
10q22.1 |
See also
- Situs inversusSitus inversusSitus inversus is a congenital condition in which the major visceral organs are reversed or mirrored from their normal positions. The normal arrangement is known as situs solitus...
- Situs solitusSitus solitusSitus solitus refers to the normal position of thoracic and abdominal organs. Anatomically, this means that the heart is on the left with the pulmonary atrium on the right and the systemic atrium on the left along with the cardiac apex...
- Chirality (mathematics)Chirality (mathematics)In geometry, a figure is chiral if it is not identical to its mirror image, or, more precisely, if it cannot be mapped to its mirror image by rotations and translations alone. For example, a right shoe is different from a left shoe, and clockwise is different from counterclockwise.A chiral object...
- AspleniaAspleniaAsplenia refers to the absence of normal spleen function and is associated with some serious infection risks. Hyposplenism is used to describe reduced splenic functioning, but not as severely affected as with asplenism.-Congenital:...
- PolyspleniaPolysplenia-Associated conditions:There are frequent associated congenital anomalies all related to deviations in the development of anatomical asymmetries in early embryonic stages...
- Ivemark syndromeIvemark syndromeAsplenia with cardiovascular anomalies or Ivemark syndrome, also known as Right Atrial Isomerism, is an example of a heterotaxy syndrome...