Situs ambiguus (), or heterotaxy, is a rare congenital defect in which the major visceral organs are distributed abnormally within the chest and abdomen. Clinically, heterotaxy spectrum generally refers to any defect of left-right asymmetry and arrangement of the visceral organs; however, classical heterotaxy requires multiple organs to be affected. This does not include the congenital defect situs inversus, which results when arrangement of all the organs in the abdomen and chest are mirrored, so the positions are opposite the normal placement. Situs inversus is the mirror image of situs solitus, which is normal asymmetric distribution of the abdominothoracic visceral organs. Situs ambiguus can also be subdivided into left-isomerism and right-isomerism based on the defects observed in the spleen, lungs and atria of the heart.
Individuals with situs inversus or situs solitus do not experience fatal dysfunction of their organ systems, as general anatomy and morphology of the abdominothoracic organ-vessel systems are conserved. Due to abnormal arrangement of organs in situs ambiguus, orientation across the left-right axis of the body is disrupted early in fetal development, resulting in severely flawed cardiac development and function in 50–80% of cases. They also experience complications with systemic and pulmonary blood vessels, significant morbidity, and sometimes death. All patients with situs ambiguus lack lateralization and symmetry of organs in the abdominal and thoracic cavities and are clinically considered to have a form of heterotaxy syndrome.
Heterotaxy with atrial isomerism occurs in 1 out of every 10,000 live births and is associated with approximately 3% of congenital heart disease cases. Additional estimation of incidence and prevalence of isomerism proves difficult due to failure to diagnose and underestimation of the disease by clinicians. Furthermore, right isomerism is much more easily recognized than left isomerism, contributing to the failure to diagnose.
Situs ambiguus is a growing field of research with findings dating back to 1973.
Signs and symptoms There are a variety of clinical manifestations of situs ambiguus. Acute symptoms can be due to both cardiac and non-cardiac defects. Cyanosis or blue skin coloration, primarily affecting the lips and fingernails, can indicate a systemic or circulatory issue. Poor feeding, failure to thrive, and rapid shallow breathing may also be observed due to poor circulation. Upon examination, arrhythmia and heart murmur may raise further suspicion of a cardiac abnormality. Non-cardiac symptoms include impairments of the liver and gastrointestinal tract. Biliary atresia, or inflammation and destruction of the bile ducts, may lead to jaundice. Vomiting and swelling of the abdominal region are features that suggest improper positioning of the intestines. Poor positioning of the intestine also makes it more prone to blockage, which can result in numerous chronic health issues. Asplenia and polysplenia are also possible features of heterotaxy.
Due to abnormal cardiac development, patients with situs ambiguus usually develop right atrial isomerism consisting of two bilaterally paired right atria, or left atrial isomerism consisting of two bilaterally paired left atria. Clinical features and symptoms can vary dependent upon assignment of left versus right atrial isomerism. In either instance, the apex of the heart will be poorly positioned, which should alert a clinician of the likelihood of atrial isomerism. It is estimated that 5–10% of isomeric patients have mesocardia, in which the heart is positioned at the center of the thorax, 25–50% have dextrocardia, in which the apex of the heart is pointed toward the right side of the thorax, and 50–70% have levocardia, in which the apex of the heart is pointed toward the left side of the thorax.
Right atrial appendage isomerism Right atrial appendage isomerism, also called right atrial isomerism, is a cardiac development defect in which the heart has bilateral right atria and atrial attachments in the muscle wall, as opposed to the normal right atrium and left atrium. In right atrial isomerism, the pulmonary blood oxygen tract is damaged due to right-left shunting of blood. In addition, the atrial septum which distinguishes the 2 atria is absent. These impairments, in addition to congestion in the pulmonary tract, allows deoxygenated blood to mix with oxygenated blood, contributing to cyanosis and possible respiratory distress. Poor systemic circulation also results due to improper positioning of the aorta.
Left atrial appendage isomerism Left atrial appendage isomerism, also called left atrial isomerism, is a cardiac development defect in which the heart has two bilateral left atria and atrial appendages in the muscle wall. Left atrial isomerism can have varied clinical manifestations, including a later onset of symptoms. Heart failure is often a concern because the inferior vena cava is disrupted due to the inappropriate morphology of the left ventricle to support the vena cava.
Conductive nodes and tissues Abnormal development of the heart results in impaired doubles of conductive nodes, as well as faulty electrical fibers throughout the ventricles. Individuals with right atrial isomerism develop two sinoatrial nodes, as they have 2 mirrored right atria, whereas those with left atrial isomerism fail to develop a sinus node at all. Thus, patients with left atrial isomerism are more likely to experience atrial fibrillation, or irregular rapid heart beat, and abnormal heart rhythm, known as atrial flutter. Development of the atrioventricular node and bundle of His largely depends on physiological looping of the ventricles. Abnormal looping of the ventricles contributes to arrhythmia and heart block in fetuses.
Bronchial tree and lungs Isomerism of the bronchial tree is not typically damaging and presents no significant clinical complications. Pulmonary valve stenosis results in issues of blood flow to the lungs.
Abdominal organs Abdominal organs, including the liver, stomach, intestinal tract, and spleen may be randomly arranged throughout the left-right axis of the body. Distribution of these organs largely dictates treatment, clinical outcomes, and further evaluation.
The liver is typically symmetrical across the left-right axis in patients with situs ambiguus, which is abnormal. A majority of left atrial isomeric patients have defects throughout the biliary tree, which is responsible for bile production, even when the gall bladder is functional and morphologically normal. This biliary atresia can lead to acute problems such as nutrient malabsorption, pale stools, dark urine, and abdominal swelling. If this condition continues without proper treatment, cirrhosis and liver failure become a major concern. Biliary atresia is not usually observed in patients with right atrial isomerism.
Random positioning of the stomach is often one of the first signals of situs ambiguus upon examination. Malrotation of the entire intestinal tract, or improper folding and bulging of the stomach and intestines, results in bowel obstruction. This impairment leads to vomiting, abdominal distension, mucus and blood in the stool. Patients may also experience abdominal pain. Intestinal malrotation is more commonly identified in patients with right atrial isomerism than in those with left atrial isomerism.
Isomeric patients often experience disruptions to splenic development during embryogenesis, resulting in an overall lack of a spleen (asplenia) or development of many spleens (polysplenia). Asplenia is most often observed in patients with right atrial isomerism. Polysplenia results in 90% of patients with left atrial isomerism. Although they have many spleens, each is usually ineffective resulting in functional asplenia. Rarely, left atrial isomeric patients have a single, normal, functional spleen. Patients lacking a functional spleen are in danger of sepsis and must be monitored.
Causes Situs ambiguus has been linked to family history of malformations and maternal cocaine use, suggesting both genetic and environmental factors play a role. Several genes in the TGF-beta pathway, which controls left-right patterning of visceral organs across the body axis, have been indicated in sporadic and familial cases of atrial isomerism. There is also overlap between genes associated with situs ambiguus and primary ciliary dyskinesia, likely due to the important role of cilia in establishing left-right asymmetry. The planar cell polarity has an important role in positioning these cilia, and thus genes within this pathway are increasingly associated with situs ambiguus.
Disrupted mitochondria function has also been recently linked to heterotaxy.
Pathophysiology
Molecular and cellular mechanism Several genes have been identified in normal development of the right-left axis. These genes have been extensively researched. Gene mutations that lead to atrial isomerism is a growing area of research. Mutations in genes that encode proteins in the TGF-beta pathway, including NODAL, NKX2-5, and ZIC3, have been linked to tetralogy of Fallot and hypoplastic left heart syndrome. Mutations in the ZIC3 gene, which encodes a zinc finger family transcription factor, is linked to a 50% risk of atrial isomerism in families. It is also an X-linked disorder, so testing for ZIC3 mutations is highly encouraged in male births.
The most prevalent and best characterized genetic associations of heterotaxy include:
Type OMIM Gene Locus HTX1 ZIC3 Xq26.2 HTX2 CFC1 2q21.1 HTX3 PA26 6q21 HTX4 ACVR2B 3p22.2 HTX5 NODAL 10q22.1HTX6614779CCDC1118q21.1
Classical pathology Cardiac looping malformations: Fallot's tetralogy transposition of the great vessels Ventricular and atrial septal defects. Deranged abdominal organ asymmetry: The stomach and spleen are prone to isolated reversal The stomach, liver, and a single adrenal gland are occasionally found in the midline. Organ malformations: asplenia and polysplenia often lead to sepsis More rarely, the head of the pancreas fails to form Horseshoe kidneys develop, which can lead to cancer, kidney stones, and/or infection. Malrotation errors cause volvulus and/or faulty peritoneal attachments, which completely obstruct the bowel. Vascular abnormalities: Interrupted inferior vena cava, Bilateral superior or inferior venae cavae Intrahepatic interruption of the inferior vena cava with connection to the azygos or hemiazygos veins Aberrant portal veins.
Bronchial tree and lungs Pathophysiology in the bronchial tree can be observed by radiography. Under normal development, the bronchial tree consists of two main bronchi that are anatomically different: Hyparterial bronchus (below the pulmonary artery): supplies blood to the bi-lobed left lung Eparterial bronchus (adjacent to the artery): supplies blood to the tri-lobed right lung In situs ambiguus, there is a duplication of either the hyparterial or eparterial bronchus. These features are not associated with any significant clinical complications. Mechanisms leading to bronchial duplication are not thoroughly understood.
In pulmonary valve stenosis, there is a reduction in blood flow to the lungs due to an obstruction of the heart at the pulmonic valve. This contributes to cyanosis and pulmonary hypertension.
Diagnosis