Lateralization disorders are divided into complete (ie, situs inversus totalis) and incomplete (ie, heterotaxy); the word heterotaxy is derived from the Greek heteros, meaning “other” and taxis, meaning “arrangement.” The disorders have been recognized since at least 1933 (complete)  and 1826 (incomplete).  Only recently have genetic alterations responsible for their occurrence in humans been identified. The discovery of kindreds in which both heterotaxy and situs inversus totalis occur strongly suggests that these are not truly separate diseases.  At least 12% of primary ciliary dyskinesia (PCD) patients have heterotaxy. 
Since PCD is present in about a quarter of situs inversus totalis patients and since situs inversus totalis is present in nearly half of individuals with PCD, this article covers both heterotaxy syndrome and PCD. Moreover, because asplenia and polysplenia can occur in the same family, [5, 6] a patient’s splenic phenotype should be viewed as merely one phenotypic aspect of an underlying laterality disorder, even though many prior reviews of heterotaxy syndrome have divided patients into 2 groups based on only splenic phenotype.
Embryology and developmental biology
Ivemark’s review of 65 cases of human patients with asplenia,  in which most but not all had heart disease, firmly established the spectrum of congenital heart lesions that occurred in patients with lateralization disorders. Four years later in 1959, the recovery of a spontaneous, autosomal recessive, viable mutation in mice was reported and was named iv, for inverted viscera. 
Although the stomach position in the iv mutant colony remained perfectly randomized over 15 years of breeding, including only one outcross, the prevalence of discordance between thoracoabdominal venous anatomy and the situs of the rest of the body decreased from 42% to 26%.  In other words, the ratio of the heterotaxy phenotype to the situs inversus totalis phenotype decreased with progressive inbreeding. This suggests that the wild type allele of the iv locus controls overall thoracoabdominal sidedness and not individual organ sidedness. Indeed, when Icardo and Sanchez de Vega examined the hearts of iv homozygotes, only 40% were abnormal, and only 36% had abnormal splenic morphology. [10, 11]
The absence of outer dynein arms in the spermatozoa and airway cilia of humans with the Kartagener triad (i.e., situs inversus totalis, sinusitis, and bronchiectasis) was also noted by Bj ö rn Afzelius (see the image below).
When the iv mouse mutation was cloned, it was found to encode a molecular motor protein, an axonemal dynein, and was named lrd, for left-right dynein (human homolog is DNAH11/DNAHC11). However, its expression at embryonic day 7.5 was confined to the few hundred ciliated cells of the ventral surface of the node, a fluid-covered, pit-shaped structure at the anterior end of the primitive streak. Because these cilia, 5 microns in length and 0.3 microns in diameter, are missing the central doublet (i.e., have a 9+0 configuration of microtubule doublets, rather than the 9+2 configuration typically seen in motile cilia), they were not believed to be motile. Although the node was known to have important roles in organizing the body plan of the mouse embryo, the function of lrd remained mysterious.
The next year, the phenotype of mice missing the kinesin Kif3b was reported. Kif3b is a molecular motor which, like dynein, is responsible for transport along microtubules within cilia; kinesins transport their cargo toward the "plus end" of the microtubule, while dyneins are "minus end directed motors". Fifty percent of the 9.5-day embryos had L-looped hearts. Closer scrutiny of the cilia on normal ventral node cells showed that they do in fact move, despite their 9+0 arrangement of microtubules. In fact, uniquely among cilia, they rotate at 600 rpm. Ventral node cells of Kif3b nulls had either sporadic, very short cilia or absent cilia.  Whereas iv heterozygotes had cilia that rotated at 600 rpm, iv homozygotes had immotile cilia.
Because of a posterior tilt in the orientation of the cilia, as well as a difference in the viscous drag at the fluid surface compared with the base of the pit, the fluid within the node pit moves unidirectionally to the left, as verified by the movement of submicron-sized fluorescent beads applied to the fluid as passive tracers.  This fluid flow sets up a left-right asymmetric distribution of signaling molecules (eg, the evolutionarily conserved nodal/Pitx2 pathway) within the embryo during gastrulation, when the 3 germ layers (i.e., ectoderm, mesoderm, and endoderm) are specified.
In the zebrafish (Danio rerio), rotatory cilia-bearing structures homologous to the mouse node have been identified; however, they do not appear to be present in chicks or pigs.  Moreover, the African clawed frog (Xenopus laevis) specifies the embryonic left-right axis long before cilia can be identified. Numerous steps in the process of embryonic development that precede the development of node cilia remain unknown. In fact, 3 temporal phases in which molecular and cellular decisions determine the left-right axis of the body plan are likely (i.e., pregastrulation, gastrulation, and organogenesis), as is shown in the image below.
For technical reasons, the pregastrulation time period is particularly difficult to study in mammals. Important left-right axis specification decisions may occur in this developmental time interval in mouse and human, and thus the frog, chick, and pig may not be “outliers.”
In addition, the underlying cellular biology of why improper left-right specification of the lateral plate mesoderm has such a profound effect on the patterning of the heart, particularly the venous inflow and arterial outflow, has yet to be understood.
Predominantly endodermal structures are described below.
The bronchial branching pattern (and lung lobation) can be normal, inversus, right isomeric, or left isomeric. Liver lobation can be normal, inversus, or symmetric. In the gallbladder and biliary tree, hypoplasia, absence, and duplication can be noted. The spleen can be normal, absent, hypoplastic, or multiple. The intestine can develop malrotation.
Predominantly mesodermal structures are described below.
The hepatic segment of inferior vena cava (IVC) can be present or absent (so-called “interrupted IVC"). The hepatic veins can be normal (join IVC just proximal to the IVC-atrial junction) or can connect independently to atria. The coronary sinus can be normal, absent, or completely unroofed. The superior vena cava (SVC) can be normal (unilateral) or bilateral. Pulmonary veins can be partially anomalous or totally anomalous. Appendage morphology can be normal, inversus, right isomeric, or left isomeric. The common atrioventricular canal (CAVC) is usually significantly malaligned toward the morphologic right ventricle (RV) but can be malaligned toward the morphologic left ventricle (LV). The ventricles can be D-loop or L-loop (supero-inferior ventricles is rare). In the outflow tract, subpulmonary stenosis or atresia is usually noted, but subaortic stenosis can be observed. Double-outlet RV is most common, but tetralogy of Fallot can occur. Transposition of the great arteries can occur.
Either left aortic arch with left upper descending aorta or right aortic arch with right upper descending aorta can occur. Double aortic arch is exceedingly rare. In cases of left aortic arch with left upper descending aorta, the abdominal aorta is left of the spine. In cases of right aortic arch with right upper descending aorta, the abdominal aorta is right of the spine (unlike the situation without heterotaxy, in which the abdominal aorta is left of the spine). Many cases have both the abdominal aorta and the IVC (or azygos, if the hepatic IVC is absent) on the same side of the spine (unlike without heterotaxy, in which the IVC is right of the spine whereas the abdominal aorta is left of the spine).
Although the genetic underpinnings of left-right patterning of the embryonic brain and spinal cord have been studied extensively in some vertebrate systems, relatively little is known about this in humans so far.
The worldwide incidence is reportedly 1 case per 10,000 births.  This closely approximates the findings of the Baltimore-Washington Infant Study, in which the incidence of cardiac malformations associated with abnormal laterality was estimated at 1.44 cases per 10,000 live births. 
The true prevalence of heterotaxy syndrome is unknown because many patients, especially those with left atrial appendage isomerism or polysplenia, have sufficiently mild heart disease such that the underlying diagnosis of heterotaxy may not even be considered by the clinician.
No predilection based on race has been identified.
In two studies, a male-to-female ratio of 2:1 was observed.
Age at presentation depends largely on the severity of the heart disease.
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