been described and can be technically simple and functionally allow for equal hepatic flow distribution With regard to biventricular repairs in the setting of isomerism, there are no particular special considerations that are necessarily present Arrangement of Thoracoabdominal Organs A jumbled-up arrangement of the abdominal organs has long been recognized as the hallmark of the “splenic syndromes.” Even though the thoracic organs are isomeric, the abdominal organs are not This means that isomerism cannot reliably be diagnosed on the basis of the disposition of the abdominal organs, in particular the arrangement of the spleen If splenic tissue is absent, this should be documented since this feature carries connotations for the immune state of the patient (see below) The spleen, nonetheless, while expected to be absent in those with right isomerism, can also be absent in those with isomeric left atrial appendages The arrangement of the remaining abdominal organs is also of significance A short mesentery can lead to intestinal volvulus and occurs in right or left isomerism.8 The pancreas tends to be short or annular only in the presence of left isomerism The morphology of the thoracic organs, in contrast, specifically the bronchial tree, is a much better guide to the presence of isomerism of the atrial appendages Bilateral long and hyparterial bronchi are indicative of left isomerism (Fig 26.18A), while bilateral short and eparterial bronchi (see Fig 26.18B) are found in those with right isomerism Not all patients with isomeric atrial appendages, however, have bronchial isomerism The bronchial angle can also be telling in those with isomerism with a tracheobronchial angle of less than 135 degrees being more consistent with left isomerism and a tracheobronchial angle of greater than 135 degrees being more consistent with right isomerism Thus, while examination of the penetrated chest radiograph was considered a useful step in the evaluation of the infant or child suspected of having isomeric atrial appendages, it is now rarely used as a diagnostic tool.68–70 FIG 26.18 (A) Posterior view demonstrating bilateral, long, hyparterial main bronchi in a patient with isomerism of the left atrial appendages Both morphologically left bronchi are below the pulmonary artery extending to the lower lobe of the lung (B) Posterior view demonstrating bilateral, short, hyparterial main bronchi in a patient with isomerism of the right atrial appendages The first branch of both morphologically right bronchi is eparterial or above the pulmonary artery that extends to the lower lobe of the lung Note the right and left pulmonary veins joining in a confluence to drain via a single vein (star) below the diaphragm Visceral Symmetry Without Isomeric Atrial Appendages Although the focus of this chapter is on isomerism of the atrial appendages, there is increasing evidence to suggest that varying degrees of thoracic symmetry may occur in the presence of usual atrial arrangement Indeed, a symmetrical arrangement of the lungs, with a jumbled-up arrangement of the abdominal organs, appears to occur with significantly greater frequency than does isomerism of the atrial appendages.16,23 In a genetic analysis of isomerism, a healthy sibling of a proband with right isomerism was noted to have left bronchial isomerism.24 Abnormal lateralization, with a tendency to thoracic symmetry, can therefore be found in patients with structurally normal hearts These observations have important implications for research into the etiology of abnormal lateralization and for genetic counseling.25 For the purposes of genetic studies, cases showing any degree of abnormal lateralization should be distinguished from the “normal.” External phenotype, and cardiac morphology, is an inadequate indicator of the overall arrangement of the organs Thus each system of organs requires individual and specific analysis It is best to avoid inferring the morphology of one group of organs based on observations in another At the same time, for these purposes above all, it is necessary to stratify the so-called “splenic syndromes,” or “visceral heterotaxy,” into the subsets of right and left isomerism