Thoracic Situs Solitus With Left Isomerism
Isomerism is derived from the Greek isos (equal) plus meros (part) and refers to bilateral similarity if not equivalence of structures. A strong association exists between left isomerism and transverse liver with inferior vena caval (IVC) interruption (infrahepatic absence of the IVC). The infrarenal segment of the IVC continues as an azygos or hemiazygos vein that joins the right or left component of bilateral superior vena cavas (SVCs). The azygos vein ascends along the right side of the vertebral column, whereas the hemiazygous ascends along the left side. Thoracic left isomerism is characterized by bilateral morphological left bronchi with bilateral morphological bilobed left lungs. Bilateral SVCs join atrial chambers with bilateral morphological left atrial appendages. Polysplenia is usually but not invariably present, and the stomach is usually right sided. The images shown here are from a 32-year-old woman with a unique discordance: thoracic situs solitus but abdomino-thoracic left isomerism. The heart was left sided (Figure 1A); a right-sided morphological right bronchus was concordant with a trilobed lung, and a left-sided morphological left bronchus was concordant with a bilobed lung (Figure 1B). These situs solitus arrangements anticipated a right-sided morphological right atrium (RA) and a left-sided morphological left atrium (LA), but the LA was positioned superior to the RA (Figure 1A). Left isomerism was represented typically by a transverse liver (Figure 1C), an interrupted IVC, and a large hemiazygos vein that continued along the left side of the vertebral column to join a solitary left SVC. Surprisingly, bilateral SVCs were absent (Figure 2). Because no junction existed between a right SVC and a morphological RA, the sinus node was absent (left-axis deviation of the P wave), so the atrial rhythm was ectopic. Hepatic veins drained into the RA (Figure 1A), and pulmonary veins drained into the LA (Figures 1B and 2⇓). Stomach and spleen were right sided, but the spleen was atypically single (Figure 1D). The patient was status postdilatation with stenting of aortic coarctation (Figure 2).
The editor of Images in Cardiovascular Medicine is Hugh A. McAllister, Jr, MD, Chief, Department of Pathology, St Luke’s Episcopal Hospital and Texas Heart Institute, and Clinical Professor of Pathology, University of Texas Medical School and Baylor College of Medicine.
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