Congenital Abnormalities situs and heterotaxy syndrome

Situs
Situs refers to orientation of certain organs to the midline. In general, the bilobed lung, left atrium, descending aorta, spleen, and stomach are all located on one side of the body. When these structures are normally placed on the left side the condition is called situs solitus. Situs inversus is a mirror image and results when these structures are on the right side and the trilobed lung, right atrium, inferior vena cava, gallbladder, and liver are on the left.
The prevalence of situs inversus is about 1 in 10,000. These patients are at an increased risk for nasal polyposis, chronic sinusitis, and bronchiectasis (Kartagener’s syndrome) and a higher than normal risk for congenital heart disease. Situs inversus can be associated with multiple spleens and other defects. Often a conventional radiograph is a good starting point in evaluating these anomalies.
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This is radiology images of the Situs inversus in an 85–year-old woman. Polysplenia is present on the right (arrows)
Heterotaxy Syndrome                The term heterotaxy, or situs ambiguous, describes complex congenital abnormalities differing from situs solitus or situs inversus. Some authors use the terms asplenia and polysplenia to subdivide heterotaxy into specific categories, but these classifications are incomplete and at times confusing. Others use the terms left and right isomerism.
               Complex familial inheritance patterns exist for heterotaxy. An X-linked recessive inheritance appears to account for a male preponderance. These patients usually have complex congenital cardiac abnormalities that tend to overshadow abdominal findings. Complex bowel rotation anomalies are encountered with both right and left isomerism. Aortic and gastric positions are variable. Right-sided isomerism results in asplenia. The liver extends across both upper quadrants of the abdomen. Some of these patients have a left-sided inferior vena cava. Gallbladder duplications occur in right-sided isomerism.
               Heterotaxy with left abdominal isomerism results in polysplenia. Cardiac anomalies tend to be less severe than in asplenia. Most patients have azygous continuation with interruption of the inferior vena cava. The spleen(s) is (are) on the same side as the stomach. An absent gallbladder is more common in left-sided isomerism. An association exists between polysplenia and biliary atresia. Midgut malrotation is a common associated finding, and midgut volvulus develops in some of these infants.
                Traditionally, heterotaxy has been evaluated with angiocardiography. While both CT and pulse and color Doppler US are helpful in outlining some anomalies, MRI is evolving as the preferred modality. Liver/spleen scintigraphy detects splenic tissue in suspected heterotaxy; in some infants with negative planar imaging, the presence of splenic tissue is shown by SPECT.
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