You are in: eMedicine Specialties > Radiology > PEDIATRICS Asplenia/PolyspleniaArticle Last Updated: Feb 26, 2008AUTHOR AND EDITOR INFORMATIONSection 1 of 11
  Author: Marta Hernanz-Schulman, MD, FAAP, Professor, Radiology, Radiological Sciences, and Pediatrics, Director, Department of Pediatric Radiology, Radiologist-in-Chief, Director, Department of Diagnostic Imaging, Vanderbilt University Medical Center, Vanderbilt Children's Hospital Marta Hernanz-Schulman is a member of the following medical societies: American Institute of Ultrasound in Medicine and American Roentgen Ray Society Editors: Henrique M Lederman, MD, PhD, Consulting Staff, Department of Radiology, The Children's Hospital of Philadelphia; Professor of Radiology and Pediatric Radiology, Chief, Division of Diagnostic Imaging in Pediatrics, Federal University of Sao Paulo, Brazil; Bernard D Coombs, MB, ChB, PhD, Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand; David A Stringer, BSc, MBBS, FRCR, FRCPC, Professor, National University of Singapore; Clinical Director, Diagnostic Imaging, National University Hospital; Head, Diagnostic Imaging, KK Women's and Children's Hospital, Singapore; Robert M Krasny, MD, Consulting Staff, Department of Radiology, The Angeles Clinic and Research Institute; John Karani, MBBS, FRCR, Consulting Staff, Department of Radiology, King's College Hospital, London Author and Editor Disclosure Synonyms and related keywords: heterotaxy, heterotaxia, asplenia, polysplenia, laterality, isomerism, right-sided isomerism, left-sided isomerism, right isomerism, left isomerism, Ivemark syndrome, situs ambiguus, situs ambiguous, partial situs inversus, polyasplenia, polyasplenia syndrome, situs solitus, visceral symmetry, visceral asymmetry, situs inversus, laterality, situs inversus totalis, Kartagener syndrome, total anomalous pulmonary venous connection, TAPVC, viscerovascular anomalies, Xq26, 18p, interrupted inferior vena cava INTRODUCTIONSection 2 of 11
  BackgroundAlthough symmetry is a characteristic of the external mammalian phenotype and of some internal organs, notably the genitourinary system, much of our internal anatomy, notably the cardiovascular, pulmonary, and gastrointestinal systems, is asymmetrical.1, 2, 3, 4 The asymmetry is specific and originates in the genetic and molecular identity of the embryonic midline developmental field complex.5, 6, 7 This normal asymmetrical arrangement is called situs solitus, a descriptive term derived from the Latin situs, which means position, and solitus, which means customary. Thus, situs solitus signifies the customary, or normal, asymmetrical arrangement of the viscerovascular anatomy. Situs inversus is the mirror image of situs solitus in toto. Although this condition is embryologically related to other situs anomalies, situs inversus must be strictly differentiated, because it has significantly different pathophysiologic and clinical implications. All other conditions occur in a spectrum called situs ambiguus, which is commonly changed to the Anglican situs ambiguous. The concept of situs or laterality of the viscerovascular anatomy is important to the understanding, imaging, and interpretation of congenital visceral and vascular malformations. Assignment of laterality can be inherently difficult and affected by specific clinical biases. For example, to the cardiologist applying the concept of segmental classification of cardiac anomalies, situs refers to atrial morphology.8 The pulmonologist is likely to assess lung lobation and bronchovascular anatomy. The gastroenterologist may focus on the presence or absence of the spleen.9 Patients with situs ambiguus tend to be grouped with those in whom right- or left-sided structures predominate. Generalizations may be made in these groups. Patients with right-sided symmetry typically lack a spleen, whereas patients with left-sided symmetry typically have a segmented spleen or multiple splenules.10 These common characteristics have led to the somewhat arbitrary classification of asplenia and polysplenia. However, heterotaxia, or situs ambiguus, occurs in a continuum; this is acknowledged in the classification of polyasplenia syndrome. This term recognizes the fact that left- and right- sided tendencies are on a continuum of heterotaxy or midline derangement. Radiologists and other clinicians must be cognizant of the viscerovascular arrangements that are possible in infants with these conditions, and they must describe the specific viscerovascular anomalies in the patient. Many infants with situs ambiguus present with severe congenital cardiac anomalies. In these patients, knowing the presence of an associated interruption of the inferior vena cava is helpful before performing cardiac catheterization.11 Situs ambiguus, or heterotaxia, is associated with other conditions of major clinical relevance, such as intestinal malrotation, biliary atresia, splenic abnormalities and consequent immunologic derangements, faulty gastric suspension mechanisms, displacement of abdominal viscera, and aberrant vascular structures and vascular connections. Each of these abnormalities is derived from an embryologic inability to determine laterality and establish the complex solitus asymmetry, whereas symmetrical structures remain unaffected.12 PathophysiologyThe control of normal human somatic asymmetry is not yet known. A mouse animal model exists in which a spontaneous mutation called the iv locus is localized to the 12th chromosome.13, 14 Homozygous animals have a random phenotype: approximately 30% have situs solitus; 30%, situs inversus; and 40%, situs ambiguus. A second murine genetic mutation has been identified in the inv gene.15, 16 Homozygous individuals have shown reversed embryonic turning, the earliest manifestation of laterality in mouse embryos. Asymmetrical expressions of genes have been identified in other animal models, notably Xenopus species and chick embryos.17 In humans, familial situs ambiguus has been related to both autosomal and X-linked inheritance patterns, although most cases arise sporadically.2, 18, 19, 20, 21 Situs inversus and situs ambiguus have been described within the same family trees; this finding indicates a phenotypic arrangement similar to that in the iv/iv mice. ZIC 3 is an X-linked gene identified in both sporadic and familial cases; affected males typically have the situs ambiguus phenotype, and females have either situs solitus or situs inversus. This particular genetic code is also associated with midline anomalies, particularly neural tube defects. A submicroscopic deletion in Xq26 and a deletion at 18p have been associated with familial situs ambiguus.22, 23 Further, balanced and unbalanced autosomal translocations have also been described in sporadic cases of situs ambiguus.24 Finally, environmental factors, including exposure to retinoic acid and maternal diabetes, have been implicated in laterality defects among the offspring of affected parents.25, 26 Asymmetrical or unilateral organs are derived from embryologic midline structures. Disruption of embryonic laterality leads to faulty development of asymmetrical organs. Importantly, in the absence of solitus control, the viscerovascular arrangement becomes largely random, within broad generalizations regarding right- or left-sided tendencies. However, the affected organs or systems are predictable and characterized by complex asymmetry. The heart is most often affected, and it is the organ that most frequently leads to clinically detectable abnormalities. The liver and spleen have a variety of patterns, ranging from malformation to malpositioning, to an absence of the spleen or biliary structures. The gastrointestinal tract has a variety of malrotation anomalies. As a result of selective resorption of embryonic paired structures, vascular structures typically demonstrate a variety of abnormalities related to malpositioning or segmental absence. FrequencyUnited StatesThe true incidence of situs abnormalities is not known. Some estimates suggest that situs inversus totalis occurs in approximately 1 per 8,000-25,000 live births, and approximately 20-25% of these are associated with immotile cilia, or Kartagener syndrome. Mortality/MorbidityThe major causes of mortality and morbidity in the heterotaxy syndromes are undoubtedly the cardiac malformations that typically occur in these conditions. These are based on the inability of the complex asymmetrical connections to develop correctly, and they predictably consist of an ambiguus and single atrium, a single ventricle, and conotruncal anomalies such as truncus arteriosus and transposition of the great vessels. Vascular malconnections that are associated with high mortality and morbidity rates include total anomalous pulmonary venous connections (TAPVCs).27 Visceral abnormalities with clinically notable adverse consequences include biliary atresia and the absence of the spleen. Malrotation may become clinically evident if obstruction develops secondary to the presence of Ladd bands or if a midgut volvulus supervenes.28 Intrinsic duodenal obstruction, such as that secondary to duodenal diaphragm, may also occur. SexAlthough most cases of heterotaxia are sporadic, many cases are familial, and some are X linked. Thus, the abnormality is more common in males than in females. AgeSitus is congenital; therefore, situs abnormalities are always present in affected neonates. However, clinical manifestations depend on the individual's specific anatomic derangements. Severe and complex cardiac abnormalities are likely to be apparent at birth or soon afterward. Individuals without congenital heart disease may present later with conditions such as biliary atresia or volvulus. In persons without clinically morbid abnormalities, these conditions may be discovered during an evaluation for an unrelated problem or complaint when they are adults.29, 30 AnatomyAnatomic pathologic findings in patients with heterotaxy syndrome are described in this section within the broad categories of right- and left-sided tendencies. Some of the more notable exceptions to these general groupings are also discussed. Splenic anomalies Patients with asplenia lack a spleen, and they typically have a right-sided tendency. In these patients, Howell-Jolly bodies may be present, and diminished immunologic defenses may result in overwhelming sepsis. Patients with polysplenia have large variations in the configurations of the splenic tissue. Splenules develop along both sides of the dorsal mesogastrium (rather than just on the left side, as in solitus asymmetry). The resultant splenic tissue is always found along the greater curvature of the stomach. It may consist of multiple small splenules or a single splenic mass with 1 or more septae (see Images 1-4). Gastrointestinal and hepatic anomalies Patients in both the asplenia and polysplenia subgroups have a variety of gastrointestinal abnormalities. The stomach may be located on the right, on the left, or in the midline. Faulty mesenteric attachments of the stomach may result in gastric volvulus. Malrotation anomalies encompass a wide spectrum that ranges from nonrotation to reversed rotation and faulty peritoneal attachments. Duodenal atresia or stenosis may also be present (see Images 3-6). The liver may lie in the midline or in the right or left side of the abdomen. In patients with polysplenia, biliary atresia may be present.29, 31, 32, 33, 34 In fact, 10% of all patients with biliary atresia also have polysplenia. This co-diagnosis is important to establish, because it has the potential for anomalous caval and portal venous connections and because it is relevant to eventual surgical planning. The gallbladder may lie in the midline or be lateralized with the bulk of the hepatic mass. The hepatic venous drainage is variable. Some patients with heterotaxy, notably those with polysplenia, may also have a congenitally short pancreas, which is the result of maldevelopment or agenesis of the dorsal pancreas (which develops in the dorsal mesogastrium).10 A midline adrenal gland or a horseshoe adrenal gland may be seen in other patients, notably those with asplenia. Approximately 1% of patients may also have renal anomalies, such as a multicystic dysplastic kidney. Central nervous system anomalies A small subset of individuals with situs ambiguus also has a variety of midline defects, including central nervous system (CNS) anomalies (eg, holoprosencephaly, neural tube defects) and caudal regression syndrome. This association underscores the derangement of the midline developmental complex, which is implicated in the early genesis of situs derangements. Vascular anomalies Interruption of the inferior vena cava is most common in patients with polysplenia. This occurs in approximately 50-60% of patients with this condition. In patients with an interrupted inferior vena cava, venous return occurs via the right- or left-sided azygous systems (see Images 7-9). This condition is rare in patients with asplenia, but it has been reported in several patients. This finding underscores both the heterogeneity and underlying similarities among all patients with situs ambiguus. As a generalization, in a patient with heterotaxy, interruption of the inferior vena cava suggests polysplenia, with very few exceptions, as discussed. In patients with an intact vena cava, both the vena cava and the aorta may be to one side of the midline; this condition was once considered characteristic of asplenia. Alternatively, the vena cava and aorta may lie on opposite sides of the midline. When the vena cava lies to the right, situs solitus is simulated. When the vena cava lies to the side opposite its atrial connection, it crosses over the aorta in piggyback fashion to reach its destination. This condition has also been reported to be characteristic of asplenia, but it can occur in any patient with heterotaxy and an intact vena cava. When the inferior cava is present, any of these anatomic arrangements may occur, regardless of a right- or left-sided tendency (see Image 10-13). In TAPVC, the common pulmonary vein fails to become incorporated into the left atrium; instead, it drains anomalously. Return may be supracardiac (via a vertical vein or directly into the superior cava or azygous vein), intracardiac (typically into the coronary sinus), or infracardiac and infradiaphragmatic. Venous obstruction can occur in any of these situations, but it is the rule in infradiaphragmatic connections (see Images 14-16, Images 22-24). TAPVC is most common in patients with asplenia, although abnormal pulmonary venous connections can occur in both asplenia and polysplenia groups; more often, the abnormal pulmonary venous connection is partial in the polysplenia group. TAPVC is one of the lesions that is associated with the overall higher mortality rate of the group of patients with asplenia. The typical differences in atrial venous connections can be conceptualized within the context of atrial isomerism. In patients with polysplenia, the common atrium has left-sided characteristics, and the inferior vena cava fails to connect to a morphologic left atrium. In patients with asplenia, the common atrium has right-sided characteristics, and the pulmonary veins in turn fail to make the appropriate connections to a morphologic right atrium. Bilateral superior vena cavae are common in both asplenia and polysplenia (see Image 17). Vitelline anatomy is also skewed in situs ambiguus. The normal retroduodenal course of the portal vein is a result of specific resorption of segments of the paired vitelline veins. When the larger portion of the left vitelline vein is resorbed, the resultant anatomic relationship of the portal vein is preduodenal. This anatomy is most often noted in patients with polysplenia, although it also occurs in the asplenia group (see Images 18-21). Cardiac anomalies Patients with asplenia and those with polysplenia often have biventricular hearts, in which large atrioventricular defects result in a single-atrium, single-ventricle configuration. However, in general, patients with asplenia tend to have complex cardiac lesions more frequently than those with polysplenia. Patients with polysplenia may have only a ventricular septal defect or no cardiac anomalies at all. (Interestingly, this is the subset of patients who most often have biliary atresia.) When the patients with cardiac abnormalities are examined, a description of the lesions based on the cardiac segmental anatomy is helpful. Atrial morphology is described in terms of the situs condition, as follows: solitus (S), inversus (I), or ambiguus (A). Ventricular morphology is described in terms of cardiac looping, either to the right or dextroposed (D), which results in the normal position of the left ventricle on the left side and the right ventricle on the right side, or to the left or levoposed (L), which results in an inverted ventricular configuration. The great arteries are described in terms of the position of the aorta relative to the pulmonary artery, either normal (ie, to the right [D]) or abnormal (ie, to the left [L]). In normal hearts, solitus atria is present, with a morphologic right atrium. This contains nodal tissue, a coronary sinus, and a wide-based atrial appendage; it receives the systemic veins; and it is located on the right.35 Also present is a morphologic left atrium, which has a narrow-based atrial appendage, receives the pulmonary veins, and is located on the left. Patients with situs ambiguus typically have atrial isomerism. In patients with asplenia, a single atrium that morphologically resembles a right atrium tends to be present, whereas in patients with polysplenia, the atrium morphologically resembles a left atrium. Abnormal ventricular looping is also present in patients with an ambiguus situs. The incidence of L-loop ventricles occurs in approximately 38% of patients with asplenia and in 30% of those with polysplenia. A normal heart, with solitus atria, a D loop, and a right-sided aortic valve with respect to the pulmonary valve, is designated SDD. Patients with congenital heart disease and situs ambiguus tend to have discordant cardiac segmental anatomic features. In patients with asplenia, conotruncal abnormalities are common. They include truncus arteriosus, conal hypoplasia with pulmonary outflow obstruction or pulmonary atresia, and anomalous pulmonary venous connections (see Images 14-16, Images 21-23). These patients tend to have diminished pulmonary blood flow and cyanosis. In patients with polysplenia, atrioventricular septal defects are common (see Image 17). Pulmonary stenosis is not common, and these patients tend to have increased pulmonary blood flow. Clinical DetailsSee the Anatomy section, above. Preferred ExaminationNo single examination is best for the evaluation of heterotaxy. What may be best, however, is an ambiguous and subjective term that can be defined as the fastest and easiest study (eg, plain radiography), the least invasive study (in terms of radiation exposure or the need for sedation), the least expensive study, or the most sensitive or specific study. Knowledge of the various anatomic derangements that occur in these patients enables the informed radiologist to make the appropriate observations on most cross-sectional images. Often, the initial examination performed in these patients is plain chest radiography, which is often performed in patients with suspected congenital heart disease, for which the patients may seek clinical attention in the first place.36, 37, 38 Patients with an abdominal catastrophe, such as midgut or gastric volvulus, also undergo plain radiography as part of their initial diagnostic imaging examination.39 On the other hand, patients with biliary atresia do not undergo plain radiography as part of their initial workup. Historically, the diagnosis often depends on the angiographic findings in the cardiac and abdominal structures or on the results of abdominal examination via an iatrogenic pneumoperitoneum. However, ultrasonography, computed tomography (CT), and magnetic resonance imaging (MRI) are all useful in assessing the various viscerovascular abnormalities in patients with heterotaxy.30, 38, 40, 41, 42, 43, 44, 45 MRI is particularly useful because it can also delineate cardiac defects. Limitations of TechniquesPlain radiographs can be helpful in delineating an abnormal visceroatrial situs. A midline heart and liver, with a midline or right-sided stomach, and symmetrical bronchial branching are diagnostic findings. However, many patients with situs ambiguus do not have any of these findings, and plain radiographic findings may erroneously suggest situs solitus or situs inversus totalis. Ultrasonography is extremely useful, but it is operator dependent, and findings may be unrevealing in cases in which the operator fails to understand the complex anatomic relationships that result from derangement of the normal laterality. DIFFERENTIALSSection 3 of 11
Situs Inversus Other Problems to Be ConsideredIsolated dextrocardia RADIOGRAPHSection 4 of 11
FindingsAs discussed previously, plain radiography is the first study conducted in patients with suspected heterotaxy. In many patients with situs ambiguus, radiographs provide the initial clues to the diagnosis, which should be made in patients with a midline liver, in those with a right-sided or midline stomach, and in those with mesocardia or dextrocardia. Special attention should be directed to the pulmonary hila for the detection of inverted or symmetrical anatomy. This evaluation is often not possible in infants, in whom the overlying thymus effectively obscures the pulmonary hila anatomy. When identified, bilateral eparterial bronchi (bronchus above the artery, as in a typical right hilum) are suggestive but not diagnostic of asplenia, whereas bilateral hyparterial bronchi (bronchus underneath the artery, as in a typical left hilum) are suggestive but not diagnostic of polysplenia. The presence of a large azygous arch with the absence of the caval shadow on the lateral view further confirms heterotaxy, and these findings are highly suggestive of the polysplenia complex because of the interruption of the inferior vena cava with a right- or left-sided azygous continuation (see Images 24-26). The laterality of the aortic arch is relatively unhelpful because a right-sided aortic arch occurs frequently in situs solitus, and the arch may be either right- or left-sided in situs ambiguus. Contrast-enhanced studies of the gastrointestinal tract invariably demonstrate a rotational anomaly, which encompasses a wide spectrum. Malrotation, nonrotation, mirror-image nonrotation, and reversed rotation can all be seen in individual patients (see Images 5-6). Duodenal atresia and/or stenosis, sometimes due to an annular pancreas, may also be present. The external indentation of a preduodenal portal vein may also be identified. Degree of ConfidenceFindings with plain radiography are accurate for the diagnosis of situs ambiguus, when present. However, in some series, radiographic results in as many as 50% of patients with situs ambiguus falsely suggest either situs solitus or situs inversus totalis. Gastrointestinal anomalies detected with contrast-enhanced examinations are present in all patients with situs ambiguus. However, these anomalies may also be present in situs solitus. Therefore, outside the clinical context, these findings are not specific for situs ambiguus. False Positives/NegativesInterruption of the inferior vena cava with azygous continuation should suggest situs ambiguus, particularly polysplenia. However, this finding is not specific, and it can be seen as an isolated anomaly in those in whom a solitus condition is otherwise present. A right-sided aortic arch is most frequently encountered in patients with situs solitus. Typically, it indicates congenital heart disease, particularly tetralogy and truncus arteriosus, if mirror-image branching is present. The presence of a vascular ring is strongly suggested in cases of non–mirror-image branching. In situs ambiguus, the aortic arch could be either right- or left-sided. Thus, a right-sided aortic arch should not be initially construed as an indicator of a derangement in the laterality. Rotational anomalies of the midgut are most often encountered in individuals with a solitus condition. CT SCANSection 5 of 11
FindingsCT scanning may be helpful in delineating viscerovascular abnormalities in patients with heterotaxy and in determining whether they should be grouped with patients who have asplenia and a right-sided tendency or with those who have polysplenia and a left-sided tendency. The collimation should be set to encompass the smallest structures of interest; therefore, the scanner setting varies with the age of the patient. In infants, 3-mm collimation is necessary to illustrate the bronchovascular anatomy. Intravenously administered contrast material is essential for delineation of the cardiovascular anatomy, as well as for adequate evaluation of the abdominal visceral anatomy. An abnormal bronchial situs is highly likely in patients with these conditions; bilateral eparterial bronchi suggest asplenia, whereas bilateral hyparterial bronchi suggest polysplenia. Bilateral superior vena cavae are often present, or a single vena cava may be present on the right or left side. Malseptation of the cardiac chambers may be seen with contrast enhancement. Conotruncal anomalies, such as transposition with anterior placement of the aorta or truncus arteriosus may be identified. Abnormal drainage of the pulmonary veins can also be defined. The common pulmonary vein is a contrast-enhancing chamber dorsal to the atrium that resembles cor triatriatum. In the abdomen, the presence of abnormal splenic tissue may be identified along the greater curvature of the stomach with contrast enhancement (see Image 4). Splenules are highly variable and may consist of multiple small structures, a few small structures, or a large or lobulated spleen. Lack of any splenic tissue and the absence of a splenic artery and vein indicate asplenia. A preduodenal portal vein is characterized by an anterior course of the superior mesenteric venous–portal junction ventral to the pancreas. Interruption of the intrahepatic inferior vena cava occurs when this vessel is absent; instead, a large right- or left-sided azygous vein is seen adjacent to the aorta in the retrocrural area. This finding is far more characteristic of polysplenia than of asplenia. When the inferior vena cava is present, it typically lies to either side of the midline in both conditions. Infradiaphragmatic drainage of TAPVC can be identified by following the vertical vessel that arises in the common pulmonary vein through the esophageal hiatus into its abdominal insertion. Malrotation is also evidenced by the lack of the normal retroperitoneal course of the duodenum and the abnormal position of the cecum. Degree of ConfidenceFor the constellation of findings described above, the degree of confidence for CT scanning is 100%. By definition, the absence of the spleen or the presence of multiple splenules are likewise diagnostic findings. However, for any one of the associated abnormalities, the degree of confidence can be substantially lower, depending on the particular abnormality. Discordant positioning of the heart and abdominal viscera is characteristic and diagnostic of heterotaxy. Ambiguity of the atrial morphology and symmetrical bronchial branching are also characteristic. However, individual lesions, including conotruncal anomalies, anomalous pulmonary venous connections, intrahepatic interruption of the inferior vena cava, malrotation, and biliary atresia, may be seen in patients with normal solitus anatomy. Bilobed spleens and accessory spleens can be seen in 10% of the healthy population. False Positives/NegativesPlease see the discussion about false-positive and false-negative findings in Radiograph, False Positives/Negatives. MRISection 6 of 11
FindingsIn many ways, MRI is an ideal modality for the examination of patients with heterotaxic conditions. As with CT scanning, the section thickness should be set to the diameter that encompasses the structures of interest. Cardiac gating is needed in the evaluation of the cardiovascular system. Abnormal bronchial and pulmonary arterial branching anatomy are evident in both the coronal and axial MRI planes. The anatomy of the superior cavae, anomalous pulmonary venous connections, conotruncal anomalies, ventricular inversion, and atrial and ventricular divisions can be identified (see Images 16-17, Images 22-23). Abdominal findings, including hepatic laterality, hepatic vascular anatomy, interruption or malpositioning of the inferior vena cava, positioning and size of the stomach, and presence or absence of splenic tissue, are clearly delineated with MRI. Intra-abdominal connections of the common pulmonary vein in cases of TAPVC can also be identified (see Images 3, 8-9, 15-16, 22-23, 27). Degree of ConfidenceThe degree of confidence for MRI is the same as that of CT scanning. ULTRASOUNDSection 7 of 11
FindingsIn addition to MRI, ultrasonography may also be an ideal imaging modality in the examination of patients with situs ambiguus. Its advantages are the same as those of other modalities: ultrasonography is noninvasive, does not require radiation, and can be used to image pathology in its own plane. The disadvantages of ultrasonography are also the same: it is operator dependent, and sound waves cannot penetrate bone or air. Therefore, bronchial branching anatomy cannot be accurately evaluated, and the abdominal examination may be relatively impeded in patients with large amounts of bowel gas. An appropriate transducer, usually one of 6-MHz or higher, should be used for neonates or young infants. The infant is best examined after feeding, to prevent him or her from swallowing large amounts of air during crying and to help the infant remain quiet or asleep, conditions that enable detailed imaging. In infants, chest ultrasonography can be used to evaluate the mediastinal anatomy via the poorly calcified chest wall with the use of large cartilaginous windows and the thymic window. Laterality or bilaterality of the superior vena cavae, great vessel anatomy, ventricular position and relationships, and intracardiac anatomy can be readily assessed. The common pulmonary vein is identified dorsal to the left atrium. These findings are routinely made with echocardiography. Abdominal ultrasonography is best begun by determining the position of the stomach. This can be found by identifying the esophagus at the midline hiatus and by following it either to the right or left. Splenic tissue, when present, is found along the greater curvature of the stomach. Usually, finding the adrenal gland ipsilateral to the stomach is best. Scanning along this plane reveals the splenic tissue, if present. The appearance of the spleen, again, is highly variable; it may range from a large, septate spleen to one with multiple, small splenules that may lie close together (see Images 1-2). In some patients with asplenia, the liver extends across the midline and overlies the stomach; it may resemble splenic tissue. However, close attention to the internal architecture of the organ reveals the important differences. Splenic tissue has large hilar vascular structures, whereas hepatic extension has only small peripheral branches, which can be followed to larger trunks within the main hepatic mass (see Image 28). The position of the liver in the abdomen, the location of the gallbladder, and the hepatic venous anatomy can be easily evaluated. The course of the portal vein, whether preduodenal or retroduodenal, can also be evaluated in both the longitudinal and transverse planes. In the longitudinal plane, the superior mesenteric vein extends anterior to the pancreas and duodenum and courses ventrally into the liver; on transverse images, its cross-section is seen anterior to the pancreas (see Images 18-21). Azygous continuation of an interrupted intrahepatic inferior vena cava is best evaluated on the transverse view. The intrahepatic cava is absent. Instead, a vessel is present adjacent to the aorta in the retrocrural region; it represents the right- or left-sided azygous continuation. Doppler ultrasonographic imaging can reveal the arterial signal of the aorta and the venous signal of the azygous vein. On coronal images, the 2 parallel vessels are seen along the retroperitoneum, entering the chest cavity. An intact inferior vena cava tends to line up with the aorta to either side of the midline and to cross over in piggyback fashion, when necessary, to enter the ambiguus atrium (see Image 7, Images 10-13). In cases of total anomalous venous connections below the diaphragm, the anomalous connecting vein is easily identified; it enters the abdomen anterior to the esophagus through the esophageal hiatus. Its course to the exact point of abdominal drainage can then be determined (see Image 14). Other anomalies, such as a dilated duodenum in cases of duodenal stenosis and/or atresia, midline horseshoe adrenal gland, and renal anomalies, can also be identified. Degree of ConfidenceThe degree of confidence for ultrasonography is similar to that of MRI. False Positives/NegativesAlthough ultrasonography is extremely useful in heterotaxic conditions, this modality is also highly operator dependent. The operator's failure to understand the anomalies that occur could lead to a significant number of both false-positive and false-negative results. For example, a failure to understand the location of splenules along the greater curvature of the stomach could prevent a diagnosis. Conversely, imaging of the hepatic extension in asplenia could lead to a false diagnosis of polysplenia or situs inversus totalis. NUCLEAR MEDICINESection 8 of 11
FindingsIn the diagnosis of asplenia versus polysplenia, resorting to the liver-spleen scan is tempting. Indeed, in healthy patients with solitus anatomy, the liver and spleen are clearly identified with the use of technetium-99m sulfur-colloid (99mTc-SC) scintigraphy. However, differentiation of hepatic versus splenic activity is considerably more difficult (1) in patients who may have no spleen and in whom the liver may extend into both upper quadrants and (2) in those who may have splenic tissue in either quadrant (see Image 19). Unlike ultrasonography and MRI, scintigraphy requires radiation exposure and venipuncture. In some patients, splenic activity may be clearly distinct from that of the liver if the splenules are clearly distinguished from the liver and from each other. Scintigraphy with a bolus of contrast material that is administered during angiography may demonstrate caval interruption with azygous continuation, if the agent is injected into a foot vein. The images may demonstrate the absence of a direct connection to the inferior atrium. However, other viscerovascular abnormalities are not typically depicted. Given the availability of ultrasonography, CT scanning, and MRI, scintigraphic evaluation is needed infrequently. Degree of ConfidenceWhen splenules are close together, the splenic mass appears as a single spleen, which is difficult to differentiate from situs solitus or situs inversus totalis. If the hepatic mass extends to both upper quadrants, identification of the splenic tissue may necessitate selective splenic scintigraphy with radiolabeled, heat-damaged red blood cells and/or hepatobiliary scintigraphy. False Positives/NegativesSee the discussions about false-positive and false-negative findings in Radiograph, False Positives/Negatives and Ultrasound, False Positives/Negatives. ANGIOGRAPHYSection 9 of 11
FindingsBefore the advent of cross-sectional imaging techniques, angiography was used to diagnose heterotaxy, define cardiac anatomy, and define abdominal viscerovascular heterotaxy. Abdominal angiograms in patients with polysplenia demonstrate a variable arterial supply to the splenules. Typically, these are supplied by 2 or more arteries, which may arise as branches of a common celiacomesenteric trunk, from terminal branches of the gastroepiploic artery, or directly from the ipsilateral side of the abdominal aorta. The splenules are identified with a blush that moves more rapidly through the splenic tissue than through the overlying liver. Although MRI and echocardiography are extensively used for the evaluation of cardiac malformations in these patients, cineangiography retains an eminent place in the preoperative cardiac evaluation in these complex cases. Abdominal angiography is no longer used for the evaluation of these conditions. Degree of ConfidenceThe degree of confidence in a reported series of angiographic evaluation of the polysplenia syndrome is high. Although venous anomalies such as azygous continuation of an interrupted intrahepatic inferior cava may occur as an isolated anomaly in situs solitus, the lobulated splenules and abnormal celiacomesenteric anatomic branching is highly suggestive of the syndrome. MULTIMEDIASection 10 of 11
REFERENCESSection 11 of 11
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