The spleen develops from mesenchymal cells that migrate during embryogenesis into the dorsal mesogastrium. Spleen development begins during the fifth week of gestation. The mesenchymal cells of the splenic precordium differentiate to form the splenic parenchyma, the connective tissue framework, and the capsule. The mesenchymal tissue elements of the developing spleen fuse into a single structure by the third gestational month, at which time the characteristic lobulated configuration of the fetal spleen is present.
Anomalous development of the spleen can lead to abnormalities of morphology or location. The most common anomaly of splenic development is isolation of splenic tissue from the main portion of the organ, resulting in the formation of an accessory spleen. Complete lack of spleen formation is a component of the most common form of heterotaxy syndrome, that is, asplenia. Heterotaxy can also be associated with multiple foci of splenic development, that is, polysplenia. Splenic-gonadal fusion is a rare developmental lesion that relates to the close proximity of the left gonadal anlage to the developing spleen in the left dorsal mesogastrium. Wandering spleen is a developmental lesion that is caused by deficient formation of various ligaments in the left upper quadrant.1,2
The major components of the spleen are lymphoid tissue, erythrocytes, and reticuloendothelial cells. It is the largest unit of lymphatic tissue in the body. The primary functions are filtering blood and participation in the immune response. The 2 anatomic divisions of the splenic parenchyma are the white pulp and red pulp. The white pulp contains lymphocytes, plasma cells, and macrophages. Structurally, the white pulp is a sheath of lymphatic tissue that surrounds the intrasplenic arteries, with thickening of this sheath in areas to form lymphatic nodules. The red pulp is the predominant component of the spleen. The red pulp comprises a network of tortuous branching venous sinuses and cylindric partitions of connective tissue, that is, the splenic cords. The red pulp contains phagocytic cells, and is the site where senescent or damaged erythrocytes are removed from the circulation. There is exaggerated splenic red blood cell destruction in patients with abnormal erythrocyte morphology (e.g., hereditary spherocytosis) or abnormal antibody coating of erythrocytes (e.g., the immune hemolytic anemias).
The spleen serves to filter certain bacteria from the blood and to produce opsonizing antibodies. These functions are particularly important during the first 2 years of life, before specific immunity to various bacteria has fully developed. The splenic bacterial filtering mechanism is most effective for those organisms that have a polysaccharide capsule. This filtering does not require the presence of specific antibodies. Children who lack a spleen or who have deficient splenic function are prone to develop infections with organisms such as Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis, Escherichia coli, Staphylococcus aureus, and group A streptococci. The spleen also contributes to the systemic immune response by producing antibodies.
The spleen is an active site of hematopoiesis in the fetus. There is progressive decrease in splenic hematopoiesis late in gestation, such that the bone marrow is the major site of red blood cell production at term. The spleen is a major site of extramedullary hematopoiesis in various bone marrow failure syndromes (e.g., myelofibrosis) or excessive erythropoietic stress (e.g., thalassemia major).
The spleen weighs approximately 15 to 20 g at birth, 60 g by 6 years of age, and 150 g at puberty. Maximum splenic mass occurs at puberty, with slow decrease thereafter. The normal adult spleen is approximately 12 cm in length. Normal splenic length (in centimeters) for children can be calculated by the formula 5.7 + 0.31 × age. Loftus and Metreweli reported a spleen-to-kidney ratio of 1.25 as the upper limit of normal in children.3 Rosenberg et al set the upper normal limit of splenic length on sonography at 12 cm for girls 15 years or older and at 13 cm for boys 15 years or older.4 Splenic weight (in grams) can be estimated by the formula 3.5 × 10–3 × 0.97 × body weight.
The variable rates of blood flow through the cords of the red and white pulp result in a heterogeneous pattern of enhancement during the arterial phase of dynamic contrast-enhanced CT. Most often, there are alternating bands of relatively high and low attenuation (arciform enhancement or “zebra spleen”). Other potential patterns include focal alterations in attenuation and diffuse heterogeneity. Images obtained of the normal spleen during the portal venous phase demonstrate homogeneous attenuation.
The MR appearance of the normal spleen varies with patient age, as the signal intensity relates to the proportions of white and red pulp. In the neonate prior to proliferation of white pulp, the spleen is somewhat hypointense relative to the liver on both T1- and T2-weighted MR images. Beginning at about several months of age, the spleen is hyperintense in comparison to liver on T2-weighted images. The normal spleen is homogeneous on sonography and is slightly hyperechoic relative to liver.1
Congenital absence of the spleen (i.e., asplenia) and congenital supernumerary spleens (i.e., polysplenia) are developmental conditions that are nearly always associated with abnormal locations of other abdominal organs, anomalous lung development, and anomalies of the heart and great vessels. Aside from the increased risk of sepsis due to absence of the spleen, the major clinical importance of developmental asplenia and polysplenia relates to the associated cardiovascular abnormalities. Asplenia and polysplenia are important components of the heterotaxy syndrome, which refers to anomalies of visceral situs (situs ambiguous).5
Patients with asplenia have a form of heterotaxy termed right isomerism, or bilateral right sidedness. In this situation, the left lung has the morphology of a normal right lung, with bilateral minor fissures and bilateral eparterial upper lobe bronchi. The left atrium morphologically resembles the normal right atrium. Positioning of the abdominal viscera varies between patients. Most often, the liver in patients with asplenia has a transverse or horizontal configuration and the orientation of the pancreas is abnormal. Intestinal malrotation is common. Clinically severe cardiac anomalies are present in nearly all children with asplenia. Common cardiac lesions include atrial septal defect, atrioventricular canal, single ventricle, pulmonary atresia or severe pulmonic stenosis, transposition of the great arteries, total anomalous pulmonary venous return, and anomalous systemic venous connection.
Most patients with asplenia are male. The clinical presentation is unrelated to splenic agenesis; these patients exhibit manifestations of severe cardiovascular disease early in infancy. Because pulmonary atresia or severe pulmonary stenosis and transposition of the great arteries are common in infants with asplenia, most are cyanotic. Individuals with asplenia have a lifetime propensity for infections, including an elevated risk for fatal sepsis. Peripheral blood smear shows the presence of Heinz or Howell-Jolly bodies; however, this finding in an infant is not specifically diagnostic of asplenia.
Congenital asplenia is associated with a variety of GI, genitourinary, and neuromuscular abnormalities. Intestinal malrotation is common. The stomach may be small, malpositioned, or duplicated. There is usually abnormal symmetry of liver morphology, with 2 mirror image right lobes and a midline gallbladder. The inferior vena cava and the abdominal aorta are usually located on the same side of the spine in patients with asplenia. Rare associations with asplenia include Hirschsprung disease, imperforate anus, intestinal duplication, and horseshoe adrenal gland.6,7
Polysplenia refers to a spectrum of anomalies, ranging from multiple small splenules to an abnormally septated spleen. An accessory spleen is not considered part of this spectrum. Spleen function in individuals with polysplenia is normal. The clinical importance of this condition rests with the associated anomalies, such as heart disease, biliary atresia, and central nervous system anomalies. The pathogenesis of polysplenia is incompletely understood, and may be multifactorial. Familial patterns have been detected, including examples of autosomal recessive, dominant, and X-linked inheritance. Chromosomal abnormalities have been detected in some patients. An insult (such as infection) to the developing fetus at approximately the fifth week of gestation could also explain the association of polysplenia with other malformations. Most individuals with polysplenia are female.8
Polysplenia is associated with a form of heterotaxy termed left isomerism, or bilateral left sidedness. Both lungs have the morphology of a normal left lung, with 2 lobes. The atria have the morphology of a normal left atrium. Symptomatic cardiac anomalies are present in approximately 90% of children with polysplenia. The most common lesions include atrial or ventricular septal defect, atrioventricular canal, left ventricular outflow obstruction, double outlet right ventricle, partial anomalous pulmonary venous connection, and bilateral superior venae cavae.5
About two-thirds of patients with polysplenia have congenital absence of the hepatic portion of the inferior vena cava. There is also in association with biliary atresia. (The bile ducts and inferior vena cava are right-sided structures.) The liver frequently has a symmetric, transverse configuration. The gallbladder may be hypoplastic or absent. Preduodenal portal vein is common in patients with polysplenia, particularly those with biliary atresia. Other associations include bowel malrotation, tracheoesophageal fistula, imperforate anus, foreshortening of the pancreas, duodenal atresia, and congenital extrahepatic portosystemic shunt. The combined mass of the multiple foci of splenic tissue in most patients with polysplenia is approximately equivalent to that of a normal spleen. The splenic foci usually are arranged as a group, either in a normal left upper quadrant position or in an ectopic location such as the right upper quadrant. Malpositions of the liver and pancreas are common in these patients.9
Patients with heterotaxy syndrome usually come to medical attention early in infancy due to the associated congenital heart disease. This is particularly true for asplenia because of the strong association with clinically severe heart lesions. Standard chest radiographs of these infants frequently provide the initial clue to an anomaly of visceroatrial situs. The liver may have an abnormal symmetric, transverse configuration. An abnormal location or size of the stomach bubble is a less common finding. Careful inspection of the lungs shows bilateral minor fissures in infants with asplenia, and absence of minor fissures in those with polysplenia.
Sonography is a useful technique for evaluating splenic morphology in infants with suspected heterotaxy syndrome. With polysplenia, multiple nodules of splenic tissue are present, either in the right upper quadrant, left upper quadrant, or both (Figure 43-1). When multiple splenules are present, they are typically located along the greater curvature of the stomach. Aside from abnormal shape and size, the tissue has the imaging characteristics of normal splenic parenchyma (Figure 43-2). In some patients with clinical features of polysplenia, sonography shows subtle clefts or septations in an otherwise normal-appearing spleen. Failure to demonstrate a spleen is suggestive of asplenia, but is not pathognomonic because this technique can miss a small or ectopic spleen. In addition, the prominent left lobe of the liver often fills the splenic fossa in patients with asplenia, and sometimes mimics the sonographic appearance of splenic tissue. Sonography serves an important role in infants with heterotaxy syndrome for the assessment for associated anomalies, such as interruption of the inferior vena cava, preduodenal portal vein, and biliary atresia. CT and MR provide similar information as sonography in patients with asplenia and polysplenia (Figure 43-3). Splenules enhance intensely and homogeneously on CT (Figure 43-4).
Spleen scintigraphy with technetium-labeled heat-damaged erythrocytes is the most sensitive imaging technique for the detection of splenic tissue. In infants with suspected heterotaxy syndrome, a radionuclide angiogram with injection in a lower extremity vein provides supplemental information about the integrity of the inferior vena. With interruption of the inferior vena cava, tracer passes superiorly through the azygos system before looping back into the right atrium via the superior vena cava. Delayed images in patients with polysplenia confirm the presence of splenic tissue, and document the location (Figure 43-5). Frequently, the spleen has an abnormal shape, appears bifid, or comprises a group of splenules (Figure 43-6). With asplenia, splenic activity is completely lacking throughout the abdomen and pelvis; most of the visible activity in this situation is within the blood pool. Splenic activity is also lacking on sulfur colloid scintigraphy; the liver usually has a transverse configuration (Figure 43-7).
Figure 43–5
Heterotaxy.
A posterior image from technetium-labeled heat-damaged erythrocyte scintigraphy shows intense uptake in the spleen. The spleen is on the left, but has an abnormal shape and is more medial than normal. Blood pool activity allows visualization of the heart, the apex of which is directed to the right (arrow). The faintly visualized liver has a transverse configuration.
Figure 43–6
Polysplenia.
A posterior technetium-labeled heat-damaged erythrocyte scintigraphic image of the abdomen shows an anomalous right-sided location of the spleen (arrows). The spleen consists of 2 masses. Uptake in the liver is fainter than in splenic tissue. This transverse morphology of the liver is common in patients with heterotaxy.
Mirror image visceral and atrial anatomy is termed situs inversus. Normal visceral and atrial arrangement is termed situs solitus. Situs ambiguous, or visceral heterotaxy, refers to abnormal symmetry of normally asymmetric structures, that is, isomerism (see above). Situs inversus is a reversal of visceroatrial situs, but is not a form of isomerism. The spleen is located in the right upper quadrant and is otherwise normal. The liver morphology is normal aside from a left-sided location. The atria have mirror image configurations compared to individuals with situs solitus. The 3 forms of situs refer to visceral and atrial orientations; the position of the cardiac ventricles does not affect situs. Therefore, mirror image visceral and atrial anatomy is classified as situs inversus even if there is levocardia. Individuals with situs inversus are slightly more likely to have congenital heart disease than those with situs solitus. Most patients with this anomaly, however, are asymptomatic.
Wandering spleen refers to congenital inferior malposition of the spleen. This is related to abnormal fetal development of the dorsal mesogastrium, resulting in laxity or absence of the splenic supporting ligaments (e.g., the lienorenal, lienogastric, pancreaticosplenic, and phrenicocolic ligaments). The elongated vascular pedicle is susceptible to torsion. Inferior extension of the spleen is common in patients with agenesis of the left kidney. There is a rare association with gastric volvulus. Other types of an ectopic spleen include malposition due to abdominal wall laxity (e.g., prune belly syndrome), trauma, surgery, and diaphragmatic defect. Wandering spleen is much more common in males than in females.
Many individuals with wandering spleen have no associated symptoms. The most common presentation is that of a palpable abdominal mass. Others suffer chronic or intermittent abdominal pain due to self-resolving episodes of torsion. Acute torsion usually precipitates rapid-onset abdominal pain; the severity of pain varies with the degree of vascular obstruction. Torsion of the vascular pedicle of the spleen impairs venous outflow. Although interruption of arterial perfusion is uncommon, the congestion caused by venous obstruction can lead to hemorrhage, infection, or gangrene.10–12
The abnormal position of a wandering spleen can be documented with scintigraphy or any of the cross-sectional imaging techniques. If there is congestion, the ectopic spleen often is enlarged. Doppler sonography sometimes shows alterations in blood flow. If there is torsion, the spleen may have relatively low attenuation on unenhanced CT and diminished or no enhancement following IV contrast administration. There is sometimes a whorled appearance of the twisted splenic vessels and adjacent fat. Sonography may show parenchymal heterogeneity and diminished echogenicity. Examination with MR also shows subnormal contrast enhancement. MR angiography can be used to demonstrate the vascular anatomy in the region of the splenic hilum. With chronic torsion, CT and MR may show a thick peripheral enhancing margin due to collateral flow, and deficient enhancement centrally.13–16
An accessory spleen (supernumerary spleen or splenule) is a developmental focus of normal functional splenic tissue that is separate from the main body of the spleen. The presumed mechanism of formation of an accessory spleen is failure of fusion of the embryonic splenic anlage during the fifth gestational week. Accessory spleens are common; the prevalence as determined by autopsy series is 10% to 30%. Studies utilizing CT and sonography have demonstrated accessory spleens in 3% to 15% of individuals. Multiple accessory spleens are present in 10% to 15% of those patients who have an accessory spleen. An accessory spleen may hypertrophy after splenectomy, particularly in patients with hypersplenism. An accessory spleen may provide enough function to mitigate some of the long-term risk for sepsis in patients who have undergone splenectomy.17,18
Cross-sectional imaging studies show an accessory spleen to have identical parenchymal characteristics (i.e., attenuation, signal intensity, and echogenicity) to normal spleen (Figure 43-8). On CT, a small accessory spleen may not enhance as intensely as the normal spleen. Enhancement is, however, homogeneous. The margins are well defined; most accessory spleens are round (Figure 43-9). An accessory spleen may be located anywhere in the abdomen, although nearly all are medial to the spleen. A site posteromedial to the spleen and adjacent to the tail of the pancreas is most common. The blood supply is via the splenic artery and drainage is via the splenic vein.17–19
The great majority of accessory spleens are asymptomatic and of no clinical significance. Rare complications of accessory spleen include torsion, infarction, and spontaneous rupture. These complications are exceedingly rare with a typical small accessory spleen, but a hypertrophied accessory spleen (e.g., status post splenectomy) with a long vascular pedicle carries a substantial risk. Torsion of an accessory spleen is heralded clinically by acute abdominal pain; the clinical manifestations are nonspecific. Infarction may lead to fever in addition to acute severe pain. The vascular pedicle of an accessory spleen most often extends from the splenic hilum, but other potential connections include vessels from the tail of the pancreas, the gastrosplenic ligament, the small bowel mesentery, or the gastric fundus.
A CT finding suggestive of ischemia or frank infarction of an accessory spleen is attenuation lower than that of normal spleen or liver. There is absent or diminished enhancement with IV contrast. An enhancing peripheral rim is commonly present in the subacute phase, representing perfusion via capsular vessels. MR may show a relatively low signal intensity mass on T1-weighted images, often with a peripheral rim of higher signal. The lesion most often produces high signal intensity on T2-weighted images, due to edema, hemorrhage, and/or necrosis. Free intraperitoneal fluid is present when there is rupture of an accessory spleen.20–23
An accessory spleen with an intact blood supply accumulates sulfur colloid in the same manner as normal splenic tissue. A small accessory spleen located immediately adjacent to the normal spleen usually cannot be discerned scintigraphically. However, scintigraphy can demonstrate accessory spleens after splenectomy or those in ectopic locations. The most sensitive scintigraphic technique for the detection of small deposits of accessory splenic tissue is imaging with heat-damaged labeled erythrocytes.
Splenogonadal fusion is a rare anomaly in which a portion of the splenic anlage fuses with developing left gonadal tissue in the embryo. Subsequent gonadal descent results in formation of a long connecting cord between the gonad and the spleen or separation and inferior displacement of a portion of splenic tissue from the remainder of the spleen. The former is termed continuous splenogonadal fusion and the latter represents the discontinuous type. The fusion may occur to the gonad, epididymis, vas deferens, or mesovarium. The continuous type of splenogonadal fusion in a male is usually associated with cryptorchidism.
The great majority of patients with splenogonadal fusion are male. The ectopic splenic tissue is often contained within the tunica albuginea. Most patients are asymptomatic, with discovery of the lesion as an incidental finding. In rare instances, there is sufficient splenic tissue to produce a palpable scrotal or left lower abdominal mass. This lesion is included in the differential of a sonographically demonstrated intrascrotal extratesticular mass. Appropriate diagnosis is important to prevent unnecessary orchiectomy.24
Heterotopic splenic tissue in a patient with splenogonadal fusion is sometimes of sufficient size to be demonstrated on cross-sectional imaging studies. Most often, however, sulfur colloid scintigraphy or spleen-specific scintigraphy with labeled heat-damaged erythrocytes is most useful. The orthotopic spleen usually has a normal appearance on imaging studies. The ectopic splenic tissue can be located anywhere along the left side of the abdomen, pelvis, or scrotum. Scintigraphy is most sensitive if the majority of the liver and spleen is excluded from the field of view.25
Primary infections of the spleen are uncommon. Most splenic infections occur in the setting of immunocompromise (e.g., oncological disease, chronic granulomatous disease, or AIDS), sepsis, or bacterial endocarditis. Hematogenous seeding is the most common pathway of infection. Direct spread can occur from an adjacent bacterial infection, such as in the subphrenic space, pancreas, or kidney. Other potential predisposing conditions include IV drug abuse and amebic dysentery. Individuals with hemoglobinopathies such as sickle cell trait and hemoglobin SC disease are at an increased risk for splenic infection. This complication is uncommon, however, in patients with hemoglobin SS disease, due to the atrophy and functional asplenia that typically occur in these patients.
Splenic infections can be bacterial, parasitic, or mycotic. In immunosuppressed individuals, common spleen infections include tuberculosis, histoplasmosis, Pneumocystis carinii, and disseminated bacterial and fungal infections. Patients with AIDS can develop splenic infections with P carinii, cytomegalovirus, Mycobacterium tuberculosis, and Mycobacterium avium intracellulare. Relatively common causes of multiple splenic granulomas in children include tuberculosis, histoplasmosis, coccidiomycosis, and chronic granulomatous disease.1
The most common organisms involved in bacterial infection of the spleen include S aureus, Streptococcus species, and various gram-negative organisms (e.g., Salmonella species and Bartonella henselae). Potential clinical manifestations of bacterial infection of the spleen include fever, left upper quadrant or flank pain, and splenomegaly. Pain occasionally radiates to the left shoulder.
Early in the course of a bacterial splenic infection, there are usually 1 or more foci of edema. Imaging studies may show nonspecific splenomegaly. There are often focal areas that are hypoechoic on sonography, decreased in attenuation on contrast-enhanced CT, and of high signal intensity on T2-weighted MR images.
At the time of clinical presentation, most bacterial infections of the spleen have progressed to abscess formation. Splenic abscesses have similar imaging characteristics to those occurring in other solid organs (Figure 43-10). Sonography may show an anechoic, hypoechoic, or mixed pattern, sometimes with fluid–fluid levels. The typical CT appearance of a pyogenic abscess is that of an oval or round low-attenuation lesion, best visualized on contrast-enhanced images; this is in contradistinction to the well-defined, wedge-shaped pattern of a splenic infarct. The margins of the abscess sometimes enhances due to inflammation. Intraluminal gas is an uncommon finding, but when present is relatively specific for the diagnosis of an abscess. Bacterial splenic infection is associated with avid accumulation of radiogallium and 99mTc-labeled leukocytes.18
Fungal infections of the spleen predominantly occur in immunocompromised individuals, including those with leukemia, lymphoma, and chronic granulomatous disease. Most of these infections are due to Candida albicans; other potential pathogens include Aspergillus and Cryptococcus. Multiple small foci of infection (microabscesses) commonly occur in patients with Candida infections. The imaging features of fungal splenic infections are similar to those of the liver.
CT is the most commonly utilized technique for the detection of splenic infections. Fungal lesions usually appear as multiple small well-defined areas of diminished enhancement (Figure 43-11). Occasionally, there is a central high-attenuation focus, producing a bull’s-eye pattern. The wheel-within-a-wheel pattern that occurs with liver lesions can also occur with splenic infection. In some patients, there is a miliary or salt and pepper pattern due to innumerable tiny foci of infection. Calcifications sometimes develop during therapy.
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