During the fifth week of embryology, the spleen develops from the mesenchymal cells located between the layers of the dorsal mesentery. The spleen is not considered an endodermal derivative of the primitive gut although it does share an arterial supply with the foregut organs. The spleen moves from its original median position toward the LUQ as the stomach rotates and the mesogastrium develops.¹
Eventually, splenic mesenchymal cells differentiate to form splenic pulp, connecting tissues, and the splenic capsule.

The spleen is an intraperitoneal, ovoid organ entirely covered by peritoneum except at a small bare area at the hilum through which the splenic artery, splenic vein, and efferent lymphatic vessels pass. The spleen is a delicate and vulnerable organ, and its long axis parallels the ninth to eleventh ribs² (Fig. 10-1). The spleen is bordered anteriorly by the stomach; medially by the left kidney, splenic flexure of the colon, and pancreatic tail; and posteriorly by the diaphragm, pleura, left lung, and ribs^1,3 (Fig. 10-2). When imaging the spleen, the neighboring organs and structures may create indentations and impressions on its visceral surface that may simulate masses.³ Therefore, knowledge of the normal variants of the spleen is essential in preventing a misdiagnosis.

Although the spleen has some mobility, the organ does not normally extend inferiorly beyond the left costal margin. As
a result, clinically palpating the spleen through the anterior lateral wall is difficult without significant enlargement of the organ.² Moderate evidence suggests that palpation may be beneficial in supporting the diagnosis of splenomegaly, but this clinical examination cannot accurately rule out alternative conditions and must be correlated with diagnostic imaging.⁴

At the hilus, the splenic arteries and veins are covered by the mesentery of the lienorenal ligament, which also houses the tail of the pancreas. The lienorenal and gastrosplenic ligaments attach the spleen to the left kidney and greater curvature of the stomach, respectively.^2,3 The phrenicocolic ligament is not directly attached to the spleen; however, it supports the inferior end of the organ. The lienorenal, gastrosplenic, and phrenicocolic ligaments work together to stabilize the spleen and hold it loosely in position. Laxity of peritoneal attachments allows for hypermobility or wandering of the spleen.⁵

The average spleen measures 12 cm in length, less than 8 cm in anteroposterior dimension, and less than 4 cm in transverse dimension.^6,7 In children, spleen length increases with age and is correlated with body parameters (height, weight, and body surface area). The formula used to determine spleen size in children is 5.7 + 0.31 × age (years). Infants who are 3 months or younger should have a spleen less than 6 cm in length. The adult splenic size is commonly compared to that of a person’s fist and weighs less than 150 g.^8,9 Splenic size and weight vary based on age, gender, and nutritional status. The normal spleen is smaller in women, decreases in volume and size with advancing age, and increases in size during digestion.^2,9 It has been reported that 3D sonographic visualization and measurement of the spleen volume have a moderate-to-high agreement when compared with CT volumetry.¹⁰

Overall, the spleen has the greatest amount of lymphoid tissue within the human body.¹ A fibrous capsule composed of dense fibroelastic connective tissue surrounds the spleen and is thickened at the splenic hilum. Strands of connective tissue project from the splenic capsule and divide the spleen into several compartments. These communicating compartments are filled with splenic lymphoid tissue termed splenic pulp, which function to filter the peripheral blood^2,8 (Fig. 10-3). The presence of white and red splenic pulp within the spleen make its texture soft and sponge-like. The white pulp is clustered around splenic arterioles and activates immune response when antigens and their antibodies are present within the blood.^1,7 The red pulp consists of a network of blood-filled venous sinuses and reticular splenic cords, termed the Cords of Billroth. The red pulp is dedicated to seizing foreign particles and old, damaged, or mutated erythrocytes.¹ Venous sinuses also act to store more than 300 mL of blood depending upon the systematic blood pressure. When the blood pressure drops, venous sinuses are constricted and may eject as much as 200 mL of blood into the venous circulation in an effort to restore blood volume.^7,8

The highly vascular spleen receives its arterial blood from the splenic artery, a branch of the celiac axis (Fig. 10-4). This artery courses along the superior pancreatic border
and divides into the superior and inferior terminal branches before entering the splenic hilum. Within the hilum, the splenic artery divides into six or more segmental branches before separating into several minor arterioles within the spleen.^2,3,6 These arterioles may be visible as small echogenic lines passing through the splenic parenchyma.¹¹ The intrasplenic arterial branches do not anastomose or communicate to create collateral flow. Without collateral flow, the spleen is at increased risk for infarction. Small branches of the segmental arteries provide blood to the white pulp before flowing into the venous sinusoids. This blood is then transported by pulp veins through splenic trabeculae to the splenic vein.⁶ Eventually, the splenic vein is joined by the inferior mesenteric vein and travels posterior to the tail and body of the pancreas. The splenic vein then unites with the superior mesenteric vein posterior to the neck of the pancreas to form the hepatic portal vein.^2,3,6

There are numerous congenital variations of the spleen.¹ Several studies divide broad-spectrum splenic anomalies associated with heterotaxia syndrome into asplenia and polysplenia. Heterotaxy is related to a disruption in the normal embryologic development of left-right symmetry. This condition results in abnormal organ positions, situs, and/or arrangement.

Asplenia is the congenital absence of the spleen, also known as Ivemark syndrome.^1,6 Splenic aplasia may be diagnosed in cases of congenital absence, surgical removal, or atrophy resulting from arterial or venous occlusions.¹² Asplenia is associated with right-sided morphology of the heart and lungs. Often, the patient’s lungs are trilobed, and both main bronchi are located above the main pulmonary arteries.¹³ Other congenital malformations associated with asplenia include cardiovascular anomalies, situs ambiguous complexes, and visceral heterotaxia.^6,14 The major causes for mortality and morbidity among patients with congenital asplenia are related to cardiac malformations.¹⁴

Splenic hypoplasia is characterized by a small pathologic spleen with reduced function resulting from abnormal development or parenchymal involution.¹⁴ Often, acquired hyposplenia is caused by sickle cell anemia. Some suggest that functional hyposplenism may be underdiagnosed as an immunodeficiency condition in children. Hyposplenic patients are at increased risk to develop solid tumors and vascular, autoimmune, and thrombolytic diseases. The major cause of mortality and morbidity among patients with hyposplenia is pneumococcal sepsis.¹

Polysplenia is characterized by the presence of multiple smaller spleens, of similar size.^1,13 In cases of polysplenia, there is a left-sided dominance in lung and cardiac morphology.⁶ Often, patients with polysplenia demonstrate bilobed lungs with the main bronchi found below the pulmonary arteries. Other associated anomalies include visceral heterotaxia, malrotations of the intestine, short pancreas, inferior vena cava (IVC) anomalies, cardiac defects, and biliary atresia.¹³

An accessory spleen is a common anatomic variant seen in approximately 10% to 30% of the population. Accessory (supernumerary) spleens occur when a portion of splenic tissue separates itself from the main body of the spleen and is found in an ectopic position.^1,15 Most accessory spleens are small and measure approximately 2.0 cm.¹⁶ Approximately 75% of accessory spleens are found near the splenic hilum or gastrosplenic ligament (Fig. 10-5). The other 20% are located within the tail of the pancreas where they may be mistaken for hypervascular pancreatic tumors. Only 5% occur along the splenic artery and in the gastrosplenic, splenocolic, or gastrocolic ligament.^1,15 Differentiating accessory spleens from pancreatic masses or from hilar lymph nodes may be difficult unless it is possible to trace their blood supply to the splenic artery. Sonographically, accessory spleens present as round, mildly echogenic, and homogeneous with posterior enhancement.¹⁷ Following splenectomy, an accessory spleen may assume the function and size of the removed organ. In cases of hematologic disorders, residual splenic tissue and accessory spleens present after laparoscopic splenectomy may lead to a relapse.^1,18 Rarely, accessory spleens undergo
torsion or infarction clinically associated with acute LUQ pain. In most cases, accessory spleens are of no clinical consequence.^6,17

The wandering or ectopic spleen is a rare occurrence and is a spleen that migrates from its normal LUQ position to another location within the abdomen or pelvis. Wandering spleen is associated with a loss or a weakening of supporting ligaments.^6,19 Developmental ligamental laxity occurs when there is an incomplete fusion of the dorsal mesentery with the posterior peritoneum or secondary to hormonal changes and maternal influences during pregnancy. With age, laxity may be related to splenomegaly, trauma, extreme weight loss, weak abdominal muscles, and gastric distension. Because the wandering spleen lacks its normal peritoneal attachments, this variant is associated with a high incidence of splenic torsion and infarction.^1,20

Clinically, patients with a wandering spleen may present as asymptomatic or have varying degrees of abdominal pain. Sonographers should extensively examine the left side of the patient from the thorax to the pelvis in cases where the normal spleen is not imaged in its anatomic position. Contrast-enhanced CT is the imaging modality of choice in the diagnosis of wandering spleen. However, variable echo patterns in sonography combined with duplex Doppler and color flow imaging are sensitive in demonstrating a lack or absence of blood flow secondary to splenic artery torsion.¹⁹, ²⁰ and ²¹ The preferred treatment for wandering spleen is splenopexy wherein a surgeon repositions the spleen in the LUQ to prevent torsion of the splenic vessels and preserve splenic function.²¹

The spleen is an organ of mystery and perplexity for physiologists. Although the spleen is rarely the primary site of disease, it is often involved in inflammatory, hematopoietic, and metabolic disorders associated with immune and hematologic diseases. Functions of the spleen overlap those of other body organs, making it possible for a person to live without a spleen. However, studies completed on individuals with splenic absence have given some indication to the importance of its function. For example, patients who underwent splenectomy often suffered from leukocytosis, decreased circulating iron, decreased immune response, and an increased presence of circulating morphologically defective blood cells.^8,9,22 Physiologically, the four major functions of the spleen include reserving, filtering, producing, and defending blood products.^7,8 Within the spleen, the red pulp is dedicated to filtration, the white pulp is dedicated to adaptive immunity, and the perifollicular zone—located between red and white pulp—acts to connect both functions.²³

The normal echogenicity of the spleen is comparable with that of the liver and equal to or slightly hyperechoic to the kidney when the tissues are evaluated at the same distance from the transducer.⁶ Splenic parenchyma is homogeneous with low to mid-level echoes, which are usually disrupted only by the arteries and veins in the area of the hilus (Fig. 10-6). Within the splenic hilum, the splenic artery, and its bifurcations, along with convergence of the splenic veins are visualized. Differentiation between these vessels is difficult without the evaluation of their Doppler signals^6,9 (Fig. 10-7).