On completion of this chapter, you should be able to:
List the normal anatomy and relational landmarks of the spleen
Discuss the size and primary functions of the spleen
Describe the normal sonographic pattern of the spleen
Explain the sonographic findings and differential diagnoses for the pathologies discussed in this chapter
The spleen is the largest single mass of lymphoid tissue in the body. It is part of the reticuloendothelial system and has a role in the synthesis of blood proteins. The spleen is active in blood formation ( hematopoiesis ) during the initial part of fetal life. This function decreases gradually by the fifth or sixth month, when the spleen assumes its adult characteristics and discontinues its hematopoietic (blood-producing) activities. The spleen plays an important role in the defense of the body. Although it is often affected by systemic disease processes, the spleen is rarely the primary site of disease.
The left upper quadrant may be rapidly assessed with sonography in patients with palpable splenomegaly or trauma to the left upper quadrant. The normal texture of the spleen is homogeneous, being slightly more echogenic than the texture of the liver; therefore pathology or blood collection secondary to a splenic rupture is usually easily identified.
Anatomy of the spleen
The spleen is an intraperitoneal organ covered with peritoneum over its entire extent, except for a small area at its hilum, where the vascular structures and lymph nodes are located. The spleen lies in the posterior left hypochondrium between the fundus of the stomach and the diaphragm. The splenic axis is along the shaft of the eight to tenth ribs with the lower pole extending forward as far as the midaxillary line ( Figure 11-1 ). The inferomedial surface of the spleen comes into contact with the stomach, left kidney, pancreas, and splenic flexure of the colon ( Figure 11-2 ). The peritoneal ligament that attaches the spleen to the stomach and the kidney is called the splenorenal ligament. This ligament is in contact with the posterior peritoneal wall, the phrenicocolic ligament, and the gastrosplenic ligament. The gastrosplenic ligament is significant in that it is composed of the two layers of the dorsal mesentery that separate the lesser sac posteriorly from the greater sac anteriorly. A protective capsule covers the spleen with peritoneum. In most adults, a portion of the splenic capsule is firmly adherent to the fused dorsal mesentery anterior to the upper pole of the left kidney, which produces a “bare area” of the spleen. This bare area can be helpful in distinguishing intraperitoneal from pleural fluid collections.
The spleen is of variable size and shape (e.g., “orange segment,” tetrahedral, triangular) but generally is considered to be ovoid with smooth, even borders and a convex superior and concave inferior surface (see Figure 11-4 ). The spleen is normally measured with ultrasound on a longitudinal image from the upper margin (near the diaphragm) to the inferior margin at the long axis ( Figure 11-3 ). Normal measurements for the average adult should be 8 to 13 cm in length, 7 cm in width, and 3 to 4 cm in thickness. The spleen decreases slightly in size with advancing age. The size of the spleen may vary in size in accordance with the nutritional status of the body.
Blood is supplied to the spleen by the tortuous splenic artery that travels horizontally along the superior border of the pancreas ( Figure 11-4 ). On entering the splenic hilum, the splenic artery immediately branches into six smaller arteries to supply the organ with oxygenated blood to profuse the splenic parenchyma. Color Doppler imaging allows the sonographer to image the vascularity of the spleen; gray-scale imaging will show small echogenic lines throughout the spleen that represent the arterial system. The splenic arteries are subject to infarction because adequate anastomoses between the vessels are lacking.
The splenic vein is formed by multiple branches within the spleen and leaves the hilum in a horizontal direction to join the superior mesenteric vein. The superior mesenteric vein returns unoxygenated blood from the bowel to form the main portal vein ( Figure 11-5 ). The splenic vein usually travels along the posteromedial border of the pancreas.
The lymph vessels emerge from the splenic hilum, pass through other lymph nodes along the course of the splenic artery, and drain into the celiac nodes. The nerves to the spleen accompany the splenic artery and are derived from the celiac plexus.
The spleen lies between the left hemidiaphragm and the stomach. The diaphragm may be seen as a bright, curvilinear, echogenic structure close to the proximal superolateral surface of the spleen. Posteriorly, the diaphragm, left pleura, left lung, and ribs are in contact with the spleen. The medial surface is related to the stomach and lesser sac ( Figure 11-6 ). The fundus of the stomach may contain gas or fluid, which may cause confusion in the left upper quadrant during attempts to demonstrate the spleen. Alteration in the patient position or ingestion of fluids may help to separate stomach from splenic tissue. The tail of the pancreas lies posterior to the stomach and lesser sac as it approaches the hilum of the spleen and splenic vessels. The spleen may serve as a good acoustic window to image the tail of the pancreas. The left kidney lies inferior and medial to the spleen.
Displacement of the spleen
The spleen is held in place by the lienorenal, gastrosplenic, and phrenocolic ligaments (see Figure 15-2 ). These ligaments are derived from the layers of peritoneum that form the greater and lesser sacs. A mass in the left upper quadrant may displace the spleen inferiorly. Caudal displacement may occur secondary to a subclavian abscess, splenic cyst, or left pleural effusion. Cephalic displacement may result from volume loss in the left lung, left lobe pneumonia, paralysis of the left hemidiaphragm, or a large intraabdominal mass. A normal spleen with medial lobulation between the pancreatic tail and the left kidney may be confused with a cystic mass in the tail of the pancreas.
The term “wandering” spleen describes a spleen that has migrated from its normal location in the left upper quadrant. It is the result of an embryologic anomaly of the dorsal mesentery that fails to fuse with the posterior peritoneum without supporting ligaments of the spleen. The patient may present with an abdominal or pelvic mass, intermittent pain, and volvulus, that is, splenic torsion. The sonographer should use color Doppler to map the vascularity within the spleen. When torsion is complete, the vascular pattern shows decreased velocity.
Complete absence of the spleen (asplenia), or splenic agenesis, is rare and by itself causes no difficulties. However, it may occur as part of a major congenital abnormality. Visceral heterotaxy is the common name that consists of a spectrum of anomalies. Asplenic or polysplenia syndromes are associated with complex cardiac malformations, bronchopulmonary abnormalities, or visceral heterotaxis (anomalous placement of organs or major blood vessels, including a horizontal liver, malrotation of the gut, and interruption of the inferior vena cava with azygos continuation). The normal arrangement of asymmetric body parts is called situs solitus. The mirror image condition is called situs inversus. The term situs ambiguous is used when the anatomy falls in between these two conditions.
Patients with polysplenia may have bilateral left-sidedness (two morphologic left lungs, left-sided azygous continuation of an interrupted inferior vena cava, biliary atresia, absence of the gallbladder, gastrointestinal malrotation, and cardiovascular abnormalities). On the other hand, patients with asplenia may have bilateral right-sidedness (two morphologic right lungs, midline location of the liver, reversed position of the abdominal aorta and inferior vena cava, anomalous pulmonary venous return, and horseshoe kidneys). Patients with agenesis of the spleen have major problems with serious infection, as their immune response is absent.
Splenic agenesis may be ruled out by demonstrating a spleen on ultrasound. The sonographer should be careful not to confuse the spleen with the bowel, which may lie in the area normally occupied by the spleen. Color Doppler helps determine the splenic vascular pattern and thus helps to separate it from the colon.
An accessory spleen, or splenunculus, is a more common congenital anomaly that may be found in up to 30% of patients ( Figure 11-7 ). The accessory spleen may be difficult to demonstrate by sonography if it is very small. However, when it is seen, it appears as a homogeneous pattern similar to that of the spleen. It usually is found near the hilum or inferior border of the spleen but has been reported elsewhere in the abdominal cavity. Lesions affecting the normal spleen would also affect the accessory spleen. An accessory spleen results from failure of fusion of separate splenic masses forming on the dorsal mesogastrium; it is most commonly located in the splenic hilum or along the splenic vessels or associated ligaments. The location of the accessory spleen has been reported anywhere from the diaphragm to the scrotum, and it is usually solitary. It usually remains small and does not present as a clinical problem. The accessory spleen may simulate enlarged lymph nodes in the area of the spleen, or a tumor of the pancreatic, suprarenal, or retroperitoneal structures. As the spleen enlarges, so does the accessory spleen.
Physiology and laboratory data of the spleen
The spleen is part of the reticuloendothelial system and is rarely the site of primary disease. It is commonly involved in metabolic, hematopoietic, and infectious disorders. Blunt abdominal trauma to the spleen may result in splenic laceration and rupture. The spleen is active in the body’s defense against disease; its major function is to filter the peripheral blood.
The spleen is a soft organ with elastic properties that allow it to distend as blood fills the venous sinuses. These characteristics are related to the function of the spleen as a blood reservoir. Within the lobules of the spleen are tissues called pulp. Two components are found within the spleen: red pulp and white pulp ( Figure 11-8 ). White pulp is distributed throughout the spleen in tiny islands. This tissue consists of splenic nodules, which are similar to those found in lymph nodes and contain large numbers of lymphocytes. Red pulp fills the remaining spaces of the lobules and surrounds the venous sinuses. The pulp contains relatively large numbers of red blood cells, which are responsible for its color, along with many lymphocytes and macrophages.
The red pulp of the spleen consists of splenic sinuses alternating with splenic cords. The blood capillaries within the red pulp are quite permeable. Red blood cells can squeeze through the pores in these capillary walls and enter the venous sinuses. The older, more fragile red blood cells may rupture as they make this passage, and the resulting cellular debris is removed by phagocytic macrophages located within the splenic sinuses. The macrophages engulf and destroy foreign particles, such as bacteria, that may be carried in the blood as it flows through the sinuses. The lymphocytes of the spleen help to defend the body against infection. The blood that leaves the splenic sinuses to enter the reticular cords passes through a complex filter. The venous drainage of the sinuses and cords is not well defined, but it is assumed that tributaries of the splenic vein connect with sinuses of the red pulp.
The white pulp of the spleen consists of the malpighian corpuscles, small nodular masses of lymphoid tissue attached to the smaller arterial branches. Extending from the splenic capsule inward are the trabeculae, which contain blood vessels and lymphatics. The lymphoid tissue or malpighian corpuscles have the same structure as the follicles in the lymph nodes; however, they differ in that the splenic follicles surround arteries, so that on cross section, each contains a central artery. These follicles are scattered throughout the organ and are not confined to the peripheral layer or cortex, as are lymph nodes.
As part of the reticuloendothelial system, the spleen plays an important role in the defense mechanisms of the body and is also implicated in pigment and lipid metabolism. It is not essential to life and can be removed with no ill effects. The functions of the spleen may be classified under two general headings: those that reflect the functions of the reticuloendothelial system and those that are characteristic of the organ itself ( Box 11-1 ). The role of the spleen as an immunologic organ involves the production of cells capable of making antibodies (lymphocytes and plasma cells); however, antibodies are also produced at other sites.
Functions of the spleen as an organ of the reticuloendothelial system
Production of lymphocytes and plasma cells
Production of antibodies
Storage of iron
Storage of other metabolites
Functions characteristic of the spleen
Maturation of the surface of erythrocytes
Disposal of senescent or abnormal erythrocytes
Functions related to platelet and leukocyte life span
Phagocytosis of erythrocytes and the breakdown of hemoglobin occur throughout the entire reticuloendothelial system, but roughly half the catabolic activity is localized in the normal spleen. In splenomegaly, the major portion of hemoglobin breakdown occurs in the spleen. The iron that is liberated is stored in the splenic phagocytes. In anomalies such as the hemolytic anemias, the splenic phagocytes become engorged with hemosiderin when erythrocyte destruction is accelerated. In addition to storing iron, the spleen is subject to storage diseases such as Gaucher’s disease and Niemann-Pick disease. Abnormal lipid metabolites accumulate in all phagocytic reticuloendothelial cells but may also involve the phagocytes in the spleen, producing gross splenomegaly.
Functions of the spleen that are characteristic of the organ relate primarily to the circulation of erythrocytes through it. In a normal individual, the spleen contains only about 20 to 30 ml of erythrocytes. In splenomegaly the reservoir function is greatly increased, and the abnormally enlarged spleen contains many times this volume of red blood cells. Transit time is lengthened, and the erythrocytes are subject to destructive effects for a long time. In part, ptosis causes consumption of glucose, on which the erythrocyte depends to maintain normal metabolism, and the erythrocyte is destroyed. Selective destruction of abnormal erythrocytes is also accelerated by splenic pooling.
As erythrocytes pass through the spleen, the organ inspects them for imperfections and destroys those it recognizes as abnormal or senescent. Pitting is the process of removing the nuclei from the red blood cells. Culling is the process by which the spleen removes abnormal red blood cells. The normal function of the spleen keeps the number of circulating erythrocytes with inclusions at a minimum.
The spleen also pools platelets in large numbers. Entry of platelets into the splenic pool and their return to the circulation are extensive. In splenomegaly, the splenic pool may be so large that it produces thrombocytopenia. Sequestration of leukocytes in the enlarged spleen may produce leukopenia.
Laboratory data include the following:
Hematocrit. The hematocrit indicates the percentage of red blood cells per volume of blood. Abnormally low readings indicate hemorrhage or internal bleeding within the body.
Bacteremia. The test for bacteremia indicates the presence of bacteria within the body. The term sepsis indicates bacteria in the bloodstream. Typical symptoms of fever and chills, along with other medical conditions, may indicate the presence of an infection.
Leukocytosis. An increase in the number of white cells present in the blood is usually a typical finding in infection. This finding may also occur after surgery, in malignancies, or in the presence of leukemia.
Leukopenia. Abnormal decrease in white blood corpuscles may be secondary to certain medications or bone marrow disorder.
Thrombocytopenia. Thrombocytopenia is an abnormal decrease in platelets, which may be due to internal hemorrhage.
Sonographic evaluation of the spleen
Ultrasound examinations are performed to assess overall splenic architecture, to examine or detect intrasplenic masses, to examine the splenic hilum and vasculature, and to determine splenic size ( Figure 11-9 and Table 11-1 ).
Patient preparation: nothing by mouth (NPO) for at least 6 hours.
Transducer selection: broadband (2.5 to 4 MHz) curvilinear or sector.
Patient position: supine; or steep left lateral if echo bed with a drop-leaf component is available.
Images and observations include the following:
Coronal scans of the long axis of the spleen should be performed.
The left hemidiaphragm, splenic hilus, and upper and lower borders of the spleen should be demonstrated.
The splenic length should be measured.
The texture of the spleen should be compared with that of the liver. The splenic parenchyma should be homogeneous with the liver.
Transverse scans of the spleen at the level of the splenic hilus should be performed. The sonographer should look for increased vascularity or splenic nodes with a sweep from the superior to inferior borders.
|Scan Plane Anatomy|
|Long spleen/left kidney; measure length; color Doppler|
|Transverse splenic hilum; measure width; color Doppler|
Normal texture and patterns
Sonographically, the splenic parenchyma should have a fine uniform homogeneous mid- to low-level echo pattern, and slightly more echogenic than the liver parenchyma ( Figure 11-10 ). As the spleen enlarges, echogenicity further increases. The shape of the spleen has considerable variation. The spleen has two components joined at the hilum: a superomedial component and an inferolateral component. On transverse scans, it has a “crescent” inverted comma appearance, usually with a large medial component and a thin component extending anteriorly. This part of the spleen may be seen to indent the fundus of the stomach. Moving inferiorly, only the lateral component is imaged. On longitudinal scans, the superior component extends more medially than the inferior component. The superomedial component or the inferolateral component may enlarge independently. The irregularity of these components makes it difficult to assess mild splenomegaly accurately. The length of the spleen usually measures greater than the length of the kidney. Splenomegaly is diagnosed when the spleen measures more than 13 cm in the adult patient, or more than normal length in the child.
Patient position and technique
The left upper quadrant may be imaged as the sonographer carefully manipulates the transducer between intercostal margins to image the left kidney, spleen, and diaphragm. The sector transducer may fit between the intercostal margins better than the larger curved-array transducer. The spleen generally lies in an oblique pathway in the posterior left upper quadrant ( Figure 11-11 ); therefore with the patient supine, the transducer should be placed in the superior left upper quadrant intercostal margin and slowly sweep anterior to posterior along the long axis of the spleen. The transducer must be positioned superior and posterior enough to image the spleen ( Figure 11-12 ). A deep inspiration may help to bring the spleen farther into the field of view from the subcostal approach. Variations in patient respiration may also facilitate imaging of the spleen; deep inspiration causes the lungs to expand with air and displaces the diaphragm; the lungs may expand so fully that the costophrenic angle is obscured and visualization of the spleen is impeded. The sonographer should observe the patient’s breathing pattern and modify the amount of inspiration to adequately image the spleen without interference from the air-filled lungs.