Chapter 17: The Spleen PDF

Chapter 17: The Spleen Kacey Davis, and Reva Arnez Curry OBJECTIVES Describe the function of the spleen. Describe the location of the spleen. Define size relationships of the normal spleen. Describe gross anatomy of the normal spleen. Describe the sonographic appearance and scanning...

Chapter 17: The Spleen Kacey Davis, and Reva Arnez Curry OBJECTIVES Describe the function of the spleen. Describe the location of the spleen. Define size relationships of the normal spleen. Describe gross anatomy of the normal spleen. Describe the sonographic appearance and scanning technique of the normal spleen. Discuss sonographic applications and normal variants of the spleen. Describe the associated physicians, diagnostic tests, and laboratory values relevant to the normal spleen. KEY TERMS Culling    --- Removal of abnormal red blood cells from the blood by the spleen. Erythrocyte    --- Red blood cell. Hematopoiesis    --- Produces erythrocytes, as well as white blood cells, in the developing fetus. In the adult, red blood cell production is performed only in cases of severe hemolytic anemia. Hemoglobin    --- Oxygen-carrying and iron-containing pigment of red blood cells. Phagocytosis    --- Removal of worn-out and abnormal red blood cells and platelets from the bloodstream by phagocyte cells in the spleen. Pitting    --- Removal of nuclei from old red blood cells by the spleen without destroying the cell. Red Pulp    --- Along with white pulp, comprises spleen parenchyma. Red pulp is where worn-out red blood cells and bloodborne pathogens are destroyed. Consists of splenic sinuses and splenic cords. Reticuloendothelial System    --- Has the responsibility of phagocytosis (engulfing and destroying) of damaged or old cells and their debris, foreign materials, and pathogens, taking them out of the circulating blood. Reticuloendothelial cells are found in the spleen, as well as in the Kupffer cells of the liver, lymph nodes, alveoli, brain, blood vessels, and mucous membranes. Splenic Artery (SA)    --- Arises from the celiac axis of the abdominal aorta and travels laterally toward the left to supply the spleen with oxygen-rich blood. Splenic Hilum    --- This portion of the spleen, located medially, is where the vasculature enters and exits. Splenic Vein (SV)    --- Conveys venous blood from the spleen; running medially along the gastrolienal ligament to its confluence with the superior mesenteric vein posterior to the neck of the pancreas to form the portal vein. White Pulp    --- Along with red pulp, comprises spleen parenchyma. White pulp is where immune functions take place. Consists of lymphatic tissue containing lymphocytes and monocytes that continually produce and are active in ingesting and digesting harmful pathogens that enter the bloodstream. Normal Measurements Anatomy Measurement Spleen long axis 8--13 cm Spleen anteroposterior diameter 7--8 cm Spleen thickness 3--4 cm Splenic volume 60--200 mL Splenic index 107--314 cm3 The spleen is an intraperitoneal organ that lies in the left upper quadrant of the abdominal cavity. It is part of the reticuloendothelial system. This system has the responsibility for phagocytosis (engulfing and destroying) of damaged or old cells and their debris, foreign materials, and pathogens, taking them out of the circulating blood. The spleen is composed primarily of lymph tissue. Although the spleen is a component of the body's defense system, it is not essential to life and can be removed without adverse effects (Fig. 17.1). FIGURE 17.1 Normal Spleen and Vasculature Splenic artery (SA) Branch of the celiac artery. Branches of SA before reaching the spleen: Pancreatic branches from the initial portion of the splenic artery. Short gastric branches, which supply superior portion of the greater curvature of the stomach. Left gastroepiploic, which supplies middle portion of the greater curvature of the stomach. Branches of the SA after reaching the spleen: 2--3 lobar arterial branches at the splenic hilum. Each lobar artery can further branch into 2--4 lobular arterial branches. Lobular arteries branch into smaller splenic arteries, which terminate in tiny capillaries. Tiny capillaries anastomose with venous sinuses. NOTE: The capillaries are permeable, meaning that red blood cells can pass through them. This provides the filtering function of the spleen. Splenic vein (SV) Splenic venous sinuses anastomose with splenic capillaries from splenic arteries. Venous sinuses unite to form venules, which merge to eventually form the splenic vein. Splenic vein exits spleen at the hilum. Courses from lateral to medial along posteroinferior border of pancreas body and tail. Joins superior mesenteric vein to form portal vein at the level of the pancreas neck. Location The spleen lies in the left hypochondrium, with its longest axis along the 10th rib. It lies posterolateral to the body and fundus of the stomach, posterolateral to the tail of the pancreas, and posterior to the left colic flexure (Fig. 17.2B and C; Table 17.1). The left kidney is located inferior and medial to the spleen. Posterior to the spleen are the diaphragm, the left lung, and the 8th, 9th, 10th, and 11th ribs. The spleen is covered by peritoneum, with the exception of the medially located splenic hilum, where the vasculature structures and lymph nodes are located (see Fig. 17.2C). Size The size of the spleen varies among individuals, and at different times it can vary in the same individual. The normal range of measurements for the spleen is 8 to 13 cm in length, 7 to 8 cm in anteroposterior diameter, and 3 to 4 cm in thickness. Normal splenic volume is 60 to 200 mL, although normal volumes of up to 350 mL have been reported. The normal splenic index is between 107 and 314 cm3. Volume is calculated automatically after measuring the perimeter, area, and longitudinal diameter, whereas the splenic index is the length × width × thickness of the organ. FIGURE 17.2 (A) Microscopic organization of the spleen. (B) Medial surface of the spleen. (C) Surrounding splenic anatomic relationships (see Table 17.1). Another technique to assess normal size is to use calipers, measuring the length and width on a longitudinal section and measuring the width and depth on an axial section (Fig. 17.3). There is mixed opinion on which method is best in assessing the size of the spleen, with studies favoring the longitudinal diameter, splenic index, or splenic volume. The spleen is generally smooth in contour, with a convex superior surface and a concave inferior surface. As with the gallbladder, the shape of the spleen is just as important as the measurements when assessing normal size. For instance, it is possible to have a normal longitudinal diameter but an enlarged volume, or an enlarged longitudinal diameter with a normal volume. It is important to assess splenic size using department protocols to improve standardization and reduce operator error. Gross Anatomy The spleen is a highly vascular mass of lymphoid tissues. Considered the largest lymphoid organ, it is ovoid and has a convex superior surface and a concave inferior surface. The spleen is entirely covered by the peritoneum except at the hilum, where all vessels enter and exit. The spleen does not have a strong capsule, which is why it is a very fragile organ. Table 17.1 Location of the Spleen Routinely Visualized With Ultrasound Spleen Anterior to 8th, 9th, 10th, 11th ribs, diaphragm, left lung Posterior to Stomach body/fundus, left colic flexure, peritoneum (except for hilum) Superior to Left kidney Inferior to Diaphragm Medial to Left lateral to Stomach body/fundus, left kidney, pancreas tail, splenic artery, splenic vein, inferior mesenteric vein, liver Right lateral to The splenic artery (SA) arises from the celiac axis of the abdominal aorta and travels laterally toward the left to supply the spleen with oxygenated blood. Initially from the celiac axis, the SA size is estimated to be 5.6 mm in diameter. It gives off pancreatic branches that supply the body and tail of the pancreas, short gastric branches that help to supply the superior portion of the greater curvature of the stomach, and the left gastroepiploic artery, which supplies the middle portion of the greater curvature of the stomach (Fig. 17.4). At the splenic hilar area, the SA branches into two to four lobar arteries. These further branch into lobular arteries, which further subdivide into smaller SAs, which will eventually terminate in tiny capillaries (see Fig. 17.1). These capillaries are permeable and help to provide the filtering function of the spleen. In one cadaver study the lobular subdivisions and areas of the spleen were too varied to draw definitive conclusions as to which lobular arteries supplied particular areas of the spleen. The same study found that the lobular arteries appear to become more tortuous in older patients. FIGURE 17.3 (A) Longitudinal section of the spleen with length and width measurements. (B) Axial section of the spleen with depth and width measurements. FIGURE 17.4 Suprapancreatic, pancreatic, prepancreatic, and prehilar sections of the splenic artery. The splenic capillaries anastomose with tiny splenic venous sinuses, which unite to form splenic venules. The venules merge to eventually form the splenic vein (SV), which conveys venous blood from the spleen at the splenic hilum and courses in a horizontal direction along the gastrolienal ligament to its confluence with the superior mesenteric vein at the pancreas to form the main portal vein. The spleen's location affords it the protection of the ribs. Consequently, the spleen is usually not palpable unless it is pathologically enlarged. The spleen contains trabeculae of connective tissue which divides the spleen into splenic nodules. The splenic nodules consist of red pulp and white pulp (Fig. 17.5). The white pulp consists of the Malpighian corpuscles, small nodular masses of lymphatic tissue that surround and follow the smaller SAs. The red pulp, which is looser and more vascular, consists of the splenic sinuses and splenic cords. The sinuses are long, slender channels lined with epithelial cells. The red pulp occupies all of the space not filled by white pulp or splenic cords (Fig. 17.2A). FIGURE 17.5 Cross-section of the Spleen.  From Herlihy, B. \[2014\]. The human body in health and illness \[6th ed.\]. St. Louis: Elsevier. Physiology As part of the reticuloendothelial system, the spleen's main function is to help remove old cells, debris, pathogens, and foreign substances from circulation. Reticuloendothelial cells are found in the spleen, as well as in the Kupffer cells of the liver, lymph nodes, alveoli, brain, blood vessels, and mucous membranes. In addition, the spleen: Produces lymphocytes, monocytes (phagocytes), plasma cells, and antibodies Stores iron and metabolites Produces red blood cells (this function primarily occurs in the fetus) Produces white blood cells throughout life Acts as a blood a reservoir Regulates platelet and leukocyte life span Four major functions of the spleen are defense, hematopoiesis, red blood cell and platelet destruction, and service as a blood reservoir. Defense Functioning as a defense mechanism, the spleen aids in the destruction and removal of microorganisms by phagocytosis. In the white pulp, lymphocytes and monocytes are continually produced and are active in ingesting and digesting harmful pathogens that enter the bloodstream. These cells are able to recognize foreign harmful substances and turn themselves into antibody-producing plasma cells and memory cells. The plasma cells destroy the invading microorganism by creating antibodies to that particular pathogen. The memory cells "remember" that particular pathogen, and should it attack the body again, the antibodies are quickly activated to destroy it. This is called the immune response. Hematopoiesis This function produces erythrocytes, also called red blood cells, as well as white blood cells in the developing fetus. In the adult, however, red blood cell production is performed only in cases of severe hemolytic anemia. Red Blood Cell Removal The spleen inspects passing red blood cells for imperfections and destroys those it recognizes as abnormal. Blood then passes through the red pulp and into the splenic sinuses. This portion of the spleen is a filter that aids in phagocytosis of degenerating red blood cells. Pitting, the removal of nuclei from old red blood cells without damaging the cells, and culling, the removal of abnormal red blood cells, occur. The hemoglobin (iron-containing pigment) in these cells is broken down. The iron is either used immediately to produce new red blood cells or is transported via the portal vein to the liver and bone marrow for storage. The globin is used to break down other proteins for use in the body. The most abundant pigment released is hemosiderin. Iron can be stored in hemosiderin until it is needed to make more hemoglobin. Heme, also a pigment, is not needed and is turned into bilirubin and excreted by the liver in bile. FIGURE 17.6 Normal Spleen.(A) Longitudinal section. (B) Longitudinal section with measurements. (C) Axial section. (D) Another axial section. (E) An axial section with measurements. Storage The ability of the spleen to store red blood cells (blood reservoir) is due to its high smooth-muscle content. The red pulp of the spleen, with its venous sinuses, holds a considerable volume of blood that can be quickly released into the circulatory system if needed. The spleen's average volume of approximately 350 mL can drop quickly and dramatically after sympathetic stimulation that causes the smooth muscle to constrict. However, if the number of cells stored becomes excessive, splenomegaly (enlargement of the spleen) will develop. Sonographic Appearance and Scanning Technique The normal spleen should have a uniform homogeneous and smooth texture (Fig. 17.6). It is medium gray in color and should be the same (isoechoic) or slightly more echogenic (hyperechoic) relative to the liver. Bright reflections may be seen throughout the spleen that represent calcifications of small arterial walls or calcified granulomatous inclusions. The significance of the latter varies according to patient history. The organ may be difficult to visualize as a result of overlying ribs or gas in the adjacent bowel. It is often easiest to scan the spleen intercostally from a lateral approach. In a coronal scanning plane, longitudinal view, the spleen and left kidney can usually be visualized if there is not too much interference from bowel gas (Fig. 17.7). The splenic hilum should also be visualized, making it easy to document splenic vasculature at the hilar area (Fig. 17.8). The spleen may be visualized with the patient in a supine position as well as the right lateral decubitus position. The transducer should be placed in the superior left upper quadrant intercostal margin with the sonographer slowly sweeping anterior to posterior along the long axis of the spleen. Deep inspirations depress the diaphragm and move the spleen inferiorly away from the bony thorax, alleviating shadows from the ribs and bowel gas. In addition, the sonographer may need to alter the amount of inspiration by the patient to adequately image the spleen without the interference from the air-filled lungs. Images should also be obtained in the transverse orientation by rotating the probe 90 degrees (Table 17.2). FIGURE 17.7 Splenic Kidney Interface.(A) Coronal scanning plane image showing a longitudinal section of the normal interface between the spleen and left kidney. (B) Same view as A, but in a different patient. Sonographic Applications The most common use of sonography in imaging the spleen is to detect enlargement or splenomegaly. Years ago, when articulated arm B scanning was in wide use, the rule of thumb was that if the spleen was visualized anterior to the aorta, it was pathologically enlarged. Currently, with real-time scanning, the determination of splenomegaly has become basically a subjective judgment; however, a long axis measurement of the spleen greater than 13 cm is suggestive of splenomegaly. The more experience the sonographer and interpreting physician have, the more accurate their judgment will be. Ultrasound can help to assess splenic masses, although primary splenic masses are quite rare. Ultrasound is also useful in assessing splenic damage from blunt trauma, such as rupture or hemorrhage. FIGURE 17.8 Splenic Hilum.(A) Left lateral approach, transverse scanning plane image demonstrating the hilar area in this axial section of the spleen. (B) Same view as A, clearly demonstrating the splenic hilum in a different patient. (C) Color flow image of B. Table 17.2 Spleen Scanning Protocol Scan Plane Approach Image 1\. Coronal Left lateral Long axis image of the spleen 2\. Coronal Left Iateral Long axis image of the spleen with measurements (length and anteroposterior diameter) 3\. Coronal Left lateral Superior longitudinal image of the spleen including the adjacent pleural space 4\. Coronal Left lateral Inferior longitudinal image including left kidney for parenchyma comparison 5\. Coronal Left lateral Longitudinal image including the splenic hilum using color Doppler 6\. Transverse Left lateral Axial image of the spleen to include both anterior and posterior margins 7\. Transverse Left lateral Axial image measuring the width of the spleen 8\. Transverse Left lateral Axial image including the anterior margin and splenic hilum with color Doppler 9\. Transverse Left lateral Axial image including posterior margin FIGURE 17.9 (A) Coronal scanning plane image showing longitudinal spleen with splenomegaly and a section of accessory spleen (between calipers). (B) Transverse scanning plane image from a left approach of same patient showing an axial section of the spleen with splenomegaly and a section of accessory spleen (between calipers). Normal Variants Accessory Spleen Accessory spleen is found in up to 10% of the general population. These islands of tissue are usually less than 1 cm in diameter. More than one accessory spleen may be present, most often near the splenic hilum or attached to the tail of the pancreas (Fig. 17.9). Asplenia This rare congenital abnormality may be associated with a congenital heart defect. If solitary, there are no complications. The liver may be visualized more distinctly to the left of the midline than usual. Splenomegaly This pathologic finding is included because it is the most common splenic abnormality. Splenomegaly may be diagnosed sonographically when the long axis of the spleen measures greater than 13 cm. It is most often due to complications of other organic disease. Splenomegaly is noted as a mass in the left upper quadrant. It may be due to recent trauma, portal venous congestion, systemic infection, or a blood disorder such as anemia. One study of 78 patients with an enlarged spleen showed that the diagnosis could be made by palpation alone, defined as palpating the spleen below the costal margin by 0.5 to 2.0 cm, in only 18 patients. In this study, 24 patients were diagnosed with splenomegaly by ultrasound longitudinal diameter alone. This measurement has been shown to correlate fairly well with three-dimensional computed axial tomography (CT) studies. Another 38 patients from the study group were diagnosed by ultrasound volume measurements. The wide range of assessment techniques highlights the care that must be taken when assessing splenomegaly. Reference Charts Associated Physicians Family physician: Often serves as referring physician, coordinating patient care. In this capacity, the physician recommends referral to specialists when necessary. Internist: Specializes in the diagnosis and treatment of internal disorders. Surgeon: Specializes in performing surgical procedures. Radiologist: Specializes in interpreting diagnostic imaging procedures. Hematologist: Specializes in treating diseases of blood. Common Diagnostic Tests X-ray: In this test, ionized electromagnetic waves create photographic images, which are then read and interpreted. It is performed by a radiologic technologist and interpreted by a radiologist. Ultrasound: Nonionized sound waves generate diagnostic images in this test. The sound waves do not penetrate bone or air. The test is performed by a sonographer and interpreted by a radiologist. Nuclear medicine: This test involves intravenous injection of radionuclides to create diagnostic images. The radionuclides "tag" specific cells, so that the resulting image is specific to the area of interest. This test is performed by a nuclear medicine technologist and interpreted by a radiologist. Computed tomography (CT) scan: The ionized waves create a cross-sectional x-ray image of the body. This test is performed by a radiologic technologist who is certified for "CT" or "CAT scan." The scan is interpreted by a radiologist. Magnetic resonance imaging (MRI): This test uses nuclear magnetic resonance to produce images. This test is performed by a radiologic technologist who is certified in "MRI." The scan is interpreted by a radiologist. Affecting Chemicals Anticoagulants: These thin the blood in patients whose blood tends to clot abnormally or who are at high risk for developing thromboembolism. Patients receiving such treatment are more likely to experience internal hemorrhage or bleeding from small cuts that does not clot in a normal manner. The patient's hematocrit should be monitored. Laboratory Values Hematocrit: The hematocrit reading indicates the percentage of red blood cells per volume of blood. Normal values for men are 40%--54%; for women, 37%--47%. An abnormally low hematocrit points to internal bleeding. Bacteremia: The presence of bacteria within the blood system, also known as sepsis. Symptoms include chills, fever, and possibly the presence of abscesses. Leukocytosis: An increase in the number of circulating leukocytes (\>10,000 per mm3). This finding is indicative of an infection of the blood. It may occur in hemorrhage, following surgery, in malignancies, during pregnancy, and in toxemia. It can also be due to leukemia. Leukopenia: An abnormally low number of leukocytes in the blood (\