Abstract
Spleen can have a wide range of anomalies including its shape, location, number, and size. Although most of these anomalies are congenital, there are also acquired types. Congenital anomalies affecting the shape of spleen are lobulations, notches, and clefts; the fusion and location anomalies of spleen are accessory spleen, splenopancreatic fusion, and wandering spleen; polysplenia can be associated with a syndrome. Splenosis and small spleen are acquired anomalies which are caused by trauma and sickle cell disease, respectively. These anomalies can be detected easily by using different imaging modalities including ultrasonography, computed tomography, magnetic resonance imaging, and also Tc-99m scintigraphy. In this pictorial essay, we review the imaging findings of these anomalies which can cause diagnostic pitfalls and be interpreted as pathologic processes.
1. Normal Anatomy of the Spleen
Spleen is an intraperitoneal organ located in the left upper quadrant with a smooth serosal surface. Its normal position is provided by two fatty ligaments: the gastrosplenic ligament, which connects the greater curvature of the stomach to the ventral aspect of the spleen, and the splenorenal ligament between the left kidney and the spleen, attaching the spleen to the posterior abdominal wall. The splenic hilum is directed anteromedially and includes splenic artery and six or more branches of the splenic vein. Splenic size changes according to the age and weight. Configuration of the spleen is also variable according to the indentations of the organs including stomach, colon, pancreas, and kidney which are in close relation to the spleen [1–4].
2. Splenic Clefts, Notches, and Lobulations
The fetal spleen is lobulated, and these lobules normally disappear before the birth. Lobulation of the spleen may persist into adult life and be typically seen along the medial part of the spleen. A persisting lobule results in a variation in shape of the spleen (Figure 1). But sometimes the lobule may extend medially anterior to the upper pole of the left kidney and less often posterior to the upper pole of the left kidney. Although these lobules are not of any clinical importance for the patient, close relation of the splenic lobule to the upper pole of the left kidney may cause misinterpretations as a mass originating from the kidney by the radiologists [1,The notches and clefts of the spleen are other congenital shape anomalies which are located on the diaphragmatic surface and especially superior border of the adult spleen. These are remnants of the grooves that originally separate the fetal lobules. Clefts occasionally may reach 2-3 cm in length and may cause misinterpretations as a splenic laceration in patients with abdominal trauma [1, 2] (Figure 2). Persistence of this appearance on delayed images of dynamic contrast enhanced CT is useful in diagnosing clefts, whereas in case of trauma these clefts fill with contrast.
3. Fusion and Location Anomalies
Accessory spleen, in other words supernumerary spleens, splenunculi, or splenules, results from the failure of fusion of the primordial splenic buds in the dorsal mesogastrium during the fifth week of fetal life. Incidence of accessory spleen in the population is 10%–30% of patients in autopsy series and 16% of patients undergoing contrast enhanced abdominal CT. Although the most common location for an accessory spleen is splenic hilum (75%) (Figure 3) and pancreatic tail (25%) (Figure 4), it can occur anywhere in the abdomen including gastrosplenic or splenorenal ligaments, wall of stomach or bowel (Figure 5), greater omentum or the mesentery, and even in the pelvis and scrotum. Accessory spleen usually measures 1 cm in diameter, but its size varies from a few milimeters to centimeters. Also the number of accessory spleens can vary from one to six [1, 5–7]. Accessory spleens are usually incidentally detected and asymptomatic, but in case of unexpected locations, accessory spleen can be of clinical importance. In malignancy patients, an unexpected location of an accessory spleen can be misinterpreted as a metastatic lymph node. But the identical imaging findings of the accessory spleen with the normal splenic tissue on CT and MRI can be helpful for the differential diagnosis. Also demonstration of feeding artery from splenic artery and use of iron containing contrast agents can be helpful for diagnosis (Figure 3). In patients with splenic trauma, an accessory spleen may become clinically important to preserve splenic tissue in case of splenectomy. But in a patient who had splenectomy for hypersplenism, a preservable splenic tissue is an undesired condition that may cause recurrent disease. Splenogonadal fusion anomaly may mimic tumors and result in unnecessary surgeries. So it is important to characterize this anomaly as noninvasively as possible by using ultrasonography, CT, MRI, and Tc-99 m sulfur colloid scintigraphy. Splenopancreatic fusion anomaly which is a form of splenopancreatic field abnormalities including ectopic splenic tissue in the pancreatic tail and ectopic pancreatic tissue in the spleen or accessory spleen may also be detected incidentally (Figure 6). This rare anomaly may result from disturbances in the embryogenesis; thus, both organs arise from dorsal mesogastrium near each other. During the embryogenesis, the close interaction between pancreatic tail and splenic hilum may result in a fusion. The clinical importance of this rare anomaly is to avoid possible complications when splenectomy or distal pancreatectomy is planned [1, 6, 80].
Wandering or ectopic spleen is a rare entity in which the spleen is located outside of its normal location. Its reported incidence in several large series of splenectomies is less than 0.5% and mainly detected in children and women between 20 and 40 years of age. The reason for the wandering spleen is the laxity or maldevelopment of the supporting splenic ligaments, and the spleen can be found in any part of the abdomen related to the length of the vascular pedicle.
Wandering spleen may be incidentally detected or may cause different degrees of abdominal pain related to acute, chronic, and intermittent torsion of the vascular pedicle. Ultrasonography and CT are the most used methods for diagnosis. Imaging findings of wandering spleen are the absence of the spleen in its normal position and a mass located anywhere in the abdomen or pelvis with enhancement pattern of a normal splenic tissue (Figure 7). In case of torsion, a “whirl” appearance of its twisted pedicle and impaired enhancement of the mass can also be helpful. The treatment choice of a wandering spleen is splenopexy. Splenectomy is required only in case of infarction, which can be diagnosed radiologically. Doppler ultrasonography and contrast enhanced CT can be used to evaluate splenic vascularization. In splenic torsion, doppler ultrasonography shows no flow within the spleen and a low diastolic velocity with an elevated resistive index in the proximal splenic artery. Contrast enhanced CT can show a total absence of or heterogeneous enhancement pattern within the spleen related to partial or total infarction (Figure 8). If there is a contraindication for contrast enhanced CT, findings of infarction on unenhanced CT are low attenuation of the spleen relative to the liver, a hyperdense intraluminal filling defect in the splenic vessels indicating an acute thrombus, and high density of the splenic capsule compared with the parenchyma (“rim” sign) [1, 6, 9, 10