Abstract
The fetal spleen is an important center of hematopoiesis until the late fetal period. The fetal spleen is usually not seen on ultrasound (US) before 18 weeks. Normal values for fetal spleen dimensions have been proposed. From a clinical point of view, the rationale for developing normal values is the association of splenic size with hematologic and infectious diseases and developmental syndromes. Fetal splenomegaly is a classic sign of fetal anemia. A hypoplastic fetal spleen is much rarer and has been described in cases of DiGeorge syndrome and sickle cell anemia. Doppler measurements of the splenic artery are another potentially clinically useful aspect of US assessment of the fetal spleen. Several groups reported the association of splenic artery pulsatility and flow velocities with fetal growth restriction and anemia. Pathologic findings of the spleen discovered during prenatal US screening examinations are mostly related to the location or size of the organ and the occurrence of malformations. Accessory spleens are a common finding in adults and are typically located near the splenic hilus or the tail of the pancreas but can be found anywhere else in the abdomen, where they can be mistaken for tumors. Congenital splenic cysts are usually diagnosed during prenatal US from week 20 onward, typically as chance findings. The complete absence of the spleen (asplenia) and a condition with multiple small spleens (polysplenia) are rare anomalies and should raise the suspicion for associated laterality and cardiac defects.
Keywords
splenic artery Doppler, splenic cysts, heterotaxy syndromes, asplenia syndrome, cardiosplenic syndromes, Ivemark syndrome, left-isomerism, right-isomerism, splenogonadal fusion defect syndrome, splenomegaly, fetal anemia, hypoplastic spleen
Introduction
The fetal spleen is an important center of hematopoiesis until the late fetal period. In postnatal life, the function of the spleen is mainly as an immune organ, with lymphocyte and monocyte production continuing throughout adult life. During fetal development, the spleen appears between the fifth and eighth gestational weeks as an aggregation of reticular mesenchymal cells in the dorsal epigastrium of the stomach. The gastrosplenic ligament connects the spleen to the greater curvature of the stomach, and the splenorenal ligament connects the spleen to the dorsal abdominal wall. The fetal spleen shows distinct lobules that disappear before birth. The developing spleen acquires its characteristic “coffee bean” shape early in the fetal period.
The fetal spleen is usually not seen on ultrasound (US) before 18 weeks, and absence of the spleen may be very difficult to prove by US. Several prospective observational studies have measured fetal spleen size and volume according to gestational age. Normal values for fetal spleen dimensions have been proposed. From a clinical point of view, the rationale for developing normal values is the association of splenic size with hematologic and infectious diseases and developmental syndromes. The proposed normal ranges for size and volume of the fetal spleen according to gestational age are summarized in Table 32.1 .
| Gestational Age (Completed Weeks) | Width Measured From Side to Side (cm) | Circumference (cm) | Area (cm 2 ) | Volume (cm 3 ) | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Schmidt et al. | Hata et al. | Aoki et al. | You et al. | Schmidt et al. | Aoki et al. | You et al. | Srisupundit et al. | Hata et al. | Aoki et al. | You et al. | You et al. | |
| 16 | 2.8 | |||||||||||
| 20 | 1.3 | 1.4 | 1.5 | 2.0 | 4.5 | 4.2 | S.2 | 4.4 | 0.6 | 1.0 | 1.3 | 0.7 |
| 24 | 2.5 | 2.0 | 2.2 | 2.3 | 6.1 | 6.1 | 6.2 | 6.0 | 1.9 | 2.2 | 1.9 | 1.2 |
| 28 | 3.1 | 2.5 | 2.9 | 3.0 | 7.4 | 79 | 79 | 7.6 | 3.2 | 3.6 | 3 1 | 2.3 |
| 32 | 3.8 | 3.1 | 3.5 | 3.5 | 8.9 | 9.5 | 9.6 | 9.1 | 4.5 | 5.2 | 4.7 | 4.0 |
| 36 | 4.8 | 3.6 | 4.0 | 4.1 | 10.9 | 11.0 | 11.2 | 10.6 | 5.8 | 70 | 6.4 | 6.3 |
| 40 | 6.2 | 4.2 | 4.3 | 13.8 | 12.3 | 12.0 | 71 | 9.0 | 7.2 | |||
Doppler measurements of the splenic artery are another potentially clinically useful aspect of US assessment of the fetal spleen. Several groups reported the association of splenic artery pulsatility and flow velocities with fetal growth restriction and anemia. The splenic artery is usually visualized in an axial plane and can be identified by its course originating from the celiac trunk in front of the aorta and running behind the stomach to the spleen ( Fig. 32.1 ). Ebbing et al. published the first systematic longitudinal study of Doppler measurements in the fetal splenic artery and the celiac artery and proposed reference ranges for splenic artery pulsatility indices (PI) and peak systolic velocity (PSV). PI and PSV in the splenic artery were strongly and positively correlated with PI in the middle cerebral artery and the umbilical artery. Because Doppler measurements of the middle cerebral artery PI are widely used to assess potential anemic conditions of the fetus, Doppler velocimetry of the splenic artery could be useful as an additional tool to establish or confirm diagnosis. The study by Ebbing et al. also showed that splenic artery flow and celiac artery flow correlate with portocaval pressure and umbilical venous supply to the right liver lobe. For this reason, Doppler measurements of the splenic artery may provide a method for the differential assessment of splanchnic circulation in the fetus. Reference values for splenic artery Doppler measurements are provided in Fig. 32.2 .
The splenic vein can also be visualized in an oblique abdominal plane using Doppler. The flow pattern of the splenic vein is not pulsatile in normal conditions. Using Doppler, Musilova analyzed the flow pattern of the splenic vein in women with preterm prelabor rupture of membranes (PPROM), reporting that a pulsatile flow pattern in the splenic vein was predominantly observed in PPROM with chorioamnionitis (likelihood ratio 13.2), and with funisitis (likelihood ratio 5.7). They suggested that Doppler evaluation of the splenic vein could be a noninvasive tool for the prediction of these inflammatory complications. However, this single report lacks confirmation from other studies.
Pathologic findings of the spleen discovered during prenatal US screening examinations are mostly related to the location or size of the organ and the occurrence of malformations. Accessory spleens are a common finding in adults and are usually spherical homogeneous masses a few centimeters or less in size and with smooth borders. They are typically located near the splenic hilus or the tail of the pancreas but can be found anywhere else in the abdomen, where they can be mistaken for tumors. The complete absence of the spleen (asplenia) and a condition with multiple small spleens (polysplenia) are rare anomalies and should raise the suspicion for associated laterality and cardiac defects. US evaluation of the spleen can be important to establish the diagnosis or aid in the identification of numerous rare developmental disorders, such as cardiosplenic syndromes, polysplenia and situs inversus, and splenogonadal fusion limb defect.
Congenital Splenic Cysts
Definition
Depending on histology and suspected etiology, splenic cysts can be divided into two types: primary cysts (approximately 25% of all splenic cysts), which are formed by cellular layers and have an inner epithelium; and secondary cysts or pseudocysts, which consist of a fibrous capsule. Primary cysts can be neoplastic and originate as dermoids, epidermoids, hemangiomas, or lymphangiomas. Secondary cysts are believed to be of posttraumatic, inflammatory, or degenerative origin.
Prevalence and Epidemiology
Congenital splenic cysts are generally considered to be rare. However, since the advent of high-resolution fetal US scans, splenic cysts have been regularly detected and reported. The exact prevalence of congenital splenic cysts is unknown. A review of 13 cases was published by Kabra.
Etiology and Pathophysiology
The exact origin of congenital splenic cysts is unclear. However, several etiologic mechanisms have been proposed, such as invasion of pluripotent stem cells into the fetal spleen, invagination of peritoneal endothelial cells during development, inclusion of cellulous mesothelium during organogenesis, or dilatation of normal lymphatic areas within the spleen.
Manifestations of Disease
Clinical Presentation
Congenital splenic cysts are usually diagnosed during prenatal US from week 20 onward, typically as chance findings. In postnatal life, larger congenital splenic cysts can become symptomatic because of expansion, rupture, and bleeding, manifesting as sudden abdominal incidents.
Imaging Technique and Findings
Ultrasound.
The typical US pattern of a congenital cyst is an anechoic cyst with smooth walls. Epidermoid cysts have a complex pattern with irregularities and thickness of the posterior walls because of epithelial peripheral trabeculations and internal echoes from blood clots. Pseudocysts occasionally may be sonolucent, but calcifications are more frequently observed. Infarcts are characteristically triangular. Fresh hematomas appear hyperechoic, with subcapsular fluid collection.
Magnetic Resonance Imaging.
There are only few reports on the use of magnetic resonance imaging (MRI) for the diagnosis of splenic cysts. On both T1-weighted and T2-weighted scans, splenic cysts usually have signal intensity equal to that of water; however, depending on the composition of the cystic fluid, the signal intensity in T1-weighted images may increase.
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