Case of the Week #530

Case of the Week #530

Javier Cortejoso
 on 09/28/2022
 on Wednesday, September 28th, 2022
I would like to make some considerations regarding the frequent finding of an abdominal mass in a routine third-trimester ultrasound with previous normal scans. The first thing we must do is something as simple as identifying the fetal sex, because if the fetus is female, the most likely diagnosis is an ovarian cyst.

In the present case we do not have this data (the authors certainly do), so we have to base our diagnosis on the location and characteristics of the tumor. To do this, the authors use a volume post-processing tool available on modern General Electric ultrasound equipment known as Tomographic Ultrasound Imaging (TUI). Once a 3D volume has been obtained, we can navigate within it by scrolling or, alternatively, we can apply the tomographic mode, a multiplanar mode display of the volume as parallel planes similar to tomographic images obtained from CT and MR workstations. Fetal heart and brain are ideal regions to be examined with this tool, but in any location tomographic mode display provides an optimal overview of the region of interest. The all-in-one view of an organ or pathological finding, along with its neighboring structures, allows accurate assessment of the characteristics and extent of the lesion.

The heterogeneous mass is located in the anterior and lateral part of the abdomen, occupying practically its entire height (since it is seen in all the images of the TUI). It is well defined and inside there are small peripheral cysts with an echogenic central stroma, a feature seen in ovarian torsion. The TUI images show that the tumor is somewhat more heterogeneous in its upper part and that it is located anterior and medial to the normal kidney.

The differential diagnosis of a fetal abdominal cyst is lengthy and therefore a systematic approach to sonographic assessment is useful. The first step is to identify normal anatomy: normal cystic structures in the fetal abdomen include the stomach, gallbladder, and urinary bladder. Use of color Doppler sonography to identify the umbilical arteries flanking the bladder can be helpful. The next steps are to evaluate the urinary system (renal cysts, macrocystic kidneys, multicystic-dysplastic kidneys, hydronephrosis, dilated upper pole in duplex kidney, and dilated ureter) and possible dilation of the gastrointestinal tract. Excluding renal causes and dilated bowel, the most common causes of intra-abdominal cysts are ovarian cysts, enteric duplication cysts, hepatic and biliary tree cysts, meconium pseudocysts and mesenteric cysts. Less common causes include adrenal cystic masses (simple cysts, evolving haemorrhage or, less frequently, cystic adrenal neuroblastoma), splenic cysts, hydrometrocolpos, urachal cysts, cystic type IV sacrococcygeal teratoma, anterior sacral meningocele, cloacal malformation and varices of the umbilical vein. In clinical practice, abdominal cysts are most frequently detected at the time of routine morphology scan at 18–20 weeks. However, some, notably ovarian, cysts do not become apparent until the third trimester.

1. Ovarian cyst is the most common lower abdominal or pelvic cyst in a female fetus during the second half of pregnancy. These cysts can be “simple” cysts (round, anechoic, unilocular, and thin walled), or “complicated”, with a more heterogeneous appearance. A highly specific (up to 100%) sign of the ovarian origin of an abdominal cyst is the “daughter cyst sign”, which consists of the existence, within the main cyst, of a small, round, anechogenic structure resembling a small cyst. The “complex” cysts are thick walled, with heterogeneous echogenicity. The imaging features include fluid/debris levels, septations, and low level echoes, thought to be the result of hemorrhage or torsion. Haemorrhagic cysts are invariably associated with torsion and associated infarction. Nussbaum and colleagues grouped the sonographic appearance of prenatal and postnatal ovarian cysts into five classes: (1) cystic; (2) cystic with a fluid-debris level; (3) cystic with a retracting clot; (4) septated with or without internal echoes; and (5) solid. They concluded that most complicated cysts can be diagnosed from sonograms of patterns 2 through 5 and, conversely, that an anechoic cyst with a sonographically imperceptible wall suggests an uncomplicated cyst (non-torsed, nonhemorrhagic). Although the ovarian cysts are usually located in the pelvic cavity, they can occasionally be found in the upper abdomen, since the ligaments are lax and allow its displacement. Rarely, ovarian cysts can be associated with polyhydramnios, ascites, or fetal anemia due to hemorrhage.

2. Enteric duplication cysts can arise at any level of the gastrointestinal tract, although 80% are intra-abdominal. They can be round or tubular solitary, thick-walled cysts. Sonographic demonstration of a double wall sign (layered appearance of the wall with echogenic mucosa, and hypoechoic muscular layer) and the presence of peristalsis support the diagnosis. Its contents might be echogenic rather than anechoic if there has been haemorrhage or accumulation of inspissated material. An enteric duplication cyst shares a wall with the adjacent gastrointestinal tract. Therefore, the diagnosis is suspected if it is possible to demonstrate the “Y” sonographic configuration of the muscle layer caused by the splitting of the shared muscularis propria between the cyst and the adjacent loop. This sign is not described for other abdominal cysts.

3. Meconium pseudocyst is a result of meconium peritonitis and bowel perforation in utero, and can present as a cystic abdominal mass. Related features such as dilated bowel, peritoneal or bowel calcification and ascites might help in the diagnosis. However, there are cases described without any of these findings.

4. Mesenteric or omental cysts are often a presumptive diagnosis for a small cyst in a variable intra-abdominal location. They are lymphatic in origin, but can be differentiated from lymphangiomas, which are frequently described as multilocular and larger than mesenteric cysts. Its content is of variable echogenicity, usually anechoic, and its walls are thin.

5. Teratomas are neoplasms that arise from pluripotent cells of the three germinal layers of the embryo. As a rule, in children, they occur in the sacrococcygeal region and less frequently at other sites such as the gonads, cervical areas, mediastinum, retroperitoneum, cranial cavity, nasopharynx, or upper jaw. Teratomas arising in the gastrointestinal tract and related organs (stomach, pancreas, omentum, mesentery) are very rare. Sonographically, they present as a circumscribed echogenic mass with multiple cystic spaces and high-intensity echoes representing foci of calcification.

6. Adrenal cysts can be detected in the third trimester in a characteristic location, cephalad to a kidney. The differential diagnosis in the fetus include simple cysts, antenatal haemorrhage and cystic neuroblastoma. Adrenal haemorrhage is most common in the perinatal period, but has been demonstrated in the third trimester, presenting as an anechoic mass that evolves in appearance over time: with a tendency towards cystic changes, and spontaneous regression, often leaving residual adrenal calcification. Congenital cystic neuroblastoma usually presents as a mixed solid and cystic mass, although purely anechoic adrenal lesions have been described.

7. Choledochal cysts are cystic dilatation of extrahepatic and/or intrahepatic bile ducts. The most common presentation in the fetus is a unilocular, simple, right upper quadrant cyst, round in the axial plane and fusiform in the longitudinal plane. Bile ducts entering into the cyst confirms the diagnosis.

8. A urachal cyst on ultrasound demonstrates a communication with the bladder and extension from the dome of the bladder to the umbilicus, a finding that would best be demonstrated on a sagittal image.

The differential diagnosis for a fetal intra-abdominal cyst is broad but can be quickly narrowed by determining the organ of origin (when possible), recognizing the cyst location within the abdominopelvic cavity, and identifying its sonographic appearance. Cysts located in the lateral and lower part of the abdomen, as in this case, can only correspond to renal pathology, adrenal hemorrhage or ovarian pathology. In the present case, it is easy to exclude a renal/adrenal origin as the cyst is independent of these organs and the ultrasound appearance (complex mass with internal echoes) suggests a complicated ovarian cyst by hemorrhage and/or torsion, located close to the bladder.

Suggested readings:

1. Chantraine F, Tutschek B. Abdominal Cysts. In: Copel JA, ed. Obstetric Imaging. Fetal diagnosis and care, 2nd ed. Elsevier, Philadelphia, PA, 2018; pg 97- 105.e1
2. Chaoui R, Heling KS. Multiplanar Display II: Tomographic Mode. In: 3D Ultrasound in Prenatal Diagnosis. A Practical Approach. Walter de Gruyter GmbH, Berlin/Boston, 2016; pages 75-92.
3. Heling KS, Chaoui R, Kirchmair F, et al. Fetal ovarian cysts: prenatal diagnosis, management and postnatal outcome. Ultrasound Obstet Gynecol. 2002 Jul;20(1):47-50. PMID: 12100417.
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8. Nussbaum AR, Sanders RC, Hartman DS, et al. Neonatal ovarian cysts: sonographic-pathologic correlation. Radiology. 1988 Sep;168(3):817-21. PMID: 3043551.
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11. Shozu M, Akasofu K, Yamashiro G, et al. Changing ultrasonographic appearance of a fetal ovarian cyst twisted in utero. J Ultrasound Med. 1993 Jul;12(7):415-7. PMID: 8355336.
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