Case of the Week # 500

Case of the Week # 500

Javier Cortejoso
 on 02/02/2023
 on Thursday, February 2nd, 2023
The images show the following:
• Images 1 and 2: cross section of the fetal abdomen shows an intraabdominal retroperitoneal cyst located in a hemiabdomen, shifting liver, stomach and intestinal loops to the contralateral side. The cyst contains a heterogeneous complex mass, with solid and cystic parts. It is possible to identify a long bone, likely a femur.
• Images 3 and 4: Tomographic Ultrasound Imaging of a volume obtained with 3D ultrasound allows us to accurately study the findings described.
• Video 1: The fetus is positioned with the spine posteriorly in the video. Anterior to the spine there is an anechoic fluid collection containing a heterogeneous mass. This mass has both a large cystic area, solid components with calcifications, and at least one limb.
• Images 5 and 6: color-Doppler ultrasound of the heterogeneous fetiform complex mass, that appears connected to the host by two blood vessels, an artery and a vein, resembling an umbilical cord. Power Doppler investigation reveals umbilical artery blood flow spectrum.

Fetus in fetu (or “inclusion twin”) involves a malformed parasitic monozygotic twin found inside the body of its co-twin, usually in the abdominal cavity. Johann Friedrich Meckel, a German anatomist, described fetus in fetu for the first time in 1871. Throughout the 19th century, several other authors including Young, Highmore and Taylor described this pathology. It is a very rare condition, with an incidence of 1 in 500 000 live births. Less than 200 cases have been published in the literature to date, which were mostly diagnosed after birth. Advances in sonography have made the diagnosis of this condition possible in the prenatal period. Prenatal diagnosis was first reported in 1983 by Nicolini et al.

Several developmental hypotheses have been proposed for fetus in fetu including (1) absorption of a diamniotic, monochorionic monozygotic twin, which is the most widely accepted and (2) a well-differentiated variant of a mature teratoma. Persistent anastomoses of the vitelline circulation lead to the absorption of one twin inside the other during the ventral folding of the trilaminar embryonic disc around the third week. In intracranial fetus in fetu, this location results from a different embryological mechanism, and occurs at a much later date than those that are imbedded in the retroperitoneum. A second small differentiation focus with slower growth than the primordial one appears in the bilaminar embryo, which will be engulfed in the invagination of cells at the Hensen node.

The coexistence of a fetus in fetu and a teratoma, as well as the occurrence of a teratoma years after removal of a twin fetus in fetu have been reported, supporting the hypothesis of a continuum including conjoined twins, acardiac twins, fetus in fetu and teratoma. Denney et al. described a sacrococcygeal fetus in fetu with vertebral bodies within the mass and formed limbs emanating from its surface.

Although the most frequent site is the abdominal cavity, fetus in fetu has also been found in the posterior mediastinum, scrotum, mouth, and skull. In 80% of cases, it develops in the retroperitoneum. Usually there is one fetal mass, but up to 11 fetuses in fetu have been reported. Most cases manifest in infancy, but it can be detected at any age, the oldest reported case being in a 47-year-old man. Characteristically the fetus in fetu complex will be composed of a fibrous membrane equivalent to the chorioamnionic complex, containing some fluid equivalent to the amniotic fluid, and a fetus suspended by a cord or pedicle. As expected from the etiology, most fetus in fetu are connected to the host by vessels originating from or around the superior mesenteric artery, a derivative of the right vitelline artery in mammals. Fetuses in fetu resemble poorly formed acardiac twins. Most every organ has been recognized in various stages of development. Different organs can be seen, including the vertebral column (91%), limbs (82.5%), central nervous system (55.8%), gastrointestinal tract (45%), vessels (40%), and genitourinary tract (26.5%).

Prenatally, fetus in fetu presents as a complex cystic mass with ill-defined solid internal components. The general appearance is a well-delineated capsule, with an echogenic mass suspended in fluid or partially surrounded by fluid. Occasionally, the diagnosis can be suggested by the recognition of a rudimentary spine. Since the fetus in fetu does not have a cardiac system, severe hypoxia is responsible for the lack of evolution. Most cases are anencephalic and ventral wall defects such as omphalocele are common. The treatment of fetus in feto remains exclusively surgical, usually in the early neonatal period.

Spencer defined a fetus in fetu as a fetiform mass located within a normal fetus, either immediately adjacent to one of the sites of attachment of conjoined twins or associated with the neural tube or the gastrointestinal system. The mass has one or more of the following characteristics: (a) enclosed within a distinct sac; (b) partially or completely covered by normal skin; (c) grossly recognizable anatomic parts; and (d) attached to the host by a pedicle containing a few relatively large blood vessels. Its growth rate is similar to that of the host in which it is found. Despite Spencer's proposal, there is no firm and absolute criteria for the definitive diagnosis of fetus in fetu. Blood group typing, karyotyping, and DNA analysis show identical findings between the fetuses and their hosts, a finding that is compatible with both monozygotic multiple pregnancy theory and teratoma theory. The monozygotic twin theory might not adequately explain the occurrence of multiple fetus in fetu and its association with teratomas. For these cases, very early splitting of the zygote could allow for dichorionic, diamniotic twining with the parasitic twin implanting on or being encompassed by the other twin, rather than implanting in the uterine wall.

Prenatally, the primary differential diagnosis is teratoma. A teratoma consists of a chaotic mixture of microscopically identifiable tissues. While these tissues can be well-differentiated, there are rarely well-defined structures beyond a tooth or a portion of intestine. Other characteristics include (a) location within the body of the host often immediately adjacent to the original position of the neural tube, alimentary tract or a derivative thereof, (b) enclosed in connective tissue, and (c) involving a broad attachment to the surrounding tissue through which it is nourished by many small blood vessels. It is capable of independent growth and may become malignant. While Willis proposed in 1935 that the diagnosis of fetus in fetu requires the presence of an axial skeleton, Gonzalez-Crussi proposed a more liberal definition of fetus in fetu that requires highly developed organogenesis organized around a vertical axis. Teratomas are disorganized congregations of pluripotential cells from all three primitive tissue layers in which there is neither organogenesis nor vertebral segmentation. They occur predominantly in the lower abdomen, not the upper retroperitoneum. Other possible differential diagnoses for abdominal calcifications include meconium peritonitis (often with ascites), neuroblastoma, viral infections, and adrenal hemorrhage.

Suggested readings:

[1] Denney JM, Stanley C, Armstrong LA, et al. Fetus in fetu in lieu of a sacrococcygeal teratoma: A case illuminating the utility of serial prenatal sonographic examinations in diagnosis. J Ultrasound Med 2017; 36: 453-455
[2] Gerber RE, Kamaya A, MD, Miller SS, et al. Fetus in fetu: 11 fetoid forms in a single fetus. Review of the literature and imaging. J Ultrasound Med 2008; 27: 1381-1387
[3] Gonzalez-Crussi F. Extragonadal teratomas. Atlas of Tumor Pathology. Washington DC: Armed Forces Institute of Pathology, 1982
[4] Hui PW, Lam TP, Chan KL, Lee CP. Fetus in fetu - from prenatal ultrasound and MRI diagnosis to postnatal confirmation. Prenat Diagn 2007; 27: 657-661
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