Fetus in fetu: use of intraoperative ultrasound for safe excision of a rare entity

Background

Fetus in fetu (FIF) is a rare developmental abnormality present in only 1 in 500 000 births with around 200 cases reported worldwide to date.1–3 Initially termed by Meckel (of diverticular fame) in the 1800s, it was subsequently defined by Willis in 1953 as a pedunculated mass containing a vertebral axis, surrounded by other organs or limbs of equal maturity.2 Unique imaging findings in the prenatal and postnatal period strongly suggest a diagnosis of FIF, although pathological evaluation is necessary to confirm the diagnosis. Surgical excision is recommended to establish the diagnosis, rule out malignant pathologies, and also frequently due to mass effect. While atypical locations—intracranial,4 sacral,5 scrotal,6 mediastinal7 and oropharyngeal8—exist, the majority of masses (70%–80%) are located in the retroperitoneum.1 2 Location and size of the mass can lead to intraoperative challenges, which must be met with innovative techniques.

In this case report, we describe the management of a rare entity that required characterisation on numerous imaging modalities (radiograph, ultrasound, CT, MRI and intraoperative ultrasound) to guide management and surgical planning. Additionally, intraoperative ultrasound was used to facilitate safe surgical excision, which has not been previously described in the literature.

Case presentation

A G2P0 mother presented with cholestasis of pregnancy at 32 weeks and 0 days gestation following a previously uncomplicated pregnancy. She underwent repeat prenatal ultrasound imaging which demonstrated a previously unidentified complex, heterogeneous, intra-abdominal mass with calcifications measuring 54×65 × 59 mm as well as dilated loops of bowel. After uncomplicated induction of labour and vaginal birth at 36 weeks and 4 days with Apgar scores of 7 and 8, the baby boy was admitted to the neonatal intensive care unit with mild supplemental oxygen needs as a result of transient tachypnoea of the newborn. On physical examination, his abdomen was soft, non-tender and non-distended with a firm, approximately 5 cm mass in the left upper quadrant; bilateral hydroceles were also present.

Investigations

During the hospitalisation, the patient had a neonatal chest and abdomen radiograph to further assess bowel gas pattern which demonstrated a left upper quadrant soft-tissue mass containing an internal calcified structure suggestive of a vertebral column (figure 1A). The bowel gas pattern was otherwise normal and feeds were initiated and tolerated. Subsequently, an abdominal ultrasound was obtained to further characterise the mass which showed a complex, multiseptated mass with both soft-tissue and cystic components (figure 1B,C). Within the septations, there were large echogenic structures with posterior acoustic shadowing consistent with osseous structures. These structures resembled long bones and a vertebral column correlating with the structure seen on abdominal radiograph. MRI abdomen and pelvis allowed for evaluation of proximity to surrounding abdominal structures and vasculature, and demonstrated a cystic mass with multiple septations and soft-tissue components (figure 2). Osseous structures were further detailed again resembling a vertebral column (figure 2A), long bones (figure 2B), and even a partial skull/orbit (not shown). Small areas of enhancement on MRI were attributed to soft-tissue components. Given that the infant was clinically doing well, had been weaned off supplemental oxygen and was tolerating feeds, the decision was made to delay surgical excision for several months to allow for growth given proximity to the visceral vasculature (figure 2C,D). Preoperative alpha-feto protein (AFP) was within normal limits.

Figure 1

Radiograph and ultrasound evaluation of the left upper quadrant mass. (A) A soft-tissue mass in the left upper quadrant contains an internal calcific structure resembling a vertebral column (solid white arrow). (B) Complex, multiseptated mass with both soft-tissue, cystic and osseus components. (C) Osseous structures within the mass resemble a vertebral column (solid white arrow) and a long bone (dashed white arrow).

Figure 2

MRI of the abdomen and pelvis with gadolinium. (A, B) Coronal and axial T2 haste images demonstrate a multiseptated cystic and solid mass in the retroperitoneum of the left upper quadrant measuring 5.1×5.4× 7.5 cm. This mass contains osseous structures resembling a vertebral column (solid white arrow) and long bones (dashed white arrow). (C, D) Axial T2 haste images demonstrate a flow void arising from the aorta (red arrows) corresponding to the superior mesenteric artery (yellow arrows) which courses through a septum of the mass.

About 2 months after birth, a CT abdomen and pelvis with intravenous contrast was completed to better evaluate the surrounding vasculature for surgical planning. The abdominal mass was similarly characterised with soft-tissue, cystic and osseous components. The superior mesenteric artery (SMA) was found to be coursing within the septum of the mass while the inferior mesenteric artery was coursing just inferior to the mass (figure 3A,B). An enhancing structure in the posterior aspect of the mass adjacent to the aorta was suggestive of a vascular pedicle with splaying of the coeliac artery and SMA (figure 3B). A vessel suggestive of portal vein was coursing anterior to the mass and anastomosing with additional vessels anterior to the mass (figure 3C). There was evidence of mass effect on surrounding structures (figure 3D–F).

Figure 3

Presurgical planning CT abdomen and pelvis with intravenous contrast. (A, B) Superior mesenteric artery courses through the central portion of the mass within a septum (yellow arrows). The coeliac axis runs along the superior aspect of the mass (dashed black arrow) while the inferior mesenteric artery courses along the inferior aspect of the mass (solid black arrow). An enhancing structure in the posterior aspect of the mass and adjacent to the aorta splays the coeliac and superior mesenteric arteries and is suspicious for a vascular pedicle (black arrowhead). (C) The main portal vein is significantly displaced and drapes over the mass anteriorly (blue arrow). (D) There is significant mass effect on the surrounding intraabdominal structures causing intrahepatic biliary ductal dilatation (green arrows), (E) umbilical hernia containing bowel (orange arrow), and (F) right inguinal hernia containing bowel (orange arrow).

Differential diagnosis

An abdominal mass in an infant may be identified prenatally on ultrasound or after birth with symptoms of mass effect: jaundice, hydronephrosis, intestinal obstruction, meconium peritonitis, respiratory distress or vomiting.9 The differential diagnosis of an abdominal mass in an infant is broad, frequently guided by age and location, and includes benign and malignant neoplasms and gastrointestinal or hepatobiliary cysts. In a neonate, an abdominal mass is most frequently due to organomegaly, most commonly renal in origin (55%)—hydronephrosis or polycystic kidney disease—followed by pelvic (15%) or gastrointestinal origins (15%). Once identified, the differential can be narrowed with subsequent imaging; ultrasound is the most common initial modality. Visualisation of an axial skeleton or vertebral segments strongly supports a diagnosis of FIF and differentiates from a diagnosis of teratoma on imaging alone.9 However, diagnosis of FIF rarely occurs prenatally given challenges in visualising discrete structures on prenatal ultrasound and its close relationship to teratomas.10

Although many researchers suggest that FIF may be a distinct pathology from teratoma, multiple demonstrations of FIF without vertebral columns and varying levels of differentiation have called into question whether FIF lies instead on the spectrum of highly differentiated, mature teratomas.5 10 In order to diagnose FIF, pathological evaluation must identify a mass containing vestigial remnants enclosed by a distinct sac partly composed of skin, grossly recognisable body parts such as osseous structures, and a few blood vessels. FIF can be differentiated pathologically from teratoma by identifying body parts such as vertebral segments and soft tissue and limbs of equal maturity while teratoma is composed of disorganised pluripotent cells without identifiable body parts.11 A vertebral column is identified in 76% of cases and limbs are seen in 83% of cases.2 9 The mass rarely contains a structure suggestive of a brain.2 12 It is imperative to differentiate between these two entities because teratoma is associated with a 17% chance of malignancy. While all FIF cases reported thus far have been benign at resection, a rare, malignant transformation has been reported following resection of a FIF.13

Treatment

The baby was taken to the operating room at 2 months of age. Intraoperatively, the mass was adherent to the mesentery which was carefully peeled off the mass preserving blood flow to the bowel. Nearby structures, including the common bile duct (figure 4A) and duodenum, were being displaced by the mass and were preserved. After the mass was isolated, several arterial structures were identified adjacent to and presumably entering the mass, but it was difficult to determine whether these were the coeliac axis, SMA, or a vascular pedicle (figure 4B,C). Thus, intraoperative ultrasound was used by paediatric radiology to identify the coeliac axis, as well as SMA take off traversing posteriorly through the mass and then exiting inferiorly (figure 4D). Contrary to the preoperative imaging, the portal vein was not found to be displaced by the mass and was posterior to it. With the vascular anatomy clearly defined, the capsule of the mass was then opened and the two hemispheres removed with successful preservation of the coeliac and SMA that were now apparent and identifiable. The vascular pedicle was identified as a branch off of the SMA within the mass. Some capsule was left behind given its adherence to the vessels. Blood flow to the bowel was excellent following excision. Right inguinal and umbilical hernias were repaired intraperitoneally (no left inguinal defect was present intraoperatively). Pathology was consistent with FIF. Both hemispheres were covered with white-tan skin and contained clearly identified, well-formed, mature vertebral column, jointed long-bones containing marrow, small bowel and umbilical cord (figure 5).

Figure 4

Intraoperative findings and ultrasound. (A) Mass effect was causing common bile duct dilatation (green arrow). (B, C) The superior mesenteric artery (yellow arrow) coursing through the posterior portion of the mass was confirmed by intraoperative ultrasound and preserved. (D) The vascular pedicle of the fetus in fetu mass (dashed red arrow) is inseparable from and splays the coeliac axis (dashed black arrow) and superior mesenteric artery (yellow arrow). The coeliac axis and superior mesenteric artery are confirmed by originating from the aorta (solid red arrow).

Figure 5

Pathology findings. (A) The two hemispheres of the mass were encased in mature skin and contained (B, C) mature vertebral column, jointed long-bones containing marrow (white arrow), small bowel (orange arrows), and umbilical cord/vascular pedicle (dashed red arrows).

Outcome and follow-up

The baby was initially monitored postoperatively in the paediatric intensive care unit. He was extubated on postoperative day (POD) 1. After resolution of a mild postoperative ileus, the nasogastric tube was removed and oral feeds were started on POD4. He was discharged home on POD8. He was subsequently seen postoperatively in the paediatric surgery clinic and was doing well. Plans were made for interval follow-up with MRI at 6 months.