Introduction
Fetus in fetu is a developmental anomaly in which tissue growth resembling a fetus develops inside the twin's body. It is a rare condition in which the malformed fetus is enclosed within the twin's body. This phenomenon was first described in 1808 by George William Young. Studies have shown that fetus in fetu is a rare anomaly in one in 500,000 live births.
What Does Fetus in Fetu Mean?
Fetus in fetu is a rare abnormality in which a tissue growth that resembles a fetus (a developing baby) is enclosed inside the body of its twin. The malformed fetus incorporates the body of its twin and develops. It usually occurs secondary to an abnormality in the embryogenesis of a monochorionic diamniotic pregnancy, in which the malformed fetus becomes enclosed within the body of the fetus that is developing typically. Two theories describe the origin of a fetus in fetu. One theory is that the mass grows as a normal fetus but becomes enclosed inside its twin. The other theory is that the tissue mass is an overdeveloped teratoma (a tumor composed of several types of tissues such as teeth, muscle, hair, or bone).
How Does a Fetus in Fetu Develop?
Research has shown that a fetus in fetu can be due to an abnormality in the embryogenesis of the diamniotic monochorionic twin pregnancy. However, two theories have been formulated to describe the development of the fetus in fetu. It includes the following:
-
Teratoma Hypothesis:
Fetus in fetu can be a differentiated form of a dermoid cyst (a cystic teratoma consisting of skin, hair follicles, and sweat glands) or a mature teratoma.
-
Parasitic Twin Hypothesis:
A fetus in fetuses can be a parasitic twin (an undeveloped twin that depends on the healthy twin for development) that develops within its host twin. Fetus in fetu is common in monozygotic twin pregnancies. In this, both fetuses share a common placenta, and one fetus envelops the other. The enveloped fetus is the parasitic twin, which depends on the host twin for survival. The parasitic twin draws blood supply from the host twin for survival. The parasitic twin lacks a brain and some internal organs and cannot survive independently. As the parasitic twin feeds on the host twin, which receives nutrients from a single umbilical cord, the parasitic twin does not survive birth.
How Does a Fetus in Fetu Present?
A Fetus in fetu may be considered alive if its tissue components have not been eliminated. Therefore, a fetus in fetu is considered similar to a tumor in that the cells remain viable through metabolic activity. However, without gestational requirements such as the amnion (a membrane that covers the fetus) and placenta (a bag-like organ that helps in the exchange of nutrients and oxygen between the mother and the fetus in the uterus), a fetus in fetu develops into a well-differentiated teratoma or metastatic teratocarcinoma in some cases. The organs of a fetus in fetu may have a blood supply from the host, but it presents with defects such as a nonfunctional heart, brain, lungs, gastrointestinal tract, or urinary tract. Though a fetus in fetu has developed a few morphological features similar to that of a normal fetus, it cannot survive outside the host twin. In addition, a fetus in fetu can pose life-threatening complications to the host twin on whom its life depends.
A fetus in fetu usually develops in the abdomen of the host twin. In 80 percent of cases, the development is usually seen in the retroperitoneal cavity of the host twin. The presence of a fetus in fetu in other locations, such as the mediastinum, cranial cavity, scrotum, and pelvis of the host twin, has been reported.
What Is the Clinical Significance of Fetus in Fetu?
The malformed fetus is due to the aberration of the diamniotic, monochorionic, monozygotic twin in which an unequal division of the developing blastocyst (an early stage of embryo in which a cluster of cells in the fertilized egg divides) results in the inclusion of a small tissue mass within the maturing embryo. Some researchers believe that a fetus in fetu can be a representation of malignant teratoma. However, the presence of an axial skeleton helps distinguish a teratoma from a fetus in fetu. The presence of a spinal cord in the fetus in fetu suggests that the fetus has passed through the primary stage of development, which involves the formation of a neural tube and a symmetrical development around the axis. The presence of a fetus in fetu in the host twin can cause symptoms such as jaundice, emesis (vomiting), abdominal distension, pressure on the renal system, and dyspnea. In some cases, this anomaly does not cause any symptoms.
How Is a Fetus in Fetu Diagnosed?
The diagnosis a depends on the radiological examination. Plain abdominal X-rays help in detecting the vertebral column and axial skeleton. A 3D (three-dimensional) ultrasonography and CT (computed tomography) scan help determine a preoperative diagnosis. A magnetic resonance imaging (MRI) scan can also diagnose a fetus in fetu. The size and weight of the fetus in fetu depend on the blood supply received. A fetus in fetu with a distinct blood supply from the host may have developed features such as a spinal cord, joints, long bones, rib cage, clavicle, scapula, and external genitalia with a well-developed pelvis and a head that contains neural tissue.
How Is Fetus in Fetu Treated?
The treatment of choice for a fetus in fetu is complete excision. Surgical excision (removal) of the fetus in fetu relieves the obstruction and prevents further compression and potential complications. Complete removal of the fetus in fetu helps confirm the diagnosis and reduces the risk of recurrence.
Conclusion
Fetus in fetu is a rare condition that causes the development of an abnormal abdominal mass in children. Though retroperitoneum is the most common development site, it can also occur in any region from the cranial cavity to the scrotal sac. It can be differentiated from teratoma by the presence of a vertebral column. A fetus in fetu is malformed and does not survive. Therefore, a complete removal is suggested to prevent the risk of complications.
