A vestigial twin is a form of parasitic twin. They are malformed and incomplete, typically consisting entirely of extra limbs or organs. Vestigial twins are always anencephalic and lack consciousness. Most vestigial limbs are non-functional. Although they may have bones, muscles, and nerve endings, they are not under the control of the host. These limbs can also cause problems for the dominant twin's body. For example, blood may be directed to the parasitic limbs instead of the primary ones, causing the dominant body to suffer from resource shortages. The presence of six or more digits on the hands and/or feet (polydactyly) is typically caused by a genetic or chromosomal anomaly and is not considered a case of vestigial twinning.
Signs and symptoms
Pregnant women do not experience any different symptoms when carrying vestigial twins compared to a normal pregnancy. Once the fetus is born, the presentation will typically consist of an extra extremity attached to the dominant twin's body. Only the dominant fetus will be viable. The limbs attached will contain nerve endings, bones, and tissue. In some cases, the fetus can be born with an attached limb as a "tail". This tail is the extension of the coccygeal vertebra. The parasitic twin's limbs can be attached to the dominant twin at more than one location, for example, the head, torso, pelvis, buttocks, back, etc.
Cause Vestigial twinning occurs when a fertilized ovum or partially formed embryo splits incompletely. The result is one dominant twin, with extra body parts belonging to the vestigial twin. It is also possible for tail-like tissue to form around the sacrum. One theory proposes that the usage of teratogenic drugs by the mother may be a factor in the development of vestigial parasitic twins. The cause for vestigial twins is still unclear. There is no way to prevent vestigial twinning from occurring. This disease is not contagious and occurs rarely in pregnancies. There are currently no Centers for Disease Control and Prevention recommendations on this phenomenon, as it is a rare occurrence.
Mechanism This disease affects pregnant women expecting twins. The reason why these twins do not fully separate is still unknown, though there are multiple theories. Possible theories include abnormalities in SHH (sonic hedgehog) proteins. SHH proteins are vital for cell development; researchers believe SHH anomalies can cause the twins to be conjoined, after which one stops developing. This leads to the fetus being born with a parasitic twin attached. Fission theory is another theory researchers believe causes vestigial twinning. Fission theory indicates that the two fetuses in the womb began as conjoined twins and the parasitic twin stopped developing, leading to vestigial twins being born. The last theory researchers believe can potentially cause vestigial twins is fusion theory. According to the fusion theory, there are two fertilized eggs in the womb that end up fusing together; this would create a fetus with the parasitic twin's limbs attached. The dominant twin is impacted because the parasitic twin is somehow attached to their body. Vestigial twinning occurs very rarely, so it is unknown why the dominant twin is born with the parasitic twin's limbs attached.
Diagnosis
Early diagnosis can be found during a prenatal ultrasound, which uses sound waves to show the fetus in the womb. However, there have been cases where it was not possible to diagnose the vestigial twin before birth using an ultrasound. There is no way to prevent vestigial twins or diagnose such twins through pregnancy symptoms. Vestigial twins can be diagnosed before birth through an ultrasound, or other imaging such as an MRI or CT scan, which can show how the twins are connected. After birth, the diagnoses can be made by looking at the parasitic vestigial twin. An obstetric and gynecology physician can treat and diagnose this patient; however, a pregnant woman would be referred to a maternal and fetal medicine specialist. It is possible to have different surgeons assigned to the case to assist with removing the parasitic twin. There have been cases where it is difficult to visualize the parasitic twins on an ultrasound, which would cause a delay in diagnosis until birth. If the expecting mother does not receive prenatal visits a delay in diagnosis would occur. However, once the vestigial twins are born they would be easily diagnosed during a physical exam.
Treatment Treatment for vestigial twins typically includes having surgery to remove the parasitic twin. If the parasitic twin is not removed, the risk for further complications increases significantly. The twin supporting the parasitic twin can have complications. Therefore, the best course of treatment would be to remove the attached parasitic twin. A healthcare provider would schedule additional tests to check to see if the fetus has any complications and treat them. Treatment for other complications would be different based on each case. After the fetus undergoes the procedure to remove the parasitic twin, there is a risk of developing infection, hernia, and typical surgical complications. There are no holistic treatment options other than leaving the parasitic twin attached; however, leaving the parasitic twin attached to the dominant twin can lead to further complications for the dominant twin. The dominant twin's body can be put under a lot of stress if still attached to the parasitic twin. Surgery is the only treatment to remove the parasitic twin from the dominant twin.
Prognosis The overall prognosis for the case studies that have been found is typically good once the parasitic twin is separated. If the host twin has some complications, such as a hernia or infection, more treatment may be needed. The occurrence does not repeat after the parasitic twin is removed. It is unclear if there is an environmental or hereditary risk factor associated with vestigial twins. However, researchers believe that abnormalities in the SHH protein can create a risk or a possible explanation for having such twins.
Epidemiology Vestigial twinning is a rare occurrence; it has been reported that it occurs in fewer than one in one million births. Women of child bearing ages who are pregnant with twins can be impacted. Twin fetuses that are still developing in the womb can become vestigial twins, although this is a rare occurrence; so rare that it is unclear whether it impacts different ethnicities or races differently. However, rachipagus parasiticus, an extremely rare type of parasitic twins, is most commonly reported in Ethiopia. In rachipagus parasiticus, the parasitic twin is attached to the spine.
Research Directions There are currently no clinical trials available for vestigial parasitic twins. Since this is such a rare occurrence, there are not enough known patients to develop any drug treatments. It is unclear if any medications can be used to prevent or treat vestigial parasitic twins; however, surgery to separate the twins has shown positive outcomes. Researchers have two theories that they believe cause vestigial parasitic twins: fission theory and fusion theory.
Fission theory proposes that a single egg began splitting but stopped prematurely. Fusion theory proposes that two initially separate fertilized eggs in the womb ended up fusing together. Another potential cause researchers are investigating are abnormalities in SHH proteins. SHH, sonic hedgehog protein, plays an important role in the development of cells. During embryonic development, the SHH protein is responsible for the shaping of the body, cell specialization, and cell development. Researchers believe that potential abnormalities in this protein can put women at higher risk for potentially having vestigial parasitic twin pregnancies.
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