# Vestigial twin

A vestigial twin is a form of parasitic twinning, where the parasitic "twin" is so malformed and incomplete that it typically consists entirely of extra limbs or organs. It also can be a complete living being trapped inside the host person, however the parasitic twin is anencephalic and lacks consciousness.
Most vestigial limbs are non-functional, and although they may have bones, muscles, and nerve endings, they are not under the control of the host. The limbs can pose problems in redirecting necessary resources such as blood to the nonfunctional limbs while the body lacks these resources. Additionally, the presence of six or more digits on the hands and feet (polydactyly) is typically caused by a genetic or chromosomal anomaly and is not considered a case of vestigial twinning.

## Signs and Symptoms

Woman who are pregnant 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 twins 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 a attached limb as a "tail." This tail is the extension of the coccygeal vertebra. The parasitic twins limbs can be attached to the dominant twin at more than one location, for example the head, torso, pelvis, buttocks, back, etc.

## Cause

This phenomenon 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 this 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, as this is a rare occurrence.

## Mechanism

This disease impacts pregnant women expecting twins. The reason why these twins do not fully separate is still unknown. Possible theories include abnormalities in SHH (sonic hedgehog) proteins. SHH proteins are vital for cell development, researchers believe this can cause the twins to be conjoined and then one stops developing. This then leads for the fetus to be born with a parasitic twin attached. Fission theory is another theory researchers believe causes vestigial. Fission theory would indicate 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 researcher 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 twins limbs attached. The dominant twin is impacted because the parasitic is somehow attached to their body. The parasitic twin can be attached at the head, torso, buttocks, back, etc. Vestigial twins occurs very rarely so it is unknown why the dominant twin is born with the parasitic twins limbs attached.

## Diagnosis

Early diagnosis can be found during a prenatal ultrasound. A prenatal ultrasound uses sound waves to show the fetus in the womb. However, there have been cases that it was not possible to diagnose the vestigial twin before birth using an ultrasound. There is no way to prevent vestigial twins or diagnose vestigial twins through pregnancy symptoms.Vestigial twins can be diagnosed before birth through an ultrasound or other imaging such as MRI or Ct scan. An MRI or CT scan can show how the twins are connected, After birth the diagnoses can be made by looking at the parasitic twin attached to the vestigial twin. An obstetric and gynecology physician can be treat and diagnose this patient, however a pregnant women 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 diagnoses until birth. If the expecting mother does not receive prenatal visits a delay in diagnosis would occur. However, once the vestigial twins are born it 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 to developing infection, hernia, and typical surgical complications. There are no holistic treatment options other then 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 twins body can be put under a lot of stress if still attached to the parasitic twin. Surgery to remove the parasitic twin is the only treatment to remove the parasitic twin from the dominant twin.

## Prognosis

The typical prognosis for the case studies that have been found is overall good once the parasitic twin is separated. If the host twin has some complications such as a hernia and 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 vestigial twins.

## Epidemiology

Vestigial twins is a rare occurrence, it has been reported that it occurs less than 1 in 1 million births. Woman of child bearing ages that 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. Since this occurrence is so sparse it is unclear whether it impacts different ethnicities or races. However, rachipagus parasiticus, an extremely rare type of parasitic twins, is most commonly reported in Ethiopia. Rachipagus parasiticus is where 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 is not enough patients or known 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 been proven to show positive outcomes. Researchers have two theories that they believe causes vestigial parasitic twins: fission theory and fusion theory.
* Fission theory is when two fetuses are in the womb and one of the conjoined twins stops developing.
* The fusion theory proposes that there are two separate fertilized eggs in the womb, however they fertilized eggs end up growing together.
Another potential cause researchers are investigating an abnormalities in SHH proteins. SHH,sonic hedgehog protein, plays an important role in development of cells. During embryonic development the SHH protein is responsible for the shaping of the body, cell specialization, cell development. Researchers believe that potential abnormalities in this protein can put woman at higher risk for potentially having vestigial parasitic twin pregnancies.