Parasitic or Heteropagus Twins are asymmetric conjoined identical or monozygotic twins in which the tissues of a severely defective twin (parasite) are dependent on the cardiovascular system of the other, largely intact twin (autosite) for survival.
The estimated incidence of heteropagus parasitic twins is approximately 1 per 1 million live births. Isolated case reports comprise most of published work on this rare congenital anomaly.
The condition only occurs when an undeveloped or underdeveloped twin is attached to parts of the body of the twin that develops and is birthed.
The parasitic twin syndrome has been noted in dozens of official cases, but is thought to occur more often due to lack of official documentation of cases in third world countries. Many of these survived, often after surgery to remove the parasitiv twin.
One case was reported in 1783. The boy born with the parasitic twin did not die from the cranio-defect but from a snake bite at the age of 4. In 2003, a girl was born with a parasitic twin and lived a little more than a year, dying after an 11 hour procedure to remove the parasitic twin. Another living case occurred in 2005 in Egypt. That baby died after surgery to remove the parasitic twin as well.
A-ke was born in the district of Yun-Ping Heen in China, with another male child of nearly the same size united to the pit of the stomach by the neck, as if his brother had plunged its head into his breast. The skin of the principal here joins that of the upper part of the neck of the parasite, quite
It seems, from the noted cases. that the only case of parasitic twin to survive was associated with leaving the extra twin head in place instead of removing the head. Generally, the outcome in all cases of parasitic twins is death.
Some cases of parasitic twin syndrome are associated with a living parasitic twin. In the case of the Egyptian baby, the external head was able to blink, smile and cry. There was no body associated with the parasitic twin. The developed baby did not die from the removal of the parasitic twin, but rather from an infection associated with the surgery.
The parasitic twin syndrome is also referred to as the "two-headed" syndrome and "conjoined twin" syndrome. The conjoined twin syndrome is not the same, but many people interchange the terms conjoined and parasitic. The parasitic twin syndrome is more associated with a twin that is unable to live without being attached to the head of the living twin.
The vanishing twin syndrome and parasitic twin syndrome are closely related. When vanishing twin syndrome occurs, the twin that dies in utero is absorbed by the living fetus or the mother. In most cases, the twin that is absorbed disappears completely with no adverse effects on the living baby. While there is no known cause for the vanishing twin syndrome or the subsequent parasitic twin syndrome, it is thought that the vanishing twin was developing with a placenta that did not provide enough support for the fetus.
IVF, or in vitro fertilization, is associated with higher than normal numbers of vanishing twins. This is due to the number of eggs implanted and the higher than normal rates of multiples. Parasitic twins and vanishing twins do not just occur with twin pregnancies. They have also been associated with triplet and quadruplet pregnancies.
Cases of parasitic twins are so very rare, that investigation into the condition is rare. Many medical procedures associated with parasitic twins are experimental due to the paucity of cases.