Craniopagus parasiticus
Craniopagus parasiticus is an extremely rare type of parasitic twinning occurring in about 2 to 3 of 5,000,000 births. In craniopagus parasiticus, a parasitic twin head with an undeveloped body is attached to the head of a developed twin. Fewer than a dozen cases of this type of conjoined twin have been documented in literature. Development. The exact development of craniopagus parasiticus is not well known. However, it is known that the underdeveloped twin is a parasitic twin. Parasitic twins are known to occur "in utero" when monozygotic twins start to develop as an embryo, but the embryo  fails to completely split. When this happens, one embryo will dominate development, while the other's development is severely altered. The key difference between a parasitic twin and conjoined twins is that in parasitic twins, one twin, the parasite, stops development during gestation, whereas the other twin, the autosite, develops completely. In normal monozygotic twin development, one egg is fertilized by a single sperm. The egg will then completely split into two, normally at the two-cell stage. If the egg splits in the early blastocyst stage, two inner cell masses will be present, eventually leading to the twins sharing  the same chorion and placenta, but with separate amnions. However, the egg can split into two, but still have one blastocyst. This will lead to one inner cell mass and one blastocyst. Then, as the twins develop, they will share the same placenta, chorion, and amnion. This is thought to be the most likely reason why conjoined twins occur, and could possibly play a role in the development of craniopagus parasiticus. One hypothesis is that craniopagus parasiticus starts with the development of two fetuses from a single zygote that fail to separate at the head region around the second week  of gestation. Some believe, however, that it occurs later in development, around the fourth week of gestation, at which time the two embryos fuse together near the anterior open neuropore. Another hypothesis is that there is joining of the somatic and placental vascular system of the twins, as well as a degeneration of the umbilical cord of the parasitic twin. This suggests that craniopagus parasiticus develops due to the lack of blood supply to one of the twins. Diagnosis. Related conditions. In addition to craniopagus parasiticus, a few other conditions involve a child's, or animal's, birth with two  heads or faces. Dicephalic parapagus. Dicephalic parapagus is a condition where two heads are side by side on a single torso, with varying degrees of twinning of organs and structures within the torso. Diprosopus. Diprosopus occurs when a single body and neck are present, but there is a duplication of the structures of the face. This is different from craniopagus parasiticus in that there is only one head, although there is a duplication of the craniofacial features. Diprosopus can range from having two fully formed faces to just a duplication of the nose or eyes. Cats with the condition are  referred to as 'Janus cats'. A cat named Frankenlouie was a famous example noted by the "Guinness Book of World Records" in 2012 for being the longest surviving Janus cat. Treatment. Few individuals survive until birth. For those who do, the only treatment available is to surgically remove the parasitic twin. Of the two documented attempts, however, one child died within hours and neither reached their second birthday. The problem with surgical intervention is that the arterial supplies of the head are so intertwined that it is very hard to control the bleeding, but it has been suggested that cutting  off the parasitic twin's arterial supply might improve the odds of the developed twin's survival. Prevalence. Only ten cases of craniopagus parasiticus have been documented, out of a total of at least eighty known cases of craniopagus in general. Only four cases have been documented by modern medicine to have survived birth: History. Only ten cases of craniopagus parasiticus have been reported in the medical research literature. Of those cases, only four have survived birth. The first case on record is that of Everard Home's Two-Headed Boy of Bengal, whose skull is preserved at the Hunterian  Museum at the Royal Society of Surgeons. Terminology. In the past, the use of terminology when describing parasitic twins has been somewhat inconsistent. By definition, a parasitic twin is joined to another twin in a certain anatomical location or position on the developed twin's body. The underdeveloped twin is termed the parasite, and the developed twin is termed the autosite. The autosite can have some abnormalities, as well. For the most part, however, it has developed enough that it can live on its own.