Parasitic twin

A parasitic twin, also known as an asymmetrical or unequal conjoined twin, is the result of the processes that also produce vanishing twins and conjoined twins, and may represent a continuum between the two. Parasitic twins occur when a twin embryo begins developing in utero, but the pair does not fully separate, and one embryo maintains dominant development at the expense of its twin. Unlike conjoined twins, one ceases development during gestation and is vestigial to a mostly fully formed, otherwise healthy individual twin. The undeveloped twin is defined as parasitic, rather than conjoined, because it is incompletely formed or wholly dependent on the body functions of the complete fetus. The independent twin is called the autosite.

Variants

* Conjoined parasitic twins joined at the head are described as craniopagus or cephalopagus, and occipitalis if joined in the occipital region or parietalis if joined in the parietal region.
* ''Craniopagus parasiticus'' is a general term for a parasitic head attached to the head of a more fully developed fetus or infant.
* ''Fetus in fetu'' sometimes is interpreted as a special case of parasitic twin, but may be a distinct entity.
* The twin reversed arterial perfusion, or T.R.A.P. sequence, results in an 'acardiac twin', a parasitic twin that fails to develop a head, arms and a heart. The parasitic twin, little more than a torso with or without legs, receives its blood supply from the host twin by means of an umbilical cord-like structure, much like a ''fetus in fetu'', except the acardiac twin is outside the autosite's body. The blood received by the parasitic twin has already been used by the normal fetus, and as such is already de-oxygenated, leaving little developmental nutrients for the acardiac twin. Because it is pumping blood for both itself and its acardiac twin, this causes extreme stress on the normal fetus' heart. Many T.R.A.P. pregnancies result in heart failure for the healthy twin. This twinning condition usually occurs very early in pregnancy. A rare variant of the acardiac fetus is the ''acardius acormus'' where the head is well developed but the heart and the rest of the body are rudimentary. While it is thought that the classical T.R.A.P./Acardius sequence is due to a retrograde flow from the umbilical arteries of the pump twin to the iliac arteries of the acardiac twin resulting in preferential caudal perfusion, ''acardius acormus'' is thought to be a result of an early embryopathy.Abi Nader Khalil, Whitten Sara Melissa, Filippi Elisa, Scott Rose-Mary, Jauniaux Eric. Dichorionic triamniotic triplet pregnancy complicated by acardius acormus. Fetal Diagnosis and Therapy 2009;26(1):45-9.

Images of parasitic twins

Human

File:Histoires prodigieuses; Homme monstueux... WMS 136 Wellcome L0025563.jpg, 40-year-old man seen in Paris, 1530
File:Les écarts de la nature page28.jpg, 30-year-old Neapolitan man, seen in 1742
File:An Indian man with a headless body attached to his trunk. En Wellcome V0007399.jpg, Indian man, 1787
File:Agan (BM 1982,U.2878).jpg, Agan, a Chinese man, 1833
File:Gustav Evrard.jpg, 9-year-old Gustav Evrard of Paris, 1839
File:Dumas.3.jpg, Blanche Dumas of France, born 1860
File:B. C. Hirst & G. A. Piersol, Human monstrosities Wellcome L0027956.jpg, Joao Baptista dos Santos and Louise L. ("la femme de quatre jambes"), 1893

Animals

File:Les écarts de la nature page12.jpg, Chicken with 2 extra lower limbs
File:Les écarts de la nature page30.jpg, Pigeon with 2 extra lower limbs
File:Les écarts de la nature page44.jpg, Cat with lower body duplication
File:Les écarts de la nature page45.jpg, Sheep with 2 extra front limbs

See also

* Dipygus
* Frank Lentini
* Lakshmi Tatma
* Lazarus and Joannes Baptista Colloredo
* Rudy Santos
* Vestigial twin

References

Further reading

*

{{Situs inversus and conjoined twins

Congenital disorders
Rare diseases