Parasitic twin

A parasitic twin, also known as an asymmetrical twin or unequal conjoined twin, occurs 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 the other. It results from the same processes that also produces vanishing twins and conjoined twins, and may represent a continuum between the two. In parasitic twins, one ceases development during gestation and is vestigial to a mostly fully formed, otherwise healthy individual twin. The undeveloped twin is termed as parasitic, because it is incompletely formed or wholly dependent on the body functions of the complete fetus. The independent twin is called the autosite. The autosite, together with the parasite, are collectively referred to as heteropagus twins.

Typically, the parasitic twin is unable to speak due to an underdeveloped or absent brain, though they may be able to move their mouths or hands. There have been no known records of a parasitic twin that has been able to speak.

Variants

thumb|[[Twin reversed arterial perfusion|TRAP syndrome. The acardiac twin, left, cannot pump its own blood. The pump twin, right, supplies blood to the parasitic twin. ]]

TRAP sequence

The twin reversed arterial perfusion, or TRAP 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 TRAP 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 TRAP/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.

Others

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.

Gallery

Human

File:Gould Pyle 54.jpg|German man, born 1516

File:Histoires prodigieuses; Homme monstueux... WMS 136 Wellcome L0025563.jpg|40-year-old man seen in Paris, 1530

File:Gould Pyle 55.jpg|Lazarus and Joannes Baptista Colloredo, born 1617

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:Gould Pyle 56.jpg|Louise L., born 1869

File:B. C. Hirst & G. A. Piersol, Human monstrosities Wellcome L0027956.jpg|Joao Baptista dos Santos and Louise L. (), 1893

File:Gould Pyle 57.jpg|Laloo, 1897

</gallery>

Animals

File:Les écarts de la nature page12.jpg|Chicken with two extra lower limbs

File:Les écarts de la nature page30.jpg|Pigeon with two 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 two extra front limbs