Dicephalic parapagus twins

Dicephalus means two-headed.[1] Parapagus means joined side by side.[7][1]

Dicephalic twins are called:

• dibrachius if they have two arms altogether (one for each twin),[3]
• tribrachius, if they have three arms altogether,[8]
• tetrabrachius, if they have four arms altogether,[7]
• dipus, if they have two legs altogether (one for each twin).[7]

Conjoined twins appear in one in 50,000 to one in 100,000 births.[9] Dicephalic twins represent about 11 percent of all conjoined twins.[9]

As late as the 1960s, some medical publications argued that newborn dicephalic twins should not be actively treated or resuscitated.[10] An attempt at surgical separation was reported in a paper published in 1982, but did not result in long-term survival of either twin.[11] In more recent cases in Turkey and Minnesota, doctors advised that separation surgery would not be appropriate.[12][13]

Giacomo and Giovanni Battista Tocci (1877–1940), were Italian male dicephalus parapagus twins who survived to adulthood.[9][14] Each had his own pair of arms.[14] They had two legs all together, one of which was controlled by each twin.[15] They were exhibited in freak shows as children and teenagers. The Toccis learned to speak several languages,[16] but never learned to walk.[14]

Abby and Brittany Hensel, born in Minnesota in 1990, also are dicephalus parapagus twins who grew up.[17] They were born with two functional arms, plus a vestigial third arm, which was surgically removed.[13] Each twin has her own complete head, heart and spine, and controls one arm and one leg.[13] They learned to walk around the same age as average children, went on to attend school, learn to drive, play sports,[13] and completed courses at college.

Twin girls conjoined like the Tocci brothers, with four arms and two legs all together, were born in Turkey in 2000.[12] As with the Toccis and the Hensels, it was found that each twin controlled one leg.[18] Because of the size of their upper body, the Turkish twins needed a special program of exercises before they could learn to walk.[18] After that, they learned to run and climb stairs without further specialist help, and at age 11 they were going to school with other children.[19]

• Craniopagus parasiticus is another form of partial twinning which results in two heads kept alive by a single torso. In these cases a second, vestigial torso is present, and the heads are joined directly to one another.[20]
• Diprosopus is a condition in which there are duplicated facial features on one head.
• Polycephaly is a general term about organisms with more than one head.