Ventrally conjoined twins are fused rostral (cephalopagus, thoracopagus, omphalopagus), caudal (ischiopagus) or lateral (parapagus). Ventral union always includes the umbilicus and the abdomen. Extensive fusion including the thorax, the head or the pelvis and perineum may be present (31). All share a common yolk sac and, as a consequence, all share some part of the gastrointestinal tract.
The cephalopagus twin is also called Janiceps twins after the Roman god Janus with the two faces, being fused from the head to the umbilicus. Each twin contributes to half of the head, thorax and abdomen. This extensive fusion complicates correct diagnosis; cephalopagus conjoined twins are often referred as a singleton pregnancy with the inconclusive finding of an enlarged head (92-93). There are always two vertebral columns, two sets of genitalia, four upper and four lower limbs. Cephalopagus twins are symmetrical with two complete faces or asymmetrical with one relatively normal face and one reduced face. Two hearts are formed which are united in the plane of fusion. In the asymmetrical cephalopagus the posterior heart can be very small and functionless. The upper gastrointestinal tract is always shared, with one esophagus, stomach and small intestine. The trachea is also shared which provides a main bronchus for a lung of each twin. This type of conjoined twins is non viable.
In many thoracopagus a shared cardiac anatomy is the most prominent feature. The complexity of the shared cardiac anatomy covers a wide range from a channel between the two atria to complete fused hearts (75%); though nearly all of these hearts are inseparable. Livers are almost invariable fused in thoracopagus conjoined twins; if the fusion is extensive this also precludes separation.
The omphalopagus is characterized by a fusion from the sternum to the umbilicus. Since both thoracopagus and omphalopagus may have an union of thorax and abdomen it may be difficult to differentiate between the two. The omphalopagus twins, however, have two separate hearts, while the thoracopagus share their hearts to some extend. In 25% of cases there are cardiac anomalies, ranging from minor to severe, influencing the prognosis. In case of normal hearts the chances of survival are high. There is a single umbilical cord artery present, and due to this associated anomalies may be present. Twenty-five percent of omphalopagus cases an omphalocele is present. Shared intestines (17%) are associated with stillborn twins in half of the cases.
The liver is shared in most omphalopagus and this is associated with the extent of union of the abdominal wall. The possibility of surgical separation depends on the vasculature of the conjoined livers. Separation is often possible with a success rate of 82% (5).
The ischiopagus twins are fused at the pelvis and lower abdomen in the cloacal membrane. This fusion can be end to end or, rarely, with the twins facing each other. As the ischiopagus twins are fused in the cloacal membrane, the two sets of external genitalia are located laterally. The lower gastrointestinal tract is shared in 70% of cases. Ischiopagus may be tetrapus (having four legs), tripus (three legs) or bipus (two legs). Separation may be possible with a success rate of about 63% (94).
The most common type of conjoined twins is the parapagus with an incidence of 28%. In case of a parapagus there may be a parapagus dicephalus (having two heads) or a parapagus diprosopus (having two faces on the same side of a single head). Parapagus twins are united in the lower abdomen and pelvis and the fusion may extend even to the cranium. Parapagus have two lower limbs and have two, three or four upper limbs. There is always one lower abdomen with one gastrointestinal tract and one pelvis, but there may be two separate thoraxes. Either one or two hearts may be present. The diagnosis of parapagus dicephalus is in most cases made by the observation of two distinct heads and one fetal body (95-97). In parapagus diprosopus twins there is a high incidence of anecephaly. Also forking of the cervical spine is an additional distinctive finding (98). Most cases of parapagus diprosopus die in utero or die during the neonatal period. Parapagus dicephalus die in most cases due to anomalies of the heart. The survival into adulthood is rarely accomplished. However, this is more often by choice than clinical impossibility. The well known Hensel twins, who were born March 1990, have now reached adulthood. Both graduated from high school in 2008 and are now university students. Separation is only possible when there are two separate hearts but will inevitably lead to severely handicapped children.
In dorsally fused twins the cranium and/or the vertebrae are always part of the fusion. The face and both thoracic and abdominal organs however are never involved in this fusion (31). There are three types of dorsally conjoined twins: craniopagus, rachipagus and pyopagus.
Craniopagus conjoined twins (5%) are fused at the cranium, but never at the foramen magnum or the base of the skull. The face, neck, thorax and abdomen are never involved in the union. The orientation of the twins fused at the cranium may be in any position. Skull, meninges and venous sinuses are always fused. Brains are separate in most cases and fusion of the cortex is present in 30% of cases. Separation without further residual handicaps for the twins is unlikely.
An extremely rare type of conjoined twins is the rachipagus, which has been described only twice in the literature (99-101). Since only two cases are known, it is highly favorable to investigate these cases with the best possible imaging techniques and preserve them (102). The limited information makes description and generalization of this type of conjoined twins difficult. Rachipagus are joined dorsally with fused vertebral columns. The fusion may involve fusion in the occiputs. Surgical separation is impossible.
Pyopagus conjoined twins are joined at the sacrum and the perineum. Each twin has a separate abdomen and therefore the gastrointestinal tract is not involved with the exception of the rectums. There are two sets of genitalia which are united dorsally. When the vertebral canals are continuous the dura and/or spinal cord may be joined which has severe implications for surgery. Separation is likely to be possible since the conjunction does not involve critical organ systems, sometimes only compromising soft tissue (68, 103). A separation success rate of 68% has been reported (94).