2001
Conjoined twins
Siamese twins.
Conjoined twins are identical twins joined in utero.
Its occurrence is estimated to range from 1 in 49,000 births to 1 in 189,000 births.
There is a somewhat higher incidence in Southwest Asia and Africa.
Approximately half conjoined twins are stillborn, and an additional one-third die within 24 hours.
Most live birth conjoined twins are female, with a ratio of 3:1.
The generally accepted theory to explain the origin of conjoined twins is fission, in which the fertilized egg splits partially.
Conjoined twins share a single common chorion, placenta, and amniotic sac.
There are some monozygotic but non-conjoined twins who also share these structures in utero.
Conjoined twins are classified by the point at which their bodies are joined.
The most common types of conjoined twins are:
Thoraco-omphalopagus (28% of cases) the two bodies are fused from the upper chest to the lower chest.
These twins usually share a heart, and may share the liver or part of the digestive system.
Thoracopagus (18.5%): the two bodies fused from the upper thorax to lower belly.
The heart is always involved in these cases,and separation of a genuinely shared heart has not offered survival, but 1 designated twin may survive if allotted the heart, sacrificing the other twin.
Omphalopagus (10%):[4] The two bodies fused at the lower abdomen. The heart is never involved but often share a liver, digestive system, diaphragm and other organs.
Parasitic twins (10%): symmetrically conjoined twins, with one twin that is small, less formed, and dependent on the larger twin for survival.
Craniopagus (6%):Fused skulls, but separate bodies. Twins conjoined at the back, front or the side of the head, but not on the face or the base of the skull.
Craniopagus conjoined twins occurs in 0.6 of every 1 million births.
Less common types of conjoined twins include:
Cephalopagus: Two faces on opposite sides of a single, conjoined head; the upper portion of the body is fused while the bottom portions are separate.
Cephalopagus twins generally cannot survive due to severe malformations of the brain.
Syncephalus: One head with a single face but four ears, and two bodies.
Cephalothoracopagus: Bodies fused in the head and thorax, there are two faces facing in opposite directions, or sometimes a single face and an enlarged skull.
Xiphopagus: The twin bodies are fused in the xiphoid cartilage, which is approximately from the navel to the lower breastbone, and almost never share any vital organs, with the exception of the liver.
Ischiopagus: Twins fused lower half of the two bodies, with spines conjoined end-to-end at a 180° angle.
These twins have four arms; one, two, three or four legs; and typically one external set of genitalia and anus.
Omphalo-Ischiopagus: Twins fused in a similar fashion as ischiopagus twins, but facing each other with a joined abdomen akin to omphalopagus.
These twins have four arms, and two, three, or four legs.
Parapagus: Twins fused side-by-side with a shared pelvis.
Craniopagus parasiticus: Like craniopagus, but with a second bodiless head attached to the dominant head.
Pygopagus or ilipagus: Twin bodies joined at the pelvis.
Rachipagus: Twins joined along the backof their bodies, with fusion of the vertebral arches and the soft tissue from the head to the buttocks.
Causes:
Theories about the development of conjoined twins: The first is that a single fertilized egg does not fully split during the process of forming identical twins.
The second theory is a fusion of two fertilized eggs occurs earlier in development.
Surgery to separate conjoined twins depends on the point of attachment and the internal parts that are shared.
Most cases of surgical separation are extremely risky and life-threatening.
In many cases, the surgery results in the death of one or both of the twins.