1999-06-01-18 Conjoined twins © Silva www.thefetus.net/
Sandra Rejane Silva, MD2, Luís Flávio de Andrade Gonçalves, MD1, Philippe Jeanty, MD, PhD3
 Florianopolis, SC Brazil firstname.lastname@example.org ,  Sao Paulo – SP mailto:[email@example.com  Nashville, TN firstname.lastname@example.org
Adapted and reproduced with permission from TKI Medcon Inc. http:/www.tki.com/
Conjoined twins are defined as monochorionic and monoamniotic twins fused at any portion of their body as a result of an incomplete division of the embryonic disk, which occurs after the 13th day of conception. The term "conjoined" is actually a misnomer, since most authors consider the pathogenesis of the condition to result from failure of complete separation, instead of fusion of twin. Yet at least on author proposes that the fusion of two normal embryos results in conjoined twins.
The minimal sonographic criteria for the prenatal diagnosis of conjoined twins is the visualization of fused portion of the bodies of monozygotic - monoamniotic twins. Aside from this basic criterion, several sonographic signs (listed below) can be observed in this condition. Careful search for these features and serial scans for confirmation are recommended to prevent misdiagnosis.
The following sonographic criteria can also be observed in conjoined twins:
- Bifid appearance of the first-trimester fetal pole ("V or Y" shaped twin pregnancy)
- Continuous skin contours at the same anatomic level
- Absence of a interamniotic membrane between the twins
- Inability to separate fetal bodies
- The presence of fetal anomalies
- Abnormal number of vessels (more than three) in the umbilical cord
- The heads and bodies of both twins are seen at the same level
- Unusual extension of the spines
- Unusual proximity of the extremities
- Permanent position of the fetus relative one to another, even after external stimulation or maternal movement
- Single heart
The presence of these signs varies according to the different types of conjoined twins. Conjoined twins must be considered whenever a monochorionic and monoamniotic pregnancy is suspected. Discordant presentation does not exclude conjoined twins.
Although the diagnosis of conjoined twins is easier during the first trimester, the type and severity of the condition is better achieved during the second trimester, when a more precise evaluation of the shared organs can be done. The diagnosis with 3D transvaginal sonography during the first trimester has also been described. If diagnosis is made before viability, termination of pregnancy can be offered.
Duplicata incompleta: notice the two fetal heads and single body
Duplicata incompleta: notice the two fetal heads and diverging spines (different case from above)
The fetus from the previous ultrasound image
Cords of conjoint fetuses may have between 2 and 6 vessels
Thoraco-omphalopagus twins. Note that these are mirror image (the 2 stomachs are on the same side of the image).
A shared heart between the 2 chest cavities. This is almost always an indication that the condition is fatal.
Same fetuses as above. Notice that these are not mirror-image of each other: the stomachs are on opposite sides.
Thoraco-omphalopagus twins: Note that one of the twin is larger than the other one.
This is a rare condition and the reported frequency varies from 0.1-0.35:10,000 births. If stillborns are excluded the estimate is 0.05:10,000. Females are more commonly affected with a male to female ratio of 1.6-3:1. No association with maternal age, race, parity or heredity has ever been observed. The recurrence risk is negligible.
Most of the conjoined twins are born prematurely, 40% are stillborn, and 35% die within 24 hours. Among the survivors, the prognosis as well as attempts of surgical separation will depend on the type of conjunction, degree of involvement of the shared organs, and the presence of associated anomalies. The most ominous prognosis is among those twins who share liver and or heart. Attempts of separation in cases of a common liver can be done as long as two biliary tracts are seen. In the presence of a shared heart, separation is only attempted if two normal hearts coexist in a single pericardium.
The method of choice for delivery is c-section to maximize survival and prevent maternal and twins trauma.
Conjoined twins are classified according to the area of the bodies where the fusion takes place and the involvement of internal organs. The symmetrical and equal forms, in which the twins have equal or nearly equal duplication of structures, are called duplicata completa. When there is an unequal duplication of structures they are called duplicata incompleta, and this category includes the most severe types of conjoined twins in which just few organs systems are duplicated. The most frequent varieties of conjoined twins are thoracopagus (40%), omphalopagus (33%), pygopagus (18%), ischiopagus (6%) and craniopagus (2%).
The classification of conjoined twins is described at table I.
Table I – Classification of conjoined twins
|Duplicata incompleta: duplication occurring in only one part or region of the body.
Diprosopus: one body, one head, two faces
Dicephalus: one body, two heads
Dipygus: one head, thorax and abdomen with two pelves, and/or external genitalia
Duplicata completa: two complete conjoined twins
Terata catadidyma: conjunction in the lower part of the body
Ischiopagus: joined by inferior portion of coccyx and sacrum
Pygopagus: joined by lateral and posterior portion of coccyx and sacrum
Terata anadidyma: conjunction in the upper part of the body
Syncephalus: joined by the face
Craniopagus: joined at homologous portion of the cranial vault
Terata anacatadidyma: conjunction in the midpart of the body
Thoracopagus: joined at the thracic wall
Xiphopagus: joined at xiphoid process
Omphalopagus: joined in the area between the xiphoid cartilage
Rachipagus: joined at he level of the spines above the sacrum
Adapted from: Romero, R., Pilu, G., Jeanty, P., Ghidini, A. and Hobbins, J.C.(1988). Prenatal Diagnosis of Congenital Anomalies, p 405.
In one attempt to universalize the current nomenclature, a new classification was proposed recently based on the theoretical site of union:
Ventral union: twins united along the ventral aspect
- Cephalopagus - fused from the top of the head down to the umbilicus. Two rudimentary (fused) faces, four arms and four legs. Lower abdomen and pelvis are separated.
- Thoracopagus - united face-to-face from the upper thorax down to the umbilicus, with heart involvement always. Four arms, four legs, two pelvises.
- Omphalopagus - joined face-to-face primarily in the area of the umbilicus, and sometimes involving the lower thorax, but always preserving two distinct hearts. There is not even a cardiac vessel in common. Two pelvis, four arms and four legs.
- Ischiopagus - united ventrally from the umbilicus to a large conjoined pelvis with two sacrums and two symphyses pubis. They appear more frequently joined end-to-end with the spine in a straight line, but they can also present face-to-face with a joined abdomen. Four arms, four legs, and in general, a single external genitalia and a single anus.
Lateral union: twins joined side-by-side with shared umbilicus, abdomen, and pelvis.
- Parapagus - twins that share a conjoined pelvis, one symphysis pubis and one or two sacrums. When the union is limited to the abdomen and pelvis (does not involve the thorax) it is called dithoracic parapagus. If there is one trunk with two heads it is called dicephalic parapagus. If there is a single trunk and a single head with two faces they are diprosopic parapagus. Two, three or four arms, and two or three legs
- Dorsal union: twins joined at the dorsal aspect of the primitive embryonic disc. There is no involvement of thorax and abdomen
- Craniopagus - united on any portion of the skull, except the face or foramen magnum. They share bones of the cranium, meninges, and occasionally brain surface. Two trunks, four arms and four legs
- Pygopagus - they share dorsally the sacrococcygeal, perineal regions and occasionally the spinal cord. There is one anus, two rectums, four arms and four legs.
- Rachipagus twins fused dorsally above the sacrum, involving different segments of the column. This type is extremely rare.
Conjoined twins have a unique presentation and the few differential diagnoses could include lympangioma, teratoma, or cystic hygroma.
Congenital anomalies of organs other than the shared ones are present in 50% of the cases of conjoined twins. Cardiac defects are the most common association (20-30%) and thus echocardiography is recommended in all cases. Neural tube defects and midline fusion defects, orofacial clefts, imperforate anus and diaphragmatic hernia are also frequently seen. Polyhydramnios is observed in 50-75% of the cases.
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