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Siamese twins

Generality

Siamese twins, or conjoined twins, are a pair of homozygous (and therefore identical) twins who are physically born, often sharing one or more organs.

The process that leads to the birth of two Siamese twins is very similar to the one that characterizes the birth of normal homozygotic twins: simplifying, in fact, depends on the division of the fertilized egg cell (zygote) into two halves, with the difference that in the case of Siamese such split is incomplete.

Siamese twins are classified according to the anatomical union modes. Some junctions involve vital organs and this represents one of the main causes of death of the two individuals.

Most conjoined twins are born dead or die a few hours after birth. Couples that survive and whose union is limited to a few anatomical structures can be subjected to surgery for their separation.

Homozygous and heterozygous twins

The twins are individuals born from a single pregnancy, which in this specific case is called plural pregnancy .

There are two main categories of twins:

  • Homozygous twins, also called monoovulars.
  • The heterozygous twins, also called biovulars.

Figure: conception of homozygous twins and heterozygous twins. As can be seen, two heterozygous twins derive from two eggs fertilized by as many spermatozoa; the resulting zygotes are two and develop according to the methods of single pregnancies. This explains why heterozygous twins can also be very different. From the site: en.wikipedia.org

Multiple pregnancies conclude with the birth of heterozygous twins in 65-75% of cases and with the birth of homozygous twins in the remaining 25-35%.

OMOZIGOTI CUFFLINKS

The homozygous twins derive from the same zygote, which in an early stage of its development (precisely between the 1st and the 12th day from conception), instead of remaining united, divides into two or more equal and distinct parts (NB : the zygote is the cell that is formed by the union of the spermatozoon with the egg cell during fertilization, this cell quickly meets various subdivisions forming the morula and then the blastocyst ).

The division of the zygote will then give way to the development of two or more embryos (the quantity depends on the number of equal parts into which the zygote is divided), which possess the same genetic heritage, are of the same sex and are almost identical (even in the somatic traits).

Possible differences, at a physical level and as regards the incidence of some diseases, are due to the different life habits and to the different environmental conditions of growth of each homozygous twin.

What can differentiate two or more homozygous twins over a lifetime?

  • Cigarette smoking and tobacco use in general
  • Type of work
  • Environmental pollution
  • Type of power supply
  • Physical activity
  • Stress

PLEASE NOTE: according to the moment in which the split occurs, the monozygotic twins assume particular characteristics; in fact if the division of the zygote occurs:

  • in the first 3 days after fertilization, pregnancy will be bicorial and biamniotic, meaning there will be two placentas and two different amniotic bags;
  • between the 4th and 8th day after fertilization, the fetuses will have only one placenta but two amniotic sacks;
  • between the 9th and 12th day, the fetuses will also share the amniotic sac.

Siamese twins

Siamese twins, more rarely called conjoined twins, are a pair of homozygous twins born physically joined together in one part of the body.

Sometimes the physical union is so deep that the twins share one or more organs.

WHY ‰ DO WE USE THE TERM SIAMESE TWINS?

Figure: the twins joint Chang and Eng Bunker, from Siam (current Thailand) and from which derives the term "Siamese twins".

The use of the term "Siamese twins" is due to Chang and Eng Bunker, two joint twins from Siam (current Thailand), famous throughout the world for their performances with the circus Phineas Taylor Barnum .

United physically at the level of the lower sternum from a strip of cartilage and with the two connected livers, Chang and Eng Bunker became real celebrities starting from 1829, on the intuition of a certain Robert Hunter, who took them to America. During the circus shows, they were presented as "the Siamese twins", a term which, from that moment on, replaced the now little used "joint twins".

If Chang and Eng Bunker had lived in more modern times, they would have been separated by surgery and could have led a different life.

Epidemiology

The birth of Siamese twins is a very rare phenomenon: according to some estimates, in fact, it would occur once every 119, 000 births (NB: this is an average value).

In most cases (between 70 and 80%), conjoined twins are born already dead (death at birth) or die shortly after giving birth. The cause of death is related to shared organs and their malformations.

For yet unknown reason, Siamese twins are more frequently female: according to some statistical research, in fact, the ratio of females to males would be equal to 3: 1.

The areas of the world in which the birth of Siamese twins is most likely to be observed are Latin America (Brazil in particular), South-West Asia and some African states.

Causes

Why are Siamese twins born?

There are two theories and both focus on the division (or separation) of the zygote, typical of multiple homozygous pregnancies.

  • According to the oldest theory, the episode that would lead to the formation of Siamese twins would be a delay in the division of the zygote; division which, instead of taking place within the 12th day of conception, would take place later, usually between the 13th and 15th day.

    This delay would ensure that the separation does not end definitively; due to this incomplete splitting the two future embryos are physically joined at one point.

  • According to the most modern theory (and by now even more accredited), the formation of Siamese twins would be due to a recast of the two parts of the zygote previously completely divided.

    In other words, according to the second theory, the separation is correct and takes place according to the normal times of the homozygotic twins (therefore within 12 days of conception), but then a partial recasting of the two parts that previously constituted a zygote would occur. only.

    The causes of the recast are still completely unknown.

RISK FACTORS

At the moment, the factors and circumstances that favor the birth of Siamese twins are not yet known.

Symptoms and Complications

Pregnant women carrying Siamese twins do not present any symptoms or particular signs, except those typical of multiple pregnancies (see table).

Typical effects induced by multiple pregnancies:

  • Very rapid uterine growth
  • Greater tiredness than in a single pregnancy
  • Nausea and vomiting more frequent than in a single pregnancy

Siamese fetuses share the chorion, the placenta and the amniotic sac . However, these features are not exclusive to them: in fact, it is possible that some non-Siamese homozygous fetuses have the aforementioned structures in common (in this regard, see the figure that depicts what happens when the division of the zygote occurs at the 1st - 3rd day, 4th-8th day, 8th-12th day and 13th-15th day after conception).

TYPES OF SIAMESE TWINS

Siamese twins are classified according to how they are physically joined together.

The most common methods of joining (or joining) are:

  • The thoracophagus junction (or thoraco-onfalopagus Siamese twins) . Characterized by the fusion of the chest and part of the abdomen, this form of union is by far the most common (28% of cases). Thoraco-omphalophagus twins very often share the heart and sometimes also the liver and part of the digestive system (small intestine and colon).
  • The joining of thoracopagus (or Siamese twins toracopaghi) . The two bodies are fused at the level of the chest and always have the heart in common. This conjunction occurs in 18.5% of cases.
  • The omphalophagus junction (or cephalophagus twins) . The fusion of the two bodies is localized at the level of the abdomen, near the navel. Siamese omphalopagus twins share the liver, part of the digestive system, the diaphragm and various other abdominal organs, but never the heart. The omphalophagus junction occurs in 10% of cases.
  • The parasite junction (or Siamese twins parasites) . The two bodies are decidedly asymmetrical and the union can take place anywhere in the body. This junction is characterized by a larger twin, provided with almost all vital anatomical structures, and by a smaller incomplete twin dependent on the other (this explains the term "parasite"). Parasitic twins represent 10% of cases.
  • The craniopagus junction (or Siamese craniophagous twins) . The two individuals are joined exclusively at the level of the skull. The craniopagus junction, which affects 6% of cases, can occur posteriorly (occipital region of the skull), frontally (ie at the level of the forehead) or laterally.

    Facial unions have never been observed.

The less common ways of joining, however, are:

  • Cephalopagus junction (or cephalophagus twins) . The two individuals have a head with two faces generally located opposite to each other. Moreover, they often also have in common the upper part of the body.

    Cerebral malformations of cephalophagus twins are not compatible with life.

  • The sinfalo junction (or Siamese twins) . The two individuals have only one head and one face, but they have four ears and two trunks.
  • The combination of cephalothoracopagus (or Siamese twins cefalotoracopaghi) . The two individuals are fused in the head and chest and can have two faces or only one face.
  • The junction of xiphopagus (or xipophagus twins) . The two bodies are joined at the level of the xiphoid cartilage of the lower sternum. Xipophagus twins usually share the liver.

    The twins Chang and Eng Bunker were xifopaghi.

  • The ischiopagus junction (or ischiopagus twins) . The two bodies are fused in the pelvic region of the spine and often have the gastrointestinal tract, liver, genitals and anus in common. The ischiopagus twins have four arms and a variable number of legs from two to four.
  • The parapagus junction (or parapaghi twins) . The two individuals are joined to one another at the level of the hips. The junction can include only the pelvis or even the abdomen.

    Some parapaghi twins have a single trunk, two arms and two faces.

Additional joining methods:

  • Pineapple junction (or iliopago)
  • Spinal joint
  • Onfaloischiopago conjugation

COMPLICATIONS

Siamese twins require cesarean delivery . Caesarean section is now a safe procedure, but remains a surgical intervention, with its possible risks and its possible complications.

As already mentioned, in most cases Siamese twins are born dead (40-60% of cases) or die within a few days of birth (25-30% of cases).

Diagnosis

The presence, in the maternal uterus, of a pair of Siamese twins can be highlighted already during the first trimester of pregnancy through a simple ultrasound scan .

In these cases, doctors advise the mother to undergo more in-depth diagnostic tests, such as a higher level ultrasound scan, an MRI scan or an fetal echocardiogram ; such investigations allow to better visualize the characteristics of the junction, the organs in sharing and the functionality of the heart.

FALSE-POSITIVE

Before the tenth week, diagnostic tests could mislead medical specialists, giving a false positive . A false-positive is a situation of false alarm, in which the instrumental tests mistakenly highlight the presence of Siamese twins, although these are actually normal homozygotic twins not joined together.

The evaluation error during the analysis of the results is usually due to the fact that the Siamese twins, like some healthy homozygous twins, share the chorion and the amniotic sac.

Treatment

If Siamese twins survive childbirth, and if their type of connection allows it, separation surgery can be used.

In general, the surgical operations for the separation of the Siamese twins are quite complex, and they become even more so when the twins share vital organs or are united in extremely delicate points (for example head or abdomen).

In recent decades, surgery has made significant progress and has allowed us to consider the intervention also for cases of Siamese twins that once would have been declared incurable.

WHAT DOES THE GOOD SUCCESS OF THE INTERVENTION DEPEND ON?

The success of the separation intervention depends on:

  • State of health of the twins . If the twins are healthy (despite their condition), the separatory procedure is more likely to succeed.
  • Place of junction . Some regions of the body are decidedly more delicate than others. For example, a union at the level of the skull is certainly more difficult to treat than a union at the level of the xiphoid cartilage.
  • Organs and other shared anatomical structures . We have already discussed how important sharing of vital organs is.
  • Type of reconstruction intervention . Once the separation is completed, an intervention is planned for the reconstruction of the anatomical structures that are incised and separated. The more complex the reconstruction, the greater the likelihood of complications arising.

Unfortunately, in some cases the separation operations, although they are feasible, end with the death of one or both of the Siamese twins.

WHAT IS THE BEST TIME TO OPERATE?

According to surgeons, the best time to operate is between the 2nd and 4th month of life.

However, it must be specified that each pair of Siamese twins represents a particular case; therefore the aforementioned indication, relating to the optimal timing of intervention, may undergo variations.

WHEN IS SEPARATION NOT RECOMMENDED?

Despite surviving birth, some pairs of Siamese twins are not separated. This happens when, after an appropriate medical evaluation, it has been established that the risks of the surgical operation are clearly superior to the benefits, and that for this reason the twins have greater hopes of life remaining united.

INFORMATION RELATING TO THE CESAREO DEPARTMENT

As already mentioned, women in utero of Siamese twins are subjected to caesarean section for precautionary reasons. This should generally be done about 2-4 weeks before the due date.

OTHER TIPS FOR THE MOTHER

Women waiting to give birth to a pair of Siamese twins, doctors advise to undergo frequent ultrasound examinations, to monitor in detail the fetal growth and the characteristics of the joint.

The more information you have on the type and degree of union of the twins, the better the preparation of the surgery (if, obviously, the Siamese twins came to light alive and survived even after the birth).

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