Siamese twins

Generality

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

The process that leads to the birth of two Siamese twins is very similar to that which characterizes the birth of normal homozygous twins: simplifying, it depends on the division of the fertilized egg cell (zygote) into two halves, with the difference that in the case of the Siamese this split is incomplete.
Siamese twins are classified according to the modalities of anatomical union. Some joints involve vital organs and this represents one of the main causes of death of the two individuals.
Most conjoined twins are born already dead or die a few hours after birth. Couples who survive and whose union is limited to a few anatomical structures can undergo surgery for their separation.

Homozygous and Heterozygous Twins

Twins are individuals born from a "single pregnancy, which in this specific case is defined as multiple pregnancy.
There are two main categories of twins:

• Homozygous twins, also called monoovular.
• Heterozygous twins, also called biovular.

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 modalities of single pregnancies. This explains why heterozygous twins can also be very different from each other. From the site: en.wikipedia.org

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

HOMOZYGOT CUFFLINKS

Homozygous twins derive from the same zygote, which in an early stage of its development (precisely between the 1st and 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 union of the spermatozoon with the egg cell during fertilization; this cell rapidly undergoes various subdivisions forming the morula and then the blastocyst).
The division of the zygote will therefore give way to the development of two or more embryos (the quantity depends on the number of equal parts in which the zygote is divided), which have the same genetic heritage, are of the same sex and are almost identical (even in somatic traits).
Any differences, on a physical level and as regards the incidence of some diseases, are due to the different habits of life and the different environmental conditions of growth of each homozygous twin.

What can differentiate two or more homozygous twins in the course of life?

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

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

• in the first 3 days from fertilization, the pregnancy will be bicorial and biniotic, that is, there will be two placentas and two different amniotic sacs;
• between the 4th and 8th day after fertilization, the fetuses will have only one placenta but two amniotic sacs;
• between the 9th and the 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 who are 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 IS THE TERM SIAMESE GEMINI USED?

Figure: the joint twins Chang and Eng Bunker, coming from Siam (now Thailand) and from which the term “Siamese twins” derives.

The use of the term "Siamese twins" is due to Chang and Eng Bunker, two conjoined twins from Siam (now Thailand), famous all over the world for their performances with the circus Phineas Taylor Barnum.
Physically united at the level of the lower sternum by a strip of cartilage and with the two livers connected, Chang and Eng Bunker became real celebrities starting from 1829, on the intuition of a certain Robert Hunter, who brought them to America. During the circus shows, they were presented as "the Siamese twins", a term that, from that moment on, replaced the "now little used" conjoined twins ".
If Chang and Eng Bunker had lived in more modern times, they would have been separated with 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 (N.B: 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 a reason still unknown, Siamese twins are more frequently female: according to some statistical research, in fact, the female: male ratio would be equal to 3: 1.
The areas of the world in which it is more probable to observe the birth of Siamese twins are Latin America (Brazil in particular), the South-West of Asia and some states of Africa.

Causes

Why are conjoined twins born?
There are two theories and they 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, which, instead of occurring within the 12th day of conception, would take place later, usually between the 13th and the 15th day.
  This delay would mean that the separation does not end definitively; due to this incomplete splitting, the two future embryos are physically united at one point.
• According to the most modern theory (and now even more accredited), the formation of the Siamese twins would be due to a recasting 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 homozygous 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.

Interesting reproduction of how homozygous twins look when zygote division occurs after 1-3 days, 4-8 days, 8-12 days and after 13-15 days. As can be guessed, this image holds the hypothesis that the Siamese twins are formed due to a delay in the division of the zygote to be correct.

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 have any particular symptoms or signs, except those typical of multiple pregnancies (see table).

Typical effects induced by multiple pregnancies:

• Very rapid uterine growth
• More tired than in a single pregnancy
• Nausea and vomiting more frequent than in a single pregnancy

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

TYPES OF SIAMESE TWINS

Conjoined twins are classified based on how they are physically joined together.
The most common ways of joining (or joining) are:

• The thoraco-omphalopagus junction (or thoraco-omphalopagus conjoined twins). Characterized by the fusion of the thorax and part of the abdomen, this method of union is by far the most common (28% of cases). Thoraco-omphalopagus twins very often share the heart and, sometimes, the liver and part of the digestive system (small intestine and colon).
• The thoracopagus junction (or thoracopagus conjoined twins). 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 omphalopagus junction (or conjoined omphalopagus twins). The fusion of the two bodies is located at the level of the abdomen, near the navel. The Siamese omphalopagus twins share the liver, part of the digestive system, the diaphragm and various other abdominal organs, but never the heart. The omphalopagus junction occurs in 10% of cases.
• The parasitic junction (or parasitic conjoined twins). The two bodies are decidedly asymmetrical and the union can take place anywhere on the body. This connection is characterized by a larger twin, equipped with almost all vital anatomical structures, and by a smaller twin incomplete and dependent on the other ( this explains the term "parasite"). Parasitic twins account for 10% of cases.
• The craniopagus junction (or craniopagus conjoined twins). The two individuals are united exclusively at the level of the skull. Craniopagus junction, which affects 6% of cases, can occur posteriorly (occipital region of the skull), frontally (i.e. at the level of the forehead) or laterally.
  No unions have ever been observed at the level of the face.

The less common ways of joining, on the other hand, are:

• The cephalopagus junction (or conjoined cephalopagus twins). The two individuals have a head with two faces generally located opposite each other. They also often have the upper body in common.
  Brain malformations of cephalopagus twins are not compatible with life.
• The sinephalus (or siamese siamese twins) junction. The two individuals have only one head and one face, but have four ears and two trunks.
• The cephalothoracopagus junction (or conjoined cephalothoracopagus twins). The two individuals are fused in the head and in the thorax and can have two faces or only one face.
• The xiphagus junction (or xiphagus twins). The two bodies are joined at the level of the xiphoid cartilage of the lower sternum. Xiphagous twins usually share a liver.
  Twins Chang and Eng Bunker were xiphopagus.
• 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. Ischiopagus twins have four arms and two to four legs.
• The parapago junction (or twin parapago). The two individuals are joined to each other at the level of the hips. The junction may include the pelvis alone or even the abdomen.
  Some parapaghi cufflinks have a single trunk, two arms and two faces.

Further ways of joining:

• Pygopagus (or iliopagus) junction
• Rachipagus junction
• Conjunction onfaloischiopago

COMPLICATIONS

Conjoined twins require caesarean delivery. Cesarean delivery is now a safe procedure, but it remains a surgical intervention, with its possible risks and possible complications.
As already mentioned, in most cases Siamese twins are born dead (40-60% of cases) or die a few days after birth (25-30% of cases).

Diagnosis

The presence, in the maternal uterus, of a pair of conjoined twins can be evidenced already during the first trimester of pregnancy through a simple ultrasound.
In these cases, doctors advise the mother to undergo more in-depth diagnostic tests, such as a "higher level ultrasound, an MRI or an echocardiogram of the fetus; these investigations allow to better visualize the characteristics of the joint, the organs in sharing and functionality of the or hearts.

FALSE-POSITIVE

Before the tenth week, diagnostic tests could mislead medical specialists by giving a false-positive. A false-positive is a false alarm situation, in which instrumental tests incorrectly highlight the presence of Siamese twins, although these, in reality, are normal homozygous twins not joined together.
The misjudgment when analyzing the results is usually due to the fact that Siamese twins, like some healthy homozygous twins, share the chorion and amniotic sac.

Treatment

If Siamese twins survive childbirth, and if their type of joining allows it, separation surgery can be used.
In general, the surgical operations for the separation of conjoined twins are somewhat complex, and become even more so when the twins share vital organs or are joined in extremely delicate places (eg head or abdomen).
In recent decades, surgery has made significant progress and has made it possible to consider surgery even for cases of conjoined twins that would once have been declared incurable.

WHAT DOES THE GOOD SUCCESS OF THE INTERVENTION DEPEND?

The success of the separation procedure depends on:

• State of health of the twins. If the twins are healthy (despite their condition), the separation procedure is more likely to be successful.
• Joint seat. Some regions of the body are significantly 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. The importance of sharing vital organs has already been discussed above.
• Type of reconstruction intervention. Once the separation is completed, an intervention is planned for the reconstruction of the incised and disjointed anatomical structures. The more complex the reconstruction, the greater the likelihood of complications.

Unfortunately, in some cases the separation operations, while being feasible, end with the death of one or both of the Siamese twins.

WHEN 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 intervention times, could undergo variations.

WHEN IS SEPARATION NOT RECOMMENDED?

Despite surviving childbirth, some pairs of Siamese twins are not separated. This happens when, after an "appropriate medical evaluation, it is ascertained that the risks of the surgical operation are clearly greater than the benefits, and that for this reason the twins have greater life expectancy remaining together."

INFORMATION CONCERNING THE CAESARIAN DELIVERY

As already mentioned, women carrying conjoined twins in utero are subjected, for precautionary reasons, to a caesarean section. This usually takes place about 2-4 weeks before the expected delivery date.

OTHER TIPS FOR THE MOTHER

For women expecting to give birth to a pair of Siamese twins, doctors recommend that they undergo frequent ultrasound examinations, to monitor in detail the fetal growth and the characteristics of the joining.
The more information you have on the type and degree of union of the twins, the better the preparation for surgery (if, of course, the two Siamese twins are born alive and survive even after giving birth).