Ischiopagus
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Ischiopagi comes from the Greek word ''ischio-'' meaning hip (ilium) and ''-pagus'' meaning fixed or united. It is the
medical Medicine is the science and practice of caring for a patient, managing the diagnosis, prognosis, prevention, treatment, palliation of their injury or disease, and promoting their health. Medicine encompasses a variety of health care practic ...
term used for
conjoined twins Conjoined twins – sometimes popularly referred to as Siamese twins – are twins joined ''in utero''. A very rare phenomenon, the occurrence is estimated to range from 1 in 49,000 births to 1 in 189,000 births, with a somewhat higher incidence ...
(Class V) who are united at the
pelvis The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton). The ...
. The twins are classically joined with the vertebral
axis An axis (plural ''axes'') is an imaginary line around which an object rotates or is symmetrical. Axis may also refer to: Mathematics * Axis of rotation: see rotation around a fixed axis *Axis (mathematics), a designator for a Cartesian-coordinate ...
at 180°. However, the most frequent cases usually structures the ischiopagus twins with two separate spines forming a lateral angle smaller than 90°. The conjoined twins usually have four arms; two, three or four legs; and typically one external genitalia and anus. It is mostly confused with
pygopagus Conjoined twins – sometimes popularly referred to as Siamese twins – are twins joined ''in utero''. A very rare phenomenon, the occurrence is estimated to range from 1 in 49,000 births to 1 in 189,000 births, with a somewhat higher incidence ...
where the twins are joined dorsally at the buttocks facing away from each other, whereas ischiopagus twins are joined ventrally and caudally at the
sacrum The sacrum (plural: ''sacra'' or ''sacrums''), in human anatomy, is a large, triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30. The sacrum situates at the upper, back part ...
and
coccyx The coccyx ( : coccyges or coccyxes), commonly referred to as the tailbone, is the final segment of the vertebral column in all apes, and analogous structures in certain other mammals such as horses. In tailless primates (e.g. humans and othe ...
. Parapagus is also similar to ischiopagus; however, parapagus twins are joined side-by-side whereas ischiopagus twins typically have spines connected at a 180° angle, facing away from one another.


Classification

Ischiopagus Dipus: This is the rarest variety with the twins sharing two legs with no lower extremities on one side. Ischiopagus Tripus: This is the most common variety. These twins share three legs, the third leg is often two fused legs, or is non-functioning. The twins also usually share only one set of external
genitalia A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction. The reproductive organs together constitute the reproductive system. In animals, the testis in the male, and the ovary in the female, a ...
. Ischiopagus Tetrapus/Quadripus: This variety has the twins at a symmetrical continuous longitudinal axis with their area of union not broken anteriorly. The axes extends in a straight line but in opposite directions. The lower extremities are oriented at right angles to the axes of the
thorax The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the cre ...
and the adjacent limbs near the union of the
ischium The ischium () form ...
belong to the opposite twin.


Embryology

During embryonic development, twins can form from the splitting of a single embryo (
monozygotic Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two ...
) which forms
identical twins Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two em ...
or the twins can arise from separate
oocytes An oocyte (, ), oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female ge ...
in the same
menstrual cycle The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that make pregnancy possible. The ovarian cycle controls the production and release of eggs a ...
(
dizygotic Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two em ...
) which forms
fraternal twins Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two em ...
. Although the latter is more frequent, monozygotic is the reason conjoined twins can develop. In
monozygotic twinning Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two em ...
for conjoined twins such as ischiopagi, the twins form by the splitting of a bi-laminar embryonic disc after the formation of the inner cell masses. Thus, making the twins occupy the same
amnion The amnion is a membrane that closely covers the human and various other embryos when first formed. It fills with amniotic fluid, which causes the amnion to expand and become the amniotic sac that provides a protective environment for the develo ...
which can lead to a conjoining of the twins as a result of the twins not separating properly during the twinning process. Separation occurring between the seventh and thirteenth days should result in a
monochorionic Monochorionic twins are monozygotic (identical) twins that share the same placenta. If the placenta is shared by more than two twins (see multiple birth), these are monochorionic multiples. Monochorionic twins occur in 0.3% of all pregnancies. ...
,
monoamniotic Monoamniotic twins are identical or semi-identical twins that share the same amniotic sac within their mother's uterus. Monoamniotic twins are always Monochorionic twins, monochorionic and are usually termed Monoamniotic-Monochorionic ("MoMo" or ...
identical twins sharing a
yolk sac The yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac'' is far ...
. If separation of the twins occur in the later stages of development prior to the appearance of the
primitive streak The primitive streak is a structure that forms in the early embryo in amniotes. In amphibians the equivalent structure is the blastopore. During early embryonic development, the embryonic disc becomes oval shaped, and then pear-shaped with the ...
and axial orientation, then it can be predicted that
conjoined twins Conjoined twins – sometimes popularly referred to as Siamese twins – are twins joined ''in utero''. A very rare phenomenon, the occurrence is estimated to range from 1 in 49,000 births to 1 in 189,000 births, with a somewhat higher incidence ...
will develop. The origin of exactly what goes wrong to produce ischiopagus or any conjoined twin is a result by either incomplete fission or double overlapping inducing centers on the same germ disc. Various studies suggest that mechanical disturbances such as shaking of the
blastomeres In biology, a blastomere is a type of Cell (biology), cell produced by cleavage (embryo), cell division (cleavage) of the zygote after fertilization; blastomeres are an essential part of blastula formation, and blastocyst formation in mammals. Hum ...
, exposure of the embryo to cold or insufficient oxygen during the early process of cleavage,
grafting Grafting or graftage is a horticultural technique whereby tissues of plants are joined so as to continue their growth together. The upper part of the combined plant is called the scion () while the lower part is called the rootstock. The succ ...
organizer onto
gastrula Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of Cell (biology), cells), or in mammals the blastocyst is reorganized into a multilayered structure known as ...
or half a
gastrula Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of Cell (biology), cells), or in mammals the blastocyst is reorganized into a multilayered structure known as ...
together, or constricting the
blastula Blastulation is the stage in early animal embryonic development that produces the blastula. In mammalian development the blastula develops into the blastocyst with a differentiated inner cell mass and an outer trophectoderm. The blastula (from ...
or early
gastrula Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of Cell (biology), cells), or in mammals the blastocyst is reorganized into a multilayered structure known as ...
can cause the incomplete separation of
monozygotic twins Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two em ...
. However, studies have shown that these disturbances must happen at critical times in the pregnancy for the conjoined twins to develop.


Complications

Conjoined twins are at high risk to being
stillborn Stillbirth is typically defined as fetal death at or after 20 or 28 weeks of pregnancy, depending on the source. It results in a baby born without signs of life. A stillbirth can result in the feeling of guilt or grief in the mother. The term i ...
or dying shortly after birth. In some cases, a healthy twin and a
parasitic twin A parasitic twin, also known as an asymmetrical or unequal conjoined twin, is the result of the processes that also produce vanishing twins and conjoined twins, and may represent a continuum between the two. Parasitic twins occur when a twin embry ...
are born. The parasitic twin has no hope for survival and dies and is then surgically separated from its twin. Depending upon how the twins are attached and what is shared among them, complications can arise from surgically separating the live twin from the dead twin. In Ischiopagus cases, the children share a pelvic region along with the
gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organ (biology), organs of the digestive syste ...
and
genital A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction. The reproductive organs together constitute the reproductive system. In animals, the testis in the male, and the ovary in the female, a ...
region. Ischiopagus twins need reconstructive surgery for the genitals and gastrointestinal tract in order for normal bowel movements and reproductive possibilities in adulthood. For the Ischiopagus twins that both survive from birth, complications and risks also arise. Usually if both twins survive labor, one twin will be healthy and strong, while the other is malnourished and weak. Thus, surgery would have to be planned in advance to understand the best option and how to keep both children alive during surgery as well as afterwards.


Treatment

Separation is the only treatment for Ischiopagus. The rarity of the condition as well as the challenge it presents in separating the twins has been difficult to understand. In recent years, with advancing medical technology, physicians have been able to successfully separate Ischiopagus twins. However, it depends on the organs shared, how closely joined the twins are, and what risks could rise from separating the twins during surgery. Since Ischiopagus twins usually share a
gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organ (biology), organs of the digestive syste ...
and other organs in the pelvic region, it takes months of planning to decide whether or not separation of the twins outweighs the complications and risks associated with surgery and reconstruction of organs. Surgery to separate conjoined twins has allowed surgeons to be able to study the mechanisms of
embryogenesis An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...
as well as the physiological consequences of
parabiosis Parabiosis, meaning "living beside", is a laboratory technique to study physiology. It combines two living organisms which are joined surgically to develop a single, shared physiological system. Parabiosis is used in the study of areas such as ob ...
. Separating ischiopagus tripus conjoined twins usually leaves the twins with one leg each.


Prognosis

Depending upon what organs are shared among the twins and if they are surgically separable, usually only one of the twins makes it through the surgery or they both die due to complications either before or during the surgery. Now that successful surgery has been reported and more findings are becoming available due to research and pre-surgery evaluation, better surgery techniques and procedures will be available in the future to help increase the survival rate of ischiopagus twins as well as other conjoined twins.


Epidemiology

Ischiopagus is a rare anomaly occurring in about 1 in every 100,000 live births and occurring in 1 out of 10 conjoined twin births. Most Ischiopagus cases are common in the areas of India and Africa. Of the varieties of Ischiopagus twins, Ischiopagus Tetrapus is more prevalent, happening in 68.75% of all Ischiopagus cases. Ischiopagus Tripus occurs in 31.25% of cases while Ischiopagus Dipus occurs in only 6.25% of all Ischiopagus cases.


References

{{Situs inversus and conjoined twins Conjoined twins Animal developmental biology