Conjoined twins refers to twins that are physically fused in utero and consequently at birth. The first mention of conjoined twins historically comes from the neolithic period. The condition is proposed to result from either fission or fusion. It happens when a monozygotic twin pregnancy cleaves more than 13 days after fertilization. Conjoined twins are monochorionic (one placenta) and monoamniotic (one amniotic sac). Conjoined twins exist in a female to male ratio of 3 to 1. This type of pregnancy is a complicated phenomenon that requires an interprofessional approach to manage it effectively.
The most commonly believed etiology of conjoined twinning is either partial fission or secondary fusion. Both of these processes can be used to explain the embryological and anatomical findings observed in conjoined twins. However, the fission and fusion theories do not unequivocally account for every possible conjunction and cannot be applied to the full spectrum of findings in conjoined twins. Conjoined twins can be broadly categorized into non-dorsally conjoined twins (which includes ventral, lateral, and caudal conjunction) and dorsally conjoined twins.
Non-dorsal conjunction may have a varied presentation but typically demonstrates a single shared umbilical cord along with the sharing of organs. The dorsal conjunction presents with two umbilical cords and mostly separate internal organs. It is proposed that non-dorsal conjunction may be initiated by duplication of the axial primordia, which leads to a change in the axial orientation and/or interaction aplasia. Dorsal conjunction may occur because of secondary fusion of initially separate monozygotic twins.
The incidence of conjoined twins is one per 50,000 to 200,000 births. The rate of stillbirth is high and is estimated to be around 60 percent. There is no increased risk in future pregnancies. Accurate epidemiology across multiple countries and ethnicities cannot be calculated because the condition is underreported. Conjoined twins are associated with a higher predominance of the female sex. The female to male ratio is 3 to 1.
Conjoined twinning is the result of cleavage or axis duplication that occurs after day 13 of fertilization. They are described according to the site of fusion.
The following are the types of conjoined twins along with their frequency:
Less commonly observed conjoined twins include:
The site of fusion and organs involved are a primary consideration for separation surgery. Typically, 25% of live births live long enough to be candidates for surgery.
A conjoined twin pregnancy presents the same way as other twin pregnancies, with a uterine size greater than expected for gestational age. Due to advances in imaging techniques, most cases these days are recognized early on prenatal ultrasonographic imaging.
First-trimester ultrasonography remains the best modality of diagnosis early in the pregnancy. Prenatal magnetic resonance imaging can also help with identifying the type of conjunction, embryological malformations, and tissue characterization. Prenatal imaging should be used to inform the postnatal magnetic resonance imaging. Modern methods like 3D printing may aid with surgical pre-planning and subsequent separation if applicable.
The conjoined twins that survive until birth can be broadly categorized into two categories, those who can be surgically separated and those who cannot. The determination of which set of conjoined twins can be separated is an interprofessional effort employing imaging modalities and multiple reviews. The conjoined twins that extensively share vital organs can generally not be separated as separation might lead to the death of one or both twins. This presents some ethical dilemmas in the surgical management of conjoined twins, but the overall decision still lies with the parents. Some cases might need the involvement of the hospital ethics committee.
Surgical management requires identification of the anatomic relationship of the conjunction between the twins. Three-dimensional modeling and simulations help the team in making surgical decisions. The use of tissue expansion preoperatively is very helpful due to issues with the closure of the abdominal wall. At least 1 simulation of the separation event is recommended preoperatively as that helps with the outcome of conjoined twins.
During medical management, some pharmacokinetic considerations must be kept in mind as the shared circulation of the conjoined twins may have a variable volume of distribution and clearance.
Conjoined twins are a rare condition that is uniquely identifiable on prenatal ultrasonography. However, the differential diagnosis might include conditions with aberrant ultrasonographic findings such as cystic hygroma, teratoma, and lymphangioma.
Conjoined twins generally have a poor prognosis. The total survival rate is 7.5%. Only 60% of the surgically separated cases survive. Antenatal imaging, postnatal surgery if applicable, use of tissue expansion in surgery, and cadaveric transplant for vital organs that are shared between the twins, might lead to a better prognosis.
The intrauterine demise of the twins is likely given the complicated nature of the pregnancy. Postnatally, complications related to the complex anatomy of the twins may lead to death. If surgery is an option, complications related to separation include organ failure, skin defects, surgical infection, bleeding, injury to internal organs and/or vasculature, and failure to complete the procedure.
Conjoined twins present a complex psychosocial challenge. Surgical separation is associated with ethical dilemmas, especially in cases of an unequal separation. Furthermore, the conjoined twins have difficulty in establishing individuality and can have a twinning reaction. The involvement of social workers is important for the management of conjoined twins. Patient education regarding the challenges of conjoined twins and the poor prognosis can help the family in taking care of the conjoined twins.
The approach to the management of conjoined twins is complicated. Conjoined twins can be separated into the following groups with regards to prognosis:
This type of classification may be preferred over anatomic classification because it directs the management specifically. It is also helpful because surgical separation, if applicable, should be performed in infancy. [Level V]
There is also an ethical dilemma associated with the management of conjoined twins. Ethical considerations should be kept in mind as they relate to prognosis. The following clinical /ethical scenarios can be observed:
Keeping these considerations in mind, the interprofessional medical team can make decisions regarding surgical treatment and palliative care. Some consider sacrificial separation (performed when it is known preoperatively that surgery will be lethal for one of the twins) unethical. The sharing of organs, in particular, the heart is an important consideration because it determines whether surgical treatment is possible and its prognostic outlook. Prenatal counseling of the family plays an important role because of the complicated nature of management and prognosis.
Timing and mode of delivery should be individualized, taking into consideration any antenatal complications and the altered anatomy based on prenatal imaging. Cesarean delivery is often required, as vaginal delivery may not be possible depending on where the twins are joined. An interprofessional team is important to address the challenges of a complicated pregnancy and birth process.  The pregnancy should be managed in conjunction with a maternal-fetal medicine (MFM) specialist and should be delivered at a tertiary care center equipped to manage the twins postnatally. Pregnancy termination is an option and should be offered in accordance with state laws.
Nutrition support is very important in the management of conjoined twins during the neonatal period. Medications should be administrated with caution and increased monitoring because of the variation of pharmacokinetic variables.
Identification of the anatomical structures is accomplished with postnatal imaging, preferably MRI, keeping prenatal imaging in mind. Three-dimensional modeling and simulation-based clinical rehearsal are important in preoperative preparation. The surgical survival rate is around 66.7%. [Level V]
Conjoined twins should be cared for at specialized centers with MFM, neonatal intensive care unit (NICU) and pediatric subspecialty availability. [Level V] Nursing care (maternal) considerations include privacy, grief counseling, if necessary, and providing emotional support to the patient and family.
The management of conjoined twins requires the intimate cooperation of an interprofessional team.
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