Laboratory Studies
Chromosomal studies are inconclusive. An abnormal X-chromosome inactivation has been proposed, but this has not been proved.
Amniocentesis with an estimation of the lecithin-sphingomyelin ratio is performed to assess fetal lung maturity and to determine the optimal time for a cesarean delivery.
Imaging Studies
Antenatal ultrasonography (US) can reveal conjoined twinning as early as 8 weeks' gestation. [30] Conversely, twins with extreme fusion may be mistaken for a singleton. The twin fetuses do not move apart with fetal movement. Polyhydramnios is frequent (75%). A monoamniotic cavity is present, and more than three umbilical vessels may be observed. Fusion sites include the following:
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Thorax (thoracopagus)
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Abdomen (omphalopagus)
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Pelvis (ischiopagus)
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Sacrum (pygopagus)
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Skull (craniopagus)
Extensive zones of fusion may be named by applying the prefix di- (meaning two), followed by the portion of the twins that is unfused. Examples include dicephalus (two heads on one body) and dipygus (double buttocks, with a single head, single torso, separate pelves, and four legs). Serial scans may be required to monitor for hydrops.
Antenatal echocardiography has better yield than postnatal echocardiography in thoracopagus twins, because surface scanning may be difficult.
Magnetic resonance imaging (MRI) of the brain is performed, along with magnetic resonance angiography (MRA) and magnetic resonance venography (MRV) to delineate structures and blood supply in the craniopagus variety.
Radionuclide angiography is performed to calculate the extent of cross-circulation. Selective angiography is usually not necessary in twins with a shared liver.
US allows for a complete anatomic examination and a thorough search for associated lethal malformations. A detailed ultrasound examination to exclude the possibility of conjoined twins is mandatory in all multiple pregnancies. Two-dimensional (2D) US is instrumental in diagnosing conjoined twins antenatally, but precise classification is difficult because of three-dimensional (3D) structures. 3D US has shown promise in improving the visualization of complex anatomic spatial relations.
Abdominal US is performed to determine how many gallbladders are present (ie, one or two) and to determine the polarity of the liver and pancreas if these organs are also conjoined. Often, however, this determination may not be precise.
Advantages of US include the following:
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Images other structures (eg, aorta, pancreas, liver)
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Identifies complications (eg, stenosis, obstruction)
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Can be rapidly performed at the bedside
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Does not involve radiation (important in pregnancy)
Disadvantages of US include the following:
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Depends on the type and extent of fusion, as well as on the operator's abilities
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Cannot image the ductal system proximal to the common bile duct
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Has less sensitivity for the site and extent of duplication or fusion as compared with computed tomography (CT) or MRI
Fetal MRI can identify shared anatomy of twins with precise detail. [30] However, this test is not 100% accurate.
Although CT has been used in some studies, [31] its yield for complex anatomy is lower than that of MRI. CT may be helpful in cases of shared bony pelvis and shared pelvic perineal muscles.
Contrast studies are performed to evaluate the extent of gastrointestinal (see the image below), genitourinary, and reproductive system fusions.
Diisopropyl iminodiacetic acid (DISIDA) scanning is a nuclear medicine study performed to visualize the biliary tract. A technetium-labeled analogue of DISIDA is administered intravenously (IV) and is secreted by hepatocytes into bile, enabling visualization of the gallbladder and biliary tree in 30 minutes.
Other Tests
On electrocardiography (ECG), a single QRS indicates that cardiac separation is not possible. However, the presence of two separate patterns does not guarantee a successful separation.
Electroencephalography (EEG) may be performed to evaluate baseline brain activity in craniopagus twins.
Procedures
Cardiac catheterization is performed to determine the nature of complex cardiac anomalies. An accurate estimation of all major inflow and outflow vessels should be made.
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This set of conjoined twins was a stillbirth. Prenatal ultrasonography failed to reveal the second head. An emergent cesarean section was performed after failure to progress.
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This is an example of double-headed (bicephalus) conjoined twins, a rare occurrence.
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Even though conjoined twinning is more common in female fetuses (75% of cases), most stillborn conjoined twins are male. A single penis and 2 testes were present in this case.
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This is a much more rare occurrence termed heteropagus twinning. One twin is not fully formed (parasitic twin) and is dependent on the well-formed one. A malformed head, upper limb, and large ears are present in the parasitic twin. Three legs (tripus) are present.
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Ethical issues arose in this set. The parents refused separation on the basis that the nonviable twin was still breathing. The wishes of the parents were respected. This set of twins was lost to follow-up.
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Conjoined twins unified at the thorax and abdomen.
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Another view of the same set of twins as shown in the previous image.
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A superior radiographic view of the same set of twins as shown in the previous 2 images. The twins have 2 hearts that are not conjoined, making this a possible operation.
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Esophagram showing union at the level of stomach.
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Ischiopagus tetrapus twins.
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Ischiopagus tripus twins.
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Thoracoomphalopagus twins.
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Pygopagus twins.
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Ischiopagus tetrapus twins.
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Craniopagus twins.
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Craniopagus twins.
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Rachipagus twins.
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Parapagus twins.
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Pygopagus twins.
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Cephalothoracopagus twins.
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Ischiopagus tetrapus twins.
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Omphalopagus twins.
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Thoracopagus twins.
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Parasitic twinning.
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Union in a transverse plane in the lower body area, so that the twins face each other.
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Union in a transverse plane in the lower body area, so that the twins face each other.
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The term conjoined twinning refers to an incomplete splitting of monozygotic twins after 12 days of embryogenesis. Image created by Samantha Cloutier and Aaron Cloutier.
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An example of parasitic twin, in which the head is not visible.
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A nonoperated case of parasitic twins, presenting late.
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The parasitic twin (sacrificed) after separation.
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Multiple limbs attached to the thorax.
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Omphalopagus twins.
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Omphalopagus twins.
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Craniopagus twins.
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A radiograph of the skull, showing the conjoined heads in the case in previous two images.
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An MRI or CT may show the brain structure, while a magnetic resonance angiography can show the extent of vascular connections.
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A case of thoracoomphalopagus twins, after separation.
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Thoracoomphalopagus twins.
