Pictures of Different Types of Conjoined Twins
Introduction
Conjoined twins are rare, but the exact prevalence is unknown. The estimated prevalence in the literature varies widely, from 1:50,000 to 1:200,000 (,1–,4). An increased prevalence is observed in parts of Southeast Asia and Africa, with reported occurrence ranging from 1:14,000 to 1:25,000 (,5). Forty percent to 60% of conjoined twins are stillborn, and almost 35% of live births do not survive 24 hours (,4). There is a female predominance on the order of 3:1 (,4). The twins are monozygotic, monoamniotic, and monochorionic; there is failure of separation of the embryonic plate between 13 and 17 days gestation (,3,,6–,8). Conjoined twins are classified according to the most prominent site of connection (,Table 1): the thorax (thoracopagus), abdomen (omphalopagus), sacrum (pygopagus), pelvis (ischiopagus), skull (craniopagus), face (cephalopagus), or back (rachipagus) (,Fig 1).
In this article, we demonstrate the preoperative appearances of conjoined twins with different sites of connection using various imaging modalities. Our experience has been gained in the clinical and surgical management of 16 pairs of conjoined twins (32 patients) since 1985; preoperative imaging was performed in nine pairs (18 patients) (,Table 2). Specific topics discussed are prenatal assessment, surgical considerations, prognosis, technical factors related to imaging, imaging considerations, and associated anomalies. The types of conjoined twins are discussed separately, and the imaging algorithms that work best to demonstrate their complex anatomy and prepare for surgical separation are described. The algorithms should be used as a basic guideline, with further investigations added as necessary to evaluate individual cases.
Prenatal Assessment
The antenatal diagnosis of conjoined twins can be made with ultrasonography (US) as early as 12 weeks gestation and is important for optimal obstetric management. More accurate evaluation of visceral conjunction is possible from 20 weeks gestation and should include fetal echocardiography (,2,,3,,10). The degree of cardiac fusion and the severity of associated cardiac anomalies determine the postnatal viability of the twins and the likelihood of successful separation. Conjoined hearts are easier to examine in utero because the amniotic fluid acts as a buffer during US. After birth, the lungs inflate with air and thoracic fusion prevents optimal access; hence, limited information may be obtained (,3,,10).
Other US features that suggest the antenatal diagnosis of conjoined twins include constant relative positions of the fetuses over time, with heads and other body parts persistently at the same level; inseparable body and skin contours; fetuses facing each other with hyperflexion of the cervical spines; fewer limbs than expected; shared organs; and a single umbilical cord with more than three vessels (,3,,11).
Accurate antenatal assessment allows the parents to be counseled in depth as to the likely outcome of the pregnancy and the chances of postnatal separation and survival. Most conjoined twins are delivered at 36–38 weeks gestation by elective cesarean section, often at centers where appropriate obstetric and pediatric surgical facilities are readily available (,3,,11).
Surgical Considerations
Our institution is a specialist children's hospital with no obstetric facilities. The twins are usually delivered at a nearby hospital and then transferred for evaluation as to potential for surgical separation. The time of transfer depends on the clinical status and ranges from immediately after delivery to several months of age. The postnatal management of conjoined twins is determined by the effect of shared viscera on stability and long-term viability. Therapeutic options include nonsurgical management, emergency separation, or planned separation. Of the 16 pairs of conjoined twins managed at our institution, four pairs were treated nonsurgically and all died; seven pairs underwent emergency separation with survival of five of 14 twins (36%); and five pairs underwent planned separation with survival of eight of 10 twins (80%).
Twins in whom the prognosis is poor for both infants (because of complex cardiac connections or other life-shortening anomalies) are treated conservatively with supportive measures and no attempt at surgical separation. Emergency separation is usually performed in the neonatal period. The indications for emergency surgery include the following: one twin is stillborn or has anomalies incompatible with survival; damage to the connecting bridge; ruptured omphalocele or other life-threatening event; and congenital anomalies that are surgically correctable but would be fatal if not treated (,12). In these cases, there may be ethical considerations, as the survival of both twins is unlikely and one twin may have to be sacrificed to save the other (,12,,13). In emergency situations, the twins are in unstable condition, and plain radiography, echocardiography, and US are usually performed as bedside examinations in the neonatal intensive care unit. The ideal is elective surgery at 6–12 months of age to allow time for growth, tissue expansion, and accurate imaging demonstration of the anatomic union and associated anomalies to aid surgical planning (,2). The goal of separation is the survival of one or both twins with a reasonable quality of life (,2,,14).
The surgical procedure performed depends on the site and complexity of the conjunction and the types of organs shared. The surgical separation is developed on an individual basis and requires precision planning and a multidisciplinary approach. During the preoperative assessment period, the surgeons hold regular, detailed, multidisciplinary meetings in which imaging plays a vital role, and the findings determine whether the specialist pediatric surgeons require assistance from experts in cardiothoracic, orthopedic, or urologic surgery. The meetings also involve anesthetists, intensive care and surgical theater nurses, therapists, surgical theater staff, and management and public relations personnel (,2,,13,,15,,16). During the preoperative planning, two separate surgical teams are established, each with responsibility for one of the twins; an experienced general pediatric surgeon usually takes charge of overall coordination (,2,,15,,16).
Accurate determination of anatomy and vascular supply is important in planning the separation of the twins (,2,,15,,17). Evaluation of vascular shunts and cross-circulation is important for anesthetic management and to allow ligation and separation of these vessels early during surgery to prevent hypovolemic shock from loss of blood volume through these shared vessels (,5). Once all organ systems have been evaluated and vascular territories have been established, the multidisciplinary team reviews the information to plan separation. Decisions are made about how organs will be distributed between the twins, the order of organ separation, anesthesia, monitoring of vital signs, and wound closure, including plans for preoperative tissue expansion and postoperative care (,2,,5,,13). Authors recommend use of diagrams, three-dimensional organ models, and complete rehearsals of the separation procedure to ensure that each member of the team is familiar with his or her role and the overall plan of the surgery so that the actual operation proceeds as smoothly as possible (,5,,12,,13,,16).
The twins are anesthetized simultaneously by two separate teams, and individual specialist surgeons perform their assigned surgical procedures under the coordination of the surgeon in charge. Once the twins have been physically separated, one team moves into an adjoining surgical theater and the teams work independently to complete reconstructive surgery, hemostasis, and wound closure (,15).
The twins must be closely monitored during surgery, as blood loss may cause rapid deterioration and instability. There should be adequate venous access, and continuous central venous and arterial pressure monitoring should be used together with pulse oximetry, capnography, and regular blood gas analysis. The electrocardiogram and urine output are also closely observed. After surgery, ventilation and sedation are continued, as is intense monitoring with special attention to fluid and electrolyte balance and signs of sepsis (,2,,5,,18).
Prognosis
The outcome of surgery and the long-term survival of separated twins depend on numerous factors. No overall mortality and morbidity data are available, but rates remain high despite advances in imaging, anesthetic and surgical techniques, and aspects of medical and surgical management. Results depend on the site and complexity of the conjunction, the extent of shared organs, and the severity of associated anomalies, as well as the general medical condition of the twins at the time of separation (,12,,16). There is a higher mortality rate for twins separated in the neonatal period, and authors with experience usually recommend delaying surgery until the twins are at least 3 months old, although earlier surgery may be unavoidable with twins in unstable condition (,12).
The outcome is better in twins who do not share vital organs such as the heart or brain, and the best results are reported for omphalopagus twins and pygopagus twins without neural involvement (,16). Cardiac conjunction and associated cardiovascular anomalies in thoracopagus twins preclude surgery in 75% of cases; when attempts have been made to salvage one twin at the expense of the other, the results have been poor (,12). The success of surgery in ischiopagus and parapagus twins depends largely on the extent of union. Separation and reconstruction of the bony pelvis, urogenital tract, and lower gastrointestinal tract are necessary (,16), and some twins may also share the liver and pericardium. After separation, there are often large areas devoid of skin; despite use of preoperative tissue expansion, primary wound closure is not always possible, increasing the risk of postoperative sepsis in these individuals (,2). There is significant morbidity and long-term disability requiring further reconstructive surgery at a later stage, especially as many of these twins will have only one leg. In craniopagus twins, sharing of neural tissue often precludes surgery; in other cases, success depends on the extent of shared dural venous sinuses, as division and reconstruction of these are difficult with a high risk of hemorrhage (,16). It is not possible to separate cephalopagus twins. Successful separation of rachipagus twins depends on whether there is spinal cord involvement.
Technical Factors Related to Imaging
Sedation and Anesthesia
The responses of conjoined twins to medication are unpredictable, as vascular shunts and cross-circulation cause mixing of their blood. The dosage requirements, cardiovascular effects, and elimination half-lives expected in normal infants do not apply to the shared systems of conjoined twins; responses often differ in each twin. Drugs administered to one twin may have an unexpected effect on the other, especially for intravenous administration when there is circulatory admixing. In general, sedation and premedication are not considered particularly safe in these infants (,2,,5,,18).
In neonates and premature infants, it is often possible to undertake imaging procedures without sedation or general anesthesia. The twins are fed immediately before the procedure and are immobilized with a combination of swaddling, vacuum beanbags, and other devices. Older infants are usually examined under general anesthesia administered separately by two experienced anesthetists, each with sole responsibility for one twin. The twins undergo intubation simultaneously.
Drugs are usually administered with intravenous injections, as absorption rates after intramuscular injection are varied and unpredictable (,19). Recommended intravenous doses of premedications, anesthetic agents, and adjuvants for the combined body weight of the twins are usually halved and then divided into two equal doses to be administered to each twin. Reduced incremental doses are titrated against response and help minimize the dangers of compounding drug effects in one twin (,2,,5).
It is possible to estimate the amount of vascular shunting, cross-circulation, and exchange of blood volume between the twins by using radio-nuclide studies. Tracers described in the literature include technetium-99m sulfur colloid, radiolabeled albumin or red blood cells, and inhaled 15O2 (,5,,20). Evaluation of circulatory mixing is useful to anesthetists to help calculate drug dosage and fluid replacement during surgery (,18), especially when one twin is larger or appears to thrive at the expense of the other (,13).
Contrast Media
Intravenous contrast material for imaging is administered with doses calculated per kilogram of combined weight of the twins. A water-soluble, nonionic, iodine-based contrast medium is used for computed tomographic (CT) and vascular studies. We use a strength of 240 mg of iodine per milliliter and give 1 mL/kg to twins with a combined weight of less than 10 kg; 2 mL/kg can be given for heavier twins, especially if there are four functioning kidneys. Contrast material–enhanced CT and vascular studies are often performed with separate injections into each twin; these studies may be performed on different days or after a sufficient delay to allow contrast material elimination. Lower doses of contrast material are used in these situations. Other authors recommend a maximum contrast material load of 3 mL/kg of combined weight for CT and angiographic studies (,19). Gadolinium-based contrast medium injections are used in magnetic resonance (MR) imaging at a dose of 0.2 mL/kg (0.1 mmol/kg).
Imaging Considerations
All conjoined twins, irrespective of the site of connection, should undergo echocardiography, as there is a higher frequency of cardiac abnormalities in all forms of conjoined twinning (,13). On arrival at our institution, twins who have not undergone imaging are initially assessed with a combination of US, echocardiography, and plain radiography. These techniques can be easily performed in the intensive care unit and do not require sedation or anesthesia. They are used to provide an overview of conjoined anatomy and identify potential problems such as hydronephrosis or bowel obstruction that may require interim procedures. Twins in stable condition in whom detailed imaging is possible undergo a full range of investigations, which are mainly determined by the site of conjunction. Multiplanar MR imaging techniques provide the best overall anatomic detail, but CT is more useful if bone detail is required. Appropriate system-based imaging is described in the relevant sections (,Table 3, ,4).
Thoracopagus
Thoracopagus twins are united face to face from the upper thorax to the umbilicus with a common sternum, diaphragm, and upper abdominal wall (,Fig 2). Ninety percent of such twins have a common pericardial sac, and there is always a degree of cardiac fusion (, , ,Fig 3); in 75% of cases, the severity of cardiac fusion precludes successful surgical separation (,3,,9,,17). The severity of the cardiac abnormality determines the prognosis, survival, and feasibility of separation of these twins and requires accurate assessment. Such assessment is best accomplished with antenatal fetal echocardiography (,3). Echocardiography should also be used as the initial postnatal investigation to establish the degree of cardiac conjunction and associated structural heart abnormalities. Twins with complex cardiac abnormalities should be further investigated by using cardiac angiography in both infants (,13). Cardiac MR imaging may also be used according to local expertise.
The liver is invariably fused, and 25% of thoracopagus twins share a biliary system (,2,,17). Initial liver assessment can be performed with US. However, in twins joined anteriorly, there is limited probe access; when viewed from the side, the conjoined liver is oriented in an oblique plane to the axis of the probe (,13). A better appreciation of liver anatomy is gained from multiplanar techniques, ideally MR imaging (,Fig 4), but sagittal reconstruction of contrast-enhanced CT will provide similar information. These investigations also yield important information regarding hepatic vessels, the intra- and extrahepatic biliary tree, the number of gallbladders, and whether the pancreases are conjoined or separate. Demonstration of separate hepatic venous drainage into the inferior vena cava and right atrium of each twin is especially important, as absent or anomalous hepatic venous drainage in one twin is incompatible with survival after surgery (,13). It is important to evaluate bile excretion by using dynamic biliary scintigraphy with Tc-99m HIDA. The tracer is injected into each twin separately, at least 24 hours apart. In normal circumstances (ie, separate biliary systems), the tracer appears first in the system of the twin injected and in the other a few minutes later. The demonstration of two gallbladders and independent excretion into separate loops of proximal small intestine indicates separate extrahepatic biliary systems. The timing of the appearance of the tracer in the noninjected twin's liver can also provide information on the degree of cross-circulation. Rapid transfer of activity from one circulation to the other suggests considerable overlap of hepatic vascular supply (,13,,17,,20,,21). In difficult cases, intraoperative cholangiography may be required; even with comprehensive imaging, complex biliary anatomy may not be resolved before surgery (,17). MR cholangiopancreatography may have a role in the future.
Contrast material studies of the upper gastrointestinal tract are also important, as 50% of thoracopagus twins have a common small intestine, which usually joins in the duodenal region and separates in the distal ileum (, , , ,Fig 5). The pelvises, large intestines, and urinary tracts are usually separate. These twins have a full complement of limbs (,2,,9,,11,,17).
Omphalopagus
Omphalopagus twins are joined ventrally in the umbilical region, often including the lower thorax (,Fig 6). The heart is never involved, although the pericardium may be shared (,9).
Liver fusion occurs in approximately 80% of cases (,2). As there is no mixing of blood in the cardiac chambers, the liver can be well assessed by using CT with intravenous injection of contrast material into one twin (, , , ,Fig 7). The liver parenchyma belonging to and receiving the hepatic arterial supply from the injected twin enhances, and the site of fusion is marked by a series of irregular lobules, which can also be recognized during surgical separation (,17) (,Fig 8). Gadolinium-enhanced MR imaging can also be used in this context. As described in the section on thoracopagus twins, biliary scintigraphy can be helpful in determining biliary drainage (,Fig 9).
The stomachs and proximal small intestines are usually separate (,Fig 10); however, in 33% of cases, the small intestines join at the level of the Meckel diverticulum in the distal ileum (,2). The shared terminal ileum and proximal colon often have a dual blood supply, and vascular studies are helpful to determine the distribution of intestine between the twins at separation (,2). The colon separates distally, and each twin has a rectum. There are four arms and four legs with no pelvic or urinary tract union.
Pygopagus
Pygopagus twins are joined dorsally, facing away from each other and sharing the sacrococcygeal and perineal regions (,9). Fusion of sacral vertebrae frequently occurs, but the spinal cords usually remain separate. Occasionally, there is a degree of neural fusion, and electromyographic studies are useful to determine the innervation of the lower limbs and pelvic floor musculature. Usually, there is a single anus with one or two rectums, the remainder of the intestine being separate. Fifteen percent of these twins share a genitourinary system with a single bladder and urethra. The upper bodies are not fused, and there are four arms and four legs (,2,,9,,11,,13).
If the results of clinical examination or electromyography suggest spinal cord fusion, MR imaging studies are important to assess the feasibility of separation. Angiography is also recommended, as the pelvic vessels anastomose freely (,2,,11,,13).
Ischiopagus
Ischiopagus twins are fused from the umbilicus to a large conjoined pelvis (,Fig 11). The spinal columns are usually separate. They may lie face to face or end to end with the vertebral columns in a straight line (,9,,22). The components of the pelvis vary; usually, there are two sacra and one or two symphyses pubis (, , ,Fig 12). The twins are termed tetrapus (four), tripus (three), or bipus (two) according to the number of legs attached to the conjoined pelvis. Tetrapus twins are the most common (,23).
Pelvic conjunction gives rise to complex anatomy requiring thorough preoperative evaluation, especially from a urologic and orthopedic point of view (,15,,23). Tripus twins are of particular interest to an orthopedic surgeon. These twins each possess one leg, and a deformed tripus leg arises from the conjoined pelvis posteriorly. The bony anatomy is well demonstrated by using CT with bone windows; three-dimensional reconstruction may be helpful in planning osteotomies for pelvic ring closure after separation (,19,,23). Arteriography to determine the vascularization of the shared limb is important for surgical planning (,13,,19, ,21,,23).
Varied urogenital anomalies accompany pelvic fusion. The imaging investigations are comprehensive, and the findings are complex and variable. The twins may have four normal kidneys or varying degrees of fusion and ectopia. One or two bladders may be present; if there are two bladders, they can be collateral, lying side by side (,Fig 13), or sagittal in the midline with one bladder draining into the other (,13,,15). Initial assessment of the urogenital tract is often performed with US, which is used to establish the number of kidneys and bladders and exclude potential problems such as hydronephrosis or a urogenital sinus (,13). CT is usually performed to evaluate the bony pelvis, and this examination also provides anatomic detail of the renal tract. More complex anomalies such as a urogenital sinus or cloaca are better evaluated by using the different sequences and multiplanar capability of MR imaging. The ureters often cross to insert into the contralateral bladder and require rerouting during surgery (,15). Intravenous urography is usually required to adequately demonstrate the path of the ureters and determine the relationship of the two bladders. There may be partial duplication of the urethra, but there is usually a single external urethral orifice. Visualization of the urethras may require cystoscopy or micturating cystourethrography; the latter is also used to exclude vesicoureteral reflux (,2,,13,,15,,19). Renal anomalies such as duplex systems, renal dysplasia, pelviureteral junction obstruction, and vesicoureteral junction obstruction are frequently present in conjoined twins (,15) and should be assessed with appropriate anatomic imaging and functional isotope renography. It is important to establish the vascular supply and venous drainage of any ectopic kidneys.
Most conjoined twins are female, and US and MR imaging are used to evaluate the genital tract and establish whether each twin has a separate uterus, vagina, cervix, and ovaries or whether the structures are shared or form a urogenital sinus (,13). In male twins, it is important to determine the status of the phallus and the number and location of testes (,13). When surgical separation of a single shared set of external male genitalia is not possible, one of the twins will not receive any genitalia or gonads and may even undergo a change of gender (,13,,21).
Upper abdominal organs are generally separate in ischiopagus twins; if shared, they can be imaged with the techniques described in the sections on thoracopagus and omphalopagus twins. The lower gastrointestinal tract is often shared, and the anus is usually involved. Anal atresia and colovesical fistulas are common. Contrast material studies of the lower gastrointestinal tract are of value in these twins to determine distal bowel anatomy (,13,,21).
Complex vascular anatomy requiring arteriographic and venographic demonstration prior to surgery is found in association with a fused bony pelvis and shared pelvic organs (, , ,Fig 14). In bipus and tripus twins, separate studies are performed, with each aorta catheterized via the groin of the respective twin (,19). Selective catheterization of celiac, mesenteric, and pelvic vessels helps evaluate the blood supply to shared organs and may reveal a dual blood supply, especially to the intestine (,19,,21). Arteriography often reveals a large pelvic vessel connecting the two aortas in the form of a shunt from one twin to the other (, , ,Fig 15). There is usually a balancing shunt at a different level (eg, within the hepatic arterial or portal venous system) (,19,,21).
Craniopagus
Craniopagus twins may be joined at any part of the skull except the face or foramen magnum (,9,,24) (,Fig 16). The fusion is vertical and parietal in over 60% of cases, but frontal fusion, occipital fusion, and extensive temporoparieto-occipital fusion have been reported (,25). The twins often share the cranium, meninges, and dural venous sinuses. The brains commonly remain separate but may be connected by a bridge of neural tissue. The trunks are not joined, and there is a full complement of limbs (,9,,11,,25).
Plain radiography can be used for initial assessment; if only extracranial tissues are involved, no further imaging is required and separation will be readily achieved. Demonstration of skull vault fusion requires evaluation with CT for osseous detail and MR imaging to establish the degree of brain involvement. MR arteriography and MR venography are useful in evaluating the main cerebral circulation and dural venous sinuses; however, when there is complex neural involvement, selective cerebral angiography is required to demonstrate small vessels and arterial connections (,25). Successful separation will also depend on the extent to which intracranial structures are fused (,11).
Parapagus
Parapagus twins lie side to side with ventrolateral fusion (,Fig 17). They share the umbilicus, abdomen, and pelvis. The conjoined pelvis usually has a single symphysis pubis and one or two sacra (,9) (,Fig 18). The thorax may be involved. The twins are termed dithoracic if the thoraces are separate and fusion involves the abdomen and pelvis only. They are termed dicephalic if the heads are separate but the entire trunk is conjoined. There are two, three, or four arms and two or three legs. Parapagus is a fairly new term, and there is some confusion in the literature; cases once termed ischiopagus with thoracic or upper abdominal union and two or three lower limbs would probably now be described as parapagus (,9,,26).
Parapagus twins have anatomic abnormalities similar to those of all twin types previously described. The range of imaging investigations used in these twins is determined by the extent of fusion. Multiplanar MR imaging provides excellent anatomic definition and can be used to guide further imaging (, , , , , , ,Fig 19). Thoracic union will require cardiac assessment, and upper abdominal fusion necessitates evaluation of the liver, biliary system, and upper gastrointestinal tract. These twins invariably have a shared pelvis, and imaging studies mirror those used for ischiopagus twins. The lower gastrointestinal tract is shared, with a single colon and rectum (, , , ,Fig 20). Anal atresia with a colovesical fistula is commonly associated.
Complex urogenital abnormalities accompany pelvic fusion; these are often unique to each pair of twins but can be accurately defined with combined US, intravenous urography, cystography, and isotope studies (,15) (, , , ,Fig 21). Vascular anatomy is also complex. Aortography and selective celiac, mesenteric, renal, and pelvic arteriography can be helpful (,26) (, , ,Fig 22). MR imaging and CT allow best assessment of musculoskeletal and bony anatomy, respectively.
Cephalopagus
Cephalopagus twins are rare. They are fused from the vertex to the umbilicus. There are two faces on opposite sides of the conjoined head; one face is usually rudimentary. The heart and liver are often conjoined, but the lower abdomen and pelvis are separate. There are four arms and four legs (,9). It is not usually possible to separate these twins.
Rachipagus
Rachipagus twins are extremely rare. They are joined dorsally and face away from each other. The occiput may be involved, along with varying segments of the vertebral column. The fusion terminates above the sacrum (,9).
Associated Anomalies
Despite genetic identity, conjoined twins often have discordant anomalies (,17); these often occur in the twin on the right (,9). Levin et al (,27) surveyed 167 pairs of conjoined twins and found that the presence of laterality defects, especially reversal of cardiac situs, depended on the orientation of the conjoined twins. Such defects were common in thoracopagus and dicephalic parapagus twins but did not occur in craniopagus or ischiopagus twins. When laterality defects were present, they occurred in the twin on the right in 86% of parapagus twins and 71% of thoracopagus twins. Chick embryo models were used to postulate that this tendency is due to the orientation of the primitive streaks at gastrulation and the effect that this orientation has on signal secretion and nodal expression. Parapagus and thoracopagus twins arise from streaks that lie adjacent to one another, potentially allowing cross-signaling to occur with the development of laterality defects (,27).
In our cohort of nine pairs of twins (18 patients), anomalies included cardiac defects (n = 4), congenital diaphragmatic hernia (n = 1) (,Fig 23), anomalous pulmonary and hepatic venous drainage (n = 1), and bowel atresia (n = 5) (, , ,Fig 24); Meckel diverticulum was found in four pairs. Complex urologic abnormalities were present in all pairs with pelvic fusion, and other renal anomalies such as duplex systems, renal dysplasia, pelviureteral junction obstruction, and vesicoureteral junction obstruction were also found.
Associated orthopedic problems found in conjoined twins include congenital dislocation of the hip, clubfeet, vertical tali, and scoliosis (,23).
Conclusions
The separation of conjoined twins presents a challenge to pediatric surgeons. Interpreting the results of preoperative imaging poses a unique challenge to the radiologist.
An imaging strategy to accurately define anatomic fusion, vascular anomalies, and associated abnormalities is important for surgical planning and prognostic information. Accurate preoperative imaging aids in successful separation of conjoined twins (, , ,Fig 25).
The site of conjunction largely determines the imaging modalities used. Cardiac assessment is mandatory in cases of thoracic fusion. MR imaging provides excellent overall anatomic demonstration, and visceral conjunction and organ position are best assessed with this multiplanar technique. CT also provides good anatomic detail; separate intravenous contrast material injections in each twin help establish whether there is separate or crossover blood supply to solid organs, especially the liver. CT performed with bone algorithms and three-dimensional reconstruction is excellent for evaluation of bone fusion abnormalities.
MR imaging and CT can be used to guide other imaging investigations. A shared liver requires evaluation of hepatic anatomy, hepatic vascularization, and biliary drainage; the latter may be difficult to define before surgery (,17). Contrast material studies of the upper gastrointestinal tract are necessary in cases of thoracoabdominal fusion, and studies of the lower gastrointestinal tract are necessary in cases of pelvic union. Urologic investigations and invasive vascular studies may be required in cases of pelvic fusion or abnormal US, CT, or MR imaging findings. Angiography helps define specific vascular supply to organs, which is useful in determining the distribution of shared structures between the twins at surgical separation. Vascular studies are also important to evaluate crossover circulation to shared viscera and shunting of blood between the twins.
Certain imaging modalities and investigations are desirable in evaluating conjoined twins and can be determined according to the primary site of conjunction. However, each set of twins is unique; although generalizations can be made, the exact imaging performed must be tailored to the twins involved and the requirements of the pediatric surgeons conducting their separation.
Figure 1. Permission to reprint this figure electronically has expired. See print version.
Figure 2. Thoracopagus twins aged 2 months. Photograph shows ventral surface union from the midsternum to the umbilicus. (Reprinted, with permission, from reference ,17.) (Throughout the figures, twin A is on the right and twin B is on the left, unless otherwise noted.)
Figure 3a. Thoracopagus twins aged 1 day. (a) Lateral chest radiograph shows a common sternum, diaphragm, and upper abdominal wall. The heart shadows merge centrally. Separate stomachs are demonstrated with nasogastric tubes in situ (arrows). (b) Nonenhanced axial CT scan of the thorax shows cardiac conjunction.
Figure 3b. Thoracopagus twins aged 1 day. (a) Lateral chest radiograph shows a common sternum, diaphragm, and upper abdominal wall. The heart shadows merge centrally. Separate stomachs are demonstrated with nasogastric tubes in situ (arrows). (b) Nonenhanced axial CT scan of the thorax shows cardiac conjunction.
Figure 4. Thoracopagus twins aged 2 weeks. Sagittal T1-weighted spin-echo MR image (repetition time msec/echo time msec = 600/30) shows the conjoined liver straddling the midline (∗). Small-bowel loops are also seen crossing the conjoined bridge.
Figure 5a. Images from a contrast material study of the upper gastrointestinal tract performed in thoracopagus twins aged 2 months. (a) A small bolus of contrast material was injected via the nasogastric tube of twin B (short thin arrow) to outline the proximal duodenal loop belonging to this twin (arrowheads). More contrast material was injected into the stomach of twin A via a nasogastric tube (thick solid arrow) to outline the duodenal loop belonging to this twin (open arrows). The site of small-bowel conjunction in the proximal duodenum is demonstrated (long thin arrow). The curved arrows indicate a conjoined bowel loop. (b) The shared small-bowel loops remain central within the conjoined bridge. Separation usually occurs in the distal ileum, but this finding is not often demonstrated on follow-through images. (c) The large intestines are separate, as is generally the case. The cecum and appendix are seen in twin B, and the colon is opacified in twin A.
Figure 5b. Images from a contrast material study of the upper gastrointestinal tract performed in thoracopagus twins aged 2 months. (a) A small bolus of contrast material was injected via the nasogastric tube of twin B (short thin arrow) to outline the proximal duodenal loop belonging to this twin (arrowheads). More contrast material was injected into the stomach of twin A via a nasogastric tube (thick solid arrow) to outline the duodenal loop belonging to this twin (open arrows). The site of small-bowel conjunction in the proximal duodenum is demonstrated (long thin arrow). The curved arrows indicate a conjoined bowel loop. (b) The shared small-bowel loops remain central within the conjoined bridge. Separation usually occurs in the distal ileum, but this finding is not often demonstrated on follow-through images. (c) The large intestines are separate, as is generally the case. The cecum and appendix are seen in twin B, and the colon is opacified in twin A.
Figure 5c. Images from a contrast material study of the upper gastrointestinal tract performed in thoracopagus twins aged 2 months. (a) A small bolus of contrast material was injected via the nasogastric tube of twin B (short thin arrow) to outline the proximal duodenal loop belonging to this twin (arrowheads). More contrast material was injected into the stomach of twin A via a nasogastric tube (thick solid arrow) to outline the duodenal loop belonging to this twin (open arrows). The site of small-bowel conjunction in the proximal duodenum is demonstrated (long thin arrow). The curved arrows indicate a conjoined bowel loop. (b) The shared small-bowel loops remain central within the conjoined bridge. Separation usually occurs in the distal ileum, but this finding is not often demonstrated on follow-through images. (c) The large intestines are separate, as is generally the case. The cecum and appendix are seen in twin B, and the colon is opacified in twin A.
Figure 6. Omphalopagus twins aged 2 months. Photograph shows that the surface union is similar to that in thoracopagus twins, with ventral union from the midsternum to the umbilicus. (Courtesy of L.S.)
Figure 7a. Omphalopagus twins aged 2 months. Axial CT images were obtained with intravenous contrast material injected into twin B. (a) At the midthoracic level, contrast material (∗) is seen to be confined to the cardiac chambers of twin B (the arrows indicate the anterior cardiac border). This finding establishes that there is no vascular connection between the two hearts, making these twins omphalopagus rather than thoracopagus. (b) The liver of twin B also enhances independently; therefore, although these twins are anatomically united, vascular supply is separate. (c) Two gallbladders are seen (arrows), indicating that there is independent biliary drainage of liver segments belonging to each twin. Four kidneys (∗) are seen at this level, showing that the urinary tract development is normal. Caudal images demonstrated separate bony pelvises, bladders, and large intestines.
Figure 7b. Omphalopagus twins aged 2 months. Axial CT images were obtained with intravenous contrast material injected into twin B. (a) At the midthoracic level, contrast material (∗) is seen to be confined to the cardiac chambers of twin B (the arrows indicate the anterior cardiac border). This finding establishes that there is no vascular connection between the two hearts, making these twins omphalopagus rather than thoracopagus. (b) The liver of twin B also enhances independently; therefore, although these twins are anatomically united, vascular supply is separate. (c) Two gallbladders are seen (arrows), indicating that there is independent biliary drainage of liver segments belonging to each twin. Four kidneys (∗) are seen at this level, showing that the urinary tract development is normal. Caudal images demonstrated separate bony pelvises, bladders, and large intestines.
Figure 7c. Omphalopagus twins aged 2 months. Axial CT images were obtained with intravenous contrast material injected into twin B. (a) At the midthoracic level, contrast material (∗) is seen to be confined to the cardiac chambers of twin B (the arrows indicate the anterior cardiac border). This finding establishes that there is no vascular connection between the two hearts, making these twins omphalopagus rather than thoracopagus. (b) The liver of twin B also enhances independently; therefore, although these twins are anatomically united, vascular supply is separate. (c) Two gallbladders are seen (arrows), indicating that there is independent biliary drainage of liver segments belonging to each twin. Four kidneys (∗) are seen at this level, showing that the urinary tract development is normal. Caudal images demonstrated separate bony pelvises, bladders, and large intestines.
Figure 8. Omphalopagus twins. Sagittal reconstruction CT image obtained after intravenous injection of contrast material into twin A shows independent enhancement of the liver. This enhancement delineates the segments of conjoined liver belonging to twin A, enabling the surgeon to perform accurate surgical separation.
Figure 9. Omphalopagus twins aged 2 months. Tc-99m HIDA scan shows liver fusion. Presence of the tracer within both gallbladders (arrows) and excretion into separate small-bowel loops (arrowheads) indicate independent extrahepatic biliary tree drainage in each twin.
Figure 10. Omphalopagus twins aged 2 months. A follow-through study was performed with both twins given oral contrast material. Image shows that the stomachs and proximal small intestines are separate, and no bowel loops are seen traversing the communicating bridge. The gastrointestinal tracts were separate in these twins.
Figure 11. Ischiopagus twins aged 3 months. Photograph shows ventral surface union from the lower thorax to the perineum. Each twin has one leg, and there is a common fused leg (arrow) arising from the pelvic ring posteriorly; such ischiopagus twins are termed tripus. (Courtesy of L.S.)
Figure 12a. Ischiopagus twins aged 5 months. Nonenhanced axial CT scans show two separate spinal columns (arrows in a) with two sacra but a single symphysis pubis (arrow in b).
Figure 12b. Ischiopagus twins aged 5 months. Nonenhanced axial CT scans show two separate spinal columns (arrows in a) with two sacra but a single symphysis pubis (arrow in b).
Figure 13. Ischiopagus twins aged 4 months. Micturating cystogram shows two bladders lying side by side; the proximal urethra is duplicated but joins to form a single distal urethra. The twins possessed only one set of external genitalia.
Figure 14a. Ischiopagus twins. Vascular studies were performed to demonstrate complex vascular anatomy. (a) Venogram obtained with contrast material injected via both femoral veins simultaneously shows two inferior venae cavae draining into separate right atria. (b) Arteriogram obtained with aortic injection into twin A shows a tapering aorta with a single iliac artery (arrow); the other vessels course toward the midline viscera.
Figure 14b. Ischiopagus twins. Vascular studies were performed to demonstrate complex vascular anatomy. (a) Venogram obtained with contrast material injected via both femoral veins simultaneously shows two inferior venae cavae draining into separate right atria. (b) Arteriogram obtained with aortic injection into twin A shows a tapering aorta with a single iliac artery (arrow); the other vessels course toward the midline viscera.
Figure 15a. Ischiopagus twins. Angiographic studies were performed to show vascular anomalies, which are common. Selective catheterization of aortic branches revealed shunting between the twins. (a) Selective celiac arteriogram obtained in twin A shows a hepatic artery (arrow) crossing over to supply part of the liver belonging to twin B. (b) Angiogram shows a balancing shunt at the pelvic level via an anomalous inferior mesenteric artery (arrow), which is usually the vessel involved.
Figure 15b. Ischiopagus twins. Angiographic studies were performed to show vascular anomalies, which are common. Selective catheterization of aortic branches revealed shunting between the twins. (a) Selective celiac arteriogram obtained in twin A shows a hepatic artery (arrow) crossing over to supply part of the liver belonging to twin B. (b) Angiogram shows a balancing shunt at the pelvic level via an anomalous inferior mesenteric artery (arrow), which is usually the vessel involved.
Figure 16. Craniopagus twins. Plain radiograph shows twins joined at the vertex with subcutaneous and bony fusion. There was no meningeal or neural fusion. (Reprinted, with permission, from reference ,24.)
Figure 17. Parapagus twins aged 3 years. Photograph shows ventrolateral fusion from the upper thorax to the perineum. These girls have soft-tissue fusion of their forearms posteriorly, although the upper arms, hands, and all bones are separate. (Courtesy of L.S.)
Figure 18. Parapagus twins aged 1 day. Anteroposterior plain radiograph shows separate crania; the spinal cords converge at a single fused pelvis. Cardiac and liver shadows bridge the midline.
Figure 19a. Parapagus twins aged 3 years. Axial (a-d) and coronal (e, f) T1-weighted spin-echo MR images (600/30) show fusion of the liver across the midline (∗). The stomachs are separate (arrowheads), and most of the small-bowel loops lie on the right side of the conjoined abdomen. There are two ectopic kidneys (solid arrows in c, long arrow in e and f) and a single shared bladder (long arrow in d). There is a single colon and rectum (short arrow in d and e). The open arrow indicates the draining vein from the right kidney.
Figure 19b. Parapagus twins aged 3 years. Axial (a-d) and coronal (e, f) T1-weighted spin-echo MR images (600/30) show fusion of the liver across the midline (∗). The stomachs are separate (arrowheads), and most of the small-bowel loops lie on the right side of the conjoined abdomen. There are two ectopic kidneys (solid arrows in c, long arrow in e and f) and a single shared bladder (long arrow in d). There is a single colon and rectum (short arrow in d and e). The open arrow indicates the draining vein from the right kidney.
Figure 19c. Parapagus twins aged 3 years. Axial (a-d) and coronal (e, f) T1-weighted spin-echo MR images (600/30) show fusion of the liver across the midline (∗). The stomachs are separate (arrowheads), and most of the small-bowel loops lie on the right side of the conjoined abdomen. There are two ectopic kidneys (solid arrows in c, long arrow in e and f) and a single shared bladder (long arrow in d). There is a single colon and rectum (short arrow in d and e). The open arrow indicates the draining vein from the right kidney.
Figure 19d. Parapagus twins aged 3 years. Axial (a-d) and coronal (e, f) T1-weighted spin-echo MR images (600/30) show fusion of the liver across the midline (∗). The stomachs are separate (arrowheads), and most of the small-bowel loops lie on the right side of the conjoined abdomen. There are two ectopic kidneys (solid arrows in c, long arrow in e and f) and a single shared bladder (long arrow in d). There is a single colon and rectum (short arrow in d and e). The open arrow indicates the draining vein from the right kidney.
Figure 19e. Parapagus twins aged 3 years. Axial (a-d) and coronal (e, f) T1-weighted spin-echo MR images (600/30) show fusion of the liver across the midline (∗). The stomachs are separate (arrowheads), and most of the small-bowel loops lie on the right side of the conjoined abdomen. There are two ectopic kidneys (solid arrows in c, long arrow in e and f) and a single shared bladder (long arrow in d). There is a single colon and rectum (short arrow in d and e). The open arrow indicates the draining vein from the right kidney.
Figure 19f. Parapagus twins aged 3 years. Axial (a-d) and coronal (e, f) T1-weighted spin-echo MR images (600/30) show fusion of the liver across the midline (∗). The stomachs are separate (arrowheads), and most of the small-bowel loops lie on the right side of the conjoined abdomen. There are two ectopic kidneys (solid arrows in c, long arrow in e and f) and a single shared bladder (long arrow in d). There is a single colon and rectum (short arrow in d and e). The open arrow indicates the draining vein from the right kidney.
Figure 20a. Images from gastrointestinal contrast material studies performed in parapagus twins aged 2 weeks. (a) There is contrast material within the stomach (∗) of twin A, and the small intestine lies mainly on the right side of the conjoined abdomen. The level of bowel union is not demonstrated. (b) A single colon is opacified (arrow) and crosses the midline. (c) Lateral view shows a single fused rectum (arrow).
Figure 20b. Images from gastrointestinal contrast material studies performed in parapagus twins aged 2 weeks. (a) There is contrast material within the stomach (∗) of twin A, and the small intestine lies mainly on the right side of the conjoined abdomen. The level of bowel union is not demonstrated. (b) A single colon is opacified (arrow) and crosses the midline. (c) Lateral view shows a single fused rectum (arrow).
Figure 20c. Images from gastrointestinal contrast material studies performed in parapagus twins aged 2 weeks. (a) There is contrast material within the stomach (∗) of twin A, and the small intestine lies mainly on the right side of the conjoined abdomen. The level of bowel union is not demonstrated. (b) A single colon is opacified (arrow) and crosses the midline. (c) Lateral view shows a single fused rectum (arrow).
Figure 21a. Parapagus twins. (a) Intravenous urogram shows a single bladder and two ectopic kidneys (arrowheads). (b) Tc-99m dimercaptosuccinic acid (DMSA) scan shows the shapes of the ectopic kidneys. The right renal pelvis appears as a photon-deficient region inferiorly (arrow). (c) Retrograde pyelogram shows vesicoureteral junction obstruction in the right kidney.
Figure 21b. Parapagus twins. (a) Intravenous urogram shows a single bladder and two ectopic kidneys (arrowheads). (b) Tc-99m dimercaptosuccinic acid (DMSA) scan shows the shapes of the ectopic kidneys. The right renal pelvis appears as a photon-deficient region inferiorly (arrow). (c) Retrograde pyelogram shows vesicoureteral junction obstruction in the right kidney.
Figure 21c. Parapagus twins. (a) Intravenous urogram shows a single bladder and two ectopic kidneys (arrowheads). (b) Tc-99m dimercaptosuccinic acid (DMSA) scan shows the shapes of the ectopic kidneys. The right renal pelvis appears as a photon-deficient region inferiorly (arrow). (c) Retrograde pyelogram shows vesicoureteral junction obstruction in the right kidney.
Figure 22a. Images from angiographic studies performed in parapagus twins. (a) Right renal angiogram shows a complex fused ectopic kidney with arterial supply (large arrow) from the right aorta. The ureter drains inferiorly (small arrow). (b) A large draining vein (open arrow) emerges medially and passes posteriorly over the upper pole to join the inferior vena cava. This vein can also be identified on an MR image (, , , , , , ,Fig 19c). Solid arrow = ureter.
Figure 22b. Images from angiographic studies performed in parapagus twins. (a) Right renal angiogram shows a complex fused ectopic kidney with arterial supply (large arrow) from the right aorta. The ureter drains inferiorly (small arrow). (b) A large draining vein (open arrow) emerges medially and passes posteriorly over the upper pole to join the inferior vena cava. This vein can also be identified on an MR image (, , , , , , ,Fig 19c). Solid arrow = ureter.
Figure 23. Congenital diaphragmatic hernia in conjoined twins aged 1 day. Plain radiograph shows multiple bowel loops within the chest of twin A. Note the nasogastric tube within the stomach (arrow), which is sited above the diaphragm. This twin had associated pulmonary hypoplasia.
Figure 24a. Anorectal atresia with associated colovesical fistula. Contrast material radiographs of the intestine show a colovesical fistula in an infant separated from a stillborn conjoined twin. The infant has two bladders; the anterior bladder (∗) was received from the nonviable twin. Contrast material injected via the defunctioned distal bowel loop enters the anterior bladder via the fistula (arrow in a); this bladder communicates directly with the posterior bladder (arrowhead), which belongs to this patient. Contrast material is expelled via the urethra (arrows in b).
Figure 24b. Anorectal atresia with associated colovesical fistula. Contrast material radiographs of the intestine show a colovesical fistula in an infant separated from a stillborn conjoined twin. The infant has two bladders; the anterior bladder (∗) was received from the nonviable twin. Contrast material injected via the defunctioned distal bowel loop enters the anterior bladder via the fistula (arrow in a); this bladder communicates directly with the posterior bladder (arrowhead), which belongs to this patient. Contrast material is expelled via the urethra (arrows in b).
Figure 25a. Successfully separated conjoined twins. (a) Photograph shows separated omphalopagus twins (same pair as in Fig 6) at 4 years of age. (Courtesy of L.S.) (b) Photograph shows separated ischiopagus twins (same pair as in Fig 11) at 8 years of age. They are now 13 years old; each has an artificial limb and occasional urologic problems. (Courtesy of L.S.)
Figure 25b. Successfully separated conjoined twins. (a) Photograph shows separated omphalopagus twins (same pair as in Fig 6) at 4 years of age. (Courtesy of L.S.) (b) Photograph shows separated ischiopagus twins (same pair as in Fig 11) at 8 years of age. They are now 13 years old; each has an artificial limb and occasional urologic problems. (Courtesy of L.S.)
| |
| |
| |
| |
References
- 1 Hanson JW. Incidence of conjoined twinning (letter). Lancet 1975; 2:1257. Google Scholar
- 2 Spitz L. Conjoined twins. Br J Surg 1996; 83:1028-1030. Crossref, Medline, Google Scholar
- 3 Barth RA, Filly RA, Goldberg JD, Moore P, Silverman NH. Conjoined twins: prenatal diagnosis and assessment of associated malformations. Radiology 1990; 177:201-207. Link, Google Scholar
- 4 Edmonds LD, Layde PM. Conjoined twins in the United States, 1970–1977. Teratology 1982; 25:301-308. Crossref, Medline, Google Scholar
- 5 Diaz JH, Furman EB. Perioperative management of conjoined twins. Anesthesiology 1987; 67:965-973. Crossref, Medline, Google Scholar
- 6 Zimmermann AA. Embryological and anatomic considerations of conjoined twins. Birth Defects 1967; 3:18-27. Google Scholar
- 7 Jones KL. Smith's recognizable patterns of human malformation 2nd ed. Philadelphia, Pa: Saunders, 1988; 594-595. Google Scholar
- 8 Rudolph AJ, Michaels JP, Nichols BL. Obstetric management of conjoined twins. Birth Defects 1967; 3:28-37. Google Scholar
- 9 Spencer R. Anatomic description of conjoined twins: a plea for standardized terminology. J Pediatr Surg 1996; 31:941-944. Crossref, Medline, Google Scholar
- 10 Sanders SP, Chin AJ, Parness IA, et al. Prenatal diagnosis of congenital heart defects in thoracoabdominally conjoined twins. N Engl J Med 1985; 313:370-374. Crossref, Medline, Google Scholar
- 11 Fitzgerald EJ, Toi A, Cochlin DL. Conjoined twins: antenatal ultrasound diagnosis and a review of the literature. Br J Radiol 1985; 58:1053-1056. Crossref, Medline, Google Scholar
- 12 Stauffer UG. Conjoined twins. In: Lister J, Irving IM, eds. Neonatal surgery. 3rd ed. London, England: Butterworths, 1990; 153-162. Google Scholar
- 13 O'Neill JA, Holcomb GW, Schnaufer L, et al. Surgical experience with thirteen conjoined twins. Ann Surg 1988; 208:299-312. Crossref, Medline, Google Scholar
- 14 Melikoglu M, Aslan A, Mete A, Ozkaynak C, Inan M, Karaveli S. A case of thoraco-omphalo-ischiopagus bipus conjoined twins. J Pediatr Surg 1997; 32:656-658. Crossref, Medline, Google Scholar
- 15 Wilcox DT, Quinn FM, Spitz L, Kiely EM, Ransley PG. Urological problems in conjoined twins. Br J Urol 1998; 81:905-910. Crossref, Medline, Google Scholar
- 16 Conjoined twins. In: Sanders RC, eds. Structural fetal abnormalities: the total picture. St Louis, Mo: Mosby, 1996; 223-227. Google Scholar
- 17 Spitz L, Crabbe DCG, Kiely EM. Separation of thoraco-omphalopagus conjoined twins with complex hepato-biliary anatomy. J Pediatr Surg 1997; 32:787-789. Crossref, Medline, Google Scholar
- 18 James PD, Lerman J, McLeod ME, Relton JES, Creighton RE. Anaesthetic considerations of omphalo-ischiopagus tripus twins. Can Anaesth Soc J 1985; 32:402-411. Crossref, Medline, Google Scholar
- 19 Donaldson JS, Luck SR, Vogelzang R. Preoperative CT and MR imaging of ischiopagus twins. J Comput Assist Tomogr 1990; 14:643-646. Crossref, Medline, Google Scholar
- 20 Mann MD, Coutts JP, Kaschula ROC, Fraser CB, Fisher RM, Cywes S. The use of radionuclides in the investigation of conjoined twins. J Nucl Med 1984; 24:479-484. Google Scholar
- 21 Spitz L, Capps SNJ, Kiely EM. Xiphoomphaloischiopagus tripus conjoined twins: successful separation following abdominal wall expansion. J Pediatr Surg 1991; 26:26-29. Crossref, Medline, Google Scholar
- 22 Spencer R. Minimally united ischiopagus twins: infraumbilical union with cloacal anomalies. J Pediatr Surg 1996; 31:1538-1545. Crossref, Medline, Google Scholar
- 23 Albert MC, Drummond DS, O'Neill J, Watts H. The orthopedic management of conjoined twins: a review of 13 cases and report of 4 cases. J Pediatr Orthop 1992; 12:300-307. Crossref, Medline, Google Scholar
- 24 Wolfowitz J, Kerr EM, Levin SE, Walker DH, Vetten KB. Separation of craniopagus twins. S Afr Med J 1968; 42:412-424. Medline, Google Scholar
- 25 Jansen O, Mehrabi VA, Sartor K. Neuroradiological findings in adult cranially conjoined twins: case report. J Neurosurg 1998; 89:635-639. Crossref, Medline, Google Scholar
- 26 Spitz L, Stringer MD, Kiely EM, Ransley PG, Smith P. Separation of brachio-thoraco-omphalo-ischiopagus bipus conjoined twins. J Pediatr Surg 1994; 29:477-481. Crossref, Medline, Google Scholar
- 27 Levin M, Roberts DJ, Holmes LB, Tabin C. Laterality defects in conjoined twins (letter). Nature 1996; 384:321. Crossref, Medline, Google Scholar
Pictures of Different Types of Conjoined Twins
Source: https://pubs.rsna.org/doi/10.1148/radiographics.21.5.g01se011187