To date, approximately 250 successful separations in which one or both twins have survived over the long-term have been recorded. The most successful series have been described from institutions with a large clinical experience in contrast to reports of individual cases. The major factors, which have resulted in improvements in surgical outcomes, have been review of long-term experience, improvements in the accuracy of radiology imaging technology, and advances in surgical techniques and anesthesia.
Incidence and Etiology
Regular monozygotic twins are more frequently male, and stillborn conjoined twins have a predominance of males as well. On the other hand, conjoined twins who survive to the point of being candidates for separation are mostly female at a ratio of three to one over males.
The exact cause of conjoined twinning is not known, but two theories have been offered. Both theories depend upon the understanding that during the first week of gestation, a fertilized egg is capable of splitting to form two developing embryos. Complete splitting of the embryo into two separate ones results in typical monozygotic identical twins. On the other hand, if the two completely separated embryos rejoin or fuse in part, conjoined twins are thought to result.
On the other hand, the other, and most commonly held, theory is that conjoined twinning results from incomplete division of the very early embryo with two individuals resulting but joined at various sites. Monozygotic identical twins, whether separate or conjoined, are identical in sex and genes, and they share a common placenta. Fraternal or dizygotic twins result from fertilization of two separate eggs, and each fetus has its own placenta and umbilical cord. Fraternal twins may be of the same or opposite sex and have different genes.
Another feature of conjoined twins is related to other abnormalities or anomalies. It appears that incomplete division of the embryo is associated with incomplete formation of the various organ systems and conjoined twins with fused organs usually have incomplete development of their hearts, liver, intestines and urinary systems. It is important to keep in mind that in almost all instances of conjoined twinning, one or both of the twins have other organ abnormalities that must be dealt with.
It is known that twinning tends to occur in families, and families with conjoined twins often have a family history of twinning. Thus far, there is no evidence that fertility drugs or other drugs are involved in producing conjoined twins.
Types of Conjoined Twins
In the order of frequency, the types of conjoined twinning are thoracopagus (chest), omphalopagus (abdomen), pygopagus (rear pelvis), ischiopagus (front pelvis), craniopagus (head), and heteropagus (unequal or asymmetric) (see Figure 2). Another commonly used classification system combines the first two as thoraco-omphalopagus because they commonly occur together.
Antenatal Diagnosis and Obstetric Management
Prenatal ultrasound is also important in planning obstetric management of conjoined twins, who frequently have a breach presentation. Caesarian section would appear to be the safest and preferable approach to delivery when the diagnosis is known ahead of time. Conjoined twins often cause premature labor, so efforts must be made to inhibit labor to permit the fetuses to become as mature as possible for survival. The condition of the infants at the time of birth determines how aggressive supportive care must be to permit the infants to undergo the necessary series of diagnostic tests. The more that is known about the twins preoperatively, the more likely separation is to be successful.
Simple diagnostic studies, such as the usual x-rays, may be performed immediately following birth, but invasive procedures must be timed according to the infants’ condition and ability to withstand stress.
Additionally, some diagnostic studies are better performed later at the time of anticipated separation when they may be more accurate and associated with less stress than if performed immediately following birth. It is often the case that the results from one study indicate that another type of study must be performed, so most conjoined twins have a large number of diagnostic studies performed before enough information is available to plan successful separation.
Some studies are strictly for imaging (radiology viewing of the organ structures) while others provide information on organ function and the effects of shared cross-circulation (blood supply). In some instances, a number of serial (sequential) studies are needed to determine when the twins’ general condition is improving or deteriorating to guide the team in terms of timing of separation. For example, because prematurity in conjoined twins is common, serial studies of lung function are necessary.
In all conjoined twins, one twin is smaller than the other; half the time, one infant has more anomalies than the other. These distinctions should be identified as they may affect the separation and reconstruction. The results of all of the various preoperative studies form the basis for the planning and order of surgical separation by the various specialty members of the team.
Preoperative evaluation of the cardiovascular system is essential in all conjoined twins, whether or not the hearts are conjoined as determined from prenatal studies. Echocardiography (cardiac ultrasound) and electrocardiography (EKG) are performed to determine the presence of anomalies in the case of separate hearts and to determine the sites where the hearts are joined and the presence of abnormalities in the case of conjoined hearts.
Cardiac (heart) catheterization was previously the main means for diagnosis of complex heart anomalies, but now three-dimensional heart magnetic resonance angiography (MRA, an MRI which focuses on showing the blood vessels and heart) is used. MRA provides precise anatomic information that may help to decide whether or not heart separation should be attempted. The presence of a single heart beat on EKG usually indicates that successful cardiac separation would not be possible, but the presence of two separate beating systems does not necessarily indicate a better outcome.
Most conjoined twins share a liver and may share a pancreas and liver, both vital organs. Ultrasound, three-dimensional MRA, and nuclear medicine scanning of the bile ducts which carry the bile from the liver to the intestines (biliary tree) with separately timed injections into each twin help to determine the degree of shared organs are the most helpful studies.
Except for craniopagus twins, all other forms of conjoined twins may share portions of the gastrointestinal (GI) tract. This is evaluated best by upper and lower GI contrast (dye) studies: separately timed injections frequently are needed to define precisely which structures are shared. In thoracopagus and omphalopagus twins, the joining of the intestines is frequently at the level of the duodenum, the first part of the small intestine. In this case only one pancreas and biliary system may be present. In ischiopagus and pygopagus twinning, the site of intestinal junction is usually in the terminal ileum, or near the end of the small intestine. In the latter case, a single colon (large intestine) and terminal ileum may be shared.
Genitourinary system evaluation is best approached by a variety of studies including ultrasound, bladder contrast (dye) studies, computed tomography (CT), nuclear medicine studies, and cystoscopy in which one can look with a scope into the bladder, as well as the vagina. The information that must be gathered from these studies includes the status of kidney function and the number and location of the kidneys, ducts that drain the kidneys (ureters), and bladders. Because most conjoined twins are female, accurate vaginal examinations are crucial to determine whether there is a single vagina or a double vagina. Determination of the number, size, and location of these cavities is crucial to planning the staged reconstructive procedures required. In males, it is important to evaluate the penis, scrotum, and testicles before separation.
When a lower extremity is shared, angiography (injection of dye into the bloodstream) may be needed to determine the exact nature of the blood supply to the shared limb, the twin to which it primarily is related, and to what degree the limb can be salvaged or possibly shared by the infants.
Three dimensional reconstruction of CT scans and MRI/MRA studies, as well as three dimensional models of conjoined twins are a new tool to help surgeons understand the degrees of connection between the conjoined twins. They help surgeons map out separation and reconstruction techniques and are becoming an increasingly valuable tool in the management of conjoined twins.
Table 2 outlines the usual separation decisions based on historical outcomes although each decision depends on the individual case.
It has already been mentioned above that information should be accumulated based on prenatal ultrasound, MRI and echocardiography, and these studies are known to be quite reliable. The pediatric surgeon who is experienced with separation of conjoined twins is probably the best person to counsel prospective parents about the likelihood of successful separation and what the outcome for the twins is likely to be given the new developments in this field. It is only with this information that the parents are in the best position to determine whether or not they wish to continue the pregnancy.
Before and following the infants’ birth, the family’s right to privacy should be respected and preserved. The pediatric surgeon must develop a trusting relationship with the family because of the nature of the complicated decisions that must be made and the absolute need for informed consent.
Detailed and repeated discussions are necessary particularly if, after all of the preoperative evaluations are done, it is evident that only one twin can survive or if one of the two will probably be left with a serious disability. Only solitary survivors may be possible in twins with conjoined hearts, twins with only one inferior vena cava draining shared livers, and twins with a single biliary tree.
In terms of such cases in which only one twin can survive, such difficult decisions can be aided by the knowledge that both children usually die without separation under these circumstances, and the situation has been made more rational with the understanding that the operation itself does not determine which twin can survive but rather the nature of the anatomy and the organ abnormalities involved. If high-quality survival is possible, most experienced pediatric surgeons recommend that twins be separated, even if only one twin can survive, rather than allowing the loss of both infants.
Shared structures and organs are usually allocated to the individual twin based on the anatomy and available information with the intent of maximizing both twins when there are no medical differences between the two. If one of the twins is significantly mentally or physically impaired, allocation may be directed best toward the healthier twin. However, in the end, the parents have the ultimate right to accept or refuse surgical separation. In all instances the situation must be approached with the utmost sensitivity for the rights and feelings of the parents and their children.
Timing of Separation and Planning
In the older age group, anesthetic management is easier, and blood loss and changes in blood pressure and other aspects of physiology are tolerated better at this time than in the immediate period after birth. If operation is delayed much beyond a year of age, conjoined twins may have difficulty developing an independent personality for a few months although they eventually adapt quite well.
Conditions that force emergency separation include the presence of a stillborn twin, intestinal blockage (obstruction), rupture of a membrane on the abdominal wall, heart failure, obstruction of the urinary tract, and lung failure. One of the main advantages of undertaking separation at nine to 12 months of age is that it provides time for thorough preoperative assessment and involvement of all of the specialists who are required for a successful outcome.
The key to successful separation of conjoined twins is when separation is performed by an experienced team of pediatric surgical specialists and anesthesiologists who know how to integrate their efforts. Team conferences, including preoperative and postoperative caretakers; all surgeons, anesthesiologists, and other specialists; and nurses and others involved in the operation and the care of the infants are necessary for review of all gathered information. A mock preoperative drill involving all team members makes the procedure more efficient because all involved know what the next step will be. The leader of the team must be prepared to make decisions during the separation, but thorough preoperative conferences and drills make it possible to anticipate most of the issues ahead of time.
With regard to the procedure itself, the main considerations include anesthetic and surgical management during the time of the separation and attention to the reconstructive needs of each individual twin following the separation. Therefore there need to be two teams of anesthesiologists, two teams of pediatric surgeons, and one or two teams of surgical specialists including plastic surgeons, neurosurgeons, orthopedists, urologists and cardiac surgeons depending upon the reconstructive needs of each twin, and two teams of nurses with two complete operative set ups.
Because fluid and blood loss are the primary risks that the anesthesiology teams must handle, the placement of special catheters that allow fluid to be given and monitoring to be performed in arteries and large veins in both twins is usually necessary. Putting breathing tubes into conjoined twins can be quite difficult depending on the area of joining.
Depending upon the nature of the twinning and the number of organ systems involved, a flexible plan for the operation is made for the order of separation and reconstruction. It is generally possible to accomplish this. While the majority of the reconstructive procedures follow standard approaches to organ reconstruction, at times, new techniques or specially tailored standard reconstruction techniques are required. This is particularly the case with regard to management of the skin, heart, biliary tree, intestine and genitourinary structures. Since so much skin is shared in most instances of conjoined twinning, preliminary procedures involving the placement of multiple skin expanders to develop extra skin are usually desirable prior to undertaking separation. This eliminates a lot of problems with body closure later. It is beyond the scope of this chapter to describe the details of separation and reconstruction of each organ system because of the wide range of complicated issues involved. However, a few examples follow:
Central Nervous System
Pygopagus and some ischiopagus twins may share varying portions of the vertebral column and the spinal cord. Several operations may be needed to separate these twins with division of the spinal cord structures initially.
Liver and Pancreaticobiliary System
The second pitfall occurs when there is a single system to drain bile (called the extrahepatic biliary tree); this must be determined ahead of time to decide where the liver should be divided. In twins with one extrahepatic biliary tree, special x-rays in the operating room (cholangiograms) may be necessary to determine how to reconstruct the biliary trees. Division of the pancreas can be quite complex as well, and usually mirrors separation of the liver and bile ducts. In a few cases with a single extrahepatic biliary tree, nothing is available for one of the twins.
Gastrointestinal (GI) Tract
Most female ischiopagus or pygopagus twins have single external genitalia and double vaginas. Fertility may be preserved in both female twins. Male conjoined twins may have one or two sets of external genitalia that must be separated appropriately. In cases with only one penis, one twin may undergo male reconstruction and the other may undergo female reconstruction.
Urinary reconstruction of ischiopagus twins usually requires multiple stages and close long-term follow-up if complications are to be avoided. It is generally possible to maintain normal kidney function and reasonable bladder control. Normal sexual activity and fertility are reasonable and achievable goals.
Skeletal System and Rehabilitation
The main orthopedic challenges are related to the management of ischiopagus conjoined twins. The first challenge is a thorough evaluation of the pelvis and a shared leg. Only after thorough evaluation of three-dimensional reconstruction of a CT scan of the pelvis can the appropriate site for division of the pelvis be determined. This method also facilitates the decision about whether a third shared leg would be given to one or a portion to both twins. As mentioned earlier, a crucial part of the evaluation and the separation procedure is evaluation of the blood supply to the extremities whether shared or not because this evaluation determines whether one or both twins provide the main blood supply.
Most conjoined twins have some degree of ongoing disability ranging from minor to considerable, but with modern reconstructive and rehabilitative techniques, the outcomes are generally good. However, in order to achieve this, long-term followup and periodic operations and procedures are required as twins grow and develop over time. Psychological adjustment is also generally quite good, but it may be affected by long-term problems such as genitourinary problems and orthopedic issues. Most surgeons experienced in this field agree that long-term outcomes are better with separation than if twins are left unseparated.
Articles and graphics adapted from O'Neill: Principles of Pediatric Surgery. © 2003, Elsevier.