Incidence

Because of the significant number of conjoined twins who are aborted or who are stillborn, the exact incidence is not known, but the reported incidence worldwide is estimated at 1:50,000 to 1:100,000 live births, with a higher incidence of 1:14,000 to 1:25,000 experienced in Africa and Asia (Hoyle, 1990). Of those that are born, many may have congenital abnormalities that are incompatible with life. The presence of associated congenital anomalies in live twins is common. Congenital diaphragmatic hernia, pulmonary hypoplasia, congenital heart disease, anomalous hepatic arterial and venous drainage, biliary tree anomalies, bowel atresia, Meckel’s diverticulum, complex urogenital anatomy, and spinal dysraphism have all been reported.

Historical perspectives

Conjoined twins are one of nature’s greatest enigmas. Cave drawings, pottery, figurines, and folklore indicate that conjoined twins have occurred since prehistoric times. Embryonic duplication also occurs in plant and other animal forms of life (Fig. 29-1) (Thomas and Lopez, 2004). The famous and most celebrated pair of conjoined twins, Chang and Eng Bunker, who were joined at their xiphisterna, was born in Siam (now Thailand) in 1811. Because of their fame and the rarity of this pathology, the term Siamese twins has become synonymous with conjoined twins. Over time, the bridge of skin and tissue between them stretched, and they were able to stand side by side. The twins were taken to America, where they were on display by Phineas T. Barnum in his circus. Surgery for separation was deemed too risky unless one of the twins was to die. The Bunker twins lived very interesting lives—they married sisters and fathered 22 children between them. They died within hours of each other at the age of 63 (Spitz, 2003). Interestingly, the twins who have become most famous have all remained conjoined, whereas there is very little literature on the lives of those who have been separated. Votteler and Lipsky (2005) have reported on the long-term results of ten separations, as have Hoyle and Thomas (1989) with their experiences studying ischiopagus twins over 23 years. The first set of omphalopagus twins separated at the Red Cross Children’s Hospital now each has children of their own (Cywes and Louw, 1967; Cywes et al., 1982).

FIGURE 29-1 Conjoined twinning occurs in plants and animals, as well as in human life. This photograph is of twin roses of the Double Delight variety.

The first successful separation of conjoined twins was performed by Konig in 1689, when he separated omphalopagus twins by slowly tightening an encircling band around the connection until it necrosed the connecting bridge. The first successful separation of thoracopagus twins with a conjoined heart was reported in 1979 when, after the interatrial bridge was interrupted, there was a single survivor (Synhorst et al., 1979).

During the 1960s and 1970s, emphasis in the perioperative management of conjoined twins was on preoperative discussions and planning, dress rehearsals, and on the then-new techniques of invasive intraoperative monitoring for blood loss and hemodynamic status (Diaz and Furman, 1987). From the 1980s onward, most improvements have occurred in obstetric and fetal ultrasound; fetal and neonatal surgery; magnetic resonance imaging (MRI); contrast computed tomography (CT); numerous radiographic technological options to help delineate anatomic variations in different twins; in the anesthetic advances of oximetry, capnography, agent analysis; and in techniques for monitoring brain oxygenation and blood flow.

For the induction of omphalopagus twins, Allen et al. (1959) described the use of cyclopropane and oxygen, supplemented by ether via the open-drop method. He describes the attempts at intubation by holding one infant above the other to facilitate easier intubation as a “near fatal mistake.” In this position, the upper twin became pale and apneic, whereas the condition of the lower infant rapidly deteriorated and it became plethoric. Once they were returned to their normal position, they were successfully intubated. He comments that “there are few vital signs available for monitoring infants of this size. The most important signs are respirations, color, and muscle tone.” Both infants survived the surgical separation.

The anesthesia provided to separate the first set of twins in 1967 in Cape Town, South Africa, was described as “uneventful” (Cywes and Louw, 1967). Preoperative investigations in these omphalopagus twins indicated that there was no cross-circulation between the infants and this was confirmed at the induction of anesthesia, when the pancuronium given to one infant had no effect on the other.

In Furman’s report (Furman et al., 1971), intubation on a set of xiphopagus twins was performed when both infants were awake, and vascular cannulation was done when they were under local anesthetic. Jarem (Jarem et al., 1977), in his classic paper, describes the double breathing circuit fashioned from readily available parts of anesthetic equipment.

Roy (1980) described his experience with craniopagus twins. His group used two anesthetic teams, but only one anesthetic machine with a non–rebreathing Jackson-Rees breathing system from the gas outlet leading to two T pieces. Monitoring included: electrocardiograph (ECG), Doppler arm-cuff pressures, esophageal stethoscopes, and rectal telethermometers. Each infant received preoperative atropine, and they were induced simultaneously with halothane, nitrous oxide, and oxygen. He continued to describe the 15-month preparation time before their separation. This was successfully achieved, but he describes the anesthetic challenges as severe hemorrhage and maintenance of a clear airway with obstruction from secretions, kinking, or accidental extubation. Harrison and his team (1985) commented on the need for modification of the breathing circuit to accommodate the process of ventilating neonatal conjoined twins simultaneously and stated that adequate intravascular fluid replacement was essential to prevent hypotension at separation. They also commented on the intraoperative challenges of these infants.

Diaz and Furman, (1987), in their well-known paper on the perioperative management of conjoined twins, highlighted many factors that are now integral to the care of these patients. Advances in anesthetic equipment, monitoring, and drugs (inhalational or intravenous) give us options for anesthesia that earlier anesthetic specialists did not have. Similarly, outcomes for surgical procedures in these patients have improved. The developments in pediatric intensive care have had a positive impact on both the preoperative and postoperative care and outcomes of these children.

Embryology

Two theories have been proposed as to the etiology of conjoined twins: fusion and fission. Although the differences between these theories have yet to be resolved, fission is the generally accepted and older theory, where the fertilized egg fails to split completely (Kaufman, 2004). The theory of fusion, proposed by Spencer (2000a, 2000b, 2003), suggests that the fertilized egg splits completely, but a secondary union of two separate embryonic discs at the dorsal neural tube or ventral yolk sac takes place at 3 to 4 weeks’ gestation. This theory was also held by Aristotle (Millar et al., 2009).

Monozygotic twins are the result of one ovum being fertilized by one sperm and then dividing into two embryos. In conjoined twins, this division, at the 13th to 17th day of the blastocyst stage, is incomplete.

Conjoined twins are identical, with what has always been thought to be an identical chromosomal pattern. With new molecular genetics, this has been shown to not necessarily be the case, and the twins may neither be completely identical nor have the same chromosome pattern (Hall, 2003). It has been hypothesized that monozygotic twinning occurs at four possible phases: up to 3 days’ postconception, after 4 to 6 days, after 7 to 12 days, and finally after 13 to 17 days. It is during this latter period that conjoined twins, fetus-in-fetu, or a teratoma may arise (Hall and Lopez-Rangel, 1996). Parasitic twins are thought to result from the embryonic death of one twin, with the remaining parts of the body vascularized by the surviving autocyte. Fetus-in-fetu are asymmetric monozygotic diamniotic intraparasitic twins (Hall and Lopez-Rangel, 1996). Conjoined triplets and quadruplets have been documented, although all have been aborted (O’Neill, 1998; Rode et al., 2006). These cases are exceptionally rare, with an even more obscure pathogenesis.

Conjoined twins are always the same gender. There is a 3:1 female-to-male incidence in live births, but more males are stillborn (Hoyle, 1990; Rejjal et al., 1992). They are monozygotic, monoamniotic, and monochorionic. Despite this, the infants usually vary in size, appearance, personality, internal anatomy, and degree of organ-sharing and duplication. Many of the challenges for all disciplines relate to the vast diversity in the anatomic variations that may possibly occur. Organs may be conjoined, duplicated, or absent. Investigations are crucial to clarifying these differences in order to make decisions about surgery, interventions, and survival. Even then, surprises occur at the time of surgery, and innovative techniques may be necessary.

Survival depends on the site and complexity of conjunction, the degree of organ sharing, and the presence of other congenital anomalies, with the extent of cardiac fusion and lung development being major determining factors (Andrews et al., 2006).

Classification, nomenclature, and terminology

Conjoined twins are typically classified by the anatomic site of conjunction (which may be complex) followed by the suffix -pagus—the Greek word meaning that which is joined (Box 29-1). There is an international collaboration attempting to create a more uniform classification of conjoined twins (Spencer, 1996). This considers a ventral (front, caudal, or lateral) or dorsal (back-to-back, sacrum-to-sacrum, or head-to-head) fusion, with further classification described in the following paragraphs (Fig. 29-2).

FIGURE 29-2 Illustrations depicting the anatomic relationships of the different types of symmetric conjoined twins.

(Modified from Spencer R: Conjoined twins: developmental malformation and clinical implications, Baltimore, 2003, John Hopkins University Press.)

Twins are described as being symmetric or asymmetric, although these terms are often misleading, because even when the twins are fully formed they are seldom truly symmetric. According to the most prominent site of conjunction, the following sites of conjunction are the most common: the chest (thoracopagus twins), the abdomen (omphalopagus twins), sacrum (pygopagus twins), pelvis (ischiopagus twins), and back (rachipagus twins) (Table 29-2).

TABLE 29-2 Types of Symmetric Conjoined Twins

Symmetric twins are two anatomically identifiable infants with a physical appearance suggestive of two possible survivors. With asymmetric, or parasitic, twins, one twin is a potential survivor and usually appears normal, but the other is incomplete and attached as a parasite. This may be externally visible, or it may be internal. Where there is an attachment of an anatomically identifiable part, but not of a total individual, the term heteropagus is used. It is in this group, that regional anesthesia is particularly useful. Within a classified group, especially with complex conjunctions, the external physical appearance and spatial arrangement of the infants may be very different (Figs. 29-3 and 29-4).

FIGURE 29-3 These ischiopagus twin girls were born via vaginal delivery to a very surprised mother and her midwife.

FIGURE 29-4 This set of twin boys has a complex conjunction at the thoracic, abdominal and pelvic levels, and despite also being ischiopagus, show no physical resemblance to the twins in Figure 29-3.

Conjoined twins are further classified by the number of limbs present and the internal organs that are involved in the conjunction. The following list shows this classification system:

The degree of cardiac fusion, or degree of cardiopagus, can be considered as follows (Andrews et al., 2006):

A: Separate hearts and pericardium

B: Separate hearts and a common/shared pericardium

C: Fused atria and separate ventricles

D: Fused atria and ventricles

Outcomes for separation in groups C and D are poor.

Ethics

In broad terms, ethics are a set of standards of professional conduct (Atkinson, 2004). Whenever planned separation of conjoined twins takes place, and when there is a possibility or probability that one twin may not survive the procedure, moral, ethical, and legal arguments are raised. Many of these issues have been addressed in the literature (Pearn, 2001; Unknown author, 2000; Spitz and Kiely, 2000; Spitz, 2000; Bratton and Chetwynd, 2004). The Hastings report identified three cardinal issues (London and Knowles, 2001):

In his review of this subject, Atkinson (2004) commented that the birth of a handicapped child is an immense burden to any parents. If they are to avoid greater challenges, the clinicians require sensitivity and patience as they steer the family along a pathway to survival. The question most often asked is whether it is justified to sacrifice one life to save another, or should both infants be allowed to die? Answers are seldom simple, and each case should be assessed on its own merits. Decisions depend on the type and complexity of the conjunction, the overall health of both infants, the laws of the country, and the religion and beliefs of the parents. If the sacrifice of one twin is necessary for the other twin to survive, it is important to recognize that both would die without separation. If they were separated, at least one would live (O’Neill et al., 1988).

Especially in centers where these operations are done more often, medical staff is more confident of their ability to provide insightful management decisions pertaining to these care of conjoined twins. They are supported by investigations that provide valuable information, allowing rational decisions to be made about the immediate and long-term futures of these infants. These investigations, performed to clarify anatomic structures include radiologic imaging, ultrasonography, CT, MRI, radioisotope studies, echocardiography, cardiac catheterization, and neuroradiologic imaging. As a consequence, in the perioperative period the pediatric surgeons and anesthesiologists in these centers have reported much higher rates of success in operating on these infants. Developments in intensive care also have a positive impact on these improved outcomes.

Ian Aird (1954, 1959) wrote that if there was a possibility of at least one twin surviving, surgery should be performed. The operation does not decide which twin will survive; this is determined by their relative conditions (O’Neill et al., 1988).

Great Ormond Street Ethical Guidelines for Conjoined Twin Separation has been adopted by a number of units. Where separation is feasible with a reasonable chance of success, it should be carried out. When surgery is not possible, custodial care should be offered, and nature should be allowed to take its course. When one twin is dead or has a lethal abnormality and cannot survive independently from its normal twin, and if there is no surgery both twins would die, separation to save the healthy twin should be attempted (Rode et al., 2006; Millar et al., 2009).

Perinatal considerations

Improved techniques of antenatal diagnosis and fetal imaging have allowed the diagnosis of conjoined twins to be made during pregnancy, so that counseling of the parents may allow them the option to have the pregnancy terminated. Conjoined twins can be identified as early as 11 weeks’ gestation, and in this series of cases studied by Sebire et al. (2000), all the parents opted for termination. Complex craniopagus and cardiopagus anatomy in particular may tilt the decision in this direction. Conjoined hearts are easier to study via ultrasound in utero, because the amniotic fluid acts as a buffer, whereas after birth the lungs inflate with air and prevent optimal visibility (Kingston et al., 2001).

Obstructed labor with difficult vaginal delivery may necessitate an emergency cesarean section, but this can be avoided by antenatal diagnosis and elective cesarean section at 36 to 38 weeks’ gestation (Millar et al., 2009). In many developing countries, the birth of conjoined twins may come as a surprise to the mother and the attending midwife or medical practitioner (Thomas, 2004; Thomas and Lopez, 2004).

Perinatal management of conjoined twins involves a close collaboration between anesthesiologists, obstetricians, and pediatricians so that birth trauma for both the mother and the infants can be minimized. Unborn conjoined twins may be referred to pediatric surgeons for advice in order to plan the delivery and the immediate perinatal management.

Careful prenatal investigations may identify cases in which emergency separation at birth is life saving (MacKenzie et al., 2002). Advances in prenatal ultrasound, as well as the use of color-flow Doppler and prenatal MRI, have improved the antenatal diagnosis of conjoined twins. In particular, prenatal and postnatal echocardiography has been shown to accurately delineate the extent of cardiac fusion, the intracardiac anatomy, and the ventricular function (Andrews et al., 2006). Planning of the antenatal course and the perinatal management of twins can be facilitated by identifying those twins at particular risk, such as twins with twin reversed–arterial perfusion sequence. In this situation, vascular communications between the two fetuses allows deoxygenated blood from one fetus (the pump twin) to perfuse the other fetus (the perfused twin), resulting in reversed flow in the umbilical vessels and the development of multiple anomalies, including acardia, in the perfused twin (Norwitz et al., 2000). Antenatal surgical intervention, removing the acardiac twin in utero, may allow for the survival of the remaining (pump) twin. If they are conjoined, however, this is not possible, and immediate surgical intervention at birth is necessary. In order to make rational and sound decisions as to the anesthetic management, anesthesia for fetoscopic fetal surgery requires knowledge of the pathophysiology of the fetus, the fetoplacental unit, and the condition of the mother (Galankin et al., 2000).

Plans may be necessary, in those units providing that option, for an ex utero intrapartum treatment (EXIT) procedure (Bouchard et al., 2002). In this operation, access to the fetus is achieved when the fetus is brought into the surgical field while it is still attached to the mother’s placenta. The planned procedure, whether for access to the fetal airway in the case of an airway tumor, or to allow separation of a conjoined twin from its non–survivable twin, is then carried out. Once it has been completed, the infant is then delivered from the mother and separated from its placenta. This procedure has also been used for airway control of both twins when the antenatal airway and cardiopulmonary status of the twins was not known (Ossowski and Suskind, 2005).

At delivery, it is optimal to have two sets of all neonatal resuscitation equipment available and a pediatrician (or anesthesiologist) present for each infant. A conventional open incubator usually provides a large enough surface area to accommodate both infants. If one twin is doing poorly, it should be attended to first. Especially with thoracopagus twins, over-vigorous ventilation of one may compromise the other because of the chest contents moving across into the chest cavity of the other. If intubation is necessary, this should be performed on one infant at a time, with the other twin being supported, if necessary, with bag face-mask oxygen and ventilation. A T piece and small mask is easier to use than an Ambu bag. While laryngoscopy is being performed on one twin, care must be taken to protect the infant who is not being intubated from trauma to the face and eyes by the laryngoscope. The aim of immediate postnatal management should be the resuscitation and stabilization of both infants, thorough physical examination and special investigations that will allow for definition of the relevant anatomy, and subsequent medical and surgical management.

Cardiopulmonary resuscitation (CPR) in conjoined twins has significant limitations. Not only can it be physically challenging, but it may also be unreliable because the anatomy is distorted and access to the heart, especially in thoracopagus twins, is limited. Damage to other organs, particularly the upper gastrointestinal tract and liver, may occur (Millar et al., 2009). The basics of neonatal and pediatric resuscitation should be followed.

Medical and surgical management

Spitz and Kiely (2002) describe the management of conjoined twins in the prenatal and postnatal stages. When complex thoracopagus or craniopagus twins are diagnosed, termination of pregnancy is recommended. The postnatal management involves the following options:

Some overlap occurs when the twins arrive for emergency surgery before the results of all the investigations have enabled decisions around survival to be made (e.g., when intestinal obstruction develops). Emergency surgery is then performed, but later findings may indicate that other anomalies that are incompatible with life are present. In all studies, emergency surgery for separation is associated with a poor prognosis.

Indications for emergency separation include the following (O’Neill, 1998, Millar et al., 2009; Rode et al., 2006; Cywes et al., 1997):

Elective separation for simple conjunctions can be performed in the neonatal period with minimal problems. No benefit is gained from waiting for the infants to grow or for further investigations to be done. The maternal hormonal influences are still present; thus, the skin is more pliable to cover the wound defect. In general, separation is planned for some time between 4 and 11 months of age when the infants are bigger and their investigations are more meaningful and when they have had adequate tissue expansion to allow closure of the skin defect (O’Neill et al., 1988).

Investigations

Accurate preoperative imaging is essential to surgical planning and prognosis, and the area(s) of fusion determine the imaging modality used. MRI and CT provide very good anatomic and bone detail and show organ positions, shared structures, and limited vascular anatomy (Kingston et al., 2001). Radiography with contrast material provides excellent gastrointestinal and urogenital evaluation. When there is liver conjunction, its anatomy, the vascular supply, and assessment of the biliary tree are required. Angiography helps clarify the vascular supply of organs and determine blood supply between the twins. Careful use of contrast material is important, because there may be overenthusiastic administration of this in an attempt to achieve better views. Evaluation of vascular shunts and cross circulation is vital for anesthesiologists—loss of blood from these during surgery can be catastrophic.

Cardiac catheterization is required to identify intracardiac connections and to clarify cardiac chambers. Echocardiographic technology is constantly improving, so the need for catheterization (and therefore anesthesia) is becoming less common. When cardiac surgery is required, however, this is the preferred investigation (Fig. 29-5).

FIGURE 29-5 A cardiac catheterization image of complex cardiopagus thoracopagus twins with a catheter and contrast material in each heart. These twins could not be separated.

Results of these tests clarify anatomic variations in each infant. Understanding the complexity of organ conjunctions enables investigators to identify those structures that are present or missing. The results of complex investigations are best evaluated by a multidisciplinary team, because many factors are raised that anesthesiologists do not consider but that are important for both anesthetic management and good overall care of the infants. Each test has its own role for all of these requirements. Box 29-2 details investigations for craniopagus twins. The order in which procedures occur is also determined by the results of the investigations.

Sedation Techniques for Investigations

The anatomic evaluations of conjoined twins require multiple tests that commonly require the administrations of sedative medications or anesthetic agents. These techniques or options may vary from a simple feed to a full general anesthetic. Regardless of the choice, a full and comprehensive presedation evaluation should be carried out. The choice of technique depends on the following factors:

• Age and size of the twins

• Site and complexity of the twins’ conjunction

• Airway anatomy of the twins

• Venue for the investigation

• Accessibility to the twins

• Level of pain caused by procedure

• Length of time taken for the investigation

• Availability and achievability of an intravenous catheter line

Options for simple sedation used include a feeding before CT or MRI, sucrose on a pacifier, swaddling and physical support, chloral hydrate, trimeprazine (with or without droperidol), and midazolam (Thomas, 2004; Thomas and Lopez, 2004). Two pediatric anesthesiologists should be present from the beginning of these investigations, rather than them having to be called in an emergency situation.

Commonly used agents include propofol, ketamine, fentanyl, dexmedetomidine, and inhalational agents. For MRI evaluations, the safest option may be a general anesthetic to ensure airway control for the procedure (Sury et al., 1994). The use of laryngeal mask airways may not always provide the ideal airway management, and rescue maneuvers in this environment are fraught with potential complications (Shank et al., 2005

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