Conjoined twins are identical twins whose bodies are joined in utero. The prevalence is estimated to range from 1 in 50 000 to 1 in 100 000 births, with a somewhat higher occurrence in Southwest Asia and Africa . The condition is more frequently found among females, with a ratio of 3:1 . Many are born with abnormalities incompatible with life. The overall survival rate for conjoined twins is approximately 20 percent .
The term conjoined twinning refers to an incomplete splitting of monozygotic twins after day 12 of embryogenesis. The fetuses are physically joined at some anatomical location as a result. The point of union is used to classify twins; the suffix used is the Greek word pagos, which means “that which is fixed” .
Thoraco-omphalopagus: Fusion from the upper chest to the lower abdomen. These twins usually share a heart, and may also share the liver or part of the digestive system.
Thoracopagus: Fusion of the upper thorax of variable extent. The heart is always involved.
Omphalopagus: Fusion at the abdomen. Unlike thoracopagus, the heart is never involved; however, the twins often share a liver, digestive system, diaphragm, and other organs.
Ischiopagus and pyopagus: Fusion at the pelvis or sacrum. Usually genitourinary and intestinal tract are involved. May have limb abnormalities.
Craniopagus: Fused skulls, but separate bodies. These twins can be conjoined at the back of the head, the front of the head, or the side of the head, but not on the face or the base of the skull. The brains may be separate or fused with variable amounts of shared brain tissue, CSF spaces, and blood vessels.
Parasitic twins: Twins that are asymmetrically conjoined, resulting in one twin that is small, less formed, and dependent on the larger twin for survival. This is known as heteropagus twinning.
Thoracopagus, thoraco-omphalopagus, and omphalopagus twins comprise approximately 70 percent of all conjoined twins. Other classification terms are symmetrical or equal conjoined twins (i.e., two well-developed babies) and asymmetrical or unequal conjoined twins (i.e., a small part of the body is duplicated, or an incomplete twin is attached to a fully developed twin). The later the incomplete embryologic separation occurs, the higher the likelihood of a complicated fusion .
Two opposing theories exist to explain the origin of conjoined twins. The older theory is fission, in which the fertilized egg splits incompletely. More recently, fusion has been proposed, which is now more generally accepted. In the process of fusion, the fertilized egg completely separates, but stem cells find like-stem cells on the other twin and fuse the twins together. Conjoined twins share a single common chorion, placenta, and amniotic sac, although these characteristics are not exclusive to conjoined twins as there are some rarely occurring monozygotic, monoamniotic twins which are non-conjoined .
Treating conjoined twins can be a formidable challenge for the multidisciplinary team. Furthermore, these cases often have religious, moral, ethical, and legal implications.
Conjoined twinning can usually be diagnosed in the first trimester by ultrasound and MRI. It is important to make the diagnosis as early as possible to identify associated anomalies, predict prognosis, plan obstetrical management as well as allow appropriate counseling for the family. Cardiac and neurologic anomalies are key factors in determining prognosis and outcomes. Perinatal care should be directed to centers that have experience managing conjoined twins. Cesarean delivery is almost always the delivery method of choice.
The moral and ethical aspects of separation must be considered once the diagnosis is confirmed, especially in the following circumstances  :
A choice must be made concerning single organ systems. The twin who receives the organ system will live and thrive, whereas the other twin will suffer or die. A similar problem arises when unequal limbs are present.
The twins are craniopagus and have complete brain junction. These twins are usually inseparable.
Additionally, the following will need to be addressed prior to separation .
One person or two? Should be considered two individuals except in the case of parasitic twins, one heart, or one head.
Acceptable operative risks: Operative mortality should not exceed 50 percent and potential for major permanent disability should be considered.
Almost every organ system will need to be investigated thoroughly, taking into consideration shared organs and body parts. Basic laboratory studies (CBC, metabolic panel, liver function, and coagulation studies) are obtained. The most widely used diagnostic tests are X-rays, ultrasound, CT, MRI/MRA/MRV and echocardiography. Catheterization may be necessary to determine complex cardiac anomalies. An accurate assessment of all major inflow and outflow tracts needs to be determined.
A systematic approach to the workup is necessary in conjoined twins. The type of conjoining determines the specific studies needed; the possible areas of fusion are predictable in each type. Three-dimensional models should be built to depict the shared anatomy . The shared structures must be divided, if possible, in order to maintain maximum functional integrity for each twin. Some shared organs (e.g., a single rectum) may not be divisible.
Shared organs need to be evaluated. Mixing of blood is an issue in shared organs as well as direct intervascular communications between twins. The anesthesia team must evaluate the condition of the twins several times before surgery (posture, preferred position, experimenting with different positions, assessing intubation difficulties). Cardiac evaluation and direct communication with a cardiologist is essential to determine potential issues perioperatively. Problems with intravenous access issues should be anticipated and addressed prior to the day of separation. Large-bore central venous access such as Broviac catheters may need to be placed in each twin by a surgeon or interventional radiologist prior to the day of surgery.
The timing of surgery is determined by several factors. Associated anomalies, which may preclude survival or lead to major handicap interfering with acceptable life, need to be determined prior to proceeding with separation. Cardiac anomalies are the most common cause preventing separation followed by brain involvement. The decision has to be made with caution after thorough investigation and several meetings with the multidisciplinary team. Preoperative team conferences should be held specifically aimed at reviewing all the information available. Additionally, moral, ethical, and legal issues will need to be fully addressed prior to separation. Separation is best performed on an elective basis at 9–12 months of age, with a preferred combined weight of at least 8 kg. Such a weight will provide greater skin surface area for coverage and blood volume to tolerate and allow for successful surgery. Experience with separation of large numbers of conjoined twins is limited to only a few centers in the world [9–12].
Staging and Planning Procedures
Most conjoined twins will require anesthesia for medical imaging studies prior to separation. The general principles of anesthesia for conjoined twins discussed below must be adhered to, with some modification. It may be safer to intubate in the operating room and transfer to the radiology suite. In addition, most will require one or more surgical operations: most of these procedures are relatively minor, short in duration, and not associated with blood loss.
Tissue expanders are usually placed several months prior to the planned separation. They are placed with the aim of recruiting skin to facilitate coverage, but may also contribute to complications such as organ compression, sepsis and breakdown, as well as possible compromise of respiration and ventilatory mechanics .
The multidisciplinary team is made up of different specialties that are involved in the management of the twins before, during, and after separation. The parents are part of the team since they must be informed of procedures and risks to which their children will be exposed in detail, the likelihood of separation and quality of life after separation. The multidisciplinary team leader is responsible for the overall management of the case. The anesthesia team leader is usually someone who has prior experience in the separation of conjoined twins. In addition, two different anesthesiologists should be assigned to manage each of the twins. Each anesthesiologist, with an assistant and technician, is assigned to manage one twin throughout surgery (Figure 33.1).
Figure 33.1 Panoramic photo of operating room after separation of twins and transfer to two separate operating tables.
Separation is usually planned at 6–12 months of age and around 8 kg. Surgical separation is a complex surgery involving different phases with several steps in each phase. Positioning of a large amount of equipment, movement, and location must be determined before surgery. Personnel and equipment assigned to each twin should be identified by different color labels and readily recognizable clothing, such as hats. Live rehearsal with the multidisciplinary team addresses these issues as well as positioning of the patients for induction, line placement, and then surgery, draping, and moving the twins after separation. The parents should be asked if they would like to attend the rehearsal/simulation to help them understand what their children will undergo.
Blood loss may be sudden and massive during the separation, necessitating that blood and component products are available in the room. Blood for each twin must also be color coded. Circulatory collapse at separation secondary to unappreciated volume and blood loss must be planned for and anticipated. Frequent laboratory testing during the procedure is necessary.
Routine noninvasive monitors, as well as adequate vascular access and arterial access are absolutely essential. If neurological function is at stake, SSEP (somatosensory evoked potentials) and MEP (motor evoked potentials) monitoring is indicated. An attempt to synchronize ventilation to some degree may be beneficial. Maintaining normothermia may be challenging secondary to the surface area exposed throughout the procedure: forced warmed air, plastic drapes, warmed fluids, etc. are useful in achieving this goal.
The longest part of the operation is the reconstruction phase after separation. Fluid requirements are substantial and blood loss, which may be concealed, can be massive. Normothermia may be especially hard to achieve during this stage of the operation. A primary closure that is too tight may compromise cardiac and pulmonary, as well as, gastrointestinal and renal function.
Prepping and draping and planning for the point of separation have several issues, mainly transferring the just-separated twins onto separate operating room tables while maintaining a sterile field. Once the babies have been separated and transferred to separate operating tables, continued sterility needs to be ensured. Moving from one to two operating tables should be done slowly and in a coordinated fashion. During the transfer gentle handling is essential since it will be necessary to protect the airway, support and protect the limbs, and reestablish lines; many hands are necessary to accomplish this. It is one of the biggest logistical challenges of the entire procedure.
Lap pad, instrument, and needle counts need to be verified, and it is not unreasonable to obtain plain films on both twins to ensure nothing has been inadvertently left inside the infants.
Of all types of conjoined twins, omphalo-ischiopagus twins are the most favorable candidates for elective surgery because of good survival rates . Historically, conjoined twins in general have been placed into three groups  :
Group 1: Those who do not survive delivery and those who die shortly after birth.
Group 2: Those who survive to undergo an elective procedure.
Group 3: Those in whom an emergent procedure is required.
Emergent conditions may arise at any time and include intestinal obstruction, rupture of an omphalocele, congestive cardiac failure, severe degree of respiratory compromise, and life-threatening illness in one of the twins. The following are indications for emergent separation :
Anesthetic Considerations and Challenges
The anesthesia team usually consists of a team leader, two lead anesthesiologists (one to care for each twin), trainees, and technicians. Executing the anesthetic plan involves careful planning and several rehearsals prior to separation; experience helps with these cases.
Premedication is generally not required since most surgical procedures are performed on young infants who have been hospitalized since birth, who are familiar with the hospital environment and personnel. If needed, oral midazolam 0.5 mg kg–1 (or an equipotent dose of intravenous midazolam) may be administered to each twin. If the twins appear equal in size the combined weight can be divided in half; however, if the twins differ in size then the weight may be estimated according to their ratio of size difference. One should not give a double dose to one twin assuming it will be distributed equally, due to incomplete cross-circulation. Ideally, intravenous access should be obtained prior to arrival in the operating room.
Cross-circulation is always present to varying degrees. The extent of the shared organs and vasculature and the effect on the pharmacokinetics and pharmacodynamics of a drug, as well as fluid and blood administration, needs to be appreciated and ideally determined before separation . Communication between the anesthetic teams is critical.
Induction of anesthesia should commence in sequential fashion, not simultaneously; the induction and intubation of one twin followed by the other. Inhalation or intravenous induction of anesthesia in one twin does not usually produce significant sedation in the other twin; however, when a drug is administered intravenously to one twin in the usual individual dose, it will distribute to the whole body and shared organs. Some of the drug that reaches the shared organs may cross over to the other twin; this dose is usually too small to produce a significant clinical effect; muscle relaxants may be an exception, because of the possibility of the premature onset of weakness in the unintubated twin. Inhalational or intravenous induction with ketamine and atropine is appropriate. The first twin is induced and usually intubated nasally without the administration of muscle relaxant. The second twin is then induced and intubated nasally after administration of a muscle relaxant if needed. The advantage of intubating the first twin without the use of muscle relaxant is to avoid the possibility of subclinical paralysis with hypoventilation and desaturation in the second twin. Elevating one twin to optimize induction/intubation of the other twin should be avoided if possible since it may lead to hypotension in the elevated twin.
Airway management in thoraco-omphalopagus twins is predictably more difficult: these twins have difficult airways until proven otherwise. Opisthotonus, hyperextension of the neck, and inability to position each twin supine may make visualizing the cords difficult. Also, since their faces are close to one another access to the mouth and larynx is often difficult, making instrumentation of the mouth challenging. Placing the tracheal tube is often difficult since it tends to get hung-up in the subglottic space due to angulation of the trachea (Figure 33.2).