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Synonyms:  Incomplete conjoined twins, incomplete monozygotic twins.

Definition: Incomplete monozygotic twinning with duplication of few organ systems.

Prevalence: 0.1-0.2:10,000 births, being more prevalent in girls^1,3,6.

Embryology: Incomplete division of the embryonic disc after the 14^th day after fertilization (see p 7594-7).

Associated anomalies: Superior and inferior conjunction defects (duplicata incompleta) are grossly anomalous.

Differential diagnosis: Multiple gestations.

Prognosis: Usually stillborn or die shortly after delivery.

Recurrence risk: Not increased.

Management: Prior to 24 weeks gestation, a vaginal delivery can be offered. Later in pregnancy, vaginal delivery may be possible for preterm conjoined twins. Near term, a cesarean section is the delivery method of choice in order to prevent trauma to the mother and decrease the risk of maternal morbidity.

Although duplication of part of a twin is different from duplication of a complete twin, none of the classifications separate the two entities. Thus the codes on page 7594-1 are used for this disorder also.

Introduction

The most severe form of conjoined twins is the duplicata incompleta type. It accounts for one in 50,000-100,000 births^1,3,6 and is more prevalent in females. The conjoined twinning process occurs when the division of the embryonic disc occurs more than 14 days after fertilization. This entity carries no chance for intrauterine or postnatal correction. Obstetrical management for delivery prior to 24 weeks gestation is via the vaginal route. For preterm labor, a vaginal delivery may be attempted since postnatal survival is impossible. Near term, a cesarean section should be performed to decrease the risk of maternal morbidity and prevent maternal trauma.

Case report

A 24-year-old white woman G₁P₀, presented at 15 weeks 4 days gestation by dates for a routine antenatal visit. The fetal heart was auscultated and found to be slightly irregular. She had no known exposures to teratogens, no known infections, and no family history of  congenital anomalies. She had a regular menstrual cycle. An in-office ultrasound evaluation was performed at 15 weeks 6 days gestation using a 3.5 MHz linear array real-time scanner (ATL Ultramark 4). A dicephalic fetus was found in a vertex presentation having independent head motion. Two distinct cervical spines were noted fusing caudally. A single fetal heart was noted having regular cardiac activity; however, a structural evaluation could not be accomplished due to a prone position. A single fetal stomach and single fetal urinary bladder were seen. The extremities were difficult to visualize; however, there appeared to be two arms and two legs. The placenta was posterior-fundal in location and grade I. The umbilical cord could not be evaluated. The amniotic fluid volume was within normal limits. The patient was informed of the ultrasound diagnosis of this very rare form of conjoined twins and was referred for a Level II ultrasound to confirm our diagnosis and be counseled. This was done and the patient expressed a strong desire to terminate the pregnancy. She had a D&E performed at 18 weeks gestation.

Figure 1: The bifurcation of the spine and two heads.

Figure 2: The two spinal cords side-by-side.

Table 1: Classification of conjoined  twins (duplicata completa and incompleta), adapted from^7-9.

Discussion

Ultrasound diagnosis

This case illustrates the early ultrasonographic diagnosis of duplicata incompleta dicephalus, an inferior conjunction conjoined twin defect, at 15 weeks 6 days gestation. This diagnosis was made based on the following ultrasonic findings:

•  two fetal heads with independent motion

•  two fetal cervical spines

• two fetal thoracic spines in close proximity fusing caudally

• one fetal body

• one fetal heart with cardiac activity

• one fetal stomach

• one fetal bladder

• one posterior-fundal placenta

• no separating membrane

Real-time ultrasound is the best method of diagnosing and initially assessing conjoined twins⁵. The diagnosis of conjoined twins should be entertained when a twin pregnancy is identified with a single placenta and no separation membrane. Polyhydramnios is frequent, occurring in 50% of the cases, as opposed to 10% of normal twins and 2% of normal single pregnancies⁸. Polyhydramnios was not noted in the case presented; however, it may have been too early to manifest. Monoovular twinning is thought not to be influenced by heredity, race, maternal age or parity³.

The pitfalls of the sonographic diagnosis of conjoined twins include not persistently visualizing an area of inseparable fetal contour at the same anatomic level, thinking that fetuses must be positioned as a mirror image, and interpreting the conjoined twins as a single fetus with a malformation. Ultrasound was the only testing modality utilized to confirm the diagnosis in this case.

Embryology

The twinning process occurs in the very early stages of embryogenesis. If division of the zygote occurs during the first 4 days after fertilization, a dichorionic, diamniotic twin gestation results because the trophoblast (chorion) differentiates at that time. If division of the zygote occurs between the 4th and 8th day after fertilization, a monochorionic, diamniotic twin gestation results because the amnion differentiates at approximately 8 days after fertilization. When the division of the zygote occurs by the 14th day after fertilization, the monochorionic, monoamniotic twin gestation results. Any division after the 14th day after fertilization results in an incomplete division, because once it becomes  an embryo, it is incapable of fission. The later the division, the more severe the effect. Duplicata incompleta is the most severe form of incomplete division. This entity is seen more frequently in females.

Pathogenesis

Duplicata incompleta is the most severe anomalous form of twinning. Because division is so late and incomplete, only a few organ systems are duplicated. This makes surgical division impossible. Many variations of conjoined twins are possible. It is customary to name these large lateral fusions, which incorporate multiple regions, on the basis of the anatomic part that remains double and separate; for example, dicephalus, two heads with fusion of the thorax and abdomen1. Guttmacher and Nichols developed a classification system to define the types of conjoined twins. These are listed in Table 1^7-9. Dicephalus, which occurs frequently in amphibians and reptiles but rarely in man,  is among the rarest types of conjoined twins, with fewer than 70 cases being reported in the literature⁴. Our presented case is of   this type.

Management

A vaginal delivery can be offered prior to 24 weeks gestation. After 24 weeks gestation a vaginal delivery can be attempted if damage to the mother can be avoided. Near term, a cesarean section is indicated in order to prevent maternal trauma and decrease the risk of maternal morbidity. Delivery in a tertiary center is usually not necessary because postnatal survival is nil. However, because these patients are usually being followed by a high-risk team in a tertiary center, they may desire to be delivered there. The wishes of the patient should always be considered. Dicephalus conjoined twins are usually stillborn or die shortly after birth, although in 1937 the Perosian twins, Gayla and Ira, lived 1 year 22 days⁴. Compassionate care must be provided for all live births.

Prognosis

Historically, the prognosis is poor and probably will not change.

References

1. Barth RA, Filly RA, Goldberg JD: Conjoined twins: Prenatal diagnosis and assessment of associated malformations. Radiology 177:201-07, 1990.

2. Jirous J, Radocha K, Hanas S: Dicephalus, tribrachius: Prenatal diagnosis and management. Acta Obstet Gynecol Scand 66:79-81, 1987.

3. Hammond DI, Okun NB, Carpenter BF: Prenatal ultrasonographic diagnosis of dicephalus conjoined twins. Can Association Radiol J 42:357-9: 1991.

4. Apuzzio JJ, Ganesh VV, Chervenak J: Prenatal diagnosis of dicephalous conjoined twins in a triplet pregnancy. Am J Obstet Gynecol 159:1214-5, 1988.

5. Fitzgerald EJ, Toi A, Cochlin DL: Conjoined twins: Antanatal ultrasound diagnosis and a review of the literature. Br J Radiol 11:94-6, 1983.

6. Chatterjee MS, Weiss RR, Verma UL: Prenatal diagnosis of conjoined twins. Prenat Diagn 3:357-61, 1983.

7. Guttmacher AF, Nichols BL: Teratology of conjoined twins. Birth Defects 3:3-9, 1967.

8. Chan DPC: Thoracompholapagus diagnosed before delivery. Med J Aust, I, pp480-3, 1976.

9. Romero R, Pilu GL, Jeanty P: Prenatal diagnosis of congenital anomalies. Appleton-Lange, Norwalk, 1988, p 405.